A gas delivery system

Abstract

A gas delivery system 1, 2 comprises a pressurised gas source 4, 16 and pressure regulator 5, 17 for regulating the pressure of the gas flow from the gas source, an input line in communication with a

Description

The present disclosure relates to a gas delivery system for a gas analysis system, in particular but not exclusively to a gas delivery system for a gas chromatograph coupled to a mass spectrometer with an atmospheric pressure chemical ionisation source. Issues exist with current gas delivery systems for gas analysis systems, particularly for atmospheric pressure gas chromatography systems and with respect to the delivery of reference gas and / or modifier gas. For the reference gas, a user is typically required to place a vial containing calibrant into a source enclosure to obtain reference peaks for calibration and tuning. This approach has numerous drawbacks: 1. Tuning of the cone gas flow, makeup (or nebulising) gas flow and auxiliary (or bath) gas flow is not possible when using a reference peak introduced in this fashion. This is because the molecules are entering the ionisation chamber from the source enclosure, and therefore gas flow optimisation leads to biasing in favour of drawing in gas from the enclosure, which is usually in opposition to ionisation of the effluent from the gas chromatography column. 2. Setting the level of reference can be an iterative process. The number and internal diameter of the wicks used in the reference vial are typically varied on a trial-and-error basis, in order to obtain reasonable strong peaks which are not saturating the system. 3. Placing the reference vial in the source enclosure and varying the wick configuration requires that the source door is repeatedly open and shut, which can introduce moisture into the source, necessitating a conditioning / settling break, and a disturbance of alignment is risked. The modifier gas is used to enhance and control protonation within the ionisation source. There are two main routes of ionisation, charge transfer and protonation. Protonation is enhanced by adding water (or other protic solvents) to the source region. However, this is detrimental to charge transfer ionisation which favours dry conditions. There are several analyses which contain analytes which favour both routes of ionisation, such as pesticides analysis or semi-volatile organic compounds (SVOCs) analysis. For these analyses it is recommended to setup the source in a “mixed-mode” where there is enough water to enhance protonation, but not so much that it suppresses charge transfer ionisation. There are challenges in setting the mixed-mode. It is an iterative process to determine the wicking arrangement of water vial(s) in the source enclosure to achieve the optimum compromise level of water. These conditions can only usually be maintained for a few days at a time before the water vial(s) need replenishing. Changes in water level can lead to serious inaccuracies in the quantitation of analytes, particularly if the internal standards rely on a different mode of ionisation to the analyte. The present invention arose in a bid to provide an improved gas delivery system, in particular such a gas delivery system addressing one or more of the shortcomings of the prior art. Representative features are set out in the following clauses, which stand alone or may be combined, in any combination, with one or more features disclosed in the text and / or drawings of the specification. According to the present invention in a first aspect, there is provided a gas delivery system for a gas analysis system, comprising a pressurised gas source, a pressure regulator for regulating the pressure of the gas flow from the gas source, an input line in communication with a reservoir for supplying a regulated gas flow from the pressure regulator to the reservoir at a pressure of at least 101 kPa, wherein the reservoir contains a material to be entrained in the gas flow, and an outlet is provided for the entrained gas flowing from the reservoir. The gas analysis system may comprise a gas chromatograph coupled to a mass spectrometer with an atmospheric pressure chemical ionisation source. The gas analysis system may otherwise comprise a thermal desorption unit, a pyrolysis unit, the evolved gas from a thermogravimetric analyser, or any other system in which a gas input is analysed. There is preferably a closed flow path between the pressure regulator and the outlet. The gas is preferably nitrogen. The material is preferably a liquid. The reservoir preferably comprises a container with a closed bottom that contains the material. The reservoir may comprise a bulb that contains the material. There may be a restriction line and a relief line in communication with the outlet, wherein the relief line provides a flow path to waste and the restriction line is in communication with a mass spectrometer. The outlet, the restriction line and the relief line may be connected through a tee. The outlet is preferably in communication with a transfer line. The outlet may be in communication with the transfer line via the restriction line. The outlet is preferably in communication with a waste line and the transfer line through one or more valves arranged to selectively direct flow through either the waste line or the transfer line. The waste line and the transfer line are preferably of equal pneumatic resistance. There is preferably a heater provided for maintaining the temperature of the gas delivery system from the input line onwards above a predetermined minimum