Method and apparatus for emission spectroscopy

Abstract

The present invention discloses a method for controlling the flow of gas through a spectrometer, comprising: passing a gas through an interior of the spectrometer, wherein light from a sample can pass through the interior along a first path to reach a first detector, and the gas is transparent to light within a spectral region analyzed by the spectrometer; transmitting light from a light source through the gas along a second path to a second detector; detecting the intensity of the light from the light source at one or more wavelengths of light at the second detector; comparing the detected light intensity with a corresponding set value corresponding to a desired transmittance of the gas in the interior of the spectrometer, and generating at least one error signal based on the comparison; and adjusting the flow rate of the gas through the interior of the spectrometer based on the error signal to, in particular, minimize the difference between the detected intensity and the set value.

Description

Technical Field

[0001] The present invention relates to the field of emission spectroscopy and, in particular, to improvements in the control of purge gases in emission spectrometers. Background Art

[0002] Emission spectroscopy is a well-known technique for analyzing samples. It is used to determine the molecular or atomic components of a sample. When atoms excited to a high-energy state relax to a lower energy state, or ground state, they emit photons. The wavelength of the emitted photons is related to the energy gap between the excited state to which the atom relaxed and the state to which it decays. Different atomic species have distinct atomic emission spectra, so spectral analysis can be used to determine the composition of a sample.

[0003] Emission lines typically occur in the infrared, visible, and ultraviolet regions of the electromagnetic spectrum. Spectrometers can be configured to detect radiation at different wavelengths across the electromagnetic spectrum. Researchers are particularly interested in detecting atomic emission lines in the ultraviolet region. Ultraviolet (UV) radiation is electromagnetic radiation located between the visible and X-ray regions of the spectrum, typically between 380 nm and 5 nm. Vacuum ultraviolet (VUV) radiation is the portion of the UV region of the spectrum that is less than 200 nm. To obtain information about the various elements in a sample, a spectrometer must be able to transmit photons with wavelengths below 200 nm, particularly below 190 nm, from the sample to the detector, as many elements emit photons in this wavelength range when relaxing to lower energy states. Spectral analysis of VUV atomic lines for quantitative emission spectroscopy is crucial for analyzing elements such as carbon, nitrogen, sulfur, and oxygen with ppm sensitivity (i.e., trace levels).

[0004] However, one problem is that VUV radiation is absorbed by air, particularly by oxygen and water in air. Therefore, any spectroscopic analysis designed to detect this VUV radiation is typically performed in a vacuum or in a non-absorbing gas environment. To prevent absorption of UV photons by air and the wavelength shifts associated with variations in the gas's refractive index (which varies with gas pressure and composition), purging the spectrometer (and typically the sample) with a substantially UV-transparent gas, such as argon, is often more convenient and less expensive than providing a vacuum environment. Spectrometers can detect radiation in other regions, such as the infrared or visible light, and in such cases, the gas is preferably substantially transparent in these regions.

[0005] Due to the high purity required, these gases are relatively expensive, and their consumption rate makes the cost of purging spectrometers with these gases one of the highest consumable expenses for laboratories using these spectrometers. Typically, a constant flow of argon gas is introduced into the spectrometer during every phase of spectrometer operation, including sample analysis, to maintain the concentration of gaseous contaminants at a controlled level. However, users often shut down the system when not in use to control the costs associated with argon consumption. Since the spectrometer housing is typically not completely airtight when the purge system is turned off, leaking gas can slowly seep into and contaminate the spectrometer. Small leaks can cause air to enter from the outside, causing the pressure within the housing to fluctuate over time, leading to changes in the refractive index and absorption rate of the gases within the system and, consequently, shifts in the wavelengths of the detected spectral lines. In particular, oxygen and water in air strongly absorb VUV light, reducing the light intensity transmitted into the spectrometer and compromising the reliability of quantitative analysis.

[0006] Therefore, when the device is turned back on, the spectrometer needs to be purged with a higher flow rate of argon to remove these impurities and restore the optical VUV transmittance to the optimal level in the shortest possible time.

[0007] The purge is typically continued for a fixed period, at least long enough to ensure that all impurities are kept below a certain threshold, even in the worst-case scenario. The first problem with this approach is that the intensive purge with argon is continued for an extended period, regardless of the actual impurity concentration. The second problem is that once the purge is complete, the transmittance within the spectrometer typically takes much longer to reach a sufficiently stable level for reliable quantitative analysis. This occurs because the purge causes the transmittance to "overshoot." That is, the transmittance level achieved within the spectrometer after the purge is so high that it is not balanced by the constant leak from the outside atmosphere and the constant argon flow when the system is operating under its steady-state conditions. A long period of time (e.g., 24 or 48 hours) may be required to reach steady-state conditions. Figure 1 An example is shown in Figure 1, which shows the normalized intensity plot of the 170.20 nm atomic emission line of iron (Fe), recorded immediately after purging the spectrometer with argon. After an initial high argon purge flow rate, the flow rate was switched to a lower, constant argon flow rate for analysis, and the intensity of the detected line continued to decrease until it finally reached a steady state after at least 350 minutes.

[0008] In view of the above, it is desirable to provide a reliable and simple method and apparatus for controlling the purge gas system of a spectrometer to optimize purge gas consumption, thereby reducing costs associated with gas usage, and / or to more quickly stabilize transmittance levels within the spectrometer so that reliable quantitative elemental analysis can be performed with minimal instrument downtime. Against this background, the present invention has been developed. Summary of the Invention

[0009] According to one aspect of the present invention, a method and an apparatus are provided. A number of preferred features of the invention are set out in the dependent claims.

[0010] The present invention utilizes the Beer-Lambert law to provide a method and apparatus for measuring the intensity of light transmitted through a purge gas within a spectrometer, as well as the transmittance of the gas. The present invention is based on the use of absorption spectroscopy to determine the transmittance of the purge gas. Transmittance measurements can be performed continuously or discontinuously over time, and a feedback loop uses the measured values ​​to adjust or control the gas flow rate. Thus, the measured transmittance is used to dynamically control, i.e., adjust, the gas flow rate through the spectrometer to purge the interior of the spectrometer. In a specific embodiment, the present invention uses absorption spectroscopy to determine the level of absorbing contaminants, such as oxygen and water, which have strong absorption in the VUV spectral region, within the interior of the spectrometer. Regulation of the gas flow rate can be achieved by controlling or regulating a gas source, such as a valve for regulating the flow of gas into the spectrometer interior, or a pump for pumping gas through the spectrometer interior. Advantages of the present invention include the ability to quickly achieve a stable transmittance level and generally reducing gas consumption.

[0011] The method according to the present invention is a method for controlling the gas flow through a spectrometer. The spectrometer may be, for example, an emission spectrometer or an absorption spectrometer.

[0012] The gas is transparent to light from the sample in the spectral region analyzed by the spectrometer. Preferably, the gas used to purge the interior of the spectrometer and increase transmittance is a substantially UV-transparent gas, such as argon or helium. Nitrogen can be used, but is rarely used in practice due to its drawback of being unable to analyze nitrogen in the sample, trace amounts of which are often important for analysis, for example in high-quality steel. If desired, the transparent gas can be a mixture of different gases, such as argon and helium. The gas can be supplied from a source, such as a pressurized bottle or tank. Gas can be pumped through the interior of the spectrometer by evacuating the interior of the spectrometer using a vacuum pump and supplying the gas from the source to the evacuated interior. The flow of gas into the spectrometer can be controlled by a controllable valve, for example, the valve control being based on feedback from transmittance measurements. The system can operate at ambient pressure or higher, using a source pressurized above atmospheric pressure to supply the interior, for example with one or more valves controlling the flow rate. This allows the purge gas from the pressurized source to be used to purge residual air from the interior at atmospheric pressure.

