Subsequent to the 1997 promulgation of the Federal Reference Method (FRM) for monitoring PM(sub 2.5) in ambient air, the United States Environmental Protection Agency (USEPA) received reports that the Dow 704 diffusion oil used in the method's WINS fractionator would occasionally crystallize during field use-particularly under wintertime conditions. While the USEPA's survey of site operators revealed that the frequency of occurrence on a nationwide basis was low, concerns were raised that crystallization of the Dow 704 oil during a given sampling event may adversely affect the event's data quality due to possible particle bounce from the crystallized impaction surface. In response to these concerns, the USEPA and the Connecticut Department of Environmental Protection (CT DEP) jointly conducted a series of specialized tests to determine if crystallized oil adversely affected the performance of the WINS fractionators. In the laboratory, an experimental setup used dry ice to artificially induce crystallization of the diffusion oil under controlled conditions.