temperature. The heater may be arranged to maintain the gas delivery system at a temperature of at least 90°C. The reservoir may contain a reference material, such that the entrained gas comprises a reference gas. The reference material may comprise Perfluorotrihexylam ine. The input line preferably terminates at a point that is at or before an inlet end of a neck of the reservoir. The relief line may be located before the restriction line. The reservoir may contain a modifier compound. The modifier compound may comprise a protic solvent. It may comprise water. A heater is preferably provided for heating the reservoir. The heater may be arranged to maintain the temperature of the reservoir at a substantially constant temperature of at least 30°C. The input line may terminate at a point that is proximal an outlet end of a neck of the reservoir, or may extend into the modifier. The relief line may be located after the restriction line. According to the present invention in a further aspect, there is provided a gas delivery system for a gas analysis system, which comprises two of the pressure regulators, two of the input lines, two of the reservoirs, and two of the outlets. A first of the reservoirs preferably contains the reference material. A second of the reservoirs preferably contains the modifier compound. According to the present invention in a yet further aspect, there is provided a gas analysis system comprising a gas delivery system as detailed in any of the above paragraphs. The gas analysis system may comprise a heated transfer line, wherein a nebulising gas flow is fed into the heated transfer line at a point upstream of the points at which the reference gas and the modifier gas from the delivery systems are fed into the heated transfer line. The gas analysis system may comprise a gas chromatograph coupled to a mass spectrometer with an atmospheric pressure chemical ionisation source. According to the present application in another aspect, there is provided a method of tuning a gas delivery system for a gas analysis system, which comprises a pressurised gas source, a pressure regulator for regulating the pressure of the gas flow from the gas source, an input line in communication with a reservoir for supplying a regulated gas flow from the pressure regulator to the reservoir at a pressure of at least 101 kPa, wherein the reservoir contains a material to be entrained in the gas flow, an outlet is provided for the entrained gas flowing from the reservoir, and a restriction line and a relief line in communication with the outlet, wherein the relief line provides a flow path to waste and the restriction line is in communication with a mass spectrometer, the method comprising modifying a split ratio of the relief line and restriction line. Preferably, the reservoir contains the reference material. According to the present application in another aspect, there is provided a method of tuning a gas delivery system for a gas analysis system, which comprises a pressurised gas source, a pressure regulator for regulating the pressure of the gas flow from the gas source, an input line in communication with a reservoir for supplying a regulated gas flow from the pressure regulator to the reservoir at a pressure of at least 101 kPa, wherein the reservoir contains a material to be entrained in the gas flow, an outlet is provided for the entrained gas flowing from the reservoir, the method comprising modifying the pressure. Preferably, the reservoir contains the modifier compound. Non-limiting embodiments of the invention will now be discussed with reference to the following drawings: Figure 1 shows a schematic view of a gas delivery system for a reference gas; Figure 2 shows a schematic view of a gas delivery system for a modifier gas; and Figure 3 shows a gas analysis system, which comprises a gas delivery system that comprises the gas delivery system of Figure 1 and the gas delivery system of Figure 2. With reference to Figure 1, the gas delivery system 1 for a reference gas comprises a pressurised gas source 4, a pressure regulator 5 for regulating the pressure of the gas flow from the gas source 4, and an input line 6 in communication with a reservoir 7 for supplying a regulated gas flow from the pressure regulator 5 to the reservoir 7 at a pressure of at least 101 kPa. The reservoir 7 contains a reference material 8 to be entrained in the gas flow. An outlet 9 is provided for the entrained gas flowing from the reservoir 7. The gas is preferably nitrogen. The reference material is preferably Perfluorotrihexylamine (PFTHxA). It may, for example, comprise PFTHxA, CAS 432-08-6, available from Apollo Scientific under part number PC6220G. As will be appreciated, however, alternative gases and reference materials may be used. One particular exemplary alternative reference material that has been used successfully is Perfluorotributylamine (PFTBA). PFTBA, however, has an upper mass of 614Da, which can be limiting for gas chromatography atmospheric pressure chemical ionisation (GC-APCI) analysis. The highest mass analyte that is routinely targeted in GC-APCI analysis is decabromodiphenylether (DecaBDE, or BDE209) with a mass of 959Da (or 971 Da for its 13Ci2 labelled internal standard), which is outside the calibration mass range of PFTBA. PFTHxA on the other hand has an upper mass of 985Da and as such allows the user to calibrate up to the highest expected analytical mass. It is also sufficiently volatile to infused into the source region. Figure 3 shows an exemplary gas analysis system 3, which includes a gas delivery