Retention Index in gas chromatography
Richard
Retention index in usually defined in reference to a homologous group
of alkanes or esters of homologous acids, etc. I am generating some
fairly complex chromatograms and there is no room to add retention
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.
Thanks for your consideration,
Richard
Tobias Kind
> 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? :-)
Richard
> 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.
Thanks Tobias,
This is what seemed a reasonable approach. I am only needing a way to
ensure internal consistency among a series of GC/MS runs. I use a TOF
mass spec that can provide mass spectra of coeluting peaks a few
hundred milliseconds apart and there is some run to run variation in
the retention times.
I couldn't find any reference to assigning RI values this way. I was
concerned that my calibration run worked by serendipity and all the
other runs might be invalid.
I will try to convert my system to conventional RI values as you
suggest. That may resolve some ambiguous library search matches.
PS. Are you guessing or is there that little anonymity on the
internet these days?
Richard