DETERMINATION GC with element-selective detection has been the primary method for the determin-

6.4 DETERMINATION GC with element-selective detection has been the primary method for the determin-

ation of pesticides for many years. These systems are used to screen a wide variety of pesticides containing heteroatoms such as halogens, phosphorus, sulfur, and nitrogen. These detectors provide not only sensitivity, but also great specificity, such that very little sample cleanup is required. Confirmation of identity of the pesticide usually required the use of a second GC with a different type of column. While element-selective detectors are still widely used, they are being replaced or complemented by mass spectrometric (MS) detectors.

The most popular GC-MS procedure for pesticide analysis of vegetal matrices is an instrument equipped with electron impact ionization and a single quadrupole mass spectrometer in the SIM mode (GC-MS=SIM). Multiresidue pesticide screening of fresh produce using GC-MS=SIM is widely used because these methods are economically efficient because any pesticide present can be screened, quantitated, and confirmed. The FDA uses GC-MS=SIM methods to detect and confirm >100 pesticides in a single injection [4,5]. Other mass analyzers used for GC analysis of pesticides besides a single quadrupole are triple quadrupole [82,144], ion trap [115,145], and time-of-flight (TOF) [146 –149]. The advantage of a triple

Determination of Pesticides in Food of Vegetal Origin 163 quadrupole mass spectrometer is that it offers enhanced specificity and sensitivity.

The first and third quadrupoles are mass filters for the parent and product, respect- ively, whereas the second quadrupole is a collision chamber where collisional dissociation of the parent is used to form the product ions. Ion trap mass spectro- meters can provide superior sensitivity and qualitative information, since MS=MS and MS n experiments can be conducted, but there are problems with quantitation.

TOF mass spectrometers provide the capability of nontarget identification, since these techniques provide either mass resolution or full-scan spectra at all times of the analyte, which is not possible with single ion or multiple reaction monitoring [56]. The major drawback of triple quadrupole and TOF techniques is that the instruments are expensive, compared with the cost of a single quadrupole mass spectrometer.

Methods based on HPLC were not as common in the past, because the traditional UV and fluorescence detectors were much less selective and sensitive than the GC detectors. This has changed with the commercial availability of HPLC-mass spectrometry (HPLC-MS), which has increased sensitivity and selectivity. The HPLC can be coupled to various types of mass spectrometers, such as ion trap, single quadrupole, triple quadrupole (or tandem), TOF, or hybrid instruments such as quadrupole –ion trap and quadrupole–TOF. For small molecule analysis, such as pesticides, these instruments operate in atmospheric pressure ionization (API), either in electrospray ionization (ESI) or in atmospheric pressure chemical ionization (APCI) in the positive or negative mode. A number of multiclass MRMs using

HPLC-MS=MS have been reported for pesticide screening in vegetal matrices [43,48,150 –157]. These multiclass MRMs have the advantage of excellent sensitivity, quantitation, and the capabilities of screening a large number and a wide variety of pesticides in a single HPLC run. The disadvantage of tandem instruments is that they are limited to targeted screening, which only permits screening of masses or transitions in the selected ion recording or multiple reaction monitoring mode, but does not allow for the discovery of unknown pesticides in the sample matrix. HPLC-TOF=MS benefits from the high resolving power of m=z signals, which enables for the accurate masses of ions for identity of the analyte [158,159]. These instruments are also sensitive, quantitative, and provide accurate mass measurements for confirmation. These characteristics allow the analysis to be conducted under full-scan conditions.