GB2513120A

GB2513120A - Method of determining an isotope ratio

Info

Publication number: GB2513120A
Application number: GB1306808.5A
Authority: GB
Inventors: Eric Wapelhorst; Hans-Jurgen Schluter; Oliver Kracht; Johannes Schwieters; Michael Krummen
Original assignee: Thermo Fisher Scientific Bremen GmbH
Current assignee: Thermo Fisher Scientific Bremen GmbH
Priority date: 2013-04-15
Filing date: 2013-04-15
Publication date: 2014-10-22
Anticipated expiration: 2033-04-15
Also published as: GB201306808D0; GB2513120B

Abstract

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. The sample gas is preferably diluted in the carrier gas prior to measuring its isotope ratio.

Description

Method of determining an isotope ratio

Field of the invention

This invention generally relates to gas inlet systems for isotope ratio analysers, to said analysers incorporating said inlet systems and to methods of isotope ratio measurement.

Background of the Invention

Isotope-ratio analysis is used to measure the relative abundance of isotopes (isotope ratio) in a gaseous sample. For instance, it is used for determining the isotope ratios 13C/12C and/or 18Q/16Q from CO2 e.g. in air.

Isotope-ratio analysis is most commonly performed by mass spectrometry (MS) but may also be performed by optical spectrometry.

Gas inlet systems for isotope ratio analysis are well known, especially for use with isotope ratio mass spectrometers. A general review of isotope ratio mass spectrometry and gas inlet systems can be found in Brenna et a!, Mass Spectrometry Reviews, 1997, 16, 227-258. Isotope-ratio mass spectrometry usually comprises comparative measurements of isotope ratios of a sample gas and of one or more reference gases of known isotope ratio.

Accordingly, isotope ratio MS (IRMS) typically requires at least one sample gas inlet and at least one reference gas inlet.

A popular solution for gas flow management is the so called open split, which comprises a mixing region that is open to the atmosphere. Gases to be analysed emerge from a line into the mixing region and whilst a large proportion of the gases is lost to the atmosphere as excess a small amount is transferred to a further line. The open split thus vents gas flow in excess of that acceptable by the isotope ratio analyser. In isotope ratio MS an example of an open split design is known from US 5,424,539, wherein the open split comprises a small glass vial open to the atmosphere with various sample, reference and carrier gas capillaries ending in the vial and a further capillary sampling the mixing region within the vial. The carrier gas is used to dilute the various sample and reference gases to achieve a desired concentration for analysis. However, the design is not as robust as would be desired and excess sample gas is lost from the system in the open split prior to analysis. A significant amount of carrier gas also has to be used. A similar open split design is described in US 5,661,038 and shown in Tobias et 8/, AnaL Chem. 1996, 68, 3002-3007. The open split concept has been refined for improved performance and automation, for example as shown in US 7,928,369 and WO 20071112876, wherein the capillaries for supplying gases are provided with drives for movement in and out of the mixing zone. In all, such open splits as described, essentially comprising an array of nested capillaries, are not simple to manufacture and can lack reproducibility in production as well as robustness in use.

A type of open split is used in gas chromatography MS (GCMS) for pressure adaption, although it is not used in isotope ratio MS. Such an open split is the Open-Split Capillary Interface Part No. 113532 from SGE (www.sge.com).

Gas inlet systems configured for autodilution of samples using an open split are also known in the form of the Thermo Scientific GasBenchTM and Thermo Scientific C0nFI0TM interfaces for isotope ratio MS (wwwthermoscientifIccom).

As mentioned above, isotope ratio MS (IRMS) typically requires at least one sample gas inlet and at least one reference gas inlet. The attained measurement precision is typically about 0.05% and accuracy is derived from that precision by use of the reference gas. However, in isotope ratio optical spectrometry (IROS) there is not currently offered an equivalent effective solution for reference and calibration. A system for calibrating the isotope ratio measurements to account for concentration dependence and a delta scale contraction is described in B. Tuzson et a!, High precision and continuous field measurements of 513C and 518Q in carbon dioxide with a cryogen-free QCLAS, AppI. Phys. B (2008), Volume 92, pp 451-458. However, a drawback with the system described in Tuzson eta/is that it utilises a significant number of separate diluted supplies of reference gases of known isotope ratio. Such reference gas/air mixtures are not commonly available when working in the field for example. Furthermore, the system described Tuzson et a/ does not employ a sample dilution.

Isotope ratio optical spectrometers differ in several aspects from isotope ratio mass spectrometers: IROS requires a higher inflow of sample; an IROS system overall is more compact, which makes it transportable, but which in turn requires extra ruggedness, and this also applies to the gas inlet system; the IROS market is more price sensitive, necessitating simple low-cost solutions. As a consequence, conventional IRMS gas inlet systems are not first choice for use as IROS inlet systems; and a cheaper, more compact and simpler system designed for higher flow rates is required for IROS measurements.

From this background it can be seen that it would be desirable to provide a gas inlet system for an isotope ratio analyser that is compact and robust, easy and cheap to manufacture, and furthermore allows comparative measurement of isotope ratios of a sample and one or more reference gases.

It would also be desirable in an inlet system to reduce the loss of excess sample gas that occurs with conventional open split configurations.

The invention has been made against this background in order to try to alleviate one or more of the aforementioned problems as well provide one or more additional advantages as hereafter described.

