Effect of the Dry Ashing Method on Cadmium Isotope Measurements in Soil and Plant Samples

A dry ashing method is commonly used to remove organic material from samples prior to geochemical analysis. In the course of this study, the Cd isotope ratios of a series of soil and plant reference materials and samples were studied to evaluate the effect of the dry ashing method on measurement results of Cd isotope ratios. The samples were pre-treated using the dry ashing method and high-pressure bomb for comparison. The results show that the digestion using high-pressure bombs did not lead to Cd loss, but using the dry ashing method would cause different proportions of Cd loss. The whole range of Cd isotope difference between two methods was from -0.07 parts per thousand to 3.01 parts per thousand. There was also an obvious difference in measured Cd isotope ratios from the same leaf sample pre-treated independently by the dry ashing method, indicating that the amount of Cd loss and the effect on Cd isotope measurement during dry ashing is related to the properties of the samples. Therefore, dry ashing may not be appropriate for the removal of organic material in Cd isotope ratio measurement, especially for samples with high organic contents. The delta Cd-114/110 values of reference materials NIST SRM 1573a and GSD-30 are reported for the first time in this study.