Pesticide Residues in Surface Water and Groundwater - Pesticides: Evaluation of Environmental Pollution - page 282

Samples compatible with SPE must be relatively clean (e.g., groundwater). When surface
water samples are analyzed, filtration prior to extraction is necessary to remove the
suspended solids. This may not be desirable for hydrophobic compounds, because a
significant fraction of the analyte may be associated with the suspended solids.
9.5.1.2 Recent Trends
Modern trends in chemical analysis are aimed at the simplification and miniaturization
of the sample preparation as well as the minimization of the organic solvent used. In view
of this aspect, there is growth in significance of solventless extraction and microextraction
techniques.
Solventless extraction techniques, such as solid-phase microextraction (SPME) and
stir bar sorptive extraction (SBSE), are used in many applications (Wardencki et al. 2007;
Sánchez-Rojas et al. 2009). SPME is performed by immersing a silica fiber coated with a
sorbent in an aqueous sample or in the headspace (HS-SPME). Then, the analytes are ther-
mally desorbed into the GC column or eluted with the mobile phase in the mode of LC
analysis. SBSE is carried out by stirring the sample with a stir bar covered with a sorbent
(polydimethylsiloxane) for a given time. Wardencki et al. (2007) and Lambropoulou and
Albanis (2007) published reviews describing recent developments and trends in this area.
Liquid-phase microextraction (LPME) can also be a promising tool to improve the per-
formance of methods used in the control of pesticides in water, particularly in the analysis
of nonpolar pesticides (Lambropoulou and Albanis 2007; Trtić-Petrović et al. 2010). LPME-
based techniques are simple, rapid, and inexpensive compared to other microextraction
techniques (Pinto et al. 2010). In LPME, the solvent can be a single microdrop suspended
from a needle (single-drop microextraction, SDME) or can be present in pores of a hydro-
phobic membrane or separated from the donor phase by a membrane interface (mem-
brane liquid-phase microextraction, MLPME). Recently, a detailed review of the developed
LPME techniques and their application in analysis of the pesticide residues in water has
been published (Pinto et al. 2010).
Finally, the application of new materials, such as carbon nanotubes or molecularly
imprinted polymers, as sorbent materials in SPE has been one of the hot research top-
ics in the last years (Petrovic et al. 2010). Regarding the extraction of pesticides, carbon
nanotubes have been shown to outperform many popular solid-phase extractants (Ravelo-
Pérez et al. 2010).
9.5.2 Analyte Detection
Pesticides belong to more than 100 substance classes, having widely different chemical
and physical properties. For example, some pesticides contain halogens, while others con-
tain phosphorus, sulfur, or nitrogen, and these heteroatoms may have relevance when
choosing an appropriate detector for their analysis. Furthermore, a number of compounds
are very volatile, but some of them do not evaporate at all. This diversity causes serious
problems in the development of a universal residue analytical method, which should have
the widest scope possible.
9.5.2.1 GC Detection Methods
Since the early 1970s to the early 1990s, most routine pesticide residue analyses were per-
formed by GC in combination with conventional detectors, such as ECD, NPD, or FID (Gan