Page:NIOSH Manual of Analytical Methods - 5525.pdf/8

 ISOCYANAT ES, TOTA L (MAP): METHO D 5525, Issue 1, dated 15 March 2003 - Page 8 of 17 EVALUATION OF METHOD: Shelf Life of MAP, MAP Solutions and M AP-C oated Filters: Pu re, so lid M AP is stab le for years when stored in a freezer. Concentrated solutions of MAP (concentrated stocks or filter-spiking solutions) are not very stable and should be kept in the freezer for a maximum of one week [7,8]. MAP -coated glass-fiber and qu artz-fiber filters are stable for 1 week at room temperature and 6 months in the freezer [7,8]. Artifacts derived from MAP are present in the chromatogram of MAP-coated filter sam ples. Although the levels are low, these impurities dictate the method LOD when they coelute with an analyte. The level of artifacts observed is not correlated well with storage time. Room temperature storage results in greate r artifact levels than sto rage in the freezer. MAP is known to be light sensitive. It is not clear what role exposure to light plays in the formation of the chromatographic interferences. It is recomm ended that exposure to light be minimized when handling MAP solutions and MAP-coated filters. No formal study has been conducted reg arding the shelf life of im pinger solutions of MAP in butyl benzoate. However, no problems have been observed when the impinger solutions are used within one week of being m ade. Im pinger sam ples are noted to have fe we r artifacts than filter sam ples. A ltho ugh this may be due in part to greater stability of the dilute M AP solutions versus M AP on filters, it ma y also be attribu table to the generally lower amount of reagent used in impinger samples as well as the solid-phase extraction procedure to which impinger sa m ples are s ubje cted. Stability of MAP-Isocyanate Derivatives: Pure, solid MAP-derivatized isocyanate monom ers are stable for years when stored in the freezer. MAPderivatized m onom ers and oligom ers of alip hatic isocyanate s are very stab le in aceto nitrile (the sam ple solve nt) for at least one year [7]. Samples containing TDI-MAP gave results 16-24% lower than the original value s wh en re ana lyzed afte r stora ge fo r nine m onth s in the freezer [8]. Filte r M ate rials: Com parison of quartz-fiber and glass-fiber filters showed no significant differences in recovery of MAPderivatized monom ers from the filters. Com parison of stainless steel and cellulose backup pads showed no significant reagent loss with either. Neither backup material contributed to the artifact production in the filter sam ple [8]. Filter Versus Impinger Sampling: A side-by-side comparison of MAP-coated IOM filters and MAP impingers, conducted in autobody shops using aliphatic isocyanate products [9], showed no significant difference in the performance of these two samplers. It can be concluded from this stu dy that losses of relatively slow-curing aliphatic isocyanates collected on filters due to curing reactions are minimal when filters are extracted in the field imm ediately after sampling. It also appears from this stu dy that th e IO M’s bette r as piratio n effic iency for rela tively large aerosol did not result in increased collection efficiency compared to the midget impinger. This suggests th at an inhalable sampler is not necessary in this type of environm ent. In general, the approp riate choice of sam pler - filter, impinger, or im pinger an d filter in se ries - is d ictated by the particular expos ure s cen ario [1,2 ]. PEEK Versus Stainless Steel HPLC Columns: PEEK and stainless steel HPLC columns were compared with respect to efficiency, tailing, and carryover. No significant differences in performance were observed [8]. Therefore, stainless steel columns are considered acceptable. However, because MAP derivatives are known to adsorb onto sta inless ste el surfaces, it is recomm ended that the frits of the column be constructed of more inert material, i.e., PEEK or titanium. pH Gradient: Method 5525 uses pH -gradient H PL C to enable m easurem ent of rela tively weak ly retained monofunctional isocyanates and diisocyanate monomers as well as relatively strongly retained oligomeric isocyanates in the sam e analysis. Several characteristics of the pH gradient have been evaluated [10]. The pH grad ient is selective, only accelerating strongly basic compounds (all com pounds containing a MA P group a re strongly basic). The strength of the pH grad ient w as evaluate d by com paring the degree to which retention is NIOSH Manual of Analytical Methods (NMAM), Fourth Edition