Isotope-ratio analysis is used to measure the relative abundance of isotopes (isotope ratio) in a sample, which may be solid, liquid or gaseous and for a wide variety of elements.  For instance, it is used for determining the isotope ratios 13C/12C and/or 18O/16O from CO2, such as in air.  Isotope-ratio analysis is most commonly performed by mass spectrometry (MS) but may also be performed by optical spectrometry. 
For optical spectrometry, an isotope ratio is generally determined in a measurement cell of the spectrometer 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 12C16O2 and another line for 13C16O2.  A convenient absorption line for CO2 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 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=c13c/c12c).  The result is referenced against international standards using the established delta notation for isotope ratio reporting (e.g. δ13C [‰]). 
A general review of isotope ratio mass spectrometry and gas inlet systems can be found in Brenna et al, Mass Spectrometry Reviews, 1997, 16, 227-258. 
In isotope ratio spectroscopy, a sample should be measured against a working standard, that is, one or more reference gases of known isotopic ratio.  Typically the concentration of the sample and the working standard differ by between 10 parts per million (ppm), per minute, in ambient measurements, and up to 40 pp/min for plant chamber experiments. 
It is observed that the measured isotope ratio depends upon the concentration of the analyte.  Therefore a calibration factor (also termed a linearity calibration or concentration dependence), which depends upon the, for example, CO2 concentration is used for each isotope ratio.  It is known to employ one or more reference gases (that is, either one reference gas or a plurality of reference gases) each at a constant (known) concentration.  The concentration of each reference gas is selected on the basis of a known concentration range of the sample, and usually provided in the form of reference gas mixtures in gas tanks.  To calculate the linearity calibration factors, the spectrometer measures the gas (for instance CO2) with the same isotopic ratio at different concentrations, or at least numerous reference gases with known isotope ratio and concentration are provided. 
To provide the sample and reference at the same intensity (concentration) in gas isotope mass spectrometry, a classical solution has been the use of adjustable bellows and a changeover valve. ( for example, see: Halsted R. E. & Nier A. O., 1950, Gas flow through the mass spectrometer viscous leak, Rev. Sci.  Instrum.,  21: 1019-1021; Or Habfast K. (1997) Advanced isotope ratio mass spectrometry I: Magnetic isotope ratio mass spectrometers; and In: Modern isotope ratio mass spectrometry, I. T. Platzner (ed.),  John Wiley & Sons, Chichester, UK: 11-82).  In this way, discrete samples are measured, after filling them into a reservoir.  At the beginning of a measurement, the intensities from both reservoirs are matched by changing the volume of (and thereby the pressure within) one or both adjustable reservoirs. 
Another system for calibrating the isotope ratio measurements to account for concentration dependence and a delta scale contraction is described in B. Tuzson et al, “High precision and continuous field measurements of δ13C and δ18O in carbon dioxide with a cryogen-free QCLAS”, Appl.  Phys.  B (2008), Volume 92, pp 451-458.  However, a drawback with the system described in Tuzson et al 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 al does not employ a sample dilution. 
U.S. Pat.  No.  7,810,376 describes an aperture to keep the partial pressure of an analyte diluted in a carrier constant by feeding back the concentration information of the sensor in a flow controller to adjust the gas flow constant.  Any changes in the sample concentration are removed by diluting the sample, such that the sample concentration of the measured sample is kept constant.  The concentration information is therefore lost.  The working standard concentration is also kept constant by a flow controller. 
WO-2007/112876 describes a unit which keeps the sample gas concentrations constant by means of flow control devices and open splits by using the sensor signal to adjust valves.  In general, samples are measured discretely, with the sample being burnt in a reactor and analysed by gas chromatography before isotope ratio analysis is performed.  In this case, the carrier gas (such as Helium) flow is controlled to keep the concentration of sample and/or reference gases constant.  This allows a single isotope ratio for each gas chromatograph peak to be determined.  Sample concentration information is not relevant. 
An improved arrangement for performing calibrations, which are required to calculate a δ-value from a ratio of spectral intensities, is described in our co-pending patent applications GB1306806.9, GB1306807.7 and GB1306808.5, the contents of which are incorporated herein by reference in their entirety. 
Although calibration improves the concentration dependency of isotopic ratio analysis by a factor of between 5 and 10, further improvements in the accuracy of calibration would be desirable.