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:
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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


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From: Stable Isotope Geochemistry on behalf of Daniel H Doctor
Sent: Thu 22/07/2010 17:31
To:
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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>  
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