Page:NIOSH Manual of Analytical Methods - 5601.pdf/14

 ORGANONITROGEN PESTICIDES: METHOD 5601, Issue 1, dated 15 January 1998 - Page 14 of 21 NOTES ON HPLC ANALYSIS OF CARBAMATE, UREA, AND SULFENIMIDE PESTICIDES It is expected that the analyst will have a well-founded background in the analytical practices and principles that are required for any method to work. The following list of cautions and reminders is provided for convenience; most will impact the success or failure of this method. A.

ANALYTES 1. Aldicarb a. The UV response for aldicarb passes through a minimum at approximately 225 nm. If only two wavelength channels are used (225 nm and 200 nm), the response for aldicarb at 200 nm should be monitored(refer to UV spectrum for aldicarb). At 205 nm the signal will be smaller, but the signal-to- noise ratio may be better on particular instruments. An alternative but lesser maximum is at approximately 245 to 246 nm. This latter maximum, though small, may have less background noise and interferences from coelutants. b. Aldicarb is a highly toxic pesticide. Care should be exercised in handling pure stock material. 2. Benomyl (see also Carbendazim) a. Benomyl breaks down rapidly by either hydrolysis in protic solventse.g., ( water or methanol) or solvolysis in nonproticsolvents (e.g., methylene chloride or acetonitrile) to carbendazim. The breakdown of benomyl can be so rapid that no benomyl will be detected at all after approximately 4 to 24 hours at room temperature.The rate of solvolysis is slower in the less polar methylene chloride andvery rapid in acetonitrile, even exceeding the rate of hydrolysis [10-12]. Benomyl standards in thesenonprotic solvents can be stabilized with the addition of 1% n-butyl isocyanate [30]. The preservative effect is lost as soon as the solution is diluted in solvents that contain or consist of protic solvents, since protic solvents also react with the isocyanates. It is generally unnecessary to add any preservative to the benomyl standard solution (it breaks down anyway). The carbendazim produced can precipitate out, if the solutions are too concentrated. In such an event, the addition of 1% n-butyl isocyanate is essential. b. Since benomyl breaks down to carbendazim, do not include both benomyl and carbendazim in the same standard mixture. c. When analyzing for benomyl, both benomyl and carbendazim must be determined. The results can be reported as either benomyl or carbendazim by converting the response of one to the equivalent response for the other at any particular wavelength. The relative response of benomyl to carbendazim at 225 nm has been determined to be approximately 1.0738, (adsorption-benomyl/ adsorption-carbendazim). This ratioshould be determined for individual instruments with equimolar solutions analyzed separately, and with the benomyl injection solution being preserved with 1% n-butyl isocyanate. To convert carbendazim response to the equivalent benomyl response so that the values can be summed, multiply the carbendazim response by 1.0738 and add this to the benomyl response. Report results as benomyl. Alternately, to report results as carbendazim, divide benomyl response by 1.0738 and then add this to the carbendazim response. Report as total carbendazim. d. See notes for carbendazim. 3. Captan a. Captan is not stable in methanol or in aqueous mixtures of methanol or acetonitrile at temperatures greater than –12 °C. Therefore, do not dilute desorbing solutions with water or methanol to weaken the injection solvent. Instead,use small injection volumes of acetonitrile solutions. b. Use methylene chloride for making stock standard solutions. See Note A16 of this Appendix. c. Captan has low absorption at 225 nm (see UV spectrum for captan). If only two wavelength channels are used (225 and 200 nm), the response for captan at 200 nm should be monitored. Although a lesser response may be obtained at approximately 205 or 210 nm, this can be accompanied by a proportionately better signal-to-noise response and may be an optional choice of wavelength if it is compatible with other analytical objectives. 4. Carbaryl (see also Diuron): Carbaryl tends to break down to 1-naphthol. Chloroacetic acid in methanol will inhibit this reaction [9], but this reagent is deleterious to other analytes. Carbaryl is NIOSH Manual of Analytical Methods, Fourth Edition