Gas inlet system for isotope ratio analyzer and method of determining an isotope ratio

Abstract

A gas inlet system for introducing gas into an isotope ratio analyser, the gas inlet system including a reference system comprising: a first supply of a reference gas having a first known isotope ratio; a supply of a carrier gas, wherein the supplies of reference gas and carrier gas are each connected by respective reference and carrier gas lines to a first mixing junction where the reference gas and carrier gas combine; a mixing zone connected downstream of the first mixing junction wherein the combined reference gas and carrier gas mix together; an exit line for transporting the mixed gas from the mixing zone to the isotope ratio analyser; and an opening on the exit line, wherein the opening is downstream of the mixing zone. Also provided is a method of determining an isotope ratio.

Claims

1. A gas inlet system for introducing gas into an isotope ratio analyzer, the gas inlet system including a reference system comprising: a first supply of a reference gas having a first known isotope ratio; a supply of a carrier gas, wherein the supplies of reference gas and carrier gas are each connected by respective reference and carrier gas lines to a first mixing junction where the reference gas and carrier gas combine; a mixing zone connected downstream of the first mixing junction wherein the combined reference gas and carrier gas mix together; an exit line for transporting the mixed gas from the mixing zone to the isotope ratio analyzer; and an opening on the exit line, wherein the opening is downstream of the mixing zone, where the opening is open to the atmosphere and the pressure at the first mixing junction with the opening is close to the atmospheric pressure.

2. A gas inlet system according to claim 1, the reference system further comprising a second supply of a reference gas having a second known isotope ratio, wherein the first and second supplies of reference gas may be each independently connected to the first mixing junction for mixing with the carrier gas.

3. A gas inlet system according to claim 1 wherein the reference gas is a substantially pure gas and the carrier gas is substantially free from the reference gas.

4. A gas inlet system according to claim 1 wherein the carrier gas flow to the first mixing junction is dynamically controllable by flow control means.

5. A gas inlet system according to claim 1 wherein the opening is in the form of an open capillary that is at least 0.5 mm in internal diameter and at least 5 mm in length.

6. A gas inlet system according to claim 5 wherein the flow rate through the open capillary is at least 0.5 ml/min.

7. A gas inlet system according to claim 1 wherein the mixing zone is at least 75 mm in length and at least 0.8 mm internal diameter.

8. A gas inlet system according to claim 1 wherein the mixing zone includes one or more bends and/or includes an angle along its length.

9. A gas inlet system according to claim 1 wherein the system further comprises a second mixing junction on the exit line from the mixing zone for mixing further carrier gas with the already mixed gas, the further carrier gas being supplied to the second mixing zone via a second carrier gas line.

10. A gas inlet system according to claim 9 wherein the carrier gas supplied to the second mixing junction is not dynamically controllable.

11. A gas inlet system according to claim 1 wherein the mixed gas that enters the analyzer comprises a CO.sub.2 in air or CO.sub.2 in nitrogen concentration in the range between about 200-1500 ppm.

12. An isotope ratio analyzer comprising the gas inlet system according to claim 1.

13. A gas inlet system according to claim 12 wherein the isotope ratio analyzer is an isotope ratio mass spectrometer or an isotope ratio optical spectrometer.

14. A gas inlet system according to claim 12 wherein the isotope ratio analyzer is for determination of a concentration dependence of an isotope ratio measurement and/or determination of an isotope ratio dependence of the isotope ratio measurement.

15. A gas inlet system for introducing gas into an isotope ratio analyzer, comprising: a supply of analyte gas; an analyte gas line for transporting a flow of analyte gas from the supply of analyte gas; a supply of carrier gas; and a carrier gas line for transporting a flow of carrier gas from the supply of carrier gas; wherein the analyte gas line joins the carrier gas line at an analyte-carrier junction to mix the analyte gas and the carrier gas, wherein the analyte-carrier junction is further connected to an exit gas line for transporting the mixed gas from the junction to the isotope ratio analyzer, wherein the analyte-carrier junction is positioned downstream of an opening on the carrier gas line, and wherein the opening is open to the atmosphere and the pressure at the analyte-carrier junction with the opening is close to the atmospheric pressure.

16. A gas inlet system according to claim 15 configured such that the flow rate of the analyte gas in the analyte line is lower than the flow rate of gas into the isotope ratio analyzer and the flow between the opening and the analyte-carrier junction is always towards the isotope ratio analyzer.

17. A gas inlet system according to claim 15 configured such that the flow rate of the analyte gas in the analyte line is higher than the flow rate of gas into the isotope ratio analyzer and the flow between the opening and the analyte-carrier junction is always towards the opening.

18. A gas inlet system according to claim 15 wherein the analyte gas is selected from the group consisting of: CO.sub.2, CH.sub.4, C.sub.2H.sub.6, C.sub.xH.sub.(2x+2), water vapour, CO, small hydrocarbons, alcohols, aldehydes, NO.sub.x, N.sub.xO.sub.y, H.sub.2S, nitrogen, oxygen, and hydrogen.

