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High accuracy measurements of CO2 concentration by isotope dilution method.

Milton, M J T; Wang, J (2003) High accuracy measurements of CO2 concentration by isotope dilution method. In: 16th Annual Meeting of the Stable Isotope Users Mass Spectrometry Users Group Conference Digest, April 2003, University of Bristol, UK.

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Long-term measurements of the composition of the atmosphere play an important role in understanding the causes of global climate change. Such measurements must be referred to standards that are stable over a period of many tens of years. These must be complemented by analytical methods with uncertainties of better than 0.05% in order to detect trends in the concentration of atmospheric gases at the levels of interest. Research at NPL is addressing this requirement using isotope-dilution mass spectrometry [1].

We report the successful operation of a GC-MS analysis using an isotope-dilution method that is independent of any external isotopic reference materials. The isotope-dilution “curve method” developed at NPL [2] is insensitive to instrumental drift and cross contamination and is demonstrated to be capable of measuring gas concentration with high accuracy and clear traceability to the SI. In the experiments reported here, the concentration of CO2 in nitrogen was measured by dilution with a “spike” of pure CO2 highly depleted in 13C. The curve method requires one blend of the unknown and two “reference” blends of a pure sample of CO2 (with natural isotopic composition). The ratio 44/45 is then measured for each of these blends relative to the ratio 44/45 in the natural CO2 . The IDMS curve method then allows the amount ratio in the unknown (x1) to be calculated using:

x1=Dx2 + (1-D)x3

Where x2 and x3 are the amount ratios of the two reference blends and D is a function of the three measured isotope ratios. A six-port gas selection valve and a sample gas valve are added to the inlet of a GC (HP6890 plus) to facilitate the inputs of all the required gas samples (the analyte, pure reference and blends). Analysis is carried out at 44, 45 and 46 by a custom-designed four input-channel magnetic sector mass spectrometer (Micromass). A 30 m long, 32 mm ID HPLT column is used to separate the CO2 from the N2 and a heart-split valve is used to control the flow into the GC. For each measurement, the injection order and time of these gas samples are programmed to minimize instrumental drift and memory effect. Recent results include the measurement of the concentration of four NPL Primary Standards of CO2/N2 primary standards by the new method with an uncertainty of 0.2% .

Reduction of the measurement uncertainty to below the 0.05% level is planned. The IDMS curve method developed here combines the speed and convenience of an on-line GC_MS measurement with the high accuracies achievable by isotope-dilution methods. It can be used to underpin stable gas concentration standards and other gas concentration measurements that require high accuracy. The addition of a combustion unit could adapt the method to the quantitative analysis of a range of organic materials.

Item Type: Conference or Workshop Item (UNSPECIFIED)
Subjects: Environmental Measurement
Last Modified: 02 Feb 2018 13:16
URI: http://eprintspublications.npl.co.uk/id/eprint/2668

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