Robert Anczkiewicz*a,
Wolfgang Müllerb,
Szymon Mianowskic,
Maria Dądelaa,
Alessia Nava
c,
Luca Bondioli
a,
Milena Matyszczaka,
Anna Jasińskaa,
Jörg Ostendorf
a,
Sofia Bakayeva
de and
Taras Yanytsky
e
aInstitute of Geological Sciences, Polish Academy of Sciences, Kraków Research Centre, Kraków, Poland. E-mail: ndanczki@cyfronet.pl
bInstitute of Geosciences, Goethe University Frankfurt, Frankfurt am Main, Germany
cDepartment of Odontostomatological and Maxillofacial Sciences, Sapienza University of Rome, Rome, Italy
dInstitute of Paleobiology, Polish Academy of Sciences, Twarda 51/55, 00-818 Warsaw, Poland
eState Museum of Natural History of the National Academy of Sciences of Ukraine, Teatralna St. 18, Lviv 79008, Ukraine
First published on 26th June 2025
The effects of isobaric interferences on the precision and accuracy of in situ Sr isotopic composition measurements in bio-apatite were investigated using different plasma conditions, interface configurations and instrumentation. Analysis of modern shark enameloid via laser ablation MC-ICPMS (LA-MC-ICPMS) Neptune and Neoma using H-type cones show similar long-term 87Sr/86Sr reproducibilities of 0.709194 ± 0.000034 and 0.709189 ± 0.000027 (2SD), respectively. Neither instrument showed detectable interference from matrix-generated 40Ca31P16O or Ca dimer/argide isobars. The ratios agree with solution-based MC-ICPMS and TIMS analyses, yielding an average 87Sr/86Sr of 0.709176 ± 0.000008 (2SD). The use of a Jet sampler and a X-type skimmer cone with Neoma increases the sensitivity 2–3 times while maintaining the same level of accuracy. The formation of 40Ca31P16O polyatomic clusters can be avoided by tuning the plasma to a low oxide level. An inherent feature of high-sensitivity cones is the production of Ca dimers/argides, which affects 84Sr/86Sr, additionally suffering from 68Zn16O isobars readily generated at higher oxide levels owing to the ubiquitously high Zn concentration in bio-apatite. The mass bias of 87Sr/86Sr analysis follows the exponential law across all instrument configurations, which is interpreted as being due to a small average mass difference between normalised and normalising ratios. We demonstrate the benefit of high-sensitivity cones by analysing the highly compositionally zoned enamel of a woolly mammoth. Higher ion beam intensity allowed improved spatial resolution down to 40 μm diameter, revealing compositional variability obscured by larger spots. Improved counting statistics allowed for shorter integration times, resulting in <10 days of temporal resolution for >10 years of enamel formation. Increased sensitivity is of particular value for analysing organisms with low Sr uptake, such as humans, and in slowly mineralizing bioapatite tissues. Another benefit is the enhanced ability to distinguish early in vivo enamel formation from subsequent maturation, which is crucial for reconstructing the palaeoecology of large mammals and human life histories.
The desolvating nebulisation increased the sensitivity of the mass spectrometers by about 4–8 times with respect to the SIS. For both introduction systems, SRM 987 was at a concentration providing 10–15 V of 88Sr. The standard Ni sampler and H-type skimmer cones (hereafter jointly referred to as H-type cones) were used. The isotope ratios were measured in static mode. A single run consisted of 50 cycles of 4.048 s integration. The collector gains were measured prior to each analytical session using the factory setup. The peak centering was performed during tuning and verified every few hours. The baseline was measured on the masses of interest, “on peak zeroes” (OPZ), performed on 2% HNO3. Washouts between samples were conducted using 3% HNO3. The total integration time of the baseline was similar to that of the sample. Krypton isobaric interferences associated with trace level contamination of the Ar plasma gas have been reported as an obstacle to accurate Sr isotopic composition measurements, and various correction methods have been proposed.7,10,13,30,31 In our setup, we observe less than 0.5 mV of 84Kr, with a typical level of <0.2 mV, and ordinary gas blank subtraction efficiently removes Kr isobars from Sr isotopes. In solution-based analyses, the 87Rb isobar is also a minor problem as samples are highly purified by ion exchange chromatography. The 85Rb/87Rb correction value was estimated from Rb–Sr mixed solutions as 0.38557, which is slightly lower than the natural reference value of 0.38571 due to the difference in mass bias between Rb and Sr. For the correction, the estimated ratio was de-normalised using the exponential mass bias law and within run 88Sr/86Sr ratio. After subtraction of the interfering isobars, the final ratios were normalized to 86Sr/88Sr = 0.1194, applying the exponential law.32 The data were filtered at a 2σ level.
