Environmental Engineering Reference
In-Depth Information
most often by the use of LLE or SPE (Gan and Bondarenko 2008). SPE is the most widely
used procedure for cleanup, as well.
LLE is one of the earliest methods used for analyzing pesticides in water samples. It
relies on the partition of analytes between two immiscible liquids, usually an aqueous
solution and an organic solvent. Because of its simplicity and also its inclusion in many
standard methods, LLE used to be the most popular sample preparation technique for
pesticide analysis in many matrices. Its advantages include relatively minimal equipment
requirements and low demand on the analyst skills, compatibility with a broad range of
pesticides, and reliability. But, there are a number of drawbacks regarding the standard
LLE. The most distinguished one is the consumption of large quantities of organic sol-
vents, which makes LLE methods less environment friendly. In addition, polar pesticides,
as well as their degradation products, cannot be extracted using this method (Kuster et al.
2006). LLE is generally labor-intensive, time-consuming, and physically demanding. For
the reasons mentioned above, LLE has been largely replaced by SPE (Petrovic et al. 2010)
in the last decade.
9.5.1.1 Solid-Phase Extraction
The ideal sample preparation method should be fast, accurate, precise, and consuming low
amounts of solvent. Furthermore, it should be easily adapted for field work and employ
less costly materials. Capability of automation is highly favored as well. SPE is the isola-
tion technique capable of meeting these expectations (Picó et al. 2007). As seen in
Tables 9.7
and 9.8, SPE is the technique most widely used for sample preparation prior to chromato-
graphic analysis of pesticide residues in water.
In SPE, the analyte is transferred from the aqueous phase onto a sorbent phase, and then
recovered for analysis. A typical SPE sequence includes the activation of the sorbent bed
(conditioning), application of the sample, removal of interferences (cleanup), elution of the
sorbed analytes, and reconstitution of the extract (Liška 2000). Exact conditions are usu-
ally specified by the manufacturer and may vary significantly in the types and volumes of
solvents used for conditioning and elution.
Sorbents available in standard SPE include the common sorbents used in LC, such as
bonded silica phases and polymers. The most popular phases used in pesticide analysis are
octadecyl- (C18) and octyl-silica (C8) and styrene-divinylbenzene copolymers (Tables 9.7
and 9.8). Pesticides are often eluted with methanol, ethyl acetate, dichloromethane, or their
mixtures. The preferred sorbent-eluent combination is primarily determined by the polar-
ity of the target analytes and the nature of the sample matrix.
Compared to conventional LLE methods, SPE has several distinctive advantages.
SPE generally reduces the analysis time, consumes lower amounts of organic solvents,
and is less costly than LLE (Gan and Bondarenko 2008). Furthermore, compatibility of
reversed-phase LC systems with aqueous samples allows on-line coupling of SPE with
the analytical system. On-line SPE systems consist basically of a binary pump, one or
more high-pressure six-port valves, and a clamping system for automated exchange of
the SPE columns (Hogenboom et al. 2000). The sample is passed through the cartridge,
and subsequent elution with the LC-gradient drives the retained compounds into the
chromatographic column. SPE-LC with tandem mass spectrometry detection appears to
be the method of choice to control the quality standards for individual pesticides and
the sum of all pesticides including their relevant metabolites, degradation and reac-
tion products set up in the Directive 2006/118/EC (Lepom et al. 2009). The disadvan-
tage of SPE is that suspended solids and salts can cause the blockage of SPE cartridges.
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