d. Sample Handling and Preparation Sample handling and preparation include those steps taken to stabilize the sample or make the sample more compatible with the analytical procedure. Sample handling considerations actually begin before sample collection. Some reagents and sampling media have limited shelf lives and require special storage conditions. For example, the nitro reagent and 9-(methylaminomethyl)-anthracene (MAMA) are known to be light sensitive.4,80 It has also been found that filters coated with 1-(2-pyridyl)piperazine show substantial reagent loss to the back-up pad during storage.81 This loss is greatly reduced by storing the reagent-coated filters in the freezer before use. Users of methods need to be aware of such problems and follow the method’s guidelines for storage of sampling media. All filter methods require extraction of the filter with a suitable solvent prior to analysis by high performance liquid chromatography (HPLC). The Iso-Chek™ method,58,59 which utilizes a reagentless filter, requires that extraction with a derivatization solution be done immediately after sampling. It has been reported that extraction in the field, preferably with a reagent solution, is beneficial for reagent-coated filters as well.66 Typically, reagent-coated filters are transported to the laboratory, where extraction takes place prior to analysis. It may be desirable to filter the extraction solution prior to analysis by HPLC.49 Impinger methods frequently require the impinger solvent to be exchanged to a more HPLC-compatible solvent prior to analysis. This is generally achieved by evaporation of the impinger solvent to dryness and the redissolution of the sample in a solvent more compatible with HPLC analysis. Toluene impingers are typically treated in this manner.77,78 If the volume of solvent used for redissolution is relatively small (or the redissolution solvent is easily concentrated), this step can also serve to concentrate the sample and improve the LOD of the method. Losses can conceivably occur because of incomplete redissolution of the sample components. Ultrasonication of the reconstituted solution may be done to facilitate redissolution.77 Evaporation/redissolution requires that the impinger solvent be somewhat volatile. Unfortunately, a volatile solvent is undesirable during sampling because of the potential for vapor exposures as well as fire hazards. If a method uses a non-volatile, HPLC-incompatible impinger solvent, solvent exchange can be accomplished by solid-phase extraction (SPE). This is the procedure used to remove the butyl benzoate in the NIOSH method currently under development. SPE has the advantage of being readily automated. Also, it may enable removal of excess reagent and impurities prior to HPLC analysis. A drawback is the potential for incomplete elution of isocyanate species. If the derivatizing reagent is not removed prior to HPLC analysis (e.g., by SPE), methods frequently recommend acetylation of the excess reagent with acetic anhydride prior to sample injection.49,77,82 Excess reagent typically gives a large, tailing peak near the beginning of the HPLC chromatogram. This tail may interfere with the quantification of analytes. Also, repeated injection of large amounts of amine reagent can degrade the analytical column. The acetylated reagent does not tail appreciably and its presence in the sample does not degrade the analytical column. In cases where a method uses an impinger solvent compatible with HPLC analysis, pre-analysis sample preparation may not be necessary. In NIOSH Method 5522,79 an aliquot of the DMSO impinger solution is injected directly into the HPLC. This is simple, saves time, and avoids losses of isocyanate that can result from sample manipulation. e. Separation 1/15/98
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