Dear Matthias,
> many thanks for your prompt and detailed answer. You are right by your
> argumentation, but the facts are not unknown. I would like to add something.
Quite right; but they are not widely published either.
> The MTBSTFA-derivatisation is very quick and an one step method. The
> problem is the instability of the samples, which sets in after about just
> 40 to 48 hours. Two other problems are the possible interaction of the
> derivates with the GC-column (we observed a deterioration of the seperation
> of the column after 200-300 injections) and the molecule size. This is the
> cause of the trouble of the last aa in the chromatogramm- the temperature
> of the column is relativly high and you have problems with the stability of
> the compounds.
>
> By the TFA you don't have all these described problems. Here is the problem
> the irreversible poisoning between the metals in the oxidation furnace and
> the F. You must change the oxidation reactor after 100 to 150 injections.
> But the price for 2-3 new ox.-furnaces is relativly lower as for a new
> GC-column. By the reduction furnace (Cu) we never saw this effect.
Which is exactly what I said. In my experience a CuO/NiO/Pt oxidation
tube can last 2 - 3 weeks depending on sample throughput. It also
protects the reduction tube since it mops up all the fluorine.
> Our experience is that the TFA-method is the best suitable derivatisation
> for the measurement of Arg. It is a very interesting aa for many problems
> in the metabolism research.
I don't disagree. I'm a great fan of TFA myself. Rock stable
derivative and wonderful GC properties.
> It is possible, that we have found special problems by the measurements of
> nitrogen. The sample size is much bigger then the size for corbon.
> Sometimes it is difficult to disregard the chemical and the physical limit
> of the coloumn and to receive enough nitrogen for the isotopical
> measurement.
Quite.
> At the moment D. Hofmann and myself prepare a paper about
> different sample preparation techniques for the nitrogen-determination of
> aa by GC/C-IRMS.
That's splendid. A long overdue contribution which will be greatly
appreciated by AA isotopists. Would you awfully mind putting me
an the waiting list for reprint recipients?
> By the way I cannot confirm the described effect from CO2 and CO. I think
> you didn't have enough oxygen in the furnace or you used an unqualified
> temperature.
Let me assure you that I know my around a GC/C-IRMS system. For
CuO/Pt and CuO/NiO/Pt I set my furnace temperature to 820 C and 960
C, respectively.
However, when I took a look at the derivatization issue I used
freshly prepared, freshly oxidized CuO/Pt oxidation tubes as on
this system any detrimental effect is bound to show up pretty soon.
The CuO/NiO/Pt system survives ill treatment longer because of the
fact that NiO is a better oxygen sink than CuO. On the CuO/Pt system
at 820 C these effects (CO and NO formation) show up immediately. By
placing a PoraPLOT Q between combustion interface and the IRMS I
could separate and identify all combustion products. The order of
elution (30 C isothermal for the PLOT column) is N2, NO, CO, CO2.
At the end of the day, it boils down to personal philosophy. If you
are happy that d15N and d13C from TFA derivatives on a CuO/NiO/Pt
(that's what you are using, isn't it?) oxidation tube can be trusted
for up to 100 injections that's fine. I still would like to see how
these measurements compare to those for NAP derivatives.
I, on the other hand, am a bit old fashioned. As soon as I suspect or
know of potential interferences that might have a negative impact on
the quality of the analysis I try to avoid them at all cost.
Best wishes,
Wolfram
P.S.: Diana, could you please send me reprints/copies of your J. Mass
Spectrom. (1997) and Isoptenpraxis (1995) articles?
Thanks ever so much.
*****************************************************
Dr. W. Meier-Augenstein, CChem MRSC
Senior Research Fellow
University of Dundee, Dept. of Anatomy & Physiology,
OMS, DUNDEE DD1 4HN, United Kingdom
Tel.: +44-(0)1382-34/5124 or /4574
Fax: +44-(0)1382-34/5514
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URL: http://www.dundee.ac.uk/anatphys/wma/wolfram.htm
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