Analytical methods and quality assurance

Abstract

A wet digestion method of soil samples has been developed for analysis of “total” concentration (acid extraction) of elements by inductively coupled plasma emission spectrometry (ICP‐AES). This HNO₃‐H₂O₂ wet digestion method is a simple, fast and safe sample preparation method with satisfactory accuracy and precision. The first examined condition was the applied digestion acid or acid mixture (HNO₃, HNO₃‐H₂O₂, HCl‐H₂O₂, HNO₃‐HClO₄, H₂SO₄‐H₂O₂), and the volume of concentrated nitric acid (65% HNO₃). Temperature and duration of predigestion, volume of concentrated hydrogen‐peroxide (30 % H₂O₂), temperature and duration of digestion were also investigated. Two different kind of soil samples (a sandy soil with low humus content, calcareous chernozem with relatively high humus content), three different dry weight values and three different values for each parameters were chosen to investigate soil sample digestion in order to select the best parameters. A LABOR MIM Electronic Block Digest Apparatus was applied for sample preparation and numerous elements (e.g. Al, As, Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, P, Pb, S, Sr, Zn) have been measured by a LABTAM 8440M Inductively Coupled Plasma Emission Spectrometer. The optimum values of parameters to digest soil sample in an electronic block digest apparatus are: a) 1 g dry weight, b) 5 cm³ HNO₃ as digestion acid, c) 30°C‐60°C temperature range for 30–60 minutes predigestion, d) 5 cm³ 30% H₂O₂, e) 120 C temperature for 270 minutes digestion. Two soil samples were digested with four methods (block digestion, Milestone microwave, Prolabo focused microwave and Hungarian standard). Results of the two microwaves and detailed block digestion methods are in well agreement in the two soil samples. Finally three Standard Reference Materials were applied to compare the appropriate results. These results showed well agreement for all elements except for aluminium and iron content. The difference between certified and measured results is dependent on their concentrations in soil.