system comprising the reference gas delivery system 1. As shown in Figure 3, the reference gas delivery system 1 is connected to a heated gas transfer line 30 of the gas analysis system 3. The gas analysis system further comprises a gas chromatograph 31, which comprises a gas chromatography column 32, and a mass spectrometer 33. In the gas analysis system 3 the mass spectrometer preferably has an atmospheric pressure chemical ionisation source, although the gas analysis system 3 need not be limited as such. The gas analysis system 3 may otherwise comprise a thermal desorption unit, a pyrolysis unit, the evolved gas from a thermogravimetric analyser, or any other system in which a gas input is analysed. A nebulising (or makeup) gas flow is fed into the heated transfer line at a point upstream of the reference gas and a modifier gas, through a nebulising gas line 35, as seen in Figure 3. The nebulising gas is preferably nitrogen although need not be limited as such. It may comprise, for example, comprise argon or air. The reference and modifier gases will thereby be entrained in the nebulising gas flow. The reference gas from the gas delivery system 1 is preferably fed into the heated transfer line 30 of the gas delivery system 3, as shown in Figure 3. It may be fed into the end of the heated transfer line 30 closest to the gas chromatograph 31. The approach of feeding the reference gas into the heated transfer line 30 has been determined to allow optimisation of the gas flows such that it reflects the performance seen with analyte eluting from the gas chromatography column. Such an arrangement can significantly speed up the process of optimisation, since it allows it to be performed in real time, rather than the prior art approach of performing gas chromatography injections of a standard and iterating through multiple parameter settings (which can take several days). It also allows for an autotune function, and further diagnostics. The reference gas delivery system 1 allows for the supply a steady level of reference compound. This is useful in the very short term (for example, <1 second timeframe), to allow tuning of the mass spectrometer 33 and source (not shown), and also in the longer term (for example, >1 month timeframe), such that the intensity of the reference peaks can highlight any changes in system sensitivity. The level may be consistent from instrument to instrument so it can be used for a first-pass assessment of system sensitivity. The exact level of reference material required has two considerations: 1) it must not cause detector saturation under normal tuning conditions; and 2) it must be high enough that the reference peaks contain sufficient ions to allow reliable calibrations and precise tuning of the source and mass spectrometer. The level required will therefore depend on the model of mass spectrometer being used. The gas delivery system 1, by its novel arrangement, is configurable such that the reference level can be set appropriately for the particular model of mass spectrometer 33 that it is interfaced to. The reference reservoir 7 preferably comprises a bulb, as shown. It should be appreciated, however, that alternative forms of reservoir will be possible. The input nitrogen supply 4 will be set to a suitable pressure. In an exemplary arrangement, it is set at 7 Bar pressure. The pressure regulator, which preferably comprises an electronic pressure regulator, will be used to hold the outlet pressure at a desired pressure (of at least 101 kPa). This pressure is preferably set at a static pressure during installation and fixed throughout the time the system is in use. However, arrangements will be possible in which the pressure may be varied by users of the system. The input line 6 transfers gas from the pressure regulator into a reference assembly that comprises the reference reservoir 7. The input line preferably terminates (i.e. its outlet is located) at a point that is at or before an inlet end of a neck of the reservoir 7. It is preferably just above the neck of the reference reservoir 7, as seen in Figure 1. The proximity of the input line outlet to the reservoir has an impact on the amount of reference material supplied to the instrument. In a nonlimiting exemplary arrangement, the input line is set to be 15cm long with an Internal Diameter (ID) of 0.28mm. The reference reservoir 7 in the present arrangement is secured into the assembly with a hand-tightened knurled nut. An O-ring is used to seal around the neck of the reservoir 7. In alternative arrangements, as will be readily appreciated, alternative attachment / sealing arrangements may be implemented. The size / volume of the reservoir 7 is not particularly limited. Notably, however, only a small volume of reference material is typically required. Purely by way of example, it has been found that 10OpL of PFTHxA will give a useable signal over around 6 months of use. Again, purely by way of example, a bulb with a diameter of 15.5mm has been determined to be suitable for holding such a volume of reference material. In use, gas flows from the input line 6 into the reservoir 7. The reference material 8 in the reservoir is entrained in the gas flow, and the entrained gas flows out through the outlet 9. There is a closed flow path between the pressure regulator and the outlet. The flow through the outlet 9 is at the desired pressure (of at least 101 kPa). A restriction line 10 and a relief line 11 are provided in communication with the outlet 9, wherein the relief line 11 provides a flow path to waste and the restriction line provides