ORGANONITROGEN PESTICIDES: METHOD 5601, Issue 1, dated 15 January 1998 - Page 19 of 21 two internal standards, when the first internal standard has a coeluting interference. 2. Alternate Calibration InternalStandards: It may be wise, especially under isocratic conditions, to select other internal standards having capacity factors closer to particular analytes of interest. There is a wide range of alkyl phenones available for late-eluting analytes, and 4-hydroxy-acetanilide (acetaminophen)for earlier eluting analytes. Their retention times are listed in Table 4. Since these longer alkyl chain phenones have increasingly lower recoveries from the XAD-2 resin, they should not be added to the extraction solutions until after the resin is removed. 3. Retention Index Reference Standards:These are optional standards and are used to establish relative retention times for qualitative purposes. These are highlighted in Table 4 and include a homologous series of alkyl phenones and acetanilides. Establishing retention times between two nearest-eluting reference standards gives a more consistent retention value than retention time alone or relative retention time using a single internal standard alone. This value, the Retention Index, varies according to column and analytical conditions, but should be relatively consistent for any one set of conditions and more reliable as a qualitative tool over a long period of time. This value is calculated as follows: RI(A)
=
Retention Index of Analyte “A”
Tr(A) Tr(RS Tr(RS where
Tr(A) Tr(RS-P) Tr(RS-F) N(RS-P)
F)
P)
Tr(RS
N(RS
P)
P)
= = = =
retention time of analyte A retention time of preceding reference standard retention time of following reference standard a number assigned to preceding reference standard, with zero assigned to the first peak in the series. (Numerical assignments for the acetanilide and phenone series are suggested in Table 4.)
In order to avoid confusion in the chromatograms, the retention index reference standards are analyzed periodically as external standards and not added to the analytical samples themselves. The use of these reference standards is optional but is suggested where application of this method is expected to encounter consistently a broad range of unknown analytes and to augment other confirmatory techniques. D. INTERFERENCES: The UV detector responds to many compounds. Retention times of some common potential interferences are provided in Table 4. Interfering compounds that may be encountered are discussed below. 1. Impurities in Mobile Phases and Additives: Only HPLC-grade solvents should be used. Triethylamine (TEA) was found to develop unidentified impurities over time which contributed to significant irregularities in the baseline. This degradation was reduced or eliminated by storing the TEA under nitrogen in a small desiccator at 0 to 4 °C [8]. 2. Organic Solvents or Fuels a. Chloroform showed a response that nearly coelutes with carbaryl (Table 4). Benomyl and captan, therefore, are made up in methylene chloride, which is more UV transparent than chloroform. b. Toluene (Table 4). c. Blends of solvents having ketones, ester, or any of the above compounds, such as lacquer solvents, gasoline, paint stripper, and cleaning solvents, are to be avoided or documented during sample collection and handling.
NIOSH Manual of Analytical Methods, Fourth Edition
