TECHNICAL FIELD

[0001] The invention relates to the field of gas chromatography and more particularly to the detection of gas impurities at the output of a gas chromatograph. BACKGROUND OF THE ART

[0002] In the quantification and qualification of gas impurities in a gas sample, gas chromatography is widely used. This technique makes use of a mobile phase (carrier gas) to carry samples through columns. The columns cause the different gases in the sample to elute at different times allowing them to be measured independently in the detection system. In laboratories or industrial environments, this technique uses different types of detectors and detection systems to measure the desired impurities in the sample.

[0003] Many types of detectors are available to achieve the desired measurement depending on the sensitivity and selectivity required by the application. The Thermal Conductivity Detector (TCD) is simple and relatively low cost. However, the lack of sensitivity of the detector is a major limitation. In most cases, measuring a low ppm concentration does not yield acceptable results or is simply not achievable. The Flame Ionization Detector (FID) is also simple and low cost. However, the explosive nature of gases like hydrogen may be problematic in plants and laboratory settings. Furthermore, since it can only detect organic compounds, it is often necessary to use another type of detector to measure the other impurities in the sample. The Discharge Ionization Detector (DID) is a more sensitive universal detection technique. This detector is sensitive and can detect ppb levels of organic and inorganic compounds. Helium is required to create the discharge and it is unfortunately getting more expensive and difficult to obtain.

[0004] The Plasma Emission Detector (PED) is based on the spectroscopic emission from the light generated by the ionization of the gas. Impurities coming in the plasma, along with the carrier gas, will generate a light spectrum at specific wavelengths. By converting light to current, each analyte can be quantified. This technology provides many advantages such as selectivity and relatively good sensitivity. By using a specific optical filter, selectivity can be obtained on some compounds. Furthermore, argon and helium can be used as the carrier gases through the gas chromatograph. Argon is typically readily available at a relatively low cost. However, the PED has some limitations in terms of dynamic range and sensitivity. Since the response amplitude is lower and the noise level is higher than other detection techniques, the sample injection volume needs to be considerable. Some impurities like CH ₄ can saturate the signal or can make tailing peaks. To overcome this issue, doping agents, such as moisture, are added to the sample at the entry of the gas chromatograph. The doping agent brings the problematic impurities to Gaussian peaks and helps to avoid saturation. Unfortunately, adding moisture increases the noise level of the PED, resulting in less sensitivity. As a consequence, argon, as a carrier gas, becomes less attractive and may not be appropriate for some applications.

[0005] Therefore, it would be desirable to provide a detection system and method to improve the peak shape and sensitivity of the detector by using a low cost gas, such as argon or nitrogen, as the carrier gas. At the same time, it would be desirable to keep the selectivity capabilities of the Plasma Emission Detector. Finally, it would be desirable to provide a system and method that can control at least some parameters of the detection system to ensure more stable and accurate measurements. SUMMARY

[0006] A detection system and a method are provided to improve at least one of sensitivity, selectivity and dynamic range in the measurement of gas impurities in a gas sample analyzed by a gas chromatograph.

[0007] According to a broad aspect, there is provided a detection system for the measurement of gas impurities in a gas sample with a gas chromatograph comprising a plasma emission detector made of a cell of quartz having electrodes on both sides with at least one window for light emission to determine gas impurities, the plasma emission detector connected to a gas chromatograph to bring impurities to be measured; a vacuum pump mounted at the outlet of the plasma emission detector operating from 0 to -30 InHg; a pressure sensor mounted between plasma emission detector outlet and vacuum pump for monitoring pressure; a make-up gas connected to the inlet of the plasma emission detector; an independent flow controller for the make-up gas; a moisture permeation device connected to the make-up gas to add moisture to the plasma emission detector; a temperature regulation system to control temperature of the permeation device; and a microcontroller device to control the temperature of the permeation device, to regulate the vacuum pressure in the plasma emission detector cell and adjust the amplification of the light signal from the plasma emission detector.

