Introduction
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.
Experimental Procedures
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 tipsTeeth  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.
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