Page:NIOSH Manual of Analytical Methods - 5601.pdf/16

 ORGANONITROGEN PESTICIDES: METHOD 5601, Issue 1, dated 15 January 1998 - Page 16 of 21 12. Oxamyl: Attention should be placed on the problem of oxamyl as it relates to its early elution time. Refer to Notes B1a and B4 of this Appendix. 13. Propham: No problems are expected with propham. 14. Propoxur a. With prolonged standing in desorbing solutions at room temperature without preservative, propoxur has been observed to break down to some degree, although much less than carbaryl. b. Propoxur elutes very close to carbofuran. See comments under carbofuran. 15. Thiobencarb: Thiobencarb is relatively nonpolar and has lower (but acceptable) recoveries from the XAD-2 resin than the other analytes. 16. Solvent for Analyte Standard Solutions: Most of the analytes (Table 1) are soluble in acetonitrile at stock standard concentrations. The exceptions are benomyl, captan, and formetanate. Appropriate solvents are described underheadings for these compounds. In each of the solvents mentioned, analytes are stable at –12 ± 1 °C for at least 30 days. 17. Desorption Solvent and Preservative for Analytes: All analytes (except benomyl) were stable in acetonitrile at –12 °C for at least 48 hours. If refrigerated HPLC autosampler trays are available and all desorbing and tumbling operations are carried out in subambient conditions, no preservative should be needed. At room temperature, several analytese.g., ( carbaryl, methiocarb, and oxamyl especially) degraded unpredictably over 24 to 48 hours. The addition of as little as 0.2% v/v of aqueous 0.1 M triethylamine phosphate (TEA-PO4) buffer, pH 6.9 to 7.1, to the acetonitrile desorption solvent stabilizes all analytes (except benomyl) from hydrolysis or solvolysis for at least 48 hours at room temperature. Methanol, isopropanol, aqueous methanol, and acetonitrile containing any of these alcohols promote degradation of several analytes (especially captan and formetanate) and poor desorption efficiencies. The use of chloroacetic acid, as is required for aqueous samples in EPA Method 531.1 [9], is destructive to at least formetanate. Since the desorption solvent is also the HPLC injection solvent, refer to note on Injection Solvent (B1) for additional comments. B. HPLC CONDITIONS 1. Injection Solvent a. Normally a solvent producing equal or lower capacity factors than the mobile phase is desirable as an injection solvent in order to produce sharp peaks for early eluting analytese.g., ( oxamyl and methomyl) [31,32]. Since the initial mobile phase mixture of this method is mostly water, the only way to achieve this would be to dilute the samples with water. Water, however, is deleterious to several of the analytes specified in this method (see note A17 above). Therefore, an acceptable alternative is to inject very small volumes, not greater than 5 µL [31], of the desorption solvent (acetonitrile in this method) on a high-resolution HPLC column. If it is known that only analytes that are stable in aqueous solutions are to be determinede.g., ( oxamyl and methomyl, but not formetanate or captan), the desorbates may be diluted with water and larger volumes injected. By diluting with water, sharper peaks may be obtained for early eluting compounds with accelerated elution conditions such as the use of shorter HPLC columns, higher percentages of organic modifier, or higher percentages of mobile phase B in the initial conditions. b. All sample extracts must be filtered through individual 0.45-micron PTFE filters in order to prolong guard column lifetimes and to protect the injection system valving. c. The accidental or intentional inclusion of significant amounts of solvents less polar than acetonitrile in the injectionsolvent, such as tetrahydrofuran or acetone, may shorten retention times and adversely affectthe peak shapes of early eluites e( .g., oxamyl and methomyl) [31]. 2. Guard Columns: Guard columns are essential to the long life and reproducibility of results on the main analytical column. There are several on the market.Those giving the lowest possible dead volume preserve good peak shapes and resolution of quality analytical columns and are preferred. NIOSH Manual of Analytical Methods, Fourth Edition