Elemental bio-imaging of trace elements in teeth using laser ablation-inductively coupled plasma-mass spectrometry

https://doi.org/10.1016/j.jdent.2011.03.004Get rights and content

Abstract

Objectives

In this study we present the application of a novel laboratory method that employs laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to construct two-dimensional maps of trace elements in teeth.

Methods

Teeth were sectioned longitudinally, embedded in resin and polished to a smooth surface. Data were generated by laser ablating the entire sectioned tooth surface. Elemental images were constructed using custom-built software.

Results

Quantified images of 66Zn, 88Sr, 111Cd and 208Pb, with a spatial resolution of 30 μm2, were generated from three teeth. Concentrations were determined by single-point calibration against NIST SRM 1486 (bone meal). Zn and Sr concentrations were determined in the μg g−1 range and Cd and Pb in the ng g−1 range. Concentrations of Pb, Zn and Cd were higher in dentine particularly in regions adjacent the pulp.

Conclusions

Elemental bio-imaging employing LA-ICP-MS is a novel method for constructing μm-scale maps of trace elements in teeth. This simple imaging method displays the heterogeneity of trace elements throughout the tooth structure that correspond to specific structural and developmental features of teeth. As a preliminary study, this work demonstrates the capabilities of LA-ICP-MS imaging in dental research.

Introduction

There is increasing recognition of the role of trace elements in oral health. A large body of evidence supports the caries-protective role of fluoride,1 and more recent reports have implicated Pb and Cd with the formation of dental caries2, 3 and periodontal disease.4 Other trace metals such as Zn, Cu, Fe, Se, Sn and Al in tooth enamel may also play a role in dental caries but the role of these elements is not clear.5, 6 Teeth not only serve as targets for the effects of metal exposure but have also been used as bio-indicators of environmental exposure to metals, infant nutrition and disease outcomes.7 Pb levels in primary teeth are a well-established indicator of environmental Pb exposure in children and adolescents, and have been linked to several neuro-developmental health outcomes.8 However, a majority of such studies have used whole teeth or fragments of dentine to estimate environmental exposure to metals and subsequent health effects. A major barrier to the detailed study of metal uptake in teeth and the further development of tooth-metal concentrations as a time-specific biomarker of environmental exposure has been the absence of readily accessible analytical methods that can reliably measure the spatial distribution of metals in teeth.

Current methods for the spatial analysis of teeth using element-specific detection include synchrotron microprobe X-ray fluorescence (SXRF),9, 10, 11 particle-induced X-ray emission (PIXE),12, 13, 14, 15, 16 and secondary ion mass spectrometry (SIMS).17 However, these methods are expensive, which prohibits their use in large epidemiologic studies. Teeth have also been acid digested and analysed by atomic absorption spectroscopy (AAS),18 inductively coupled plasma-optical emission spectroscopy (ICP-OES) and inductively coupled plasma-mass spectrometry (ICP-MS).7, 19

Elemental bio-imaging employing laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a new method for imaging trace elements in biological tissues.20 Our laboratory has applied this technique to the imaging of trace elements in mouse brains,21 osteoarthritic joints22 and human lymphatic tissues.23, 24 LA-ICP-MS is a relatively simple technique that does not require a high vacuum ion source or suffer from severe matrix effects. Complicated and often hazardous digestion procedures are not required as it is a solid-state analyzer. Additionally, the lower spatial resolution compared to PIXE (0.1–10 μm) is better suited to imaging the entire tooth. LA-ICP-MS applications have been limited to spot ablation and small area rastering to determine approximate spatial distribution of trace elements in teeth.25, 26 Detailed elemental maps of entire tooth sections using LA-ICP-MS have not previously been reported. In this paper we apply this novel method for quantitative imaging of trace elements in teeth. This preliminary study demonstrates the potential for using LA-ICP-MS imaging in studying tooth microchemistry and monitoring trace elements concentrations, including important developmental nutrients and environmental toxicants.

Section snippets

Sample preparation

Naturally shed deciduous incisors collected from children living in a rural community of Australia were analysed. These teeth were part of a larger sample set examining the impact of trace element distribution in dental development. Three incisors that were free of dental caries, any obvious developmental defects, and also histologically unremarkable (samples BH3, BH4 and BH10) were selected for analysis. Written consent was obtained from the parents/guardians of the participants. Ethics

Results

Fig. 1 shows a sample image created using the single-point calibration LA-ICP-MS method described. A 30 μm diameter laser beam was used for all experiments, traversing the sample at a rate of 30 μm s−1. Ablation lines were spaced precisely 30 μm apart. Integration parameters for the MS were set so as to acquire one data point per second for each measured mass. Consequently, each pixel represents the signal intensity corresponding to a 30 μm2 area of ablation. These values were converted to

Discussion

Sr was observed at concentrations over 120 μg g−1 in all teeth, with notably higher concentrations present in the dentine approaching the pulp cavity. Zaichick et al. reviewed 20 publications that determined the Sr content in the enamel of intact permanent human teeth and found the measured range of Sr in these reports was from 50 to 318 μg g−1.29 Sr concentrations exhibit heterogeneity throughout the tooth structure, and environmental exposures have been implicated as a major factor in this

Conclusions

LA-ICP-MS imaging is a powerful tool for mapping the spatial distribution of trace elements in teeth. Preliminary results indicate that quantitative data can be recoded through the ablation of an appropriate matrix-matched standard, such as NIST SRM 1486 Bone meal. Clear demarcation of trace element deposition in regions of teeth associated with developmental periods were observed for Sr, Zn and Pb. Concentration ranges (ng g−1 for Cd and Pb, μg g−1 for Sr and Zn) were consistent with previously

References (35)

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