I was worried you might have been trying to measure d13C on your TC/EA and your latest e-mail seems to confirm this.
Your TC/EA reactor contains glassy carbon chips, a glass carbon tube and a graphite crucible, all in all 3 major sources of carbon (alien to sample C) besides the carbon your samples are comprised of. So, in the same way one cannot determine d18O values of samples on an EA-IRMs due to the oxygen contribution /mixing from O2 and chromium oxide one cannot determine d13C of samples on a TC/EA due carbon contribution /mixing from the carbon present in the reactor.
You are right, in theory stable isotopic composition of your cylinder gases should not matter if results are properly scale normalized. However, in practice it does if, as in your case, stable isotopic composition is miles away from what one could consider normal (i.e. way outside the ballpark as our international reference materials (and samples)). We made a similarly bad experience once with an H2 cylinder that was about -780 o/oo, i.e. more than 300 o/oo more negative than SLAP. We changed supplier until we found one that supplied H2 with a d2H value of about -180 o/oo.
In short, try to source a CO cylinder with abundance values between -50 and +50 o/oo.
Prof. Dr W Meier-Augenstein, CChem, FRSC
Stable Isotope Forensics & Analytical Sciences
Robert Gordon University
School of Pharmacy and Life Sciences
The Ian Wood Building
E-mail: [log in to unmask]
From: Stable Isotope Geochemistry [[log in to unmask]] On Behalf Of Wei Huang [[log in to unmask]]
Sent: 01 December 2015 21:07
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] slop of two-point normalization for cellulose analysis
Dear Tom, Gerry and Wolfram,
Thank you very much for the detailed information. We do have thermo TCEA with zero blank auto-sampler. I had carefully packed the combustion tube with little funnel on top of the crucible. But the glassy carbon could be compressed when it was heated up, so that the samples did not fall into the hot zone. I need to double check with it. I did monitor background before I started the samples. it looked good. I also did CO on/off and linearity. They all looked fine, too.
I also ran C3, CH6 and CH7 on EA-IRMS, and got good correlation with slope ~1. Does it mean samples were not fully combusted at TCEA?
Another thing other colleague mentioned to me is that our reference CO has too negative isotopic values with d13C ~ -500 per mil and d18O ~ -300 per mil. Even though I think two-point normalization should not be affected too much by reference gas isotopic value, could the too negative reference gas cause the calculation error?
We plan to use IAEA C3 and IAEA CH6 (-24.91 - -10.45 per mil) to anchor d13C of cellulose, and IAEA 601 and 602 (23.3 - 71.4 per mil) to anchor d18O. I guess they are the common standards that most people are using for cellulose analysis. However, some cellulose samples I am going to analyze has d13C ~ -26 per mil and d18O ~20 per mil. Could anybody suggest any other standard with more negative d13C or d18O? Or they (C3/CH3, CH6, 601 and 602) are the best choice so far.
Sorry for so many questions. As always, I appreciate any information or suggestion.
On Tue, Dec 1, 2015 at 5:07 AM, Wolfram Meier-Augenstein (aps) <[log in to unmask]<mailto:[log in to unmask]>> wrote:
Dear Wei Huang,
While seeing occasionally a slope of <1 is not unusual, even on these occasions slopes are generally >0.9. In line with Tom’s and Gerry’s comments I too think the slopes you are calculating are outside the ball park.
I am not sure what exactly may be the root cause of your unusual results but reading your e-mail prompted several questions.
How were d13C values measured, i.e. what working conditions did you use for your EA-IRMS runs? Why chose IAEA-CH-3 and IAEA-CH-6 as 13C scale anchors? Their d13C values are only 14.32 ‰ apart.
Similarly, setting aside the fact neither CIAAW nor IAEA have published internationally accepted d18O values for either IAEA-CH-3 or IAEA-CH-6 which in itself suggests they should not be used for scale normalization, their unofficial d18O values are only 4.2 ‰ apart, i.e. less than a 1/10 of the d18O range as defined by the VSMOW/SLAP scale.
Choice of scale anchors is not a trivial matter. For scale normalization to work as intended scale anchors used must cover a delta value range that is equal or close to the range covered by the scale defining reference materials.
International reference material pairs for scale normalization of d18O values to VSMOW on the VSMOW/SLAP scale cover a range of >45 ‰: e.g. 48.14 ‰ (IAEA-601/IAEA-602); 53.18 ‰ (USGS32/USGS34); 55.5 ‰ (VSMOW/SLAP).
My suggestion would be to run a sample sequence on your TC/EA-IRMS comprised of IAEA-601 and IAEA-602 (to use as scale anchors) together with IAEA-CH-3 and IAEA-CH-6 as “unknowns” to see if scale normalization of the latter two will yield expected d18O values. Also bear in mind both IAEA-CH-3 and IAEA-CH-6 are hygroscopic and should be vacuum dried for several days prior to analysis.
Similarly, on your EA-IRMS run a sample sequence comprised of IAEA-CH-7 and IAEA-CH-6 (to use as scale anchors) together with IAEA-CH-3 as “unknown”. Alternatively, run USGS40 and USGS41 as scale anchors and include both IAEA-CH-3 and IAEA-CH-6 as “unknowns”.
The results of these two experiments should provide you with solid framework for any troubleshooting (if required).
From: Stable Isotope Geochemistry [mailto:[log in to unmask]<mailto:[log in to unmask]>] On Behalf Of Wei Huang
Sent: 30 November 2015 22:47
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: [ISOGEOCHEM] slop of two-point normalization for cellulose analysis
I was wondering what is the slop (delta true vs. delta measured) between IAEA C3 and IAEA CH6 for two-point normalization. Should it be close to 1? My results showed 0.6 for d13C normalization slop and 0.8 for d18O slop, based on four IAEA-C3 samples and four IAEA-CH6 samples. The precision was not bad: 0.2 per mil for d13C and 0.1 per mil for d18O respectively for both IAEA-C3 and IAEA-CH6, based on four samples each. Does anybody have any idea what could cause the normalization slope<1?
Basic information: This was my first time to run cellulose samples on TC/EA (brand new TC/EA). Gradually increased reactor temperature to 600 degree C and kept it overnight, then gradually increased it to 1400 C for analysis; carrier gas flow 90 ml/min; GC column pre-bake at 300 degree C for 14h, then back to 70 C; no leaks.
I appreciate any information or suggestion.
Wei Huang, Ph.D.
Stable Isotope Lab Manager
Lamont-Doherty Earth Observatory of Columbia University
61 Route 9W
Palisades, NY 10964
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