Portable X-ray fluorescence of zinc and selenium with nail clippings - visit 3 of the Mother and Infant Nutrition Investigation (MINI).

Introduction: Recent advances in portable X-ray fluorescence (XRF) have opened up the possibility of a rapid and cost-effective method for measuring elemental content in nail clippings. Two elements of particular interest for intake assessment through nail clipping measurement are zinc and selenium. The New Zealand-based Mother and Infant Nutrition Investigation (MINI) provides an ideal testing ground for the application of portable XRF in the measurement of zinc and selenium. Two aspects requiring further study with portable XRF are the optimal time of measurement and the optimal approach to energy spectrum analysis.

Methods: A subset of nail clippings from 20 mother-infant pairings were initially selected for analysis over three separate MINI study visits. This paper considers measurements from visit 3 (12 months postpartum). For portable XRF, a mono-energetic X-ray beam was used to excite characteristic X-rays from zinc and selenium. Individual XRF measurements were made for 60 s (real time). In each case, analysis of the elemental signal was performed in four different ways: (1) automated system output concentration; (2) element signal to total signal ratio; (3) element signal to Compton scatter signal; (4) element signal to coherent scatter signal. XRF results were compared against zinc and selenium concentrations obtained from a "gold standard" method of inductively coupled plasma mass spectrometry (ICP-MS).

Results: ICP-MS results from the visit 3 nail clippings determined a mean zinc concentration of 104 µg/g and a mean selenium concentration of 0.606 µg/g. Analysis of XRF energy spectra from 60 s measurements showed that zinc was clearly detected in all cases. The selenium signal was more challenging, but was also detected from all spectra. For both zinc and selenium, correlations between XRF results and ICP-MS concentrations were highly significant (p < 0.01) in all cases, with the zinc results more strongly correlated than selenium. Of the four approaches to XRF signal analysis, the three approaches involving an external software package provided superior results relative to the automated system output. Correlations of XRF results with ICP-MS concentrations were similar between these three approaches.

Conclusion: ICP-MS concentrations for both zinc and selenium were found to be lower from visit 3 relative to results from visit 1 and visit 2. This may indicate overall dietary changes postpartum. Correlations of XRF results with ICP-MS concentrations suggest that a relatively short 60 s measurement time is adequate for a reliable zinc assessment, but not for selenium. Although correlations of XRF results with ICP-MS concentrations were similar between three different methods of spectra analysis, practical considerations may favor use of the element signal to total signal approach.