Molecular rotational resonance spectrometer for synchronous measurements and methods of use thereof

Abstract

Molecular rotation resonance (MRR) spectroscopy is a structure-specific high-resolution spectroscopy technique that can identify chemical species, including enantiomers, that cannot be distinguished by other analytical techniques. In MRR spectroscopy measurements, a gas phase sample is injected into a vacuum chamber in which the gas phase sample is excited with millimeter wave or microwave radiation, and in response, the gas phase sample emits a free induction pulse (FID). The spectrum of the FID pulses provides information about the chemical structure of the sample. In MRR ratio method measurements, the sample is irradiated simultaneously by two excitation pulses at different frequencies (e.g., broadband pulses and narrowband pulses of different frequency bands or narrowband pulses at different frequencies). The pulses stimulate simultaneous or overlapping FID emissions of different components of the sample. The spectra of these FID emissions are compared, resulting in a ratio of the components in the sample.

Description

[0001] Cross-referencing related applications

[0002] This invention claims priority to U.S. Application No. 63 / 486,235, filed February 21, 2023, pursuant to 35 USC 119(e), which is incorporated herein by reference in its entirety for all purposes. Background Technology

[0003] Molecular rotational resonance (MRR) spectroscopy (also known as molecular rotational spectroscopy or microwave spectroscopy) identifies and characterizes molecules, including compounds, by exciting them with electromagnetic radiation based on their characteristic rotational angular momentum transitions and measuring the coherent radiation emitted by the molecules through free induction decay (FID) in response to the excitation. The emitted radiation is detected as a time-varying signal. The spectrum generated by Fourier transforming the signal can be used to identify the characteristic rotational angular momentum of molecules in a sample, which in turn can be used for the identification and / or quantification of analytes and their components.

[0004] The energy levels of a molecule are quantified based on its three-dimensional mass distribution, which is represented by the moment of inertia I. This (in one dimension) is defined as I = Σm i r i 2 , where m i It is the mass of atom i in the molecule, and r i This is the distance between atom i and the molecular center of mass. The rotational spectrum of a molecule is described by the Hamiltonian, which depends on the molecule's moments of inertia along its three spatial axes. By using rotational spectroscopy, molecules can be clearly distinguished by their structural differences. Given that molecules have numerous and extremely narrow spectral lines (typically requiring a very narrow spectral resolution...), this method is highly efficient. ≈ 10 –5 Therefore, the high-resolution rotational spectrum of each molecular structure is unique.

[0005] MRR spectroscopy is typically performed in a vacuum chamber that provides a low pressure environment (e.g., less than 100 mTorr) using electromagnetic radiation in the microwave to millimeter-wave spectrum for excitation (e.g., the microwave region is 1-40 GHz and the millimeter region is 30-3000 GHz). The extremely high resolution of MRR spectroscopy means that different molecules can be resolved directly in a mixture without separation (e.g., a plot (spectrum) of each molecule). Moreover, the structure of the plot depends only on the three-dimensional structure (mass distribution and electronic charge distribution) of the molecule, which can be accurately and efficiently calculated by commercially available quantum chemistry software. With MRR spectroscopy, molecules can be identified directly in a complex mixture without the need for pure reference standards, which can be very expensive and difficult to produce.

[0006] For example, FIG. 1 shows 13 CH3CN and CH3 13 CN, both of which are isotopologues with the same mass, where a clearly resolved spectral plot of each molecule can be observed. Additionally, it is noted that the MRR spectrum of each molecule can be calculated with high precision. Thus, using MRR, molecules can be unambiguously identified without reference samples. SUMMARY

[0007] MRR spectroscopy is typically performed in one of two ways, either broadband measurement or targeted measurement. Broadband measurement is performed by exciting a sample of an analyte with one or more broadband pulses of electromagnetic radiation (e.g., a chirped pulse of microwave or millimeter-wave radiation). Compared to targeted measurement, broadband measurement can obtain a sample spectrum with a relatively large bandwidth (e.g., greater than or equal to 5 MHz) at the expense of lower sensitivity and / or lower spectral resolution. Broadband measurement is typically used to characterize most or all components (in some cases) in a sample (e.g., a mixture), including unknown or unexpected molecules.

[0008] Targeted measurement is performed by exciting a sample with a narrowband pulse of electromagnetic radiation (e.g., a pulse of microwave or millimeter-wave radiation that resonates with a cavity containing the sample). Compared to broadband measurement, the spectrum of the sample acquired in targeted measurement has a relatively small bandwidth (e.g., less than or equal to 1 MHz), which can provide higher sensitivity (e.g., 10 to 100 times) because the excitation power is concentrated in a smaller frequency range for the same measurement time and / or higher spectral resolution. Targeted measurement is typically used to quantify and / or detect known components (e.g., species) in a sample.

[0009] MRR spectroscopy can provide sufficient sensitivity to distinguish between individual components (e.g., isotopic isomers) in a sample and / or to quantify trace amounts of particular components. In principle, multiple MRR measurements can thus be used to distinguish and compare relative abundances of different chemical species within a sample or between different samples, particularly for chemical species that are difficult to distinguish and / or quantify using other analytical chemistry techniques. For example, broadband MRR measurements and targeted MRR measurements can provide complementary information about a sample. Broadband MRR measurements can detect and monitor most or all (in some cases) known and unknown components in a sample, while targeted MRR measurements can selectively detect and quantify known components with higher sensitivity and lower detection limits. In another example, a first targeted MRR measurement can detect and quantify a first component in a sample, and a second targeted MRR measurement can detect and quantify a second component in the same sample to compare relative amounts of the first and second components.

[0010] However, conventional MRR spectrometers or MRR spectroscopy systems typically can only perform one measurement on a sample at a time. In other words, conventional MRR spectrometers cannot simultaneously perform multiple broadband and / or targeted measurements on the same sample. As a result, various factors that affect the repeatability of measurements, such as sample-to-sample variations or changes in environmental conditions over time, can make any quantitative analysis incorporate data obtained from multiple MRR measurements, resulting in greater measurement error and loss of precision. For example, conventional MRR spectroscopy is typically performed by injecting a controlled amount of sample (also referred to herein as a “sample pulse”) into a vacuum chamber. However, even using the same inlet to inject multiple sample pulses from the same sample source, the composition of the sample pulses can vary significantly. In some cases, the total number density of molecules can vary by at least 10% between different sample pulses. In another example, MRR spectrometers are typically susceptible to long-term drift due to thermal effects in the spectrometer and / or changes in the sample, vacuum level of the chamber, and / or pulse nozzle over time (e.g., due to changes in solenoid characteristics or wear of a poppet valve). As a result, even when characterizing the same sample, multiple MRR measurements taken over an extended period of time (e.g., a 24-hour period) can yield different results.

[0011] Accordingly, the present disclosure relates to various inventive MRR spectrometers that are capable of simultaneously performing multiple simultaneous measurements on the same sample. The simultaneous measurements can include, for example, a combination of at least one broadband measurement and at least one targeted measurement or a combination of two or more targeted measurements. The MRR spectrometer includes a vacuum chamber for housing a sample during the measurements. The vacuum chamber provides a low vacuum environment, partially generated by a vacuum pump, and measured by a pressure gauge operably coupled to the vacuum chamber.

[0012] In one non-limiting example, the MRR spectrometer can include a broadband spectrometer and a target spectrometer to perform broadband and target measurements on a sample, respectively. The broadband spectrometer can include several components disposed in a vacuum chamber, such as a source horn antenna to transmit broadband pulses of electromagnetic radiation (e.g., chirped pulses of microwave or millimeter wave radiation) and a receiver horn antenna to receive electromagnetic radiation emitted by components in the sample in response to the broadband pulses. The target spectrometer can include several components disposed in the vacuum chamber, such as a fixed mirror and a movable mirror with a translation stage that together form an adjustable Fabry-Perot resonator, a source antenna to emit narrowband pulses of electromagnetic radiation, and a receiver antenna to receive electromagnetic radiation transmitted by components in the sample in response to the narrowband pulses. The MRR spectrometer can further include one or more signal generators to generate the broadband and / or narrowband pulses, and an analog-to-digital converter (ADC) or digitizer to receive the emitted radiation received by the receiver antennas of the broadband and target spectrometers and convert it from an analog to a digital signal for subsequent processing and analysis. Other MRR spectrometers of the present invention can include different combinations of spectrometers, such as two target spectrometers.

[0013] The MRR spectrometer of the present invention can further include at least one sampling inlet to inject an analyte sample into the vacuum chamber. The sampling inlet can operate in a variety of ways. In one example, the sampling inlet can be coupled to a pulse valve driver to controllably inject sample pulses into the vacuum chamber at desired time intervals. The sample pulses can propagate through the vacuum chamber before being removed by the vacuum pump. In another example, the sampling inlet can inject the sample as a continuous flow into the vacuum chamber, and the vacuum pump can continuously remove the sample so that it does not accumulate within the vacuum chamber. In some examples, multiple sampling inlets can be used to selectively inject samples and references into the chamber for measurement.

[0014] In one non-limiting example, the sampling inlet can be a needle orifice nozzle that injects the sample as a fine spray with a carrier gas in which the sample is maintained in the gas phase. The sampling inlet can also be coupled to a sample source. The sample source can include a reservoir that receives a sample of analyte from, for example, a flow reactor or syringe, and a carrier gas source that provides a flow of carrier gas to facilitate injection of the sample through the nozzle and / or to purge various fluid lines after each measurement. The sample source can further include a gas or liquid chromatograph to separate individual components of the sample before injection into the vacuum chamber.

[0015] The manner in which multiple MRR measurements are considered simultaneous or parallel can vary depending on the manner in which the sample is injected into the vacuum chamber.

[0016] When the sampling inlet injects a sample into the vacuum chamber in a series of pulses (also referred to herein as“sample pulses”), multiple MRR measurements can be considered to be performed simultaneously if corresponding measurements are made on the same sample pulse. For example, the corresponding source antennas of the broadband spectrometer and the target spectrometer can emit broadband pulses and narrowband pulses that illuminate the same sample pulse, and the corresponding receiver antennas of the broadband spectrometer and the target spectrometer can detect FID emissions from the same sample pulse in response to the broadband pulses and the narrowband pulses.

[0017] When the sampling inlet injects a sample into the vacuum chamber as a continuous flow, multiple MRR measurements can be considered to be performed simultaneously if corresponding measurements are made on similar or identical portions of the sample flow within a sufficiently short period of time such that the composition of the portion of the sample flow being characterized remains substantially the same. For example, the source antenna of the broadband spectrometer can emit a broadband pulse to illuminate a first portion of the sample flow, and the source antenna of the target spectrometer can emit a broadband pulse to illuminate a second portion of the sample flow. The second portion of the sample flow should substantially overlap the first portion of the sample flow (e.g., it can overlap by 50%, 60%, 70%, 80%, 90%, or more). Additionally, both the broadband and target MRR measurements should be made within a predetermined period of time (e.g., less than or equal to 1 millisecond).

[0018] In general, the period of time that a sample pulse or portion of a sample flow can be exposed to broadband pulses and narrowband pulses can be significantly longer than the dephasing time of a FID (e.g., the duration of a sample pulse can be 1-4 ps, while the FID dephasing time can be between 10-40 ps). In some cases, the MRR spectrometer can probe a particular sample pulse or portion of a sample flow with multiple broadband pulses and / or narrowband pulses and detect corresponding FID emissions in response to the broadband pulses and / or narrowband pulses. Although the MRR spectrometers disclosed herein can perform multiple MRR measurements simultaneously, it should be understood that individual MRR measurements (e.g., individual broadband MRR measurements, individual target MRR measurements) can be performed on a sample individually as needed.

[0019] The MRR spectrometers further include a timing controller for synchronizing operation of at least the multiple spectrometers and the sampling inlet (e.g., through a pulsed valve driver). For example, the timing controller can control when the target MRR spectrometer and the broadband MRR spectrometer produce respective excitation pulses for measurement, when the detectors (e.g., receiver antennas and ADCs) receive and record signals (e.g., FID emission signals) produced by excitation pulses interacting with the sample, and when the sampling inlet injects sample pulses into the system (e.g., relative to the production of excitation pulses and / or the reception of signals by the detectors). For example, this can be accomplished through the timing controller including a frequency standard (e.g., a rubidium atomic clock) that provides a reference clock signal for synchronizing operation of components in the MRR spectrometers.

[0020] When multiple MRR measurements are performed simultaneously, multiple pulses (e.g., broadband pulses, narrowband pulses) and FID emissions can be present in the vacuum chamber at the same time, which can create additional noise for the measurements. To reduce or in some cases mitigate the noise that can be produced by performing, for example, a broadband measurement and a target measurement simultaneously, components of the spectrometers can be arranged to reduce the amount of electromagnetic radiation associated with one spectrometer that reaches the detector of another spectrometer.

[0021] In one non-limiting example, the vacuum chamber can be shaped as a six-way cross with a first axis for the broadband spectrometer, a second axis for the target spectrometer, and a third axis for various other hardware in the MRR spectrometers (e.g., ports for connecting to a vacuum pump, a pressure gauge, and / or other instrumentation to facilitate operation of the MRR spectrometers). The first, second, and third axes can be orthogonal to one another. Generally, the direction of an FID emission is similar to the direction of the excitation pulse that produced it. By placing components of the broadband spectrometer and the target spectrometer on different axes, FID emissions associated with broadband pulses are less likely to propagate or travel along the axis of the target spectrometer, and in particular, FID emissions associated with narrowband pulses are less likely to propagate or travel along the axis of the broadband spectrometer. Additionally, RF-absorbing material can be disposed on the interior walls of the chamber to reduce the effects of cavity ringing, particularly at the frequencies probed by the broadband MRR spectrometer.

[0022] It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the inventive subject matter disclosed herein. In particular, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the subject matter disclosed herein. It should also be appreciated that terminology explicitly employed herein that also can appear in any disclosure incorporated by reference should be accorded a meaning most consistent with the particular concepts disclosed herein. BRIEF DESCRIPTION OF DRAWINGS

[0023] The skilled person will appreciate that the drawings are primarily for illustrative purposes and are not intended to limit the scope of the inventive subject matter described herein. The drawings are not necessarily to scale; in some instances, various aspects of the inventive subject matter disclosed herein can be shown exaggerated or enlarged in the drawings to facilitate an understanding of different features. In the drawings, like reference numerals refer to like features (e.g., functionally similar and / or structurally similar elements).

[0024] Figure 1 shows the molecular rotational resonance (MRR) spectra of the two isotopologues of acetonitrile, which demonstrate the selectivity for isobaric compounds and the extremely high resolution of MRR spectral lines.