Data obtained by MC ICPMS Neptune for SRM 987 shows long-term reproducibility expressed as 2SD (standard deviation) for >10 years period of 87Sr/86Sr = 0.710260 ± 0.000011 (n = 213) and 0.710251 ± 0.000013 (n = 107) for the wet and dry plasma, respectively (Fig. 1a). The external precision is similar to the internal precision of an individual run in both cases, which is expressed as 2SE (standard error). Notably, the dry plasma conditions provide slightly more accurate data (relative to TIMS values), although, in a strict sense, within uncertainties, the wet and dry plasma results are indistinguishable. The corresponding 84Sr/86Sr ratios are 0.056492 ± 0.000023 and 0.056499 ± 0.000030, which are also accurate. The internal precision of 84Sr/86Sr is nearly three times better (absolute error) than that of long-term reproducibility. Although the latter values are practically the same and agree well with the TIMS ratio, the long-term reproducibility of data obtained using Aridus II is worse than that of SIS. This is interpreted as being due to a low 84Sr signal (17 to 170 mV) and the higher importance of accurate baseline correction. In the case of the desolvating nebuliser, it requires a lot of patience due to the excessively long washout time, and thus, incomplete washout likely affected some measurements. Some contribution to lowering the precision could be associated with 84Kr and 86Kr interferences. However, as mentioned above, the Kr content in our analyses is minor, and the baseline subtraction practically corrects it to zero (mean measured 83Kr/86Sr = 0.000001 ± 0.000007). In the absence of any major isobaric interferences, the main factor potentially affecting the accuracy of isotope ratio determination is the mass bias correction, if it departs from the exponential law of ref. 32, as demonstrated for Nd isotopes e.g. ref. 34–37. Ionisation in the ICP source generates approximately 10 times larger isotope fractionation than the thermal source of TIMS, but unlike TIMS, the bias is relatively stable throughout a single measurement. The prediction of an exponential mass bias law is that log natural 88Sr/86Sr versus 87Sr/86Sr ratios corrected for baseline and interferences form a linear trend with a gradient equal to 0.50359 (Fig. 1b and c). The linear regression parameters estimated for ratios in LN(87Sr/86Sr)m vs. LN(87Sr/86Sr)m coordinates yield 0.50382 ± 0.00049 (MSWD = 0.8) and 0.50362 ± 0.00054 (MSWD = 1.0) for wet and dry plasma conditions, respectively (Fig. 1c and d). Both values are in excellent agreement with the theoretical prediction, demonstrating that the exponential law perfectly describes the mass discrimination of Sr isotopes during MC-ICPMS measurements. This remarkable agreement makes the Sr isotope system unique. However, in our view, this relationship arises from the small average mass difference between the normalised 87Sr/86Sr and the normalising 88Sr/86Sr ratios. As shown for Nd isotopes, the smaller the mass difference, the closer the agreement with the exponential law; conversely, larger mass differences result in greater deviation from the exponential law.35–37 Our data suggest that this model is also valid across lower mass ranges. Analogous assessment for 84Sr/86Sr gave −1.028 ± 0.010 (MSWD = 3.6) and −1.052 ± 0.019 (MSWD = 9.3) gradients for the wet and dry plasma, respectively (Fig. 1d). Both values approximate the expected −1.023 gradient, however, the lower precision of the 84Sr/86Sr measurements limits a more detailed assessment of mass bias effects. Nonetheless, under wet and dry plasma conditions, the short- and long-term reproducibility of Sr isotope ratios is accurate at high precision.