Summary of the Invention

According to a first aspect of the invention there is provided a gas inlet system for introducing gas into an isotope ratio analyser, comprising: a supply ot 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 junction is further connected to an exit line for transporting the mixed gas from the junction to the isotope ratio analyser, and wherein the junction is positioned downstream of an opening on the carrier gas line. The junction and the opening on the carrier gas line are arranged whereby when the flow rate of the analyte gas in the analyte line is arranged to be lower than the flow rate of gas into the isotope ratio analyser the flow between the opening and the analyte-carrier junction is always towards the isotope ratio analyser, and whereby when the flow rate of the analyte gas in the analyte line is arranged to be higher than the flow rate of gas into the isotope ratio analyser the flow between the opening and the analyte-carrier junction is always towards the opening. Accordingly, the gas inlet system preferably is configured in one mode of operation 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 analyser. Accordingly, the gas inlet system may be configured in another mode of operation 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 analyser. It is preferred in any case that the gas inlet system is configured such that the flow rate of the analyte gas in the analyte line is not substantially identical to the flow rate of gas into the isotope ratio analyser.

Also provided is an isotope ratio analyser comprising the gas inlet system according to the first aspect of the invention.

Diffusion of the analyte gas backwards, i.e. upstream, to the opening is restricted by the magnitudes of the flow rates and the distance from the analyte-carrier junction to the opening. The first aspect of the invention thus provides a substantially complete analyte gas transfer from the analyte gas supply into the isotope ratio analyser, i.e. with substantially no loss of analyte gas through the opening, which instead carries away only excess carrier gas, preferably to the atmosphere. The carrier gas is typically less valuable (usually cheaper) than the analyte gas and therefore it can afford to be wasted more. In other words, the analyte-carrier junction and the opening and the distance between are configured such that when the flow rate of the analyte gas in the analyte line is arranged to be lower than the flow rate of gas into the isotope ratio analyser the flow between the opening and the analyte-carrier junction is always towards the isotope ratio analyser.

The system preferably comprises a supply of analyte gas to supply analyte gas to the analyte gas line. The analyte gas may be a sample gas (e.g. of unknown isotope ratio and/or unknown concentration, which are to be measured) or a reference gas (e.g. of known isotope ratio for calibration of the analyser). Preferably, the analyte gas is a sample gas. The system also comprises a supply of carrier gas to supply carrier gas to the carrier gas line.

The carrier gas supply is preferably substantially free from the analyte gas. A preferred embodiment is wherein the analyte gas is CO2 and the carrier gas is a gas substantially free from 002, e.g. C02-free air or C02-free nitrogen (N2).

The analyte and carrier gas lines respectively transport the flow of analyte and carrier gas from their supplies towards the analyser. In some embodiments there may be more than one analyte gas supply and/or more than one analyte gas line. In some embodiments there may be more than one carrier gas supply and/or more than one carrier gas line.

According to a second aspect of the invention there is provided a gas inlet system for introducing gas into an isotope ratio analyser, the gas inlet system including a reference system comprising: at least a first supply of a reference gas having a first known isotope ratio; a supply of a carrier gas (preferably the same supply of a carrier gas as may supply the carrier gas line in the first aspect of the invention), the carrier gas preferably being free of reference 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, preferably wherein the carrier gas flow to the first mixing junction is controllable by flow control means; 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.

The reference (also termed calibration) system preferably further comprises at least a second supply of a reference gas having a second known isotope ratio. In preferred embodiments, the chemical composition of the first and second reference gas is the same (e.g. both may be Ca2) and they differ only in the isotope ratio. The first and second supplies of reference gas may each be independently (e.g. alternately or not simultaneously) connected to the first mixing junction for mixing with the carrier gas as required. The first mixing junction is preferably a I-junction. Herein the term T-junction means any junction of three flow channels, i.e. it has three arms. It may be provided as a 1-piece, or Y-piece, or a junction of three orthogonal channels, and may be either two-dimensional wherein the three channels lie in the same plane or three dimensional (e.g. in the form of a three-dimensional "tripod" configuration) wherein the three channels do not all lie in the same plane.

Herein, a mixing junction where two gases meet and are mixed is preferably not open to the atmosphere, in contrast to prior art systems where mixing occurs at open splits. This enables the critical flows for mixing to be under full control.

The opening is preferably open to atmosphere. The opening to atmosphere ensures that the flow in the exit line does not exceed that which can be handled by the isotope ratio analyser. The opening is preferably in the form of an open capillary. The opening is preferably situated on a junction (opening junction), such as a T-junction, positioned downstream of the mixing zone. This opening should not offer a marked restriction to the gas flow so that the pressure at the junction with the opening is always very close to the atmospheric pressure. This means that the flow rates through this opening will not be too high. The pressure drop across the opening preferably is arranged to be 250 mbar or less, especially 50 mbar or less. On the other hand, the flow rate through this opening should be high enough to make sure that back-diffusion of the reference gas against the carrier gas flow is small enough; otherwise this would lead to fractionation (altering of the isotope ratio). As an example, the opening in the form of an open capillary may be at least 0.5mm, e.g. 0.5mm to 2mm, in internal diameter and preferably at least 5 mm, or at least 10 mm in length. A preferred flow rate to the junction with such open capillary is at least 1 mI/mm and a preferred flow rate through such open capillary is at least 0.5 mI/mm. The loss of flow through the open split is then typically < 1/1 000 of input flow.

