Critical evaluation of a modified automatic wet-only precipitation collector for mercury and trace element determinations

In 1993, the University of Michigan Air Quality Laboratory (UMAQL) designed a new wet-only precipitation collection system that was utilized in the Lake Michigan Loading Study. The collection system was designed to collect discrete mercury(Hg) and trace element samples on either an event or weekly basis. Performance characteristics such as collector opening efficiency, collocated precision, spike recovery, field blanks, and bottle blanks were rigorously evaluated from October 1994 through October 1995 at a semi-rural site near Dexter, MI. Comparisons for Hg were made between HCl preserved versus unpreserved event collection, wet-only event versus manual event collection, and weekly volume-weighted wet-only event versus weekly bulk collection. Results of this investigation indicated that with proper preparation and ultra-clean collection techniques, wet-only precipitation collection utilizing the UMAQL modified MIC-B collector can provide virtually contamination-free Hg and trace element samples. On average, field blanks contributed <1% of the Hg and trace element sample mass. The absolute mean difference for collocated samples was 8.1% for Hg and from 3.6% to 14.1% for 21 other trace elements. Wet-only precipitation collection for Hg was found to be statistically equivalent (alpha = 0.05) to manual event collection (absolute mean difference of 9.8%), demonstrating the suitability of the automatic wet-only technique.