In Fig. 2, we compiled the LA MC ICPMS analyses of shark dental enameloid collected over 10 years. An individual analysis represents a mean ratio for the integration time between 60 and 600 seconds, with the vast majority lasting from 150 to 250 s (Fig. 2a and b). The intensity of 88Sr varied from ca. 1 to 20 V, indicating variable spot size, laser pulse repetition rate, and, to a lesser degree, MC-ICPMS settings. The 87Sr/86Sr ranges from 0.70915 to 0.70922, with the internal precision expressed as 2RSE (relative standard error) ranging from 120 ppm for signals ≤1 V of 88Sr to approximately 20 ppm for signals >10 V. It is noteworthy that low-intensity measurements, although less precise, remain accurate. The internal precision well approaches or occasionally even matches that achieved for the SRM 987 solution-based measurements (ca. 15 ppm 2RSE) for comparable integration time and intensities > ca. 10 V of 88Sr. However, the long-term reproducibility (0.709195 ± 0.000025; 2SD) is approximately two times lower than that of the solution analyses. This is attributed to the instability of the transient signal and imperfections of natural solid materials (defects, zonation, etc.). Overall, the long-term reproducibility, expressed as arithmetic mean with 2SD (standard deviation) uncertainty for all data, yielded 87Sr/86Sr = 0.709194 ± 0.000034 (Fig. 2a and b), which is satisfactory for resolving compositional differences in a broad range of applications requiring high spatial resolution.
Sr isotope analyses of shark teeth are unaffected by isobars of doubly charged REE ions simply because there is no detectable uptake of these elements during the animals' lives. The Kr level in our setup is the same as that in the solution mode analyses; thus, ordinary gas blank subtraction efficiently strips the interference from the peaks. Thus, the primary concerns are the 87Rb and matrix-related isobars. The Rb concentration in shark tooth is negligible. The measured 85Rb/86Sr (Fig. 2d) normalized using Sr mass bias spanned from below the detection limit of Rb to 0.0004 with the average 0.000088 ± 0.000103 (2σ outliers rejected). Such small interference can be effectively corrected using the standard peak stripping method. Some of the data are slightly overcorrected (negative 85Rb/86Sr values, Fig. 2d) due to a “more intense washout” of 85Rb and contrasting cone memory effects affecting Rb vs. Sr such that 85Rb can drop below the corresponding baseline level observed in some runs. This is illustrated in Fig. 3, which shows a decrease in the 85Rb signal below the blank level during ablation. Such inadequate baseline correction may change the 87Sr/86Sr ratio by as much as 15 ppm. This effect is undetectable in materials with higher Rb abundance because the baseline correction does not lead to negative values. Only long pre-conditioning of the cones with apatite matrix seems to prevent more intense Rb washout during the ablation.
A debated obstacle in obtaining accurate 87Sr/86Sr ratio is the 40Ca31P16O isobar generated during the ablation of apatite matrix.9,18,41,42 This, however, seems specific to the instrument or setup, as some studies essentially do not observe the formation of this compound.11 The production of this cluster is strongly linked to the oxide level in the plasma.9,18 We de-tuned the torch position and gas flows to achieve 232Th16O/232Th of approximately 10%, but we observed no detectable effect on the accuracy of 87Sr/86Sr ratio determination, indicating that even relatively high oxide production is insufficient to generate enough 40Ca31P16O ions to affect 87Sr/86Sr ratio within the obtained precision, which agrees with the observation of ref. 11. Other apatite matrix-related isobars include Ca-dimers and argides.9,13,31,43,44 The Ca dimers and argides affect all Sr isotopes, but due to the low abundance of 84Sr (ca. 0.59%), the 84Sr/86Sr ratio is a particularly sensitive indicator of these interferences. This is well illustrated with the anomalous group of 84Sr/86Sr ratios showing values from 0.0568 to 0.0578 (Fig. 2a). They show a clear positive linear correlation with mass 82X used as a monitor of Ca argide/dimer (42Ca40Ca, 42Ca40Ar) (Fig. S1a and b†). The measured intensity of 82X was 20–120 μV, with