Detecting trace levels of a gas component in a gas sample

Abstract

The invention is directed to a method for detecting trace levels of an active gas component as contained in a gas sample by detection of said active gas component using a gas chromatograph in fluid communication with a detector sensitive for the gas component. The following steps are performed in a sequence: (i) feeding a carrier gas doped with a quantity of the active gas component to the gas chromatograph for a certain doping time; (ii) feeding a carrier gas which is not doped with the active gas component to the gas chromatograph for a certain flushing time and (iii) adding the gas sample to the carrier gas of step (ii) and feeding the resulting mixture to the gas chromatograph.

Description

[0001] DETECTING TRACE LEVELS OF A GAS COMPONENT IN A GAS SAMPLE

[0002] The invention is directed to a method for detecting trace levels of an active gas component as contained in a gas sample by detection of said active gas component using a gas chromatograph in fluid communication with a detector sensitive for the active gas component. The invention is also directed to a gas chromatographic system suitable for this method.

[0003] Many analyses within gas chromatography suffer from loss in the system due to reaction or adsorption of active gas components, ie the components of interest, with or on active sites in the gas chromatographic system. Adsorption or reactivity of components of interest, particularly at lower concentration levels, result in bad detection limits, repeatability, linearity and equimolarity. The level of adsorption of the components of interest will depend on the component itself as well as on the activity of the different surfaces that the component comes into contact with during transport through the gas chromatographic system. Common practice within gas chromatography is to minimize such active surfaces by treating these surfaces with coatings like e.g. Sulfinert. But even after these treatments, active surfaces may remain. The existence of such remaining active surfaces is especially a problem when measuring active components like ammonia or hydrogen sulfide at low levels in hydrogen or propylene gas samples.

[0004] Active surfaces can be deactivated by making multiple sample or calibration standard injections containing a certain amount of the component of interest. The problem with this method is that it takes time before you can do the actual sample analysis. Further, it sometimes remains uncertain if all of the active surface is deactivated and for how long. This method is further not suited when very low concentrations of the component of interest are to be measured.

[0005] US2012 / 0131987 describes a chromatographic method wherein the active component is first measured in a first carrier gas, followed by flushing the system using a different carrier gas and measuring the sample using the different carrier gas. US5612489 describes a method to deactivate the active surfaces in a gas chromatographic system by doping the carrier gas with a certain deactivating component. This deactivating component may be the component of interest. The problem with deactivation by the component of interest is that it will increase baseline noise and it will also create issues when performing sample injection by valve switching. When measuring very low concentrations of the component of interest in the sample it is found that the peak of the component of interest is not quantifiable. The method is also difficult to perform when making use of a sample injection by valve switching, the so-called gas sampling valve injection (GSV- injection).

[0006] The object of this invention is to provide a method for detecting trace levels of an active gas component as contained in a gas sample which does not have the disadvantages of the prior art methods.

[0007] This is achieved by the following method. A method for detecting trace levels of an active gas component as contained in a gas sample by detection of said active gas component using a gas chromatograph in fluid communication with a detector sensitive for the gas component, wherein the following steps are performed in a sequence wherein:

[0008] (i) feeding a carrier gas doped with a quantity of the active gas component to the gas chromatograph for a certain doping time,

[0009] (ii) feeding the carrier gas of (i) which is not doped with the active gas component to the gas chromatograph for a certain flushing time and

[0010] (iii) adding the gas sample to the carrier gas of step (ii) and feeding the resulting mixture to the gas chromatograph.

[0011] Applicant found that with this method an improved signal to noise ratio is achieved enabling measurement of low levels of the active gas component in the sample gas. Also the method results in a good linearity. Without wishing to be bound by the following theory, applicants believe that by first feeding the gas chromatograph with the carrier gas doped with a quantity of the active gas component in step (i), active sites in the system will be chemically bonded. In addition, a quantity of unbonded active gas component will be present in the gas chromatograph system of which the gas chromatograph is part of. By performing step (ii) the unbonded active gas component are flushed from the gas chromatograph system while the chemically bonded active gas component on active sites remains. By subsequently performing step (iii) the active gas component of the sample will not interact with the active surface because of this presence of the chemically bonded active gas component on the active sites. The sequence of steps (i)-(ii) is referred to in this description as intermittent carrier doping. As explained, the component of interest as present in the sample gas will reach the detector, largely unaffected by adsorption losses, such to create a normal positive peak. This peak will have the best possible signal to noise (S / N) ratio because the surplus doping is timely removed from the system by performing step (ii) prior to step (iii) and thus will not increase baseline noise.

