Mass calibration

Abstract

In one aspect, a mass calibration method for use in mass spectrometry is disclosed, which includes generating a plurality of mass spectra corresponding to at least two calibrant ions having different m / z ratios for a plurality of different intensities of said at least two calibrant ions, using the mass spectra to determine an m / z ratio for each of said calibrant ions and for each of said ion intensities. For each of said calibrant ions and each of said ion intensities, a shift of said determined m / z ratio is determined relative to the m / z ratio of that calibrant ion and the m / z shifts are used to generate an intensity-dependent correction factor for correcting a measured m / z ratio of a target ion associated with an analyte in a sample.

Description

[0001] IDF-25318

[0002] MASS CALIBRATION RELATED APPLICATIONS

[0003] The present application claims priority to Provisional Application No. 63 / 738, 602_, filed on December 24, 2024 and Provisional Application No. _63 / 793,968, filed on April 24, 2025. Both of these provisional applications are herein incorporated by reference in their entirety.

[0004] TECHNICAL FIELD

[0005] The present disclosure relates generally to (A) mass spectrometry and, in particular, to (B) methods and / or systems for generating mass calibration data and / or utilizing the mass calibration data to apply correction factors to masses of target ions computed based on ion detection signals generated by a mass analyzer, such as a time-of-flight (ToF) mass analyzer, and / or (C) a mass calibration method, (D) a mass spectrometer calibrated in accordance with the mass calibration method, (D) a sample tested by the mass spectrometer, (E) mass spectrum data generated by the mass spectrometer in response to measurement of a sample by the mass spectrometer, (F) a computer system having computer-usable memory tangibly storing computer-executable code configured to execute the mass calibration method, and (G) computer-usable memory tangibly storing computer-executable code configured to execute the mass calibration method.

[0006] BACKGROUND

[0007] Mass spectrometry (MS) is an analytical technique for determining the structure of chemical substances with both qualitative and quantitative applications. MS can be useful for identifying unknown compounds, determining the composition of atomic elements in a molecule, determining the structure of a compound by observing its fragmentation and quantifying the amount of a particular chemical compound in a mixed sample. Mass spectrometers detect chemical entities as ions, so a conversion of the analytes to charged ions must occur.

[0008] Mass spectrometers use an ion source to ionize a sample to generate ions corresponding to target analytes. An ion detector of a mass spectrometer can detect the ions and generate ion detection signals that can be processed to derive information about the masses of the target ions.IDF-25318

[0009] Typically, the ion detector generates an analog signal having an amplitude that is proportional to the ion intensity at the detector. An analog-to-digital converter (ADC) is employed to digitize the analog signals, thereby generating digital ion detection signals. Such ion detection signals can be employed to derive the masses of the target ions.

[0010] SUMMARY

[0011] After much study and technical investigation, it is believed that various factors can, however, introduce errors in a mass derived for an ion based on an ion detection signal associated with that ion. By way of example, and without limitations, some sources of error can include detector noise, saturation of an analog-to-digital converter processing an ion signal, space charge, etc.

[0012] In one aspect, a mass calibration method for use in mass spectrometry is disclosed, which includes generating a plurality of mass spectra corresponding to at least two calibrant ions having different m / z ratios for a plurality of different intensities of said at least two calibrant ions and using the mass spectra to determine an m / z ratio for each of the calibrant ions and for each of the ion intensities. For each of the calibrant ions and each of the ion intensities, a shift of the determined m / z ratio relative to a reference m / z ratio of that calibrant ion is determined and the mass shifts are used to generate an intensity-dependent correction factor for correcting a measured m / z ratio of a target ion associated with an analyte in a sample.

[0013] In various embodiments, the plurality of mass spectra is generated using any of a time-of-flight (TOF) mass analyzer and an electrostatic trap analyzer. For example, the m / z ratio for each of the calibrant ions and each of the ion intensities can be generated based on a flight time of that ion through said TOF mass analyzer. Further, the m / z shift for each of the calibrant ions and each of the ion intensities can be generated based on a shift of a flight time of that ion relative to a reference flight time for that ion through the TOF mass analyzer.

[0014] A variety of calibrant ions can be employed in various embodiments of the present teachings. By way of example, and without limitation, one of the calibrant ions can have a mass of less than about 200 Da and another one of the calibrant ions can have a mass greater thanIDF-25318

[0015] about 600 Da. Further, in some embodiments, the target ion has a mass less than about 800 Da, e.g., less than about 100 Da.

