Stability of trace element concentrations in frozen urine - Effect on different elements of more than 10 years at - 80 °C

Background: Analysis of urinary trace elements is widely used in Human Biology, especially in occupational and environmental biomonitoring. Collections of urine samples are of great interest for those studying trace elements but many of them are actually unused, in part perhaps because of a lack of knowledge about the stability of trace element concentrations under such storage conditions. The aim of this study was to evaluate the impact of a long-term frozen storage on the measurement of the urinary concentration of 10 trace elements.

Method: Forty-eight urinary samples were re-analysed by inductively coupled plasma mass spectrometry (ICP-MS) for the quantification of As, Cd, Co, Cr, Mn, Ni, Pb, Sb, Tl, and Zn, after 11-13 years of frozen storage at - 80 °C.

Results: A slight water loss likely occurred, even at - 80 °C, but seems to be compensated by using creatinine-adjusted concentrations. Concentrations of As, Cd, Pb, Tl, Zn, and possibly Mn, did not suffer from major modification during storage, while the plastic container likely contaminated samples with Sb. The technological evolution over 13 years may have affected some results, especially those with lower concentrations, and must be taken into account when comparing data over time.

Conclusion: This study provides some promising preliminary data on the stability of trace element concentrations during long-term frozen storage, and some evidence that urine samples in existing biobanks remain a valuable resource, even if they were collected many years ago.