[0013] Light from the sample can pass through the interior along a first path to reach the first detector, and thus the first detector is referred to as a sample detector. The sample is typically located in a sample chamber. The interior through which the gas passes is typically contained within the spectrometer housing. The first detector is also typically contained within the spectrometer housing. Therefore, light from the sample chamber passes along the first path to reach the first detector. The interior and housing typically contain a spectrograph for analyzing the light from the sample.

[0014] The first (i.e., sample) detector is a photodetector for detecting one or more wavelengths of light from the sample, which is used to determine the composition of the sample, such as the elemental composition. The first or sample detector may include, for example, one or more photomultiplier tubes (PMTs), and / or one or more CCD detectors, or other types of multi-dimensional pixel detectors. The first detector may include an array photodetector, such as a CCD (single-line or multi-line CCD), for detecting multiple parallel spectral lines, i.e., a spectrum. The first detector may include one, two, or more so-called fixed detection channels for detecting specific spectral lines. In addition to array detectors, one or more fixed channel detectors may also be provided. Such fixed detection channels are preferably provided by respective dedicated detectors, such as photomultiplier tubes (PMTs) or photodiodes, and are configured to detect specific spectral lines, such as those characteristic of an element of interest.

[0015] A method for controlling gas flow includes transmitting light from a light source (not a sample) along a second path through the gas to a second detector. The light source and the second detector are configured to measure the transmittance of the gas. Thus, the second detector is a transmittance detector. The light source is adapted to emit light, particularly light of one or more wavelengths that are absorbed by contaminants in the gas, such as oxygen or water. The light source typically emits light of wavelengths in the spectral region in which the light from the sample is analyzed by a spectrometer. The wavelengths emitted by the light source can be infrared (IR), visible, or UV. More useful wavelengths are in the VUV or near IR (NIR) regions. Thus, in some embodiments, the light used is VUV light, while in other embodiments, the light used is NIR light.

[0016] The advantage of the VUV region is that air (especially water and oxygen) has strong absorption there, but sources of these gases can be relatively expensive. It's best to use a bandpass VUV filter with a VUV source to improve the signal-to-noise ratio. The advantage of the NIR region is the high stability, relatively low cost, and long life of NIR light sources. However, the absorption of these gases is much weaker in this region, and there are no common absorption bands for water and oxygen, making simultaneous measurement of both gases impossible. The light source can be a laser, or a UV or IR lamp. This can be a VUV laser source or an IR laser source, such as a diode laser.

[0017] In some embodiments, the light source can be configured to emit light at a single wavelength or a single (continuous) wavelength band. In other embodiments, the light source can be configured to emit light at two or more non-continuous wavelengths or two or more non-continuous wavelength bands. To this end, the light source can include two or more independent light sources, for example, two or more lasers emitting light at different wavelengths. When detecting two or more gas components that do not share a common absorption band in a particular spectral region, it may be preferable to use a light source emitting at two or more different wavelengths. In some embodiments, the device can include two or more light sources for transmitting light in the gas along two or more second paths, respectively, and two or more second detectors for detecting the intensity of the light transmitted along the corresponding second or multiple paths, respectively. Each light source emits light at a different wavelength, or each second detector detects light at a different wavelength, and the light intensity is detected at two or more non-continuous wavelengths or two or more non-continuous wavelength bands.

[0018] The second detector can be any suitable type of photodetector, such as a photomultiplier tube (PMT), a photodiode, or a CCD. For example, a GaP photodiode or PMT can be used for VUV detection. For example, a silicon photodiode or CCD can be used for NIR detection.

[0019] The method includes detecting the intensity of the light at one or more wavelengths at a second (i.e., transmittance) detector and comparing the detected light intensity with a corresponding set value, the set value representing a desired transmittance of the gas within the spectrometer (in other words, the set value corresponds to a desired detected intensity). Based on the comparison, at least one error signal is generated, typically based on a difference. For example, the error signal may be based on the difference between the detected intensity and the set value, and is typically proportional to the difference. The method then adjusts the flow rate of gas through the interior of the spectrometer based on the error signal to minimize the error, thereby minimizing the difference between the detected intensity and the set value. Preferably, the difference between the detected intensity and the set value (and the resulting error) is below a threshold, and more preferably, the detected intensity is substantially aligned with the set value. A controller, particularly an automatic controller, may be used to compare the detected light intensity provided by the second detector with the set value, generate the error signal, and adjust the gas flow rate. The controller may generate an output signal to control the gas flow rate, for example, by controlling a valve and / or pump based on the output signal. In some embodiments, intensity detection is performed at two or more wavelengths, thereby obtaining two or more detected intensities corresponding to the respective wavelengths. Each detected intensity is then compared with a corresponding set value and a corresponding error signal is generated. Thus, multiple error signals can be used. In a specific embodiment, two error signals are used.

[0020] The detected intensity and setpoint are typically provided as representative voltage values. The detected intensity provided by the detector may be a voltage value. The setpoint may be stored in the controller and provided by a voltage source. Signal processing (e.g., amplification, etc.) may be performed on the detected intensity before comparison with the setpoint. The automatic controller may include a proportional-integral-derivative (PID) controller. The automatic controller, preferably a PID controller, may include a comparator for comparing the detected intensity (process variable PV) with a setpoint (SP), for example, by comparing voltages, and generating an error signal, typically a voltage value. The error signal typically represents the difference between the detected voltage and the setpoint voltage. In some embodiments, the error signal may be a simple voltage difference. The automatic controller may include a single-input-single-output (SISO) controller or a multiple-input-single-output (MISO) controller, which converts a single error signal or multiple error signals into an output signal, respectively. The output signal is used, for example, to control gas flow by controlling a power regulator that controls a gas flow valve and / or pump. The output of the automatic controller typically depends on the magnitude of one or more error signals. For example, the output of a PID controller depends on the magnitude, duration, and / or rate of change of one or more error signals. The power regulator then regulates the flow, for example by means of a valve and / or by controlling a pump, to reduce large variations in transmittance that are too high or too low. Therefore, the present invention provides an automatic control feedback loop. The automatic control feedback loop is run regularly or at predetermined time intervals to establish and maintain the transmittance of the gas in a steady-state condition. The time interval may depend on the measured detection intensity signal, for example the rate of change between consecutive measured intensity signals. In one embodiment, the time interval for running the automatic control feedback loop may remain unchanged or preferably decrease if the rate of change between consecutive measured intensity signals decreases. In one embodiment, the time interval for running the automatic control feedback loop may remain unchanged or preferably increase if the rate of change between consecutive measured intensity signals increases.

[0021] In some embodiments, when the spectrometer is turned on after being shut down (e.g., for a long or short period of time), the gas flow control process is initiated such that the control loop is activated at the start of the gas purge (either at the start of the gas purge or after). In this way, the gas transmittance stabilizes within a sufficient time before reliable analysis can begin. A computer control system can be used to initiate the gas flow control process when the spectrometer is turned on and gas begins flowing.

