M ICRO -LLE

9.2.2 M ICRO -LLE

9.2.2.1 Principles and Procedures Micro-LLE is a miniaturization of standard LLE in that only a very small amount of

solvent is used for extraction. For instance, Zapf et al. [7] developed a micro-LLE method for the analysis of 82 various pesticides in tap water. Briefly, a 400 mL tap water sample in a 500 mL narrow-necked bottle was saturated with 150 g NaCl and buffered to a pH value of 6.5 –7.0. The water sample was spiked with analyte mixtures in 100 mL methanol to achieve concentrations of 50, 100, and 500 ng=L. After addition of 500 mL toluene, the bottle was sealed and shaken for 20 min at 420 rpm. After phase separation, the solvent layer was brought up to the bottleneck by addition of a saturated NaCl solution using a Pasteur pipette connected to a separating funnel. About 150 mL of the toluene phase was transferred into 200 mL vials and 2 mL was injected into a gas chromatograph (GC) with electron capture detector (ECD) or nitrogen phosphorus detector (NPD) for detection. For 68 com- pounds, the recoveries were higher than 50%. The mean relative standard deviations (RSD) at spiking levels of 50, 100, and 500 ng=L were 7.9%, 6.6%, and 5.2%, respectively. In most cases, compounds were reproducibly detected at concentrations well below 0.1 mg=L.

de Jager and Andrews [8] have described a micro-LLE method, in which a single drop of water-immiscible solvent is attached to the tip of a syringe needle, for the analysis of organochlorine pesticides in water samples. This method is also called solvent microextraction (SME) or single-drop microextraction (SDME) [9]. In this method, a 2 mL drop of hexane containing 100 ng=mL of decachlorobiphenyl as internal standard was used as the extraction solvent and immersed in the stirred sample solution for a 5 min extraction time. The sample solution was stirred at a rate of 240 rpm, and a Hamilton 10 mL 701SN syringe fitted with a Chaney adapter (Hamilton, Reno, NV, USA) was used in all extractions and injections. By using the Chaney adapter, the maximum syringe volume was set to 2.2 mL and the delivery volume was set to 2.0 mL. For the extraction, 2.2 mL of hexane was drawn into the syringe and the plunger was depressed with the stop button engaged,

236 Analysis of Pesticides in Food and Environmental Samples causing 0.2 mL to be expelled. The microsyringe was then positioned in the

extraction stand in such a way that the tip of the extraction needle protruded to a depth of about 8 mm below the surface of the aqueous solution. The syringe plunger was then completely depressed causing a 2 mL drop to form on the needle tip. The drop was suspended from the needle for 5 min at which time the plunger was withdrawn to 2.2 mL with the needle tip still submerged in the sample solution. The contents of the syringe were then injected into the GC for analysis. Total analysis time was less than 9 min, allowing 11 samples to be screened per hour. This method was therefore useful for quick screening of organochlorine compounds in water. Using a similar method, Liu et al. [9] was able to detect fungicides such as chlorothalonil, triadimefon, hexaconazole, and diniconazole in water at 0.006 –0.01 m g=L with RSD < 8.6%.

9.2.2.2 Advantages Micro-LLE is advantageous over the conventional LLE in that only a very small

amount of organic solvent is used. As a significant fraction or all of the organic phase is used for detection, good sensitivity may be achieved. Micro-LLE is therefore far less time consuming and inexpensive.

9.2.2.3 Disadvantages Micro-LLE operates at a phase ratio that does not favor pesticide enrichment into the

organic phase. It is difficult to automate, and performance is likely dependent on the analyst’s skills. The solvent chosen must be completely immiscible with water, and therefore micro-LLE is suitable only for nonpolar pesticides. Inconsistency in recovery may be overcome by using an internal standard at the extraction step. This method is more appropriate for rapid screening, rather than for routine analysis.