[0008] According to another broad aspect, there is provided a method for the measurement of gas impurities in a gas sample with a gas chromatograph comprising providing gas impurities to be analyzed out of a gas chromatograph; introducing the gas impurities in a plasma cell of a plasma emission system made of a cell of quartz having electrodes on both sides with at least one window for light emission to determine gas impurities; measuring the intensity of light from the plasma emission system; adjusting the vacuum pressure in the plasma cell to get a desired signal response; adjusting the temperature of the permeation device to get the desired signal response; adjusting the amplification of the light signal to get the desired signal response; controlling a stable temperature of the permeation device, a stable flow in the make-up gas, a stable vacuum of the plasma cell and the amplification signal of the light emission from the plasma cell using a micro controller device; wherein one of air, noble gas, nitrogen, hydrogen or oxygen is used as a carrier gas for the gas chromatograph.

[0009] According to another broad aspect, there is provided a detection system for the measurement of gas impurities in a gas sample with a gas chromatograph, comprising: a gas sample source for providing the gas sample; a carrier gas source for providing carrier gas, the carrier gas being one of air, noble gas, nitrogen, hydrogen and oxygen; a gas chromatograph in gas line connection with the gas sample source and the carrier gas source for separating the gas sample into analytes of gas impurities to be measured; a plasma emission detector, the gas chromatograph being in gas line connection with the plasma emission detector to bring the analytes to the plasma emission detector; a vacuum pump mounted at the outlet of the plasma emission detector for providing a sub-atmospheric pressure in the plasma emission detector; a make-up gas source connected to an inlet of the plasma emission detector to provide make-up gas to the plasma emission detector; a moisture permeation device in gas line connection with the make-up gas source and the plasma emission detector to create moisture from the make-up gas and add the moisture in the plasma emission detector; an optical detector for transforming a light signal emitted from the plasma emission detector into an analog signal representing a measurement of the gas impurities; a microcontroller device in communications with the optical detector, the vacuum pump and the permeation device to provide control commands for controlling at least one of the flow of make-up gas in the permeation device and the vacuum pressure in the plasma emission detector cell using the analog signal.

[0010] According to another broad aspect, there is provided a detection system and method for the measurement of gas impurities in a gas sample with a gas chromatograph. The system comprises a plasma emission detector; a gas chromatograph; a vacuum pump for the plasma emission detector; a moisture permeation device to create moisture and add the moisture to the plasma emission detector by bypassing the gas chromatograph; a microcontroller device to control operation of the detection system, the microcontroller device controlling the pressure of the plasma emission detector using the vacuum pump and the moisture added to the plasma emission detector using the moisture permeation device.

[0011] According to another broad aspect, there is provided a detection system for the measurement of gas impurities in a gas sample with a gas chromatograph, comprising: a plasma emission detector made of a cell of quartz having electrodes on both sides with at least one window for light emission to determine gas impurities; a gas chromatograph, the gas chromatograph being connected to the plasma emission detector to bring impurities to be measured to the plasma emission detector; a vacuum pump mounted at the outlet of the plasma emission detector operating from 0 to 30 InHg; a pressure sensor mounted between an outlet of the plasma emission detector and the vacuum pump for monitoring pressure; a makeup gas source connected to an inlet of the plasma emission detector to provide make-up gas to the plasma emission detector; an independent flow controller for the make-up gas source; a moisture permeation device connected to the make-up gas source to create moisture and add the moisture to the plasma emission detector; a temperature regulation system to control temperature of the permeation device; a microcontroller device to control the temperature of the permeation device, to regulate the vacuum pressure in the plasma emission detector cell and adjust the amplification of the light signal from the plasma emission detector.

[0012] According to another broad aspect, there is provided a method for the measurement of gas impurities in a gas sample with a gas chromatograph, comprising: providing gas impurities to be analyzed out of a gas chromatograph, the gas chromatograph operating with one of air, noble gas, nitrogen, hydrogen, oxygen as a carrier gas; introducing the gas impurities in a plasma cell of a plasma emission system; placing the plasma emission system at sub-atmospheric pressure; measuring the intensity of light emitted from the plasma emission system; adjusting, to get a desired signal response, at least one of the vacuum pressure in the plasma cell; the flow of make-up gas of the permeation device; the temperature of the permeation device; and the amplification of the light signal; outputting an analog signal indicative of a measurement of the gas impurities in the gas sample.

