G AS C HROMATOGRAPHY Capillary gas chromatography (GC), coupled to selective detectors, such as electron

7.3.1 G AS C HROMATOGRAPHY Capillary gas chromatography (GC), coupled to selective detectors, such as electron

capture (ECD), nitrogen –phosphorus (NPD), atomic emission (AED), flame photo- metric (FPD), or to nonspecific detectors, such as flame ionization (FID), is still one of the most used techniques for the determination of pesticide residues in foods (Table 7.2). Several applications describe the use of such detectors. However, more and more methods are using mass spectrometric (MS) detection, because it allows to identify, quantify, and confirm the compounds present in the sample on basis of their structure in one single run.

Nowadays, GC-MS is the primary analytical technique used for confirmation of results obtained with classical detectors. In addition, EU requirements indicate that all confirmatory methods for pesticide residues in animal foods must use MS 72 detection. There are three modes of GC-MS available, electron impact (EI), positive chemical ionization, and negative chemical ionization; the first one is the most widely used in this field. Due to its adequate sensitivity and selectivity, GC-MS in selected ion monitoring (SIM) is commonly used in the determination of different

10 14 15 classes of pesticides in animal tissues, 16 milk, butter, egg, honey, –22

and fish and shellfish. –39 On the contrary, the less sensitive method of mass scanning, full-

scan mode, has been applied only when concentrations of investigated pesticides were high enough. 26

GC coupled to tandem mass spectrometry (MS=MS) commonly provides higher sensitivity and selectivity, as well as degree of certainty, than GC-MS in SIM mode,

192 Analysis of Pesticides in Food and Environmental Samples

TABLE 7.4 Advantages and Drawbacks of Different Detection Techniques for Chromatographic Determination of Pesticides

Detection Advantages Drawbacks ECD

Fair sensitivity for OCs Interferences from other halogen-containing species Purchase cost

Limited linear range

Maintenance cost

Very low selectivity

Easy-to-use NPD

Fair sensitivity for OPs and ONs Interferences from other halogen-containing species Purchase cost

Limited linear range

Maintenance cost

Very low selectivity

Easy-to-use FPD

Fair sensitivity for OPs Interferences from other halogen-containing species Purchase cost

Limited linear range

Maintenance cost

Very low selectivity

Easy-to-use EI-LRMS

Good selectivity

Low sensitivity

Relatively cheap and easy-to-use ECNI-LRMS

Relatively cheap and easy-to-use Frequent source maintenance required Good sensitivity for

Limited number of applications organohalogens Good selectivity QTrap-MS

Relatively cheap Needs adequate optimization Good sensitivity

Consistent but sometimes unpredictable Very good selectivity

fragmentation

IT-MS=MS Relatively cheap Needs adequate optimization Good selectivity

Limited linear range

No isobaric interferences Variable sensitivity for different pesticide classes HR-TOFMS

Full-scan spectra and fast Limited dynamic range scanning rate

Matrix can saturate detection system Spectral deconvolution and

Quantitation can be difficult identification of unknown pesticides or metabolites

Benchtop high resolution easy- to-use system Excellent screening tool Can also be used in ECNI mode

HRMS Good sensitivity Purchase and maintenance cost Very good selectivity

Experienced analyst required Exclusive use in EI mode

LC-MS Q-MS

Relatively cheap

Low sensitivity

Easy-to-use system Relatively low selectivity

Determination of Pesticides in Food of Animal Origin 193

TABLE 7.4 (continued) Advantages and Drawbacks of Different Detection Techniques for Chromatographic Determination of Pesticides

Detection Advantages Drawbacks QqQ-MS

Good selectivity Needs adequate optimization Good sensitivity

Purchase cost

IT-MS=MS Good selectivity Needs adequate optimization Fair sensitivity

Cutoff limitations for daughter ions Limited linear range

Abbreviations: LR, low resolution; HR, high resolution; ECNI, electron capture negative ion; TOF, time-of-flight; QqQ, triple quadrupole.

because it involves at least two stages of mass analysis, separated by a fragmentation step. The most common tandem mass spectrometers for GC, ion trap (IT), and triple quadrupole (QqQ) are important tools in food analysis. However, a limited number of examples are presented in the literature on application of GC-MS=MS in the area of pesticide residue determination in foods of animal origin. GC-IT-MS=MS has been employed in the determination of multiclass pesticide in different kinds of

milk 11 and in the analysis of OCs in fish samples. 37 –39 However, GC-QqQ-MS=MS has been less frequently used to determine pesticide residues in animal tissues such as muscle, 5 liver, 6 and fat (Figure 7.1). 9 In all the previously summarized GC-MS applications, fused capillary columns with bonded phases of different polarities (nonpolar BP-1; low polar VF-5, DB-5, LM-5, ZB-5MS, HP-5MS, RTX-5MS, DB-XLB, SE-54, HT-8, or CP Sil 8; low-=midpolarity (DB-1701); and medium polar DB-17), various lengths (10 –60 m), internal diameters (0.20 –0.53 mm), and film thickness (0.10–0.30 mm) have been used (Tables 7.1 and 7.2).

7.3.1.1 Enantioselective Gas Chromatography Several pesticides have optically active or chiral isomers (e.g., a-HCH, o,p 0 -DDT,

cis=trans-chlordane, or heptachlor). 73 As a consequence, biotransformation reactions in biological samples can result in nonracemic patterns in environmental samples.

Crucial for chiral analysis is the availability of chiral capillary GC columns such as those with various cyclodextrins chemically bonded to a polysiloxane. These phases

are relatively heat stable and have low bleed. 73 Current methods range from the simple use of 30 m chiral columns to a two-dimensional ‘‘heart-cutting’’ technique,

providing higher peak capacity and generally further separation of chiral com- pounds. 73 While use of chiral GC separations is not part of routine pesticide analysis,

it is a well-developed technology that is relatively easy to implement in existing GC-ECD and GC-MS instruments.

194 Analysis of Pesticides in Food and Environmental Samples

FIGURE 7.1 Total ion chromatogram obtained by GC-QqQ-MS=MS of a spiked sample of chicken liver with 34 pesticides (organochlorine þ organophosphorus) at 50 mg=kg, and detail of extracted ion chromatograms of (b1) vinclozoline, (b2) parathion methyl, (b3) pirimiphos methyl, and (b4) malathion. (Modified from Garrido Frenich et al., 2007, J. Chromatogr ., 1153, 194.)