[0016] In various embodiments, the correction factor can be computed based on calibration data from the determined m / z shifts of said at least two calibrant ions at each of said intensities. By way of example, the calibration data can be generated via interpolation between the m / z shifts of said at least two calibrant ions at each of said plurality of ion intensities.

[0017] In various embodiments, the correction factor can be applied to the measured m / z ratio of the target ion when an intensity of an analog ion detection signal associated with the target ion exceeds an output threshold of an analog-to-digital converter (ADC) employed to digitize the analog ion detection signal.

[0018] In various embodiments, the correction factor can be applied to the measured m / z ratio of the target ion when an output of an ADC receiving an ion detection signal associated with the target ion at its input is greater than half of ADC’s pre-saturation full range.

[0019] In various embodiments, the m / z ratio of the target ion can be generated by processing an ion detection signal generated by an ion detector receiving the target ion.

[0020] In a related aspect, a mass calibration method for use in mass spectrometry is disclosed, which includes: for a plurality of ion intensities, determining mass shifts associated with at least two calibrant ions having different m / z ratios, thereby generating intensity-dependent calibration data, and using the generated intensity-dependent calibration data to apply a correction factor to a mass of a target ion, wherein said mass of the target ion was determined based on generation of an ion detection signal associated with detection of the target ion.

[0021] In various embodiments, the intensity-dependent calibration data is generated using ion detection signals generated by any of a TOF (time-of-flight) mass analyzer and an electrostatic trap analyzer.

[0022] In various embodiments, one of the at least two calibrant ions can be a low-mass calibrant ion and the other can be a high-mass calibrant ion. By way of example, and without limitation,IDF-25318

[0023] the low-mass calibrant ion can have a mass in a range of about 50 to about 200 Da and the high-mass calibrant ion can have a mass in a range of about 600 to about 1000 Da.

[0024] The present teachings can be applied to correct measured masses of a variety of target ions. By way of example, in various embodiments, the present teachings can be particularly effective in providing mass corrections for ions having a mass less than about 800 Da, and more particularly, a mass less than about 100 Da.

[0025] In a related aspect, a mass calibration method for use in mass spectrometry is disclosed, which includes determining, for a plurality of ion intensities, mass shifts associated with at least two calibrant ions having different m / z ratios, thereby generating intensity-dependent calibration data. At least one ion detection signal associated with a target ion is generated and a mass of the target ion is determined based on said at least one ion detection signal. The intensity-dependent calibration data can then be used to apply at least one correction factor to the mass of the target ion. In various embodiments, one of the at least two calibrant ions can be a low-mass ion and the other calibrant ion can be a high-mass ion.

[0026] In various embodiments, the at least one ion detection signal can be generated using an ion detector, e.g., an ion detector of a TOF mass analyzer or an electrostatic trap analyzer. In some such embodiments, the ion detector can include a plurality of ion detection channels and said at least one ion detection signal can include a plurality of ion detection signals each generated by one of said ion detection channels. In some such embodiments, an independent correction factor can be computed for each of the ion detection channels for application to a mass of the target ion generated based on the ion detection signal associated with that channel. The correction factors can then be applied to the masses determined via the ion detection signals of those ion detection channels.

[0027] In various embodiments, the correction factor of the target ion mass obtained from an ion detection signal can be achieved by determining a correction factor based on interpolating between the mass shifts associated with the low mass and the high mass ions at an ion intensity that is substantially equal to an intensity associated with the ion detection signal.

[0028] In various embodiments, the ion detection signals associated with the calibrant ions and the target ion can be generated using a TOF or an electrostatic trap analyzer, by way of example.IDF-25318

[0029] In a related aspect, a computer-readable medium is disclosed, which stores instructions for executing a mass calibration method according to any of the above methods.

[0030] In yet another related aspect, disclosed is a computer system including computer-usable memory tangibly storing computer-executable code configured to execute a mass calibration method according to any method described herein.

