SPE D ISKS

9.3.2 SPE D ISKS

9.3.2.1 Principle and Procedures In a special form of SPE, the sorbent is bonded to a solid support that is configured

as a disk. During filtration, using SPE disks, the pesticides sorb to the stationary phase and then are eluted with a minimal amount of organic solvent. Empore disks (3 M, St. Paul, MN), bonded with a C18 or C8 solid phase, have been the most commonly used SPE disks [18]. The general procedure for using Empore disks is as follows, although details may vary for specific applications and for the types of SPE disks used [19].

Before use, condition Empore disks by soaking in a solvent (e.g., acetone).

Pass the water sample through the disk under vacuum on an extraction manifold. In some applications, a small amount of solvent modifier (e.g., methanol) is added to the water sample to improve pesticide recovery [20]. It is usually recommended that the disk should not be allowed to become dry during the extraction.

After sample extraction, elute the disks with a small amount of solvent (e.g., dichloromethane –ethyl acetate mixture) or extract the disk by mixing the disk in an extracting solvent in a closed vessel.

Evaporate the solvent extract to a small volume, and an aliquot of the final sample extract is injected into GC or LC for detection.

9.3.2.2 Advantages Like SPE cartridges, the use of SPE disks also greatly reduces the volume of

solvents, decreases sample preparation time and labor, and sometimes increases extract purity from water samples [21 –23]. SPE disks can also be used for temporary pesticide storage [24,25], field extraction of pesticides [26], and shipping pesticides from one location to another [27,28].

Field extraction capability adds a new dimension to the sampling of natural water samples. When using the conventional approach, water samples are collected in glass containers and transported or shipped to a laboratory for extraction and analysis. With SPE disks, it is possible to extract pesticides from water in the field and transport only the disks to the laboratory for elution and analysis [26]. This elimin- ates the risk of glass breakage during collection, transport, and shipping, in addition to greatly reducing freight costs, and preserves some pesticides that are prone to hydrolysis. Numerous studies have shown that SPE disks can be used to extract pesticides from water and to preserve sample integrity until laboratory analysis [18,28 –30]. Pesticide stability studies using Empore disks show that some pesticides have greater stability on C18 disks than in water at 48C [25]. For instance, Aguilar et al. [27,31] stored SPE cartridges at room temperature, 48C, and 208C for 1 week or

3 months, and found minimal losses of pesticide for the lowest temperature at both time intervals. A multistate regional project showed that the pesticides atrazine, chlorpyrifos, and metolachlor could be retained on SPE disks and shipped to another laboratory for analysis with little pesticide losses [27].

Determination of Pesticides in Water 241

9.3.2.3 Disadvantages The main difficulties encountered with any kind of SPE configurations are caused by

the presence of suspended particles in the sample. The particles of the alkyl bonded silica act as a mechanical filter that retains suspended soil or sediment particles, and the result is a loss of filtration due to clogging. This is very inconvenient when large volumes of sample are processed. To resolve this problem, acidification to a pH value of 2 is widely applied. Alternatively, the water sample is filtered prior to extraction. However, this treatment may not be desirable if the purpose of the analysis is to determine the total chemical concentration. In addition, although many studies have demonstrated the stability and good recovery of many pesticides from SPE disks, recoveries may vary with pesticide chemistry. It has also been shown that pesticide recovery from turbid water samples is less than that from deionized water samples [32]. Recoveries for compounds such as chlorpyrifos can

be low and variable [29]. Therefore, field spikes, surrogates, and other quality assurance measures must be considered when using SPE disks for field samples.

9.3.2.4 Trends

A couple of problems may be encountered when using Empore SPE disks for pesticide extraction at one site followed by shipment to another site for elution and analysis. Once removed for shipping, it is impossible to perfectly realign disks onto another laboratory’s extraction manifold so that the entire impregnated portion of the disk is exposed to the elution solvent. Realignment problems can result in reduced recovery from incomplete pesticide elution. This problem can be solved by combining the disks with the elution solvent in screw cap tubes, which are mixed on a shaker to extract pesticides from the disks [27]. In addition, surface water with high levels of particulates clogs disks and requires a filtration step prior to passing the water sample through the disk. Speedisks (J.T. Baker, Phillipsburg, NJ) offer an alternative to the use of traditional Empore SPE disks. Speedisks contain the extraction sorbent in a plastic housing, which is placed directly onto an extraction manifold, eliminating the realignment problems as noted earlier. The combination provides one-step filtration and extraction.

9.3.2.5 Applications Numerous studies have reported the use of SPE disks for extracting or preserving

pesticides from water samples. C18 Empore disks have been reported to extract some fungicides [33], carbamates and herbicides [34], or polar pesticides and herbicides [20] from waters. C8 Empore disks have been used to recover organo- chlorine pesticides, triazine herbicides, and other compounds from spiked water samples [35], and organochlorine, organophosphorus insecticides, triazine, and neutral herbicides from drinking water [23]. For instance, in Ref. [36], Empore C18 disks were used to extract a range of organophosphate compounds, including bromophos ethyl, bromophos methyl, dichlofenthion, ethion, fenamiphos, feni- trothion, fenthion, malathion, parathion ethyl, and parathion methyl. Using GC=MS or GC=FTD, MDLs were in the range of 0.01 –0.07 mg=L and the recovery was from 60.7% to 104.1%.

242 Analysis of Pesticides in Food and Environmental Samples