[0025] Figure 2 shows a block diagram of an inventive MRR measurement apparatus configured to perform a broadband measurement and a target measurement simultaneously.

[0026] Figure 3 shows a three-port sampling interface suitable for use with the inventive MRR spectrometer.

[0027] Figure 4A shows a front end of the inventive MRR spectrometer with separate channels and analog-to-digital converters (ADCs) for the broadband signal channel and the target signal channel.

[0028] Figure 4B shows a front end of the inventive MRR spectrometer with frequency multiplexed channels and a single ADC for the broadband signal channel and the target signal channel.

[0029] Figure 5A shows a schematic of the inventive MRR spectrometer configured to perform a broadband measurement and a target measurement simultaneously on an analyte gas sample.

[0030] Figure 5B shows a MRR ratio meter instrument with two Balle-Flygare type target spectrometers arranged with orthogonal cavities and a single nozzle.

[0031] Figure 5C shows a MRR ratio meter instrument with two Balle-Flygare type target spectrometers arranged with orthogonal cavities and a pair of symmetric nozzles.

[0032] Figure 5D shows a dual polarization MRR ratio meter with two moveable planar reflectors (metallic mirrors and wire grid polarizers) and two pulsed nozzle sources for injecting samples into the same volume of the vacuum cavity of the MRR ratio meter.

[0033] Figure 5E shows a folded cavity MRR ratio meter with a wire grid polarization beamsplitter fixed within a confocal microwave cavity and a side-mounted nozzle.

[0034] Figure 5F shows a folded cavity MRR ratio meter with a wire grid polarization beamsplitter fixed within a confocal microwave cavity and a coaxially mounted nozzle.

[0035] Figure 5G illustrates an MRR ratiometer with a dual polarization cavity enhanced detection system (target spectrometer) with a fixed flat mirror and a movable spherical mirror arranged in a semi-confocal mirror geometry.

[0036] Figure 5H illustrates an MRR ratiometer with a dual polarization cavity enhanced detection system (target spectrometer) with a fixed spherical mirror and a movable flat mirror arranged in a semi-confocal mirror geometry.

[0037] Figure 6A illustrates a flowchart of an example method of performing a broadband measurement and a target measurement simultaneously.

[0038] Figure 6B illustrates an example timing diagram of a sample pulse of an analyte, a broadband excitation pulse from a broadband spectrometer, and a target excitation pulse from a target spectrometer.

[0039] Figure 7A illustrates chiral labeling by gas phase formation of different diastereomers with different spectra by combining a chiral tag and an enantiomer.

[0040] Figure 7B illustrates enantiomeric excess analysis from MRR spectroscopy. Measurements are made using racemic or even pure enantiomers of a chiral tag.

[0041] Figure 8 is a calibration curve for determining enantiomeric excess of 1,1,1- trifluoropropan-2-ol (TFIP) using chiral tag rotational spectroscopy.

[0042] Figure 9A illustrates signal changes of individual MRR spectroscopy transitions observed in TFIP measurements.

[0043] Figure 9B illustrates changes or fluctuations in signal in a back-to-back measurement of transitions of TFIP dimers.

[0044] Figure 9C illustrates changes or fluctuations in signal of transitions of TFIP monomers.

[0045] Figure 10 illustrates the 13 C isotopic isomers and 12 C "normal species" transition intensity ratio fluctuations.

[0046] Figure 11 illustrates the 13 C: 12 C isotopic signal ratio MRR ratiometer measurements.

[0047] Figure 12 illustrates the 13 C: 12a series of 60 sequential measurements of the C ratio.

[0048] FIG. 13 is a plot of the difference between consecutive measurements in the series of 60 sequential measurements of FIG. 12.

[0049] FIG. 14 is a plot of EE determination of a high enantiomeric purity sample of (R)-TFIP using the MRR ratio meter of FIG. 5A. DETAILED DESCRIPTION

[0050] The following is a more detailed description of various concepts and embodiments thereof related to MRR spectrometers capable of performing multiple MRR measurements simultaneously. The multiple MRR measurements include, for example, a combination of at least one broadband MRR measurement and at least one target MRR measurement or a combination of two or more target MRR measurements. The various concepts introduced above and discussed in greater detail below can be implemented in a number of ways. The specific embodiments and examples provided for the purposes of illustration are not meant to limit the embodiments of this disclosure to the embodiments and alternatives described herein. Embodiments of this disclosure are capable of being practiced with other embodiments and alternatives that are apparent to those skilled in the art.

[0051] The drawings and example embodiments described below are not meant to limit the scope of the embodiments of this disclosure to a single embodiment. Other embodiments are possible by varying some or all of the elements described or illustrated. Moreover, where certain elements of the disclosed example embodiments can be used partially or completely in implementing a certain portion of the disclosed example embodiments, in some instances only those portions of such known elements that are necessary for an understanding of the embodiments of this disclosure have been described, and detailed descriptions of other portions of such known elements have been omitted so as to not obscure the embodiments of this disclosure.

[0052] In the discussion below, various examples of MRR spectrometers of the present disclosure are provided, where a given example or set of examples illustrates one or more particular features of the vacuum chamber, the broadband spectrometer, the target spectrometer, the sampling inlet, the timing circuitry, and various hardware that facilitates operation of the above components, such as hardware that operates the sampling inlet or moves the movable mirror in the target spectrometer. The features discussed in connection with a given example of a MRR spectrometer can be used in other examples of MRR spectrometers according to the present disclosure, such that the various features disclosed herein can be readily combined in a given MRR spectrometer according to the present disclosure (provided that the respective features are not incompatible with one another).

[0053] The terms "approximately," "about," "generally," and / or "similarly" are used herein to describe certain dimensions and features of the MRR spectrometer. As used herein, the terms "approximately," "about," "generally," and / or "similarly" indicate that each dimension or feature so described can not be strictly bounded or exact, and that a functional equivalent can be used. Unless the context or description indicates otherwise, the use of the terms "approximately," "about," "generally," and / or "similarly" in connection with a numerical parameter means that the numerical parameter includes variations that would be recognized by those of ordinary skill in the art as being equivalent in function or effect to that which is described by the numerical parameter.

[0054] 1. MRR ratio meter

[0055] The present MRR ratio meter can simultaneously perform two Fourier transform microwave (FTMW) spectroscopy measurements on the same sample. These measurements can be performed on different molecular components or species of the sample, which can be a mixture of different compounds or components, and can be used to determine a relative indication of the components, such as a number density ratio or mass ratio of the components. Both measurements can be targeted (narrowband) measurements, or one measurement can be targeted and the other measurement can be broadband. In either case, the resonant frequencies of the two species being measured should be different, allowing both species in the same sample volume to be measured simultaneously. Simultaneously measuring two chemically different species eliminates signal fluctuations caused by variations in the number of molecules injected by the pulsed nozzle source in each measurement cycle.

[0056] In the present MRR measurement apparatus, at least one of the FTMW spectroscopy measurements can be performed using cavity-enhanced FTMW spectroscopy. The enhanced sensitivity of the cavity-enhanced FTMW spectroscopy shortens the time to acquire a signal of a low-abundance molecular species to a specified detection limit. The cavity can be confocal or semi-confocal.

[0057] The present MRR ratio meter can also be capable of injecting two different samples (analyte and reference) into the same spatial region of the spectrometer's vacuum cavity at different times. The reference provides a comparison to the analyte to calibrate the measurement. The analyte sample and the reference sample can each be introduced into the vacuum cavity by pulsed jet expansion of a dilute sample mixture in an inert gas (typically 0.1% of a molecular sample diluted in neon), respectively. Furthermore, switching between the reference sample and the analyte sample during the measurement (e.g., at a rate of once per second or faster) can reduce or eliminate long-term drift in the ratio determination. For example, the present MRR measurement apparatus can switch between the reference sample and the analyte sample on a pulse-to-pulse basis at a pulse rate of 10 Hz (5 Hz of reference pulses and 5 Hz of analyte pulses).

[0058] For example, in stable isotope measurements used to prove the authenticity of a sample, the reference sample can be a sample with a known ratio of a given pair of species, in which case a difference in the ratio of the pair of species in the analyte can indicate a forgery. In cases where different chemical species are considered, all carbon atoms in the molecule are 12 C isotopes, and in the case of a single substituted isotopologue, one carbon atom in the molecule is 13 C replaced.

[0059] In enantiomeric excess measurements using chiral tag rotational spectroscopy, the reference sample can be a mixture of the chiral sample under study and a racemic sample of the tag, and the analyte can be a mixture of the sample under study and a high enantiomeric purity sample of the tag. In this case, the chemical species being monitored can be homochiral and heterochiral complexes formed between the chiral sample under study and the tag in pulsed jet expansion.

[0060] The present MRR ratiometer can include a pulse generation system that can produce two microwave pulses of different frequencies in a substantially simultaneous manner. These pulses are applied to different sample components being measured; each pulse is resonant with a selected rotational transition of a different molecular species. (When broadband and target measurements are performed simultaneously, the broadband pulse can be resonant with two or more species, and the target pulse can be resonant with only one of the species or with a different species.) In some cases, the pulse generation system uses microwave amplifiers to achieve sufficient pulse power to optimally excite the sample, i.e., the so-called p / 2 pulse condition in coherent excitation measurements. These amplifiers can be temperature stabilized to reduce long-term drift in the amplifier gain, which can cause fluctuations in the pulse power and signals from the reference and analyte.

[0061] The present MRR ratiometer can include a coherent detection system that measures FID signals from both the reference sample and the analyte sample. The amplitude of the oscillating electric field in the FID (which can be obtained by a Fourier transform of the FID) is proportional to the number density of the molecular species being monitored. The electric field amplitude of the FID signal is used to determine the relative indication of the two different molecular species being measured, such as the number density ratio or the mass ratio. The detection system can use separate receivers and digitizer systems to record the FIDs from the two different molecular species. As is common in FTMW spectroscopy, the receiver design can employ down-conversion using a mixer and a local oscillator (LO) signal to convert the signal to a lower frequency at the digitizer. The receivers typically use low-noise microwave amplifiers in the signal processing. These can be temperature stabilized to reduce fluctuations in the amplifier gain.

[0062] For high precision measurements, the signals from the two molecular species being measured can be combined using a microwave power divider / combiner circuit element (such as a Wilkinson power divider) prior to any subsequent signal processing (signal amplification and / or frequency conversion). Combining the signals in the microwave domain can reduce fluctuations in receiver performance by providing the same signal path through the receiver (amplifier, frequency conversion components, and digitizer) for both measured frequencies to detect two separate molecular signals at different resonance frequencies.

[0063] The ability of the inventive MRR ratio meter to perform multiple MRR measurements simultaneously can provide considerable benefits for various applications. In one example, a broadband measurement can identify new compounds in a sample, and then target measurements can quantify the abundance of each new compound. In another example, multiple MRR measurements can be performed to compare the relative abundance of different compounds in a sample. For example, the high sensitivity of a target measurement can be utilized to quantify the amount of a first trace component in a sample. Another target measurement (e.g., by a second target spectrometer) can be performed to quantify the amount of a second trace component in the sample. Alternatively, a broadband measurement can also be used to quantify the amount of the second component in the sample if the second compound is more abundant and / or produces a stronger FID emission signal.

[0064] In another example, the inventive MRR ratio meter can be used to characterize isotopically labeled compounds. Given knowledge of the parent structure of a given compound and / or the location of the label in the sample, a broadband measurement can be used to identify the isotopic variants of the compound in the sample, and target measurements can be used to quantify the amount of each isotopic variant, thus providing the distribution of isotopic variants present in the sample. Additional details regarding the use of the inventive MRR ratio meter can also be found below.

[0065] In another example, the inventive MRR ratio meter can be used for high-throughput screening applications. For example, a collection of samples obtained by the same chemical reaction but under different experimental conditions can be characterized to determine which sample, and thus which experimental conditions, provided the highest yield and / or higher conversion efficiency. A broadband measurement can be used to screen the respective samples for unexpected impurities, and target measurements can be used to quantify the components of the samples to determine product yield.

[0066] In another example, the inventive MRR ratio meter can be used for reaction monitoring. For example, the sample inlet of the inventive MRR ratio meter can receive a sample of a reaction mixture from a flow reactor. As the reaction occurs, the inventive MRR ratio meter can monitor and quantify various components of the reaction mixture to assess reaction completion, reaction kinetics, and / or production of impurities. Broadband measurements can be used to screen for unexpected impurities in the respective sample, and targeted measurements can be used to quantify components of the sample to determine product yield.

[0067] In another example, the inventive MRR ratio meter can be used to assess raw material purity in a sample. Broadband measurements can be used to monitor the primary species of the sample (e.g., to monitor the remaining species if the sample is pure), and targeted measurements can be used to identify and quantify known impurities in the sample.

[0068] In another example, the inventive MRR ratio meter can be used to analyze a complex mixture. Given a batch sample of a mixture, the inventive MRR ratio meter can identify and / or quantify components of the sample to assess batch-to-batch consistency of the mixture. Broadband measurements can be used to determine whether new, unknown compounds are present in the sample, and targeted measurements can be used to quantify the amount of compounds present.

[0069] 2. MRR spectrometer for simultaneous broadband and targeted measurements

[0070] FIG. 2 illustrates a block diagram of an example MRR spectrometer 200 that can perform broadband and targeted measurements simultaneously. As shown, the MRR spectrometer 200 includes a vacuum chamber 202 for housing a sample (e.g., a sample pulse of an analyte, a continuous flow of a sample), a pump system 204 operably coupled to the vacuum chamber 202 to generate and maintain a low vacuum environment within the vacuum chamber 202, and a sample inlet 206 (e.g., a nozzle) coupled to a sample source (not shown) to inject a sample into the vacuum chamber 202. The pump system 204 can include a vacuum pump, a pressure gauge, and / or a controller that manages operation of the vacuum pump and the pressure gauge. The sample inlet 206 can be coupled to a pulse valve driver 208 that can be used to controllably inject sample pulses into the vacuum chamber 202 at desired time intervals (see, e.g., FIG. 6). The sample inlet 206 can also inject a continuous flow of a sample into the vacuum chamber.

