Hi Wolfram,

Thanks so much for your insights. I now have a clearer sense of the issues involved with peak integration for calculating delta values.  You are right, I was referring to a bulk sample prep such as from carbonate digestion or combustion on an EA, an important distinction from compound-specific analysis.  Like you said, we do use a GC column to separate out the permanent gases, and chromatographic effects do occur.  

What you say about H2 is very interesting, and I will keep it in mind.  

Kind regards,
Dan


From: Wolfram Meier-Augenstein <[log in to unmask]>
To: [log in to unmask]
Date: 07/23/2010 05:22 AM
Subject: Re: technical question--peak area or peak height?
Sent by: Stable Isotope Geochemistry <[log in to unmask]>





 
Hi Dan,


When you say CF(-IRMS) data I take you mean gas peaks coming from a bulk sample prep unit such as carbonate digest or EA?

Even here gas peaks of converted samples are subject to the chromatographic isotope effect since they pass through a GC column prior to entering the ion source. The reason why we get away with cutting off a few data points at either end is the comparatively huge signal size and favourable signal-to-noise ratio (and greater material transfer rate through the "interface") compared to GC-IRMS. Any error caused by missing a couple of seconds worth of data point at either end will be smaller than the overall error (or at least not greater). However, there is a limit to how far you slice off peak information at the front and the end of the peak before calculated delta-values become significantly affected. A good rule of thumb to gauge at which points significant changes will occur is to look at the ratio trace to see at which point in time its value changes significantly from the background ratio value. For a CO2 peak in EA-IRMS with a peak maximum of 30 nA (or 9 V if the cup resistor is 3*10e8 Ohm) it is possible to stop peak integration once the peak tail has reached 0.3 nA even if the background signal is 0.01 nA without seriously compromising accuracy However, this doesn't mean one can expect the same to be true if one only integrates say 6% of the entire peak (approx. a 2 s wide time slice).

Things are even more interesting for the H2 peak. Since it is not retained by the GC column it is not subject to the chromatographic isotope effect that results in peak front being more enriched in the heavier isotope as indicated by the S-deflection of the ratio trace. However, this does not mean the H2 peak is isotopically uniform across the peak either.  Here we see mass discrimination resulting from differences in diffusion (coefficients) and viscosity between the isotopologues, which results in the H2 peak front being depleted in 2H while the peak tail is enriched.  While isotope data from most CF-IRMS generated gas peaks are virtually impervious (within reason) to detrimental effects cause by peak shape deformation (if that's a word) such as peak broadening, the H2 peak is quite sensitive to changes carrier gas flow and, hence in H2 peak shape.

In CF-IRMS (e.g. EA-IRMS) your standards do not undergo the exact same preparatory process as your actual samples since the standards will be run as a separate sample. Furthermore, most reference materials a no matrix match for the samples we analyse, which may result in subtle differences in the way (speed, efficiency) in which they combust. Examining the entire peak instead of a time slice is therefore the (only) way to mitigate against such differences.


Best,

Wolfram

________________________________

From: Stable Isotope Geochemistry on behalf of Daniel H Doctor
Sent: Thu 22/07/2010 20:15
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] technical question--peak area or peak height?



Hello Wolfram,

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.

Dan
 
***********************************************
Daniel H. Doctor
U.S. Geological Survey
Eastern Geology and Paleoclimate Science Center
12201 Sunrise Valley Drive, MS 926A
Reston, VA 20192
tel: 703-648-6027
http://profile.usgs.gov/professional/mypage.php?name=dhdoctor <http://profile.usgs.gov/professional/mypage.php?name=dhdoctor>  



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Re: technical question--peak area or peak height?                

Wolfram Meier-Augenstein                  to:                  ISOGEOCHEM                  
07/22/10 01:13 PM

Sent by:                  Stable Isotope Geochemistry <[log in to unmask]>                

Please respond to Stable Isotope Geochemistry


________________________________





Dear Dan,


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


Best,

Wolfram


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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?



Dear Isogeochemists,

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.

Dan

***********************************************
Daniel H. Doctor
U.S. Geological Survey
Eastern Geology and Paleoclimate Science Center
12201 Sunrise Valley Drive, MS 926A
Reston, VA 20192
tel: 703-648-6027
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