19. A gas inlet system according to claim 15 wherein the carrier gas is selected from the group consisting of: air, nitrogen, helium and argon.

20. A gas inlet system according to claim 15 wherein the flow rate to the opening junction is at least 10 ml/min.

21. A gas inlet system according to claim 15 wherein the gas inlet system further includes a calibration system comprising: at least a first supply of a reference gas having a first known isotope ratio, wherein the supplies of reference gas and carrier gas are each connected by respective gas lines to a mixing junction where the reference gas and carrier gas combine; a mixing zone connected downstream of the mixing junction wherein the combined reference gas and carrier gas mix together; an exit gas line for transporting the mixed reference and carrier gas from the mixing zone to the isotope ratio analyzer; and an opening to atmosphere on the exit line from the mixing zone, wherein the opening is downstream of the mixing zone.

22. A method of determining an isotope ratio, comprising: providing a reference gas at a first known isotope ratio; measuring the isotope ratio of the reference gas with the first known isotope ratio at a plurality of concentrations in a carrier gas; determining a concentration dependence of the isotope ratio measurements of the reference gas with the first known isotope ratio; providing a sample gas of unknown isotope ratio and unknown concentration; measuring the isotope ratio and concentration of the sample gas; and correcting the measured isotope ratio of the sample gas by the determined concentration dependence.

23. A method of determining an isotope ratio according to claim 22, wherein the method further comprises: providing the reference gas at a second known isotope ratio; measuring the isotope ratio of the reference gas with the second known isotope ratio, at one or more concentrations in the carrier gas that lie within the range of measured concentrations of the reference gas with the first known isotope; determining an isotope ratio calibration from the measured isotope ratios of the reference gas with the first and second known isotope ratios; and further correcting the measured isotope ratio of the sample gas by the determined isotope ratio calibration.

24. A method of determining an isotope ratio according to claim 22 wherein there is a single supply of reference gas with the first known isotope ratio, which is dynamically diluted with the carrier gas to provide the plurality of concentrations used for measuring the isotope ratio.

25. A method of determining an isotope ratio according to claim 22 wherein the reference gas is CO.sub.2, the carrier gas is CO.sub.2-free air or CO.sub.2-free nitrogen and the sample gas comprises CO.sub.2.

26. A method of determining an isotope ratio according to claim 25, wherein the plurality of concentrations in a carrier gas comprise a CO.sub.2 in air or CO.sub.2 in nitrogen concentration in the range between about 200- 4,000 ppm.

27. A method of determining an isotope ratio according to claim 26, wherein the plurality of concentrations in a carrier gas comprise a CO.sub.2 in air or CO.sub.2 in nitrogen concentration in the range between about 200-1500 ppm.

28. A method of determining an isotope ratio according to claim 25 wherein the measured isotope ratio is the ratio .sup.13C/.sup.12 C or .sup.18O/.sup.16O.

Description

LIST OF FIGURES

(1) FIG. 1 shows a schematic layout of an isotope ratio optical spectrometer interfaced to a gas inlet system in accordance with the present disclosure.

(2) FIG. 2 shows a schematic layout of a gas inlet and referencing system in accordance with the present disclosure.

(3) FIG. 3 shows a schematic layout of the referencing section of the system shown in FIG. 2.

(4) FIG. 4 shows a schematic layout of the referencing section as shown in FIG. 3 with optional valve for closure of an opening to atmosphere.

(5) FIG. 5 shows a schematic layout of the referencing section as shown in FIG. 3 with optional zero air supply in an opening to atmosphere.

(6) FIG. 6 shows a schematic layout of the sample inlet section of the system shown in FIG. 2.

(7) FIG. 7 shows an embodiment of a system in accordance with the schematic layout of FIG. 2.

(8) FIG. 8 shows an embodiment of a machined valve block for use in a system as shown in FIG. 7.

DETAILED DESCRIPTION OF EMBODIMENTS

(9) In order to assist further understanding, but without limiting the scope thereof, various exemplary embodiments are now described with reference to the Figures.