for communication, ultimately, with the mass spectrometer 33. As is seen in Figure 1, the outlet 9, restriction line 10 and the relief line 11 are preferably connected through a tee. It should be appreciated, however, that alternative fluidic connections may be implemented. The relief line selection / configuration is preferably the main means of setting the amount of reference material transferred to the source. The relative dimensions of the relief line 11 and restriction line 10 define how much of the reference compound is transferred I can be used to set the split ratio, wherein the reference level can be set by changing the dimensions of the relief line 11. The system is preferably configured such that flow through the lines approximately follows Poiseuille’s Law: F = ^-Po^ 128 t]L ' ' Where D is the pipe diameter and L is the pipe length. In a non-limiting exemplary arrangement, the relief line 11 has a length of 30cm and an inner diameter (ID) of 0.25mm, and the restriction line, used to control the flow of reference compound to the mass spectrometer 33, has a has a length of 30cm and an inner diameter (ID) of 0.125mm. In this example, the two pipes only differ in their diameter by a factor of 2, so the split ratio will be around 16:1. Clearly, in dependence on required settings, the relative dimensions of the relief line 11 and restriction line 10 may be varied. The above split ratio may, for example, be suitable for a highly sensitive mass spectrometer 33, such as the Xevo (RTM) TQ-XS from Waters Corporation, where less sensitive mass spectrometers may require at least ten times more reference material to obtain a useful signal, wherein this may be achieved by setting a lower split ratio, using an alternative relief line 11 of higher restriction. Notably, an alternative approach that could be implemented is to block off the exit of the relief line 11, and therefore remove the split altogether. Based on the above example, this would give around 17 times more signal, which may be adequate for less sensitive systems. Clearly, such an arrangement would allow for easy setup on different mass spectrometers. As shown in Figure 1, the relief line 11 is preferably located before the restriction line 10. The outlet 9 is in communication with a transfer line 13 via the restriction line 10. A valve 15, which preferably comprises a 3 port, 2 position valve, is preferably provided for selectively directing flow through either the transfer line 13 or a waste line 14. By such an arrangement the reference material may either be sent to waste or to the mass spectrometer 33. Such an arrangement is useful to ensure there is a continuous flow path for the reference material, so as to avoid surges and prolonged settling times when the valve 15 is turned off and on. The settling time with such an arrangement has been found to be within 3 seconds. The waste line 14 and the transfer line 13 are preferably of equal pneumatic resistance. This may be achieved by ensuring they have the same dimensions and are held at the same temperature. In an exemplary non-limiting arrangement, a metal tube having a length of 20cm and an inner diameter of 0.80mm may be used for both lines. A heating means of any suitable form may be incorporated into the system for maintaining the temperature of the reference gas delivery system 1 from the input line 6 to the transfer line 13 above a predetermined minimum temperature. This may be a temperature of at least 90°C. The temperature may be maintained at around 100°C. Maintaining the system at an elevated temperature can be beneficial for preventing condensation of the reference material, which has the potential to lead to a long term variation (drop) in the reference level. The transfer line 13 is connected to the heated transfer line 30, as seen in Figure 3. It may be fed into a centre of the heated transfer line 30 through a suitable end piece, which may be located within the gas chromatography oven, preferably within a wall of the gas chromatography oven. The end piece is preferably held at an elevated temperature, most preferably above 100°C. The elevated temperature will prevent reference material condensation occurring at this point. With reference to Figure 2, the gas delivery system 2 for a modifier gas comprises a pressurised gas source 16, a pressure regulator 17 for regulating the pressure of the gas flow from the gas source 16, and an input line 18 in communication with a reservoir 19 for supplying a regulated gas flow from the pressure regulator 17 to the reservoir 19 at a pressure of at least 101 kPa. The reservoir 19 contains a modifier compound 20 to be entrained in the gas flow. An outlet 21 is provided for the entrained gas flowing from the reservoir 19. By such an arrangement, it is possible to provide a very stable, reproduceable and tuneable level of modifier compound to the source, such that mixed mode analysis is simple to achieve and maintain for extended periods. The system may be configured such that the modifier compound is electronically controlled, whereby a user can turn the modifier on and off as part of the experiment conditions or can set the level of modifier. The level of modifier may be modified by varying the pressure. A user interface for the system may be configured such that there are pre-set levels of modifier, e.g. “medium” or “high”, with the levels corresponding to different predetermined pressures. The gas is preferably nitrogen. It need not be limited as such. The modifier compound comprises a protic solvent. Whilst any protic solvent may be used, the