[0031] In another related aspect, a mass spectrometer is disclosed that is calibrated according to methods according to the present teachings. By way of example, such a mass spectrometer can include an ion source for generating a plurality of ions, at least one ion guide configured to receive the plurality of ions and generate an ion beam, and a mass analyzer that is positioned downstream of the at least one ion guide and is configured to receive the plurality of ions or product ions generated via fragmentation of the plurality of ions and to generate ion detection signals. The mass spectrometer can further include a mass analysis module that is configured to receive the ion detection signals and process those ion detection signals to generate a mass spectrum of the plurality of ions or the product ions. The mass analysis module is configured to receive ion detection signals generated by the mass analyzer corresponding to at least two calibrant ions having different m / z ratios for a plurality of different intensities of the at least two calibrant ions, process the ion detection signals to generate a plurality of mass spectra corresponding to the at least two calibrant ions, use the mass spectra to determine an m / z ratio for each of the calibrant ions and for each of the ion intensities. For each of the calibrant ions and each of the ion intensities, a shift of the determined m / z ratio relative to a reference m / z ratio of that calibrant ion can be determined and the m / z shifts can be used to generate an intensitydependent correction factor for correcting a measured m / z ratio of a target ion associated with an analyte in a sample.

[0032] In various embodiments, the mass spectrometer can include a mass filter that is positioned between the ion guide and the mass analyzer and is configured to allow passage of ions having an m / z ratio in a target range.

[0033] In various embodiments, the mass spectrometer can further include an ion dissociation device that is positioned downstream of the mass filter for receiving ions exiting the mass filter and causing dissociation thereof to generate the product ions.IDF-25318

[0034] In various embodiments, the analysis module is configured to generate the m / z ratio of each of the calibrant ions and each of the ion intensities based on a flight time of that ion through the TOF mass analyzer. The analysis module can be configured to generate the m / z shift for each of the calibrant ions and each of the intensities based on a shift for a flight time of that ion relative to a reference flight time for that ion through the TOF mass analyzer.

[0035] The analysis module can be configured to compute the correction factor based on calibration data from the determined m / z shifts of said at least two calibrant ions at each of said ion intensities.

[0036] As noted above, a variety of calibrant ions can be employed in the practice of the present teachings. By way of example, and without limitation, one of the calibrant ions can have a mass of less than about 200 Da and another one of the calibrant ions can have a mass greater than about 600 Da. Further, the mass of the target ion can be, for example, less than about 800 Da, such as less than about 100 Da.

[0037] In another related aspect, mass spectrum data is disclosed, which is generated by a mass spectrometer calibrated according to methods according to the present teachings.

[0038] BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow chart depicting various steps for performing an embodiment of a method according to the present teachings,

[0039] FIG. 2 schematically depicts a mass spectrometer according to an embodiment of the present teachings,

[0040] FIG. 3 schematically depicts a computer system configured to perform a method according to the present teachings,

[0041] FIG. 4 depicts an example of a mass shift as a function of ADC signal output area, FIG. 5 depicts another example of a mass shift as a function of ADC signal output area,IDF-25318

[0042] FIG. 6 shows uncorrected and corrected TOF ion detection signals at different intensities obtained for cesium iodide (CsI) at m / z = 132.9049 with a time-of-flight of about 23527.43 ns.IDF-25318

[0043] Detailed Description

[0044] It will be appreciated that, for clarity, the following discussion will explicate various aspects of embodiments of the applicant’s teachings while omitting certain specific details wherever convenient or appropriate to do so. For example, the discussion of like or analogous features in alternative embodiments may be somewhat abbreviated. Well-known ideas or concepts may also, for brevity, not be discussed in any great detail. The skilled person will recognize that some embodiments of the applicant’s teachings may not require certain of the specifically described details in every implementation, which are set forth herein only to provide a thorough understanding of the embodiments. Similarly, it will be apparent that the described embodiments may be susceptible to alteration or variation according to common general knowledge without departing from the scope of the disclosure. The following detailed description of embodiments is not to be regarded as limiting the scope of the applicant’s teachings in any manner.

[0045] As used herein, the terms "about" and "substantially equal" refer to variations in a numerical quantity that can occur, for example, through measuring or handling procedures in the real world; through inadvertent error in these procedures; through differences in the manufacture, source, or purity of compositions or reagents; and the like. Typically, the terms "about" and "substantially" as used herein mean 10% greater or less than the value or range of values stated or the complete condition or state. For instance, a concentration value of about 30% or substantially equal to 30% can mean a concentration between 27% and 33%. The terms also refer to variations that would be recognized by one skilled in the art as being equivalent so long as such variations do not encompass known values practiced by the prior art.

