Comparing The Efficacy Of Two Acid Digestion Methods For Analysis Of Fe, Ca, Al, Mn, Cd, and Pb In Santa Cruz Mudstone.

Heavy metal cations naturally become bioavailable through chemical weathering of bedrock. The experiment was conducted to determine the more precise and accurate method between two acid digestion procedures for analysis of Fe, Ca, Al, Mn, Cd, and Pb in Santa Cruz mudstone. All samples were Santa Cruz mudstone and were collected every 10-20 feet at a depth up to 600 feet from a well drill site on Swanton Pacific Ranch near Davenport, CA. The quality control and quality assurance parameters using 1:1 HNO₃:HCl and 1:1 HNO₃ matrices were determined at the method and instrument level. The concentrations of Fe, Ca, Al, Mn, Cd, and Pb in Santa Cruz mudstone were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). The percent recoveries (%R) of SRM 2711 and 8704 for Ca (107-109%) and Pb (113-120%) using HNO₃ were within the target range of 80-120% for method accuracy; the 1:1 HNO₃:HCl method provided no SRM %R  ranges within the target threshold. The standard deviations of the duplicate percent relative difference provided data supporting the precision of the HNO₃:HCl method. The low reading of Al relative to its abundance in the Earth’s crust suggested inefficient digestion of Al, further supported by the low %R of 2% for both SRMs using both methods. The heavy metals (Mn, Cd, Pb) were present in lower concentrations than the macro metals (Fe, Ca, Al) at all depth increments. The high concentration of Ca indicates the sampled Santa Cruz mudstone likely contains Ca-feldspar minerals. The HNO₃ showed greater method accuracy, but the 1:1 HNO₃:HCl method suggested greater precision.