Comparison of Digestion Methods for the Determination of Trace Elements and Heavy Metals in Human Hair and Nails

Background: Microwave is the most reliable sample digestion method. However, it requires expensive microwave digester automation and has relatively low productivity. In this study, three non-automated digestion methods, i.e. wet acid digestion using nitric acid (HNO₃) and hydrogen peroxide (H₂O₂), wet acid digestion using HNO₃, and dry washing, are compared in order to determine the best approach.
Methods: Certified reference material IAEA-086 (International Atomic Energy Agency, Austria) and hair and nail samples from 20 female students of Universiti Kebangsaan Malaysia, aged 19 to 30 years, were collected and analysed using the three digestion methods.
Results: For hair samples, analysis of variance of repeated measures showed significant differences in the level of all elements (P < 0.001) between the three methods. For nail samples, only the copper (Cu) level showed no significant difference (P = 0.100) between methods. Wet acid digestion using HNO₃ and H₂O₂ showed the best within- and between-run relative standard deviation (RSD) values, with within-run RSD for all elements, except for selenium (Se), < 5%. The between-run precision ranges from 6.14% to 17.96% for hair and from 3.53% to 11.52% for nail samples. Wet acid digestion using HNO₃ and H₂O₂ showed both good accuracy and precision for manganese (Mn) and magnesium (Mg), with percentage recoveries of 110% and 96.9%, respectively. All elements show higher method detection limit (MDL) values than the previous study: 0.05 μg/g Mg for wet acid digestion using HNO₃, 0.02 μg/g Se for wet acid digestion using HNO₃ and H₂O₂, and 0.2 μg/g Mg for dry ash method.
Conclusion: Wet acid digestion using HNO₃ and H₂O₂ proved to be the best method in terms of precision, accuracy, recovery, and MDL. However, only Mn and Mg showed adequate precision, accuracy, and percentage of recovery.