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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> 



 <
http://exchange3/exchange/wmeier/Drafts/_1_201714302017105C0069B2E385257768
> 
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


****************************************
"Facts do not cease to exist because they are ignored."

Aldous Huxley, "Proper Studies", 1927
****************************************
Dr W Meier-Augenstein, CChem, FRSC
Principal Scientist - Stable Isotopes
Stable Isotope Laboratory
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http://www.isotopesignatures.com/


Senior Lecturer - Stable Isotope Forensics, University of Dundee
Associate Professor (Adjunct), Southern Illinois University
Centre for Anatomy & Human Identification
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Dundee: <http://www.isotopesignatures.com/> 
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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
<http://eu.wiley.com/WileyCDA/WileyTitle/productCd-0470517050.html> 
http://profile.usgs.gov/professional/mypage.php?name=dhdoctor < <
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