Page:NIOSH Manual of Analytical Methods - 5601.pdf/18

 ORGANONITROGEN PESTICIDES: METHOD 5601, Issue 1, dated 15 January 1998 - Page 18 of 21 holdup time). The value of keeping analytes from eluting too close to the solvent front cannot be overemphasized. d. Buffer: For the compounds evaluated, formetanate, carbendazim, and benomyl required a buffer in order to obtain good peak shapes. Of several pHs tested, pH 6.8 to 7.1 gave the best results. Buffer concentration should be 0.01 to 0.05M. The concentration and pH of the bufferhad a great effect on exact retention time of formetanate. 5. Dwell Volumes: Dwell volumes can affect the retention times and peak shapes of analytes. This is internal volume of the system from the point of mixture of the mobile phases A and B to the head of the column. It includes the volume of the transfer lines; any in-line filters (which should be between the pump and the sample injector if at all, and definitely not after the sample injector); the sample injector; the guard column; and the head of the analytical column. This volume can range from approximately 0.6 to 3.8 mL. At a flow rate of 1 mL/minute, each mL dwell volume represents a minute delay between the time that the pump produces a given change in a mixture of solvent and the time when the analytical column experiences that change. In effect, it is equivalent to a solvent program delay [13,14]. For this method, lower dwell volumes and no program delays gave better peak shapes for the early eluites. 6. Dead Volumes: Dead volumes can cause the method to fail seriously. Important areas to look for are larger bore and longer-than-necessary transfer lines between the sample injector and the analytical column [15]. Another area of potential large dead volumes are poorly designed or connected guard columns and sample injectors. 7. UV Wavelength Selection a. The wavelengths specified in this method are a best compromise for all of the analytes listed in Table 1. There are maxima for each of the pesticides that would give greater sensitivity and/or signal-to-noise ratio. If only a selected few of the pesticides in Table 1 are to be determined, other wavelengths may be considered. For this purpose, Table, an orthographic projection of the UV absorbances, is provided. From this table, a wavelength can be selected in the vertical column that intersects the zones of greater UV absorbance for the compounds of interest. For example, if only ureas are to be determined, the selection of an absorbance band of 240 to 250 nm would give greater sensitivity for these compounds and less interference from others. b. Spectra are also provided in the Backup Data Report [1] for many analytes and potential interferences. These were obtained under actual operating conditions. Because of unavoidable errors in background subtractions, the profiles of these spectra are subject to errors at the low wavelength ends of the spectra (190 nm to 210 nm). This could be attributed to the absorbance from the alcohol modifier to the mobile phases, which, in spite of efforts to add an equal amount to both phases, produces a slight baseline rise near the end of the chromatograms. c. Many of the O-aryl carbamates and the sulfenimides absorb well only in the low UV range (<215 nm). This is generally an area of great background noise. Many contaminants (plasticizers and solvents) also absorb in this range better than at higher wavelengths. Selecting a longer wavelength that is not at the absorption maximum may give a better signal-to-noise ratio and thus actually increase sensitivity. This should be determined by experimentation for selected analytes. d. Bandwidth: A bandwidth of 15 nm was used for evaluation of this method. C. INTERNAL STANDARDS 1. Calibration Internal Standards: Internal standards are essential for obtaining the precisions listed in Table 2 [1,16]. Acetanilide was found to be unretained by the media during desorption, and therefore, could be added to the extraction fluid. It was also found to be relatively stable and not to interfere with the retention times of other analytes. A second compound, acetophenone, may be added as a back-up internal standard. It is also unretained by all media except XAD-2, on which it has about a 95% recovery. It may be used whenever the acetanilide is interfered with by coeluting analytes or contaminants. It alsoserves to indicate, by monitoring relative response between the NIOSH Manual of Analytical Methods, Fourth Edition