(10) Referring to FIG. 1, there is shown schematically an isotope ratio optical spectrometer (100) interfaced to a gas inlet system (120) in accordance with the present disclosure. It will be appreciated that the isotope ratio optical spectrometer could in other embodiments be replaced with an isotope ratio mass spectrometer interfaced to the gas inlet system. The optical spectrometer is a laser spectrometer. A sample (or reference) gas to be measured is transported from the gas inlet system (120) through a multi-pass measurement cell (112) in the laser spectrometer by a vacuum pump (114), such as a membrane pump, in the outlet (115) from the spectrometer that pumps the cell. The measurement cell has a total optical pathlength of approximately 5.4 m. The incoming gas is directly and completely transferred into the measurement cell (112). A filter (not shown) upstream of the cell prevents transfer of particles into the cell. The inlet flow into the measurement cell in this embodiment is limited by a fixed flow restriction (111) and is set to allow a gas flow rate of 80 ml/min into the cell for atmospheric pressure at the inlet ports (101, 104) and 0.5 bar (g) at inlet ports (102, 103). The actual flow through the measurement cell however depends on the pressure of the delivered gas. The pressure in the measurement cell (112) is kept constant by controlling the pump speed, for example in this embodiment by feedback to the pump (114) of signals generated from a pressure gauge (113) connected to the cell (112). It is also possible in other embodiments to have an adjustable valve between cell (112) and pump (114) and control the valve instead of the pump (114), e.g. using the feedback from pressure gauge (113). In this way, the pressure in the cell is desirably maintained generally in the range 20-200 mbar (a), such as 40-200 mbar (a), even 40-150 mbar (a). The pressure in the measurement cell is typically kept constant at approximately 100 mbar (a) (or in the range 20 to 150 mbar (a), or even to 200 mbar (a)). The operating measurement range of the cell is 200-4,000, such as 200-3,500, ppm of CO.sub.2 in air or in N.sub.2 with highest performance of detection between 200-1,500 ppm, especially 300-1,500 ppm of CO.sub.2.

(11) An isotope ratio is generally determined in the measurement cell by measuring two separate spectral absorption lines, typically in the infrared region, one line for each different isotopic species (isotopologue), e.g. an absorption line for .sup.12C.sup.16O.sub.2 and another line for .sup.13O.sup.16O.sub.2. A convenient absorption line for CO.sub.2 is the line at or about 4.3218 μm. If more lines are available per isotope (e.g. a doublet or triplet) it is possible to measure and use the information from more than one line, e.g. for other gases than CO.sub.2 or in other spectral ranges that might be interesting. The ratio of the intensities of the spectral absorption lines is a measure of the ratio of the abundance of each of the isotopic species (and hence the isotope ratio, e.g. .sup.13C/.sup.12C) The outputs of the spectrometer are thus ratios of different isotopic lines (e.g. R.sub.13C=C.sub.13c/C.sub.12c). The result is referenced against international standards using the established delta notation for isotope ratio reporting (e.g. δ.sup.13C [%.sub.o]). The means for performing calibrations, which are required to calculate a δ-value from a ratio of spectral intensities, are described below.

(12) On the gas inlet line into the measurement cell a multiport valve (shown schematically as distinct valves 107-110 for illustration purposes) allows switching between four different gas inlet ports (101-104). One of these ports (101) is connected to a gas inlet and referencing system according to the disclosure as described in more detail below (see FIG. 2). The remaining ports (102-104) can optionally be used, for example, for additional sample gas (e.g. ambient air at port 104) and/or calibration gases for additional concentration calibration (102, 103). The latter requires one or two references with known concentration. The inlet and referencing system connected to port (101) is typically used for calibration of the concentration dependence of the isotope ratio measurement and for the isotope ratio dependence of the isotope ratio measurement as described below.

(13) The apparatus and method of the disclosure is illustrated in the embodiments below with the example of a CO.sub.2 analysis system (i.e. CO.sub.2 as sample and reference gas) but it should be appreciated that the disclosure is applicable to any other gas that is susceptible to isotope ratio analysis, either by optical spectrometry or mass spectrometry or other spectrometry technique. In those cases, the reference gas will not be CO.sub.2 but will the same gas as the particular sample gas being analyzed. Similarly, the apparatus and method is illustrated in the embodiments below with the example of an optical spectrometer but it should be appreciated that the disclosure is applicable to a mass spectrometer or other spectrometer.

(14) It has been found that the isotope ratio reported by the spectrometer differs with gas concentration and therefore a correction factor (also termed linearity calibration or concentration dependence) that depends on the CO.sub.2 concentration is required for each ratio. To calculate these linearity calibration factors, the spectrometer needs to measure CO.sub.2 with the same isotopic ratio at different concentration, or at least numerous reference gases with known isotopic ratio and concentration are needed.

(15) In addition, it is known that there is an isotope ratio dependence of the isotope ratio measurement (the so-called delta scale contraction). Typically (at least) two reference gases of known and different isotope ratio are necessary to calculate the delta scale contraction. The highest accuracy is achieved if the reference gases used for the delta scale contraction have a similar concentration to that of the measured sample(s) and their delta values narrowly frame the delta value(s) of the sample(s). Therefore the two references gases should be diluted to give a CO.sub.2 gas concentration in the range 100-4,000 ppm CO.sub.2 in air, which is the operating measurement range of the spectrometer, such as 200-4,000 ppm, even 200-3,500 ppm, and optimally 400-2,000 ppm CO.sub.2 in air. For each different type of gas, there can be a range of gas concentrations in the analyzer with a width of approximately 1 decade for the given analyzer configuration (e.g. optical path, and analyzer sensitivity). The position of this range can be adjusted by changes to the configuration. Thus, for each gas the requirement is to match the concentration of the gas reaching the analyzer with the dynamic range of the analyzer.

