he use of mercury-based dental amalgam fillings as a dental restorative is a well-established practice that was first introduced in France in 1826 ( 1 ). When correctly formulated, and neglecting any concerns related to toxicity, mercury dental amalgams present an almost ideal substance for restorative work, with easy installation, low creep, minimal dimensional change, and high compressive strength. Typically, amalgam fillings will outlast other restorative materials ( 2 ), and it has been estimated that the average American adult has seven mercury amalgam dental fillings; while formulations vary, most modern amalgams have a typical final composition in the range of 40 - 50% Hg, 30 - 40% Ag, 5 - 15% Sn, and smaller amounts of other metals including Cu and Zn or Pd. However, dental amalgam is also one of several sources of human exposure to potentially toxic mercury ( 3 ), and its disposal can also be a source of environ- mental mercury pollution ( 4 ). Because of this, in recent decades, the use of mercury-containing dental amalgams has become controversial, and several countries including Sweden and Norway have recently banned its use ( 5 ). In North America, the traditional view is that the advantages of mercury-containing amalgam outweigh any possible health risks ( 3 ), and for this reason, mercury-containing dental amalgams are still widely used. Nevertheless, the debate continues, and dentists are divided about whether the benefits outweigh the risks or vice versa, and more research is needed to quantify and characterize mercury exposure from dental amalgam ( 6 ). A number of sources of human exposure are known, and these include evaporation of mercury from the surface of the filling and subsequent inhalation and leaching of mercury into saliva, in which bacterial action may be involved. Other sources of mercury exposure have also been recently demonstrated, specifically, migration of mercury through the tooth dentinal tubules ( 6 ) from where it could enter the blood supply to the pulp. One source of human exposure to mercury ( 7 ) from dental amalgam is through the surface of the filling, and the chemical composition of the surface is therefore of interest. In this preliminary study, we have used a surface-sensitive spectro- scopic tool s electron yield Hg L III X-ray absorption spectro- scopy s to probe the chemical nature of mercury at the surface of fresh and exposed fillings. Information obtained from this study will assist in understanding released metal toxicological behavior at the molecular scale and will provide much-needed insights into the potential hazards or otherwise of the use of dental amalgam.
Sample Preparation. Fresh amalgam fillings were prepared using dental amalgam from a commercial capsule (Valliant PhD #1, Ivoclar Vivadent Inc., Amherst, NY; amalgam final weight composition: Hg, 47.9%; Ag, 27.3%; Cu, 9.1%; Sn, 15.5%; and Pd, 0.2%) into a melamine replacement tooth (model A5-200 typodont, Kilgore International, Inc., Coldwater, MI). Drilling, installation of the restorative, and burnishing were performed exactly as if for human installation. The filling had a distinctive bright metallic luster. For the aged amalgam, extracted molar teeth were obtained from the University of Saskatchewan Dental Clinic’s tooth bank. The tooth examined in this study was a mandibular second molar with a mesio-occlusal restoration and had a discernible dark patina. The exact history and age of the restoration is unknown, but we estimate approximately 2 decades of exposure to normal aural conditions. The tooth was carefully washed with distilled water and allowed to dry in air. dental laser tips
Teeth were mounted with conducting aluminum adhesive tape to the backplane of the electron yield detector, taking care to place a slight overlap with the edge of the amalgam filling to maintain electrical connectivity and to avoid sample charging artifacts ( 8 ). Chemicals were obtained from Sigma- Aldrich and were of the best quality available. Samples of Ag - Hg amalgam and Sn - Hg amalgam were prepared by allowing liquid elemental mercury to come in contact with a small piece of silver or tin metal foil (Goodfellow Metals, Huntingdon, United Kingdom) whereupon the mercury was quickly absorbed to form an amalgam containing approximately 50:50 mol ratios of the two metals (established by weighing the samples). A sample of pure metallic mercury suitable for spectroscopic measurements was precipitated from aqueous HgCl 2 solution by reduction with a slight excess of sodium borohydride, which yielded a gray-milky solution, which was then frozen prior to spectroscopic examination.