[0012] Step (i) is suitably performed for a time sufficient for enabling the active gas component to chemically bond with all or nearly all of the active sites in the gas chromatographic system. This minimum time may depend on the active gas component and the gas chromatographic system itself. For most applications using the gas chromatograph system step (i) is continuously performed apart from when steps (ii) and (iii) are performed. This allows to measure multiple samples in a consecutive order.

[0013] Step (ii) is suitably performed for a time sufficient to flush the unbonded active gas component from the gas chromatograph system while the chemically bonded active gas component on active sites remain. The carrier gas is the same carrier gas as in step (i) except that it contains no active gas component. The optimal time to perform step (ii) may depend on the active component and the chromatographic system and can be easily determined. Preferably the active component of step (iii) elutes to the detector just behind the carrier gas doped with a quantity of the active gas component of step (i). Because the component of interest is identical to the doping component the active gas component of step (iii) will not be able to catch up with the doping component of step (i) while eluting though the gas chromatographic system. This means that step (ii) is preferably performed for a very short period. If next to the active components other components of the sample are to be detected which elute faster through the gas chromatographic system it is theoretically possible that these other component or components catch up with the doping component of step (i). In such a situation it may be preferred to perform step (ii) for a longer period. For most applications and especially for the applications described in this application the flushing time is preferably between 10 and 300 seconds. A longer flushing time may cause the chemically bonded active gas component to leave the active sites which is disadvantageous for the reasons described earlier.

[0014] Step (iii) is suitably performed immediately after performing step (ii). This may be achieved by making use of switch valves as is normal practice in gas chromatographic systems. In step (iii) the sample gas is added to the carrier gas. This is typically performed by injecting a small quantity of the sample gas to the carrier gas.

[0015] After performing step (iii) step (i) may be performed such that the gas chromatographic system can be used for a next measurement. The timing at which step (i) is performed after performing step (iii) depends on the active component, the further composition of the gas sample and the chromatographic system. Optimally step (i) is performed when all the peaks of interest are detected by the detector. For most applications and especially for the applications described in this application step (i) is preferably performed after step (iii) within 300 seconds. By performing step (i) any active sites from which the active components have been removed in a previous steps (ii) and (iii) may again be chemically bonded with the active components.

[0016] Between performing step (iii) and a step (i) of a new measurement carrier gas which is not doped with the active component may be fed to the gas chromatograph. When a new sample is to be analysed steps (i)-(iii) will then be performed. This avoids unnecessary consumption of the active component.

[0017] The active gas component may be any component which requires to be detected in a sample gas. Examples of active gas components are ammonia, hydrogen sulphide, hydrogen cyanide, carbon monoxide, carbon dioxide, oxygen, hydrogen, fluorine, chlorine, sulphur dioxide, formaldehyde, formic acid, hydrogen chloride and water and their mixtures.

[0018] The sample gas is preferably composed for more than 50 vol% of hydrogen, carbon monoxide, carbon dioxide, methane, ethane, propane, butane, air, nitrogen, helium, ethylene, propylene or their mixtures.

[0019] Preferably the sample gas is composed for more than 80 vol.% of hydrogen, more preferab+ly more than 95 vol.% and even more preferably more than 99 vol.% hydrogen and the active gas component is any one or more of ammonia, hydrogen sulphide, formic acid, sulphur dioxide, formaldehyde, oxygen, carbon monoxide, carbon dioxide, hydrogen chloride and water. More preferably the sample gas is composed of only hydrogen and trace amounts of impurities comprising at least one or more of the following active gas components chosen from ammonia, hydrogen sulphide, formic acid, sulphur dioxide, formaldehyde, oxygen, carbon monoxide, carbon dioxide, hydrogen chloride and water. By trace amounts is meant that the total of these impurities does not exceed 100 ppm.

[0020] Preferably the sample gas is composed for more than 80 vol.% of propylene and the active gas component is any one or more of hydrogen sulphide, sulphur dioxide and mercaptans. More preferably the sample gas is composed of only propylene and trace amounts of impurities comprising at least one or more of the following active gas components chosen from hydrogen sulphide, sulphur dioxide and mercaptans. By trace amounts is meant that the total of these impurities does not exceed 100 ppm.

