Refinement of Methodology for Cadmium Determination in Soil Micro-Arthropod Tissues

Highly precise and reliable determination of heavy metals in soil micro-arthropod tissues remains a challenge because of the small size of the animals and their typical low abundance in metal-contaminated agricultural soils. The present study sought to develop a method for cadmium (Cd) determination in soil micro-arthropods by optimizing the sample digestion procedure, reducing sample weight, modifying sample pre-treatment and validating the methodology with field samples. The optimized digestion conditions comprised a sample mass of 50-150 µg, digestion reagent of nitric acid:hydrogen peroxide (3:1), digestion temperature of 105 °C, digestion period of 3 h and digestion volume of 30 µL. Defecation of the standard Collembola Folsomia candida (92 h) and the indigenous Collembola Onychiurus yodai (42 h) and ultrasonic cleaning of F. candida increased the accuracy of Cd determination. The recovery of Cd using the refined procedure was 98.9% and the limits of detection and quantification were 0.002 and 0.008 µg L−1, respectively. The within-batch precision values were < 3%. The Cd concentrations in the tissues of the Collembola Isotoma sp. collected from a range of metal-contaminated fields determined by the improved method were consistent with the Cd concentrations in the field soils. The results indicate that the optimized method can be used for more accurate or reliable determination of Cd concentrations in soil micro-arthropod tissues.