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beezoboar@hotmail.com (Richard) wrote in message news:<9fa77d4c.0305140445.cf2bdff@posting.google.com>...

> time standards to the sample.  I have had luck identifying a number of
> the major peaks in the chromatogram as retention time standards in the
> part of the GCMS software that does the retention index operation on
> the chromatogram.
> 
> It returns reasonable-looking retention indices for all the peaks and
> with the retention index standards correctly labeled in multiples of
> 100.
> 
> Question is, is this valid?  Retention index, as described in the
> literature I am familiar with, is for a series of compounds with a
> regular retention time increase during temperature programming.  My ad
> hoc RI standards do not meet this criteria.  Even though everything
> looks OK, am I setting things up to fail when retention indices are
> needed the most.


Dear Richard,

you are totally free to define a retention index (RI) system
by yourself. Nevertheless it does not make sense, because you can
not compare your values to other published RI-values. But if 
your RI-values fit your own experimental values its OK. 

Export your retention times and RIs to EXCEL/Origin and 
try to interpolate with a linear or non-linear equation.
If you experimental settings and the temperature programme 
of your GC is stable, you have nothing to fear. Take this
equation as your personal interpolation function. 
In a second step you can proof this concept (if you don't
trust your GC-software) and export only retention times
and assign the RI-values from your identified substances 
manually. The values of both functions after interpolation 
should be the same. If not, the second equation is your
personal equation to work with.

If you defined your "known" substance with values
from LEE or KOVATS you can even compare your values to 
other RIs.

BTW. Your approach is a very common one for old GC-MS-data.
If you have regular patterns of known compounds with fixed
RI data - lets say alkanes or metylsiloxanes - 
in different chromatographic runs (belonging 
together, under same conditions) you can assign RI values
to this data and can do a proper data evaluation. This is 
a nice and smart approach.

If you work with AMDIS you can now use the freely available
RIZA GCMS database for coupling RI-values and mass spectra 
together! http://www.minvenw.nl/rws/riza/home/gcms/index.htm

Automated storage of gas chromatography–mass spectrometry data in a
relational database to facilitate compound screening and identification 
J.A. Staeb, O.J. Epema, P. van Duijn, J. Steevens, V.A. Klap, I.L. Freriks
Volume 974, Issues 1-2 , 18 October 2002, Pages 223-230 
Journal of Chromatography A 
http://dx.doi.org/10.1016/S0021-9673(02)01309-2

With kind regards
Tobias Kind
www.amdis.net

PS: pepsi or coke? :-)