[0021] The trace level of the active gas component in the gas sample is preferably less than 100 ppm, more preferably less than 10 ppm and even more preferably less than 5 ppm and most preferably less than 1 ppm.

[0022] The carrier gas may be any inert gas. Preferably the carrier gas may be selected from the group comprising hydrogen, helium, argon and nitrogen. The content of the active component in the carrier gas as used in step (i) may be in the range of the content of the active component in the sample of step (iii). However also higher contents may be used, for example as high as 100 times the content of the active component in the gas sample. Because a flushing step (ii) is performed any excess active component which is not chemically bonded in the gas chromatographic system will be removed prior to the actual measurement in step (iii). Doping of the active component into the carrier gas can be performed by known methods, such as for example dynamic dilution, permeation or calibrated leak. Doping by dynamic dilution is performed by consistent mixing of the carrier gas with a stream containing either pure active component or a mixture of the active component and the carrier gas. Doping by permeation is performed by consistently diffusing the active component from a separate reservoir through a permeable material into the carrier gas. Calibrated leak is performed by constantly injecting a quantity of the active component through controlled leak typically consisting of an orifice or capillary tube into the carrier gas. Alternatively, doping could be performed by using a mixture of carrier gas and the active component dispensed from an industrial gas cylinder.

[0023] The carrier gas in step (i) may contain one or more active gas components. When the sample contains more than one components to be analysed the carrier gas in step (i) may contain more than one active gas components. Preferably this carrier gas in step (i) contains one active gas component and preferably the most reactive active gas component. For example when a mixture of sulphur compounds as listed above is present in the sample the active component in the carrier gas in step (i) is preferably hydrogen sulphide.

[0024] The pressure of the gas sample and optionally also the carrier gas in steps (i)- (iii) may be ambient or higher. Higher pressures of the gas sample and optionally also the carrier gas of between 10 and 500 kPa may be advantageous to further improve the signal to noise (S / N) ratio. The method is particularly useful for applications making use of gas sampling valve injection (GSV-injections) but it may also be useful for GO applications using Liquid Sampling Valve (LSV) or Automatic Liquid Sampling (ALS) injections.

[0025] The above method is preferably performed in a gas chromatographic system as described below. The invention is also directed to this system. The gas chromatographic system comprises a gas chromatograph column having an upstream and downstream end, a detector fluidly connected to the downstream end of the gas chromatograph column, a first switch valve fluidly connected to a second switch valve, each switch valve having different switch valve positions. The first switch valve has an inlet for a carrier gas, an inlet for a doped carrier gas and an outlet for the carrier gas or for the doped carrier gas depending on the position of the first switch valve. The outlet for the carrier gas or for the doped carrier gas is fluidly connected to an inlet of the second switch valve. The second switch valve is further provided with an inlet for a gas sample, an outlet to a vent conduit and a second switch valve outlet fluidly connected to the upstream end of the gas chromatograph column. The first and second switch valves have switch valve positions (a)-(b) where there is

[0026] (a) a fluid communication between the inlet for a doped carrier gas and the upstream end of the gas chromatograph column via the second switch valve outlet,

[0027] (b) a fluid communication between the inlet for a carrier gas and the upstream end of the gas chromatograph column via the second switch valve outlet, and

[0028] (c) a fluid communication between the inlet for a carrier gas, the inlet for a gas sample and the upstream end of the gas chromatograph column via the second switch valve outlet. In switch valve position (a) step (i) of the method of this invention may be performed. In switch valve position (b) step (ii) of the method of this invention may be performed. In switch valve position (c) step (iii) of the method of this invention may be performed.

[0029] When the method uses doping by permeation it is preferred that the first switch valve has an outlet for carrier gas fluidly connected to an inlet for doped carrier gas via a flow path comprising a chamber which contains a permeation tube and wherein the inlet for doped carrier gas is fluidly connected to the outlet for a doped carrier gas.

[0030] When the method is performed using a higher sample pressure than ambient it is preferred to provide the vent conduit with a back pressure regulator. By setting a gas sample pressure higher than ambient on the back pressure regulator a larger amount of sample will be introduced into the system when injecting.

[0031] The column and detector of the gas chromatographic system suitable for the process of this invention may be any known column and detector.

[0032] The gas chromatographic system is illustrated by the following three figures 1- 3.

