Headspace samples were injected into the gas chromatograph using Precision Sampling Corp. syringes (Pressure-Lok, series D) of 5, 2 and 1mL capacity and Hamilton syringes (1000 series) of 500, 250 and 100^L capacity.

Purge and trap analyses were conducted using the above chromatograph and column; however, a Tracor 700 Hall electrolytic conductivity detector was used. The detector was operated at 900°C with a hydrogen reaction gas flow of 40cm3 min-1. The electrolytic fluid flow was set at 1.2mL min-1; detector conductivity range was 10. The chromatographic column employed a nitrogen flow of 60mL min-1 and was operated at 80°C. The injector was 90°C and the column to detector transfer line was maintained at 120°C. Liquid sample concentration was provided by a Tekman LSC-1 (all Tenax trap). The Tekmar trap effluent port was directly interfaced to the gas chromatograph injection port with 0.03in (0.8mm) i.d. (1/16 in (1.6mm) o.d.) stainless steel capillary tubing. The desorption heater was a modified replacement heater (P/N 12082) which provides a rapid temperature ramp (180°C in 40s). The Tekmar unit was operated using a 15min sample purge of 20mL min-1 of nitrogen, a 4 min desorption at 200°C, and 5mL samples. The purge and trap method was calibrated using aqueous standards (and dilutions thereof). Integration of detector signals was conducted as described above. Observed retention times (from initiation of desorption) for the respective components CHCl , CCl , C HCl and C Cl were 4.5, 6.2, 7.2

and 14.6min. Single or multiple component primary aqueous standards were prepared. The primary aqueous standard contained the following halocarbon concentrations: 50ppb of chloroform, 2ppb of carbon tetrachloride, 20ppb of trichloroethylene and 20ppb of tetrachloroethylene.

Dietz and Singley [82] also showed that equilibration of the aqueous sample with the headspace with no agitation is very slow. Even after 2h, equilibration is far from complete when vials are not shaken. One minute of agitation is sufficient for sample phase equilibration. Equilibrium is completely attained by rapid hand agitation which is comparable to a long-term agitation on a mechanical shaker.

When very carefully conducted, analytical precision of approximately 3% relative standard deviation (r.s.d.) can be achieved by this method. For routine analyses of many samples, a precision of from 5 to 10% (r.s.d.) is readily attained. Table 5.2 presents a study in which 10 lake water samples were collected during a 1h period. These samples were collected, transported and stored for 30 days at 4°C, then analysed. The first five samples (group A) were analysed very carefully; a precision of 3% (r.s.d.) was observed. The second set of samples (group B) was analysed using less stringent attention to detail. In this case, a precision of about 5% (r.s.d.) was noted. The analytical method thus provides excellent precision even for rapid screening of samples.

Table 5.2 Results from analyses of 10 lake water samples

Table 5.2 Results from analyses of 10 lake water samples



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