[0071] The MRR spectrometer 200 includes both the target spectrometer 220 and the broadband spectrometer 230 operably coupled to the vacuum chamber 202 to make target and broadband measurements, respectively, on the same sample at (substantially) the same time. The broadband spectrometer 230 can include two horn antennas 232 in the vacuum chamber 202: a source antenna that emits a broadband pulse (e.g., a chirped pulse) for excitation and a receiver antenna that receives the FID emission from the sample in response to the broadband pulse (see, e.g., the source horn antenna and the receiver horn antenna in FIG. 3). The target spectrometer 220 can also include two antennas in the vacuum chamber 202: a source antenna that emits a narrowband pulse (e.g., a resonant pulse) for excitation and a receiver antenna that receives the FID emission from the sample in response to the narrowband pulse (see, e.g., the source antenna and the receiver antenna in FIG. 3). The target spectrometer 220 can also include a pair of mirrors that form a Fabry-Perot cavity 222 to amplify the excitation of the sample by the resonant pulse and the resulting FID emission. One of the mirrors can be further movable to facilitate adjusting the resonant frequency of the Fabry-Perot cavity 222 as needed, e.g., to correspond to the desired frequency of the target measurement. For example, the movable mirror can be coupled to a translation stage as described below. Other inventive MRR spectrometers can include two target MRR spectrometers.

[0072] The MRR spectrometer 200 includes hardware control circuitry and / or electronics 210 for controlling the operation of, e.g., the sample inlet 206, the target spectrometer 220, and / or the broadband spectrometer 230 in the MRR spectrometer 200. In one example, if pulsed operation is used, the hardware control circuitry 210 can set and maintain the desired rate of the sample injected into the vacuum chamber 202 through the sample inlet 206 and / or the timing interval between sample pulses. In another example, the hardware control circuitry 210 can adjust the position of the movable mirror in the target spectrometer 220 through the translation stage described above.

[0073] The MRR spectrometer 200 also includes timing control circuitry and / or electronics 212 to synchronize the operation of at least the sample inlet 206, the target spectrometer 220, and the broadband spectrometer 230. The timing control circuitry 212 can ensure that the broadband measurement and the target measurement are performed on the same sample at the same time. Additional aspects of the timing control are discussed in more detail below.

[0074] The MRR spectrometer 200 also includes a processor 216, such as a field programmable gate array (FPGA), which is operably coupled to the hardware control circuitry 210, the timing control circuitry 216, the target spectrometer 220, the broadband spectrometer 230, and the pump system 204. The processor 216 can also receive FID signals detected by the receiver antennas in the target spectrometer 220 and the broadband spectrometer 230. These FID signals can be converted from analog to digital signals by one or more digitizers 214 before being sent to the processor 216. In one non-limiting example, the processor 216 can include or be operably coupled to a display device (e.g., a computer monitor) to render and display a graphical user interface and at least one user input device (e.g., a mouse and / or a keyboard) for a user to interact with the graphical user interface. For example, the graphical user interface can provide control for the MRR spectrometer 200 (e.g., selecting desired frequency ranges for broadband and target measurements, timing intervals for sample injection). The processor 216 can perform a Fourier transform on the detected FID emissions to produce a corresponding MRR spectrum and display the MRR spectrum to the user through the graphical user interface.

[0075] The MRR spectrometer 200 generally measures the MRR spectrum of a sample by illuminating the sample with one or more excitation pulses of microwave and / or millimeter wave radiation. The excitation pulses can be generated, for example, by one or more signal generators, such as an arbitrary waveform generator, a direct digital synthesizer, or a pulse pattern generator, and can be filtered, frequency multiplied, and / or upconverted with appropriate circuitry. The MRR spectrometer 200 can include a single signal generator shared by the broadband spectrometer 230 and the target spectrometer 220, or one signal generator for the broadband spectrometer 230 and another signal generator for the target spectrometer 220. If the MRR spectrometer 200 includes multiple signal generators, these signal generators can be locked to a common time base / frequency standard to ensure that the signal generators are synchronized and phase coherent. The excitation pulses generated by the signal generator can include both broadband pulses and narrowband pulses generated simultaneously. The respective source antennas in the broadband spectrometer and the target spectrometer emit the broadband excitation pulses and the narrowband excitation pulses, respectively, to probe the sample in the vacuum chamber.

[0076] If the excitation pulse includes a frequency component that matches the characteristic rotational frequency of an analyte, a component of the sample can be excited by the excitation pulse or pulses. Once the sample component is excited, the sample component can emit a FID signal within a few microseconds. The receiver antenna in the broadband spectrometer and the target spectrometer detect the FID signal emitted by the sample component in response to the broadband pulse and the narrowband pulse, respectively (e.g., using heterodyne detection). Each FID signal can then be digitized by the digitizer 214. In some cases, the FID signal can be mixed down to an intermediate frequency range so that it can be digitized with a lower bandwidth digitizer, as discussed in more detail below with reference to FIGS. 3B and 3C. The processor 216 receives the time-domain digitized FID signals and performs a Fourier transform on them to produce a MRR spectrum.

[0077] The processor 216 can then use a library of MRR spectra containing previously characterized molecules to identify and / or quantify individual components of the analyte based on the MRR spectrum. For example, data obtained from the broadband measurement and the target measurement can be used to determine a relative indication of one chemical species to another chemical species in the sample, such as a number density ratio or a mass ratio. More generally, multiple MRR measurements can be used to determine various ratios of components in the sample, including but not limited to enantiomer ratios, isomer ratios, isotopologue ratios, and isotopomer ratios. Peaks in the MRR spectrum that are not assigned can be further analyzed in order to be characterized using theoretical predictions of related species.

[0078] The processor 216 can record the FID signal at a fine temporal resolution (e.g., at a sampling rate of 5-10 Hz) because the digitizer can achieve the desired throughput rate and the processor can perform data processing operations without dead time. The processor 216 can continuously measure and record the MRR spectrum of the sample as the sample is injected into the vacuum chamber. In some cases, the processor records and processes all MRR spectra. In other cases, the processor 216 records all time-domain data and only performs a Fourier transform on those segments corresponding to the “region of interest” output from the sampling inlet in order to conserve processing resources and reduce the overall processing time. The processor can discard time-domain and / or Fourier-domain data that is unprocessed or unchecked.

[0079] The following is an additional description of various components of the MRR spectrometer 200. Additional details regarding examples demonstrations of operation of MRR spectrometers and / or MRR spectroscopy can be found in U.S. Pre-Grant Publication No. 2022 / 0196582 Al, filed November 5, 2021, and entitled “Highly Selective Chromatography-Molecular Rotational Resonance Spectroscopy Systems and Methods,” which is incorporated by reference herein in its entirety.

[0080] 2.1 Wideband Spectrometer and Target Spectrometer

[0081] The wideband spectrometer 230 of the MRR spectrometer 200 performs wideband measurements by generating a wideband excitation pulse, illuminating the sample with the excitation pulse with a source antenna (e.g., a horn antenna), and receiving the FID emission of the sample in response to the excitation pulse with a receiver antenna (e.g., another horn antenna). The wideband spectrometer 230 can include a dedicated signal generator to generate the wideband excitation pulse. Alternatively, a single signal generator can be shared between the wideband spectrometer and the target spectrometer (see, e.g., MRR spectrometer 400 in FIG. 4A described below). For example, the signal generator can simultaneously generate both a wideband pulse and a narrowband pulse, and send the respective pulses to the source antennas in the target spectrometer and the wideband spectrometer accordingly.

[0082] In one non-limiting example, broadband spectrometer 230 can perform broadband measurements based on a chirped pulse Fourier transform technique involving illuminating the sample with one or more pulses of chirped microwave or millimeter wave radiation, and detecting and Fourier transforming FID signals emitted by the sample in response to the chirped pulses. For more information on chirped pulse Fourier transform MRR spectroscopy, see, e.g., the following U.S. patents, which are incorporated herein by reference in their respective entirety: U.S. Patent No. 9,046,462 entitled “Chirped Pulsed Frequency-Domain Comb for Spectroscopy”; U.S. Patent No. 9,921,170 entitled “Apparatus and Techniques for Fourier Transform Millimeter-Wave Spectroscopy”; and U.S. Patent No. 10,107,744 entitled “Frequency Hopping Spread Spectrum (FHSS) Fourier Transform Spectroscopy”. Broadband measurements can be taken across the entire frequency band at once or over segments of the entire frequency band (e.g., 2 GHz or 4 GHz segments of a 6-18 GHz frequency band) to reduce data rates, as disclosed in U.S. Patent No. 8,873,043 entitled “Segmented Chirped-Pulse Fourier Transform Spectroscopy”, each of which is incorporated herein in its entirety by reference.

[0083] Target spectrometer 220 of MRR spectrometer 200 performs target measurements by generating a narrowband excitation pulse, illuminating the sample with the excitation pulse through a source antenna, and receiving FID emissions from the sample in response to the excitation pulse through a receiver antenna. The target measurements can be facilitated in part by a Fabry-Perot cavity 222, which can be used to amplify excitation and / or FID emissions at desired frequencies, and / or suppress undesired excitation and / or FID emissions at undesired frequencies. Target spectrometer 220 can include a dedicated signal generator to generate the narrowband excitation pulse. Alternatively, as described above and below, a single signal generator can be shared between the broadband spectrometer and the target spectrometer. Target spectrometer 220 can acquire, process, and store target measurement data relatively quickly, for example at a rate of 2 Hz, 3 Hz, 5 Hz, 10 Hz, or faster. This measurement rate can be improved by performing a Fourier transform directly on the acquired data to a field programmable gate array (FPGA).

[0084] Target spectrometer 220 can operate without “species identification” capability, as the one or more excitation frequencies and the target species are known in advance. Target MRR measurements typically involve looking at the single-line time versus intensity, as the measured intensity is proportional to the species concentration. If the target species and / or other information (e.g., separation parameters of a gas chromatography or liquid chromatography system) are known in advance, then target spectrometer 220 can be programmed in advance to target different spectral lines or bands for different species.

[0085] The broadband spectrometer and the target spectrometer can generally make broadband measurements and target measurements in one or more frequency bands in the microwave spectrum (e.g., on a frequency band of 4-18 GHz, including all values and subranges therebetween) and / or the millimeter wave spectrum (e.g., 75-110 GHz, 260-290 GHz, or 520-580 GHz, including all values and subranges therebetween). The bandwidth of the target measurements performed by the target spectrometer can generally be less than the bandwidth of the broadband measurements performed by the broadband spectrometer. For example, the bandwidth of the target measurements can be less than or equal to 1 MHz, including all values and subranges therebetween. The bandwidth of the broadband measurements can be greater than 5 MHz.

[0086] MRR spectrometer 200 can perform broadband measurements and target measurements at non-overlapping frequencies. For example, broadband spectrometer 230 can make measurements between 8-18 GHz, while target spectrometer 220 can make measurements at 6 GHz.

[0087] 2.2 Vacuum chamber

[0088] The vacuum chamber 202 can generally be designed such that multiple spectrometers (e.g., the broadband spectrometer and the target spectrometer in the MRR spectrometer 200) can interact with the same volume of sample injected into the chamber. When multiple MRR measurements (e.g., broadband and target measurements) are performed simultaneously, multiple pulses (e.g., broadband and narrowband pulses) and FID emissions in response to the pulses can be present in the vacuum chamber 202 simultaneously, which can distort the measurement results. To reduce or in some cases mitigate the interference between the respective broadband and target measurements, the components of the broadband spectrometer 230 and the narrowband spectrometer 220 can be arranged to reduce the amplitude of the electromagnetic radiation related to the broadband spectrometer 230 that reaches the detector of the target spectrometer 220, and vice versa.

[0089] When both broadband and target MRR measurements are performed simultaneously, the cavity ringing modes (e.g., resonant modes determined by the shape and size of the vacuum chamber cavity) can also be excited, due to the broad frequency range of e.g., the broadband excitation pulse. To reduce the impact of the cavity ringing on the measurements, RF absorbing material can be added on the inner walls of the vacuum chamber 202 to absorb at least a portion of the cavity ringing modes. The quality factor of the vacuum chamber 202 can also be designed to be low enough such that any cavity ringing modes that are excited dissipate within about 5 microseconds or less. The FID signals generated in response to the broadband pulses can also be time-gated to reduce detection of the cavity ringing modes, as the cavity ringing modes typically dissipate faster than the FID emissions. When no sample is present, any artifacts in the detected FID signals caused by the cavity ringing can also be digitally removed from the MRR spectra by measuring a background spectrum.

[0090] 2.3 Sampling Inlet and Sample Source

[0091] The sampling inlet 206 can generally be fluidically coupled to a sample source (not shown) that provides a sample of an analyte to be analyzed by the MRR spectrometer 200. The sampling inlet 206 can inject the sample into the vacuum chamber in a variety of ways, including but not limited to continuous flow, pulsed injection at predetermined time intervals, and / or trigger-based pulsed injection. Additionally, the sample source of the MRR spectrometer 200 can include various components for preparing the sample for measurement, including but not limited to a gas chromatograph, a liquid chromatograph, a programmable temperature vaporizer, and the like. The MRR spectrometers disclosed herein can generally include one or more sampling inlets 206 fluidically coupled to the same or different sources. For example, some of the MRR spectrometers described below include two sampling inlets: one for injecting an analyte sample into the vacuum chamber 202, and another for injecting a reference sample into the vacuum chamber 202.

[0092] FIG. 3 shows a sampling inlet 306 that can be used in the MRR spectrometer of FIG. 2 to inject both the analyte sample and the reference sample through a shared pulsed solenoid valve 368. This sampling inlet 306 has a three-port sample feed design: it includes an analyte sample port 362 for receiving the analyte, a reference sample port 364 for receiving the reference sample, and a purge port for purging the shared sample volume when switching between the reference sample and the analyte sample. This design allows for a high degree of sample introduction matching of the reference sample and the analyte sample, as the same pulsed nozzle source 368 is used to inject both the reference sample and the analyte sample into the vacuum chamber 202. The sampling inlet 306 is particularly useful in applications requiring high measurement accuracy, such as stable isotope analysis, as it provides the same sample injection characteristics for the reference and analyte.

[0093] An alternative sampling interface can include different electromagnetic injectors for the reference and analyte to eliminate sample cross-contamination. The two sample injection systems can be placed in symmetric locations in or around the vacuum chamber 202 so that the pulsed jet expansion occurs primarily within the same effective volume of the vacuum chamber 202. Alternatively, two separate sample injection systems that use flexible tubing coupled to gas sources can be placed on a movable stage that can translate the nozzles of each sample injection system into the same injection location. It is also possible to use a coaxial orientation beam resonator design to embed two separate nozzles in one of the cavity mirrors.

[0094] In one non-limiting example, the sample source of the MRR spectrometer 200 can include or be coupled to a gas chromatograph (GC; not shown) with a temperature-regulated flow interface. The GC has a carrier gas source that flows a carrier gas, such as helium, hydrogen, neon, or argon, through a column. The carrier gas pushes an analyte, which can have many different chemical constituents, including isomers, isotopes, isotopic isomers, and isotopic conformers, through the column and through the flow interface into the vacuum chamber. This analyte can (periodically) be siphoned out of a continuous stream or flow of gas or liquid, accumulated and volatilized (if appropriate), and injected into the column, such that the gas chromatograph effectively samples the continuous stream much like an analog-to-digital converter (ADC) samples an analog signal.

