Each sample was air-dried at ambient temperature, disaggregated, and sieved to <2 millimeters (mm). The <2-mm material was then crushed to <150 micrometers (µm) prior to chemical analysis. For analysis of arsenic , the sample was fused in a mixture of sodium peroxide and sodium hydroxide at 750 °C. The fused mixture was then dissolved in hydrochloric acid. Selenium was determined after dissolution in a mixture of nitric, hydrofluoric, and perchloric acids.

Hageman, P.L., Brown, Zoe Ann, and Welsch, Eric, 2002, Arsenic and selenium by flow injection or continuous flow-hydride generation-atomic absorption spectrometry, chap. L of Taggart, J.E., Jr., ed., Analytical methods for chemical analysis of geologic and other materials, U.S. Geological Survey: U.S. Geological Survey Open-File Report 02-223, 7 p.