[0013] According to another broad aspect, there is provided a method for the measurement of gas impurities in a gas sample with a gas chromatograph, comprising: providing gas impurities to be analyzed out of a gas chromatograph, the gas chromatograph operating with one of air, noble gas, nitrogen, hydrogen, oxygen as a carrier gas; introducing the gas impurities in a plasma cell of a plasma emission system; measuring the intensity of light emitted from the plasma emission system; adjusting the vacuum pressure in the plasma cell to get a desired signal response; adjusting the temperature of the permeation device to get the desired signal response; adjusting the amplification of the light signal to get the desired signal response; maintain a stable temperature of the permeation device, a stable flow in the make-up gas, a stable vacuum of the plasma cell and a stable amplification of the light emission signal.

[0014] According to another broad aspect, there is provided a detection system for measuring at least one gas impurity in a gas sample, comprising: a gas sample source for providing the gas sample having the at least one gas impurity; a carrier gas source for providing carrier gas, the carrier gas being one of air, noble gas, nitrogen, hydrogen and oxygen; a gas chromatograph in gas line connection with the gas sample source and the carrier gas source for separating the gas sample into analytes of the at least one gas impurity to be measured; a plasma emission detector, the gas chromatograph being in gas line connection with the plasma emission detector to bring the analytes to the plasma emission detector; a vacuum generator for controlling a sub-atmospheric pressure in the plasma emission detector; a make-up gas source connected to an inlet of the plasma emission detector to provide makeup gas to the plasma emission detector and the moisture generator; a moisture generator in gas line connection with the make-up gas source and the inlet of the plasma emission detector to create moisture from the make-up gas and add the moisture in the plasma emission detector thereby bypassing the gas chromatograph; an optical detector for capturing and transforming a light signal emitted from the plasma emission detector into an analog signal representing a measurement of the at least one gas impurity in the gas sample; a microcontroller device in communications with the optical detector, the vacuum generator and the moisture generator to provide control commands for controlling at least one of a flow of make-up gas in the moisture generator and the sub-atmospheric pressure in the plasma emission detector cell using the analog signal.

[0015] In one embodiment, the carrier gas source and the make-up gas source are a single gas source in gas line connection with both the gas chromatograph and the moisture generator. [0016] In one embodiment, the detection system further comprises a purifier at an output of the single gas source for purifying the carrier gas and make-up gas. [0017] In one embodiment, the detection system further comprises a pressure sensor mounted between an outlet of the plasma emission detector and the vacuum generator for providing a pressure reading corresponding to a pressure in the cell, wherein the microcontroller device is in communications with the pressure sensor to provide control commands to the vacuum generator using the pressure reading.

[0018] In one embodiment, the detection system further comprises a flow controller for the make-up gas source, the flow controller being adapted to control a flow of the make-up gas into the moisture generator.

[0019] In one embodiment, the microcontroller device is in communications with the flow controller to provide control commands to the flow controller.

[0020] In one embodiment, the moisture generator comprises a temperature regulation system adapted to adjust a temperature of a chamber of the moisture generator.

[0021] In one embodiment, the temperature regulation system includes at least one of a heater and a temperature sensor.

[0022] In one embodiment, the microcontroller device is in communications with the temperature regulation system to provide control commands to the temperature regulation system.

[0023] In one embodiment, the vacuum generator is adapted to create a sub-atmospheric pressure from 0 to 30 InHg in the plasma emission detector.

[0024] In one embodiment, the detection system further comprises an amplifier with an adjustable gain, the amplifier receiving the analog signal, wherein the microcontroller device is in communications with the amplifier, the microcontroller device being adapted to provide control command for controlling the adjustable gain of the amplifier. [0025] In one embodiment, the optical detector is a set of at least one optical detector, each optical detector of the set including at least one photodiode and at least one optical filter.