[0022] In some embodiments, the second path for measuring gas transmittance is located within the spectrometer. This second path may be partially or entirely within the spectrometer. In other embodiments, the second path is located within a measurement cell that is in fluid communication with the interior. This second path may be partially or entirely within the measurement cell. The measurement cell may be arranged to be in fluid communication with the interior via a closed-loop fluid circuit. For example, the closed-loop fluid circuit may be pumped by fluid communication with a pumping system within the spectrometer. Gas may circulate through the fluid circuit, which allows a portion of the gas from the interior of the spectrometer to enter the measurement cell and returns gas from the measurement cell to the interior of the spectrometer. Gas may be pumped into the measurement cell from the interior and passed through the closed-loop fluid circuit. A gas pumping system may be provided to pass a purge gas through the purge space. In some embodiments, the purge gas may be circulated through the purge space multiple times, optionally with each cycle passing through one or more gas purification or filtration stages to remove contaminants, such as UV-absorbing gases. A gas recirculation circuit may be provided to recover the gas in the manner described above. In other embodiments, the purge gas may be exhausted to the atmosphere after the interior is purged.

[0023] In some embodiments, the second path for measuring transmittance is a single pass through the gas. In some embodiments, the second path includes multiple passes through the gas. A preferred light source is a laser, particularly in embodiments using multiple passes through the gas. In embodiments using multiple passes through the gas, a multipass measurement cell is typically provided. For transmittance measurements based on absorption bands with low absorption cross sections, a multipass through the gas is more preferred. When using a multipass measurement cell, the optical path length of the second path can be increased to 1-100 meters.

[0024] In some embodiments, the intensity of light from a light source can be detected at a single wavelength or a single wavelength band (i.e., a single continuous set of wavelengths forming a single band). In some particularly preferred embodiments, the intensity of light is detected at one or more absorption wavelengths of water and / or molecular oxygen. The absorption of light by these gases enables detection with ppm sensitivity. In some embodiments, light is preferably detected at wavelengths absorbed by two or more contaminant gases (e.g., water and oxygen), i.e., at a common absorption wavelength. In other embodiments, the intensity of light can be detected at two or more discrete wavelengths or two or more discrete wavelength bands.

[0025] The purge gas stream can be passed through the interior at ambient pressure (i.e., atmospheric pressure), preferably at elevated pressure (higher than atmospheric pressure) (to reduce air leakage into the interior), or by drawing the interior under reduced pressure (vacuum) by a vacuum pump. In embodiments employing high pressure conditions, the pressure can be 1000-1100 mbar. In embodiments employing vacuum conditions, the gas pressure within the interior can be below atmospheric pressure to about 1 mbar, for example, 1-500 mbar, or 1-100 mbar. In some embodiments, a vacuum pressure below 1 mbar can be employed, for example, as low as 0.1 mbar, 0.01 mbar, or 0.001 mbar. The purge gas stream can be discharged to the atmosphere after passing through the interior, or it can be recirculated or returned to the interior in an air circulation loop, optionally after drying and / or filtering. The flow rate can range, for example, from 100 to 5,000 ml / min, although flow rates above or below this range can also be employed.

[0026] The spectrometer can be any spectrometer, such as an emission spectrometer or an absorption spectrometer, in which a gas is used to purge the interior of the spectrometer, which contains (all or part of) the optical path of light from the sample, which is analyzed at the detector. Emission spectrometers that use a UV-transparent purge gas (e.g., argon) are particularly suitable for use with the present invention. Spectrometers used to perform spark optical emission spectroscopy (Spark-OES) or laser-induced breakdown spectroscopy (LIBS) are two examples of such spectrometers. Other plasma source optical emission spectrometers, such as inductively coupled plasma (ICP-OES) or glow discharge optical emission spectrometers, can also be used.

[0027] An emission spectrometer can be used to perform emission spectroscopy. The emission spectrometer includes a spectrograph for recording the spectrum of emitted light, which travels along a first path from a sample to a first detector. The spectrograph can analyze the light based on its wavelength, for example by using one or more gratings to separate the light according to its wavelength and detecting the separated light, or by using an energy dispersive detector as the first detector to detect light based on different energies (i.e., wavelengths). Therefore, the spectrograph can be a wavelength dispersive or energy dispersive spectrograph. The spectrograph can include a dispersive device, such as a grating, to spatially disperse the light according to its wavelength. The first detector can detect the dispersed light to produce a spectrum. Thus, the spectrograph can record variations in intensity over the wavelength spectrum. The spectrum of the emitted light can be used to infer the composition of the sample material.

[0028] The spectrometer can include analyzing and detecting light from a sample chamber, such as a plasma chamber, which houses a sample to be analyzed, wherein a plasma excites a portion of the sample. Excited sample material in the sample chamber can emit light. The sample excited by the plasma can also emit light. The sample chamber can be a spark chamber, where a spark generates a plasma from the sample, or a laser in the sample chamber (e.g., in LIBS). Thus, plasma can be generated by applying an electrical discharge (spark or arc) or a laser to the sample.

[0029] One type of emission spectroscopy may use a plasma (such as an inductively coupled plasma (ICP) or microwave induced plasma (MIP) or glow discharge) to excite a sample to emit light at a wavelength characteristic of one or more elements in the sample.

[0030] A specific type of emission spectrometry uses a spark or arc to excite a sample into emitting light at a wavelength characteristic of one or more elements in the sample. For convenience, as used herein, the term spark emission spectrometry refers to any emission spectrometry method that uses an electrical discharge, such as a spark or arc, to excite the sample, and the term spark chamber refers to a chamber used to conduct any type of electrical discharge. In spark emission spectrometry, a solid sample is typically mounted on the surface of a spark stand, which is part of the spectrometer. The spark stand also includes a spark chamber, in which an electrode is positioned with its tapered end facing the sample surface. The spark stand's surface has an opening in the spark chamber wall, into which the sample is mounted, typically with a hermetic seal, facing the end of the electrode. Except for the tapered end, the rest of the electrode is surrounded by an insulator. A series of discharges is initiated between the electrode and the sample, with the sample acting as the counterelectrode. The insulator encourages the discharge to proceed toward the sample rather than the chamber walls. Localized sample material subjected to the discharge is vaporized, and some of the vaporized atomic material rises to an excited state. During relaxation, atomic materials emit photons whose energies are characteristic of the elements in the material. Spectral analysis of these emitted photons allows inferences about the sample material's composition. Therefore, a portion of the emitted light, caused by the discharge, travels along a first path from the spark chamber to a spectrometer for spectral analysis. A substantially UV-transparent gas, such as argon, is passed through the interior of the spectrometer, which contains both the first path and the spectrometer.

[0031] The method of the present invention may be used as part of an emission spectroscopy method. That is, the present invention provides an emission spectroscopy method comprising a method for controlling the flow of gas through a spectrometer as described herein.

[0032] The emission spectroscopy method may include: providing a sample (preferably a solid sample) to be analyzed; exciting the sample to emit light (e.g., using a spark, laser, or plasma); spectroscopically analyzing the emitted light using a spectrometer having a first detector to determine one or more elements in the sample, wherein the emitted light travels along a first path through the interior to the first detector; passing a substantially (UV) transparent gas through the interior; and controlling the flow of the substantially (UV) transparent gas through the interior using the method according to the present invention. In particular, the emission spectroscopy method may include transmitting light from a light source along a second path through the gas to a second detector; detecting the intensity of the light from the light source at one or more wavelengths of light at the second detector; comparing the detected light intensity with a corresponding set value corresponding to a desired transmittance of the gas in the interior and generating at least one error signal based on the comparison; and adjusting the flow rate of the gas through the interior based on the error signal.

