Thank you for the prompt response, and
the references--I will look at them.
I appreciate your answer, and in fact
had always followed the same reasoning. I understand that chromatographic
effects in GC-IRMS cause the ratio trace of a peak to vary along the whole
peak, and that any one point along that trace would not be representative
of the entire sample. However, when dealing with CF data from the mass
spec, the early and late tails are often removed from the peak area, usually
below some baseline value. So, we don't truly use the entire peak--some
of the peak is "sliced" out. My understanding is that this is
done because of unreliable measurements at low response (out of the linear
range of the instrument).
I have been wondering if perhaps taking
the process to the extreme, i.e. making the peak slice very narrow, might
improve precision (if not accuracy) when correcting the data according
to standards treated in the same way? Most ratio traces will show a relatively
flat-topped region near the peak maximum, so might correcting the sample
peak ratio in this region (or simply at the peak-maximum) to that of a
standard in the same peak region yield the most reliable data? Since the
accuracy of the corrected delta value of a sample is relative to the accuracy
of a standard (and by standard I mean a standard that has gone through
the same preparatory process as the sample), and both chromatograms were
treated equally, would this not work?
I understand that this may not work
for analysis of multiple peaks derived from the same sample, such as in
GC/C-IRMS analysis of carbon isotope composition of multiple organic compounds.
In this case, one cannot easily compare a sample matrix to that of the
standard. I envision this as a possible means of correcting runs in which
there is only one peak per element of interest.
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
The reason for that is the chromatographic isotope effect, which is a variation
of the mass discrimination associated with almost any two-phase partitioning
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,
Caimi, R. J. & Brenna, J. T. 1993, "High-precision liquid chromatography-combustion
isotope ratio mass- spectrometry", Analytical Chemistry, vol. 65,
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,
"Facts do not cease to exist because they are ignored."
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.
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