The Effect of Dry-Ashing Method on Copper Isotopic Analysis of Soil Samples with Organic Matter

Abstract

BACKGROUND When analyzing copper isotopes in soil samples, organic matter can cause serious interference on chemical purification and analysis processes. Therefore, the complete removal of organic matter from the soil is critical to obtain high-precision copper isotope data without changing the copper isotope composition of the sample. OBJECTIVES To carry out the condition experiments before using the dry-ashing method, because the method may influence the compositions of volatile elements, such as Cu, in soil samples. METHODS Soil samples containing organic matter were treated by dry-ashing and then digested by the high-pressure wet digestion method. The copper isotope composition was measured by Multi-collector Inductively Coupled Plasma-Mass Spectrometry after purification. Through the comparison of the results of the two treatment methods, the extent of influence of the dry-ashing method on the copper isotopic composition of soil samples was investigated. RESULTS The results demonstrate that the wet-digestion method can provide more precise results, while the dry-ashing method can significantly fractionate Cu isotopes of soil samples. The deviation of δ 65Cu values varies from -0.61‰ to 3.46‰ for the dry-ashing method, which was caused by volatilization loss of Cu. Moreover, the degree of influence is controlled by a few factors such as sample nature and ashing temperature. CONCLUSIONS The dry-ashing method is not an appropriate method for soil sample digestion. The wet-digestion method is a better choice to prepare samples for Cu isotopic analysis.

Keywords

• soil samples
• organic matter
• dry-ashing method
• volatilization loss
• isotope fractionation
• multi-collector inductively coupled plasma-mass spectrometry