[0033] The emission spectrometry method may include other well-known emission spectrometry steps, such as any, preferably all, of the following steps: providing a solid sample to be analyzed, the sample being mounted typically with the sample surface presented to the end of an electrode in a spark chamber of an emission spectrometer and / or typically with the sample positioned above an opening in a wall of the spark chamber facing the end of the electrode, typically with a gas-tight seal; inducing one or more, typically a series of, discharges between the electrode and the sample, with the sample acting as a counter electrode; vaporizing material from the sample and exciting at least a portion of the vaporized material such that the excited material emits photons having an energy (i.e., wavelength) characteristic of the elements in the material; and spectrally analyzing the emitted photons using a spectrometer to determine (i.e., identify) one or more elements in the sample, wherein, in use, a purge gas stream, preferably an inert gas (e.g., argon) is passed through an interior comprising the spectrometer, and light from the sample passes through the interior along a first path to a detector. The interior through which the purge gas stream passes preferably includes the spectrometer, the spark chamber, and any interior connecting the spectrometer and the spark chamber through which the emitted light travels.

[0034] In one particular aspect, the present invention provides an emission spectroscopy method comprising: providing a solid sample to be analyzed so that the surface of the sample is presented to the end of an electrode in a spark chamber; inducing one or more discharges between the electrode and the sample, wherein the sample serves as a counter electrode; vaporizing material from the sample and exciting at least a portion of the vaporized material, thereby exciting the material to emit light; performing spectral analysis on the emitted light using a spectrometer having a first detector to determine one or more elements in the sample, wherein the emitted light passes through an interior along a first path to reach the first detector, and the method comprises passing a substantially UV transparent gas through the interior; transmitting light from a light source through the gas along a second path to a second detector; detecting the intensity of the light from the light source at one or more wavelengths of the light at the second detector; comparing the detected light intensity with a corresponding set value corresponding to a desired transmittance of the internal gas, and generating at least one error signal based on the comparison; and adjusting the gas flow rate through the interior based on the error signal.

[0035] The present invention provides an apparatus for controlling the flow of gas through a spectrometer, comprising: a housing containing an interior through which light from a sample can pass along a first path to reach a first detector; a gas source for passing a substantially UV-transparent gas through the interior; a light source for transmitting light through the gas along a second path; a second detector for detecting the intensity of light from the light source at one or more wavelengths of light transmitted through the gas along the second path; and a controller for comparing the detected light intensity with a corresponding set value corresponding to a desired transmittance of the gas in the interior, generating at least one error signal based on the comparison (i.e., based on a difference between the detected intensity and the set value), and adjusting the flow rate of the gas through the interior of the spectrometer based on the error signal. The present invention provides an emission spectrometer including the gas flow control apparatus. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 Shown is a plot of the normalized intensity of the 170.20 nm atomic emission line of iron (Fe), recorded after purging the spectrometer with argon.

[0037] Figure 2A and Figure 2B Components that may be used for single-pass absorption measurements according to embodiments of the present invention are schematically shown.

[0038] Figure 3 Components that may be used for multi-pass absorption measurements according to an embodiment of the present invention are schematically shown.

[0039] Figure 4 An embodiment of a control loop system for controlling the flow of gas through the interior of a spectrometer using single-pass absorption measurements based on feedback of the detected intensity of light transmitted through the gas is schematically shown.

[0040] Figure 5 Another embodiment of a control loop system is schematically shown for controlling the flow of gas through a spectrometer based on feedback of the detected intensity of light transmitted through the gas, wherein the spectrometer uses two light sources to perform multi-pass measurements to monitor the intensity of light transmitted through the gas at two different wavelengths.

[0041] Figure 6 An embodiment of a spark emission spectrometer is schematically shown, including means for controlling the flow of a purge gas through the spectrometer, the spectrometer using single-pass absorption measurements.

[0042] Figure 7 An embodiment of a spark emission spectrometer is schematically shown, including means for controlling the flow of a purge gas through the spectrometer, the spectrometer using multi-pass absorption measurements.

[0043] Figure 8 A flow chart of a method according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0044] In order to further understand the characteristics of the present invention, various embodiments will be described below as examples only.

[0045] The present invention utilizes the Beer-Lambert law, which states that when light passes through an absorbing gas, the measured transmittance decays exponentially with the product of the gas concentration (c), the optical path length (d), and the absorption cross section (σ), which is an intrinsic property of the gas composition.

[0046] One embodiment of the present invention is based on Figure 2A and Figure 2B Schematic diagram of a single-pass absorption measurement. A UV-transparent purge gas stream is passed through the interior of the spectrometer (not shown), which contains the optical path from the sample to the detector. This gas purge increases the transmittance within the spectrometer, thereby improving the stability and sensitivity of the sample analysis. Figure 2A The measurement cell 102 is shown filled with flowing purge gas (e.g., argon) from a spectrometer, which has a high concentration of contaminants such as absorbing gas components, such as oxygen (O2) and water (H2O). Contaminants may be present in the purge gas due to leaks into the spectrometer purge space and may accumulate after the spectrometer purge system has been shut down for a period of time. Figure 2B A measurement cell 102 is shown filled with a gas (eg, argon) having relatively low concentrations of contaminant-absorbing gas components (eg, oxygen (O 2 ) and water (H 2 O)).

[0047] In some embodiments of the present invention, measurement cell 102 is in fluidic communication with the interior of a gas-purged spectrometer (not shown), such as an emission spectrometer, to increase transmittance within the spectrometer. Light from a sample travels along a first path within the gas-purged spectrometer, preferably containing a spectrograph, to reach a first detector (not shown). Gas from the spectrometer, referred to herein as purge gas, is in fluidic communication with the measurement cell and enters the measurement cell through gas inlet 106 and exits the measurement cell through gas outlet 108, e.g., back to the spectrometer. Thus, the gas is in fluidic communication with the interior of the spectrometer via a closed-loop fluid circuit. In some embodiments, gas can be pumped from the interior into the measurement cell and through the closed-loop fluid circuit.

[0048] The vacuum ultraviolet (VUV) light source 101 in the embodiment shown is arranged at one end of a measuring cell 102 to allow VUV light to pass through the measuring cell by using a suitable VUV-transparent low-reflectivity window 104. For example, the light source 101 can be a deuterium lamp or a xenon flash lamp. The windows 104 are arranged at both ends of the measuring cell. The measuring cell can be a stand-alone unit as in the present embodiment, or it can be part of a spectrometer, for example, arranged inside the spectrometer. In the latter case, the analysis of the gas transmittance is performed directly inside the spectrometer. Figure 2A and Figure 2B In the former case shown, the gas from the interior of the spectrometer is pumped through the gas inlet 106 into the independent gas measurement cell 102 and discharged through the gas outlet 108 in a closed loop, and the analysis is performed in the closed loop. Light from the light source 101 passes through the gas in the measurement cell along a second path 110 and is detected using a VUV sensitive detection system 103 (such as a GaP photodiode or a photomultiplier tube (PMT)) as a second detector. The measurement cell has a path length d. In some embodiments where the measurement cell is integrated into the spectrometer, the path length d for a single pass can be 5-30 cm. In some embodiments where the measurement cell is located outside the spectrometer, the path length d for a single pass can be 10-50 cm. The intensity of the received light depends on the concentration of the absorbing gas components (water and oxygen) and can be detected by the detection system 103. Compared to higher concentrations of absorbing gases (such as Figure 2A As shown in Figure 2), when the concentration of the absorbing gas is low, the detected intensity is higher (due to the higher transmittance of the gas), as shown in Figure 2. Figure 2BAs shown in the figure, due to leaks in the system, absorbing gases such as water and oxygen are introduced from the air outside the spectrometer. When the water and oxygen levels are detected to be too high (indicated by lower light intensities), the flow rate of the UV-transparent purge gas entering the spectrometer should be increased. When the water and oxygen levels are detected to be too low (indicated by higher light intensities), the gas transmittance is too high, and the flow rate of the UV-transparent purge gas entering the spectrometer should be reduced. Therefore, the gas flow rate can be dynamically adjusted based on the detected light intensity, effectively achieving a stable gas flow rate into the spectrometer in a short period of time with minimal gas consumption. Further details on controlling the gas flow rate based on the detected gas transmittance are described below.