[0046] As used herein the term "and / or" includes any and all combinations of one or more of the associated listed items and may be abbreviated as

[0047]

[0048] The term “a low-mass ion,” as used herein, refers to an ion having a mass equal to or less than about 200 Da and the term “a high-mass ion,” as used herein, refers to an ion having a mass equal to or greater than 600 Da.IDF-25318

[0049] The present disclosure relates generally to generating intensity-dependent calibration data using at least two calibrant ions and a plurality of ion intensities and utilizing the calibration data to apply mass corrections to masses derived for a target ion based on an ion detection signal associated with that ion. It has been discovered that ion masses determined using time-of-flight (TOF) mass analyzers can exhibit intensity-related mass shifts. Further, such mass shifts can correlate approximately with molecular size and ion current. For example, when measuring ion masses using a TOF mass analyzer, the shift in the determined mass (relative to the correct mass) can become more pronounced (it can typically exponentially increase) for low masses due to the proportionality of the drift time to the square of the ion mass. By way of example, and without limitation, mass shift corrections may be less important for ion masses greater than about 800 Da, as a difference in flight time for such ions can be small for a wider range, i.e., smaller mass drift for large mass ions. In contrast, in many cases, a noticeable mass shift may be observed for low-mass ions, e.g., ions with a mass less than about 200 Da.

[0050] Without being limited to any particular theory, such mass shifts may be at least partially due to TOF mass analyzer asymmetry (an ion cloud for a given ion is not a perfect sphere), electrical pulse asymmetry, electrical pulse distortions due to signal saturation, etc.

[0051] Without loss of generality, in the following discussion, it is assumed that the mass analyzer is a TOF analyzer; however, the present teachings can apply to other mass analyzers as well. In fact, the present teachings have general applicability and can be applicable to any mass spectrometer to provide a correction for intensity-related mass shifts observed based on an ion detector output. By way of example, in various embodiments, such a correction can remove errors due to space charge or contamination, which are generally not predictable and, therefore, not suitable for calibration. While in many embodiments, the present teachings can be employed in mass spectrometers that employ TOF or electrostatic trap analyzers incorporating a microchannel plate (MCP) detector, it may also be useful in other mass spectrometers as well, such as triple quadrupole mass spectrometers.

[0052] With reference to the flow chart of FIG. 1, in various embodiments, a method for applying a correction factor to a mass determined using a mass analyzer is disclosed, which includes generating mass calibration data by determining mass shifts for at least a low-mass calibrant ion and at least a high-mass calibrant ion at a plurality of ion intensities and using theIDF-25318

[0053] intensity-dependent calibration data to apply a correction factor to a mass of a target ion determined based on generation of an ion detection signal associated with detection of the target ion.

[0054] Since the mass shift is linear in time, only two calibrant ions can be utilized to obtain a correction (though more than two calibrant ions can also be employed, if desired). A variety of calibrant ions can be utilized in the practice of the present teachings. Further and by way of example, in various embodiments, the low-mass calibrant ion can have a mass less than about 200 Da and the high-mass calibrant ion can have a mass greater than 600 Da.

[0055] For example, for each of the calibrant ions, the ion beam intensity can be ramped from low to high while tracking the shifts associated with masses determined for that calibrant ion at a plurality of ion beam intensities. For example, during the ramp of the ion beam intensity, each ion detection signal corresponding to a particular ion beam intensity can provide a mass shift relative to the known mass of the calibrant ion. An ion detection signal can correspond to the output of an analog-to-digital converter (ADC) that receives an analog signal from an ion detector detecting the calibrant ion and digitizes the analog signal to generate a digital output signal.

[0056] In some cases, an output signal of the ADC can exhibit saturation at high ion beam intensities. In such cases, a saturation correction can be applied to the ADC’s output to correct an area associated with the ADC’s output, which can be utilized as a measure of the ion intensity. By way of example, the systems and methods disclosed in Published International Application No. PCT7IB2023 / 054303 titled “Systems and methods for determining position and time of clipped ADC ion response signals in mass spectrometry,” which is herein incorporated by reference in its entirety, can be employed for such saturation correction.

[0057] The mass shifts calculated for the low-mass and the high-mass calibrant ions at a plurality of ion beam intensities provide calibration data that can be employed to correct a mass determined for a target ion using the mass analyzer. In various embodiments, the calibration data can be in the form of two mass shifts (one corresponding to a low-mass calibrant ion and another corresponding to a high-mass calibrant ion) for each of a plurality of ion beam intensities.IDF-25318

[0058] By way of example, and without limitation, the ion intensity ramp can start at an ion rate of 1 ion count per TOF push to a maximum ion current during mass shift tuning.

