The simple answer is No, it is not a good idea to calculate accurate (and precise) isotope ratios from voltage (or current) readings at peak maximum, especially Not for chromatographic peaks.
Simple reason, the isotopic composition of such a CO2 (or H2) peak slice is not representative of the entire isotopic composition of its parent organic material.
The reason for that is the chromatographic isotope effect, which is a variation of the mass discrimination associated with almost any two-phase partitioning process.
The underlying principle is the same physico-chemical principle (solute / stationary phase interaction governed by van der Waals forces) organic MS people exploit when using perdeuterated internal standards for compound quantification in organic GC/MS. In the same way e.g. perdeuterated benzene (C6D6) will elute earlier than its natural abundant analogue C6H6, a GC (or HPLC) peak of a near natural abundant compound is 13C rich at its peak front and 13C poor at its peak end. Below is a list of publications you might find useful.
Rautenschlein, M., Habfast, K., & Brand, W. A. 1990, "High-Precision Measurement of 13C/12C Ratios by On-Line Combustion of GC Eluates and Isotope Ratio Mass Spectrometry," in Stable Isotopes in Paediatric, Nutritional and Metabolic Research, T. E. Chapman et al., eds., Intercept Ltd., Andover, pp. 133-148.
Caimi, R. J. & Brenna, J. T. 1993, "High-precision liquid chromatography-combustion isotope ratio mass- spectrometry", Analytical Chemistry, vol. 65, pp. 3497-3500.
Matucha, M. 1995, "Isotope Effects (IEs) in Gas Chromatography (GC) of Labelled Compounds (LCs)," in Synthesis and Applications of Isotopically Labelled Compounds, J. Allen, ed., John Wiley & Sons Ltd, pp. 489-494.
Matucha, M., Jockisch, W., Verner, P., & Anders, G. 1991, "Isotope effect in gas-liquid-chromatography of labeled compounds", Journal of Chromatography, vol. 588, pp. 251-258.
Meier-Augenstein, W. 1999, "Applied gas chromatography coupled to isotope ratio mass spectrometry", Journal of Chromatography A, vol. 842, no. 1-2, pp. 351-371.
Meier-Augenstein, W. 2004, "GC and IRMS Technology for 13C and 15N Analysis of Organic Compounds and Related Gases," in Handbook of Stable Isotope Analytical Techniques, P. A. de Groot, ed., Elsevier B.V., Amsterdam, pp. 153-176
"Facts do not cease to exist because they are ignored."
Aldous Huxley, "Proper Studies", 1927
Dr W Meier-Augenstein, CChem, FRSC
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From: Stable Isotope Geochemistry on behalf of Daniel H Doctor
Sent: Thu 22/07/2010 17:31
To: [log in to unmask]
Subject: [ISOGEOCHEM] technical question--peak area or peak height?
Many of us are familiar with the problem of instrument linearity affecting the isotopic ratios obtained for a sample or standard as a result of varying yields in the gas of interest. Those of us who run samples with a Continuous Flow (CF) method are also familiar with the problem of chromatography affecting the isotopic composition of a sample or standard. Moreover, those of us who run samples with unknown yields a priori are familiar with the struggle of correcting for both linearity and chromatography. Finally, there are those among us who run hundreds if not thousands of samples with unknown yields in an automated CF mode, creating a very complex situation when dealing with the simultaneous correction of both linearity and drift, over and above considering chromatography and possible memory effects.
My question is this: would it be better to calculate and correct isotope ratios for samples run in CF mode using the peak height (or maximum voltage response) rather than the peak area? Of course, this would only apply to the standards and the samples in the run that result in chromatographic peaks, not the reference gas peaks, which are flat-topped.
Any feedback is appreciated.
Daniel H. Doctor
U.S. Geological Survey
Eastern Geology and Paleoclimate Science Center
12201 Sunrise Valley Drive, MS 926A
Reston, VA 20192
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