DETERMINATION OF PESTICIDE RESIDUES Gas and liquid chromatography are the most widely used analytical techniques for

8.3 DETERMINATION OF PESTICIDE RESIDUES Gas and liquid chromatography are the most widely used analytical techniques for

the determination of pesticide residues in soil. Thermal stability and volatility are the main characteristics that a pesticide must possess in order to be suitable for gas chromatographic analysis. Initially, GC was performed with short glass or steel columns packed with a stationary phase; however, nowadays fused silica capillary columns are almost exclusively employed. The stationary phases used are usually polysiloxanes with different functional groups to increase the polarity.

Table 8.7 summarizes the GC methods used to determine pesticide residues in soil. Electron-capture detection (ECD) is adequate for halogenated compounds or

TABLE 8.7 GC Methods Used for the Determination of Pesticide Residues in Soil

Detector Compound

References ECD

LOD (mg=kg)

Dinitroanilines 10 [18] Organophosphorus

[65] MS EI Benzonitriles

Organophosphorus

0.5 –100 mg=L

1 [6] Dinitroanilines

10 [18] Organochlorines

[62] Phenoxy acids

2 –100 ng=L

[60,61] MS=MS EI Organochlorines

[61] ECD, electron-capture detector; NPD, nitrogen –phosphorus detector; FPD, flame photometric detector;

Pyrethroids

MS, mass spectrometry; EI, electron impact; NCI, negative chemical ionization; MS=MS, tandem mass spectrometry. LOD, limit of detection.

218 Analysis of Pesticides in Food and Environmental Samples those that contain electronegative atoms such as oxygen or sulfur, pyrethroids and

OC pesticides being typical examples. A chromatogram of a mixture of fungicides analyzed by GC –ECD is depicted in Figure 8.2. On the other hand, the determination of pesticides that contain nitrogen or phosphorus atoms, such as triazines and OP pesticides, has been carried out with nitrogen –phosphorus detection (NPD) or flame photometric detection (FPD). Atomic emission and flame ionization detectors have also been employed in the determination of pesticide residues in soil.

Although these selective detectors allow quantitating residues at trace levels, the confirmation of the identity is achieved by mass spectrometry (MS) coupled to GC. The ionization technique most commonly used in GC –MS analysis is electron impact (EI), which produces characteristic ion fragments of compounds that are

Time (min)

Counts 9 8 7 6 5 4 3 2 1

6 8 1 1 1 1 1 2 (b)

Time (min)

FIGURE 8.2 GC–ECD chromatograms. (a) A soil sample fortified at 0.05 mg=g and (b) a blank soil sample. Peak identification: 1 ¼ Quintozene; 2 ¼ chlorothalonil; 3 ¼ tolclofos-methyl;

4 ¼ dichlofluanid; 5 ¼ triadimefon; 6 ¼ procymidone; 7 ¼ myclobutanil; 10 ¼ ofurace; 12 ¼ nuarimol; and 13 ¼ fenarimol. (From Sánchez-Brunete, C. et al., J. Chromatogr. A, 976, 319, 2002. With permission.)

Determination of Pesticides in Soil 219 collected in spectral libraries. Full scan and selected ion monitoring (SIM) are the

two working modes for EI-MS; SIM mode is more sensitive and selective than full scan. Most of the multiresidue methods developed in the last few years use MS as detection system as it offers the possibility of the simultaneous determination and identity confirmation of a large number of pesticides from different chemical classes in a single injection. Chemical ionization (CI) is a useful tool when molecular ions are not observed in EI mass spectra that can work with two different polarities, positive (PCI) and negative (NCI). Time of flight mass spectrometry (TOF-MS) is the result of the significant advances undergone by the analytical instrumentation that is beginning to be applied in the determination of pesticides since full mass-range spectrum and exact mass determination can be obtained for each pesticide without compromising sensitivity. Tandem mass spectrometry (MS=MS) coupled to GC has also been used to determine pesticides in soil with good selectivity and high sensibility.

HPLC is an analytical tool adequate for the determination of pesticides that are not thermally stable or not volatile. Reversed-phase HPLC has been widely used in the analysis of pesticides as most of these compounds present a low polarity. The HPLC methods developed for the determination of pesticides in soil are summarized in Table 8.8. Ultraviolet (UV) detection has been the most frequently used technique in liquid chromatography, although other selective detectors such as FL present higher selectivity and sensitivity. The drawback of FL detection is that it is limited

TABLE 8.8 HPLC Methods Used for the Determination of Pesticide Residues in Soil Detector

Compound LOD (mg=kg) References UV

[52] Phenoxy acids

n.a.

[8,34] 1 –50 mg=L

[49] MS APCI

[74] MS=MS APCI

Glyphosate 5 [9] Multiresidue

[33] UV, ultraviolet detector; FL, fluorescence detector; MS, mass spectrometry; APCI, atmospheric pressure

0.15 –7.5 a

chemical ionization; ESI, electrospray ionization; MS=MS, tandem mass spectrometry; n.a., not available. a LOQ (limit of quantitation, mg=kg) instead of LOD (limit of detection).

220 Analysis of Pesticides in Food and Environmental Samples

(c) 10 20 30 40 min FIGURE 8.3 HPLC-Fl chromatograms. (a) A soil sample fortified at 0.1 mg=g, (b) a blank soil

sample, and (c) a soil sample spiked at the LOQ level (0.01 mg=g). Peak identification: 1 ¼ oxamyl, 2 ¼ methomyl, 3 ¼ propoxur, 4 ¼ carbofuram, 5 ¼ carbaryl, 6 ¼ methiocarb. (From Sánchez-Brunete, C. et al., J. Chromatogr. A, 1007, 85, 2003. With permission.)

to compounds that fluoresce or else derivatization to obtain a fluorescent compound is required. Figure 8.3 shows a representative chromatogram of a mixture of carba- mates that has gone through a postcolumn derivatization process.

The preparation of thermally stable derivatives for the subsequent gas chroma- tographic analysis is an alternative that nowadays is seldom applied because of the high sensitivity and selectivity achieved with liquid chromatography coupled with mass spectrometry (LC –MS). The implementation of robust ionization interfaces, such as electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), is considered one of the main instrumental improvements. The selection of

Determination of Pesticides in Soil 221 the ionization interface depends on the nature of the analyzed pesticide; APCI is

adequate for moderately nonpolar pesticides such as triazines and phenylureas, whereas ESI is suitable for polar and ionic pesticides. Tandem mass spectrometry is also used to determine pesticides in soil with the advantage of achieving a better selectivity owing to the selection of daughter ions.

The analysis of pesticides has also been carried out with nonchromatographic methods. Capillary electrophoresis (CE) is an alternative analytical tool that has been applied in the determination of residues in soil samples [27,83,84]. CE presents different working modes, and micellar electrokinetic chromatography (MECK), capillary zone electrophoresis (CZE), and capillary electrochromatography (CEC) are the most frequently used. The application of sensors and biosensors in the determination of pesticides in environmental samples is also rapidly increasing. These portable analytical devices offer the possibility of in situ analysis [85]. Immunoassays, such as enzyme-linked immunoabsorbent assay (ELISA), have been also used to determine pesticides [86]. This technique, as well as the biosensors, is usually applied as screening tests rather than to quantitate residue levels, and the chromatographic methods are a more suitable alternative for this purpose.