[0026] According to another broad aspect, there is provided a method for measuring at least one gas impurity in a gas sample, comprising: providing the at least one gas impurity to be analyzed out of a gas chromatograph, the gas chromatograph operating with one of air, noble gas, nitrogen, hydrogen, oxygen as a carrier gas; introducing the at least one gas impurity in a plasma emission detector in gas line connection with the gas chromatograph; controlling a sub-atmospheric pressure in the plasma emission detector using a vacuum generator; providing moisture to the plasma emission detector using a moisture generator in gas line connection with the plasma emission detector, the gas line connection between the moisture generator and the plasma emission detector bypassing the gas chromatograph; measuring an intensity of light emitted from the plasma emission detector using an optical detector; adjusting, to obtain a desired signal response for the analog signal, at least one of the sub-atmospheric pressure in the plasma emission detector; a flow of make-up gas of the moisture generator to adjust a level of the moisture; and a temperature of the moisture generator to adjust a level of the moisture; outputting an analog signal representing a measurement of the at least one gas impurity in the gas sample using the intensity of light.

[0027] In one embodiment, the method further comprises a step of adjusting an amplification of the analog signal using an amplifier with an adjustable gain, the amplifier receiving the analog signal.

[0028] In one embodiment, the step of adjusting to obtain the desired signal response for the analog signal includes analyzing a shape of a peak for the impurity in the analog signal and to emit a control signal to adjust the shape, the control signal being adapted to control at least one of the vacuum generator and the moisture generator; repeating the steps of controlling, providing the moisture, measuring the intensity of light and adjusting. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Reference will now be made to the accompanying drawings, showing by way of illustration example embodiments thereof and in which:

[0030] FIG. 1 is a block diagram of example components of the detection system according to an example embodiment;

[0031] FIG. 2 is a flow chart of main steps of the detection method according to an example embodiment;

[0032] FIG. 3 (prior art) is a graph showing a chromatogram of 10 ppm CH ₄ without added moisture added and without vacuum in the plasma cell; [0033] FIG. 4 (prior art) is a graph showing a chromatogram of 10 ppm CH ₄ with added moisture and without vacuum in the plasma cell;

[0034] FIG. 5 is a graph showing a chromatogram of 10 ppm CH ₄ without added moisture and with vacuum in the plasma cell;

[0035] FIG. 6 is a graph showing a chromatogram of 10 ppm CH ₄ with added moisture and with vacuum in the plasma cell;

[0036] FIG. 7 is a graph showing a chromatogram of 10 ppm CH ₄ with added moisture, with amplified gain and with vacuum in the plasma cell;

[0037] FIG. 8 is a graph showing a chromatogram of 100 ppm CH ₄ with the same parameters as the embodiment of FIG. 6 for 10 ppm; [0038] FIG. 9 is a graph showing a chromatogram of 100 ppm CH ₄ for the embodiment of FIG. 8 with readjusted moisture;

[0039] FIG. 10 is a graph showing a chromatogram of 1 % CH ₄ with the same parameters as the embodiment of FIG. 8 for 100 ppm; [0040] FIG. 11 is a graph showing a chromatogram of 1 % CH ₄ for the embodiment of FIG. 10 with readjusted moisture;.

DETAILED DESCRIPTION

[0041] Described herein are a system and a method to improve detection of gas impurities at the output of a Gas Chromatograph (GC). The system includes a Plasma Emission Detector (PED), a vacuum generator to generate a vacuum environment in the plasma cell of the PED, a temperature controlled moisture generator to add moisture to the carrier gas through a makeup gas line and an adjustable amplification system for the signal generated from the PED. A microcontroller device is used to control the different components of the system. [0042] The present detection method and system can be implemented with any GC and PED system and is aimed at improving the results obtained for the detection of the gas impurities regardless of the quality of the sample or of the GC unit.