The mixing zone, which typically comprises a length of capillary between the first mixing junction and the opening junction, may be a straight capillary but preferably includes one or more bends and/or includes an angle between the two junctions (e.g. 90 degrees). Mixing within the mixing zone may be increased in this way. In addition or alternatively, the mixing zone may comprise one or more changes of internal diameter (internal cross section area). Other means to improve mixing in the mixing zone include: use of carrier gas with high interdiffusion coefficient, heating the mixing zone, modifying the junctions so that the addition of the reference gas is in the middle of the cross section of the mixing zone capillary, bends in the mixing zone and/or the use of baffles or other mixing devices (passive or active).

The flow control means to control carrier gas flow to the first mixing junction enables controlled dilution of the reference gas so that measurements of isotope ratio of the reference gas may be made by the analyser at a plurality of different concentrations. The flow control means enables the carrier gas flow to be regulated or dynamically controlled, e.g. wherein the flow control means comprises a mass flow controller or proportional valve.

The flow control means may be any flow controller or regulated valve. The flow control means may be, for example, a mass flow controller or proportional valve, or volume flow controller, or a switchable combination of fixed flow restrictions allowing the flow to be adjusted in discrete steps as described in US 7,928,369 and WO 2007/112876. The flow control means may be automatically or manually operated. The flow control means may comprise at least one automatic or manual pressure regulator combined with at least one flow restriction downstream of the pressure regulator. The flow control means may be an automatic, electronic or digital flow controller, for example as described in WO 2007/112876. One example of a flow control means is the C0nFI0IVTM from Thermo ScientificTM.

The calibration system preferably comprises two stages of dilution of the reference gas with the carrier gas. Preferably, only one of the dilution stages has the flow of carrier gas dynamically regulated, e.g. by mass flow controller. More preferably, the first stage of dilution has the flow of carrier gas dynamically regulated. In preferred embodiments, the calibration system further comprises a second mixing junction on the exit line from the mixing zone wherein further carrier gas can be mixed with the already mixed gas. In this case, the gas emerging from the mixing zone after a first stage of dilution is a pre-mixture of reference gas and carrier gas, which is further diluted with additional carrier gas in the second stage of dilution. The second mixing junction is preferably downstream of the opening on the exit line. The carrier gas supplied to the second mixing junction is preferably from the same supply of carrier gas that supplies the first mixing junction, i.e. there is preferably only one supply of carrier gas. The same supply of carrier gas is preferably also to supply the carrier gas in the first aspect of the invention. The carrier gas is preferably transported to the second mixing junction by a second carrier gas line. The carrier gas supplied to the second mixing junction is preferably not regulated or dynamically controlled, i.e. a mass flow controller or proportional valve is not used in that case. Thus, dilution of the reference gas is preferably regulated by the control of carrier gas flow to the first mixing junction. In some embodiments it may be possible to regulate dilution of the reference gas by dynamic control of the carrier gas flow to the second mixing junction instead of to the first mixing junction although this is less preferred.

The exit line from the mixing zone preferably has a flow restriction upstream of the second mixing junction, but preferably situated downstream of the opening junction. The second carrier gas line preferably also has a flow restriction upstream of the second mixing junction. These two flow restrictions ensure a flow ratio between the flow of the pre-mixture and the flow of the carrier at the second mixing junction. The flow ratio (pre-mixture:carrier) may be for example about 1:30. In the case of a CO2 reference gas, the CO2 concentration of the pre-mixture is preferably in the range from about 13,000 ppm down to 4,000 ppm (parts per million). In the diluted gas that enters the analyser (i.e. after second dilution where a second dilution is used) the CO2 concentration is preferably in the range between about 200-4000 ppm, preferably 200-3,500 ppm (CO2 in air or CO2 in nitrogen concentration), with the range between about 200-1500 ppm being more optimal. This is optimized for measuring ambient CO2, which is typically at least 400 ppm. These concentration ranges are especially applicable for a measurement cell of an analyser that is a laser cell for performing laser absorption measurements on the gas. These concentration ranges may also apply to some other analyte gases, optionally with some variation. For gases other than C02, e.g. methane, it is desirable to adjust the system for the optimum concentration of the gas to be measured. In general, the preferred concentration of gas that enters the analyser will depend on the application and factors such as the type of analyte gas, the strengths of absorption lines, and the sensitivity of the analyser.

Preferably, the supply of reference gas that is supplied to the apparatus of the invention or used for the method of the invention is a pure gas, or is diluted by a factor not more than about 2, or not more than about 5, or not more than about 10.

The invention thus provides a system capable of allowing determination of a concentration dependence of the isotope ratio measurement (linearity calibration), especially relevant for isotope ratio optical spectrometry. In further preferred embodiments, it can further provide determination of an isotope ratio dependence of the isotope ratio measurement (delta scale contraction).

According to a third aspect of the invention there is provided 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.

In a preferred embodiment, 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, preferably 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.

Preferably, the sample gas is also diluted in the carrier gas prior to measuring the isotope ratio. Preferably, the sample gas is also diluted to a concentration in the carrier gas that lies within the range of concentrations of the reference gas with the first known isotope. The sample gas may be diluted using the system according to the first aspect of the present invention. In some cases, the sample gas may not be diluted in the carrier gas prior to measuring the isotope ratio, i.e. it may be used as supplied.