[0095] Some or all of the constituents of the analyte propagate through the column at different rates, and thus can appear at the end of the column at different points in time. If these points in time are separated widely enough, the constituents can be resolved at the output of the column. Other components (e.g., isomers) can co-elute, i.e., the other components can appear at the output of the column at the same or nearly the same time and thus cannot be resolved using the GC alone.

[0096] The carrier gas pushes the (at least partially separated) components through the interface and into the vacuum chamber, such that the MRR spectrometer 200 can measure the MRR spectrum of the components. This interface enables the possibility to inject the sample through a GC column or directly into the vacuum chamber (e.g., for pure compounds or simple mixtures that do not require GC separation). In other words, some samples can require GC separation, while other samples can not require GC separation. Samples that do not require GC separation can inject the sample directly into the MRR spectrometer (not through a GC), while other samples can be injected through a GC.

[0097] In another non-limiting example, the sample source of the MRR spectrometer 200 includes or is coupled to a liquid chromatograph (LC). In this example, the sample source can include a volatilization interface to volatilize at least a portion of the analyte for injection into the vacuum chamber. For more information on volatilization interfaces, see U.S. Pre-Grant Publication No. 2021 / 0302340 Al, which is incorporated by reference herein in its entirety.

[0098] The sampling interface 206 can also be coupled to (in fluid communication with) a second carrier gas source. The second carrier gas source flows a second carrier gas to the interface in order to push or propel the analyte components into the vacuum chamber. The first and second carrier gases can be different, for example, the first carrier gas can be helium or hydrogen, and the second carrier gas can be neon or argon, as described in more detail below.

[0099] The sampling interface 206 can be coupled to (in fluid communication with) a (chiral) tag source. Mixing the chiral tags from the chiral tag source with the analyte components in the reservoir in the interface causes the chiral tags to attach themselves to different components. The chiral tags change the moment of inertia of different enantiomers in the analyte components, which enables the resolution and quantification of the enantiomers from the MRR spectrum of the components as described below. The tag source can store and supply other types of tags, including for labeling polar molecules that do not have an MRR spectrum with non-polar molecules to create a complex that has a dipole moment and thus can be detected by MRR.

[0100] The sample source can include an auxiliary (universal) detector such as a thermal conductivity detector (TCD) that can trigger MRR measurements based on the output of the GC column. This auxiliary detector can be inline (sampling the same gas stream) or separate (such as a FID or MS detector, where the analyte is destroyed). When the auxiliary detector detects a peak in the GC output, the auxiliary detector sends a trigger signal to the processor, which in turn triggers the emission of excitation pulses from the broadband spectrometer and the target spectrometer and measures and analyzes the resulting FID signal.

[0101] The MRR spectrometer disclosed herein can also be coupled to a sample source that does not include a GC or LC. For example, the sample source can include a programmable temperature vaporizer to vaporize an analyte, and thus inject the analyte into the vacuum chamber of the MRR spectrometer 200.

[0102] The processor 216 of the MRR spectrometer 200 can also record all MRR data, as explained above, and discard MRR data that is not mapped to a chromatographic peak sensed by the auxiliary detector. Additionally, the auxiliary detector data can be combined with the MRR data to provide a more complete analysis of the analyte, for example, the auxiliary detector can sense components that do not have a dipole moment, while the isotopic information from the MRR spectrometer 200 can complete the characterization of other components. For more information on triggering MRR spectroscopy measurements, see, for example, U.S. Patent No. 10,620,138 entitled “Methods and Apparatus for Direct Multiplication Fourier Transform Millimeter Wave Spectroscopy,” which is incorporated by reference herein in its entirety.

[0103] The MRR spectrometer 200 can also be configured to analyze samples having high molecular weight analytes. In typical MRR spectroscopy systems having a flow cell that operates at room temperature or higher, the upper limit of molecular weight can be about 150 amu, and sensitivity at molecular weights of 100-150 amu can be limited because MRR spectra of room temperature molecules tend to become very weak above 150 amu. However, using a pulsed jet supersonic expansion source, a continuous wave jet, or a buffer gas cooled cell rotationally cools molecules for MRR analysis while also keeping the molecules in the gas phase. Thus, in some cases, the sampling inlet 206 of the MRR spectrometer 200 can include a pulsed jet expansion source, a continuous wave jet, or a buffer gas cooled cell to analyze molecules having higher molecular weights (e.g., up to 400 amu or higher) through such rotational cooling.

[0104] In one non-limiting example, a pulsed-jet supersonic nozzle can be used to inject analyte into the measurement cell of the MRR spectrometer 200. The nozzle can receive one or more gas flows, including but not limited to a flow inlet for analyte components, an exhaust valve inlet for rapid removal of solvent or other volatile matrix components, and an optional inlet for (additional) carrier gas. The inlets can be made from 1 / 16" tubing and can be heated to at least 300°C to receive samples at high temperature (e.g., direct inlet from a GC column). The tubing can be formed from PEEK plastic, although the connections can instead be made from metal with better thermal properties. A gas connection for carrier gas (e.g., 1 / 4" gas connection) is optionally provided and can be used to purge gas for rapid cleaning of the sample. The multiple gas flows can be combined in a reservoir, which can have a volume of about 500 mL or less.

[0105] The combined gas flow can be co-expanded with a rare carrier gas through a pinhole nozzle with a diameter of about 1 mm. The pinhole nozzle is rapidly opened and closed by a solenoid valve sealed with Teflon poppet valves. On each gas pulse (duration of about 1 millisecond), the analyte components travel through the pinhole and into the MRR spectrometer measurement cell, which is held at high vacuum (about 10 –6 Torr). Along the path through the pinhole, the molecules experience multiple collisions with the carrier gas. These collisions reduce the rotational temperature of the analyte components to about 1-2 K.

[0106] In one non-limiting example, analyte components separated by a GC column can be injected into a vacuum chamber with a pulsed-jet supersonic nozzle. The carrier gas (e.g., neon) pressure can be set to about +2 to about +5 pounds per square inch gauge (psig). The nozzle is pulsed at a frequency of 10 Hz. On each pulsed injection, the nozzle injects about 1 nmol of analyte components and 1 pmol of neon into the vacuum chamber. This corresponds to about 10-15 mL / min (STP) of carrier gas through the MRR spectrometer 200. This is comparable to the typical flow rate of a wide-bore GC column. Injecting the neon carrier gas at the nozzle can enhance the pulsed valve operation. If neon carrier gas is injected at the nozzle, the GC column can be operated with a different carrier gas (such as hydrogen or helium), as the neon will dominate the rotational cooling caused by the supersonic pulsed injection of the analyte components into the vacuum chamber. Both of these carrier gases can be injected into the vacuum chamber. The pulsed-jet supersonic nozzle can be used with a GC column or other sample source components, including but not limited to LC and programmable temperature vaporizers.

[0107] 2.4 Timing Controller

[0108] The MRR spectrometers disclosed herein can include a timing controller (e.g., timing circuitry and / or electronics) to synchronize the operation of multiple spectrometers (e.g., broadband spectrometer and target spectrometer) and / or sampling inlets (e.g., via a pulse valve driver). For example, the timing controller can synchronize the emission of a broadband excitation pulse and the emission of a resonant excitation generated by a signal generator associated with the broadband spectrometer and target spectrometer. The timing controller can further synchronize the receivers to receive the corresponding FID signals emitted in response to the broadband excitation pulse and resonant excitation pulse and / or the operation of the processor (e.g., to process the digitized FID signals and determine the ratio of the analyte).

[0109] In one non-limiting example, the timing controller can include a frequency standard (e.g., a rubidium atomic clock) that provides a reference clock signal (e.g., a 10 MHz clock signal) for use in synchronizing the operation of various components in the MRR spectrometer. For example, the broadband spectrometer and target spectrometer can each include a dedicated signal generator for generating broadband pulses and narrowband pulses and a detector (e.g., a receiver antenna) for detecting sample emissions. The signal generators used to trigger the pulse valve driver, the processor, and the electronics can be operably coupled to the frequency standard and locked to the same reference clock signal. In another non-limiting example, the timing controller can include a pulse generator and / or a pulse pattern generator to generate timing signals for the respective spectrometers and / or sampling inlets of the system.

[0110] If the system includes a single signal generator that generates both the broadband excitation pulse and the target excitation pulse, the signal generator can also be used to trigger the pulse valve driver or emit the broadband excitation pulse and target excitation pulse in response to a trigger pulse from another circuit or component that controls the pulse valve driver. This signal generator can also emit one or more local oscillator signals for heterodyne detection of the FID pulses emitted by the sample in response to the broadband excitation pulse and target excitation pulse. Whether or not the signal generator is locked to a separate frequency reference, if the broadband excitation pulse and target excitation pulse and the local oscillator signals are all generated by the same signal generator, they should be phase coherent with each other.

[0111] In another example, the system can include separate signal generators for the target measurement and broadband measurement. In this case, both signal generators can be locked to the same frequency reference, with one signal generator triggering the other signal generator and other electronics, or both signal generators responding to a trigger pulse from another circuit or component that is locked to the same frequency reference. In this case, each signal generator emits a local oscillator (LO) for heterodyne detection of the corresponding FID pulses.

[0112] In yet another example, the system can include a separate generator that produces the respective LO signals in parallel with the signal generator that produces the excitation pulses. The separate generator can be, for example, circuitry incorporated into a broadband spectrometer or target spectrometer.

[0113] 2.5 Heterodyne signal detection and digitization

[0114] Figs. 4A and 4B illustrate analog front-ends 440a and 440b, respectively, for detecting and digitizing FID pulses emitted by a sample in response to broadband and target excitation pulses. The analog front-end 440a in Fig. 4B has separate channels for the broadband and target FID pulses. Each channel includes a mixer 442a, 442b for mixing the corresponding FID pulse with a corresponding local oscillator (LO) signal to produce an intermediate frequency (IF) output at a lower center frequency (e.g., 100 MHz for the FID pulses versus 2-18 GHz). Each channel also includes its own digitizer (ADC) 444a, 444b for digitizing the respective IF output and providing the resulting digitized FID pulse to a corresponding input channel of the processor 216, which Fourier transforms the digitized FID pulses to produce broadband and target MRR spectra of the sample. The bandwidth of each ADC 444a, 444b can be up to about 3 GHz. The bandwidth of the FID pulses can be up to about 2 GHz.

[0115] The analog front end 440b in FIG. 4B also includes separate mixers 442a, 442b for the FID pulses emitted by the sample in response to the broadband excitation pulse and the target excitation pulse. However, in this case, the FID signals are mixed with LO signals at different frequencies fl and f2 to different non-overlapping IF bands (e.g., 75-125 MHz and 150-200 MHz). More generally, the IF can be in a range of between about 0 GHz and 3 GHz, including all values and sub-ranges therebetween. The LO frequencies can be in a range of about 2 GHz to about 18 GHz, depending on the FID center frequency and the target IF band. The bandwidth of each frequency channel can be about 30 MHz. The spacing between the different IF ranges can be in a range of about 500 MHz to about 1500 MHz. These down-converted FID signals emitted by the mixers 442a, 442b are digitized by the same digitizer 446, which provides the digitized output to the processor 216 for spectral generation and other further processing. In other words, the mixers 442a, 442b and LOs are used to frequency multiplex the broadband and target FID signal frequencies onto separate frequency multiplexed channels within the frequency band of the same ADC 446, allowing the ADC 446 to digitize both signals simultaneously. The processor 216 can separate or de-multiplex the signals by filtering the signals in the frequency domain. Spurious signals in the signals can also be filtered during post-processing using background measurements.

[0116] FIGS. 4A and 4B are non-limiting examples. In yet another example, the broadband and target FID pulses can be combined prior to mixing and / or amplification. In this example, the analog front end includes a single mixer and a single ADC.

[0117] The analog front ends in FIGS. 4A and 4B can also be used in MRR spectrometers that include two target spectrometers, where the FID pulses generated for each target spectrometer are mixed and / or digitized in separate channels.

[0118] 3. MRR Spectrometer Cavity

[0119] FIGS. 5A-5H illustrate different cavity designs for MRR ratiometers. Each cavity design features two spectrometers: one target spectrometer, and another target spectrometer or broadband spectrometer. The two spectrometers are arranged to excite and detect FID signals from the same or overlapping volume of sample (e.g., analyte sample or reference sample) in the vacuum chamber at the same or overlapping times.

[0120] Figure 5A shows an example MRR ratio meter 500a having a vacuum chamber 502 shaped as a six-way cross with a first axis 521 for a target spectrometer 520, a second axis 531 for a broadband spectrometer 530, and a third axis (not shown) for other hardware (e.g., ports for connecting to a vacuum pump and / or a pressure gauge). The first, second, and third axes can be orthogonal to each other. For example, the third axis can point into and out of the page of Figure 5A.

[0121] The target spectrometer 520 is a Ball-Flygare spectrometer with a Fabry-Perot cavity formed by a fixed mirror 522a and a movable mirror 522b at opposite ends of the vacuum chamber 502 along the first axis 521. A motorized translation stage 524 can move the movable mirror 522b back and forth along the first axis 521 over a range of travel L to change the resonance frequency of the target spectrometer. The target spectrometer 520 also includes a source antenna 526a and a receiver antenna 526b mounted coaxially with the fixed mirror 522a. The broadband spectrometer 530 includes a source horn antenna 532a and a receiver horn antenna 532b in the vacuum chamber 502 at opposite ends of the second axis 531.

[0122] The MRR ratio meter 500a also includes hardware control circuitry 510, timing control circuitry 512, an analog front end and digitizer 514, a processor 516, and one or more signal generators 518. These electronics function as described above, with the signal generators 518 generating the target excitation pulses and broadband excitation pulses; the analog front end and digitizer 514 amplifying, filtering, and digitizing the FID signals; and the processor 516 computing the Fourier transforms of the FID signals and the ratio of the species of interest. The hardware control circuitry 510 and timing control circuitry 512 synchronize the emission of excitation pulses with the injection of analyte or reference sample into the vacuum chamber 502 through a sampling interface or nozzle 506 fluidly coupled to the sample source 508.