the majority of measurements ranging from 20 to 60 μV, which translates into max. 0.2 mV of 44Ca interference on 84Sr ref. 13 report about 100 mV of total Ca-argide/dimer signal and shift in 84Sr/86Sr ratios similar to those in our study. The anomalous 84Sr/86Sr ratios show a positive correlation with 82X/86Srm (Fig. S1a and b†). Linear regression defines a zero intercept at 0.05658 ± 0.00018, which agrees with the reference value of 0.05650 (both ratios normalized using Sr mass bias). The slope of the regression defines 84(CaCa, CaAr)/82(CaCa, CaAr) as 3.2 ± 1.1 (MSWD = 6.3), which broadly agrees with the expected value of ca. 3.2241 for 44Ca/42Ca. The data scatter may be due to the small instability of the plasma conditions that change the dimer/argide production or the contribution from other isobar(s) affecting 82X/86Sr ratio (Fig. S1a†). Although it is impossible to differentiate between the Ca dimer and argide clusters, as pointed out by ref. 13, the relative abundances of key isotopes are similar, and the same correction will accommodate both interfering compounds. The group of inaccurate 84Sr/86Sr data was generated by a single user during several consecutive sessions. We investigated all the operator-controlled parameters of laser ablation and MC-ICPMS, but no correlation was found with the anomalous data. Moreover, we did not intentionally tune the MC-ICPMS to stimulate the Ca-dimer/argide production to a visible level. The accidental use of the X-type skimmer cone is the most probable cause of the observed systematics. Nonetheless, even such significant dimer/argide production does not affect the corresponding 87Sr/86Sr ratio, which, uncorrected for dimer/argide isobars, is 0.709181 ± 0.000032 (2SD, n = 28) and perfectly agrees with the reference value. All the remaining 84Sr/86Sr measurements, uncorrected for dimer/argide, yielded an accurate mean value of 0.056501 ± 0.000049 (2SD, n = 328). The small positive correlation with 82X/86Sr (Fig. S1b†) is certainly not because of the dimers/argides, as the relationship in the 82X/86Sr and 84Sr/86Sr coordinates does not follow the argide/dimer trend (Fig. S1a†). A potential reason could be a “residual”, uncorrected by blank subtraction, Kr interference. However, assuming that all mass 82 is 82Kr, the performed correction reduces the slope of the observed trend from 0.96 ± 0.14 (2SD, MSWD = 6.2) to 0.52 ± 0.17 (2SD, MSWD = 6.7). This changes the mean 84Sr/86Sr ratio to 0.056496 ± 0.000053, resulting in a negligible difference in accuracy and precision.
The determination of the mass bias trajectory, as for the SRM 987 (Fig. 2f and S1c, d†), in the case of 87Sr/86Sr ratio, shows a gradient of 0.050337 ± 0.000054 (MSWD = 0.96), which closely follows the exponential mass bias law (Fig. 2f). In the case of the 84Sr/86Sr ratio, the fractionation trajectory with a slope of −0.999 ± 0.010 (data affected by dimer/argide interferences excluded) significantly deviates from the expected value of −1.048 (Fig. S1d†) and the data obtained in solution mode (Fig. 2d). This discrepancy is likely due to the low precision of the measurements (0.1% 2RSD), which limits the detection of mass bias effects. Nevertheless, from a practical perspective, the measured 84Sr/86Sr ratio agrees with the TIMS value within uncertainty, indicating that any deviation from the exponential law did not significantly distort the results.
The internal precision achieved for LA-MC-ICPMS shark analyses approaches that of SRM 987 solution-based analyses when comparing data acquired with signals of similar intensity and integration time. For 88Sr >6 V, 87Sr/86Sr ratios show an internal precision of 35 to 20 ppm 2RSE and yield a mean ratio of 0.709182 ± 26 (36 ppm, 2RSD). The solution mode analyses have similar internal and external precision at the level of approximately 15 ppm (for measurements at about 15 V of 88Sr). Lower external precision is expected in the case of transient signal analyses of a natural standard with small-scale heterogeneities, local mechanical defects, or zonation in element concentration, matrix effect, etc. Nonetheless, the long-term reproducibility of LA-MC-ICPMS at the level presented in this study is satisfactory for most applications and can certainly be improved.