[0033] Figure 1 shows a schematic representation of a gas chromatographic system

[0034] (I) according to this invention allowing gas sampling valve injection (GSV-injections) provided with a gas chromatograph column (2) having an upstream end (3) and downstream end (4), a detector (5) fluidly connected to the downstream end (4) of the gas chromatograph column (2). A first switch valve (6) is shown which is fluidly connected to a second switch valve (7). In Figures 1-3 the switch valves (6) and (7) are shown having different switch valve positions. The first switch valve (6) has an inlet (8) for a carrier gas, an inlet (9) for a doped carrier gas and an outlet (10) for the carrier gas or for the doped carrier gas depending on the position of the first switch valve (6). The inlet for inlet (9) for a doped carrier gas is fluidly connected to a zone

[0035] (II) where carrier gas contacts a permeation tube containing the active gas component. The carrier gas itself flows from a source of carrier gas (12) via the first switch valve (6) to zone (11). The source (12) of carrier gas may be two gas reservoirs which are intermittently used as source for the carrier gas and the doped carrier gas.

[0036] The outlet (10) for the carrier gas or for the doped carrier gas is fluidly connected to an inlet (13) of the second switch valve (7). The second switch valve (7) is further provided with an inlet (14) for a gas sample, an outlet (15) to a vent conduit (16) and a second switch valve outlet (17) fluidly connected to the upstream end (3) of the gas chromatograph column (2). A sample loop (18) is present having an upstream end (19) and a downstream end (20).

[0037] In Figure 1 the first (6) and second (7) switch valves have switch valve positions for carrying out step (i) of the method of the invention. The switch valve positions result in a fluid communication between the source (12) of a carrier gas, the zone (11), inlet (9) for a doped carrier gas and the upstream end (3) of the gas chromatograph column (2) via the second switch valve outlet (17), The upstream end (19) of the sample loop (18) is connected to the inlet (14) for a gas sample and the downstream end (20) of the sample loop (18) is connected to the vent conduit (16) provided with a back pressure regulator (22). Thus in step (i) the sample loop (18) is filled with the gas sample.

[0038] In Figure 2 the first (6) and second (7) switch valves have switch valve positions for carrying out step (ii) of the method of the invention. The switch valve positions result in a fluid communication between the source of the carrier gas (12), the inlet (8) for a carrier gas and the upstream end (3) of the gas chromatograph column (2) via the second switch valve outlet (17). The doped carrier gas obtained while performing step (ii) is vented via vent (23). The sample loop (18) is connected as in Figure 1 resulting in that the sample loop (18) is filled with the gas sample at the end of step (ii).

[0039] In Figure 3 the first (6) and second (7) switch valves have switch valve positions for carrying out step (iii) of the method of the invention. The switch valve positions result in a fluid communication between the inlet (8) for a carrier gas, the inlet (13), second valve outlet (17), the sample loop (18) and the upstream end (3) of the gas chromatograph column (2) via the second switch valve outlet (17). The inlet (14) for a gas sample is connected to vent conduit (16).

[0040] The gas chromatographic system (1) according to this invention as shown in Figure 1 can also be duplicated wherein in each parallel system a different active component can be analysed using a different detector (5). The sample can then be fed to a second (7) switch valve of one system and subsequently to a second (7) switch valve of the next system. For example in one system trace levels of ammonia can be measured in hydrogen using a Nitrogen Chemiluminescence Detector and trace levels of sulphur components in hydrogen using a Sulphur Chemiluminescence Detector.

[0041] The invention will be illustrated by the following examples.

[0042] Comparative Experiment A

[0043] A 10 ppm ammonia in nitrogen sample and a 1 ppm ammonia in nitrogen sample have been analysed by a gas chromatographic method at ambient conditions where no doping is carried out to reduce the active surface. The column used was a 60m x 0.53mm x 7um MXT-1 as obtained from Restek and the detector was a Nitrogen Chemiluminescence Detector (NCD). Figure 4a shows the resulting chromatogram for the 10 ppm sample and Figure 4b shows the resulting chromatogram for the 1 ppm sample, wherein ‘A’ is the ammonia peak. The chromatograms show that at the concentration level of 10 ppm the system activity is already creating significant tailing on the ammonia peak. At a level of 1 ppm the ammonia peak is completely lost to adsorption in the system.

