S TANDARD SPE

9.3.1 S TANDARD SPE

9.3.1.1 Principles The trend in pesticide analysis in water has moved away from LLE to SPE. This is

due to the better extraction efficiencies, ease of use, less use of solvents, potential for automation, and better selectivity of SPE. Compared with most other methods, SPE is a widely used and mature method. In SPE, the analyte is transferred from the aqueous phase onto a sorbent phase, which can then be recovered for analysis. Sorbents available in standard SPE include the common inorganic adsorbents used in liquid chromatography (LC), such as silica gel, as well as activated charcoal, bonded silica phases, and polymers [10]. The most popular phases are octadecyl (C18) and octyl-silica (C8), styrene-divinylbenzene copolymers, and graphitized carbon black.

Alkyl-bonded silica sorbents : The peak tailing and poor selectivity of silica gel led to the development of silica-based phases with an alkyl- or aryl-group substituted

Determination of Pesticides in Water 237 silanol. The functionality properties of the sorbent depend on the percentage of

carbon loading, bonded-silica porosity, particle-size, and whether the phase is end- capped. Endcapping is used to reduce the residual silanols, but the maximum percentage of endcapping is 70%. The most popular sorbents from this group are C18 and C8.

Carbon sorbents : An important gain of graphitized carbon black (GCB) as the sorbent is that the recoveries do not decrease when environmental waters with dissolved organic carbon (DOC) are extracted. This is due to the fact that fulvic acids, which represent up to 80% of the DOC content in surface waters, are adsorbed on the anion-exchange sites of the GCB surface, and therefore they cannot compete with nonacidic pesticides for adsorption on the nonspecific sites of the sorbent. GCB has three main disadvantages: the collapsing of the sorbent, desorption problems during elution, and the possibility of reactions between the analytes and the sorbent surface, leading to incomplete sorption and desorption.

Polymeric resins : With these sorbents, the retention behavior of the analytes is governed by hydrophobic interactions similar to C18 silica, but, owing to the aromatic rings in the network of the polymer matrix, one can expect strong electro- donor interactions with aromatic rings of solutes.

Mixed phases : The advantages of each sorbent can be combined in the form of a mixture of sorbents used in the same SPE column.

9.3.1.2 General Procedures

A typical SPE sequence includes the activation of the sorbent bed (wetting), removal of the excess of activation solvent (conditioning), application of the sample, removal of interferences (cleanup) and water, elution of the sorbed analytes, and reconstitution of the extract [10]. Exact conditions are usually specified by the manufacturer, and may vary significantly in types of solvents used for conditioning and elution. A general procedure for using SPE cartridges is as follows [11]:

Wash the cartridge with a small amount of relatively nonpolar solvent (e.g., ethyl acetate, acetone), followed by a relatively polar solvent (e.g., methanol), and finally water.

Without letting the cartridge become dry, pass the water sample (e.g., 1 L) through the column under vacuum at a relatively fast rate (e.g., 15 mL=min).

If the water sample contains an appreciable amount of suspended solids, filter the sample to remove suspended solids before loading.

After the sample is loaded, wash the cartridge with a small amount of water and dry the cartridge by passing air for a short time.

Elute the SPE cartridge with the same solvents used at the preparation step, except in a reversed order.

The eluate is dried with a small amount of anhydrous sodium sulfate and further evaporated to dryness under a gentle stream of nitrogen.

The residue is recovered in a small amount of solvent appropriate for GC or LC analysis.

238 Analysis of Pesticides in Food and Environmental Samples

9.3.1.3 Advantages Compared with conventional LLE methods, SPE has several distinctive advantages.

SPE generally needs a shorter analysis time, consumes much less organic solvents, and may be less costly than LLE [11]. SPE also offers the great advantage for easier transportation between laboratories or from the field to the laboratory, and for easier storage. For example, water samples can be processed at a remote site, and only the cartridges need to be transported back to the laboratory, which makes sampling at remote sites feasible. Automation or semiautomation may be potentially achieved for either off-line or on-line use of SPE, although manual, off-line is likely the dominant form that has been used.

9.3.1.4 Disadvantages There are many different types of sorbents and configurations (e.g., mass of sorbent

per tube), and each SPE is inherently best suited for a specific class of pesticide compounds. This, when combined with operational factors such as flow rate, con- ditioning, and elution, and the effect of sample matrix, can make the recovery of pesticides highly variable [11]. In addition, suspended solids and salts are known to cause blockage of SPE cartridges. Samples compatible with SPE must be relatively clean (e.g., groundwater). When surface water samples are analyzed, prefiltration is generally necessary to remove the suspended solids. This may not be desirable for hydrophobic compounds, because a significant fraction of the analyte is associated with the suspended solids.