The sample gas may have a range of concentrations. The reference gas preferably is diluted to at least a concentration in the carrier gas that lies within the range of concentrations of the sample gas.

The reference gas is preferably provided as a substantially pure gas and the carrier gas is preferably provided as substantially free from the reference gas. Where the analysis is of C02, the reference gas is CO2 and the isotope ratio is preferably the ratio 13C/12C or 180/160, especially 13C/12C. The carrier gas therefore is preferably a COrfree gas such as C02-free air or C02-tree N2. The sample gas therefore will also comprise CO2 to be measured. Other gases that may be measured for their isotope ratios and therefore used as a reference include, for example: CH4, C2H6, CXH(2X+2) where x=integer, water vapour (e.g. in ambient air), CO, small hydrocarbons, alcohols, aldehydes, N0, N0 where x=1, 2 and y= 1, 2, 3, 4 or 5 (e.g. NO, NO2 or N2O), H2S, nitrogen, oxygen, or hydrogen, or other gases.

A list of some examples of analyte gases and some of their measurable isotope ratios is given below.

Gas Isotope ratio C02, CO: 13C/12C 180/160 1716 CH4, other alkanes: 13C/12C N20: a-15N114N 13-15N14N 1801160 170/160 N0: 16N/14N 180/160 1701160 The carrier gas may be selected from: air, nitrogen, helium or argon, or mixtures of any two or more of the foregoing.

Advantageously, a single supply of reference gas with the first known isotope ratio is preferably employed, which is dynamically diluted with carrier gas to provide the plurality of concentrations used for measuring the isotope ratio and determining the concentration dependence of the isotope ratio. This removes the need for a plurality of prepared reference gases at different known concentrations.

Preferably, the dilution of reference gas comprises two stages of dilution, for example in the manner of the calibration system described.

Preferably, the supply of reference gas that is supplied to the apparatus of the invention or used for the method of the invention is a pure gas, or is diluted by a factor not more than about 2, or not more than about 5, or not more than about 10. Such supply of reference gas is then preferably subjected to the mentioned dynamic dilution, especially by the mentioned two stages of dilution.

Preferably, the measurements of the isotope ratio of the reference and sample gas are made using an optical spectrometer, but may be made using a mass spectrometer, or other spectrometer or measurement device.

The method of the third aspect may be performed using the systems of the first and second aspects.

The flow of the carrier gas in the third aspect is preferably at least partially regulated or dynamically controlled by the flow control means. The flow control means preferably comprise a mass flow controller or proportional valve, which is further preferably computer controlled (i.e. under the control of software). In this way, the dilution of the reference gas can be controlled. The dilution of reference gas comprises two stages of dilution with carrier gas.

Preferably, a flow of the carrier gas in a first stage of dilution of the reference gas is dynamically controlled by flow control means. Preferably, a flow of the carrier gas in a second stage of dilution is not dynamically controlled.

Preferably, the method further comprises connecting the reference gas and carrier gas by respective reference and carrier gas lines to a first mixing junction where the reference gas and carrier gas combine; mixing the combined reference gas and carrier gas in a mixing zone downstream of the first mixing junction; transporting the mixed gas from the mixing zone to the isotope ratio analyser along an exit line; and providing an opening to atmosphere on the exit line downstream of the mixing zone.

Preferably, the sample gas is diluted with the carrier gas at a junction, wherein the junction is further connected to an exit line for transporting the mixed gas from the junction to the isotope ratio analyser, and wherein the junction is positioned downstream of an opening to atmosphere on a carrier gas line, whereby the flow rate of the analyte gas to the junction is arranged to be lower than the flow rate of gas into the isotope ratio analyser whereby the flow between the opening and the junction is always towards the isotope ratio analyser.

Preferred features and further details of the invention will now be described.

The sample gas and/or reference gas is preferably CO2. The isotope ratio to be measured for the sample gas and the reference gas is thus preferably 13C/12C and/or 180/160. The reference gas is preferably a pure gas, e.g. pure CO2. This compares to the use of reference gases pre-diluted at a known concentration in prior art calibration systems. The dilution from a pure gas source enables complete flexibility in setting the concentration of the gas admitted to the analyser. Advantageously, a commercial-size (e.g. 10 litre of C02) bottle of pure reference gas, which can be diluted with the carrier gas, can last a very long time, in some cases perhaps for the working lifetime of the analyser. Thus, changing bottles of reference gas can be hardly ever required. The carrier gas is conversely preferably reference gas-free, e.g. for CO reference gas, the carrier is preferably a C02-tree gas, e.g. C02-free air or CO2-free N2. Advantageously, such carrier gas can be generated in the field.

Herein the term gas line refers to any channel, conduit, tube, capillary or the like for transporting gas. Situated on a line may be any number of devices, for example devices such as junctions, valves, flow restrictions, flow controllers, gauges and the like.

The junctions mentioned herein are preferably T-junctions as defined above and/or preferably one or more junctions and one or more portions of the gas lines are provided in a machined block, i.e. in one mechanical piece.

