Anion exchange resin and slow precipitation preclude the need for pretreatments in silver phosphate preparation for oxygen isotope analysis of bioapatites

Sequential extraction of the phosphate and collagen fractions of small bone samples for analysis of multiple isotope systems (δ18OPO4, δ13C, δ15N)

“…Although our study design did not include targeted measures to determine the exact level of possible remaining organic contaminants in the Ag 3 PO 4 precipitates, the absence of nitrogen peaks in the IRMS analysis of all the H 2 O 2 ‐treated samples indicates relatively effective and consistent removal of organics. This is noteworthy, since previous studies comparing pretreatment methods have reported the relative ineffectiveness of H 2 O 2 in removing the organic fraction from solid sample powders 55,62,63 . We hypothesize that the use of oxidants after the dissolution step, as applied in our method, and earlier by Lamb et al 2 and Tudge, 47 could be more effective, because the organic fraction in solution is no longer shielded by the bone mineral matrix, and thus such treatments are likely to be more efficient in removing organic contaminants (cf 46,55,62 …”

“…The direction and magnitude of these shifts depend on the fraction of PO 4 3− possibly left in the solution, and the O isotope compositions of the precipitation solution and that of the sample 32 . It is therefore possible that the isotopic shifts observed by Pederzani et al, 55 attributed to oxidizing treatments in general and NaOCl in particular, are at least partly caused by the difference in the precipitation method of the control and test procedures, rather that the effects of the pretreatments themselves. To summarize, while the extensive study of Pederzani et al 55 demonstrated that anion exchange resin treatment is an effective method to remove organic contaminants from bioapatite materials, the above considerations taken together with previous assessments of pretreatments 32,62 show that the call for the discontinuation of the use of NaOCl as a pretreatment agent 55 awaits further experimental support.…”

“…The organic matter present in the bone is most commonly removed prior to acid dissolution by oxidizing bone powder with H 2 O 2 50 or NaOCl, 51 but in the early works of Tudge 47 and Kolodny et al 48 organics were removed after acid dissolution by using KMnO 4 , or recently by Lamb et al 2 by using H 2 O 2 and HNO 3 in solution. Further purification may be conducted with ion exchange resins, 24,54 or ion exchange resins may be used as the only means to remove organic contaminants from dissolved bone without prior oxidative steps 46,55,56 . The Ca 2+ released in the acid dissolution step is isolated from the PO 4 3− ‐containing solution as CaF 2 either by using HF in the acid dissolution, by later addition of HF 33,52 or KF, 57 or by using ion exchange chromatography 34 .…”

“…Finally, the works by Grimes and Pellegrini 62 and Pederzani et al 55 comparing analytical protocols for δ 18 O analysis of skeletal phosphate have raised some concerns that the oxidative chemical pretreatments themselves, which are used to purify skeletal material from organic contaminants as described above, may cause changes in the O isotope composition of the phosphate. The inclusion of O‐bearing organic matter in Ag 3 PO 4 can alter the obtained δ 18 O values when the less effective oxidant H 2 O 2 is used, 55 but the data of Grimes and Pellegrini 62 indicate that the H 2 O 2 pretreatment tends to also systematically increase the δ 18 O values. The reason for this increase is unclear, but possible O‐isotope exchange between H 2 O 2 and the phosphate O is implied 62 .…”

“…In contrast to their findings, Grimes and Pellegrini 62 and Mine et al 32 observed small, if any, isotopic shifts when treating highly mineralized material (phosphate rock, synthetic hydroxyapatite, tooth enamel) with NaOCl, compared with non‐treated materials (see also Wiedemann‐Bidlack et al 33 ). It is notable that the Ag 3 PO 4 precipitates in the H 2 O 2 and NaOCl pretreatment experiments in Pederzani et al 55 were prepared with a fast precipitation method, while the control samples, purified using anion exchange resin, were precipitated using a slow Ag 3 PO 4 precipitation method. For their work, Grimes and Pellegrini 62 only used slow precipitation methods.…”