Part III. Gas Chromatography
Barbara Kanegsberg and Ed Kanegsberg, March 2005
As we discussed, chromatography is a powerful device for separating compounds,
thus allowing easier identification. In chromatography, compounds are separated
by their relative affinity between a stationary phase and a mobile phase.
We then gave an overview of ion chromatography as an example of liquid
chromatography using water as the mobile phase and discussed post-column
modification. High pressure liquid chromatography involves one or more
organic compounds as the mobile phase.
GC is a column chromatographic technique. The mobile phase is a chemically
non-reactive gas like nitrogen or argon; the stationary phase is a thin
film of a very high boiling organic liquid. The GC column may be packed
with specific materials to support the liquid phase. In capillary or
open tubular GC columns, the liquid adheres directly to the walls
or to a thin
layer of support material. The sample is volatilized, injected onto the
column, and eluted at temperatures above the boiling point of the materials
to be identified.
GC has a number of advantages. It has a high resolution; closely-related
compounds can be separated. Many suitable stationary phases are available;
and GC can be used to separate many, but not all classes of organic
compounds. Once separation has been completed, several sensitive
detectors are available.
Perhaps most important, because the mobile phase is a gas, GC can be
combined with mass spectroscopy (MS). MS in itself constitutes a family
of sophisticated
analytical techniques; we previously discussed static and dynamic Secondary
Ion MS[1]. GC/MS is a two-dimensional separation technique, allowing
more definitive resolution of components of complex mixtures. GC/MS
is therefore
often used forensically. However GC and GC/MS are not universal techniques;
and only on prime time television are unambiguous results successfully
obtained in a pro-forma manner.
For successful GC, the analytical chemist has to have an understanding
of the likely components of the mixture. Compounds must be sufficiently
volatile that they are in the gas phase during injection; this means
we have to have some idea of the boiling point. The temperature has
to be
appropriately and precisely controlled during elution. Minimal temperatures
can give better resolution, but increase analysis time. In liquid
chromatography, the liquid mobile phase chemical is sometimes gradually
changed during
the separation process for complex mixtures; in GC, temperature programming
can be used to separate mixtures with widely varying boiling points.
GC is not appropriate for all organic compounds. Thermal stability
is of concern; if the contaminant in question breaks down at high
temperature, it will be difficult to accurately identify the material.
GC is limited
to nonpolar or slightly polar molecules. An estimate places these
at 20%
of all organic compounds, but about 60 to 80% of organic contaminants
of interest are in this non-polar group [2]. However, as water-based
chemicals
are increasingly used in fabrication processes, the proportion
of polar contaminants may increase. It is sometimes possible to do
a
pre-column
modification or reaction of the compound to make it less polar;
but, again, we have to have some idea of what we are looking for.
Then there is the choice of detectors; all have advantages and limitations. Non-selective detectors such a Flame Ionization Detector (FID), may respond to most organic compounds. FIDs destroy the sample. Other detectors may be more specific; in some cases, a single compound may be detected. In summary, the more information you provide the analyst, the better he or she will be able to select the optimal detection system.
References
1 Kanegsberg & Kanegsberg, A2C2, December, 2003.
2 “Applications of CG and HPLC in Environmental Studies,” Advanced
Analytical Center for Environmental Sciences Tutorial, http:
//www.uga.edu/srel/AACES/GCtutorial/page6.html
NOTE: The diagram in February’s column, “Fundamentals of Ion
Chromatography” was courtesy Dionex Corporation.
Barbara Kanegsberg and Ed Kanegsberg
are independent consultants in critical cleaning, precision cleaning, surface
preparation, and contamination control. They are the editors of “Handbook
for Critical Cleaning,” CRC Press. Contact them at BFK Solutions
LLC., 310-459-3614; info@bfksolutions.com; www.bfksolutions.com.
