Analytical Chem istry - DePauw University

Chapter 7 Collecting and Preparing Samples345sample is 10? Which is the best sampling scheme if the cost of collectinga single sample is 7, and the cost of analyzing a single sample is 3?15. Maw, Witry, and Emond evaluated a microwave digestion method forHg against the standard open-vessel digestion method. 26 The standardmethod requires a 2-h digestion and is operator-intensive. The microwavedigestion is complete in approximately 0.5 h and requires littlemonitoring by the operator. Samples of baghouse dust from air-pollution-controlequipment were collected from a hazardous waste incineratorand digested in triplicate before determining the concentrationof Hg in ppm. Results are summarized in the following tables.ppm Hg Following Microwave DigestionSample Replicate 1 Replicate 2 Replicate 31 7.12 7.66 7.172 16.1 15.7 15.63 4.89 4.62 4.284 9.64 9.03 8.445 6.76 7.22 7.506 6.19 6.61 7.617 9.44 9.56 10.78 30.8 29.0 26.2ppm Hg Following Standard DigestionSample Replicate 1 Replicate 2 Replicate 31 5.50 5.54 5.402 13.1 12.8 13.03 5.39 5.12 5.364 6.59 6.52 7.205 6.20 6.03 5.776 6.25 5.65 5.617 15.0 13.9 14.08 20.4 16.1 20.0Does the microwave digestion method yields acceptable results in comparisonto the standard digestion method?16. Simpson, Apte, and Batley investigated methods for preserving watersamples collected from anoxic (O 2 -poor) environments that containhigh concentrations of dissolved sulfide. 27 They found that preservingwater samples with HNO 3 (a common method for preserving aerobicsamples) gave significant negative determinate errors when analyzing26 Maw, R.; Witry, L.; Emond, T. Spectroscopy 1994, 9, 39–41.27 Simpson, S. L.: Apte, S. C.; Batley, G. E. Anal. Chem. 1998, 70, 4202–4205.