[0059] In various embodiments, to generate the mass calibration data, the mass shift of each of the low-mass calibrant ion and the high-mass calibrant ion can be tracked as a histogram with a bin size in ADC units (area of the output signal) after saturation correction. By way of example, ADC can have a range of about 0 to 100,000 (area of electrical pulse) and saturation correction can extend to about 1,000,000 ADC units area. Again, by way of example, and without limitation, the bin size can be typically chosen to be about 50,000 ADC units area, i.e., every 50,000 units area, and the observed mass shifts can be binned together. As the observed mass in a TOF mass analyzer is related to the drift time of the ion, tracking mass shifts corresponds to tracking shifts in observed ion flight time.

[0060] The calibration data can then be utilized to compute a mass correction for a measured mass of a target ion. By way of example, for a given ion detection signal corresponding to the target ion, the ADC intensity bin associated with the calibration data that would correspond to the ADC intensity bin associated with the target ion detection signal can be identified. The mass calibration data includes, for that ADC intensity bin, a time shift (e.g., in units of picoseconds) corresponding to the low-mass ion and a time shift (e.g., in units of picoseconds) corresponding to the high-mass ion. A correction time shift for the target ion associated with the ADC intensity bin can then be computed by interpolating between the two time shifts.

[0061] In various embodiments, the correction of the observed ion mass does not commence until the ADC output is greater than half of its pre-saturation correction full range. By way of example, and without limitation, in various embodiments, the correction may start at 50,000 ADC units.

[0062] The time-of-flight (T) for an ion having a mass (m) and a kinetic energy KE through a drift region of a TOF mass analyzer is provided by the following relation:

[0063]

[0064] Thus, the mass of the ion can be calculated as:

[0065] m = (2K£')T2Eq. (2)IDF-25318

[0066] In various embodiments, a corrected mass (mcorr) calculated by computing a mass shift calibration constant (a) according to various embodiments of the present teachings can be provided by the following relation:

[0067]

[0068] In various embodiments, most ions do not require the application of a mass shift correction to their observed masses. As such, in some embodiments, the mass shift corrections are applied when the detected ion intensity exceeds an ADC output threshold, e.g., 50,000 ADC units.

[0069] By way of example, in various embodiments, if an ADC intensity bin has a calibration value of less than about 25 ps, no mass correction shift will be applied. Calibration thresholds other than 25 ps may also be employed. In general, the calibration threshold can be selected based on a desired mass accuracy.

[0070] For TOF mass spectrometers in which multiple, independent ion detection channels are employed concurrently for detecting an ion detection signal, the mass shift correction according to various embodiments can be applied to each ion detection channel independently of the other ion detection channels. By way of example, and without limitation, in some embodiments, the number of independent ion detection channels can be four (4), and the mass shift correction can be determined for each of these four (4) channels for application to masses derived based on ion detection signals generated by those channels.

[0071] In various embodiments, the mass shift correction can be applied to each ion detection signal during runtime, while in other embodiments, the mass shift correction can be applied post data acquisition.

[0072] In various embodiments, the application of mass shift correction methods according to the present teachings can significantly enhance the precision for the determination of the m / z ratios of ions. By way of example, and without limitation, in some embodiments, the use of mass correction methods according to the present teachings can lead to the determination of the m / z ratios of ions with a precision in a range of about 100 ppm to about 1000 ppm. In particular, as noted above, the mass shift methods according to various embodiments of the present teachings can be particularly advantageous for improving the precision by which the m / z ratiosIDF-25318

[0073] of ions with a mass less than about 100 Da can be determined using, e.g., a TOF or an electrostatic trap mass analyzer, though the present methods can also be employed for correcting the measured masses of heavier ions.

[0074] The mass correction methods and systems according to the present teachings can be incorporated into a variety of mass spectrometers. By way of example, FIG. 2 schematically depicts a mass spectrometric system 200 (herein also referred to as a mass spectrometer) according to an embodiment that includes an LC column 202 that can receive a sample and separate a plurality of analytes in the sample based on their elution times from the LC column. The mass spectrometric system 200 further includes an ion source 204 that receives an eluate exiting the LC column and ionizes one or more analytes contained in the eluate to generate a plurality of precursor ions.