(16) Regular referencing and calibration of the spectrometer ensures high data quality and accurate analysis. This regular referencing and calibration is facilitated by the referencing system of the gas inlet as described below.

(17) For linearity calibration and delta scale contraction, mixtures of reference CO.sub.2 with carrier gas, using CO.sub.2 from two different sources (with different known isotope ratios), are required. The disclosure provides a convenient way to supply these different concentration gases to the spectrometer by mixing pure CO.sub.2 with CO.sub.2-free air (also termed zero air), or other CO.sub.2-free gas. As reference gases for isotope analysis gases are typically expensive and air-CO2 mixtures require large gas cylinders it is an advantage to use pure CO.sub.2 as a reference gas instead of a pre-mixture as described in the prior art. Pure CO.sub.2 with various certified isotope values as reference for isotope ratio analysis is commercially available and 1 kg CO.sub.2 may last for the whole instrument lifetime. The dilution is performed to give the required 200-4,000 (such as 200-3,500) ppm CO.sub.2 in air (or in N.sub.2). Further advantageously, CO.sub.2-free air can either be produced in the field using a CO.sub.2 absorber or delivered in gas tanks. The CO.sub.2 may be supplied, for example, in standard 13 liter (L) containers or in 1 L (low pressure) containers. The latter may be conveniently shipped or mailed with standard air freight. It is possible to produce more than 15,000 m.sup.3 of gas mixture at 350 ppm CO.sub.2 in air using one commercial 13 L gas cylinder of CO.sub.2 by mixing it with CO.sub.2-free air. Thus, the working lifetime of a 13 L CO.sub.2 gas cylinder in various embodiments may be several years. To have a better stability of the gas flow and avoid possible isotope fractionation by a flow controller the gas inlet system is used. In contrast to the prior art, the flow of CO.sub.2-free air (zero air) is controlled by a flow control means (e.g. flow controller or a proportional valve) and instead the CO.sub.2 flow is kept constant. For higher dynamic range and flow matching the dilution can be done in two steps as described in more detail below.

(18) Another advantage of using pure CO.sub.2 (or other pure reference gases) as reference gas, is that this makes it easy to switch to another carrier gas (for example to use helium, argon or nitrogen instead of zero air). In a laser spectrometer, the absorption spectrum of an analyte (sample) gas is also influenced by the surrounding gases. Therefore, the main components of the gas mixture should ideally be the same or similar for the reference gas and the sample gas. In the system according to the present disclosure, it is possible to switch the carrier gas (e.g. to be the same or similar to the gas surrounding the sample gas) without changing the valuable reference gas.

(19) Referring to FIG. 2, there is shown a gas inlet and referencing system according to the disclosure. Firstly, it is noted how the system connects with the spectrometer shown in FIG. 1. The exit line (16) of the system shown in FIG. 2 connects with port (101) of the gas inlet system shown in FIG. 1. Thus, gases exiting from the system shown in FIG. 2 enter the spectrometer shown in FIG. 1 for isotope ratio measurement. The system is configured to be able to deliver sample gas and reference gas to the optical laser spectrometer.

(20) Supplies of two pure CO.sub.2 reference gases (ref 1 and ref 2) are provided (41, 42). The isotope ratio (.sup.13C/.sup.12C and/or .sup.18O/.sup.16O) of each CO.sub.2 supply is known. The flow of each supply of CO.sub.2 is controlled by a respective valve (44, 45), which are constant pressure valves, and a respective flow restriction (1, 2) on the supply lines. The two valves V1 and V2 (3, 4) on the reference gas line allow switching between the two reference gases as well as shutting them both off from the rest of the system to save reference gas. A supply of a carrier gas, which is CO.sub.2-free air, is also provided (40), the flow of which is controlled by a respective valve (43), which is a constant pressure valve. For simplicity, FIG. 3 shows the flow scheme of the referencing section of the inlet system alone.

(21) To avoid fractionation of the CO.sub.2, a constant flow of CO.sub.2 of 400 μl/s (24 ml/min) from a selected one of the CO.sub.2 reference supplies is mixed into a variable flow of CO.sub.2-free air (or other carrier gas) (flow rate 3 to 100 ml/min) using a mass flow controller or a proportional valve (9) on the carrier gas line. The mass flow controller or a proportional valve (9) is in this embodiment computer controlled (such as most or all valves shown in the system are computer controlled). That is, the CO.sub.2 is not subject to variable mass flow control, thus avoiding fractionation, but rather it is the CO.sub.2-free air that is dynamically flow controlled. The gases first mix at T-junction (50), which is a first mixing junction. The gases further mix downstream inside a mixing zone (5), which is a tube with 0.8 mm inner diameter and a minimum length of 75 mm to get a homogeneous mixture of the two gases. The flow restrictions (1, 2) and constant input pressure valves (44, 45) of the CO.sub.2 references define the constant CO.sub.2 flow into the mixing zone (5). The CO.sub.2 concentration of this resultant pre-mixture is designed to be in the range from 4,000 ppm to 13,000 ppm. The mixing zone (5) is necessary to ensure that the CO.sub.2 and the zero air are thoroughly mixed. As the flow rate here can be larger than 100 ml/min, the residence time of the gas in the mixing zone (5) may be very short. With such flow rates, for a 75 mm length and 1 mm width (i.e. internal diameter (id)) of the mixing zone, the residence time would be only 35 msec. Under these conditions the flow is still laminar and mixing occurs by diffusion with nearly no concentration gradient across the capillary. In view of such considerations, the mixing zone can be at least 75 mm in length and at least 0.8 mm id.

