Comparison of four digestion procedures not requiring perchloric acid for the trace-element analysis of plant material

Abstract

Perchloric acid (HClO/sub 4/) is often used to destroy organic material contained in plant tissue during sample preparation for trace-element analysis. However, since perchloric acid is an extremely strong oxidizing agent that can cause fire and explosion when in contact with combustible materials, its use is best avoided when proper safety equipment and training is unavailable. A comparison was made of four digestion procedures that do not require perchloric acid: wet digestion with nitric and sulfuric acids; wet digestion with nitric acid alone; a repeated wet digestion with nitric acid; and direct dry ashing. Each procedure was used to digest National Bureau of Standards orchard leaves (SRM 1571). To investigate the effect of possible filter paper adsorption on the determination of trace elements, digested samples were either filtered or not filtered before analysis. Atomic absorption spectrophotometry was employed to determine concentrations of As, Be, Cd, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sr, and Zn in each digested sample. Recoveries of each element and the relative error of each determination for each digestion procedure were then calculated. A statistical analysis of these data indicates that the direct dry ashing procedure is best suited for multi-element analysis. Dry ashing is appropriate to recover As, Be, Cr, Cu, Fe, Mn, Mo, Pb, and Zn. The nitric-sulfuric acids, nitric acid, and repeated nitric acid digestion procedures were deemed poor for multi-element analysis; however, each proved useful for the recovery of certain individual elements, including Cd, Pb, and Zn. Sample filtration significantly (p less than or equal to 0.05) lowered the recovery of Cr, Mn, Pb, and Zn from the digested samples. Conversely, the recovery of As, Mo, and Sr was significantly (p less than or equal to 0.05) higher in samples filtered before analysis when compared to the recovery of these elements in unfiltered samples.