Microhomogenization of individual zooplankton species improves mercury and methylmercury determinations

A new method for processing zooplankton improves the accuracy and precision of mercury (Hg) and methylmercury (MeHg) determinations and permits the analysis of Hg species and ancillary variables, such as dry weight and C, N, and protein contents on subsamples of the same tissue homogenate using clean technique. In this study, 10-50 individual zooplankton were sorted from live samples, homogenized in 500 mu L of low-Hg water, subsampled, and then digested for analysis without contamination. Detection limits ranged from 4 to 12 pg for Hg and from 0.8 to 4.3 pg for MeHg. Procedural blanks averaged 18.7 pg Hg and 5.3 pg MeHg. Yield from certified reference materials was consistently high on small masses of tissue homogenate (96% for Hg and 89% for MeHg), and recovery from spiked samples was good (99% for Hg and 92% for MeHg). We also present results for Daphnia and Chaoborus from Mud Lake, Wisconsin, processed in this manner. The new method improves our ability to determine the distribution of Hg species at lower levels of aquatic food webs and to investigate factors potentially regulating the bioaccumulation of Hg and MeHg in specific taxa.