modifier compound is preferably water. The input line 18, which may be dimensioned as discussed above in respect of the input line 6, preferably terminates at the base of the neck of the reservoir, as seen in Figure 2. Such an arrangement is beneficial to ensure that the gas sweeps the head space within the reservoir 19 and forces the modifier compound (water vapour) 20 out. The arrangement is preferably such that the input line 18 is not immersed in the modifier compound, since whilst this works well at getting more modifier compound into the gas flow, the input line 18 itself can act as a wick, allowing the modifier compound to rise up along its outside and eventually block the exit path by forming a plug. Nevertheless, arrangements may be implemented where the input line 18 is immersed. The modifier reservoir 19 preferably comprises a bulb, as shown. It should be appreciated, however, that alternative forms of reservoir will be possible. The size / volume of the reservoir 19 is not particularly limited. Notably, however, only a small volume of modifier compound is typically required. Purely by way of example, it has been found that 7mL of water is sufficient to supply water to the source for around 1 week. Again, purely by way of example, a bulb having an outer diameter of 30mm has been determined to be suitable for holding this volume of water without the meniscus touching the base of the neck of the reservoir. There may be a heater (not shown) provided for heating the reservoir. The heater may be arranged to maintain the temperature of the reservoir at a substantially constant temperature of at least 30°C, at least 35°C, or at least 50°C. Maintaining the reservoir at an elevated temperature is preferable, in order to ensure that the modifier compound’s temperature is not affected by variations in the ambient temperature. In use, gas flows from the input line 18 into the reservoir 19. The modifier compound 20 in the reservoir 19 is entrained in the gas flow, and the entrained gas flows out through the outlet 21. There is a closed flow path between the pressure regulator and the outlet. The flow through the outlet 21 is at the desired pressure (of at least 101 kPa). A restriction line 22 and a relief line 23 are provided in communication with the outlet 21, wherein the relief line 23 provides a flow path to waste and the restriction line 22 provides for communication, ultimately, with the mass spectrometer 33. As is seen in Figure 2, the outlet 21, restriction line 22 and the relief line 23 are preferably connected through a tee. It should be appreciated, however, that alternative fluidic connections may be implemented. The relief line 23 is preferably located after the restriction line 22, as shown in Figure 2. This arrangement has been found to be more effective for the modifier gas than the above preferred arrangement for the reference gas. The relief line 23 in a non-limiting exemplary arrangement is 5cm long with an inner diameter of 0.50mm. Regardless of the particular dimensions of the relief line 23, which may be as discussed or different, in an exemplary non-limiting arrangement, the split ratio relative to the transfer line 27 is about 60:1. The selection of the relief line 23 may be based on an iterative process, to ensure mixed mode conditions at the predetermined pressure. The restriction line 22 in a non-limiting exemplary arrangement is made from 1 m of steel tubing having an inner diameter of 0.28mm. Such an inner diameter, whilst not essential, has been determined to suitably avoid risks of the line plugging. The inner diameter may, of course, be varied. The outlet 21 is in communication with a transfer line 27 via the restriction line 22. A valve 26, which preferably comprises a 3 port, 2 position valve, is preferably provided for selectively directing flow through either the transfer line 27 or a waste line 25. By such an arrangement the modifier compound may either be sent to waste or to the mass spectrometer 33. The waste line 25 and the transfer line 27 are preferably of equal pneumatic resistance. This may be achieved by ensuring they have the same dimensions and are held at the same temperature. In an exemplary non-limiting arrangement, a metal tube having a length of 15cm and an inner diameter of 0.28mm may be used for both lines. A heating means of any suitable form may be incorporated into the system for maintaining the temperature of the modifier gas delivery system 2 from the input line 18 to the transfer line 27 above a predetermined minimum temperature. This may be a temperature of at least 90°C. The temperature may be maintained at around 100°C. Maintaining the system at an elevated temperature can be beneficial for preventing condensation of the modifier compound. The transfer line 27 is connected to the heated transfer line 30, as seen in Figure 3. Numerous alternative arrangements and modifications to the embodiments as described herein will be readily appreciated by those skilled in the art within the scope of the appended claims. In particular, features of the described embodiments may be readily combined. When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components. The features disclosed in the foregoing description, or the following claims, or 5 the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. 10 Although certain example embodiments of the invention have been described, the scope of the appended claims is not intended to be limited solely to these embodiments. The claims are to be construed literally, purposively, and / or to encompass equivalents.