In other words, manufacturing of at least part of the system can be by machining out of a bulk material (e.g. a metal block), which allows better reproducibility in production. Such construction provides improved rigidity to the system and allows integration of the opening (open split) and other parts of the gas inlet into one mechanical piece. Using 1-junction configurations at the junctions with openings, with or without manufacture in a machined block, ensures that the critical flows through the openings are under full mechanical control (in the prior art the open split was conventionally provided as an array of nested tubes in an open vial), which in turn allows control over the mathematics of the flow and management of fractionation processes at such open splits'. The 1-junction design enables well separated diffusion paths, which facilitates calculating the system because the flow properties are well determined In another aspect, the present invention provides a gas inlet system for an isotope ratio analyser comprising: at least a supply of each of a reference gas, a sample gas and a carrier gas for diluting the reference gas or sample gas, one or more 1-junctions for allowing the carrier gas to dilute the reference gas or sample gas, and one or more separate 1-junctions having an opening to atmosphere (i.e. one of the arms of the T-junction opens to atmosphere).

In a further aspect, the present invention provides a gas inlet system for an isotope ratio analyser, comprising: at least a supply of each of a reference gas, a sample gas and a carrier gas for diluting the reference gas or sample gas, one or more junctions for allowing the carrier gas to dilute the reference gas or sample gas, and one or more junctions having an opening to atmosphere, wherein at least some gas flow channels for the gases (and/or the junctions) are provided as machined channels in a mechanical block. The one or more junctions having an opening to atmosphere are separate to the one or more junctions allowing the carrier gas to dilute the reference gas or sample gas. Where the gas flow channels extend outside of the mechanical block they are preferably provided as capillaries, e.g. fused silica, or metal, or polymer (e.g. PEEK) capillaries.

Thus the gas inlet system of the present invention is one which can be provided in a compact and robust form, especially where junctions between gas lines are provided as 1-junctions, compared to the prior art open splits where the junction between flows, especially for dilution purposes, is provided in the form of an open vial. Providing at least part of the gas lines in a machined block is another advantage for system robustness and reproducibility in manufacture.

Using the present invention, the concentration of the sample gas and reference gas can be matched to one another and to the optimum concentration range suitable for the isotope ratio analyser used.

The invention also provides numerous improvements to so-called open splits for isotope ratio analyser gas inlets, especially for isotope ratio optical spectrometers. Further advantages of the invention include that arranging an opening on a carrier (dilution) gas line upstream of where a sample gas line joins the carrier gas line can provide for little or no sample loss, and an addition of only a minimum of carrier gas. In addition, an open split can be configured such that only a minimum of a reference or sample gas is split away from the flow into the isotope ratio analyser.

The gas inlet system can also be used with an isotope ratio analyser for concentration measurements and a calibration (linearity calibration) for the concentration dependence of isotope ratio measurements can be determined and used for correction of sample measurements.

The isotope ratio analyser may be any type of analyser capable of measuring an isotope ratio, especially an isotope ratio mass spectrometer or an isotope ratio optical spectrometer but the invention is particularly advantageous in the case of an isotope ratio optical spectrometer (i.e. optical absorption spectrometer), especially a laser spectrometer (i.e. laser absorption spectrometer). Such spectrometers typically operate in the infrared region, more preferably the mid-infrared region (e.g. 2.Spm -6pm).

Accordingly, preferably the isotope ratio analyser is an isotope ratio optical spectrometer, moreover preferably comprising a measurement cell for performing optical absorption measurements on the gas to be analysed. More preferably, the measurement cell is a laser cell for performing laser absorption measurements on the gas to be analysed. The laser cell is preferably a multi-pass cell. The optical pathlength in the cell provided by the sum of the multiple laser passes may be in the ranges 1 to 1o5 m, or 1 to i04 m, 1 to i03 m or 1 to 102 m, or 1 to 10 m. A single measurement cell is preferably employed for measurement of both reference and sample gases. Such laser cell is typically pumped by a pump on the outlet from the cell. A preferred laser measurement cell satisfies the inequality l*L*C C 1.6*1 012, where: = Spectral intensity in cm L= length of the optical path in cm C= Concentration of the measured gas in ppm.

The isotope ratio is generally determined in such an analyser by measuring two (or more) separate spectral absorption lines, typically in the infrared region, at least one line for each different isotopic species (isotopologue), e.g. a line for 12C1602 and another line for 13C1602. A preferred absorption line for CO2 is the line at or about 4.329pm. 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 13C/12C in this case (which can be expressed as the established delta notation, 613C)). In general, the isotope ratios herein can be expressed as the established delta notation (6).

The isotope ratio analyser may be configured for measuring the isotope ratio 13C/12C and/or 180/160 from CO2. It will be appreciated that it is also suitable for measuring other isotope ratios such as H/D in CH4 analysis as well as referencing isotope ratios (e.g. 13C/12C or 180/160 or 15N14N) in other gases such as N20 or CO for example. The types of other gases and their isotope ratios that can be analysed by the present invention are not particularly limited and further examples are described elsewhere herein. It will be appreciated that for other gases, a suitable adjustment of the gas concentration and flow rate may be required depending on the nature of the measurement and the sensitivity of the analyser. Herein, measuring the isotope ratio13C/12C and/or 180/160 (or other isotope ratio) may comprise determining the ratio specifically or a quantity representative of the ratio (e.g. the delta value).

The above features are further described below, along with other details of the invention, with reference to the Figures.

List of Figures Figure 1 shows a schematic layout of an isotope ratio optical spectrometer interfaced to a gas inlet system in accordance with the present invention.

Figure 2 shows a schematic layout of a gas inlet and referencing system in accordance with the present invention.