[0043] Referring now to FIG. 1, example components of the system according to an example embodiment 100 are shown. Carrier and make up gas source 103 provides gas to a gas purifier 101 which in turn supplies the gas chromatograph (GC) 102 with purified carrier gas 117. It also supplies make up gas 118 to the moisture generator. In the example embodiment of FIG. 1, the moisture generator is permeation device 106. The choice of which gas to use for the carrier and make up gas is made by the person skilled in the art depending on the application. The carrier and make up gas can be a noble gas, nitrogen, hydrogen or oxygen, for example.

[0044] The gas chromatograph 102 receives a sample gas 122 from a sample source 104. The sample gas 122 contains at least one impurity to be detected and/or measured. The gas chromatograph 102 produces a series of separated analytes 120 at its output. The internal configuration of the gas chromatograph 102 is selected by the person skilled in the art depending on the application. [0045] Each analyte of interest is introduced in the Plasma Emission Detector (PED) cell 105. In this example embodiment, the PED cell is a Dielectric Barrier Discharge (DBD) PED cell. Other types of PED cells could also be used. The PED cell 105 can be made of quartz, borosilicate, glass, synthetic diamond or another inert transparent material. In this example embodiment, the PED cell 105 is made of highly purified quartz. The PED cell 105 consists of an air tight sealed monolithic environment with gas inlet and gas outlet passageways to admit carrier gas with its analytes 120 and the wet make up gas 1 19 to pass through the PED cell 105. The PED cell 105 has its ionization zone located in between two electrodes on which the discharge voltage is applied. An optical detector is mounted in front of the ionization area of the PED cell 105 to allow collecting the photons. The optical detector can include an arrangement of a minimum of one optical filter 113 and one photodiode 1 14. In this example embodiment, only one optical filter 1 13 and one photodiode 1 14 were used to focus on methane detection. Other optical filters and photodiodes can be added to the PED cell 105 for detection of more species, each generating a distinct analog signal. [0046] Moisture is provided to the PED cell 105 through the wet make up gas line 119. The moisture is produced by a permeation device 106 which is connected to the inlet of the PED cell 105. The addition of moisture created by the permeation device 106 therefore bypasses the gas chromatograph 102 to directly supply the PED cell 105. It can therefore be controlled independently from the gas chromatograph 102 in order to adjust the moisture content of the wet make up gas 1 19 without modifying the moisture content of the carrier gas with separated analytes 120. In this example embodiment, the permeation device 106 is made from a Polytetrafluoroethylene (PTFE) tube filled with demineralized purged water. The water-filled PTFE tube is typically purged with the same carrier and make up gas type as that provided by the carrier gas source 103 prior to installation in the system to allow good stability. The choice of PTFE material used for the permeation device 106 depends of the permeability rate required for the application. The material is selected by the person skilled in the art depending on the application. A higher permeability rate is required when a high level of moisture is necessary for the application. The length and diameter of the permeation device 106 are also selected by the person skilled in the art as a function of the required moisture rate. The dimensions of the permeation device are also selected by the person skilled in the art depending on the application. The permeation device 106 can also be replaced by another subsystem for generating moisture. [0047] The permeation device 106 is temperature controlled with an isolated enclosure

107 which includes a heater 108 and a temperature sensor 109. In this example embodiment, the temperature sensor 109 is a resistance temperature detector (RTD) type sensor. Other temperature sensor types can be used as well. The temperature of the permeation device 106 can therefore be measured by the temperature sensor 109 and adjustments can be made using the heater 108. The make-up gas 118 used by the permeation device 106 can be obtained from the purifier 101 and is then the same as the purified carrier gas 1 17 supplied to the gas chromatograph 102. An optional flow controller 1 10 controls the flow of the make-up gas into the permeation device 106. The flow controller 110 allows adjusting the moisture rate for generating different levels of moisture in the wet make up gas 1 19. The flow controller 1 10 is also used to keep the flow stable to ensure a constant level of moisture. Higher flow rates increase the moisture level and reduced flow rates decrease the moisture level.

