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Hair and nails

Scalp hair (n=114) and fingernail (n=96) samples from a population living in north-east Sweden (42% males, 58% females, age range 1-76 y with mean = 33y and S.D. = 17y) were analyzed (5). Only samples from subjects without known occupational exposure to metals were used.

All samples were thoroughly pre-washed using different solvents in sequence. They were then digested with nitric acid and hydrogen peroxide in closed PFA containers in a microwave digestion system. After dilution with ultra-pure water, the digests were analysed by Inductively Coupled Plasma Sector Field Mass Spectrometry (ICP-SFMS).

The analysis comprised 71 elements. For both hair and nails, all except 9 elements were detected in all samples. For 39 elements, detection limits in the sub-ng/g range were achieved. Some examples of found concentrations are given in Table 7. Compared to previously published ranges, concentrations found in this study tended to be somewhat lower.

The accuracy of the method was assessed (4) by analysis of the certified reference material GBW07601 human hair, and by

analysis of samples from an interlaboratory comparison program (from Le centre de toxicologie du Québec, Canada). For selenium, comparison was also made with another analytical technique (Atomic Fluorescence Spectrometry). In total, accuracy could be assessed for more than 40 elements.

Significant correlation between hair and nail concentrations was found for 15 elements. This correlation, which was especially close for Hg, Cd, Pb, Sb, and Bi, is of importance for the usefulness of these sample media in biological monitoring.

Some findings in the population data may be mentioned. Concentrations of a group of elements (Th, Al, Ti, rare earths) showed a well-defined peak for hair and nails in the age interval 2-7 years. Since this group of elements is associated with soil, the pattern could be attributed to behavioural characteristics of this age group. Cadmium concentrations were five and ten times higher for smokers than for non-smokers in hair and nails, respectively. Some smokers also showed an anomalous pattern of rare earth elements (as shown by chondrite normalization) in hair and nails which could be attributed to flint in disposable lighters, containing La, Ce, Pr, and Nd.

Table 7. Element ranges (central 90% fraction) found in hair and nails. Concentrations in µg/g (5). For other elements, see Element List.

Element	Hair	Nails	Element	Hair	Nails
Zinc	68-198	80-191	Platinum	0.00002-0.00061	0.00002-0.0011
Copper	8.5-96	4.2-17	Mercury	0.053-0.927	0.028-0.311
Iodine	0.13-3.31	0.077-0.810	Cadmium	0.010-0.356	0.013-0.438
Arsenic	0.034-0.319	0.065-1.09	Antimony	0.007-0.122	0.014-0.128
Lead	0.22-7.26	0.27-4.75	Uranium	0.006-0.436	0.002-0.047

As an alternative to digestion, hair and nail samples can be analyzed by ICP-SFMS following laser ablation, a sample introduction technique which enables analyses directly on solid samples (8). An important advantage of laser ablation is the possibility of spatial resolution in the analysis, e g along the growth axis on a single nail clipping. Detection capability is roughly comparable to analysis following acid digestion. Many elements have detection limits in the low pg/g range, others in the ng/g range. The majority of the latter require high mass resolution in the ICP-SFMS instrument, which results in lower sensitivity.

Studies on elemental distribution along the length of the hair (5, 8) showed that the concentrations of some elements (e. g. Ca, Sr, Au) increased from the proximal to the distal end of the hair, while others (e. g. Re) showed the opposite trend. A third group contains elements (e. g. Se, P, Zn, Cu) that are uniformly distributed along the hair.

Analyses along the growth axis of a nail were also demonstrated, with a spatial resolution of ca 40 µm (8).

Hair and nails may be regarded as complementary to body fluids in biological monitoring. The former enable monitoring of elements accumulated over a time span up to several months. They are also more easily sampled, handled and transported, and less prone to post-sampling contamination, partly because of generally higher elemental concentrations. On the other hand, their usefulness is limited by difficulties in differentiating between endogenous and exogenous deposition, even when using elaborate washing procedures. Large differences in concentrations were found between surface and interior layers of nails, and contamination was traced as deep as 100 µm in the nail (8). For most of the elements, concentrations in the interior layer were much lower than previously reported for human nails. Selective analysis of deeper layers using laser ablation may thus be an attractive approach to nail analysis.