[0049] The advantages of using VUV light are: a) it enables the simultaneous measurement of water and oxygen by exploiting common electronic transitions in the 130-190 nm wavelength range; b) both gases have strong molecular absorption cross sections in this wavelength range (10 -19 centimeter -1 Moore -1 centimeter 3 ), and c) the ability to perform high-sensitivity measurements in a compact single-pass absorption cell. The disadvantages of using VUV light are that VUV light sources tend to be relatively expensive and often require the use of bandpass VUV filters to achieve a good signal-to-noise ratio.

[0050] Another embodiment of the present invention is based on Figure 3 Multi-pass absorption measurement is shown schematically. In addition to the features related to the multi-pass configuration, other features and principles are the same as Figure 2A and Figure 2B The single-pass system shown in FIG is the same as that shown in FIG, such as the gas inlet and outlet, etc.

[0051] Figure 3 The illustrated embodiment uses a near infrared (NIR) light source 201, such as a laser diode, to detect gaseous water or oxygen in the NIR portion of the electromagnetic spectrum (wavelengths between 760-950 nm). Laser wavelengths in the NIR may include 830 nm and 940 nm for water absorption and 760 nm for oxygen absorption. Figure 2A and Figure 2B The principle of multi-pass measurement is the same as the single-pass measurement embodiment shown in the figure. However, the absorption cross section of strong molecules of water or oxygen in the NIR region is 2-4 orders of magnitude lower than that in the VUV region (10 -23 –10 -21 centimeter -1 Moore -1 centimeter 2 ). Therefore, a longer optical path length is required to achieve high enough sensitivity to perform measurements with ppm accuracy.

[0052] exist Figure 3 In the embodiment shown, a NIR light beam 207 from an NIR light source 201 passes through a gas measurement cell 206 containing a purge gas to measure the transmittance of the gas, wherein the light beam enters and leaves the measurement cell through a NIR transparent low reflectivity window 204. By using highly reflective NIR mirrors 203 placed at opposite ends of the cell, the light is reflected multiple times between the mirrors, thereby increasing the optical path length of the light beam through the measurement cell by 2-4 orders of magnitude to improve sensitivity (compared to a single pass). The NIR transparent window 204 is located within the mirror 203. In the embodiment shown, the window 204 is located within the same mirror, but in other embodiments, the window may be located in an opposing mirror. The reciprocating path of the light beam through the cell (the second path according to the present invention) is shown by the dotted line 207. By adjusting the incidence or angle of incidence θ (202) of the light beam 207 on the surface of the mirror 203, the number of reflections can be increased rapidly, thereby enabling the design of compact gas cells with an optical path length of 1-100 meters (e.g., 1-10 decimeters). 3 ). Typically, the angle of incidence θ is 1°-10°. After multiple reflections, the light beam leaves the cell and is received by an NIR-sensitive detector 205 (e.g., a charge-coupled device (CCD) or a silicon photodiode), i.e., the second detector according to the present invention. The advantages of the NIR multi-pass method are: a) the high stability and long life of the NIR light source; and b) low initial and maintenance costs. A disadvantage of this method is that oxygen and water cannot be detected simultaneously because these two gases do not have a common absorption band in the infrared region of the electromagnetic spectrum. To address this disadvantage, information about the gas transmittance can be obtained by measuring only one of the gases, preferably gaseous water, because water can adhere to surfaces through hydrogen bonds and takes longer to clear from a closed container than oxygen. Alternatively, two independent measurement systems (e.g., two independent multi-pass systems) can be used to measure gaseous water and gaseous oxygen separately. Each independent measurement system includes a light source, a beam path, and a second detector.

[0053] Figure 4An embodiment of a control loop system is schematically illustrated for controlling the flow of gas through a spectrometer based on feedback from the detected intensity of light transmitted through the gas (gas transmittance). The system includes an automatic control loop based on proportional-integral-derivative (PID) control. The diagram shows measurement system 401, which includes a light source 410 that transmits light along a path through UV-transparent gas in a measurement container 412 and is collected at a detector 414. Gas enters container 412 through a gas inlet 416 and exits through a gas outlet 418. A pump (not shown) that can be fluidically connected to the gas outlet pumps the gas through the container, or circulates it through the container in a closed loop. During operation, the gas pressure in the container is typically above, and preferably slightly above, atmospheric pressure (e.g., 1100 mbar) to minimize gas leakage into the container. The flow of gas into the container is controlled by a variable valve 407. Container 412 can be positioned within the interior of a spectrometer, with light from a sample passing through the interior of the spectrometer along a first path to a first detector for analysis, and the interior is purged with UV-transparent gas. In an alternative embodiment, the container 412 may be a separate measurement unit in fluid communication with the interior of the spectrometer.

[0054] The detector 414 of the measurement system generates a detection signal 402 based on the detected light intensity, which represents the measured gas transmittance. The detection signal 402 is fed to an automatic controller 408 including a PID controller. The signal 402 is typically a potential voltage, which is a process variable (PV) input value 403 for the PID control of the automatic controller 408. As needed, the input value 403 is compared with a predefined set value (SP) 404, which corresponds to the desired or predetermined concentration of the absorptive contaminant gas (such as water and oxygen). The set value is stored in the controller 408 as a voltage provided by a voltage source (not shown). Through the comparison, an error signal (E) is output. The single-input single-output (SISO) PID controller 405 reads the error signal and outputs a reference value or signal 420 to the power regulator 406, so that the reference value or signal 420 depends on the magnitude, duration and rate of change of the error signal, that is, it is based on Proportion 、 integral and differentialThe power regulator 406 then outputs a signal, in this case a DC current 409, based on a reference value from the PID controller to control (i.e., adjust) the valve 407, thereby regulating the gas flow through the valve. If the error signal is large, for example due to a large difference between the detected light intensity value 403 and the set point 404 due to low gas transmittance (under-purging) or high gas transmittance (over-purging), the reference value 420 used to control the power regulator output 409 (to the valve 407) is adjusted to regulate the gas flow rate and minimize the difference between the detected light intensity and the set point as quickly as possible. As the error signal decreases, i.e., based on the difference between the detected light intensity value 403 and the set point 404, the reference value 420 used to control the power regulator output 409 (to the valve 407) is adjusted accordingly to further reduce the difference between the detected light intensity and the set point, while preventing or limiting the gas transmittance from becoming too high or too low relative to the set point. In this way, the system mitigates large transmittance variations caused by over- and under-purging of the gas. When the error signal approaches zero, the control loop system continues to operate to maintain a steady-state gas transmittance. The automatic control loop system operates at predetermined time intervals (sampling intervals) or continuously, such as regular sampling intervals or sampling intervals determined based on a previous reference value or error signal, to maintain the gas transmittance within the container at steady-state conditions. Therefore, the automatic PID controller continuously measures to ensure that there is no significant difference between the detected signal voltage and the set voltage (the error signal remains substantially zero). In some embodiments, the control loop system can be controlled by a processor or computer 425 that can execute a computer program containing instructions that cause the control loop system to perform the methods described herein. The computer program can be stored on a computer-readable medium. In some embodiments, the computer can control the system so that when the spectrometer is powered on after a shutdown (e.g., a long or short shutdown), the control loop (light source, detector, automatic controller, and power regulator) is activated as soon as a gas purge begins (once or after the gas purge begins). In this way, the gas transmittance reaches a stable state within a sufficient time before reliable analysis can begin.