[0048] The wet make up gas 119 from the permeation device 106 and the carrier gas with separated analytes 120 are combined prior to entry in the PED cell 105 or within the enclosure of the PED cell 105. [0049] A pressure sensor 11 1 reads the pressure at the outlet of the PED cell. A vacuum generator 112 is connected at the outlet of the PED cell 105 to control the pressure inside the cell. In most applications, the vacuum generator 1 12 will be used to generate sub-atmospheric pressure inside the cell. Example vacuum generators include venturi systems and vacuum pumps. As will be readily understood, additional components may be required to create a venturi system, such as a flow controller, a source of gas at a controlled pressure, gas connections to a gas at another pressure, etc. [0050] At least one optical filter 113 can be installed at an output window of the PED cell to discriminate the wavelength of the light emitted from the plasma. Optical filters 113 are chosen by the person skilled in the art depending of the analytes to be measured. An optical detector or photodetector, such as photodiode 114, converts the detected light into a current value which allows measuring the impurities. An amplifier 115 with adjustable gain is used to increase the amplitude of the signal from the photodiode 114.

[0051] A microcontroller unit (MCU) 116 is used to control the sub-systems. It reads the temperature sensor 109 and controls the heater 108 of the enclosure 107 containing the permeation device 106. The microcontroller 116 also reads the pressure from the pressure sensor 111 and controls the vacuum generator 112 to keep stable sub-atmospheric pressure in the PED cell 105. The microcontroller 116 also reads the signal from the amplifier 115 and can adjust its gain. The microcontroller unit 116 can control the flow controller 110 of the make-up gas to control flow of the make-up gas 118 into the permeation device 106.

[0052] MCU 116 is a computer processor which executes computer executable statements and instructions, such as code, stored on a computer readable memory to perform the data analysis steps described herein. MCU 116 is in electronic communications with the optical detector, the vacuum generator and the moisture generator to provide control commands for controlling at least one of a flow of make-up gas in the moisture generator, the temperature of the moisture generator and the pressure in the plasma emission detector cell using said analog signal.

[0053] MCU 116 is further in electronic communications with an output device 123 to output the signal representing the measurement of the gas impurities in the gas sample. As will be readily understood, it may be necessary to input data for use by the MCU 116 and/or other sub-systems. Input device(s) 124 can therefore be provided for this purpose. An output of the analog signal may be viewed graphically on output device 123. Additional or alternative control commands may be inputted into the MCU 116 using input device 124 or may be provided directly to each sub-system (moisture, pressure, gain) through other input devices (not shown).

[0054] The following example steps are carried out by the system in use. Impurities to be measured elute from the gas chromatograph 102 and enter the PED cell 105 which is kept at the desired sub-atmospheric pressure by the vacuum generator 1 12. The operating sub- atmospheric pressure can be kept in the range of -30InHg to atmospheric pressure (0 InHg). In this example embodiment, the vacuum pressure was kept in the range of -28InHg to -20InHg. The signal from the amplifier 115 is acquired by the microcontroller unit 1 16. The level of moisture in the plasma cell is adjusted by changing the flow rate in the permeation device 106 with the flow controller 110 and/or by changing the temperature of the permeation device 106 in its enclosure 107. The level of moisture is adjusted to obtain the desired peak shape, namely a Gaussian peak shape desirable and acceptable for the application, with the maximum response to be measured for each impurity. In this example embodiment, the H ₂ 0 (moisture) level is varied from Oppm to 80ppm. [0055] As will be readily understood by the person skilled in the art, the Gaussian peak shape yields an adequate measurement of the impurity. The Gaussian peak shape is commonly known in the art and can be summarized as a symmetrical bell-shaped peak with substantially the same level at the beginning and at the end of the peak. Tailing of the curve is not desirable.

[0056] Optionally, the signal from the photodiode 114 is amplified with the amplifier 115 to a desired response to obtain an adequate signal-to-noise ratio after finding the optimal level of moisture. Microcontroller unit 1 16 keeps the pressure in the PED cell 105 stable by reading the pressure sensor 111 and adjusting the vacuum generator 112. The microcontroller unit 116 keeps the level of moisture provided to the PED cell 105 stable by adjusting the flow controller and keeping the temperature of the enclosure 107 constant by reading the temperature sensor 109 and adjusting the heat production of the heater 108.