Both low and enhanced recoveries have been observed when SPE is used for extracting pesticides from water samples. For instance, when using C18 SPE cart- ridges for the determination of 23 halogenated pesticides, Baez et al. [11] found that recoveries depended on the pesticides, and losses occurred with heptachlor, aldrin, and captan. Recoveries for vinclozolin and dieldrin from groundwater were lower than those obtained from nanopure water. In river water, losses of these compounds were higher. High losses were also observed for trifluralin, a-BHC, g-BHC, tri- allate, and chlorpyrifos. In a follow-up study, Baez et al. [12] evaluated the use of C18 SPE columns for the determination of organophosphorus, triazine, and triazole- derived pesticides, napropamide, and amitraz. Under general extraction conditions, losses were found for amitraz, prometryn, prometon, dimethoate, penconazole, and propiconazole. At 100 ng=L, enhanced responses were observed for mevinphos, simazine, malathion, triadimefon, methidathion, and phosmet, which was attributed to matrix effects.

9.3.1.5 Trends Current trends include the use of SPE on-line, coupling with selective or sensitive

detectors, the use of stable isotopes to overcome the issue of variable recoveries, and automation. Bucheli et al. [13] reported a method for the simultaneous iden- tification and quantification of neutral and acidic pesticides (triazines, acetamides, and phenoxy herbicides) at the low ng=L level. The method included the

Determination of Pesticides in Water 239 enrichment of the compounds by SPE on GCB, followed by the sequential elution

of the neutral and acidic pesticides and derivatization of the latter fraction with diazomethane. Identification and quantification of the compounds was performed with GC

–MS using atrazine-d5, [ C6]-metolachlor, and [ C6]-dichlorprop as internal standards. Absolute recoveries from nanopure water spiked with 4 –50

and the phenoxy acids, respectively. Recoveries from rainwater and lake water spiked with 2 –100 ng=L were 95 zines, the acetamides, and the phenoxy acids, respectively. Average method precision determined with fortified rainwater (2 –50 ng=L) was 6.0

MDLs ranged from 0.1 to 4.4 ng=L. Crescenzi et al. [14] reported the coupling of SPE and LC=MS for determining 45 widely used pesticides having a broad range of polarity in water. This method involved passing 4, 2, and 1 L, respectively, of drinking water, groundwater, and river water through a 0.5 g GCB cartridge at 100 mL=min. In all cases, recoveries of the analytes were better than 80%, except for carbendazim (76%). For drinking water, MDLs ranged between

0.06 (malathion) and 1.5 (aldicarb sulfone) ng=L. Kampioti et al. [15] reported

a fully automated method for the multianalyte determination of 20 pesticides belonging to different classes (triazines, phenylureas, organophosphates, anilines, acidic, propanil, and molinate) in natural and treated waters. The method, based on on-line SPE-LC-MS, was highly sensitive with MDLs between 0.004 and 2.8 ng=L, precise with RSDs between 2.0% and 12.1%, reliable, and rapid

(45 min per sample).

9.3.1.6 Applications Fernandez et al. [16] performed a comparative study between LLE and SPE with

trifunctional bonding chemistry (tC18) for 22 organochlorine and 2 organophos- phorus pesticides, 2 triazines, and 7 PCBs. Mean recovery yields were higher with the LLE method, although SPE for most of the 33 analytes surpassed 70%. The MDLs for both techniques were below 5 ng=L, except for parathion (7 ng=L),

methoxychlor (8 ng=L), atrazine (35 ng=L), and simazine (95 ng=L). Patsias and Papadopoulou-Mourkidou [17] reported a rapid multiresidue method for the analy- sis of 96 target analytes in field water samples. Analytes were extracted from 1 L filtered water samples by off-line SPE on three tandem C18 cartridges. The sorbed analytes eluted with ethyl acetate were directly analyzed by GC-ion trap MS (GC –IT–MS). The mean recoveries, at the 0.5 mg=L level, for two-thirds of the analytes ranged from 75% to 120%; the recoveries for less than one-third of the analytes ranged from 50% to 75% and the recoveries for the 10 relatively most polar analytes ranged from 12% to 50%. The MDLs for 69 analytes were below 0.01 mg=L; the MDLs for 18 analytes were below 0.05 mg=L; for captan, carbofenothion, deltamethrin, demeton-S-methyl sulfone, fensulfothion, deisopro- pylatrazine, and metamitron, the MDL was 0.1 mg=L and for chloridazon and

tetradifon, the MDL was 0.5 mg=L.

240 Analysis of Pesticides in Food and Environmental Samples