Overview of Liquid Extraction Methods for Pesticides Recovery
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The variety of standardized methods for pesticide analysis were developed. Among these, there were presented methods for organochlorine and organophosphorus compounds, polychlorinated biphenyls and chlorobenzenes (EN ISO 6468, USGS O-1104-83, ASTM 6630), USGS O-3106-83 method for triazines, USGS 3107-83 method for the selected carbamates (Liska 2006).
Various extraction techniques for pesticides were developed by the US Environmental Protection Agency: EPA 505 for organohalide pesticides (Winfield 1989), EPA 507 for nitrogen- and phosphorus-containing pesticides (Engels 1987), EPA 508 for chlorinated pesticides (Lichtenberg, Engels & Graves 1989). All EPA methodologies include not only detailed description for extraction and determination, but also the directions for qualitative and quantitative determinations, instructions for quality control and estimation of method performance, as well as guides for analysis of the complex samples.
EPA Methods for CIPC and other Pesticides
The EPA Method 1699 (Environmental Protection Agency 2007) proposed usage of methylene chloride in continuous liquid-liquid extraction with application of a rotary evaporator or heating mantle. The Method 505 (Winfield 1989) presented the determination of 26 pesticides in raw source water or drinking water. The extraction was performed from a 35-ml sample by 5 ml of hexane. The Method 507 (Engels 1987) is applicable for CIPC. It described the procedures for manual and automatic extraction with methylene chloride, pH adjustment to 7 (with phosphate buffer) and sodium chloride addition. The detection limit for CIPC reported as 0.5 μg/l (0.5-3.8 μg/l for other analytes). The addition of sodium chloride increases the ionic strength, decreases analyte water solubility, and the recovery rate increases. The same effect was observed by Cheng et al. (2011). The research reported the improvement of extraction efficiency in presence of low concentration of sodium chloride. This is explained by the salting-out effect.
The Method 508 for determination of chlorinated pesticides in water samples (Lichtenberg, Engels & Graves 1989) is methodologically similar to Method 507. The main difference is in detection limits, which was set significantly lower and varied between 0.0025 and 0.025 μg/l.
Novel Modifications of LLE Extraction
The classical LLE extraction requires improvement due to low one-step recovery rates, large amounts of hazardous organic solvents used, and high running cost (due to multiple stages). In addition, a large volume of the sample is often required for trace analysis. Intense academic research developed various modifications or alternatives for LLE. The effective extraction technique has to be simple in operation and sample preparation; it has to concentrate an analyte in minimum stages (preferably one), and consume low quantities of solvent. Apparently, it has to exhibit a high recovery rate and enrichment factor.
Extraction Improvement Techniques
The extraction is intensified by agitation or heating. Thus, microextraction (Jia et al. 2010), microwave-assisted (Smith 2003), vortex-assisted (Li et. al. 2012) extraction methods were developed. If agitation or heating cannot be used, ultra-sound assisted (Cheng et al. 2011) extraction is applicable.
Intensification by heating is inapplicable for CIPC recovery, since carbamate pesticides and their metabolites are thermally instable, thus heating as cannot be applied for CIPC (García de Llasera & Bernal-González 2001).
As an alternative, application of various solvents can exhibit different recovery rates (Smith 2003). The solvent used for extraction should be immiscible with water; its density should be lower than water density; it has to exhibit high extraction efficiency for the analyte (Barrio et al. 1996).
Barrio et al. (1996) developed two improved LLE methods for several organic pesticides. These was a single solvent method with two consecutive extractions and a sequential combination of two solvents. The optimum pH range for the effective recovery was 3-7. It was found that the solvents with low polarity (in this study represented by isooctane) gave very low extraction yields, though they might be effective for extraction of non-polar analytes. Significantly better yields (preconcentration factor 2.5-2.8) were obtained with mixture of the solvents, for example, isobutylmethylketone and xylene; ethyl acetate and xylene, when the method with two consecutive extractions was applied. The results of the extraction studies with application of two solvents combination showed almost no positive effect and thus are inadvisable.
Simultaneous Separation of Several Compounds
The simultaneous separation of several compounds is a complicated task and often involves sophisticated procedures. For example, the group of scientists from China Agricultural University (Li et al. 2012) developed a vortex-assisted surfactant-enhanced-emulsification LLE microextraction for separation of five herbicides. The method requires surfactant and cosurfactant application, along with vortex agitation. The recovery rates varied between 80 and 100% for all analytes.
However, simultaneous extraction becomes a complicated task when solubilities of analytes vary significantly. This is the case for CIPC and 3CA separation, which solubilities are 89 mg/l and 2.2g/l, respectively (IUPAC 2011). Thus, development of the procedure for simultaneous CIPC and 3CA separation is an important research task.
Choice of a Solvent for LLE
The application of dispersive liquid-liquid microextraction requires the use of solvents with higher densities than water. The choice of a solvent is a vital stage for the effective recovery. Thus, chlorobenzene, carbon tetrachloride, tetrachloroethylene, carbon disulfide are reported effective for polychloric aromatic hydrocarbons, antioxidants, phenols and phthalate esters. Another prospective solvent is ionic liquid (for example, 1-alkyl-3-methylimidazolium hexafluorophosphate) used for LLE and microwave-assisted extraction. It is a good solvent for aromatic compounds with apolar group, including chloroanilines, exhibiting high affinity and good performance in HPLC column. When used as extraction solvents, the recovery rates of organophosphorus pesticides are 75-80% for ionic liquids, 45-65% for carbon tetrachloride and chlorobenzene (35 μl of a solvent used). The disperser solvent is methanol. The application of various disperser solvents (ethanol, acetonitrile, and acetone) had almost no effect and recovery rates were 55-60%, and methanol provided the maximum recovery. The procedure requires a 5-minute centrifugation (He et al. 2009).
In the work of Wu et al. (2009) the dispersive liquid-liquid microextraction of carbamate pesticides with acetone as a disperser solvent and chloroform, trichloromethane, or tetrachloroethylene as extraction solvent (50-70 μl) resulted in 20-40% extraction recovery. The study of the salting-out effect with sodium chloride was also performed and showed no efficiency.
LLE Method for CIPC and 3CA Simultaneous Extraction
Development of an effective method for CIPC and 3CA simultaneous extraction should route the following stages:
1) choice of equipment and vessels that provide consistent recovery rates;
2) the appropriate solvent choice (miscible or immiscible);
3) selection of improvement techniques (agitation, vortex, microvawe, etc.). For CIPC and 3CA, heating is inapplicable;
4) testing the salting-out effect or effect of pH
For each method, the recovery rate is the performance criterion.
The rotary evaporator 1 provides consistent results of CIPC and 3CA recovery, and for DCM solution the recovery is close to 100%. On the contrary, the methodology of 3CA extraction at rotary evaporator 2 has a systematic fault, which results in inconsistent and low analyte extraction. The extraction of CIPC at rotary evaporator is efficient, and shows 100% recovery for all the studied cases.
The difference between two rotary evaporators used for the experiment is vacuum, 638 mmHg for Rotary 1 and 760 mmHg for Rotary two. During the procedure, water is evaporated, and vacuum is used for process intensification. 3CA is relatively well soluble in water (2.2 g/l, 25 times higher than CIPC), and it carries over with water stream during evaporation; therefore, 3CA recovery rate does not exceed 40% for all the studied cases.
The rotary evaporator 1 should be used for the further research to ensure the consistent recovery results.