Evaluation of neodymium isotope analysis of human dental enamel as a provenance indicator using 1013â€¯Ω amplifiers (TIMS).

Human provenance studies employing isotopic analysis have become an essential tool in forensic and archaeological sciences, with multi-isotope approaches providing more specific location estimates compared to single isotope studies. This study reports on the human provenancing capability of neodymium isotopes (143Nd/144Nd), a relatively conservative tracer in the environment. Neodymium isotope ratios have only recently been determined on human remains due to low concentrations in human dental enamel (ppb range), requiring thermal ionisation mass spectrometry (TIMS) using 1013â€¯Ω resistors. Dental elements (third molars) from 20 individuals born and raised in the Netherlands were analysed for Nd concentration (n = 12) and Nd isotope ratios (n = 15). The geological control on Nd isotope composition was examined using coupled Nd-Sr isotope analysis of the same third molar. Teeth from different geological environments were also analysed (Caribbean, Columbian, and Icelandic, n = 5). Neodymium elemental concentrations in dental elements ranged between 0.1 and 7.9 ppb (median 0.5 ppb). The Dutch 143Nd/144Nd ratios of the provinces of Limburg and Friesland were between 0.5118 and 0.5121, with Dutch 87Sr/86Sr ratios in agreement with the previously established local range (0.708-0.710). The current findings were compared to previously published results on Nd concentration and composition from Dutch individuals. The concentration of Nd and 143Nd/144Nd ratios were weakly correlated (R2 = 0.47, n = 17) in Dutch human dental enamel. The majority (n = 25, 83.3%) of individuals had Nd and Sr isotope values isotopically indistinguishable from the geological environment in which their third molars formed and mineralised. However, the Nd isotope ratios of the Icelandic individual and several Dutch individuals (n = 4) suggested that Nd in enamel is not solely influenced by geological environment. In order for neodymium isotopes to be quantitatively applied in forensic and archaeological settings further analyses of individuals from various geographical regions with well-defined dietary Nd isotope data are required.

Measurement of small ion beams by thermal ionisation mass spectrometry using new 10(13) Ohm resistors.

We tested 5 newly manufactured - prototype - 10(13)Ohm resistors in the feedback loop of Faraday cup amplifiers to measure small ion beams by Thermal Ionisation Mass Spectrometry (TIMS). The high Ohmic resistors installed in the TRITON Plus at the VU University Amsterdam theoretically have 10 times lower noise levels relative to the default 10(11)Ohm resistors. To investigate the precision and accuracy of analyses using these new amplifiers we measured Sr and Nd isotopes of reference standards at a range of ion currents (3.2×10(-16) to 1×10(-12) A, corresponding to intensities of 32 µV to 100 mV on a default 10(11)Ohm amplifier) and on small amounts of material (100 and 10 pg). Internal precision and external reproducibility for Sr and Nd isotope ratios are both better when collected on 10(13) compared 10(12)Ohm resistors and to the default 10(11)Ohm resistors. At an (87)Sr ion current of 3×10(-14) A (3 mV on a 10(11)Ohm amplifier) the internal precision (2 SE) of (87)Sr/(86)Sr is 5 times better for 10(13)Ohm resistors compared to 10(11)Ohm resistors. The external reproducibility (2 SD) at this beam intensity is 9 times better. Multiple 100 and 10 pg Sr standards, ran to exhaustion, yielded low (87)Sr/(86)Sr compared to the long term average (e.g. 10 pg average=0.710083±164 (n=11) instead of 0.710244±12, n=73). The average off-set for 10 pg standards can be explained by a loading blank contribution of 1.3 pg. In contrast, Nd data on 100 pg and 10 pg samples are accurate suggesting that Nd loading blanks do not compromise the data. The external reproducibility of (143)Nd/(144)Nd on 100 pg samples is 125 ppm and 3.3‰ on 10 pg samples (2 RSD=relative standard deviation, n=10). Thus, variability in Nd and Sr isotope ratios in the 4th decimal place, e.g. (143)Nd/(144)Nd 0.5110-0.5119 or (87)Sr/(86)Sr 0.7100-0.7109, can be resolved in 10 to 100 pg samples provided that the procedural blanks and chemical separation are optimal. For measurements in the beam intensity range usually covered by ion counting (<3 mV or 2×10(5) cps) we obtain a (143)Nd/(144)Nd internal precision (2 SE) of 480 ppm for a (143)Nd intensity of 6.25×10(4) cps (1 mV) and 1% at an intensity of 2×10(3) cps (32 µV on a 10(11)Ohm amplifier). We find that at intensities higher than 2×10(4) cps the precision using the 10(13)Ohm resistors is better than for ion counting owing to instability and non-linearity behaviour of the ion counting system. Our results indicate that between 2×10(4) cps and an ion current of 2×10(-13) A (20 mV on a 10(11)Ohm amplifier) it is beneficial to use the high gain amplifiers instead of (multi) ion counting or Faraday cups equipped with the standard 10(11)Ohm resistors. This finding suggests that the newly developed high gain resistors could potentially be valuable in applications that currently use (multiple) ion counting to measure small ion beams (e.g. U-series, Re-Os, Pu, Pb). In addition to improved precision, the use of Faraday cups equipped with high resistance amplifiers is more practical in terms of the required calibration procedure and in the flexibility in the collector set-up compared to using multiple ion counting arrays.

Problems in obtaining precise and accurate Sr isotope analysis from geological materials using laser ablation MC-ICPMS.

This paper reviews the problems encountered in eleven studies of Sr isotope analysis using laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) in the period 1995-2006. This technique has been shown to have great potential, but the accuracy and precision are limited by: (1) large instrumental mass discrimination, (2) laser-induced isotopic and elemental fractionations and (3) molecular interferences. The most important isobaric interferences are Kr and Rb, whereas Ca dimer/argides and doubly charged rare earth elements (REE) are limited to sample materials which contain substantial amounts of these elements. With modern laser (193 nm) and MC-ICPMS equipment, minerals with >500 ppm Sr content can be analysed with a precision of better than 100 ppm and a spatial resolution (spot size) of approximately 100 microm. The LA MC-ICPMS analysis of 87Sr/86Sr of both carbonate material and plagioclase is successful in all reported studies, although the higher 84Sr/86Sr ratios do suggest in some cases an influence of Ca dimer and/or argides. High Rb/Sr (>0.01) materials have been successfully analysed by carefully measuring the 85Rb/87Rb in standard material and by applying the standard-sample bracketing method for accurate Rb corrections. However, published LA-MC-ICPMS data on clinopyroxene, apatite and sphene records differences when compared with 87Sr/86Sr measured by thermal ionisation mass spectrometry (TIMS) and solution MC-ICPMS. This suggests that further studies are required to ensure that the most optimal correction methods are applied for all isobaric interferences.