[0075] In many implementations, one or more ion guides 206 receive the precursor ions and provide focusing of the ions to generate an ion beam that is received by a mass filter 208. By way of example, the ion guide(s) can include a plurality of rods arranged in a quadrupole configuration to which RF and DC voltages generated by an RF voltage source 210 and a DC voltage source 212 can be applied in a manner known in the art to provide radial confinement of the received ions.

[0076] In other embodiments, an ion mobility spectrometer (IMS), such as a differential mobility spectrometer (DMS), can be utilized as a separation device to separate ions based on their mobility with the ions exiting the IMS being received by the one or more ion guides 206.

[0077] The mass filter 208 provides an ion transmission window that allows transmission of ions having m / z values within an m / z range through the mass filter. By way of example, the mass filter 208 can include a plurality of rods arranged in a quadrupole configuration to which RF voltages as well as a discriminating DC voltage can be applied via the RF and the DC voltage sources 210 and 212, respectively, to generate an ion transmission window. The RF and DC voltage sources are controlled by a controller 218. In this implementation, the ions passing through the mass filter 208 are received by an ion fragmentation device 214 that causes fragmentation of the precursor ions to generate a plurality of product ions.IDF-25318

[0078] The product ions are received by a time-of-flight (TOF) mass analyzer 220 that detects the ions and generates ion detection signals in response to the detection of the ions. A mass analysis module 222 receives the ion detection signals from the TOF mass analyzer and processes those signals to generate a mass spectrum. Further, in various embodiments, the analysis module can be configured to perform a mass correction of the measured mass peaks in accordance with the present teachings. By way of example, the analysis module can perform such mass correction during data acquisition or post data acquisition.

[0079] The analysis module 222 can be implemented in hardware, firmware and / or software using known engineering techniques as informed by the present teachings. By way of example, FIG. 3 is a block diagram that illustrates an example of an implementation of the analysis module as a computer system 300. Computer system 300 includes a bus 302 or other communication mechanism for communicating information, and a processor 304 coupled with bus 302 for processing information. Computer system 300 also includes a memory 306, which can be a random-access memory (RAM) or other dynamic storage device, coupled to bus 302 for determining base calls, and instructions to be executed by processor 304.

[0080] Memory 306 also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor 304. Computer system 300 further includes a read only memory (ROM) 308 or other static storage device coupled to bus 302 for storing static information and instructions for processor 304. A storage device 310, such as a magnetic disk or an optical disk, is provided and coupled to bus 302 for storing information and instructions.

[0081] Computer system 300 may be coupled via bus 302 to a display 312, such as a cathode ray tube (CRT) or liquid crystal display (LCD), for displaying information to a computer user.

[0082] An input device 314, including alphanumeric and other keys, is coupled to bus 302 for communicating information and command selections to processor 304. Another type of user input device is cursor control 316, such as a mouse, a trackball or cursor direction keys for communicating direction information and command selections to processor 304 and for controlling cursor movement on display 312. The computer system 300 further includes aIDF-25318

[0083] communication module 318 that allows the computer system to communicate with other devices and systems, and in particular, with a database 320.

[0084] Computer system 300 can perform the present teachings. Consistent with certain implementations of the present teachings, results are provided by computer system 300 in response to processor 304 executing one or more sequences of one or more instructions contained in memory 306. Such instructions may be read into memory 306 from another computer-readable medium, such as storage device 310. Execution of the sequences of instructions contained in memory 306 causes processor 304 to perform the process described herein. Alternatively, hard-wired circuitry may be used in place of or in combination with software instructions to implement the present teachings. Thus, implementations of the present teachings are not limited to any specific combination of hardware circuitry and software.

[0085] By way of example, in various embodiments, the computer system 300 can perform a method according to the present teachings by generating mass calibration data via the determination of mass shifts for at least a low-mass calibrant ion and a high-mass calibrant ion at a plurality of ion intensities and using the mass-dependent calibration data to apply a correction factor to a mass of a target ion determined based on the generation of an ion detection signal associated with the detection of that target ion. By way of example, the instructions for performing the method can be stored in the ROM 308 and can be transferred to the RAM 306 during runtime by the processor for execution.