[0057] The dynamic range for the detection is increased with the present system and method and the noise level is also reduced, thereby providing better sensitivity. In experiments carried out with the present detection system and with a prior art detection system for a same gas chromatograph, the dynamic range was increased from 10 ² to 10 ⁶ and the noise level was reduced to achieve a 19-fold increase in sensitivity. By simply changing controllable parameters of the system, impurity concentrations ranging from percentages down to ppb can be measured without changing hardware components of the GC. An extended dynamic measuring range can be achieved without a split injector commonly used in prior art systems.

[0058] The detection method can be summarized as follows with reference to FIG. 2. An example embodiment 200 for the method for measuring at least one gas impurity in a gas sample comprises the following steps: providing the at least one gas impurity to be analyzed out of a gas chromatograph and introducing the at least one gas impurity in a plasma emission detector in gas line connection with the gas chromatograph 202; controlling a sub-atmospheric pressure in the plasma emission detector using a vacuum generator 204; providing moisture to the plasma emission detector using a moisture generator in gas line connection with the plasma emission detector 206, the gas line connection between the moisture generator and the plasma emission detector bypassing the gas chromatograph; measuring an intensity of light emitted from the plasma emission detector using an optical detector 208; analyzing a shape of the analog signal 210; emitting a control signal 212, the control signal being adapted to control at least one of the vacuum generator and the moisture generator to adjust at least one of the sub-atmospheric pressure in the plasma emission detector 214 and a level of moisture by adjusting a flow of make-up gas of the moisture generator and/or a temperature of the moisture generator 216; repeating the steps of controlling, providing the moisture, measuring the intensity of light and adjusting the parameters of moisture and pressure until the analog signal has a desirable shape 210, namely a Gaussian shape, and outputting an analog signal representing a measurement of the at least one gas impurity in the gas sample 220. Optionally, the method can further include a step of adjusting an amplification of the analog signal using an amplifier with an adjustable gain 218. [0059] EXPERIMENTS

[0060] A series of experiments were carried out to demonstrate the improvements that the present system and method bring to the measurement of impurities in a gas sample. For each experiment, a constant optical filter, photodiode and GC configuration was used to ensure a comparison of the different PED modes of operation under the same conditions.

[0061] A sample gas containing lOppm CH ₄ and lOppm CO with Argon forming the balance was introduced in the GC system. FIG. 3 (prior art) shows a chromatogram having 10 ppm CH ₄ and CO in an argon balance obtained with a PED at atmospheric pressure. The CH ₄ peak is tailing and it affects the CO separation and its integration. This peak shape is not desirable because of its lack of repeatability and accuracy.

[0062] In the prior art, moisture is added to the carrier gas to overcome this issue. The moisture can be added from a permeation tube in the gas path of the gas chromatograph outlet bringing the impurities to the detector system. However, in such systems, the carrier flow changes caused by the valve switching in the GC system affect the level of moisture in the plasma cell. Variations in the moisture level affect the signal of the plasma emission detector and its results. Such prior art systems are therefore unstable.

[0063] In the present system, the permeation device receives its own supply of make-up gas, with flow control, and therefore generates a constant level of moisture, independent of the carrier gas fluctuations. [0064] FIG. 4 (prior art) shows the chromatogram of the same sample of FIG. 3 with a Gaussian CH ₄ peak which does not tail. This chromatogram was obtained with an appropriate level of moisture, namely a temperature of 45 °C in the permeation device enclosure with a flow rate of 30 mL/min. FIG. 4 shows the separation of CHi and CO allowing appropriate peak integration for both impurities. However sensitivity was decreased in comparison to FIG. 3. For the same amount of impurity, the peak height for CH ₄ was reduced from 609 mV to 510 mV. Noise was also increased from 5 mV to 6 mV. Therefore, the signal-to-noise ratio (SNR) became 85 (510 mV / 6 mV) instead of 122 (610 mV / 5 mV) for the embodiment of FIG. 3. The Lowest Detection Limit (LDL) is set at three times the noise level. This yields a LDL of 0.35 ppm for this embodiment.