Figure 3 shows a schematic layout of the referencing section of the system shown in Figure 2.

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

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

Figure 6 shows a schematic layout of the sample inlet section of the system shown in Figure 2.

Figure 7 shows an embodiment of a system in accordance with the schematic layout of Figure 2.

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

Detailed description of embodiments of the invention In order to assist further understanding of the invention, but without limiting the scope thereof, various exemplary embodiments of the invention are now described with reference to the Figures.

Referring to Figure 1, there is shown schematically an isotope ratio optical spectrometer (100) interfaced to a gas inlet system (120) in accordance with the present invention. 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 5.4m. 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 mI/mm 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) or preferably 40-200 mbar(a) or more preferably 40-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, preferably 200-3,500, ppm of 002 in air or in N2 with highest performance of detection between 200-1 500 ppm, especially 300-1,500 ppm of CO2.

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 12C1602 and another line for 13C1602. A convenient absorption line for 002 is the line at or about 4.3218pm. 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 CO2 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. 13C/12C).The outputs of the spectrometer are thus ratios of different isotopic lines (e.g. R13c c13jc120). The result is referenced against international standards using the established delta notation for isotope ratio reporting (e.g. 513C [%o]). The means for performing calibrations, which are required to calculate a 5-value from a ratio of spectral intensities, are described below.

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 invention as described in more detail below (see Figure 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.

The apparatus and method of the invention is illustrated in the embodiments below with the example of a CO2 analysis system (i.e. CO2 as sample and reference gas) but it should be appreciated that the invention 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 CO2 but will the same gas as the particular sample gas being analysed. Similarly, the apparatus and method of the invention is illustrated in the embodiments below with the preferred example of an optical spectrometer but it should be appreciated that the invention is applicable to a mass spectrometer or other spectrometer.

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 CO2 concentration is required for each ratio. To calculate these linearity calibration factors, the spectrometer needs to measure CO2 with the same isotopic ratio at different concentration, or at least numerous reference gases with known isotopic ratio and concentration are needed.

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 CO2 gas concentration in the range 100-4,000 ppm CO2 in air, which is the operating measurement range of the spectrometer, more preferably 200-4,000 ppm, still more preferably 200-3,500 ppm, and optimally 400-2,000 ppm CO2 in air. For each different type of gas there is a preferred range of gas concentrations in the analyser with a width of approximately 1 decade for the given analyser configuration (e.g. optical path, and analyser 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 analyser with the dynamic range of the analyser.

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.

For linearity calibration and delta scale contraction, mixtures of reference CO2 with carrier gas, using CO2 from two different sources (with different known isotope ratios), are required. The invention provides a convenient way to supply these different concentration gases to the spectrometer by mixing pure CO2 with CO2-free air (also termed zero air), or other C02-free gas. As reference gases for isotope analysis gases are typically expensive and air-C02 mixtures require large gas cylinders it is an advantage to use pure CO2 as a reference gas instead of a pre-mixture as described in the prior art. Pure CO2 with various certified isotope values as reference for isotope ratio analysis is commercially available and 1kg CO2 may last for the whole instrument lifetime. The dilution is performed to give the required 200-4,000 (preferably 200-3,500) ppm CO2 in air (or in N2). Further advantageously, C02-free air can either be produced in the field using a CO2 absorber or delivered in gas tanks. The CO2 may be supplied, for example, in standard 13 litre (L) containers or in 1 L (low pressure) containers. The latter may be conveniently shipped or mailed with standard air freight etc. It is possible to produce more than 15,000 m3 of gas mixture at 350 ppm CO2 in air using one commercial 13 L gas cylinder of CO2 by mixing it with C02-free air. Thus, the working lifetime of a 13 L CO2 gas cylinder in the present invention 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 of the present invention is used. In contrast to the prior art, the flow of CO2-free air (zero air) is controlled by a flow control means (e.g. flow controller or a proportional valve) and instead the CO2 flow is kept constant. For higher dynamic range and flow matching the dilution is preferably done in two steps as described in more detail below.

Another advantage of using pure CO2 (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 invention, 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.

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

Supplies of two pure CO2 reference gases (ref 1 and ref 2) are provided (41, 42). The isotope ratio (13C/12C and/or 180/160) of each CO2 supply is known. The flow of each supply of CO2 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 Vi 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 C02-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, Figure 3 shows the flow scheme of the referencing section of the inlet system alone.

To avoid fractionation of the C02, a constant flow of CO2 of 400 pIts (24 mI/mm) from a selected one of the CO2 reference supplies is mixed into a variable flow of C02-free air (or other carrier gas) (flow rate 3 to 100 mI/mm) 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 (preferably, most or all valves shown in the system of the invention are computer controlled). That is, the CO2 is not subject to variable mass flow control, thus avoiding fractionation, but rather it is the C02-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 CO2 references define the constant CO2 flow into the mixing zone (5). The CO2 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 CO2 and the zero air are thoroughly mixed. As the flow rate here can be larger than 100 mI/mm, 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 1mm 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 is preferably at least 75 mm in length and at least 0.8 mm id.

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.

Preferably, the dimensionless number (D*l/j) > 0.67 Where (in SI units) D: (lnter)Diffusion coefficient of the analyte gas in the carrier gas (m2Is), I: length of the mixing tube (m), j: flow in this tube (m3/s) 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*lfj) > 0.94 the concentration at the end of the mixing tube differs at no point across the area cross section more than 1 per mu 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 iH. 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.