(22) Whilst gas mixing can occur where the mixing zone is a straight mixing tube between the two T-junctions with constant cross section along its length, it is desirable to provide features to improve mixing in the mixing zone.

(23) In various embodiments, the dimensionless number (D*l/j)>0.67 Where (in SI units) D: (Inter)Diffusion coefficient of the analyte gas in the carrier gas (m.sup.2/s), l: length of the mixing tube (m), j: flow in this tube (m.sup.3/s).

(24) Thus, it is ensured that the concentration at the end of the mixing tube differs at no point across the area cross section more than 1% from the mean concentration. If (D*l/j)>0.94 the concentration at the end of the mixing tube differs at no point across the area cross section more than 1 per mil from the mean concentration. Mixing can be assisted by one or more of the following measures: i. Providing an angle (e.g. a 90° angle) along the length between the two T-junctions ii. Bending, e.g. including knotting or meandering, of the mixing tube iii. Periodically or arbitrarily changing the area cross section along the tube iv. Using a carrier gas with a higher interdiffusion coefficient v. Heating the tube vi. Modifying the T-junctions so that the addition of the reference gas is in the middle of the cross section of the mixing tube.

(25) The CO.sub.2 pre-mixture is further mixed with more CO.sub.2-free air (carrier gas) at a second T-junction or flow splitter (52). This is thus a second mixing junction. A second dilution of the reference flows is set to an appropriate fixed ratio (for example 1:30). The flow to the second mixing T-junction is defined by two flow restrictions (7, 8), which in this embodiment ensure a ratio between the pre-mixture and CO.sub.2-free air of 1:30. That is, flow restriction (7) restricts flow of pre-mixture and flow restriction (8) restricts flow of carrier gas. The flow of the pre-mixture is defined by the flow controller (9) and is always higher than 1/30 of the gas flow into the laser spectrometer. This two stage dilution can be due to limitations of the dynamic range of down-mixing in practice. The progressive linearity calibration of the concentration dependence is then performed by use of flow controller (9).

(26) The input pressure of both restrictions (7) and (8) is kept equal at approximately atmospheric pressure by two openings in the form of open tubes or capillaries (6, 14) on the pre-mixture exit line and the carrier gas line respectively. Thus, the rest of the pre-mixture is blown out of the opening (6) positioned after (downstream of) the mixing zone, between the mixing zone (5) and the flow restriction (7). The flow of the CO.sub.2-free air towards the second mixing split is defined by a flow restriction (13) and the constant pressure in the supply (43) of the CO.sub.2-free air. The gas flow at the restriction (13) is always higher than the gas flow to the laser spectrometer. The differential amount of CO.sub.2-free air carrier gas is blown out of an opening (14) on the carrier gas line.

(27) The openings (6, 14) are situated on T (or Y) piece connections (60, 62). The openings (6, 14) are dimensioned such that the gas velocity is always higher than the diffusion velocity of CO.sub.2 in air to avoid contaminations of the reference gases. From the above it can be seen that the reference gas flows are very low and should not be dynamically regulated or actively controlled (i.e. valves 1 and 2 (at locations 3, 4) are typically on/off valves). Thus, the reference gas flows from the reference gas supplies, via valves 1 and 2, are not changed when changing the CO.sub.2 concentration in the spectrometer. Instead, a first dilution of the reference gas flow is dynamically regulated by controlling flow of the zero air (using computer controlled valve (9)). Moreover, the costly conventional open split arrangement is replaced by a simple T (or Y) connection placed after the mixing zone. The opening (6) on the T connection (60) is a length of capillary, and the length of the opening is calculated to balance diffusion versus flow to avoid fractionation and thus change of the isotope ratio of the reference at the open split. The capillary of the opening (6) should not offer a marked restriction, so that the pressure at the T connection (60) is always very close to the atmospheric pressure. This means that the flow rates in this capillary will be not too high. The pressure drop across the opening (6) can be arranged to be 250 mbar or less, especially 50 mbar or less. On the other hand, the flow speed is controlled to be high enough to make sure that back-diffusion of the reference gas against the carrier gas flow is low enough, otherwise it could lead to fractionation (altering of the isotope ratio). An example of dimensions for the capillary of the opening (6) is: 1 mm internal diameter (id), 1 cm length. Generally similar considerations apply to the opening (14).

