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for less than 10-6 of the total number of atoms present in the plasma region, and thus there is little or no
quenching effect due to the presence of the sample. The ions pass through a series of skimmers to
remove the gas, and are eventually electrostatically deflected into the analyzer of the mass
spectrometer. This type of ionization has the advantages that ionization proceeds at atmospheric
pressure, all the different elements are ionized, and only singly charged ions are produced. The
detection limit is about 1 part in a trillion. Today, capillary columns are mostly used in GC/MS
instruments, in conjunction with single or triple quadrupole mass spectrometers. Employing the above
interfaces, the GC/MS tandem system has been successfully applied to wide range of analytical
problems, but has been particularly useful in environmental and pharmaceutical analysis and in forensic
chemistry. A number of other less common interfaces have also been developed. Some of these include
the membrane interface that is fabricated to specifically transport selected materials from one gas or
liquid stream to a stream of helium; a combustion interface for isotope ratio monitoring; and a pyrolysis
interface developed for identifying particulate matter from its chromatographic and mass spectrometric
fingerprints. Derivatization techniques are sometimes used to increase the partial pressure of certain
samples, and thus make them amenable to separation by GC, and allow them to be introduced into the
MS by a direct inlet probe. Samples that have low volatility due to their high molecular weight cannot
be rendered volatile by derivatization. Conversely, strongly polar, low molecular weight materials can
usually be derivatized to provide volatile products. Acids can often be esterified with appropriate
alcohols, catalyzed with hydrochloric acid or boron trifluoride and also reacted with diazomethane.
Hydroxyl compounds can be acetylated with acetic anhydride or acetyl chloride. Derivatization can take
place before the separation (pre-column) or after the separation, (post-column) depending on the nature
of the sample.

References

1. J. C. Holmes and F. A. Morrell, Appl. Spec., 11(1957)86.

2. R. Ryhage and E. von Sydow, Acta Chem. Scand., 17(1963)2025.

3. J. T. Watson and K. Bieman, Anal. Chem. 37(1965)844.

4. M. S. B. Muson and F. H. Field, J. Am. Chem. Soc., 88(1966)2621.