[0055] For embodiments where a NIR light source is used to measure gas transmittance, the absorption of the aforementioned absorbing gases (water and oxygen) is much weaker in this region, and water and oxygen do not have a common absorption band in this region, so a single light source (e.g., a single laser) cannot be used to measure both gases simultaneously. In one embodiment, feedback related to the transmittance (i.e., purity) of the purge gas inside the spectrometer is obtained by measuring the absorption of only one absorbing gas component in the air pollutants. When only one absorbing gas component is measured, it is preferred to measure only gaseous water because water can adhere to surfaces through hydrogen bonds and takes longer to purge from a closed container than oxygen. Of course, if desired, only oxygen can also be measured. In the case of measuring only one gas component, a control loop system can be used that is compatible with the gas component. Figure 4 If a NIR light source is used, it is better to use multi-pass measurement to measure the transmittance of the gas due to the weak absorption in this area, such as Figure 3 shown, but can be used with Figure 4 The same control system is used to control the gas flow.

[0056] In another embodiment, where an NIR light source is used to measure the transmittance of an argon purge gas, the absorption of two or more absorbing gas components of the air contaminants (e.g., water and oxygen) is measured separately to provide feedback related to the transmittance (i.e., purity) of the purge gas within the spectrometer. Because these absorbing gas components do not share a common absorption band in the NIR region, two (or more) independent light sources are used. Figure 5 Schematically illustrates an embodiment of a control loop system for controlling the flow of gas through an interior of a spectrometer based on feedback of the intensity of light transmitted through the gas detected, wherein similar to Figure 3 As shown, multi-pass measurement is performed using two light sources simultaneously to monitor the intensity of light transmitted through a gas at two different wavelengths. The figure shows a measurement system 501, which includes an NIR light source with two separate NIR laser sources 510a and 510b. Each transmits light of a different wavelength along its own multiple-reflection path. The light passes through a UV-transparent gas (argon) in a multi-pass measurement container 512 and is collected at respective photodetectors 514a and 514b. One wavelength matches the absorption band of water, and the other matches the absorption band of oxygen (O2).

[0057] Argon gas from a gas source (not shown) enters container 512 through gas inlet 516 and exits through gas outlet 518. A vacuum pump (not shown) connected to the interior of the container forces gas (from the gas source) through the container. The flow of gas into the container is controlled by variable valve 507. Container 512 can be positioned within a spectrometer, purged with UV-transparent gas, and through which light from a sample passes along a first path to a first detector for analysis. In alternative embodiments, container 512 can be a separate measurement unit in fluidic communication with the interior of the spectrometer.

[0058] By using this system, the detected light intensity from each light source is measured on the corresponding detector, which represents the transmittance of each absorbing gas component, namely water and oxygen. For each detected light intensity, a potential voltage is output by the respective detector (503a and 503b for water and oxygen measurements, respectively) and fed as input to the automatic controller 508. As needed, the controller compares these input voltage values ​​with the corresponding predefined set points SP1 and SP2 (504) corresponding to the desired concentration of the gas and derives an error signal as output based on the comparison. A multiple input single output (MISO) PID controller (505) reads these error signals (E1 and E2) and generates an output strategy (roughly divided into cooperative and non-cooperative) based on the predefined settings of the PID controller 505. In the cooperative strategy, each detector is assigned a weight and the response of the PID depends on the weighted average of the two sensors. For example, oxygen is typically eliminated from the system at a much faster rate than water. If the oxygen elimination rate is 10 times faster than the water elimination rate, the oxygen detector will have 1 / 10 the overall weight compared to the water detector when determining how the PID control will react when receiving measurements from both detectors. In a non-cooperative system, it is assumed that one system takes precedence over the other. In this example, the PID will only react to the gas with the highest concentration and will operate based solely on that single gas. However, in the case of a switchover of the dominant system, both gases need to be monitored. In this case, the PID response will also switch to the gas with the highest concentration at a given point in time. The PID controller 505 then outputs a reference value 520 to the power regulator 506 based on the magnitude, duration, and rate of change of the error signal. The power regulator then outputs a DC current that regulates the flow through the valve 507 to limit large variations in transmittance, both high and low, and to achieve a steady-state transmittance for the gas in the shortest possible time with minimal purge gas consumption, as described above. The automatic control loop system operates at predetermined intervals to achieve and maintain steady-state conditions.

[0059] refer to Figure 6, schematically illustrates the configuration of a spark emission spectrometer 600, including a device for controlling the flow of a UV-transparent purge gas through the spectrometer. The spectrometer includes a spectrometer 601, which comprises optics, a grating, and a detection system (not shown) for analyzing light emitted by a sample material, which has been vaporized and excited by applying a spark discharge to a solid sample in a spark chamber 607. The atmosphere in spectrometer 601 is purged by purging the interior of the spectrometer with argon gas. Argon gas enters the interior from a pressurized gas source (not shown) through a gas inlet 616, and its flow rate is controlled by an adjustable valve 605. A vacuum pump (not shown) is connected to a gas outlet 618 to maintain the pressure in the spectrometer between atmospheric pressure and approximately 1100 mbar, thereby minimizing gas leakage during the purge.

[0060] A UV light source 602 (e.g., a laser) is located outside the spectrometer and directs a light beam into the interior of the spectrometer through a UV-transparent window. A detector 603, located outside the spectrometer and opposite the light source, receives the transmitted light through another UV-transparent window. A control loop system is provided for controlling the flow of gas through the interior of the spectrometer based on feedback from the detected transmitted light intensity (gas transmittance). The detected light intensity measured at detector 603 represents the transmittance level of the absorbing gas components (water and oxygen) and is used to generate a voltage (603a), which is fed to an automatic PID controller 608. Controller 608 compares the input voltage with a setpoint corresponding to the desired transmittance (gas concentration) of the gas and generates an error signal based on the comparison. The PID controller then generates an output reference signal 620 for the power regulator 606 based on the magnitude, duration, and / or rate of change of the error signal. Then, based on the reference signal, the power regulator 606 outputs a DC current for controlling the valve 605 to avoid the argon flow being too high or too low and to reach the steady-state transmittance of the gas in the spectrometer in the shortest time with the least argon consumption.

[0061] Figure 7 An embodiment of a spark emission spectrometer 700 is schematically shown, comprising means for controlling the flow of a purge gas through the spectrometer using multi-pass absorption measurement. The figure shows a partial cross-section of the spectrometer, showing the interior of the spectrometer and the beam path.

[0062] Many components of the spectrometer are Figure 6The spectrometer 600 shown is generic. The apparatus is also used to control the flow of a UV-transparent purge gas through the spectrometer. The spectrometer includes a spectrometer 701, which includes optics, a grating, and a detection system (not shown) for analyzing light emitted by a sample material that has been vaporized and excited by applying a spark discharge to a solid sample in a spark chamber (not shown). The emitted light is received into the spectrometer 701 through an aperture 707.