The effect of Zn oxide on Sr isotopes has rarely been explored and was found to be insignificant in solution-based work with Zn/Sr ratios up to approximately 1.44 Dental tissues are rich in Zn, especially in the outer enamel,48,49 and their abundance and Zn/Sr ratio are typically strongly zoned. In sharks, the Zn concentration is on the order of several hundred ppm, reaching even ca. 0.2 wt%, approaching the Sr concentration. Oxygen and zinc form a range of interfering isobars on masses 83, 84, 85, 86, 87 and 88. The most abundant 66Zn and the most abundant oxygen isotope 16O form the highest interference, making mass 82 a sensitive monitor of ZnO isobars. The most significant consequence is on 84Sr due to the formation of 68Zn16O, which is the second most abundant Zn isotope (18.5%). The accurate 84Sr/86Sr ratio measured with H-type cones, both with Neptune and Neoma, indicates that ZnO oxide, if present, is negligible in such a configuration. In the case of high-sensitivity cones, where minor 84Sr isotope and mass 82 are additionally affected by Ca-dimer/argide, assessing the potential effect of superposed ZnO isobars is more difficult. Hence, we performed experiments in solution mode, aspirating only the NIST 683 Zn standard using Aridus II connected with Neoma. Experiments were conducted for different oxide levels controlled by lowering the sampling depth while keeping all other parameters unchanged (Fig. 5). At 232Th16O/232Th of ca. 2–3% or lower, the ion beams at 82 and 84 masses are at the acid blank level (or the effect is too small to be detected with a Faraday collector). Increasing oxides to about 10% causes a significant rise at mass 82 (66Zn16O) and a smaller rise at mass 84 (68Zn16O). This also changed the signal on mass 85 to the level below the corresponding blank, yet had no effect on masses 86 and 83, the latter being the most reliable Kr interference monitor. Higher oxides caused the rise of interferences in all monitored isotopes, with the biggest change for masses 82 (66Zn16O) and 84 (68Zn16O). A further increase in the number of oxide magnifies the effect, which is best manifested by the changes in mass 82 (Fig. 5). Noteworthy, the effect of 70Zn16O isobars on mass 86, although present, is negligible as 70Zn constitutes only 0.6% of total Zn. The effect of 70Zn17O, involving a minor oxygen isotope, on mass 87 is even smaller. Hence, the effect of ZnO is expected to be minor or even negligible on the 87Sr/86Sr ratio. However, it is important for the 82X/86Sr ratio used for argide/dimer correction, if Zn concentration or oxide level is high. A complementary test for potential ZnO interferences was directly performed on the dental enamel. High Zn enrichment in the outer enamel layer is an expression of amelogenesis.48,50,51 We measured the invariant in nature, the 84Sr/86Sr ratio, as a line scan from the inner to outer enamel of a woolly mammoth (see below). Subsequently, we measured the intensity of 66Zn as a measure of element concentration along the same line. The estimated Zn/Sr ratio changes from approximately 0.2 in the inner enamel to nearly 4 in the outer enamel. The first scan was conducted with an oxide level of ca. 2%. We observe a strong increase in 82X/86Sr and 84Sr/86Sr in the outer enamel layer, where the zinc concentration abruptly rises. A small effect is also visible on 83Kr/86Sr ratios (Fig. 6a and b). Applying only the correction for dimers, the 84Sr/86Sr ratio is accurate, but notably, it gently rises in the Zn-rich zone, and the data is more scattered. Setting up oxides to a very high level of approximately 50% did not reveal any extra features and resulted only in the magnification of the changes observed for the low oxide production (Fig. 6c and d). Under such conditions, the final 84Sr/86Sr ratio becomes highly inaccurate. A “by-product” of our experiments is the observation of a rise in the 82X/86Sr ratio across the enamel, even when the Zn content gently decreases. We did not observe any chemical change in the studied profile that could explain this observation, which certainly deserves further investigation. Overall, our experiments show that even at fairly low oxide levels, ZnO is a significant obstacle in obtaining accurate 84Sr/86Sr ratios in materials with elevated Zn/Sr ratios.
Better plasma sampling and ion extraction with the Jet-X cones also results in smaller isotopic fractionation, which was also observed for the wet plasma, solution mode Sr isotopic composition analyses.20 The latter authors also reported a good agreement between the fractionation trend and the exponential law. Our data show a narrow range of the beta factors from −1.25 to −1.08 for nearly all our measurements (Fig. 4g). The fractionation trend has a gradient of 0.4978 ± 0.0057 (MSWD = 0.6) and deviates from the expected exponential law prediction by 1.15% but overlaps with it within uncertainty and does not have any bearing on the accuracy of 87Sr/86Sr measurements (Fig. 4h). The higher error of the estimated gradient is due to the smaller amount of data and the smaller range of isotope fractionation. Smaller isotopic fractionation is also reflected in the slightly smaller elemental fraction between Rb and Sr, with 85Rb/86Sr = 1.138.
