S OLID -P HASE M ICROEXTRACTION

9.3.3 S OLID -P HASE M ICROEXTRACTION

9.3.3.1 Principles and Procedures Although SPE methods use less amount of solvents, they are multiple-step proced-

ures and are still somewhat time consuming. In 1990, an alternative extraction procedure employing SPME was introduced by Pawliszyn and coworkers [37,38]. In SPME, a thin fiber is coated with a sorbent and is exposed to the aqueous solution or the headspace of an aqueous sample to cause partitioning of some of the target analyte into the sorbent phase of the fiber. The fiber is then withdrawn, and introduced directly into a GC inlet to thermally desorb the enriched analyte into the GC column or eluted with the mobile phase in the mode of LC analysis. This technique fuses sample extraction and analysis into a single, continuous step, is compatible with GC and LC, and eliminates the use of any solvent for extraction. SPME is an equilibrium process that involves the partitioning of analytes between the sample and the extraction phase. Sampling conditions must therefore be system- atically optimized to increase the partitioning of analytes in the coated fiber. Besides sampling conditions and analyte properties, the type of fiber coating is one of the most important aspects of optimization. Supelco (Bellefonte, PA, USA) is the main supplier of commercialized SPME fibers. Depending on the coating phase, the commercially available SPME fibers can be divided into absorbent- and adsorbent- type fibers. Absorbent-type fibers extract the analytes by partitioning of analytes into

a ‘‘liquid-like’’ phase (e.g., polydimethylsiloxane or PDMS) whereas adsorbent-type fibers (e.g., activated carbon) extract the analytes by adsorption.

SPME consists of two extraction modes. One is the direct immersion mode, in which analytes are extracted from the liquid phase onto an SPME fiber, and the other is the headspace mode (HS –SPME), in which analytes are extracted from the headspace of a liquid sample onto the SPME fiber [39]. In general, direct SPME is more sensitive than HS –SPME for analytes present in a liquid sample, although HS –SPME gives lower background than direct SPME [40].

SPME can be coupled with either GC or LC. Coupling of SPME –GC is suitable for nonpolar and volatile or semivolatile pesticides. However, thermal desorption at high temperature creates practical problems such as degradation of the polymer, and furthermore, many nonvolatile compounds cannot be completely desorbed from the fiber. Solvent desorption is thus proposed as an alternative method through SPME –LC coupling. An organic solvent (static desorption mode) or the mobile phase (dynamic mode) is used to desorb the analytes from the SPME fiber.

9.3.3.2 Advantages Several advantages can be pointed out in relation to SPME: it is solvent free, uses the

whole sample for analysis, and requires only small sample amounts. The fibers are highly reusable (up to more than 100 injections). The success of SPME is based on its combining sampling, isolation, and concentration into a continuous step, and its compatibility with GC or LC.

Determination of Pesticides in Water 243

9.3.3.3 Disadvantages SPME suffers drawbacks such as sample carry-over, high cost, and a decline in

performance with increased usage. The reluctance to adopting SPME in some cases can be also due to the steep learning curve expected for new users. To achieve good reproducibility, conditions such as fiber exposure time, solution stirring speed, fiber immersion depth, and fiber activation time and temperature must be precisely con- trolled, which may prove to be difficult if a manual assembly is used. In general, the use of manual SPME is tedious and gives low sample throughput. However, precise and easy handling of SPME can be realized using an automated SPME sampler such as the Combi-PAL autosampler made by Varian (Palo Alto, CA, USA).

9.3.3.4 Trends In addition to the general purpose PDMS and polyacrylate (PA)-coated fibers, a large

number of fiber coatings based on solid sorbents are available, namely the PDMS – divinylbenzene (PDMS –DVB), Carbowax–DVB (CW–DVB), CW–templated resin (CW –TR), Carboxen–PDMS, and DVB–Carboxen PDMS coated fibers [41]. SPME

fibers with bipolar characteristics can be very useful for the simultaneous analysis of pesticides representing a wide range of polarities.

In-tube SPME is a new variation of SPME that has recently been developed using GC capillary columns as the SPME device instead of the SPME fiber. In-tube

SPME is suitable for automation, and automated sample handling procedures not only shorten the total analysis time but also usually provide better accuracy and precision relative to manual SPME. In Ref. [42], an automated in-tube SPME method coupled with LC=ESI –MS was developed for the determination of chlorin- ated phenoxy acid herbicides. A capillary was placed between the injection loop and the injection needle of the autosampler. A metering pump was used to repeatedly draw and eject sample from the vial, allowing the analytes to partition from the sample matrix into the stationary phase. The extracted analytes were directly des- orbed from the stationary phase by mobile phase, transported to the LC column, and then detected. The optimum extraction conditions were 25 draw=eject cycles of

30 ml of sample in 0.2% formic acid (pH ¼ 2) at a flow rate of 200 ml=min using

a DB-WAX capillary. The herbicides extracted by the capillary were easily desorbed by 10 ml acetonitrile. The calibration curves of herbicides were linear in the range

0.05 –50 mg=L with correlation coefficients above 0.999. This method was success- fully applied to the analysis of river water samples without interference peaks. The MDL was in the range of 0.005 –0.03 mg=L. The repeatability and reproducibility were in the range of 2.5% –4.1% and 6.2%–9.1%, respectively.

9.3.3.5 Applications Choudhury et al. [43] evaluated the use of SPME –GC analysis of 46 nitrogen- and

phosphorus-containing pesticides defined in the EPA Method 507. Effects of pH, ionic strength, methanol content, and temperature on extraction were determined. Analytes were extracted into a PDMS fiber and then thermally desorbed in a GC

244 Analysis of Pesticides in Food and Environmental Samples injector and analyzed. When analyzed by SPME GC=NPD or by SPME GC=MS,

34 and 39 pesticides, respectively, were measured at levels lower than the EPA MDLs and precision requirements. This method was applied to the analysis of contaminated well water, watershed, and stream water and compared to U.S. EPA Method 507 findings. The results demonstrated that SPME was a valuable tool for the rapid screening of 39 EPA Method 507 nitrogen- and phosphorus-containing pesticides in water.

Jackson and Andrews [44] evaluated the use of SPME under nonequilibrium conditions for analysis of organochlorine pesticides. SPME is typically performed for a length of time that nears the equilibrium time of the analyte in the sample. However, equilibrium times for organochlorines fall in the range of 30 –180 min. Studies show that linear responses having good precision are possible by using extraction times well short of equilibrium times [37,45]. With a 2 min extraction time and 100 mm PDMS fiber, analysis of a sample took less than 10 min, with MDLs in the order of 10 ng=L.

Chafer-Pericas et al. [46] compared the advantages and disadvantages of two different configurations for the extraction of triazines from water samples, on-fiber SPME coupled to LC, and in-tube SPME coupled to LC. In-tube SPME used a packed column or an open capillary column. In the on-fiber SPME configuration, the fiber coating was PDMS–DVB. The MDLs obtained with this approach were

between 25 and 125 mg=L. The in-tube SPME approach with a C18 packed column (35 mm 3 0.5 mm I.D., 5 mm particle size) connected to a switching microvalve provided the best sensitivity; under such configuration, the MDLs were between 0.025 and 0.5 mg=L. The in-tube SPME approach with an open capillary column coated with PDMS (30 cm 3 0.25 mm I.D., 0.25 mm of thickness coating) connected to the injection valve provided MDLs between 0.1 and 0.5 mg=L.