On Fri, 14 Jun 2002, Lichao wrote:

> ISOGEOCHEMDear all,
>
> I am measuring H/D isotopic values of individual alkanes (C12,C14,C16,C18
> C20,C22,C25,C28,C30,C32) by GC/TC/IRMS (DELTA+XL, Finnigan MAT) now.(1) I get a hard
> problem: the peak shape of each compound show obvious pre-tailing whatever split or
> splitless mode for injection.I injected about 400ng per compound on column and obtained
> a signal of about 4.2V.HP-5ms column (30x0.32x0.25) was used and He was used as the
> carrier gas, operating at constant flow mode with a rate of 1.5ml/min, the purge flow
> of 30ml/min was opened at 0.75min after injection when in splitless mode. GC
> program:80C(0min) to 290C(30min)at a rate of 3C/min.(2) I found another problem: the
> Delta D values of same compound are diferrent(amount to 5 o/oo) when injection was
> performed in split mode and splitless mode respectively.Maybe,this phenomenon is not
> significant for 13C measurement, but it seem significant for D/H measurement.Howeever,I
> found that lotof D/H data provided by literatures are determinated in split mode. Can
> anyone tell me why?
>
> Thanks in advance,
>
> Lichao
>             [log in to unmask]
>

Lichao,

I never heard about "pre-tailing". I guess it is what I know as
"fronting".

Some additional questions ...

        1)      Do *ALL* peaks show fronting to the same extent?
        2)      Do you use a retention gap?
        3)      Do you use the same temperature program for split and splitless
                injections?
        4)      What kind of solvent do you use?
        5)      What about the temperature of the injector and the shape of the
                insert liner?
        6)      What amount of solvent did you inject in split and in
                splitless mode?

I have no experience with D/H measurements (yet), but I think a 5 pmil
difference between split and splitless injections might well be due to
different peak shapes or different signal intensities. What about your
background? Does the peak shape look similar for split and splitless?

Have a nice weekend ...


--
        Uli Flenker

        Institute of Biochemistry
        German Sport University Cologne
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