[0123] In operation, the source antenna 526a emits a target excitation pulse, and the receiver antenna 526b receives an FID signal from the sample in response to the target excitation pulse. Similarly, the source horn antenna 532a emits a broadband excitation pulse, and the receiver horn antenna 532b detects an FID signal from the sample in response to the broadband excitation pulse. In general, the FID emission travels in the same direction as the corresponding excitation pulse. Thus, the FID emission stimulated by the broadband pulse is likely to propagate along the second axis 531, and is unlikely to propagate along the first axis 521. Similarly, the FID emission associated with the narrowband pulse is unlikely to enter the axis 531 of the broadband spectrometer 530. In addition, the detection frequency range of each spectrometer 520, 530 is generally different. Thus, even if a portion of the FID emission associated with the broadband (narrowband) pulse travels along the axis associated with the target (broadband) spectrometer 520 (530), the detector of the spectrometer should not detect the FID emission.

[0124] FIGS. 5B and 5C illustrate MRR ratio meters 500b and 500c, respectively, each of which includes two target cell enhanced FTMW (Balle-Flygare) spectrometers 520a, 520b with orthogonal cell axis orientations. The reference and analyte samples are introduced into a region 501 of the spectrometers in which the cell volumes of the two FTMW spectrometers 520a, 520b overlap. This can be achieved by placing the pulsed nozzle sources 506b for the analyte and reference samples above (and / or below) the plane of the FTMW spectrometers 520a, 520b, as shown in FIG. 5B. Alternatively, as shown in FIG. 5C, the reference and analyte samples can be introduced with nozzles 506, 506’ on a 45° diagonal.

[0125] FIG. 5D shows an MRR ratio meter 500d with a pair of coaxial, overlapping, polarization multiplexed microwave cavities in a vacuum chamber. The first cavity is defined by a fixed curved mirror 582 on a first translation stage 585 and a movable flat mirror 584. The second cavity is defined by the fixed curved mirror 582 and a movable wire grid polarizer 586 located between the fixed curved mirror 582 and the movable flat mirror 584. The movable wire grid polarizer 586 is located on a second translation stage 587, which can be actuated independently of the first translation stage 585, and can be mounted on the first translation stage 585, as shown in FIG. 5D. Two nozzles 560, 562 inject the analyte and reference samples, respectively, into a common sample region between the fixed curved mirror 582 and the movable wire grid polarizer 586. Source and receiver antennas (not shown) can be mounted with or through the fixed curved mirror 582 or the movable flat mirror 584, just like the source antenna 526a and the receiver antenna 526b in FIG. 5A, although mounting them in the fixed curved mirror 582 tends to simplify the design and construction of the MRR ratio meter 500d.

[0126] The wire grid polarizer 526 transmits radiation polarized orthogonal to the grid lines and reflects radiation polarized parallel to the grid lines. As a result, radiation polarized orthogonal to the grid lines resonates in the first cavity and radiation polarized parallel to the grid lines resonates in the second cavity. The first cavity has a resonant frequency defined by the distance between the fixed curved mirror 582 and the movable flat mirror 584, and the second cavity has a resonant frequency defined by the distance between the fixed curved mirror 582 and the movable wire grid polarizer 526. Both cavities are semi-confocal because they have a movable flat reflective surface and a (shared) curved mirror.

[0127] The MRR spectrometer 500d in FIG. 5D has at least two advantages over the crossed cavity design shown in FIGS. 5A-5C: (1) it has a smaller footprint, making it more suitable for analytical chemistry laboratory spaces; and (2) the cylindrical symmetry of the cavity axes of the two molecular species makes it easier to position the dual-pulsed jet sample source so that it injects the sample into the same effective volume of the spectrometer.

[0128] Wire grid polarizers with excellent performance are commercially available. For example, Millitech, Inc. manufactures a wire grid polarizer for microwave and THz applications that can be made up to 9" in diameter. The microwave optics use tungsten wire that is 0.001" in diameter. A wire grid polarizer with 200 wires per inch has a transmittance of > 99% for 200 GHz radiation when the electric field is polarized orthogonal to the wires, and less than 1% when the electric field is parallel to the wire windings. Performance at lower frequencies used in MRR spectroscopy (e.g., below 20 GHz) can be even better. Wire grid polarizers are also excellent reflectors at THz frequencies. Future advances in manufacturing technology can make it possible to manufacture high performance wire grid polarizers with a spherical shape for use as a confocal cavity mirror.

[0129] FIGS. 5E-5H show MRR ratio meters 500e-500h, respectively, with folded cavities that use a wire grid polarizer 540 as a polarization beamsplitter. In each of these MRR ratio meters 500e-500h, the wire grid polarizer 540 is fixed at a 45° angle with respect to the axis of the microwave cavity of the Balle-Flygare (target) spectrometer. Again, source and receiver antennas (not shown) can be mounted with or through any of the mirrors in these ratio meters, although it is often simpler to design and construct them in the fixed mirrors.

[0130] In FIGS. 5E and 5F, the folded cavity is a confocal cavity defined by fixed curved mirrors 542, 542' on one side of wire grid polarizer 540 and a pair of movable curved mirrors 544 on the other side of wire grid polarizer 540. In FIGS. 5G and 5F, the folded cavity is a semi-confocal cavity defined by fixed flat mirror 546 and movable curved mirror 544 or by fixed curved mirror 542 and movable flat mirror 544'.

[0131] In FIGS. 5E, 5G, and 5H, analyte nozzle 560 injects the analyte sample from the top into the vacuum chamber and reference nozzle 562 injects the reference sample from the side into the vacuum chamber. In FIG. 5F, analyte nozzle 560 and reference nozzle 562 inject the samples into the vacuum chamber through fixed curved mirror 542'. In all of these MRR ratio meters 500e-500h, the nozzles inject the analyte and reference samples into substantially the same or overlapping volumes in the vacuum chamber to achieve more consistent FID excitation and detection. Both MRR ratio meters 500e and 500f include source and receiver antennas (not shown) mounted with or through fixed curved mirrors 542, 542', just like source antenna 526a and receiver antenna 526b in FIG. 5A.

[0132] In all of these MRR ratio meters 500e-500h, wire grid polarizer 540 acts as a polarization beam splitter, transmitting microwave radiation polarized normal to its wires and reflecting microwave radiation polarized parallel to its wires. The transmitted radiation is reflected back from one movable mirror 544, 544' through wire grid polarizer 540 to the receiver antenna and fixed curved mirror 542, 542'. The reflected radiation is reflected back from the other movable mirror 544, 544' to wire grid polarizer 540, which reflects the radiation back to the receiver antenna and fixed mirror 542, 542', 546.

[0133] In these MRR ratio meters 500e-500h, movable mirrors 544, 544' can be moved independently of each other to tune the cavity resonances of the orthogonal polarizations to different molecular resonance frequencies. Having separate movable mirrors to define the cavity lengths for the two orthogonal electric field polarizations provides additional space for placing antennas for coupling microwave pulses into or out of the cavity for FTMW measurements.

[0134] Some of the MRR ratiometer designs described herein can be extended to measure two different ratios simultaneously. For example, an MRR ratiometer with dual-polarization horn antennas as shown in FIG. 5A can monitor two different "strong" signals simultaneously using two orthogonally polarized. Incorporating wire grid polarizers in the spectrometer cavities makes it possible to measure two separate "weak" signals (from sample impurities) simultaneously. This MRR ratiometer can perform two different ratio measurements simultaneously, provided that the pulse generation system and receiver system are extended to allow for additional excitation pulses and coherent FID signal measurements. Likewise, if two cavity instruments incorporating wire grid polarizers are arranged in a crossed configuration, the resulting MRR ratiometer can monitor four separate frequencies per sample injection, or make two separate ratio measurements.

[0135] 4. MRR Ratiometry Methods

[0136] FIGS. 6A and 6B illustrate an example method for simultaneously performing broadband measurements and target measurements to determine a ratio of one component of a sample to another component of the sample (e.g., a ratio of isomers, isotopologues, and / or isotopomers, enantiomer ratio). The steps of this method can be synchronized using a timing controller for the injection of analyte into the vacuum chamber, the broadband measurements, and the target measurements. For example, the steps of this method can be performed according to the frequency standards as described above.

[0137] As shown in FIG. 6A, an MRR ratiometry measurement involves injecting a sample (analyte or reference sample) into a vacuum chamber of an MRR ratiometer (602). Once the sample is in the chamber, the MRR ratiometer measures a first MRR spectrum of the sample with a first excitation pulse (604) and simultaneously measures a second MRR spectrum of the sample with a second excitation pulse (606). Both excitation pulses can be resonant pulses with different resonance frequencies, or one excitation pulse can be a broadband (e.g., chirped) pulse and the other pulse can be a resonant pulse. The MRR ratiometer quantifies a first component and a second component (species) of the sample based on the first and second MRR spectra, respectively (608, 610), compares the quantities of the first and second components (612), and determines a ratio of the first and second components (e.g., a ratio of isomers, isotopologues, and / or isotopomers, enantiomer ratio) (614).

[0138] In general, the time period that a sample pulse or a portion of a sample stream can be exposed to broadband and narrowband pulses can be significantly longer than the phase- shifting time of an FID. For example, as shown in FIG. 6B, the time period that a sample pulse is present in the vacuum chamber can be a few milliseconds, while an FID emission can occur within a few microseconds. In some applications, an MRR ratiometer can probe a particular sample pulse or a portion of a sample stream with multiple broadband pulses and / or narrowband pulses and detect corresponding FID emissions in response to the broadband pulses and / or narrowband pulses (see, e.g., the timing diagram in FIG. 6B). The duration of the excitation pulses on the spectrometer depends on the analyte and can be the same or different. Ideally, the detection is performed completely simultaneously (although this is not a strict requirement; the pulses can be performed alternately to reduce or avoid cross-talk).

[0139] For example, an MRR spectrometer can irradiate a sample pulse with the same broadband pulse and / or narrowband pulse multiple times to improve the signal-to-noise ratio of the detected FID signal (e.g., by adding the spectra obtained from multiple broadband pulses or narrowband pulses). In another example, an MRR spectrometer can perform a frequency sweep on the same sample pulse, where multiple narrowband pulses covering a set of frequencies are generated one after another, and FID signals are recorded in response to the narrowband pulses at different frequencies.

[0140] While the MRR ratiometer disclosed herein can perform broadband measurements and target measurements on the same sample simultaneously, it can also perform the broadband measurements and target measurements on separate samples, respectively, as needed.

[0141] 5. Example use cases

[0142] Molecular rotational resonance (MRR) spectroscopy, or simply rotational spectroscopy, is a high-resolution spectroscopy method with high structural specificity. The technique measures spectral transitions between quantized energy levels of rotational kinetic energy about the molecular center of mass in the Hamiltonian. The parameters in the Hamiltonian for the rotational kinetic energy of a rigid molecule, the parameters A, B, C, known as the “rotational constants,” are inversely proportional to the moments of inertia calculated in the principal axis system. Thus, any differences in the mass distribution of the molecular structure produce differences in the rotational constants and distinguishable spectra. This enables MRR spectroscopy to distinguish isomers of molecules, including isotopologues, and provides a unique analytical chemistry capability. Advances in quantum chemistry have made it possible to estimate the rotational constants of molecules with high accuracy using optimized (equilibrium) geometries. Thus, by comparing experimental and theoretical rotational constants, different chemical species can be identified with high confidence. Finally, the spectrometer used for MRR spectroscopy uses pulsed jet expansion of the analyte seeded in an inert gas, typically neon, into a vacuum, resulting in unparalleled spectral resolution in the field of analytical chemistry. This feature of the instrument for MRR spectroscopy makes it possible to analyze individual chemical species within complex sample matrices without the need for prior chemical separation by chromatography.

[0143] MRR spectroscopy is particularly useful for measurements that are difficult or impossible with well-established analytical chemistry techniques such as nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (often coupled with gas chromatography), infrared spectroscopy, and electronic spectroscopy. Two application areas that have emerged in MRR spectroscopy are chiral analysis and stable isotope analysis. The MRR ratiometer described in this document provides unique measurement capabilities in both of these areas. The MRR ratiometer of the present invention is also generally useful for accurately determining the relative abundance of pairs of chemically distinct species.

[0144] 5.1 Chiral Analysis

[0145] Chirality is a subtle aspect of molecular structure. A molecule is chiral when it has a non-superimposable mirror image. These "handed" molecules are called enantiomers. Individually, enantiomers have identical chemical and physical properties.

[0146] However, when placed in a homochiral or chiral environment, enantiomers can exhibit different chemical behavior. One application where chirality can have an impact is pharmaceutical chemistry, where the homochiral environment of the body can cause the two enantiomers of a drug to have different safety and efficacy. As a result, the current guidance from the U.S. Food and Drug Administration is that active pharmaceutical ingredients (APIs) should be manufactured as single enantiomers. The challenge of enantiomer-specific drug production has driven developments in both synthetic chemistry and chemical measurement. Additionally, new methods of API production employing the concept of continuous manufacturing using flow chemistry methods can also require the use of analytical chemistry techniques. Specifically, for these production techniques, high-speed chiral analysis within high enantiomeric purity limits is desired, which is difficult to meet with chiral chromatography analytical methods. The MRR ratiometer of the present invention can perform spectroscopic chiral analysis with high precision and speed.

[0147] Chiral analysis using the MRR ratio meter of the present application is based on chiral tagging rotational spectroscopy. Chiral tagging measurements provide accurate determination of the enantiomeric excess (EE) of an analyte. The EE is a measure of the relative abundance of two non-superimposable mirror image stereoisomers. This measurement approach uses chiral derivatization to create spectroscopically distinguishable chemical species from the enantiomers (where the enantiomers have identical rotational spectra without derivatization). In chiral tagging rotational spectroscopy, derivatization is achieved using non-covalent attachment of a small chiral molecule or tag through cluster formation in the pulsed jet expansion of a mixture of the analyte and tag molecules. Unlike the chiral derivatization approach in NMR spectroscopy, no additional chemical synthesis steps are performed. Additionally, through this approach, the derivatization does not affect the stereochemistry of the chiral center (a process known as racemization, which would compromise the accuracy of the analysis). It has been shown that small amounts of small, volatile chiral molecules effectively complex with a wide range of chemical analytes and allow for accurate determination of the enantiomeric excess of these compounds within the mixture. For more information on chiral tagging, see, e.g., U.S. Patent No. 11,237,104 entitled “Cavity-Enhanced Fourier Transform Spectroscopy for Chiral Analysis,” which is incorporated by reference herein in its entirety. Additional details on the use of the MRR spectrometer for chiral analysis described herein can be found in the Appendix.

