Since the intention was to use a simple nitric acid digestion
procedure (see Digestion Procedures) the early commissioning
of the VGA-76 involved the use of mercury standards prepared
in different concentrations of nitric acid. The first parameter
investigated was the delay time. This is the time in seconds
which elapses between the probe of the autosampler reaching
the fully down position and the absorbance reading becoming
stable. For a 0.02 μg/mL mercury standard in 40% v/v nitric
acid the delay time required is 55 seconds (Figure 1). The
same concentration of mercury in 20%, 30% and 50% v/v nitric
acid requires the same delay time, but higher concentrations
of mercury take a few seconds longer to reach a steady signal.
The delay time now used routinely is 70 seconds.