Agreed.
My post was chiefly meant as response to Mark sharing a common experience, not so much as a "this-is-it" solution to the original question about the sudden drop in peak height. Named AAs were meant purely as examples.
You are right in suggesting there could be a number of culprits such as GC injector, GC column, leaking unions, hairline crack in reactor ceramic tube etc.
Best,
Wolfram
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Prof. Dr W Meier-Augenstein, CChem, FRSC
Stable Isotope Forensics & Analytical Sciences
Robert Gordon University
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From: Stable Isotope Geochemistry <[log in to unmask]> on behalf of Patrick Griffin <[log in to unmask]>
Sent: 12 October 2017 18:56:15
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] GC-IRMS sudden peak height reduction
Asn and Gln do not form stable derivatives, at least by any method I'm aware of. Lysine does, and like those two, it has two functional groups that would be trifluoroacylated, but its abundance is so low that HF from the other AAs would be an overwhelming contribution if we were concerned with which AAs did the most damage.
All of this seems tangential, though, since it doesn't seem that these problems are consistent with only a reactor or plumbing poisoned by HF. A new reactor that's seen only a few samples is unlikely to be feeling the effects of HF, or anything else that's been run through it, assuming anything nearing a reasonable concentration.
I am also suspicious of plumbing -- a reduction in signal intensity and an increased retention time are both consistent with pure plumbing problems. We've seen the exact same thing from tiny bits of crumbled ferrule. While that might seem like a static problem, the expansion and contraction of a few runs has been enough to turn a good flow into a bad one. But before I took anything apart to sonicate and clean, I would try streaming Ar if the first leak detection pass was with a He sniffer.
-patrick
Patrick Griffin
Department of Earth and Atmospheric Sciences
Indiana University
1001 E. 10th St.
Bloomington IN 47408
On Thu, Oct 12, 2017 at 1:18 PM, Wolfram Meier-Augenstein (pals) <[log in to unmask]<mailto:[log in to unmask]>> wrote:
Hi Marilyn,
Did I put Asp? Stupid boy. I meant of course Asn. Same as Gln (I put Gln, not Glu) Asn has two amino functions and both will be acetylated. These were only meant as examples not as an exhaustive list.
You are right of course wrt Asp and Glu; here the two carboxyl groups will be propylated.
If you didn’t have any problems, good for you. However, it would seem others (incl. ickle me) have experienced problems. Once I had measured the impact on sample conversion once TFA derivatised AAs had gone throught my GC/C-IRMS I decided to play it safe. My philosophy is and always has been, if I know there is something fishy going that may / will impact negatively on my results I will do my best to avoid it / find a work-around. Preparing O-propyl, N-acetyl derivatives of AAs is not rocket science and avoids any risk of affecting incomplete sample conversion and thus fractionation or damage to one’s instrument. Granted, GC peak shapes are not as sharp as with TFA but, hey one can ‘t have everything. Besides, playing with stationary phases of different polarity one can get decent peaks and separation with O-propyl, N-acetyl derivatives of AAs.
Best,
Wolfram
From: Stable Isotope Geochemistry [mailto:[log in to unmask]<mailto:[log in to unmask]>] On Behalf Of Marilyn Fogel
Sent: 12 October 2017 17:45
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: Re: [ISOGEOCHEM] GC-IRMS sudden peak height reduction
Hi Wolfram,
Asp and Glu only react with one TFAA molecule—there are two of the isopropanol groups, and 1 TFA. Serine and Threonine have a 2nd TFAA on the hydroxyl group.
In my +20 years of using these derivatives and publishing numerous papers, I have had minimal problems with the small amounts of HF generated from combustion.
Marilyn
From: Stable Isotope Geochemistry <[log in to unmask]<mailto:[log in to unmask]>> on behalf of Wolfram Meier-Augenstein <[log in to unmask]<mailto:[log in to unmask]>>
Reply-To: Stable Isotope Geochemistry <[log in to unmask]<mailto:[log in to unmask]>>
Date: Thursday, October 12, 2017 at 5:48 AM
To: <[log in to unmask]<mailto:[log in to unmask]>>
Subject: Re: [ISOGEOCHEM] GC-IRMS sudden peak height reduction
In neutral AAs TFAA introduces one trifluoro-acetyl group, in AAs such as Asp, Gln or Lys two TFA groups are introduced per AA. That’s a lot of F. ;-)
Fluorine does not only cause problems downstream of the combustion reactor, as Mark discovered to his detriment it already causes problems within the combustion reactor no matter which type / built vintage. Fluorine will react with the metal wires serving as oxygen sink / oxygen donor and will form highly stable metal fluorides that cannot be recharged by flooding the reactor with oxygen. For example, nickel fluoride has a melting point of 1,474°C (its boiling point is 1,750°C).
A reduction in oxidation capacity as a result of fluorine in the combustion reactor will almost immediately result in reduced (= non-quantitative) conversion of sample compound peaks. I have tested this myself years ago on a CuO/Pt reactor comparing CO2 yield (m/z 44 peak area) obtained from Leu as O-propyl-N-acetyl derivative (9 carbons) compared to Leu as O-propyl-N-trifluoro-acetyl derivative (9 carbons). I also compared injections of methyl-octanoate (9 carbons) to Leu as O-propyl-N-trifluoro-acetyl derivative (9 carbons). In all instances, injection of the TFA derivative yielded 40% less CO2 than either of the two other compounds.
Wolfram
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Mark Haught
Sent: 11 October 2017 19:30
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: Re: [ISOGEOCHEM] GC-IRMS sudden peak height reduction
What derivatization method are you using? We were using a TFAA for acylation and discovered the fluorine was poisoning the catalyst in the combustion reactor.
Mark
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Jenan Kharbush
Sent: Wednesday, October 11, 2017 11:04 AM
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: [ISOGEOCHEM] GC-IRMS sudden peak height reduction
Hello again,
We're having a new problem with our Delta V + GC Isolink, running d15N of amino acids. Everything was fine until, from one run to another during a sequence, the peak size suddenly decreased by half (e.g. mass 28 amplitude went from ~400 to ~200). This results in later peaks, which are always shorter and less sharp, being too small to integrate or disappearing entirely. The decrease in peak size was accompanied by an ~10 second increase in retention time for all of the peaks. I don't know if that means anything, because the retention times returned to normal the next day, but the peak size has remained abnormally small.
I have re-oxidized the reactor, changed the syringe, changed the septum and inlet liner, and checked for leaks but found none. Also trimmed the inlet end of the GC column. Still the peaks are small and the later peaks especially so. The peaks are also wider than they were.
The inlet is set to a temperature of 270 (not PTV mode) and the initial column temperature is 90, but none of that has been changed from before. We recently replaced the entire turret of the TriPlus autosampler so it seems unlikely to be that. I tried a manual injection just in case and it looked a little better but not much. There don't seem to be any leaks in the usual suspected places but maybe there is somewhere less obvious where I haven't thought to check.
Has anyone experienced an issue like this? Any advice would be very appreciated.
Thanks,
Jenan
--
Jenan J. Kharbush, PhD
Laboratory for Molecular Biogeochemistry and Organic Geochemistry
Department of Earth and Planetary Sciences
Harvard University
20 Oxford St.<https://maps.google.com/?q=20+Oxford+St.%0D+Cambridge,+MA&entry=gmail&source=g>
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