[0063] The spectrometer 701 is purged of air by purging the interior of the spectrometer with argon. Argon gas from a pressurized gas source (not shown) enters the interior through gas inlet 716, and the flow rate through the gas inlet is controlled by adjustable valve 705. A vacuum pump (not shown) is connected to gas outlet 718 to generate an argon pressure (between 1100 mbar and atmospheric pressure) within the spectrometer, thereby removing most of the air. Argon gas is introduced into the interior during the purge process to purge the air inside. The argon gas is then exhausted from the interior through gas outlet 718.

[0064] A UV light source 702 (e.g., a laser) is located outside the spectrometer and allows a light beam 714 to enter the spectrometer through a UV transparent window (not shown). A detector 703 is also located outside the spectrometer and, in this example, is located on the same side of the spectrometer as the light source. The detector 703 receives the transmitted light from the spectrometer through another UV transparent window (not shown). Figure 6 The illustrated embodiment differs in that the optical path within the spectrometer includes multiple optical paths 715 through the gas, due to repeated reflections between reflective elements 710 (e.g., highly reflective surfaces or mirrors). A first reflective element 710 is mounted on the inner side 711 of the spectrometer, visible in the cross-sectional view; a second reflective element (not shown) is mounted on the opposite inner side 712 of the spectrometer, opposite the first. Consequently, the optical path 715 in this embodiment has a zigzag shape.

[0065] A control loop system is provided for controlling the flow of gas through the interior of the spectrometer based on feedback from the detected transmitted light intensity (gas transmittance). The detected light intensity measured at detector 703 represents the transmittance level of the absorbing gas components (water and oxygen) and is used to generate a voltage value (703a), which is fed into an automatic PID controller 708. Controller 708 compares the input voltage with a setpoint corresponding to the desired transmittance of the gas (gas concentration) and generates an error signal based on the comparison. The PID controller then generates an output reference signal 720 for power regulator 706 based on the magnitude, duration, and / or rate of change of the error signal. Based on the reference signal, power regulator 706 then outputs a DC current 704 for controlling valve 705 to avoid excessively high and low argon flow rates and to achieve a steady-state transmittance of the gas in the spectrometer in the shortest possible time with minimal argon consumption.

[0066] It should be understood that, in addition to proportional-integral-derivative (PID) control, other mechanisms for controlling gas flow can also be used in the control loop. A simple comparator can be used to compare the detected light intensity (representing gas transmittance) with a set point and, based on the difference between the measured light intensity and the set point, change the output reference value used to control a gas supply flow valve or pump.

[0067] It will be appreciated that while the aforementioned embodiments are considered preferred, different configurations may be used. For example, any of the mentioned light sources (UV, VUV, visible, IR or NIR) may be used to perform single or multi-pass measurements.

[0068] In view of the above embodiments, Figure 8 A flow chart of a method according to the present invention is shown. In step 800, first, a purge gas flow is passed through the spectrometer. In step 802, a light source is started to transmit light through the gas to a detector, and the intensity of the light is detected at the detector. In step 804, the controller compares the detected light intensity signal with a set value corresponding to the desired transmittance of the gas. In step 806, an error signal is generated based on the difference between the detected signal and the set value. In step 808, the controller determines whether the error is lower than / higher than a predetermined threshold error. The threshold represents the minimum difference or zero difference between the detected signal and the set value. If the error is higher than the threshold, the controller adjusts the flow rate of the purge gas in step 810 and restarts the process in step 802. If the comparison result of the detected light signal and the set value shows that the detected signal is too low (the transmittance of the gas is too low), the gas flow rate can be increased. Similarly, if the comparison result shows that the detected signal is too high (the transmittance of the gas is too high), the gas flow rate can be reduced. In a preferred embodiment, PID control is used for adjustment so that it is based on Proportion 、 integral and differential Item. The process is iteratively executed to minimize the error until, at step 808, it is determined that the error is no longer above the threshold, thereby eliminating the need to adjust the flow rate and maintaining the flow rate at step 812. Thereafter, as shown by the dashed line, the process is executed at intervals to ensure that the error signal remains below the threshold. If the error rises above the threshold at any time, the controller adjusts the flow rate of the purge gas at step 810, and so on.

[0069] From the above description, the following preferred features can be derived, but these features are not exhaustive.

[0070] Preferably, the flow rate of the gas through the interior of the spectrometer is adjusted to minimize the difference between the detected intensity and the set value.

[0071] Preferably, the spectrometer is an emission spectrometer. Preferably, the spectrometer is a spark emission spectrometer or a LIBS spectrometer.

[0072] Preferably, the second path is internal to the spectrometer.

[0073] Preferably, the second path is within the measuring cell which is in fluid communication with the interior. Preferably, the measuring cell is in fluid communication with the interior via a closed-loop fluid circuit. Preferably, gas is pumped from the interior into the measuring cell and through the closed-loop fluid circuit.

[0074] In some embodiments, the second path is a single light path through the gas. In some other embodiments, the second path includes multiple light paths through the gas.

[0075] In some embodiments, preferably, the light is VUV light. In some other embodiments, preferably, the light is near IR (NIR) light.

[0076] In some embodiments, the intensity of the light is preferably detected at a single wavelength or a single wavelength band. In some other embodiments, the intensity of the light is preferably detected at two or more non-contiguous wavelengths or two or more non-contiguous wavelength bands. In such embodiments, the light source preferably comprises two or more discrete light sources for transmitting light along two or more second paths in the gas, respectively, and two or more second detectors for detecting the intensity of the light transmitted along the corresponding second or multiple paths, respectively, wherein each light source emits light at a different wavelength and / or each second detector detects light at a different wavelength, and the intensity of the light is detected at two or more non-contiguous wavelengths or two or more non-contiguous wavelength bands. Preferably, the different wavelengths correspond to the absorption of gaseous water and gaseous oxygen, respectively. Therefore, such embodiments preferably allow the measurement of gaseous water and gaseous oxygen using two independent measurement systems (e.g., two independent multi-pass systems), each of which includes a light source, a second beam path, and a second detector.

[0077] Preferably, the intensity of light is detected at one or more absorption wavelengths of water and / or molecular oxygen.

[0078] In some embodiments, the steps of comparing the detected light intensity with a corresponding setpoint, generating at least one error signal, and adjusting the gas flow rate are preferably performed using proportional-integral-derivative (PID) control. Preferably, control is performed by a controller comprising a PID controller. Preferably, the controller comprises a single-input-single-output (SISO) controller or a multiple-input-single-output (MISO) controller, which converts a single error signal or multiple error signals, respectively, into an output signal for controlling the gas flow rate. Preferably, the output signal is used to control a gas flow valve and / or pump.

[0079] It will be appreciated that modifications may be made to the foregoing embodiments of the present invention and still fall within the scope of the present invention.

[0080] Unless otherwise stated, the use of any and all examples or exemplary language ("for example," "such as," and similar language) described herein is intended merely to better illustrate the invention and does not limit the scope of the invention. No language in this specification should be construed as indicating any non-claimed element essential to the practice of the invention.

[0081] As used herein, including in the claims, unless the context indicates otherwise, terms in the singular should be construed to include the plural, and vice versa. For example, unless the context indicates otherwise, the singular forms "a" or "an" herein, including in the claims, refer to "one or more."

