So, the doctor asks the patient, “What’s wrong?” The patient replies, “You’re the doctor— you tell me.” The doctor would wish for a bit more collaboration. It is also important to collaborate with the analytical chemist when chromatographic separation and analysis are performed, as exemplified by Gas Liquid Chromatography (GC or GLC).
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. 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 withwidely 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 . 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.
1 Kanegsberg & Kanegsberg, A2C2, December, 2003.
2 “Applications of CG and HPLC in Environmental Studies,” AdvancedAnalytical 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 and 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; firstname.lastname@example.org; www.bfksolutions.com.