[0148] FIGS. 7A and 7B illustrate the idea behind chiral tagging. Briefly, chiral tagging involves “tagging” an analyte with a small chiral molecule (such as the verbenone / butyrolactone system in FIG. 7A) that has known stereochemistry. In a pulsed supersonic expansion nozzle, weakly bound complexes stabilized by a combination of hydrogen bonding, van der Waals, and other forces can be effectively formed. The resulting diastereomeric complexes have different moments of inertia and can be resolved by the energy of the MRR spectroscopy as described above and illustrated in FIG. 7B. Chiral analysis is important in a wide range of applications, including pharmaceuticals, environmental analysis, and metabolite analysis.

[0149] More specifically, FIG. 7A shows that the enantiomers of verbenone (an example chiral molecule) have the same MRR spectrum because the enantiomers have the same moment of inertia. However, when a chiral tag (in this case, (S)-3-butyne-2-ol) is complexed with both enantiomers, a difference in the moment of inertia results, which produces two species with different moments of inertia and thus different MRR spectra. These two complexes are referred to as heterochiral (i.e., (R,R)-verbenone + (S)-3-butyne-2-ol) or homochiral (i.e., (S,S)-verbenone + (S)-3-butyne-2-ol).

[0150] FIG. 7B shows two spectral lines, one for each of the heterochiral and homochiral complexes in FIG. 7A. In the case of a racemic mixture of butyne alcohol tags, the intensities of the two complexes are observed to be approximately equal (the two complexes differ due to complex dipole moments, formation ratios, and other factors). With pure (S)-3-butyne-2-ol as the tag, different intensities of the spectral lines are observed. These spectral line ratios can be used to calculate the enantiomeric ratio and / or enantiomeric excess of verbenone in the sample.

[0151] Chiral monitoring can be integrated into the present MRR ratiometer as an auxiliary system for introducing a gas-phase chiral tag. For example, the present MRR ratiometer can include an additional valve coupled to a source of a gas-phase chiral tag at an interface of a gas chromatography outlet prior to a sampling inlet (e.g., a nozzle). The chiral tag can be mixed with an eluting sample post-column (e.g., after the sample exits the gas chromatography outlet). Further, a spectral library containing known compounds and their corresponding MRR spectra can include compounds with chiral tags. For the MRR ratiometer 200 in FIG. 2, broadband measurements and target measurements can be used to determine the enantiomeric ratio between two enantiomers in a sample, the enantiomeric excess (e.g., the purity of a chiral substance), the absolute configuration of a chiral component in a sample, and / or the achiral purity of a sample.

[0152] 5.2 Chiral tag enantiomeric excess (EE) analysis as a ratiometric determination

[0153] Using the intensity information in the rotational spectrum of the weakly bound complex formed in the pulsed jet expansion of a chiral analyte with a small chiral molecule (chiral tag or simply tag) used to distinguish between the analyte enantiomers, it is possible to make quantitative measurements of the EE. A homochiral complex is defined as a 1 : 1 cluster of the analyte and tag molecules, where both species have the same chiral designation. Here, the designation is defined by the sign of optical activity (+) or (-). A heterochiral complex is formed from a cluster of opposite chirality. The geometry of the homochiral and heterochiral complexes have different mass distributions (and hence different rotational constants), resulting in different MRR spectra. The derivation of the formula for determining the EE has one assumption. In the pulsed jet expansion, the number density of the homochiral and heterochiral complexes is linearly proportional to the number density of the tag and analyte. Therefore, the transition intensity in the homochiral and heterochiral MRR spectra can be written as:

[0154] I 同 = C 同 ([(+)-analyte][(+)-tag] + [(–)-analyte][(–)-tag]) (1)

[0155] I 异 = C 异 ([(–)-analyte][(+)-tag] + [(+)-analyte][(–)-tag]) (2)

[0156] Here, the constants C 同 and C 异 include all the instrumental response and spectral terms that relate the measured signal to the number density (or concentration) of the species in the sample.

[0157] The chiral tag measurement uses the transition intensities from two different samples. In one case, a racemic sample of the tag is used (this is the reference sample). The second measurement uses a high enantiomeric purity tag sample (in the case where the EE is known from separate analysis). The EE determination uses the normalized transition intensities defined in terms of the intensities in the spectra using the racemic and enantiomeric purity tag samples as follows:

[0158] (3)

[0159] and the same for the heterochiral spectral intensity.

[0160] Using the normalized intensities, the ratio R is

[0161] (4)

[0162] The final analysis result is

[0163] (5)

[0164] Here, the fraction ee of the tag and analyte is used (EE = 100 ee). From each pair of transitions of the homochiral and heterochiral spectrum, it is possible to determine EE.

[0165] An example of the precision and accuracy of chiral tag rotational spectroscopy is shown in the EE determination of 1,1,1-trifluoro-propan-2-ol (trifluoroisopropyl alcohol, TFIP). This analysis is a special case of chiral tag spectroscopy, referred to as “auto-tagging”. In this case, the analyte acts as both tag and analyte, and the analysis uses the homochiral and heterochiral dimers of TFIP formed in the pulsed jet expansion. In this case, EE is determined from:

[0166] (6)

[0167] (7)

[0168] Figure 8 shows the measured calibration curve, where each sample was measured in at least triplicate. Reference samples of known EE were made from mixtures of enantiopure samples of TFIP (EE = 99) and racemic TFIP samples. The amount of enantiopure and racemic samples used in each mixture was determined gravimetrically. The EE of each reference sample was measured in a broadband chirped-pulse FTMW spectrometer using the “auto-tagging” variant of chiral tag rotational spectroscopy, where the dimers of TFIP were used for the analysis. The EE determination of each reference sample was performed in triplicate. Chiral tag rotational spectroscopy gives EE directly from the measurement, so there is no need for analyte samples of known EE to calibrate the measurement. The properties of this measurement can be summarized as follows: (1) the measured EE is exactly the EE of the reference sample (that is, there is no need to use samples of known EE using a different chiral analysis technique to calibrate the measurement); (2) the method is accurate over the entire range of EE; and (3) the measurement has high precision, as indicated by the small spread in the triplicate measurements of each reference sample.

[0169] The chiral tag method can be implemented in a MRR ratiometer. As shown in equation (5), the EE determination is based on the accurate determination of the quantity R, defined using equations (3) and (4):

[0170] (8)

[0171] The second expression indicates that the MRR ratio meter should measure two different transitions simultaneously, one from the spectrum of the homochiral complex and one from the heterochiral complex. In addition, it should have two different sample injections. One uses an enantiopure tag sample. The second uses a racemic tag sample for calibrating the instrument response and is a kind of "reference" measurement. There is no requirement for enantiomeric purity of the analyte used in the reference measurement, so the racemic tag sample does not have to be a known EE analyte for analysis.

[0172] Using the formulation in equation (8), the present MRR ratio meter can: (1) use a cavity-enhanced FTMW spectrometer to reduce the measurement time to measure the weaker signal in the enantiopure tag measurement; (2) measure both the homochiral signal and the heterochiral signal simultaneously at each sample injection; and (3) inject two different samples into the instrument, one using an enantiopure tag and the other using a racemic tag sample for the reference measurement.

[0173] Overall, the accuracy and precision of chiral tag rotational spectroscopy exceeds that of the "gold standard" method of chiral gas chromatography. However, chiral tag rotational spectroscopy has the potential for high speed operation. The MRR ratio meter described here has the potential to realize the potential of the technique.

[0174] 5.3 Current EE measurement limitations

[0175] High speed EE measurements have been demonstrated using a cavity-enhanced Fourier transform microwave (FTMW) spectrometer based on a Balle-Flygare instrument. In the current instrument design, the FTMW spectrometer only allows measurements at a single resonant frequency. In a chiral tag measurement, this means that the signal levels for the homochiral and heterochiral complexes are measured separately by retuning the resonator when switching between the two species. In the case of low abundance chiral tag complexes that exist at low levels, there is a significant limitation to the analysis of high enantiopurity samples (EE ~ 100). Therefore, significant signal averaging is typically used to measure the rotational spectroscopy signal associated with this chiral tag complex until the limit of quantification (typically a signal to noise ratio of 10: 1).

[0176] During the measurement process, several drift sources exist in the signal: time-dependent changes in sample composition, temperature variations altering the amplifier gain in microwave electronics, and positional drift of the cavity resonator mirrors relative to the molecular transition frequencies. Therefore, accurate EE determination at high EE limits is challenging, a significant limitation in chemical manufacturing. The MRR ratiometer instrument described here has two important design additions to combat signal drift: simultaneous measurement of two signals used to determine EE (from rotational transitions of chemically distinct isochiral and heterochiral tagging complexes), and the ability to rapidly switch between the measured sample (analyte) and a reference sample used for measurement calibration.

[0177] 5.4 Stable Isotope Analysis

[0178] The second analytical chemistry application enabled by the MRR ratiometer of this invention is site-specific stable isotope analysis. Stable isotope analysis is a well-established field in analytical chemistry, measuring small changes in the relative abundance of isotopes relative to a reference sample. For example, common analyses determine the abundance of a chemical substance relative to a reference sample. 13 C: 12 Variations in the carbon isotope ratios. These variations are small fractions of the average natural abundance of stable isotopes. For 13 C stable isotope analysis, with changes reported at the part-of-thousand level:

[0179] (9)

[0180] 13 C: 12 Changes in the C ratio are caused by small kinetic isotopic effects in the synthetic pathway of the resulting sample. Therefore, stable isotope analysis can distinguish molecular samples produced by different natural or human-made processes. For example, this measurement capability can be used to verify the origin of chemical samples. This method can be used to verify the origin of expensive chemicals and combat counterfeiting of chemical products.

[0181] Currently, the most common method for stable isotope analysis is isotope ratio mass spectrometry. In this analysis, the analyte is converted into CO2 through combustion, and the CO2 content is measured relative to a reference CO2 sample. 13 C: 12 C ratio. The limitation of this method is that it loses all position-specific information about isotopic variations, namely the δ¹⁸O value of each different carbon atom in the molecular structure. 13 C measurement. This limitation is acceptable for mass spectrometry-based detection because different monosubstituted... 13 C isotope species have the same mass. That is, isotopic isomers, i.e., those with the same amount of carbon. 13Isotopomers that differ in isotopic substitution but have different structural positions are not easily distinguished by mass spectrometry.

[0182] To unlock 13 All of the chemical information in stable isotope measurements involves a technique that provides a distinct, fully resolved signature for each isotopomer and can detect the signal at a signal-to-noise ratio of 100,000: 1 (or better) so that changes in natural abundance can be measured with sufficient precision. Except for very small molecules for which infrared spectroscopy is applicable, only two analytical chemistry techniques have the potential to meet these measurement standards: NMR spectroscopy and MRR spectroscopy. Attempts to develop robust analytical methods using NMR spectroscopy have been reported. These reports show that, 13 C: 13 C: 12 C There are significant site-specific variations in isotope ratios (e.g., in the case of vanillin). However, there are challenges in ensuring instrument transferability of results. In addition, NMR spectroscopy is a slow measurement method that requires expensive large instruments and requires a great deal of maintenance.

[0183] The MRR ratio meter disclosed herein has the potential to extend the field of site-specific stable isotope analysis to a wide range of analytes. MRR spectroscopy can achieve the 100,000: 1 signal-to-noise ratio requirement that many molecules desire. In addition, each isotopomer has a different mass distribution and, therefore, a different rotational spectrum. Finally, the high spectral resolution of the MRR spectrometer ensures that these different spectral features can be measured without spectral overlap. Rewriting Equation (9) shows that stable isotope measurements can be made in a MRR ratio meter where the two signals monitored are for 12 C isotopomers and singly substituted 13 C isotopomers:

[0184] (10)

[0185] In addition, the measurements should be made on two different samples (analyte and reference). To make the MRR ratio meter successful, the measurements should preferably remove the effects of number density fluctuations of the amount of sample injected in each measurement cycle so that the measurement precision is limited primarily, or in some cases, only by the signal-to-noise ratio of the MRR transitions of the 13 C isotopomers.

[0186] 5.5 Characterization of signal fluctuations in pulsed-jet MRR measurements

[0187] The fluctuations of individual rotational transitions in MRR spectroscopy measurements were characterized by measurements on a broadband chirped-pulse FTMW spectrometer. This measurement is an idealized case: (1) all transitions of the same valve pulse are detected in the broadband measurement, so that signal variations caused by the sample injection system are apparent; (2) all transitions are measured using a single excitation pulse, so that variations in the pulse amplitude do not affect the measurement characterization; and (3) all transitions are detected by the same receiver system, so that there are no variations between different detector components. The broadband MRR spectrometer provides excellent performance for ratio measurements. However, the measurement is impractical. In applications such as EE determinations of high enantiomeric purity samples and stable isotope analysis using low abundance stable isotopes, many spectral averages are typically required to reduce the noise level to the point where weaker "impurity" species can be detected with sufficient sensitivity for accurate ratio determinations. Both the long measurement time and the high sample consumption of broadband measurements preclude this approach as a useful analytical chemistry method. The MRR ratiometer described herein includes a cavity-enhanced FTMW detection system to measure lower abundance impurities, so that the measurement time and sample consumption are significantly reduced.

[0188] FIGS. 9A-9C demonstrate signal characteristic measurements using TFIP. This sample was also used to validate ratiometer measurements for EE determinations and stable isotope measurements, as described below. The pulsed jet expansion of TFIP produces clusters (for chiral tag EE determinations), so that both monomer and dimer signals are observed. The measurement data set is 10 back-to-back spectral acquisitions with 40,000 averages of broadband free induction decay signals.

[0189] FIG. 9A shows the signal variation of individual MRR spectroscopy transitions observed in the TFIP measurement. FIG. 9B and FIG. 9C analyze the signal variation or fluctuation in the back-to-back measurements of the TFIP dimer transitions and the TFIP monomer transitions. In both cases, the intensity fluctuation (measured as the standard deviation over the ten individual measurements) is proportional to the intensity of the transition. In addition, the proportionality constant for the dimer transitions is twice that of the monomer transitions. These results indicate that the intensity fluctuation comes from the number density variation in the sample injection cycle.

[0190] The measurements in Figures 9A-9C show that the variation in sample amount from pulse jet injection cycle to pulse jet injection cycle causes a significant fluctuation in signal level. The cause of the fluctuation was determined by observing the signal standard deviation in ten measurements for both the monomer signal and the dimer signal. The signal level is proportional to the number density or concentration of the species in the pulse jet expansion. If the signal variation is caused by a change in number density, then the fluctuation (measured by the standard deviation of the signal in ten trials) should be proportional to the average signal. Furthermore, since the number density of dimers is proportional to the square of the number density of monomers, the percent fluctuation in the dimer signal should be twice that of the monomer signal. The expected signal fluctuation caused by a change in number density from sample to sample was met, as shown in Figures 9A-9C.