(28) A first example of parameters for the reference opening is: i. Reference: CO.sub.2 in zero air ii. Minimum flow through open split (6) capillary: 0.5 ml/min (i.e. a flow of at least 0.5 ml/min) iii. Open split capillary diameter: 1 mm iv. Minimum flow to open split: 1 ml/min (i.e. a flow of at least 1 ml/min) v. Loss of flow through open split < 1/1000 of input flow vi. Fractionation <0.3 per meg on mass 46

(29) This fractionation is achieved for all lengths >1 cm.

(30) A second example of parameters for the reference opening is: i. Reference: CO.sub.2 in zero air ii. Minimum flow through open split capillary: 0.5 ml/min iii. Open split capillary diameter: 2 mm iv. Minimum flow to open split: 1 ml/min v. Loss of flow through open split < 1/1000 of input flow vi. Fractionation <0.5 per meg on mass 46

(31) This fractionation is achieved for all lengths >3.7 cm.

(32) In some situations, it might be convenient to block opening (6) against diffusion of contaminating components from the air (e.g. humidity), such as when the reference gas is switched off and when there is no carrier gas flow through the mass flow controller (9). This can be done in two ways for example: the opening can be blocked by a valve (70) as shown in FIG. 4, or a zero air flow (80) can protect the opening (6) from being in contact with the contamination as shown in FIG. 5. It will be appreciated that opening (14) likewise can optionally be provided with a similar blocking valve or zero air flow.

(33) The referencing system is designed to allow dilution of the supplied gases by mixing different gases with each other to change concentrations of the desired gas species (e.g. CO.sub.2 in zero air). It can be seen that by varying the flow of the carrier gas using mass flow controller (9), the concentration of CO.sub.2 reference gas in the CO.sub.2-free carrier gas can be varied. The referencing system allows any concentration of the CO.sub.2 for linearity calibration in the measurement range of the spectrometer from 100-4,000 ppm, such as 200-3,500 ppm. In this way, isotope ratio measurements can be taken in the spectrometer at a plurality of different CO.sub.2 concentrations to enable a concentration dependence of the isotope ratio measurement to be determined.

(34) An exit line (16) takes the flow from the mixing zone into the optical laser spectrometer after the second stage of dilution. The output flow into the optical laser spectrometer is defined by the spectrometer itself and is ideally 80 ml/min. The pressure at the outlet (16) that interfaces to the spectrometer is designed to be around atmospheric pressure.

(35) In the described embodiments, it is possible to switch between the two reference gases with different isotopic ratios. Therefore, the same reference gases used for determination of the concentration dependence (linearity calibration) can be used to perform a delta scale calibration. As the reference gases can be diluted to any concentration, the delta scale calibration can be done at any concentrations or even at more than one concentration to achieve a high accuracy over a wide range. The measured sample should be close to the concentration of the reference gases to avoid linearity effects of the analyzer. For ambient (air) applications, the described setup enables the user also to dilute or mix the reference into the concentration range of the sample to avoid linearity effects of the analyzer. It is also possible to measure the sample with unknown concentration first and afterwards measure the reference with a similar concentration as the sample.

(36) In addition to the referencing system described above, the gas inlet system shown in FIG. 2 further comprises a sample inlet system for introducing a sample gas (i.e. of unknown isotope ratio and/or concentration) into the spectrometer. For simplicity, the sample inlet part of the system in FIG. 2 is shown alone in FIG. 6 and described in more detail below. Sample gas from the sample inlet system (FIG. 6) and reference gas from the referencing system (i.e. shown in FIGS. 3 to 5) can be periodically supplied to the measurement cell. In this way, the software controlled valve switching allows intermittent injection of reference gas for quality assurance and/or calibration of the spectrometer.

(37) In the arrangement shown in FIG. 6, the flow of CO.sub.2-free air, which is defined by a fixed flow restriction (13), is mixed with a CO.sub.2 sample flow coming from a sample inlet port (12) to which is connected a supply of sample gas (CO.sub.2). In this embodiment, the supply of CO.sub.2-free air for the sample flow is the same supply (40) as used for dilution of the reference gases in the referencing system described above.

(38) The sample inlet system according to the disclosure ensures 100% sample transfer from the sample input port ((12) in FIG. 6) to the laser spectrometer (16). A constant flow to the laser spectrometer is ensured by filling the differential volume between the sample flow and the flow to the laser spectrometer with a carrier or dilution gas, which in this case is CO.sub.2-free air (zero air). No sample is wasted and the concentration of CO.sub.2 in the gas flow to the laser spectrometer is kept constant and in the optimal range. In some embodiments, using a variable flow of CO.sub.2-free air, any sample vials with a variable CO.sub.2 concentration over time can be flushed out through the sample inlet port to ensure the concentration of CO.sub.2 in the gas flow to the laser spectrometer is kept constant and in the optimal range.

