ORGANONITROGEN PESTICIDES: METHOD 5601, Issue 1, dated 15 January 1998 - Page 15 of 21 stabilized in acetonitrile desorbing solutions containing the TEA-PO 4 preservative for at least 24 hours at room temperature, long enough to permit HPLC analysis. Any 1-naphthol that is detected may represent breakdown of carbaryl prior to desorption or may come from another source. Carbaryl has been stabilized with the TEA-PO 4 buffer/preservative for up to three weeks at room temperature. 5. Carbendazim a. See notes for benomyl. b. Carbendazim can break down to 4-aminobenzimidazole and other compounds. In acetonitrile containing TEA-PO4 preservative, this breakdown appears to be inhibited for up to 24 hours at room temperature, long enough to permit analysis of desorbates. c. Carbendazim is itself used as a pesticide. There are also otheranalytes such as thiophanate methyl that can break down to carbendazim. Therefore, any carbendazim should not automatically be assumed to represent benomyl. 6. Carbofuran a. In most instances carbofuran and bendiocarb coeluted under all conditions tested. Their UV spectra are so nearly alike as to be practically indistinguishable. Any positive identification for carbofuran must be tempered by this knowledge and would require an alternate confirmation such as by mass spectrometry or a knowledge of the history of the sample. b. Propoxur and carbofuran elute very close together. The separation of the two compounds with nearly baseline resolution is a good test of resolving power of the HPLC system for neutral compounds on a C18 HPLC column. If they cannot be resolved, there may be problems with large dead volumes in the system, analytical column quality, or injection quality. 7. Chlorpropham: No serious problems are expected to be encountered. 8. Diuron: The breakdown product of carbaryl, 1-naphthol, elutes immediately after diuron under the conditions given. On certain columns, it may coelute with diuron, giving false positive responses. The retention time of 1-naphthol relative to diuron must be determined for any particular column if carbaryl is anticipated to have been present in the sample. 9. Formetanate a. At analytical concentrations formetanate is not stable either in methanol or in aqueous mixtures of methanol or acetonitrile with or without chloroacetic acid. Therefore, do not dilute desorbing solutions with methanol or water to weaken the injection solvent. Do not use chloroacetic acid as a preservative. Use only small injection volumes of acetonitrile solutions. b. The breakdown of formetanate in analytical solutions is inhibited in acetonitrile containing TEA-PO4 preservative at room temperature for up to 24 hours, long enough to permit analysis. c. At stock standard concentrations, formetanate dissolves better in 50:50 mixtures of methanol: acetonitrile and is stable if kept in freezer at –12 ± 1 °C. d. Formetanate is basic and possesses a single positive charge in solution at neutral pH (6.9 – 7.1). The use of TEA-PO4 buffer and/or base-deactivated HPLC columns is essential for the analysis for this compound. e. The exact retention time of formetanate is more affected by exact pH and ionic strength of the buffer (or any other electrolytes) than are the noncharged analytes. Day-to-day inconsistencies in the preparation of mobile phases will cause differences in retention time. If the analyte is eluting too close to methomyl (which just precedes it under the specified conditions), its retention time may be delayed by either raising the pH slightly (by 0.1 to 0.2 units), or by lowering the ionic strength of mobile phase A. 10. Methiocarb: Methiocarb has the same problems as carbaryl, only to a lesser extent. As methiocarb breaks down, there are several other peaks that arise. These have been shown to be alkylthiophenol and/or oxidation products. 11. Methomyl: Attention should be placed on the problem of methomyl as it relates to its early elution time. Refer to Notes B1a and B4 of this Appendix.
NIOSH Manual of Analytical Methods, Fourth Edition