The analyses were performed with the setups described above, namely, MC-ICPMS Neptune with H-type cones and MC-ICPMS Neoma with (1) the H-type and (2) the high sensitivity (Jet-X) cones for different laser beam diameters. As the S-155 laser sample cell was large enough to fit an entire dental plate of a mammoth molar, the measurements were taken as a single line scan (Fig. 7a). The ablation lines were placed near the enamel–dentine junction (EDJ) and proceeded from the cusp (early forming enamel) to the cervix (late forming enamel). The results obtained with the Neptune for the longest centrally located tooth plate (Fig. 7a), summarised in Fig. 7b–d, show twelve cyclic changes in the 87Sr/86Sr ratio ranging from 0.7099 to 0.7119 and invariant 84Sr/86Sr with a mean value of 0.056507 ± 0.000010 (2SD). The latter ratio is accurate and indicates the absence of significant dimer production, with the mean of the interference monitor ratio 82(Kr + CaCa + CaAr)/86Srm = 0.000025 ± 0.000014 (2SD). The accurate 87Sr/86Sr ratio obtained for the accompanying matrix-matched shark tooth analyses proves 40Ca31P16O to be negligible, as expected for low-oxide plasma. This leaves 87Rb as the only significant isobaric interference. The 85Rb/86Sr ratio in the shark (≤0.0001) is more than an order of magnitude lower than that observed in terrestrial animals (Fig. 7c), and the shark tooth cannot serve as an optimal verification of the accuracy of the 87Rb interference correction. Instead, we relied on the reference ratio determined using the Rb–Sr mixed solutions, which demonstrates the reliability of the 85Rb isobar correction well above 85Rb/86Srm of 0.025. Even a slightly higher interference can be accurately corrected.9,11
The cusp to cervix variations along the EDJ in Sr concentration show an 88Sr ion beam from 3.5 to 8.2 V. Smoothing the 87Sr/86Sr data using a moving average with a span of 50 seconds, corresponding to 0.8 mm of ablated enamel, gives an internal precision of 40 to 280 ppm (2RSE), with the vast majority of data (about 90%) falling within the range of 40 to 120 ppm (excluding locally mechanically damaged zones). The precision varies greatly depending on the compositional gradient of the Sr isotopic composition (Fig. 7b–d). Our histomorphometric record, with a close-to-a-week resolution chronology of enamel secretion, suggests that the observed compositional cyclicity reflects 12 annual cycles interpreted as diet changes during the cold and warm seasons. The decreasing ‘wavelength’ of the cycles towards the cervix represents the slowing down of the recruitment rate of new ameloblasts in the secretion front along the EDJ with time. At each stage of enamel formation, the precision obtained is more than satisfactory for resolving temporal variations in 87Sr/86Sr at the sub-seasonal level.
The results obtained using Neoma with the H cone for different dental plates from the same tooth are in excellent agreement with the data obtained using Neptune. The compositional details can be easily followed and correlated with the analyses of both instruments (Fig. 7 and 8). The non-radiogenic 84Sr/86Sr ratio is also accurate and shows no indication of the presence of polyatomic Ca dimer/argide interferences. The internal precision obtained for the time equivalent fragments of dental enamel is comparable for both instruments, ranging from 70 to 110 ppm (2RSE). Measurements of contemporaneously mineralised enamel on the opposite side of the same tooth plate, with the same laser parameters but using Jet-X high-sensitivity cones, are in perfect agreement with the H-type cones (Fig. 8c). Higher sensitivity (4.5 to 8.0 V of 88Sr) led to improved internal precision (50–90 ppm 2RSE). However, the normalised 84Sr/86Sr ratio fluctuates strongly throughout a run, ranging from approximately 0.060 to 0.062 (Fig. 8c and d). An additional correction for Ca dimer/argide isobars gives a stable and accurate 84Sr/86Sr = 0.056472 ± 0.000005 (2SE). A small region at the beginning of the scan deviates from this value because of intense fracturing, which also affects the corresponding 87Sr/86Sr ratio (Fig. 8d). Reducing the spot size to 40 μm reduces the ablated sample volume almost 7 times. The intensity of 88Sr ranged from approximately 1.0 to 2.5 V. Despite the lower internal precision, all compositional details are visible (Fig. 8e and f). The lower precision did not obscure the high spatial resolution advantage. As expected, it revealed more compositional details. This is well illustrated by magnifying the same peak measured with the 3 different settings (Fig. 9a–c). The region with the highest 87Sr/86Sr ratio, visible in the 40 μm spot measurement, is only detected as a sharp peak in the 104 μm spot measurements. Similarly, smaller peaks are much better defined in the regions