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Hi Dana,

Have you calculated the concentration of your current injections to
determine if you are now and were then exceeding the capacity for a column
of specific dimensions?  Rood estimates a .32 mm diameter column with .5 um
film at 125-250 nanograms per compound (peak).  If you are overloading your
column and have compounds with similar retention times you will have
problems with coelution.  This can be sneaky, since amino acids require
derivatization that may increase mass by a great deal without a concomitant
increase in the element you are analyzing.

In particular, a number of papers that look at compound-specific dD
measurements advocate a peak height of 1-4V to achieve linearity.  However,
if those authors computed the amount of sample they injected, they would
find they were exceeding the column capacity by a factor of 10.  That is
fine if you are looking at one or two compounds that aren't near one
another but in the case of the amino acids that is a highly non-trivial
issue.  It's a dynamic trade-off between the parameters that work best for
the IRMS and the GC.

The two most obvious solutions are to a) buy a very high capacity column
such that an injection at a concentration below capacity yields large
enough peaks or b) carefully calibrate peak height against isotope ratio of
a standard of known value.

-patrick


On Mon, Sep 23, 2013 at 11:08 AM, Dana Biasatti <[log in to unmask]> wrote:

> Hello,
>
> We are running amino acid standards on a Trace GC with an HP-Ultra 1
> column and Delta V Plus and are getting very small peak sizes (<1V). Our
> column parameters are: length=50m; 0.32 diameter; 0.52 film thickness.
> Carrier flow rate is 2.0 ml/min. We recently changed from a 30m column with
> a 0.25 diameter. We were getting much larger peaks (~10X the size) with the
> old column, but were not getting enough separation. Can anyone tell us how
> we might increase peak size with our new column? Is it a matter of needing
> to increase the concentration of our samples by about 10X or are there
> changes in instrument settings that  you may suggest? Thank you very much.
>
> Dana
>
> Dana Biasatti
> Assistant Research Scientist
> University of Maryland Center for Environmental Science
> Chesapeake Biological Laboratory
> 146 Williams Street
> Solomons, MD 20688(410) 326-7452
>
>