[0044] Comparative Experiment B

[0045] The same samples of Experiment A were analysed using the same gas chromatographic system in terms of column and detector except that the helium carrier gas was doped with 10 ppm ammonia (doped carrier gas). This method of carrier doping is the method as described in US5612489. Figure 5a shows the resulting chromatogram for the 10 ppm sample and Figure 5b shows the resulting chromatogram for the 1 ppm sample, wherein ‘A’ is the ammonia peak.

[0046] The chromatograms show a high baseline caused by the excess ammonia in the chromatographic system eluting to the detector. The 10 ppm ammonia chromatogram displays a normal peak shape but the peak looks smaller than expected which can be explained by the fact that the baseline drops when the injected sample plug reaches the detector. This can be explained by the fact that the sample plug in the used GSV-injection does not contain the background / doping of ammonia, which means that before a peak is created by the ammonia present in the sample this lack of background first needs to be compensated for. This will make the peak smaller. So even at a 10 ppm level the doping will decrease the response somewhat. This is more clearly shown by the analysis of the 1 ppm ammonia sample where the chromatogram shows that the ammonia peak is not quantifiable any more due to the drop of the baseline when the injected sample plug reaches the detector.

[0047] Besides the issue of the GSV-injection the introduction of doping also increases the detector noise which results in a reduced S / N ratio. This makes the analysis of ammonia at this concentration level and lower like 0.1 ppm more problematic. In an ideal situation the doping of the carrier is such that it is just enough to take up all the active sites in the system resulting in almost no increase in baseline. This way the drop in the baseline caused by the GSV injection will be minimal as will be the increase in S / N ratio. In practice this is very difficult to achieve and maintain.

[0048] Example 1

[0049] Comparative Experiment B was repeated except that before the sample was injected the doped carrier gas was replaced by carrier gas not containing ammonia. After injecting the sample, the carrier gas not containing ammonia was replaced by the doped carrier gas. Figure 6a shows the resulting chromatogram for the 10 ppm sample and Figure 6b shows the resulting chromatogram for the 1 ppm sample, wherein ‘A is the ammonia peak. The example was performed in a gas chromatographic system of Figures 1-3 and the pressure of the gas sample in the gas sample loop was ambient.

[0050] The chromatograms show a lowered signal baseline just before elution of the peak of interest, ie the ammonia peak, towards the detector. The direct drop in the baseline is believed to be caused by the removal of the unbonded ammonia from the system, ie by flushing. Because the bonded ammonia will more slowly elute from the system it is believed that most active sites will still be bonded with ammonia originating from the doped carrier gas when the sample plug passes these sites. This bonded ammonia will thus prevent adsorption of the ammonia present in the injected sample plug. The intermittent carrier doping of this example enables the ammonia of the gas sample to reach the detector, largely unaffected by adsorption losses, where it will create a normal positive peak. This peak will have the best possible S / N ratio because the surplus doping is timely removed from the system and thus will not increase baseline noise.

[0051] Example 2

[0052] Example 1 was repeated wherein the pressure of the gas sample in the gas sample loop was 150 pKa. Figure 7a shows the resulting chromatogram for the 10 ppm sample and Figure 7b shows the resulting chromatogram for the 1 ppm sample, wherein ‘A’ is the ammonia peak. As can be seen when comparing the chromatograms of Figure 6a, 6b with the corresponding chromatograms of Figure 7a, 7b an improvement in S / N ratio is observed when the method is performed at higher pressures.

[0053] Example 3

[0054] Example 2 is repeated for a 3 ppm mole ammonia in nitrogen gas sample. Figure 8a shows the resulting chromatogram, wherein ‘A is the ammonia peak. Figure 8b represent a gas chromatograph for the same sample where no doping is applied. As can be seen the ammonia peak of chromatograph of Figure 8a has a much higher S / N ratio than the chromatograph of Figure 8b.

[0055] Example 4

[0056] Example 1 was repeated for a 100 ppb ammonia in pure hydrogen. Step (i) of the invented process was carried out for 0.08 minutes, step (ii) was carried out for 2.99 minutes and step (iii) was carried out for 2.90 minutes. The measurement was repeated 3 times (n=3). The results are presented in table 1.

[0057] Example 5

[0058] Example 4 was repeated for a pure hydrogen gas sample containing 1 .00 ppb H2S, 1.00 ppb COS, 1.00 ppb methyl sulphide (MeSH), 0.94 ppb ethyl sulphide (EtSH), 1.00 ppb dimethyl sulphide (DMS). Step (i) of the invented process was carried out for 0.01 minutes with a carrier having H2S as the active component, step (ii) was carried out for 2.99 minutes and step (iii) was carried out for 2.90 minutes. The column used was a 60 m x 0.53mm x 7um MXT-1 as obtained from Restek and the detector was a Sulphur Chemiluminescence Detector (SCD). The results are presented in table 1 .