The CO2 pre-mixture is further mixed with more C02-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 C02-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/ of the gas flow into the laser spectrometer. This two stage dilution is preferable 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).

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 C02-free air towards the second mixing split is defined by a flow restriction (13) and the constant pressure in the supply (43) of the C02-free air. The gas flow at the restriction (13) is always higher than the gas flow to the laser spectrometer. The differential amount of C02-free air carrier gas is blown out of an opening (14) on the carrier gas line.

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 CO2 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 CO2 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) preferably is 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 diameterOd), 1 cm length. Generally similar considerations apply to the opening (14).

A first example of parameters for the reference opening is: i. Reference: CO2 in zero air ii. Minimum flow through open split (6) capillary: 0.5 mI/mm (i.e. a flow of at least 0.5 mI/mm) iH. Open split capillary diameter: 1mm iv. Minimum flow to open split: 1 mI/mm (i.e. a flow of at least 1 mI/mm) v. Loss of flow through open split c 1/1000 of input flow vi. Fractionation c 0.3 per meg on mass 46 This fractionation is achieved for all lengths> 1 cm.

A second example of parameters for the reference opening is: i. Reference: CO2 in zero air ii. Minimum flow through open split capillary: 0.5 mI/mm iH. Open split capillary diameter: 2mm iv. Minimum flow to open split: 1 mI/mm v. Loss of flow through open split < 1/1 000 of input flow vi. Fractionation C 0.5 per meg on mass 46 This fractionation is achieved for all lengths> 3.7 cm.

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 Figure 4, or a zero airflow (80) can protect the opening (6) from being in contact with the contamination as shown in Figure 5. It will be appreciated that opening (14) likewise can optionally be provided with a similar blocking valve or zero air flow.

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. CO2 in zero air). It can be seen that by varying the flow of the carrier gas using mass flow controller (9), the concentration of CO2 reference gas in the C02-free carrier gas can be varied. The referencing system allows any concentration of the CO2 for linearity calibration in the measurement range of the spectrometer from 100-4,000 ppm, more preferably 200-3,500 ppm. In this way, isotope ratio measurements can be taken in the spectrometer at a plurality of different CO2 concentrations to enable a concentration dependence of the isotope ratio measurement to be determined.

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 mI/mm. The pressure at the outlet (16) that interfaces to the spectrometer is designed to be around atmospheric pressure.

In the described invention 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 analyser. 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 analyser. It is also possible to measure the sample with unknown concentration first and afterwards measure the reference with a similar concentration as the sample.

In addition to the referencing system described above, the gas inlet system shown in Figure 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 Figure 2 is shown alone in Figure 6 and described in more detail below.

Sample gas from the sample inlet system (Figure 6) and reference gas from the referencing system (i.e. shown in Figures 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.

In the arrangement shown in Figure 6, the flow of C02-free air, which is defined by a fixed flow restriction (13), is mixed with a CO2 sample flow coming from a sample inlet port (12) to which is connected a supply of sample gas (C02). In this embodiment, the supply of C02-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.

The sample inlet system according to the invention ensures 100% sample transfer from the sample input port ((12) in Figure 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 C02-free air (zero air). No sample is wasted and the concentration of CO2 in the gas flow to the laser spectrometer is kept constant and in the optimal range. In some embodiments, using a variable flow of C02-free air, any sample vials with a variable CO2 concentration over time can be flushed out through the sample inlet port to ensure the concentration of CO2 in the gas flow to the laser spectrometer is kept constant and in the optimal range.

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 1-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 1-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.

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 CO2 concentration at (14) is close to zero).

Diffusion of CO2 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 C02-free air at (13). The excess C02-tree 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 C02-free air (40) (which is zero concentration by definition). In all cases, the CO2 isotope ratios at (12), (14) and (16) are substantially identical, which is important.

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).

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 analyser is relatively high at typically 100 mI/mm. Preferably, the minimum flow to the open split (opening (14)) should be 10 mI/mm. Furthermore, it should be ensured that the flow through the open capillary (14) is always at least 0.5 mI/mm. Preferably, a length of the open capillary (14) is at least 5 mm or 6mm, more preferably at least 10mm, and an internal diameter is preferably at least 1.0 mm, e.g. 1.3 mm. The loss of flow through the open split is then typically < 1/1 000 of the input flow and there is little or no fractionation effect. The pressure drop across the opening (14) preferably is arranged to be 250 mbar or less, especially 50 mbar or less.

An example of parameters for the sample inlet opening (14) is: i. Sample: 002 gas in zero air (e.g. dilution of sample:carrier = 1:1 to 1:0).

ii. Minimum flow through open split capillary: 0.5 mI/mm (i.e. the flow should be at least 0.5 mI/mm) iH. Open split capillary diameter: 1mm iv. Minimum flow to open split: 10 mI/mm (i.e. the flow should be at least mI/mm) v. Loss of flow through open split c 1/1 000 of input flow vi. Fractionation c 0.3 per meg on mass 46 This fractionation is achieved for all lengths> 6 mm.

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 mI/mm iH. Open split capillary diameter: 0.5mm iv. Minimum GO flow to open split: 0.8 mI/mm v. Loss of flow through open split C 1/100 of input flow vi. Fractionation c 60 per meg (0.06 per miD for the H2/HD ratio This fractionation is achieved for all lengths> 1.52 cm.