[0065] FIG. 5 shows a chromatogram obtained with the same sample gas used in FIG. 3 and FIG. 4 when a vacuum pump is used to create a sub-atmospheric pressure in the plasma cell without moisture. The CH ₄ impurity cannot be quantified because of its low response that decreases down to 41mV and its bad peak shape that is not Gaussian. The CH ₄ peak is not detectable in this mode of operation. Furthermore, the CH ₄ peak polarity is inverted on this chromatogram and shows a strong tailing which indicates a serious change in PED equilibrium. A PED cell system configured in a dry condition at sub-atmospheric pressure does not improve the detection of the impurities.

[0066] When moisture is added to the PED cell and a vacuum generator, such as a vacuum pump, is used to create a sub-atmospheric pressure in the plasma cell, the CH ₄ peak appears with an adequate Gaussian shape, as shown in FIG. 6. The peak width is even thinner than in the other experiments, which is highly desirable in gas chromatography. However, the peak height is considerably reduced and, at first sight, this could indicate a loss of sensitivity compared to the scenario of FIG. 4 (prior art). However, noise was reduced by an even greater factor indicating that sensitivity has improved. The SNR is 1090 ( 327mV / 0.3 mV). Therefore, an embodiment with moisture and under sub-atmospheric pressure yields adequate results for many applications. The desired sub-atmospheric operating pressure for the PED cell during this experiment was between -24InHg to -26InHg. The optimal moisture range was selected between 2ppm to 5ppm H ₂ 0. It was achieved with a temperature of 60°C for the enclosure of the permeation device and a flow rate of 25 mL/min.

[0067] By amplifying the analog signal from the plasma emission detector, the peak height can be increased without unduly affecting the noise level. The amplification system is adjusted by the person skilled in the art to find the optimal signal gain to increase the peak response while considering the noise increase. FIG. 7 shows a chromatogram for such an embodiment in which the analog signal is amplified, yielding a SNR of 1592 (1115 mV / 0.7 mV). The amplification was set at 3 times the signal. In contrast to the LDL of the embodiment of FIG. 4 which is 0.35 ppm, a LDL of 0.018 ppm is achieved with the embodiment of FIG. 7. [0068] FIG. 8 shows a chromatogram of 100 ppm CH ₄ and 100 ppm CO with the same conditions, moisture rate and sub-atmospheric pressure in the plasma cell as in the embodiments of FIGS. 5 and 6. A tailing peak for CH ₄ appears and the result is not as desired. More moisture is then added to the plasma cell to obtain a Gaussian peak without tailing. The moisture level is increased by increasing the temperature of the permeation device and/or by increasing the flow rate of the make-up gas at the input of the permeation device 106. FIG. 9 shows that by amplifying the signal at the right level, the peak height is increased and the SNR and sensitivity are improved. The desired sub-atmospheric operating pressure for the PED cell during this experiment was between -24InHg and -26InHg. The optimal moisture range was selected between lOppm and 15ppm H ₂ 0. The temperature of the enclosure of the permeation device was 75 °C with a flow rate of 10 mL/min. The gain was adjusted to 3 times the signal.

[0069] Higher concentrations of impurities can be measured in a similar way. FIG. 10 shows a chromatogram with 1% CH ₄ and CO in Argon using the same moisture, vacuum and amplification settings as for the 100 ppm concentration of FIG. 9. The tailing peak is resolved and adequate sensitivity is achieved by readjusting the moisture level. The resulting curve is shown in FIG. 11. The desired sub-atmospheric operating pressure for the PED cell during this experiment was between -24InHg and -26InHg. The optimal moisture range was selected between 40ppm and 50ppm H ₂ 0.

[0070] Although the above description relates to example embodiments as presently contemplated by the inventors, it will be understood that the invention in its broad aspect includes equivalents and variants of the elements described herein. The embodiments described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the appended claims.