Claims

1. A gas delivery system for a gas analysis system, comprising a pressurised gas source, a pressure regulator for regulating the pressure of the gas flow from the gas source, an input line in communication with a reservoir for supplying a regulated gas flow from the pressure regulator to the reservoir at a pressure of at least 101 kPa, wherein the reservoir contains a material to be entrained in the gas flow, and an outlet is provided for the entrained gas flowing from the reservoir.

2. A gas delivery system as claimed in Claim 1, wherein there is a closed flow path between the pressure regulator and the outlet.

3. A gas delivery system as claimed in Claim 1 or 2, wherein the gas is nitrogen.

4. A gas delivery system as claimed in any preceding claim, wherein thereservoir comprises a bulb that contains the material.

5. A gas delivery system as claimed in any of Claims 1 to 4 comprising a restriction line and a relief line in communication with the outlet, wherein the relief line provides a flow path to waste and the restriction line is in communication with a mass spectrometer.

6. A gas delivery system as claimed in Claim 5, wherein the outlet, the restriction line and the relief line are connected through a tee.

7. A gas delivery system as claimed in any preceding claim, wherein the outlet is in communication with a transfer line.

8. A gas delivery system as claimed in Claim 7, when dependent on Claim 5, wherein the outlet is in communication with the transfer line via the restriction line.

9. A gas delivery system as claimed in Claim 7 or 8, wherein the outlet is in communication with a waste line and the transfer line through one or more valves arranged to selectively direct flow through either the waste line or the transfer line.

10. A gas delivery system as claimed in Claim 9, wherein the waste line and the transfer line are of equal pneumatic resistance.

11. A gas delivery system as claimed in any preceding claim comprising a heater for maintaining the temperature of the gas delivery system from the input line onwards above a predetermined minimum temperature.

12. A gas delivery system as claimed in Claim 11, wherein the heater is arranged to maintain the gas delivery system at a temperature of at least 90°C.

13. A gas delivery system as claimed in any preceding claim, wherein the reservoir contains a reference material, and the entrained gas comprises a reference gas.

14. A gas delivery system as claimed in Claim 13, wherein the reference material comprises Perfluorotrihexylamine.

15. A gas delivery system as claimed in Claim 13 or 14, wherein the input line terminates at a point that is at or before an inlet end of a neck of the reservoir.

16. A gas delivery system as claimed in any of Claims 13 to 15, when dependent on Claim 5, wherein the relief line is located before the restriction line.

17. A gas delivery system as claimed in any of Claims 1 to 12, wherein the reservoir contains a modifier compound.

18. A gas delivery system as claimed in Claim 17, wherein the modifier compound comprises a protic solvent, preferably water.

19. A gas delivery system as claimed in Claim 17, which comprises a heater for heating the reservoir.

20. A gas delivery system as claimed in Claim 19, wherein the heater is arranged to maintain the temperature of the reservoir at a substantially constant temperature of at least 30°C.

21. A gas delivery system as claimed in any of Claims 17 to 20, wherein the input line terminates at a point that is proximal an outlet end of a neck of the reservoir, or extends into the modifier.

22. A gas delivery system as claimed in any of Claims 17 to 20, when dependent on Claim 5, wherein the relief line is located after the restriction line.

23. A gas delivery system as claimed in any preceding claim, comprising two of the pressure regulators, two of the input lines, two of the reservoirs, and two of the outlets.

24. A gas delivery system as claimed in Claim 23, when dependent on any of Claims 13 to 16, wherein a first of the reservoirs contains the reference material.

25. A gas delivery system as claimed in Claim 23 or 24, when dependent on any of Claims 17 to 22, wherein a second of the reservoirs contains the modifier compound.

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