The gas line downstream of the sample introduction point is a mixing zone for the sample CO2 in air and is preferably 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.

The system of the invention is preferably realised as a compact, robust system. One such embodiment is shown in Figure 7, where the same reference numerals are used to denote the same components as shown in Figure 2. A pad 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 Figure 2 with the block itself shown in Figure 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 Vi (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 VS (2/2) is for switching flow of the diluted reference gas into the analyser 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 Figure 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 Figure 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.

It can be seen from the description herein that the gas inlet system of the invention is a compact device to deliver sample and reference gas to an isotope ratio analyser, 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 the invention comprises two functional units: a reference section and a sample inlet section.

The invention 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. CO2 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 analyser to which it is interfaced can be dedicated to analyze the isotope ratio of 13C/12C and/or 180/160 from 002 in air or other carrier gases (e.g. N2), or it can be designed to analyze these or other isotope ratios of other gases.

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 Figures 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 Figures 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.

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, preferably 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.

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.

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

It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention.

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.

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 the invention and does not indicate a limitation on the scope of the invention 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.

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

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 preferred features of the invention are applicable to all aspects of the invention and may be used in any combination. Likewise, features described in non-essential combinations may be used separately (not in combination).

Claims

Claims 1) 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.
2) A method of determining an isotope ratio according to claim 1, 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.
3) A method of determining an isotope ratio according to claim 1 or 2, wherein the sample gas is diluted in the carrier gas prior to measuring its isotope ratio.
4) A method of determining an isotope ratio according to claim 3 wherein the sample gas is diluted to a concentration in the carrier gas that lies within the range of concentrations of the reference gas with the first known isotope.
5) A method of determining an isotope ratio according to claim 3 wherein the reference gas is diluted to a concentration in the carrier gas that lies within a range of concentrations of the sample gas.
6) A method of determining an isotope ratio according to any preceding claim wherein the reference gas is provided as a substantially pure gas and the carrier gas is provided substantially free from the reference gas.
7) A method of determining an isotope ratio according to claim 6 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.
8) A method of determining an isotope ratio according to claim 7 wherein the dilution of reference gas comprises two stages of dilution with carrier gas.
9) A method of determining an isotope ratio according to claim 8 wherein a flow of the carrier gas in a first stage of dilution of the reference gas is dynamically controlled by flow control means.
10) A method of determining an isotope ratio according to claim 9 wherein a flow of the carrier gas in a second stage of dilution is not dynamically controlled.
11) A method of determining an isotope ratio according to any of claims 6 to 10 wherein the reference gas is 002, the carrier gas is C02-free air or 002-free nitrogen and the sample gas comprises 002.
12) A gas inlet system according to any of claims 6 to 10 wherein the reference gas and the sample gas is selected from the group consisting of: 002, OH4, C2H6, CH(2+2), water vapour, Ca, small hydrocarbons, alcohols, aldehydes, N0, H2S, nitrogen, oxygen, and hydrogen.
13) A gas inlet system according to any of claims 6 to 10 or 12 wherein the carrier gas is selected from the group consisting of: air, nitrogen, helium and argon.
14) A method of determining an isotope ratio according to claim 11, wherein the plurality of concentrations in a carrier gas comprise a CO2 in air or CO2 in nitrogen concentration in the range between about 200-4,000 ppm, preferably between about 200-1500 ppm.
15) A method of determining an isotope ratio according to claim 11 or 14 wherein the measured isotope ratio is the ratio 13C/12C or 180,160 16) A method of determining an isotope ratio according to any preceding claim wherein the sample gas is diluted with the carrier gas at a junction, wherein the junction is further connected to an exit gas line for transporting the mixed gas from the junction to the isotope ratio analyser, and wherein the junction is positioned downstream of an opening to atmosphere on a carrier gas line, whereby the flow rate of the analyte gas to the junction is arranged to be lower than the flow rate of gas into the isotope ratio analyser whereby the flow between the opening and the junction is always towards the isotope ratio analyser.17) A method of determining an isotope ratio according to any preceding claim wherein the method further comprises connecting the reference gas and carrier gas by respective reference and carrier gas lines to a first mixing junction where the reference gas and carrier gas combine; mixing the combined reference gas and carrier gas in a mixing zone downstream of the first mixing junction; transporting the mixed gas from the mixing zone to the isotope ratio analyser along an exit line; and providing an opening to atmosphere on the exit line downstream of the mixing zone.18) A method of determining an isotope ratio according to claim 17 wherein the method further comprises providing a second mixing junction on the exit line from the mixing zone for mixing further carrier gas with the already mixed gas; supplying further carrier gas to the second mixing zone via a second carrier gas line, wherein the second mixing junction is downstream of the opening on the exit line.19) A method of determining an isotope ratio according to claim 18 wherein the carrier gas supplied to the second mixing junction is from the same supply of carrier gas that supplies the first mixing junction.20) A method of determining an isotope ratio according to any of claims 16 to 19 wherein each of the junctions are T-junctions.21) A method of determining an isotope ratio according to any of claims 16 to 20 wherein one or more portions of the gas lines are provided in a machined block.22) A method of determining an isotope ratio according to any preceding claim, wherein the measurements of the isotope ratio are made using an isotope ratio optical spectrometer, or an isotope ratio mass spectrometer.