Pesticide Analytical Manual Vol. I SECTION 301 Experiences with such effects are usually noted in a method so use of particular solvents can be avoided Polarity. Increasing the polarity of an extraction solvent may improve a methods ability to extract particular residues, but it usually also increases the amount of co- extractives. The presence of polar solvents may also affect subsequent cleanup steps, so residues may need to be transferred to a different solvent before the next step of the method is performed Boiling Point. Solvents with a low boiling point are preferred, if evaporation to accommodate detector compatibility or appropriate polarity is necessary. In some temperature if an azeotrope is fi nigh boiling point can be evaporated at a lower cases, a solvent with a relatively h formed by addition of another solvent. Several types of evaporation apparatus exist(Section 202 C), and choice of which to use is often related to the boiling point of a particular solvent Toxicity. Solvents vary in toxicity, and laboratories should choose the least toxic among equivalent choices. Certain solvents(benzene, carbon tetrachloride) should no longer be used in residue analysis Concentration and evaporation steps must be performed in an adequately ventilated hood, and other standard safety precau- tions must be followed (Section 207) Extraction The necessity of using water-miscible solvents to extract pesticide residues from high moisture products has long been established, as has the necessity of a"blend ing type"extraction process [1-4]. Acetone (Section 302), acetonitrile(Section 03), and methanol (Sections 401, 403) are used in PAM I multiclass and selective MRMs to extract nonionic residues from fruits and vegetables. Variations in polar ity may affect the degree to which each can extract any particular residue [5-8] Because extraction capabilities of these solvents are similar, other characteristics affect which solvent a developer chooses to use in a method. For example, devel- tile (Section 303)because it is less toxic, has a lower boiling point(57 C us 82 C), does not affect detectors adversely, and does not form a two-phase system with water during analysis of fruit, as acetonitrile does [9] Liquid-liquid partitioning of residues from initial extractant to nonaqueous sol- vent is a step common to most MRMs. Nature of the solvent(s)used in this step affects the degree of transfer of both residues and co-extractives. For example, in Section 302 El, petroleum ether is included in the separator with aqueous acetone and methylene chloride to reduce the amount of polar plant constituents that partition into the organic phase. However, in a method variation targeted at the highly polar methamidophos, petroleum ether is replaced with acetone to im- prove partitioning of methamidophos from the aqueous to the organic layer [10j Any MRM is applied with the understanding that certain residues are particularly difficult to extract, e. g, the polar residue methamidophos, above. In such cases notation of partial recovery is made in the table(s) of data that accompany the method description. Tentative identification of a residue known to be incom- pletely extracted by the method in use should then lead to re-analysis by another method or variation 301-5Pesticide Analytical Manual Vol. I SECTION 301 301–5 Transmittal No. 94-1 (1/94) Form FDA 2905a (6/92) Experiences with such effects are usually noted in a method so use of particular solvents can be avoided. Polarity. Increasing the polarity of an extraction solvent may improve a method’s ability to extract particular residues, but it usually also increases the amount of co￾extractives. The presence of polar solvents may also affect subsequent cleanup steps, so residues may need to be transferred to a different solvent before the next step of the method is performed. Boiling Point. Solvents with a low boiling point are preferred, if evaporation to accommodate detector compatibility or appropriate polarity is necessary. In some cases, a solvent with a relatively high boiling point can be evaporated at a lower temperature if an azeotrope is first formed by addition of another solvent. Several types of evaporation apparatus exist (Section 202 C), and choice of which to use is often related to the boiling point of a particular solvent. Toxicity. Solvents vary in toxicity, and laboratories should choose the least toxic among equivalent choices. Certain solvents (benzene, carbon tetrachloride) should no longer be used in residue analysis. Concentration and evaporation steps must be performed in an adequately ventilated hood, and other standard safety precau￾tions must be followed (Section 207). Extraction The necessity of using water-miscible solvents to extract pesticide residues from high moisture products has long been established, as has the necessity of a “blend￾ing type” extraction process [1-4]. Acetone (Section 302), acetonitrile (Section 303), and methanol (Sections 401, 403) are used in PAM I multiclass and selective MRMs to extract nonionic residues from fruits and vegetables. Variations in polar￾ity may affect the degree to which each can extract any particular residue [5-8]. Because extraction capabilities of these solvents are similar, other characteristics affect which solvent a developer chooses to use in a method. For example, devel￾opers of the method in Section 302 used acetone as extractant instead of acetoni￾trile (Section 303) because it is less toxic, has a lower boiling point (57° C vs. 82° C), does not affect detectors adversely, and does not form a two-phase system with water during analysis of fruit, as acetonitrile does [9]. Liquid-liquid partitioning of residues from initial extractant to nonaqueous sol￾vent is a step common to most MRMs. Nature of the solvent(s) used in this step affects the degree of transfer of both residues and co-extractives. For example, in Section 302 E1, petroleum ether is included in the separator with aqueous acetone and methylene chloride to reduce the amount of polar plant constituents that partition into the organic phase. However, in a method variation targeted at the highly polar methamidophos, petroleum ether is replaced with acetone to im￾prove partitioning of methamidophos from the aqueous to the organic layer [10]. Any MRM is applied with the understanding that certain residues are particularly difficult to extract, e.g., the polar residue methamidophos, above. In such cases, notation of partial recovery is made in the table(s) of data that accompany the method description. Tentative identification of a residue known to be incom￾pletely extracted by the method in use should then lead to re-analysis by another method or variation