(39) The samples are frequently only available in minute quantities. Thus, the sample inlet system must ensure that little or no sample is lost. This is achieved in the embodiment shown by means of the open split in the form of the opening (14) constructed with the T-junction technology as described above. The sample from inlet (12) is introduced via its gas line into the stream of the carrier gas after (i.e. downstream of) the open split (14) at sample introduction point (64), which is also termed herein an analyte-carrier junction. Such sample introduction point is also a T-junction. This is in contrast to the prior art open splits where sample is introduced into a carrier or dilution gas at the open split itself such that significant sample loss occurs. The T-junction sample introduction and separate T-junction opening to atmosphere not only preserve the sample in the carrier without sample loss but the T-junction construction is simpler to manufacture and more robust in use.

(40) It is required that the sample flow (12) must not be identical or very close to the flow at the output (16). As long as the sample flow is lower than the flow at the output (16) into the laser spectrometer, the flow between the opening (14) and the sample introduction point (64) is always directed towards the laser spectrometer (and if the sample flow is sufficiently lower than the flow at the output (16) the CO.sub.2 concentration at (14) is close to zero). Diffusion of CO.sub.2 sample backwards is thereby prevented by the relative magnitudes of the flow rates. This guarantees a 100% sample transfer from the sample inlet port (12) into the laser spectrometer. The sample, however, is diluted and the dilution factor is defined by the sample flow rate. That is, when the sample flow is lower than the flow at the output (16), the concentration of sample at (16) is lower than that of the sample inlet flow (12). The difference between the flow to the laser spectrometer (16) and the sample input flow (12) is balanced by the CO.sub.2-free air at (13). The excess CO.sub.2-free air is blown out through the opening (14) on the carrier gas line. If the sample inlet flow (12) is higher than the flow at the output (16), which is also a workable embodiment, the concentration at opening (14) represents a mixture of the concentration at sample inlet (12) and that at the supply of CO.sub.2-free air (40) (which is zero concentration by definition). In all cases, the CO.sub.2 isotope ratios at (12), (14) and (16) are substantially identical, which is significant.

(41) An additional three port valve (valve V4) in the sample inlet (12) is used to be able to flush the sample line through inlet port (20). In operation of the sample inlet system, the carrier gas flow is only provided through restriction (13), not through variable flow controller (9).

(42) The opening (14) opens to atmosphere in the same way as opening (6) described above, and the distance between the T-junction with the opening (14) and the T-junction with the sample introduction point (64) is selected such that substantially no back-diffusion into the opening (14) occurs. In particular, the flow rate and line length and cross section are such that substantially no back-diffusion occurs. Thus, only the inexpensive dilution gas is wasted. The considerations for the line (15) between both T-junctions are the similar to the reference gas split. The flow into the analyzer is relatively high at typically 100 ml/min. In various embodiments, the minimum flow to the open split (opening (14)) should be 10 ml/min. Furthermore, it should be ensured that the flow through the open capillary (14) is always at least 0.5 ml/min. In various embodiments, a length of the open capillary (14) is at least 5 mm or 6 mm, such as at least 10 mm, and an internal diameter can be at least 1.0 mm, e.g. 1.3 mm. The loss of flow through the open split is then typically < 1/1000 of the input flow and there is little or no fractionation effect. The pressure drop across the opening (14) can be arranged to be 250 mbar or less, especially 50 mbar or less.

(43) An example of parameters for the sample inlet opening (14) is: i. Sample: CO.sub.2 gas in zero air (e.g. dilution of sample:carrier=1:1 to 1:0). ii. Minimum flow through open split capillary: 0.5 ml/min (i.e. the flow should be at least 0.5 ml/min) iii. Open split capillary diameter: 1 mm iv. Minimum flow to open split: 10 ml/min (i.e. the flow should be at least 10 ml/min) v. Loss of flow through open split < 1/1000 of input flow vi. Fractionation <0.3 per meg on mass 46

(44) This fractionation is achieved for all lengths >6 mm.

(45) In the case of interfacing a gas chromatography (GC) system to an isotope ratio mass spectrometer (IRMS), an example of parameters for the inlet opening is: i. Gas: Hydrogen in He ii. Minimum flow through open split capillary: 0.5 ml/min iii. Open split capillary diameter: 0.5 mm iv. Minimum GC flow to open split: 0.8 ml/min v. Loss of flow through open split < 1/100 of input flow vi. Fractionation <60 per meg (0.06 per mil) for the H.sub.2/HD ratio

(46) This fractionation is achieved for all lengths >1.52 cm.

(47) The gas line downstream of the sample introduction point is a mixing zone for the sample CO.sub.2 in air and can be subject to similar considerations of design parameters as the mixing zone (5) of the reference section described above. Moreover, it can be improved as a mixing zone by any of the measures described above in relation to the mixing zone (5) of the reference section of the system.

