0.01 ppm. During the second and third year of monitoring, we tested new 
procedures designed to lower the detection limit. 
The contribution of Hg from various brands of nitric acid was 
determined. Baker “analyzed reagent grade” contained <0.5 parts per billion 
(ppb) Hg, the lowest concentration of the acids tested. Baker “ultrex”™ 
contained 2 ppb Hg, and Mallinckrodt nitric acid contained 1.3 ppb Hg. During 
the checks of HCl0,, we found that some bottles of GFS double distilled HC10, 
contained 5 ppb Hg- We ultimately selected GFS double distilled from Vycor 
which was found to contain <0O.5 ppb Hg. The Hg concentration of each new 
bottle of acid and of every other reagent was determined before the reagent 
was used for analysis. The Hg contribution from the combined reagents was 
reduced to 0.5 ng+0.1 ng. 
We cried to lower the detection limit by increasing the sample size. 
Subsamples weighing 1 g were analyzed with various combinations of nitric and 
perchloric acids. The results were not encouraging because digestion was 
incomplete using small acid volumes or because blanks were too high when large 
acid volumes were used. The high sediment concentration in suspension during 
the gas-stripping procedure may have adsorbed some of the Hg, accounting for 
the lower concentration measured for large samples. 
Another method of increasing sample size involved successive plating of 
Hg vapor from three 200-mg aliquots onto the gold foil of the induction 
furnace. This technique yielded poor reproducibility among replicates and 
decreased the number of samples that could be analyzed in a day by a factor of 
3. 
The selection of reagents having the lower Hg concentration, addition of 
a digital readout voltmeter, and optimization of the optical system in the 
cold-vapor AA detection system (manufactured by Laboratory Data Control, Inc.) 
reduced the detection limit of our procedure from 0.01 ppm to 0.005 ppm. 
16 