[0191] As can be seen from Figure 9A, when the signal fluctuation is caused by pulse-to-pulse changes in the number density of the pulse valve used to inject the gas mixture into the vacuum chamber, there is a correlation in the signal level for all of the transitions. This behavior enables a ratiometric design. By taking the measurement of the signal intensity for both transitions simultaneously and determining the ratio for each measurement, the effect of the number density change can be removed. By considering the fluctuation in the ratio, the effectiveness of taking a ratio measurement in each signal collection cycle can be evaluated. Considering the measured signal intensities Ii and I2 for measuring the ratio R,

[0192] (11)

[0193] Under the condition that the fluctuations in the two signals are uncorrelated, with a normal distribution and a non-zero mean (where the standard deviation is much smaller than the intensity). In this case, the fluctuation in the ratio is related to the fluctuations in both signals, as

[0194] (12)

[0195] Considering the measurement of TFIP, 13 C: 12 The results in panel C of Figure 2 show that for separate measurements of the two isotope signals, one different sample injection cycle, the fractional fluctuation in the isotope ratio is expected to be determined by the pulse-to-pulse change in the number density,

[0196] (13)

[0197] Because the fractional fluctuation in each monomer signal is expected to be about 0.01 in a 40 kavg measurement. However, if the intensity fluctuations are completely correlated, as Figure 9A indicates, then 13 C signal and 12 The measurement uncertainty in the C signal is determined by the noise limit in the measurement,

[0198] (14)

[0199] And because 13 The signal from the C transition is much weaker, and its signal-to-noise ratio dominates the fluctuations in the ratio.

[0200] (15)

[0201] SNR 13C It is used in isotope ratios 13 Signal-to-noise ratio of C transition.

[0202] The results in equations (13) and (15) exhibit different behaviors. For example, in equation (13), the behavior will differ from that observed in the specific monitored data. 13 The same fluctuations in the signal intensity (and therefore signal-to-noise ratio) of C isotope isomer transitions are independent of the ratio determination. Conversely, if the ratio measurement removes the effect of the pulse-to-pulse number density variation during sample injection from the ratio determination, then the isotope ratio variation in sequential measurements will show the same fluctuations. 13 Dependence of C transition strength.

[0203] Figure 10 shows the results from the broadband spectrometer. 13 C: 12 The C isotope ratio measurements are shown. These results indicate that the ratio measurement using intensity measurements simultaneously within the same sample injection cycle is consistent with the effect of the removal number density change on the ratio determination. Three MRR spectroscopic transitions were used for the ratio determination. The x-axis is given. 12 The reciprocal of the strength of the C transition (which is related to...) 13 The intensity of the C transition is proportional. TFIP has three carbon atoms, therefore for these... 12 Each of the C transitions has three measurable parameters. 13 C: 12 The C-ratio is shown as individual data points. Fluctuations in the ratio are presented on the y-axis. The isotopic ratio was measured against the average of three different numbers of signals.

[0204] The solid line in Figure 10 is calculated based on equation (15) and the measured noise characteristics of the chirped pulse FTMW spectrometer used for the measurements. The isotope ratio fluctuation depends on the number of signal averages and the ratios used. 13both. These are hallmarks of a ratio measurement that removes the effects of number density variations associated with the sample injection system. In the specific case of stable isotope analysis, the measurement precision of the isotopic ratio is limited only by the noise level of the spectrometer detection system, and this can be reduced by additional signal averaging. These results show that, in the ideal measurement case, the ratio method mitigates the measurement limitations caused by the pulsed jet source used to inject the sample into the spectrometer. Below, it can be seen that the ratio meter design described in this disclosure has enhanced measurement sensitivity to the weak transitions (e.g., C transition intensity) of the measured ratio, maintaining the improved measurement characteristics demonstrated here in the ideal case. 13

[0205] 6. Experimental ratio and enantiomeric excess measurements

[0206] Figures 11-14 show the performance of this MRR ratio meter in the EE determination and 13 C stable isotope ratio measurements of the molecular sample TFIP. As described above, the MRR ratio meter design in Figure 5A is an extension of the Balle-Flygare cavity enhanced FTMW spectrometer, where two horn antennas are used to make broadband MRR measurements across the cavity of the FTMW spectrometer. The horn antennas do not require any mechanical motion control to tune the measurement system to resonance. In addition, since there is no resonance frequency limitation, the horn antennas can monitor multiple transition intensities in each measurement cycle. This capability increases the flexibility of the analytical chemistry measurement method.

[0207] The MRR ratio meter of Figure 5A is particularly useful in cases where the impurity species is present at low abundance, a key measurement challenge in analytical chemistry. Since one species is present at much higher abundance, the signal can be detected with good sensitivity without the enhancement provided by the cavity resonator FTMW spectrometer. The pulsed jet source used to inject the sample into the vacuum chamber can be oriented perpendicular to the cavity and horn-antenna axis (for 13 C measurements), or can be placed in the cavity mirror in a coaxial beam resonator arrangement.

[0208] Figures 11-14 show the performance of this MRR ratio meter in the EE determination and 13 C stable isotope ratio measurements of the molecular sample TFIP. As described above, the MRR ratio meter design in Figure 5A is an extension of the Balle-Flygare cavity enhanced FTMW spectrometer, where two horn antennas are used to make broadband MRR measurements across the cavity of the FTMW spectrometer. The horn antennas do not require any mechanical motion control to tune the measurement system to resonance. In addition, since there is no resonance frequency limitation, the horn antennas can monitor multiple transition intensities in each measurement cycle. This capability increases the flexibility of the analytical chemistry measurement method. 13 C stable isotope ratio measurements of the molecular sample TFIP. As described above, the MRR ratio meter design in Figure 5A is an extension of the Balle-Flygare cavity enhanced FTMW spectrometer, where two horn antennas are used to make broadband MRR measurements across the cavity of the FTMW spectrometer. The horn antennas do not require any mechanical motion control to tune the measurement system to resonance. In addition, since there is no resonance frequency limitation, the horn antennas can monitor multiple transition intensities in each measurement cycle. This capability increases the flexibility of the analytical chemistry measurement method. 13 ​C stable isotope test measurements, where the sample is injected into a shared active sample volume using a nozzle placed above the plane of the instrument and centered on the common active volume. EE measurements were made using an instrument that injects the sample coaxially with a cavity-enhanced measurement system (COBRA arrangement).

[0209] FIGS. 11-13 show 13 Results of C stable isotope measurements. FIG. 11 demonstrates that the ratiometric design of FIG. 5A removes the effects of back-to-back number density variations in pulsed-jet sample introduction. The black data points show the average of ten sequential 10 kavg measurements of 12 C (normal species) and 13 Measured values of C isotope isotope transition intensities. By themselves, 12 C and 13 Both C transition intensity measurements have measurement fluctuations, measured by a standard deviation of about 10% of the ten different intensity measurements. This result, a constant percentage fluctuation of the signal, is consistent with number density fluctuations as the cause of signal variation, as discussed above. If the measurements were uncorrelated, these fluctuations would make 13 C: 12 The C stable isotope signal ratio produces a 14% change in percentage. However, the data in FIG. 11 demonstrates that the C and C transition intensities measured simultaneously in the same pulsed-jet expansion are highly correlated (i.e., the data points are linearly distributed, rather than the circular pattern exhibited for uncorrelated transition intensities). The ratio measurement is shown by the lighter shaded data points. The fluctuation in the ratio is 0.8%, limited by the signal-to-noise ratio of the C transition monitored in the instrument. This performance demonstrates that with the MRR ratiometric design, the effects of number density fluctuations, which would otherwise cause a 14% change in the signal ratio, are mitigated. 12 C and 13 The C and C transition intensities have a high degree of correlation (i.e., the data points are linearly distributed, rather than the circular pattern exhibited for uncorrelated transition intensities). The ratio measurement is shown by the lighter shaded data points. The fluctuation in the ratio is 0.8%, limited by the signal-to-noise ratio of the C transition monitored in the instrument. This performance demonstrates that with the MRR ratiometric design, the effects of number density fluctuations, which would otherwise cause a 14% change in the signal ratio, are mitigated. 13

[0210] In addition to mitigating the effects of back-to-back measurement fluctuations in the number density of the sample introduction system, a successful ratiometer should reduce, or more preferably eliminate, long-term drift in the signal ratio. In the present MRR ratiometric design, long-term drift is removed by using one nozzle or a pair of nozzles to inject the analyte and reference samples separately into the vacuum chamber. The alternation between the analyte and reference samples on a time scale faster than the instrument drift eliminates these effects in the measurement of the difference between the analyte and reference sample properties, such as the stable isotope measurement shown in equation (9).

[0211] FIG. 12 shows the C: 13 C: 12 ​C stable isotope ratios (1 kavg per measurement, approximately 1.5 minutes per observation). Slow drift in the ratios can be clearly observed. The effect of switching between the reference and analyte samples, and the difference in measured ratios as shown in Equation (9), can be simulated by treating all other measurements as “reference” acquisitions. In this case, the analyte and reference samples are identical, and therefore δ 13 The average value of C is expected to be zero, and this value should not be fixed. The results of this analysis are shown in Figure 13. The average value of the reference-analyte switching model is zero within the measurement uncertainty range: δ 13 C = 3.7‰ ± 10‰. δ 13 The uncertainty of C (σ = 10‰) and the weaker 13 The consistent signal-to-noise ratio of the C transition allows for deeper signal averaging, which reduces measurement variability.

[0212] Test measurements of the EE determination using TFIP also demonstrate that the MRR ratiometer in Figure 5A mitigates the effects of pulsed sample injection number density fluctuations, resulting in highly accurate measurement results. Furthermore, the ability to incorporate both analyte and reference samples provides highly accurate EE determinations using the results of equations (7) and (8). The (R)-TFIP used in the test measurements yielded an EE of 95.9 in a separate analytical chemistry determination. Analyte measurement presents challenges for EE determination applications because the second enantiomer is present in low abundance. Therefore, cavity-enhanced detection is typically used, and even in this case, significant signal averaging may be required. To achieve the desired measurement accuracy, the MRR ratiometer can remove the effects of sample number density fluctuations by simultaneously measuring the signals of isochiral (strong) and heterochiral (weak) dimer transitions.

[0213] Five back-to-back measurements were performed in three TFIP test measurements. As expected based on the results shown in Figures 9A–9C, individual isochiral and heterochiral measurements showed constant percentage fluctuations: both cavity-enhanced heterochiral dimer complex transition intensities and isochiral transition intensities were 7%. If these signals are uncorrelated, then equation (8) The ratio will fluctuate by 10%. However, the ability to measure two signals simultaneously reduces the fluctuation by 2%.

[0214] Figure 14 shows the results of EE determinations in five back-to-back measurements of (R)-TFIP using the MRR ratiometer of Figure 5A. Each measurement reports four separate EE values. This is possible because the chirped pulse is used to measure a strong isochiral signal on the horn antenna detection axis, and four strong isochiral transitions are observed in each measurement cycle, which can be used as a basis for calculating the EE. The homochiral signal in the ratio. In twenty determinations, the average EE determination was EE = 95.7. This value is in excellent agreement with a separate EE determination performed on the sample (EE = 95.9). The measurement precision, as measured by the standard deviation of the twenty measurements, was 0.14. This measurement precision exceeds the performance typically observed in chiral GC / MS.

[0215] Further examples, embodiments, and / or modifications of embodiments of the application as described herein are described by one or more of the following numbered clauses.

[0216] Clause 1. A molecular rotational resonance (MRR) measurement device comprising: a vacuum chamber for housing a sample; at least one signal generator for emitting a first excitation pulse and a second excitation pulse; a first transmitter operably coupled to the at least one signal generator for illuminating the sample with the first excitation pulse; a first receiver for receiving a first free induction decay (FID) signal emitted by the sample in response to the first excitation pulse; a second transmitter operably coupled to the at least one signal generator for illuminating the sample with the second excitation pulse while the first transmitter is illuminating the sample with the first excitation pulse; a second receiver for receiving a second FID signal emitted by the sample in response to the second excitation pulse; and a processor operably coupled to the first receiver and the second receiver for determining a relative indication of at least two components of the sample based on the first FID signal and the second FID signal.

[0217] Clause 2. The MRR measurement device of clause 1, wherein the first transmitter is configured to illuminate the sample with the first excitation pulse along a first axis, and the second transmitter is configured to illuminate the sample with the second excitation pulse along a second axis different from the first axis.

[0218] Clause 3. The MRR measurement device of clause 1, wherein the first excitation pulse is a first resonant excitation pulse, and the second excitation pulse is a second resonant excitation pulse.

[0219] Clause 4. The MRR measurement device of clause 1, wherein the first excitation pulse comprises a broadband excitation pulse, and the second excitation pulse comprises a resonant excitation pulse that encompasses a narrower bandwidth than a bandwidth of the broadband excitation pulse.

[0220] Clause 5. The MRR measurement device of clause 4, wherein the at least one signal generator is configured to generate the resonance excitation pulse at a frequency outside of the wideband excitation pulse band.

[0221] Clause 6. The MRR measurement device of clause 5, wherein the resonance excitation pulse and the wideband excitation pulse include frequency components in a range of 2 GHz to 18 GHz.

[0222] Clause 7. The MRR measurement device of clause 4, wherein the wideband excitation pulse is a chirped excitation pulse.

[0223] Clause 8. The MRR measurement device of clause 4, further comprising: a pair of mirrors disposed in the vacuum chamber and forming a Fabry-Perot cavity having a resonance corresponding to the resonance excitation pulse.

[0224] Clause 9. The MRR measurement device of clause 1, wherein the first excitation pulse and the second excitation pulse include frequency components in a range of 2 GHz to 18 GHz.

[0225] Clause 10. The MRR measurement device of clause 1, wherein the first receiver is configured to mix the first FID signal to a first intermediate frequency range, and the second receiver is configured to mix the second FID signal to a second intermediate frequency range different from the first intermediate frequency range.

[0226] Clause 11. The MRR measurement device of clause 10, wherein the first intermediate frequency range and the second intermediate frequency range are between about 0 GHz and about 3 GHz.

[0227] Clause 12. The MRR measurement device of clause 1, further comprising: a timing controller operably coupled to the sampling interface and the at least one signal generator for triggering emission of the first excitation pulse and the second excitation pulse in coordination with injection of the sample.

[0228] Clause 13. The MRR measurement device of clause 12, wherein the timing controller includes a frequency standard configured to generate a reference frequency clock signal to synchronize the emission of the first excitation pulse with the emission of the second excitation pulse.