Results and Discussion
XAS arise from excitation of a core electron (e.g., a 1s electron fo r a K edge or a 2p 3/2 electron for an L III edge). Such excitation creates a core hole, which relaxes via dipole-allowed decay of an outer electron with either the emission of an X-ray fluorescent photon or the emission of an Auger electron. XAS is commonly detected by one or more of three methods s transmittance, fluorescence yield, and electron yield ( 11 ). Transmittance is conceptually simplest and is recorded by monitoring the intensity of a monochromatic X-ray beam before and after the sample. Fluorescence yield detection monitors the X-ray fluorescence that (under ideal circumstances) is propor- tional to the X-ray absorption and is the most sensitive method of detection. Electron yield monitors the X-ray-induced electron emission, and because of the low electron path length, it is predominantly sensitive to the top 20 - 30 Å of the sample. It therefore constitutes a surface-sensitive probe and is well-suited to our purposes of characterizing amalgam-filling surfaces. XAS can be arbitrarily divided into two overlapping regions s the near-edge spectrum, which is the structured region within approximately 50 eV of the absorption edge, and the extended X-ray absorption fine structure (EXAFS), which is an oscillatory modulation of the absorption on the high-energy side of the absorption edge and which can be interpreted in terms of a local radial structure. Near-edge spectra are comprised of transitions from the core level (1s fo r a K edge) to unoccupied molecular orbitals of the system. Intense transitions are dipole- allowed ∆ l )( 1 and, thus, for K and L III edges are to levels containing predominantly p and d orbital character, respectively. Near-edge spectra are therefore sensitive to electronic structure and give a fingerprint of the chemical species of the metal or metalloid concerned. The advantage of the near-edge region of the spectrum is that it can be collected relatively quickly with good signal-to-noise. In contrast, EXAFS is more challenging to collect with adequate signal-to-noise and may not be practical on dilute samples. A unique benefit of XAS is that it requires no pretreatment or extraction and thus provides a tool that can probe chemical species in situ, although a disadvantage of electron yield detection is that it cannot be used with wet samples, as a film of water would essentially eliminate the electron yield signal. Figure 1 compares the Hg L III near-edge spectra of a series of solid standard compounds. As we have discussed previously ( 10 ), differences between Hg L III spectra are more subtle than other near-edge spectra. With K edges, the valence orbitals have substantial p orbital character, giving rise to intense dipole- allowed transitions and rich chemical variability. In contrast, for Hg L III edges, the dipole-allowed transitions will be to orbitals with d character transitions, and as a result, there is a more subtle variability between the spectra. The consequence of this is that data of significantly better signal-to-noise (when measured relative to the edge jump) are required for Hg than for K-edge spectra, but the spectra can nevertheless give the desired information. Figure 2 compares the electron yield Hg L III near-edge spectra of fresh and aged amalgam fillings. The quantity of the element of interest (in this case Hg) is, to a first approximation, proportional to the edge jump, which is essentially the difference in signal just below and above absorption edge. The edge jump is accurately estimated by our background subtraction proce- dures and is used for normalization of the data. The electron yield edge jump of the aged amalgam filling was observed to be only 5% that of the fresh filling, suggesting that the surface material of the aged filling is substantially depleted in mercury, having lost up to 95% of its mercury. We note that part of the reason for the decrease in signal strength may be due to a lowering of the surface conductivity resulting from chemical changes at the surface or changes in the electron path length ( 12 ), and the value of 95% for mercury surface depletion should thus be considered an upper bound. Dental amalgam is known to lose mercury both by evaporation of mercury vapor and by leaching of mercury into saliva ( 3 ). Some extent of surface mercury depletion is therefore expected, although this is normally difficult to quantify. The spectrum of the fresh filling appears metallic in nature but is distinct from those of elemental Hg, Hg - Ag amalgam, and Hg - Sn amalgam. Examination of the EXAFS data (not illustrated) indicates predominantly metal - metal contacts, consistent with a metallic surface for the freshly prepared filling. Structural examination of the fresh amalgam surface structure by analysis of the EXAFS will be the subject of future investigations. The spectrum of the aged filling (Figure 2) shows subtle but significant differences from that of the fresh fillin.