Types of Gas Chromatography

This type of gas chromatography falls into the general classification of chromatography. 

In this technique, the stationary phase is a porous solid (such as graphite or silica gel) and the mobile phase is a gas. This type of gas chromatography demonstrates very high performance in the analysis of gas mixtures or components that have a very low boiling point. 

Two types of gas chromatography exist gas-liquid chromatography (GLC) and gas-solid chromatography (GSC). Other classification schemes such as GSC. GLC plus capillary gas chromatography (CGC) are outdated because nowadays GLC and GSC can be performed both in packed columns and in capillary or open tubular columns,... 

The first set of experiments describes the application of gas chromatography. These experiments encompass a variety of different types of samples, columns, and detectors. Most experiments maybe easily modified to use available equipment and detectors. 

Characterization of various types of damage to DNA by oxygen-derived species can be achieved by the technique of gas chromatography-mass spectrometry (GC-MS), which may be applied to DNA itself or to DNA-protein complexes such as chromatin (Dizdaroglu, 1991). For GC-MS, the DNA or chromatin is hydrolysed (usually by heating with formic acid) and the products are converted to volatile derivatives, which are separated by gas chromatography and conclusively identified by the structural evidence provided by a mass spectrometer. Stable isotope-labelled bases may be used as internal standards... 

Molecular sieves (zeolites) are artificially prepared aluminosilicates of alXali metals. The most common types for gas chromatography are molecular sieve 5A, a calcium aluminosilicate with an effective pore diameter of 0.5 nm, and molecular sieve 13X, a sodium aluminosilicate with an effective pore diameter of 1 nm. The molecular sieves have a tunnel-liXe pore structure with the pore size being dependent on the geometrical structure of the zeolite and the size of the cation. The pores are essentially microporous as the cross-sectional diameter of the channels is of similar dimensions to those of small molecules. This also contrilsutes to the enormous surface area of these materials. Two features primarily govern retention on molecular sieves. The size of the analyte idiich determines whether it can enter the porous... 

REPRESENTATIVE PROPERTIES OF DIFFERENT COLUMM TYPES IN GAS CHROMATOGRAPHY... 

Tetranuclear iron-sulfur clusters of the type [Fe4S4(SR)4]2, where R = CH2C6H5 and C6H5, were found138 to catalyze the reduction of C02 in DMF solutions. Controlled-potential electrolyses were carried out in a C02-saturated 0.1 M tetrabutylammonium tetrafluoroborate (TBAT)-DMF solution at a mercury pool cathode. In the absence of a catalyst, C02 was substantially reduced only at potentials more negative than -2.4 V versus SCE, while in the presence of a cluster, the reduction took place at around -1.7 V thus, potential shift of ca. 0.7 V was achieved. The products were analyzed by means of gas chromatography and isotachophoresis. Without a catalyst, oxalate was the main product, and addition of small amounts of water to the DMF solution favored formate production, whereas in the presence of the catalyst, formate was produced predominantly even in a dry DMF solution. This result was interpreted in terms of indirect reduction of C02, proceeding by electron transfer from the reduced cluster to C02 in the bulk... 

Work on the determination of chlorinated insecticides has been almost exclusively in the area of gas chromatography using different types of detection systems, although a limited amount of work has been carried out using liquid chromatography and thin-layer chromatography. 

One advantage of gas chromatography is the availability of detectors which respond specifically to certain types of compound. The best known are the electron capture detector for chlorine compounds and the flame photometric detector for nitrogen and phosphorus compounds. If one wants to detect very small molecules such as water or CSj, the standard flame ionisation detector must be replaced by a thermal conductivity detector. 

The authors gratefully acknowledge the comments and assistance of W.F. Todd and E.R. Kennedy (NIOSH/Cincinnati) and H.J. Ettinger (LASL). We also appreciate the assistance of W.B. Nelson (LASL) in the area of computer graphics, G.O. Wood (LASL) in the area of gas chromatography, and H.H. Kutac and S.E. Medina in the typing and assembling of this document for the printer. 

Compounds that have been separated by way of gas chromatography and liquid chromatographic techniques cannot be identified by their retention time alone as many types of compounds have similar or identical retention times. Increasingly, in recent years, this problem has been overcome by connecting a mass spectrometer to the outlet of the separation column applications of mass spectrometry are discussed in Chapters 5 (high performance liquid chromatography) and 16 (gas chromatography). 

Through the use of the various methods of recovery discussed, it is often possible for the forensic chemist to obtain a satisfactory sample of accelerant residue for examination purposes. Through utilization of gas chromatography, the identification of the accelerant can often be effected and differences and similarities between recovered and known standard specimens can be shown. However, success in the recovery and identification of accelerant residues is highly dependent upon the type and quantity of material received for examination and the care that has been taken in the preservation of the items to be examined. 

In both these areas, chemical derivatisation has traditionally played a role and with the advent of gas chromatography an even more important role. The reasons for preparing a derivative suitable for GC analysis are many and varied and have been discussed thoroughly in a number of books and reviews (l- >). For convenience they are summarised in Table I. As can be seen, two different types of chemical derivatisation techniques are mentioned under Item 4 of Criteria, There is the chemical derivatisation of a pesticide as a pre-requisite of the method of analysis, e.g. esterification of the chlorophenoxy acids, as well as derivatisation as a method for confirmation of identity. The former must meet all the requirements associated with a practical, viable analytical procedure while for the latter the emphasis is on speed, ease of operation and reproducibility. 

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