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HI Dan--

We basically do the same as Robert.  For the GC-IRMS, the standards from 
Indiana are pretty good.  If you're looking at alkanes, their mix A, up 
to carbon chain length of 30,  usually can be run on a nice temp ramp 
with all the peaks at rough the same concentration, and area and 
height.  If you through in a 36 carbon chain alkane, it comes off while 
holding at 320 and has the same area, but he height it much smaller 
(broader peak).  As long as the system is behaving well, we'll see the 
same linearity for all the compounds as long as we plot deviations from 
standard values vs intensity (assuming all are in the working range).  
Depending on GC conditions and contaminants, one or more of the standard 
peaks will have to be discarded.  Sometimes inflection points in the 
temperature gradient will skew the values, and you'll see a contaminant 
being picked up from the septa on the vials (polyethylene glycol?) grow 
in over time that can affect a couple of the peaks.

As for slicing peaks up differently, you can pick your own peak and 
baselines and see what happens.  If you're doing GC work and have a 
rising background, but have a flat baseline--extending the peak would 
skew the numbers.  Making the peak shorter would minimize that, but 
you'll eventually cut into the real data and through the numbers off.  
You can also try the sloping baselines in the software to see what 
effect that has.  It's a good way to see how robust your numbers are.

One of the issues that's annoying is, esp. on the GC runs, a small 
contaminant on the leading edge of a peak.  They usually aren't big 
enough to really affect the value for the peak directly, but they are 
big enough to though off the baseline calculation, which does affect the 
value of the peak.  Sometimes we'll have issues on some EA runs, if the 
switch over between N to C is late and the baseline jump between N and C 
is incorporated into the calculated baseline--oops.  Then it's manually 
select baseline, or pick a fixed baseline time at the end of the run.

take care


gerry

p.s.  nice chapter Wolfram...
p.p.s.  Column bleed can be a real pain when you are looking at higher 
boilers, esp. at low concentrations.



On 7/22/10 3:15 PM, Daniel H Doctor wrote:
>
> 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
>
>
> 	
> *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
> SCRI
> Invergowrie, DD2 5DA
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>
>
> Senior Lecturer - Stable Isotope Forensics, University of Dundee
> Associate Professor (Adjunct), Southern Illinois University
> Centre for Anatomy & Human Identification
> University of Dundee
> Dundee: http://www.lifesci.dundee.ac.uk/CAHId/ < 
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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://profile.usgs.gov/professional/mypage.php?name=dhdoctor < 
> <http://profile.usgs.gov/professional/mypage.php?name=dhdoctor>http://profile.usgs.gov/professional/mypage.php?name=dhdoctor> 
>
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