[0229] Clause 14. The MRR measuring device of clause 12, wherein the timing controller is further configured to trigger the first receiver to receive the first FID signal, the second receiver is further configured to receive the second FID signal, and the relative indication is a ratio of a first component to a second component.

[0230] Clause 15. The MRR measuring device of clause 1, further comprising: a sampling interface in fluid communication with the vacuum chamber for injecting the sample into the vacuum chamber.

[0231] Clause 16. The MRR measuring device of clause 1, further comprising: at least one analog-to-digital converter operably coupling the first receiver and the second receiver to the processor for converting analog outputs of the first receiver and the second receiver to digital signals for the processor.

[0232] Clause 17. The MRR measuring device of clause 16, wherein a bandwidth of the at least one analog-to-digital converter is in a range of 0 GHz to 3 GHz.

[0233] Clause 18. The MRR measuring device of clause 1, wherein the first excitation pulse is in a first polarization state and the second excitation pulse is in a second polarization state different from the first polarization state, and the MRR measuring device further comprises: a wire grid polarizer disposed in the vacuum chamber for transmitting the first excitation pulse and reflecting the second excitation pulse.

[0234] Clause 19. The MRR measuring device of clause 18, wherein the wire grid polarizer is arranged to reflect the second excitation pulse back to the second transmitter.

[0235] Clause 20. The MRR measuring device of clause 18, wherein the wire grid polarizer is arranged to reflect the second excitation pulse at an angle relative to the second transmitter.

[0236] Clause 21. The MRR measuring device of clause 1, wherein the relative indication of the at least two components is one of an enantiomeric ratio, a ratio of two isotopic species, or a ratio of two different chemical species.

[0237] Clause 22. A method of measuring a ratio of a first component of a sample to a second component of the sample, the method comprising: injecting the sample into a vacuum chamber; obtaining a first molecular rotational resonance (MRR) spectrum of the sample; obtaining a second MRR spectrum of the sample with a second excitation pulse while obtaining the first MRR spectrum of the sample; and determining the ratio of the first component to the second component based on the first MRR spectrum and the second MRR spectrum.

[0238] Clause 23. The method of clause 22, wherein measuring the first MRR spectrum comprises emitting a first excitation pulse in a first direction, and measuring the second MRR spectrum comprises emitting a second excitation pulse in a second direction different from the first direction.

[0239] Clause 24. The method of clause 22, further comprising: quantifying a ratio of isomers, isotopologues, and / or isotopomers between components of the sample based at least in part on the first MRR spectrum and the second MRR spectrum.

[0240] Clause 25. The method of clause 22, further comprising: quantifying the first component of the sample based at least in part on the first MRR spectrum; quantifying the second component of the sample based at least in part on the second MRR spectrum; and comparing the amounts of the first component and the second component.

[0241] Clause 26. The method of clause 22, wherein: the sample comprises about 10 to about 100 compounds; measuring the first MRR spectrum comprises measuring free induction decay (FID) signals from about 10 to about 100 compounds in the sample; and measuring the second MRR spectrum comprises measuring at least one FID signal from a single compound in the sample.

[0242] Clause 27. The method of clause 22, further comprising: identifying at least one unknown component of the sample based at least in part on the second MRR spectrum.

[0243] Clause 28. The method of clause 22, wherein the first excitation pulse is a first resonant excitation pulse, and the second excitation pulse is a second resonant excitation pulse emitted at a frequency different from a frequency of the first resonant excitation pulse.

[0244] Clause 29. The method of clause 22, wherein the first excitation pulse is a broadband excitation pulse, and the second excitation pulse is a resonant excitation pulse.

[0245] Clause 30. The method of clause 29, further comprising: measuring a response of the vacuum chamber to the broadband excitation pulse, wherein measuring the second MRR spectrum comprises taking into account the response of the vacuum chamber to the broadband excitation pulse.

[0246] Clause 31. The method of clause 22, further comprising: prior to measuring the first MRR spectrum and the second MRR spectrum, attaching a chiral tag to at least one component in the sample; and identifying enantiomers in the sample based on one of the first MRR spectrum or the second MRR spectrum and / or determining an enantiomeric excess of components of the sample based on the first MRR spectrum and the second MRR spectrum.

[0247] 7. CONCLUSIONS

[0248] All parameters, dimensions, materials, and configurations described herein are meant to be exemplary and actual parameters, dimensions, materials, and / or configurations will depend upon the specific application for which a teaching of the present application is used. It is to be understood that the above description is intended to be illustrative, and not restrictive. Many embodiments of the present application will be apparent to those of skill in the art upon reviewing the above description. The scope of the present application should, therefore, be determined not with reference to the above description, but instead with reference to the appended claims, along with their full scope of equivalents.

[0249] Additionally, any combination of two or more such features, systems, articles, materials, kits, and / or methods, if such features, systems, articles, materials, kits, and / or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure. Other alternatives, modifications, variations, and equivalents of the exemplary embodiments described herein can be apparent to those of ordinary skill in the art in light of the above descriptions. The use of numerical ranges is not exclusive of equivalents within the range excluding the endpoints unless the context clearly indicates otherwise.

[0250] Also, various inventive concepts can be implemented as one or more methods, wherein at least one example is provided. In some cases, the acts performed as part of the method can be performed in a different order, sequentially or concurrently. Accordingly, in some embodiments, the respective acts of a given method can be performed in an order different than described specifically shown, which can include concurrently performing some acts (even though such acts are shown as sequential acts in illustrative embodiments).

[0251] All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety.

[0252] All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and / or ordinary meanings of the defined terms.

[0253] The indefinite articles "a" and "an," as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean "at least one."

[0254] The phrase "and / or," as used herein in the specification and in the claims, should be understood to mean "either or both" of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with "and / or" should be construed in the same fashion, i.e., "one or more" of the elements so conjoined. Other elements can optionally be present other than the elements specifically identified by the "and / or" clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to "A and / or B" when used in conjunction with open-ended language such as "comprising" can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including additional elements); etc.

[0255] As used herein in the specification and in the claims, "or" should be understood to have the same meaning as "and / or" as defined above. For example, when used in either the in the alternative (e.g., "one of A or B") or in the conjunctive sense (e.g., "one of A, B, and C"), "or" should be interpreted in the same manner as "and / or" except that the conjunctive sense has a meaning inherently different from "and / or." Specifically, the phrase should be interpreted in the same manner as "one of A, at least one of B, and C," i.e., "exactly one of A, at least one of B, and at least one of C." As used herein in the specification and in the claims, "or" should not be understood as having only the inclusive, "both," meaning when used in the conjunctive sense (e.g., "A or B" should not be understood as meaning "A and / or B").

[0256] As used herein in the specification and claims, the phrase "at least one" relating to a list having one or more elements should be understood to mean at least one element selected from any one or more elements in the element list, but not necessarily including at least one of every element specifically listed in the element list, and does not exclude any combination of elements in the element list. This definition also allows for the optional presence of elements other than those specifically identified in the element list referred to by the phrase "at least one," whether related to or unrelated to those specifically identified elements. Thus, as a non-limiting example, in one embodiment, "at least one of A and B" (or equivalently, "at least one of A or B," or equivalently, "at least one of A and / or B") may mean at least one element that optionally includes more than one A, has no B (and optionally includes elements other than B); in another embodiment, it may mean at least one element that optionally includes more than one B, has no A (and optionally includes elements other than A); in yet another embodiment, it may mean at least one element that optionally includes more than one A, and optionally includes at least one element that optionally includes more than one B (and optionally includes other elements); and so on.

[0257] In the claims and in the foregoing description, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “accommodating,” “constituting,” “made of,” etc., shall be understood as open-ended, that is, meaning including but not limited to. As described in Section 2111.03 of the U.S. Patent Examination Procedure Manual, only the transitional phrases “composed of” and “substantially composed of” shall be closed or semi-closed transitional phrases, respectively.

Claims

1. A molecular rotational resonance (MRR) measurement apparatus comprising: a vacuum chamber for housing a sample; at least one signal generator for emitting a first excitation pulse and a second excitation pulse; a first transmitter operably coupled to the at least one signal generator for illuminating the sample with the first excitation pulse; a first receiver for receiving a first free induction decay (FID) signal emitted by the sample in response to the first excitation pulse; a second transmitter operably coupled to the at least one signal generator for illuminating the sample with the second excitation pulse while the first transmitter is illuminating the sample with the first excitation pulse; a second receiver for receiving a second FID signal emitted by the sample in response to the second excitation pulse; and a processor operably coupled to the first and second receivers for determining a relative indication of at least two components of the sample based on the first and second FID signals.

2. The MRR measurement apparatus of claim 1, wherein the first transmitter is configured to illuminate the sample with the first excitation pulse along a first axis, and the second transmitter is configured to illuminate the sample with the second excitation pulse along a second axis different from the first axis.

3. The MRR measurement apparatus of claim 1, wherein the first excitation pulse is a first resonance excitation pulse, and the second excitation pulse is a second resonance excitation pulse.

4. The MRR measurement apparatus of claim 1, wherein the first excitation pulse comprises a broadband excitation pulse, and the second excitation pulse comprises a resonance excitation pulse that encompasses a narrower bandwidth than a bandwidth of the broadband excitation pulse.

5. The MRR measurement apparatus of claim 4, wherein the at least one signal generator is configured to generate the resonance excitation pulse at a frequency outside a frequency band of the broadband excitation pulse.

6. The MRR measurement apparatus of claim 5, wherein the resonance excitation pulse and the broadband excitation pulse include frequency components in a range of 2 GHz to 18 GHz.

7. The MRR measurement apparatus of claim 4, wherein the broadband excitation pulse is a chirped excitation pulse.

8. The MRR measurement apparatus of claim 4, further comprising: a pair of mirrors disposed in the vacuum chamber and forming a Fabry-Perot cavity having a resonance corresponding to the resonance excitation pulse.

9. The MRR measurement apparatus of claim 1, wherein the first excitation pulse and the second excitation pulse include frequency components in a range of 2 GHz to 18 GHz. ​ 10. The MRR measurement apparatus of claim 1, wherein the first receiver is configured to mix the first FID signal to a first intermediate frequency range, and the second receiver is configured to mix the second FID signal to a second intermediate frequency range different from the first intermediate frequency range.

11. The MRR measurement apparatus of claim 10, wherein the first intermediate frequency range and the second intermediate frequency range are between about 0 GHz and about 3 GHz.

12. The MRR measurement apparatus of claim 1, further comprising: a timing controller operably coupled to the sampling interface and the at least one signal generator for triggering emission of the first excitation pulse and the second excitation pulse in coordination with injection of the sample.

13. The MRR measurement apparatus of claim 12, wherein the timing controller comprises a frequency standard configured to generate a reference frequency clock signal to synchronize the emission of the first excitation pulse with the emission of the second excitation pulse.

14. The MRR measurement apparatus of claim 12, wherein the timing controller is further configured to trigger the first receiver to receive the first FID signal, the second receiver is further configured to receive the second FID signal, and the relative indication is a ratio of a first component to a second component.

15. The MRR measurement apparatus of claim 1, further comprising: a sampling interface in fluid communication with the vacuum chamber for injecting the sample into the vacuum chamber.

16. The MRR measurement apparatus of claim 1, further comprising: at least one analog-to-digital converter operably coupling the first receiver and the second receiver to the processor for converting analog outputs of the first receiver and the second receiver to digital signals for the processor.

17. The MRR measurement apparatus of claim 16, wherein a bandwidth of the at least one analog-to-digital converter is in a range of 0 GHz to 3 GHz.

18. The MRR measurement apparatus of claim 1, wherein the first excitation pulse is in a first polarization state and the second excitation pulse is in a second polarization state different from the first polarization state, and the MRR measurement apparatus further comprises: a wire grid polarizer disposed in the vacuum chamber for transmitting the first excitation pulse and reflecting the second excitation pulse.

19. The MRR measurement apparatus of claim 18, wherein the wire grid polarizer is arranged to reflect the second excitation pulse back to the second transmitter.

20. The MRR measurement apparatus of claim 18, wherein the wire grid polarizer is arranged to reflect the second excitation pulse at an angle relative to the second transmitter.

21. The MRR measurement device of claim 1, wherein the relative indication of the at least two components is one of an enantiomeric ratio, a ratio of two isotopic species, or a ratio of two different chemical species.

22. A method of measuring a ratio of a first component of a sample to a second component of the sample, the method comprising: injecting the sample into a vacuum chamber; obtaining a first molecular rotational resonance (MRR) spectrum of the sample; obtaining a second MRR spectrum of the sample with a second excitation pulse while obtaining the first MRR spectrum of the sample; and determining the ratio of the first component to the second component based on the first MRR spectrum and the second MRR spectrum.

23. The method of claim 22, wherein measuring the first MRR spectrum comprises emitting a first excitation pulse in a first direction, and measuring the second MRR spectrum comprises emitting a second excitation pulse in a second direction different from the first direction.

24. The method of claim 22, further comprising: quantifying a ratio of isomers, isotopomeric species, and / or isotopologues between components of the sample based at least in part on the first MRR spectrum and the second MRR spectrum.

25. The method of claim 22, further comprising: quantifying the first component of the sample based at least in part on the first MRR spectrum; quantifying the second component of the sample based at least in part on the second MRR spectrum; and comparing the amounts of the first component and the second component.

26. The method of claim 22, wherein: the sample comprises about 10 to about 100 compounds; measuring the first MRR spectrum comprises measuring free induction decay (FID) signals from about 10 to about 100 compounds in the sample; and measuring the second MRR spectrum comprises measuring at least one FID signal from a single compound in the sample.

27. The method of claim 22, further comprising: identifying at least one unknown component of the sample based at least in part on the second MRR spectrum.

28. The method of claim 22, wherein the first excitation pulse is a first resonant excitation pulse, and the second excitation pulse is a second resonant excitation pulse emitted at a frequency different from a frequency of the first resonant excitation pulse.

29. The method of claim 22, wherein the first excitation pulse is a broadband excitation pulse, and the second excitation pulse is a resonant excitation pulse.

30. The method of claim 29, further comprising: measuring a response of the vacuum chamber to the broadband excitation pulse, wherein measuring the second MRR spectrum comprises taking into account the response of the vacuum chamber to the broadband excitation pulse.

31. The method of claim 22, further comprising: attaching a chiral tag to at least one component in the sample prior to measuring the first MRR spectrum and the second MRR spectrum; and identifying enantiomers in the sample based on one of the first MRR spectrum or the second MRR spectrum and / or determining an enantiomeric excess of a component of the sample based on the first MRR spectrum and the second MRR spectrum.

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