of less radiogenic Sr when scanned with a smaller ablation spot diameter (Fig. 9c). Despite some loss of precision, the increased spatial resolution is certainly beneficial in terms of obtaining a more detailed reconstruction of the temporal changes in the mineralised enamel. The increased sensitivity provided by the Jet-X cones not only allows the use of a smaller laser beam diameter but also reduces the integration time owing to the improved counting statistics, resulting in improved spatial and temporal resolution of measurements even with a larger laser beam. Reducing the integration region from 50 to 25 seconds, which represents 0.4 mm of ablated enamel, corresponding to approximately 10 days of enamel growth, reveals much more detail, similar to analyses using the smaller laser spot (Fig. 9e). The same reduction of the integration region in the case of the 40 μm beam gives no improvement, which is simply a consequence of the poorer counting statistics. Reducing the integration region even further, to 10 seconds (0.17 mm of mineralised enamel = 4 days), does not bring any improvement, although the major compositional changes can still be depicted and might still be useful for mobility studies. Although the high spatial and temporal resolution obtained cannot generally be interpreted with the same level of detail when reconstructing migratory paths (the regional coverage of Sr isotopic composition is not known in sufficient detail), such high-resolution data provide additional insight into the habits of the animals, especially when combined with other techniques such as dental histology, stable isotope, trace element data and isoscapes. The use of high-sensitivity Jet-X cones will be even more appreciated in the analysis of enamel with low extension (formation) rates and in the analysis of higher trophic level animals, particularly humans, whose dental enamel contains considerably less Sr. The reduced laser beam diameter also allows the separation of different stages of enamel formation and the recovery of the least disturbed in vivo record of bio-apatite mineralisation.
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Fig. 8 Cusp to cervix Sr isotopic composition measurements of woolly mammoth enamel performed using LA-MC-ICPMS Neoma. Measurements were conducted with the H-type cones (a and b) and high-sensitivity Jet-X cones on the opposite side of the same dental plate (c and d). A shorter dental plate was analysed using Jet-X cones with a 40 μm laser beam diameter (e and f). Green rectangles show equivalent regions mineralized at approximately the same time (see also Fig. 9). |
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Fig. 9 Comparison of measurement precision using LA-MC-ICPMS Neoma with H-type (a) and Jet-X cones (b and c) in an enamel region corresponding to approximately 1 year of mineralization. Laser beam diameters of 104 μm (a and b) and 40 μm (c) were applied. Precision is shown for 50, 25, and 10 s integrations corresponding to enamel mineralization times of approximately 20 (a–c), 10 (d–f) and 4 (g–i) days, respectively. The optimal compromise between precision and highest temporal resolution is represented by the analyses with Jet-X cones, 104 μm diameter and 25 seconds integration time (e). The highest spatial resolution was achieved with a 40 μm diameter and 50 second integration time (c). The selected region is marked with a green rectangle in Fig. 8a, c and d. The heavy red line represents the moving average, and the light red lines represent ± 2SE uncertainty. |
We tested different LA-MC-ICPMS configurations to examine seasonal variations in Sr isotopic composition in woolly mammoth enamel, obtaining high-precision measurements with sub-monthly resolution. The use of Jet-X cones enhanced the sensitivity, enabling improved spatial resolution. Reducing the laser spot diameter to 40 μm results in lower precision, yet still sufficiently high, allowing for the detection of finer compositional details that would otherwise be averaged out with a larger beam size. Similarly, shortening the integration time at higher signal intensities resulted in a resolution of <10 days for moderate enamel extension rates. Enhanced instrument sensitivity, leading to higher spatial and temporal resolution, is vital for analysing organisms at higher trophic levels, incorporating much lower amounts of Sr, such as humans, and bio-apatite tissues mineralizing at a slower pace. Another advantage is the improved ability to separate the in vivo record in the earliest formed enamel from the later maturation overprint, which is critical for reconstructing the palaeoecology of large mammals and the life histories of humans.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ja00146c |
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