[0086] The term “computer-readable medium” as used herein refers to any media that can participate in providing instructions to processor 304 for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 310. Volatile media can include dynamic memory, such as memory 306. Transmission media can include coaxial cables, copper wire, and fiber optics, including the wires that comprise bus 302.

[0087] Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, punch cards, papertape, any other physical medium with patterns of holes, aIDF-25318

[0088] RAM, PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, or any other tangible medium from which a computer can read.

[0089] Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to processor 304 for execution.

[0090] In accordance with various embodiments, instructions configured to be executed by a processor to perform a method according to various embodiments of the present teachings are stored on a computer-readable medium. The computer-readable medium can be a device that stores digital information. For example, a computer-readable medium includes a compact disc read-only memory (CD-ROM) as is known in the art for storing software. The computer-readable medium is accessed by a processor suitable for executing instructions configured to be executed.

[0091] The following examples are provided for further elucidation of various aspects of the present teachings and are not provided necessarily to indicate the optimal ways of practicing the present teachings or optimal results that may be obtained.

[0092] Examples

[0093] Example 1

[0094] A TOF mass spectrometer was employed to generate mass shift data for a low-mass calibrant ion having a mass of 118 Da and a high-mass calibrant ion having a mass of 829 Da at a plurality of ion beam intensities.

[0095] Table 1 below lists the time shifts in the units of picoseconds for a plurality of intensities provided in units of ADC area for the two calibrant ions.

[0096] Table 1

[0097]

[0098] IDF-25318

[0099]

[0100] Further, FIGS. 4 and 5 provide plots of this data as time shift versus ADC area. The calibration mass shift data was collected in 21 intensity bins. Such calibration mass shift data can be utilized in a manner discussed herein to determine a correction factor for application to the measured mass of a target ion that would require such a correction.

[0101] Y1IDF-25318

[0102] Example 2

[0103] FIG. 6 shows uncorrected and corrected TOF ion detection signals at different intensities obtained for cesium iodide (CsI) at m / z = 132.9049 with a time of flight of about 23527.43 ns. The calibration was achieved by using one calibrant ion at m / z equal to 118 with a time of flight of 22168.94 ns and another calibrant ion at m / z equal to 829 with a time of flight of 58759.92 ns. The data corresponds to a single channel from a 4-channel MCP and the mass shift at each intensity was obtained via interpolation between the masses of the calibrant ions.

[0104] The above descriptions of various implementations of the present teachings have been presented for purposes of illustration and description. It is not exhaustive and does not limit the present teachings to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practicing the present teachings. Additionally, the described implementation includes software but the present teachings may be implemented as a combination of hardware and software or in hardware alone. The present teachings may be implemented with both object-oriented and non-object-oriented programming systems.

[0105] Depending on certain implementation requirements and / or embodiments of the present teachings, the controller can be implemented in hardware, firmware and / or in software.

[0106] In some embodiments, the instructions for operating the optical system can be stored using a non-transitory storage medium such as a digital storage medium, for example, a DVD, a Blu-Ray, a CD, a ROM, a PROM, an EPROM, an EEPROM, and / or a FLASH memory, having electronically readable control signals stored thereon, which cooperate (or are capable of cooperating) with a programmable computer system such that the respective method is performed. Therefore, the digital storage medium may be computer-readable.

[0107] While various embodiments have been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; embodiments of the present disclosure are not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing embodiments of the present disclosure, from a study of the drawings, the disclosure, and the appended claims.IDF-25318

[0108] In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other processing unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

[0109] Those having ordinary skill in the art will appreciate that various changes can be made to the above embodiments without departing from the scope of the present teachings.

Claims

IDF-25318What is claimed is:

1. A mass calibration method for use in mass spectrometry, comprising:generating a plurality of mass spectra corresponding to at least two calibrant ions having different m / z ratios for a plurality of different intensities of said at least two calibrant ions,using the mass spectra to determine an m / z ratio for each of said calibrant ions and for each of said ion intensities,for each of said calibrant ions and each of said ion intensities, determining a shift of said determined m / z ratio relative to a reference m / z ratio of that calibrant ion, and using said m / z shifts to generate an intensity-dependent correction factor for correcting a measured m / z ratio of a target ion associated with an analyte in a sample.

2. The mass calibration method of Claim 1, wherein said plurality of mass spectra is generated using any of a TOF mass analyzer and an electrostatic trap analyzer.

