Effect of Metal Chlorides on the Sintering and Densification of Hydroxyapatite Adsorbent

Preparation of matrix-matched standards for the analysis of teeth via laser ablation-inductively coupled plasma-mass spectrometry

Mineralised tissue such as teeth can serve as a retrospective, chronological bioindicator of past exposure to toxic metals. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) can be used to determine the presence and spatial distribution of toxic metals in teeth, giving a record of when an exposure occurred. Concentrations of these metals are often determined by a one-point calibration against NIST glass using an equation that requires an internal standard factor that accounts for differences in ablation behaviour between the glass and the tooth. However, an ideal external calibration would contain multiple matrix-matched standards to obtain a calibration curve. Here, we investigated optimal procedures for preparing synthetic hydroxyapatite (HA) doped with elements of interest as a calibration material. The materials were examined for homogeneity of metal incorporation, matrix-matched ablation characteristics, linearity, and limits of detection. A homogenised and pelleted HA was the most suitable material, providing improved ablation characteristics over previous HA materials and NIST glass for the analysis of teeth. An ablation yield of 1.1 showed its suitability to analyse teeth, the metals were homogeneously incorporated, and it produced excellent linearity with limits of detection ranging from 0.1-2 μg kg^-1 for magnesium, aluminium, nickel, copper, zinc, cadmium, barium and lead. A juvenile incisor from a remote indigenous community in Australia and an adult molar from Sri Lanka were assessed for toxic metal exposure. The molar showed evidence of exposure to cadmium and lead. The synthetic HA material was straightforward to prepare, and will improve confidence in the analysis of teeth and other biomineralised material when assessing toxic metal exposure.

Formation of hydroxyapatite-layer on glass plate and its removal–regeneration properties for aqueous cadmium

A glass plate was coated with calcium hydroxyapatite (CaHAp) by a dip-coating method and employed for the removal of aqueous cadmium. Sol obtained from alcoholic solution of (NH(4))(2)HPO(4) and Ca(NO(3))(2) x 4H(2)O was employed for the precursor of the CaHAp layer. The preparation of CaHAp from the sol needed a rather low calcination temperature of 573 K and the resulting solid mainly contained CaHAp. It was shown that the glass plate coated with CaHAp with the sol could be employed for the removal of aqueous cadmium. Furthermore, it was found that cadmium immobilized on the coated plate could be regenerated into weak acidic solution. A dissolution-precipitation mechanism was suggested for the removal-regeneration of aqueous cadmium.

Preparation of alkaline earth phosphates with sol containing sodium alginate and sodium diphosphate

Magnesium hydrogen phosphate, calcium hydroxyapatite, and strontium hydroxyapatite were successfully prepared from sol consisting of sodium alginate and Na4P2O7 with Mg2+, Ca2+, and Sr2+ in the corresponding nitrates, respectively. It is revealed that the order of the addition of those substrates and the role of sodium alginate are important factors for the preparation of desired phosphate compounds. According to the previous paper on the preparation of calcium hydroxyapatite, sodium alginate was mixed with aqueous Na4P2O7, followed by the addition of the aqueous divalent cations, resulting in the poor formation of the target phosphates. However, as a revised sol-gel technique, sodium alginate was added to the mixture of Na4P2O7 and aqueous Mg2+ and Sr2+, resulting in a rather favorable formation of MgHPO4 and strontium hydroxyapatite, respectively, while the sol thus obtained was stable within a few days. However for aqueous Ca2+, calcium hydroxyapatite could not be obtained through the revised sol-gel technique. In the preparation of magnesium hydrogen phosphate, sodium alginate contributes mainly to the sol formation of the precursor. The ion exchange between Na+ in sodium alginate and aqueous Ca2+ was important for the preparation of calcium hydroxyapatite. In contrast, the reaction of sodium alginate with the mixture of Na4P2O7 and aqueous Sr2+ afforded strontium hydroxyapatite at the specific ratio of those three substrates. The structure of calcium and strontium phosphates prepared from the revised sol-gel process evidently depended on the amount of sodium alginate introduced into the mixture of Na4P2O7 and the corresponding divalent cations.