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provide the first indications of surface contamination and alert the authorities to a pollution problem in
its early stages. Consequently, analytical techniques for measuring water contaminants at very low
concentrations, have been the subject of much development research carried out over the last decade.

Buszka et al. [22] employed a GC/MS combination to determine trace amounts of organic chlorine
compounds in ground water. They collected the samples by a purge-trap procedure and separated the
extracted materials on a GC column. The column separation was followed by single ion monitoring.
Single ion monitoring is a particularly valuable method of dealing with very complex mixtures, as the
detection can be made highly specific and, as a consequence, less resolution is required from the
separation technique.

The purge vessel had 100 ml capacity and was fitted with a sintered glass sparge inlet. The volatile
materials sparged from the sample passed through a trap, 25 cm long, 2.7 mm I.D., packed with one
third Tenax, one third silica gel, and one third charcoal, followed by a 1 cm length of OV-1 GC column
packing. The gas chromatograph was the Hewlett-Packard 5996 Model, fitted with a DB-624 megabore
capillary column, 30 m long and 530 µm I.D.; the stationary phase had a film thickness of 3 µm. The
sample was sparged with helium for 15 min, at a flow rate of 40 ml/min, while the trap was held at
room temperature. The trap was then heated to 180°C, and the contents desorbed through a transfer line
maintained at 100°C, onto the column, which was held at 10°C. The column was held isothermally at
10°C for 15 minutes after sampling, and then programmed to 190°C at 6°C/min. The trap was baked at
190°C after sample regeneration to prepare it for the next sample. The GC/MS transfer line was held at
170°C and the source and analyzer at 200°C. The contaminant materials they were assaying were
dichlorodifluoromethane, trichlorofluoromethane, cis-l,2-dichloroethane, trichloroethane,
tetrachloroethylene and the isomers of dichlorobenzene. It was found that by employing selected ion
monitoring, the GC/MS system provided detection levels that ranged from 1-4 ng/l (1 to 4 x 10-12
g/ml) in water.