[0059] Table 1

[0060] In table 1 RSD% is the “relative standard deviation” over the results. LOQ is the “Limit of Quantification”. The LOQ is calculated as follows: LOQ = (10 * Signal I Noise) * Concentration. LOD is the “Limit of Detection”. The LOD is calculated as follows: LOD = (3 * Signal / Noise) * Concentration

Claims

CLAIMS1 . A method for detecting trace levels of an active gas component as contained in a gas sample by detection of said active gas component using a gas chromatograph in fluid communication with a detector sensitive for the gas component, wherein the following steps are performed in a sequence wherein :(i) feeding a carrier gas doped with a quantity of the active gas component to the gas chromatograph for a certain doping time,(ii) feeding the carrier gas of (i) which is not doped with the active gas component to the gas chromatograph for a certain flushing time and(iii) adding the gas sample to the carrier gas of step (ii) and feeding the resulting mixture to the gas chromatograph.

2. A method according to claim 1 , wherein the flushing time is between 10 and 300 seconds3. A method according to any one of claims 1-2, wherein after performing step (iii) step (i) is performed within 600 seconds.

4. A method according to any one of claims 1-3, wherein the active gas component is ammonia, hydrogen sulphide, hydrogen cyanide, carbon monoxide, carbon dioxide, oxygen, hydrogen, fluorine, chlorine, sulphur dioxide, formaldehyde, formic acid, hydrogen chloride and water and their mixtures.

5. A method according to any one of claims 1-4, wherein the sample gas is composed for more than 50 vol% of hydrogen, carbon monoxide, carbon dioxide, methane, ethane, propane, butane, air, nitrogen, helium, ethylene, propylene or their mixtures.

6. A method according to claim 5, wherein the sample gas is composed for more than 80 vol.% of hydrogen or propylene and the active gas component is at least one or more of the following active gas components chosen from ammonia, hydrogen sulphide, formic acid, sulphur dioxide, formaldehyde, oxygen, carbon monoxide, carbon dioxide, hydrogen chloride and water.

7. A method according to claim 6, wherein the sample gas is composed of more than 99 vol.% hydrogen.

8. A method according to any one of claims 1-7, wherein the trace level of the gas component in the gas sample is less than 100 ppm.

9. Method according to any one of claims 1-8, wherein the pressure of the gas sample in step (iii) is between 10 and 500 kPa.

10. A method according to any one of claims 1 -9, wherein gas sampling valve injection is used.11 . Gas chromatographic system comprising a gas chromatograph column having an upstream and downstream end, a detector fluidly connected to the downstream end of the gas chromatograph column, a first switch valve fluidly connected to a second switch valve, each switch valve having a different switch valve positions, wherein the first switch valve has an inlet for a carrier gas, an inlet for a doped carrier gas and an outlet for the carrier gas or for the doped carrier gas depending on the position of the first switch valve, wherein the outlet for the carrier gas or for the doped carrier gas is fluidly connected to an inlet of the second switch valve, wherein the second switch valve is further provided with an inlet for a gas sample, a sample loop, an outlet to a vent conduit and a second switch valve outlet fluidly connected to the upstream end of the gas chromatograph column andwherein the first and second switch valves have switch valve positions (i)- (iii) where there is(i) a fluid communication between the inlet for a doped carrier gas and the upstream end of the gas chromatograph column via the second switch valve outlet,(ii) a fluid communication between the inlet for a carrier gas and the upstream end of the gas chromatograph column via the second switch valve outlet, and(iii) a fluid communication between the inlet for a carrier gas, the sample loop and the upstream end of the gas chromatograph column via the second switch valve outlet.

12. Gas chromatographic system according to claim 11 , wherein the first switch valve has an outlet for carrier gas fluidly connected to an inlet for doped carrier gas via a flow path comprising a chamber which contains a permeation tube comprising an active component and wherein the inlet for doped carrier gas is fluidly connected to the outlet for a doped carrier gas.

13. Gas chromatographic system according to any one of claims 11-12, wherein the vent conduit is provided with a back pressure regulator.

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