A NALYSIS BY G AS C HROMATOGRAPHY

10.3.1 A NALYSIS BY G AS C HROMATOGRAPHY

ECD and NPD are very sensible and selective detectors, but few recent studies used these detectors. They have been used by various authors such as Millet et al., 48

69 24 27 Sanusi et al., 31 Epple et al., Quaghebeur et al., Bouvier et al., and Kumari et al. 52 for the analysis of current-used pesticides in air and rainwater. The detection

and analysis by ECD was commonly used for the analysis of organochlorine pesticides and GC –NIMS (negative ionization MS) tend to replace this detector especially because of the uncertainty on the identification with ECD.

The use of GC –MS is more developed since it provides sensitivity, specificity, and selectivity. Indeed, with mass spectroscopic detection, the identification of the compound can be done together with the identification of coeluted compounds.

Columns used are generally nonpolar of semipolar columns (30 m 3 0.25 mm,

0.25 mm film thickness) and helium is used as carrier gas. A 5% phenyl=95% polydimethylsiloxane (type DB-5, HP-5, Optima-5, etc. depending on manufactur- ers) was used in many cases. 26,36,46,49,50,68,69

10.3.1.1 Analysis by GC–ECD and GC–NPD For the separation and analysis of pesticides (see Table 10.1 for the list) by GC

–NPD, Epple et al. used a SE-54 column (30 m 3 0.25 mm, 0.25 mm film thickness; J&W Scientific, Folsom, CA, USA) and helium as carrier gas. The injection (1 mL) was made in the splitless mode and the temperature of the injector and detector was maintained at 2508C. Because of the fluctuating sensitivity of the detector, quantifi-

cation of pesticides extracted by C 18 cartridges was carried out by the internal standard method by using 2,3-diethyl-5-methylpyrazine and quinazoline. Detection limits and uncertainty of all the methods (extraction and GC –NPD analysis) are presented in Table 10.1.

Authors, because of uncertainty of the identification by GC –NPD, for most of the GC –NPD analysis, a verification by GC–MS using a GC HP 5890 II Plus, a MS

Sampling and Analysis of Pesticides in the Atmosphere 281 5989 B Engine, a column HP 5 MS (30 m 3 0.25 mm, 0.25 mm film thickness), and

crosslinked (Hewlett-Packard, Palo Alto, CA, USA) have been performed. Identifi- cation was performed by comparing the retention time and mass –peak relations with the standard substance.

Millet et al. 46 and Sanusi et al. 69 used GC –ECD for the analysis of organochlorine pesticides in atmospheric samples (air, fog, and rainwater) after fractionation of the samples by HPLC.

Detection limits obtained by Millet et al. 46 varied between 0.01 and 0.8 mg L 1 corresponding to 33 and 333 pg m 3 for a 24 h sampling at 12.5 m 3 h 1 .

10.3.1.2 Analysis by GC–MS GC –MS is employed for the analysis of pesticides in atmospheric samples for its

capacity to deliver results with high sensitivity and guaranty on the identification. In many cases, quadripole GC –MS in the SIM is employed and quantification is performed by the internal standard method by using various deuterated compounds, including pesticides. 22,33,63,64,70

Ion trap was also employed in the SIM mode by Ferrari et al. 68 for the analysis of

11 pesticides in confined atmosphere after atmospheric sampling using SPME. Ion 41 36 trap was also used in the MS –MS mode by Sauret et al. and Scheyer et al. for air samples and by Scheyer et al. 49,50 for the analysis of pesticides in rainwater after

SPME extraction. The use of MS –MS permits a better sensitivity, a higher specificity, and a more important structural information on molecules in comparison to single MS and is also better for the quantification. To improve the specificity of the detection, in MS –MS only the daughter ions characteristic of the studied pesticides were used for quantification. The parent ion was systematically excluded from the quantitative analysis, since this parent ion could be obtained from several molecules and conse- quently have a low specificity. Indeed, the presence of the parent ion on the MS=MS spectrum meant that a fraction of this ion had not been fragmented by the Collision Induced Dissociation (CID) phenomenon, necessary to produce daughter ions.