The optimal process conditions were found to be a sample size of 150 μL, drying at 120°C for 2 min 20 sec, 1 min ramp to 650°C, and finally decomposition at 650°C for 3 min 30 sec. This set of conditions was cho-sen based on precision and reproducibility over a number of sampling days, as well as completeness of combustion. Sample volumes greater than 150 μL tended to boil over in the oven and overwhelm the catalyst tube with an ac-cumulation of ash. However, at smaller volumes, there was not enough absolute mercury present in the sample for reliable analysis.
Development of the drying/decomposition ramp was also important to avoid boiling the sample out of the boats too quickly. Too high an initial drying temperature would result in boats bubbling over and loss of volume. In these cases, a blackened crust was observed on the top and sides of the boat. The ramp was used to slowly heat the dried samples, avoiding volume loss. The decompo-sition temperature recommended by the EPA method for mercury analysis in wastewater [8] was too low to prop-erly combust the complex matrix of whole blood and would leave blackened residue in the boats after heating. Raising the temperature to 650°C on the DMA 80.3 re-sulted in complete combustion.
The remaining instrument parameters for sample ana- lysis included a purge time of 30 sec, the amalgam time was set to 12 sec and the recording time measured 30 sec. Before a quartz boat could be used again after a blood analysis, it needed to be cleaned by going through one cycle at a drying temperature of 300°C for 1min and de-composition at 650°C for 3 min.