Determination of the Zinc Concentration in Human Fingernails by Laser-Induced Breakdown Spectroscopy

The absolute concentration of zinc in human fingernail clippings tested ex vivo was determined by 1064 nm laser-induced breakdown spectroscopy and confirmed by speciated isotope dilution mass spectrometry. A nail testing protocol that sampled across the nail (perpendicular to the direction of growth) was developed and validated by scanning electron microscopy energy dispersive x-ray spectroscopy. Using this protocol, a partial least squares regression model predicted the zinc concentration in five subjects’ fingernails to within 7 ppm on average. The variation of the zinc concentration with depth into the nail as determined by laser-induced breakdown spectroscopy was studied and found to show no systematic variation for up to 15 subsequent laser pulses in one location. The effects of nail hydration (dehydrated and overhydrated) and nail surface roughness on the assay were investigated to explain an anomalously large scatter observed in the measurements. This scatter was attributed to the layered nature and the fibrous structure of the fingernails which resulted in non-uniform ablation as determined by scanning electron microscopy. This work demonstrates that a protocol consisting of low pulse energy (\u3c10 mJ) 1064 nm laser pulses incident on human fingernail clippings in an argon environment can produce quantifiable zinc emission in the laser-induced plasma and the measured zinc intensity can be used to accurately predict the nail zinc concentration