[0082] In the detailed description and claims of the specification, the words "comprises," "comprising," "having," and "containing" mean "including but not limited to," and are not intended to (and do not) exclude other elements.

[0083] Prior art references

[0084] 1) L.B. Kreuzer, Journal of Applied Physics, Vol. 42, No. 7, 1971, pp. 2934–2943

[0085] 2) V. Ebert, T. Fernholz, H. Pitz, and T. Li, eds., OSA Trends in Optics and Photonics Series, vol. 36 (Optical Society of America, 2000), paper SaB4.

[0086] 3) V. Ebert, K.-U. Pleban, and J. Wolfrum, "Laser Applications to Chemical and Environmental Analysis," Technical Digest (Optical Society of America), pp. 206-209 (1998)

[0087] 4) J.U. White, Journal of the Optical Society of America, Vol. 32, No. 5, pp. 285-288 (1942)

[0088] 5) Hanst, PL, Applied Spectroscopy, 1970, 24(2), 161–174.

[0089] 6) P. G. Wilkinson, H. L. Johnston, J. Chem. Phys. 18, 190 (1950).

[0090] 7) R. W. Ditchburn, D. W. O. Heddle, Proceedings. Mathematical, physical, and engineering sciences / the Royal Society, Vol. 220, No. 1140, pp. 61–70, 1953.

[0091] 8) WO2014 / 062419

[0092] 9) US2017 / 0102315

[0093] 10) US2015 / 0177131

[0094] 11) US2012 / 0113426

[0095] 12) US2018 / 0259452

[0096] 13) US2017 / 0139182

[0097] 14) EP1664691

Claims

1. A method for controlling gas flow through a spectrometer, comprising: passing a purge gas through an interior of the spectrometer, wherein light from a sample can pass through the interior along a first path to reach a first detector, and the purge gas is transparent to light in a spectral region analyzed by the spectrometer; transmitting light from a light source along a second path through gas in a measurement cell to a second detector, wherein the measurement cell is in fluid communication with an interior of the spectrometer purged with the purge gas, and wherein the second path is within the interior of the spectrometer; detecting, at the second detector, an intensity of light from the light source at one or more wavelengths of the light; comparing the light intensity detected by the second detector with a corresponding set value corresponding to a desired transmittance of the purge gas in the interior of the spectrometer, and generating at least one error signal based on the comparison; as well as A flow rate of the purge gas through an interior of the spectrometer is adjusted based on the error signal. 2 . The method according to claim 1 , wherein a flow rate of the purge gas through the interior of the spectrometer is adjusted so that a difference between the detected intensity and a set value is below a threshold value. The method according to claim 1 , wherein the spectrometer is an emission spectrometer. The method according to claim 3 , wherein the spectrometer is a spark emission spectrometer or a LIBS spectrometer. The method of claim 1 , wherein the measuring cell is in fluid communication with the interior via a closed-loop fluid circuit. The method according to claim 1 , wherein the second path is a one-way path through the gas in the measurement cell.

7. The method of claim 1 or 2, wherein the second path comprises a multi-pass path through the gas in the measurement cell.

8. The method of claim 1 or 2, wherein the light is VUV light or near IR (NIR) light.

9. The method according to claim 1 or 2, wherein the second detector detects the intensity of the light at a single wavelength or a single wavelength band.

10. The method of claim 1 or 2, wherein the second detector detects the intensity of the light at two or more non-contiguous wavelengths or two or more non-contiguous wavelength bands.

11. The method according to claim 1 or 2, wherein the second detector detects the intensity of the light at one or more absorption wavelengths of water and / or molecular oxygen.

12. The method of claim 1 or 2, wherein the steps of comparing the light intensity detected by the second detector with a corresponding set value, generating at least one error signal, and adjusting the purge gas flow rate are performed using proportional integral derivative (PID) control.

13. An emission spectroscopy method comprising the following steps in a spectrometer: Provide samples to be analyzed; Exciting the sample to emit light; performing a spectroscopic analysis on the emitted light using a spectrometer having a first detector to determine one or more elements in the sample, wherein the emitted light passes through the interior along a first path to reach the first detector; passing a substantially UV transparent purge gas through the interior; as well as The flow of a substantially UV transparent purge gas through the interior is controlled using the method of any one of claims 1 to 12.

14. A device for controlling the flow of gas through a spectrometer, comprising: a housing in which light from the sample can pass along a first path through the interior of the spectrometer to reach the first detector; a gas source for passing a substantially UV transparent purge gas through an interior of the spectrometer; a light source for transmitting light along a second path through a gas in a measurement cell, wherein the measurement cell is in fluid communication with an interior of the spectrometer purged with the purge gas, and wherein the second path is inside the spectrometer; a second detector for detecting an intensity of light from the light source at one or more wavelengths of the light, the light being transmitted along a second path through the purge gas; as well as a controller for comparing the light intensity detected by the second detector with a corresponding set value corresponding to a desired transmittance of a purge gas in an interior of the spectrometer, generating at least one error signal based on the comparison, and adjusting a flow rate of the purge gas through the interior of the spectrometer based on the at least one error signal. 15 . The apparatus of claim 14 , wherein the controller is configured to adjust a flow rate of the purge gas through an interior of the spectrometer so that a difference between the detected intensity and the set value is below a threshold.

16. The apparatus according to claim 14 or 15, wherein the spectrometer is an emission spectrometer. The device according to claim 16 , wherein the spectrometer is a spark emission spectrometer or a LIBS spectrometer.

18. The apparatus of claim 14, wherein the measurement unit is in fluid communication with an interior of the spectrometer via a closed-loop fluid circuit.

19. The apparatus of claim 14 or 15, wherein the second path is a one-way path through the gas in the measurement cell.

20. The apparatus according to claim 14 or 15, further comprising two mirrors for reciprocally reflecting light from the light source through the gas in the measurement cell, wherein the second path comprises a multi-pass path through the gas in the measurement cell.

21. The device according to claim 14 or 15, wherein the light source is a vacuum ultraviolet (VUV) light source or a near IR (NIR) light source.

22. The apparatus of claim 14 or 15, wherein the second detector detects the intensity of the light from the light source at a single wavelength or a single wavelength band.

23. An apparatus according to claim 14 or 15, wherein the second detector is adapted to detect the intensity of the light at one or more absorption wavelengths of water and / or molecular oxygen.

24. The apparatus according to claim 14 or 15, comprising two or more light sources for transmitting light along two or more second paths through the gas in the measurement cell, respectively, and two or more second detectors for detecting the intensity of the light transmitted along the second or more second paths, respectively, wherein each light source emits light of a different wavelength and each second detector detects light of a different wavelength, wherein the intensity of the light is detected at two or more non-continuous wavelengths or two or more non-continuous wavelength bands.

25. The apparatus of claim 24, wherein the different wavelengths correspond to absorption by gaseous water and gaseous oxygen, respectively.

26. The apparatus of claim 14 or 15, wherein the controller comprises a proportional-integral-derivative (PID) controller.

27. The apparatus of claim 14 or 15, wherein the controller comprises a single-input single-output (SISO) controller or a multiple-input single-output (MISO) controller, respectively converting a single error signal or multiple error signals into an output signal for controlling the purge gas flow rate.

28. The apparatus of claim 27, wherein the output signal is used to control a flow valve and / or a pump of the purge gas.

29. An emission spectrometer comprising the device for controlling the flow rate of a purge gas according to any one of claims 14 to 28.

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