3. The mass calibration method of Claim 2, wherein said m / z ratio for each of said calibrant ions and each of said ion intensities is generated based on a flight time of that ion through said TOF mass analyzer.

4. The mass calibration method of Claim 3, wherein said m / z shift for each of said calibrant ions and each of said intensities is generated based on a shift of a flight time of that ion relative to a reference flight time for that ion through said TOF mass analyzer.

5. The mass calibration method of any one of the preceding claims, wherein one of said at least two calibrant ions has a mass of less than 200 Da and the other one of said at least two calibrant ions has a mass of greater than about 600 Da.

6. The mass calibration method of any one of the preceding claims, wherein the mass of the target ion is less than about 800 Da.IDF-253187. The mass calibration method of Claim 6, where the mass of the target ion is less than about 100 Da.

8. The mass calibration method of any one of the preceding claims, wherein said correction factor is computed based on calibration data from said determined m / z shifts of said at least two calibrant ions at each of said ion intensities.

9. The mass calibration method of Claim 8, wherein said calibration data is generated via interpolation between the m / z shifts of said at least two calibrant ions at each of said plurality of ion intensities.

10. The mass calibration method of any one of the preceding claims, wherein said correction factor is applied to said measured m / z ratio of said target ion when an intensity of an analog ion detection signal associated with said target ion exceeds an output threshold of an analog-to-digital converter (ADC) employed to digitize said analog ion detection signal.

11. The mass calibration method of any one of Claims 1 - 9, wherein said correction factor is applied to said measured m / z ratio of said target ion when an output of an analog-to- digital converter (ADC) receiving an ion detection signal associated with said target ion at its input is greater than half of the ADC’s pre-saturation full range.

12. The mass calibration method of any one of the preceding claims, wherein generating said measured m / z ratio of the target ion comprises processing an ion detection signal generated by an ion detector receiving said target ion.

13. A mass spectrometer, comprising:an ion source for generating a plurality of ions,IDF-25318at least one ion guide configured to receive the plurality of ions and generating an ion beam,a mass analyzer positioned downstream of said at least one ion guide and configured to receive said plurality of ions or product ions generated via fragmentation of said plurality of ions and to generate ion detection signals, anda mass analysis module configured to receive said ion detection signals and process said ion detection signals to generate a mass spectrum of said plurality of ions or said product ions,wherein said mass analysis module is further configured to:receive ion detection signals generated by said mass analyzer corresponding to at least two calibrant ions having different m / z ratios for a plurality of different intensities of said at least two calibrant ions,process said ion detection signals to generate a plurality of mass spectra corresponding to said at least two calibrant ions,use the mass spectra to determine an m / z ratio for each of said calibrant ions and for each of said ion intensities,for each of said calibrant ions and each of said ion intensities, determine a shift of said determined m / z ratio relative to a reference m / z ratio of that calibrant ion, and use said m / z shifts to generate an intensity-dependent correction factor for correcting a measured m / z ratio of a target ion associated with an analyte in a sample.

14. The mass spectrometer of claim 13, further comprising a mass filter positioned between said ion guide and said mass analyzer and configured to allow passage of ions having an m / z ratio in a target range.

15. The mass spectrometer of claim 14, further comprising an ion dissociation device positioned downstream of the mass filter for receiving ions exiting the mass filter and causing dissociation thereof to generate said product ions.IDF-2531816. The mass spectrometer of any one of claims 13 - 15, wherein said mass analyzer comprises any of a TOF mass analyzer and an electrostatic trap analyzer.

17. The mass spectrometer of claim 16, wherein said analysis module is configured to generate said m / z ratio of each of said calibrant ions and each of said ion intensities based on a flight time of that ion through said TOF mass analyzer.

18. The mass spectrometer of claim 17, wherein said analysis module is configured to generate said m / z shift for each of said calibrant ions and each of said intensities based on a shift of a flight time of that ion relative to a reference flight time for that ion through said TOF mass analyzer.

19. The mass spectrometer of any one of claims 13 - 18, wherein one of said at least two calibrant ions has a mass of less than about 200 Da and the other one of said at least two calibrant ions has a mass greater than about 600 Da, wherein optionally said mass of the target ion is less than about 800 Da or less than about 100 Da.

20. The mass spectrometer of claim 19, wherein said analysis module is configured to compute said correction factor based on calibration data from said determined m / z shifts of said at least two calibrant ions at each of said ion intensities.

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