(48) The system can be realized as a compact, robust system. One such embodiment is shown in FIG. 7, where the same reference numerals are used to denote the same components as shown in FIG. 2. A part of the system is provided in a machined metal block, which is denoted as the valve block and indicated schematically by the dotted line in FIG. 2 with the block itself shown in FIG. 7. In the shown embodiment, the valve block is made of metal, but a suitable polymer (e.g. PEEK) or other suitable material could be used to make it as well. It is also possible to make the block comprising of several multilayers bonded together with buried channels. Valves V1-V6 are housed in the valve block. Valves V1 (3/2) and V2 (2/2) are for switching the entry of the reference gases (41, 42) into the system as described above; valve V3 (3/2) is for switching flow of the carrier gas from the mass flow controller (9) to direct it to the mixing zone (5), or to a separate outlet (10); valve V4 (3/2) is for switching the flow of sample gas from the sample inlet (12) into the carrier gas and the spectrometer; valve V5 (2/2) is for switching flow of the diluted reference gas into the analyzer from the mixing zone (5); and valve V6 (2/2) is for switching flow of the carrier gas along the line that is not controlled by the mass flow controller. The valve block structure is shown in more detail in FIG. 8, where the same reference numerals are again used. The positions of the valves in the block and some of the connecting flow channels are shown. The channels are typically drilled in the block. The dimensions indicated in FIG. 8 are in mm. The flow restrictions in the system, such as restrictions (1, 2, 7, 8 and 13) can each be provided as a metal capillary or as metal capillary having a crimp and the openings to atmosphere (6,14) are provided simply as T junctions using capillaries. Thus, the system can be assembled largely from stock components.

(49) It can be seen from the description herein that the gas inlet system is a compact device to deliver sample and reference gas to an isotope ratio analyzer, especially an optical spectrometer. The main target is to allow comparative measurement of isotope ratios of a sample and one or more reference gases. The concentrations of the sample and reference gas should be matched to one another and to the optimum concentration range suitable for the isotope ratio analyzer used. In embodiments of the disclosure comprise two functional units: a reference section and a sample inlet section. Various embodiments are designed to allow dilution of the supplied gases by mixing different gases with each other to change concentrations of the desired gas species (e.g. CO.sub.2 in air). Software controlled valve switching allows intermittent injection of reference gas for quality assurance and/or calibration of the spectrometer. The gas inlet system with the analyzer to which it is interfaced can be dedicated to analyze the isotope ratio of .sup.13C/.sup.12C and/or .sup.18O/.sup.1O from CO.sub.2 in air or other carrier gases (e.g. N.sub.2), or it can be designed to analyze these or other isotope ratios of other gases.

(50) Using the system shown in the Figures, an isotope ratio of a sample may be measured and corrected for concentration dependence of the spectrometer. The concentration dependence may be determined by selecting a reference gas (e.g. Ref 1 (41)), which has a first known isotope ratio, and measuring in the laser cell the isotope ratio of the reference gas with the first known isotope ratio at a plurality of concentrations in a carrier gas, wherein the concentrations can be varied as described above by using the referencing system shown in FIGS. 2 and 3. From the measurements at the plurality of concentrations, a concentration dependence of the isotope ratio measurements of the reference gas is determined (e.g. from a plot of isotope ratio against concentration). A sample gas of unknown isotope ratio and unknown concentration may be admitted into the system using the sample inlet system shown in FIGS. 2 and 6 and its isotope ratio and concentration measured in the laser cell. The isotope ratio measurement of the sample gas may be corrected by the determined concentration dependence.

(51) Furthermore, using the system shown in the Figures, the isotope ratio of the sample may be corrected by an isotope ratio calibration (or delta scale contraction) of the spectrometer. This method includes selecting the reference gas (e.g. Ref 2 (42)), which has a second known isotope ratio, and measuring in the laser cell the isotope ratio of the reference gas with the second known isotope ratio, such as at one or more concentrations in the carrier gas that lie within the range of measured concentrations of the reference gas with the first known isotope (Ref 1). An isotope ratio calibration can then be determined from the measured isotope ratios of the reference gas with the first and second known isotope ratios and the measured isotope ratio of the sample gas can be further corrected by the determined isotope ratio calibration.

(52) As used herein, including in the claims, unless the context indicates otherwise, singular forms of the terms herein are to be construed as including the plural form and vice versa.

(53) Throughout the description and claims of this specification, the words “comprise”, “including”, “having” and “contain” and variations of the words, for example “comprising” and “comprises” etc., mean “including but not limited to”, and are not intended to (and do not) exclude other components.

(54) It will be appreciated that variations to the foregoing embodiments can be made while still falling within the scope of the disclosure. Each feature disclosed in this specification, unless stated otherwise, may be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

(55) The use of any and all examples, or exemplary language (“for instance”, “such as”, “for example” and like language) provided herein, is intended merely to better illustrate embodiments of the disclosure and does not indicate a limitation on the scope of the disclosure unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

(56) Any steps described in this specification may be performed in any order or simultaneously unless stated or the context requires otherwise.

(57) All of the features disclosed in this specification may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. In particular, the features of the disclosure can be applicable to all aspects of the disclosure and may be used in any combination. Likewise, features described in non-essential combinations may be used separately (not in combination).