J.M. Koornneef

Measurement of small ion beams by thermal ionisation mass spectrometry using new 1013 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.

Use of 1012 ohm current amplifiers in Sr and Nd isotope analyses by TIMS for application to sub-nanogram samples

We have investigated the use of current amplifiers equipped with 1012 ohm feedback resistors in thermal ionisation mass spectrometry (TIMS) analyses of sub-nanogram sample aliquots for Nd and Sr isotope ratios. The results of analyses using the 1012 ohm resistors were compared to those obtained with the conventional 1011 ohm amplifiers. We adopted a method where long baselines were measured before and after data acquisition to save time during sample analysis. The noise level on the new design 1012 ohm resistors is 3–5 fold lower compared to the 1011 ohm resistors, which results in a two fold better analytical precision on 143Nd/144Nd and 87Sr/86Sr analyses for beam intensities of 10 mV for the 143Nd and 87Sr masses, respectively. Reproducibility of 87Sr/86Sr on repeat analyses of 1 ng Sr sample loads is better using 1012 ohm resistors compared to 1011 ohm resistors and is a factor of 2 better compared to previous studies. The external reproducibility of 143Nd/144Nd for 1 ng Nd sample loads is a factor of 1.4 better compared to previously published Nd+ results and is similar to those obtained using the more time-consuming NdO+ techniques. Reproducibility tests on 100 pg Nd and Sr samples of reference materials yield 176 ppm of 143Nd/144Nd and 92 ppm of 87Sr/86Sr. These data suggest that we can resolve variability in the fourth decimal place of Nd and Sr isotope ratios in geological samples as small as 100 pg.

Pb isotope analysis of ng size samples by TIMS equipped with a 1013 Ω resistor using a 207Pb–204Pb double spike

The use of the double spike technique to correct for instrumental mass fractionation has yielded high precision results for lead isotope measurements by thermal ionisation mass spectrometry (TIMS), but the applicability to ng size Pb samples is hampered by the small size of the 204Pb ion beam in the natural isotope composition analysis. To overcome this limitation, we successfully demonstrate the application of a 1013 Ω resistor in the Faraday cup amplifier feedback loop used for the collection of 204Pb in combination with a newly produced 207Pb–204Pb double spike to correct for instrumental mass fractionation. The use of a 1013 Ω resistor for the collection of the small 204Pb ion beam leads to a tenfold improvement in the signal-to-noise ratio, but necessitates an external gain correction using a secondary standard and careful monitoring of the ion beam stability. SRM 981 aliquots of 5 ng display unparalleled reproducibility of 90–125 ppm (2 SD) and are in excellent agreement with recommended values: 206Pb/204Pb = 16.9404 ± 0.0016, 207Pb/204Pb = 15.4977 ± 0.0019 and 206Pb/204Pb = 36.7193 ± 0.0042 (2 SD, n = 22). Comparable high quality data have been obtained for 5 ng aliquots of geological reference materials AGV-1 and BCR-1 (e.g.206Pb/204Pb = 18.9399 ± 0.0011 and 18.8208 ± 0.0011 respectively), indicating that analytical blank contribution is negligible for sample sizes down to 5 ng. With further reduction in, or precise correction for, analytical blanks, the combination of the double spike technique with high-resistance amplifiers has the potential to produce highly accurate and precise (<200 ppm) Pb isotope data for samples below the ng level. This methodology will open a range of research possibilities presently impeded by the low amount of Pb available for analysis.

In situ analysis of 230Th–232Th–238U ratios in titanite by fs-LA-MC-ICPMS

The potential of femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometry (fs-LA-MC-ICPMS) for in situ analysis of U–Th disequilibria in titanite was investigated. The aim of the study was to resolve spatial variations in (230Th/238U) ratios (where parentheses denote activity) in titanite from slowly cooled magma bodies. An in-house titanite glass (TG2), determined to be in secular equilibrium by solution mode MC-ICPMS (i.e. (230Th/238U) = 1), was used to correct for U–Th elemental fractionation by sample standard bracketing. The effect of instrument operating conditions on the accuracy and reproducibility of (230Th/238U), (232Th/238U) and (230Th/232Th) ratios was studied by analyses of titanite minerals with known composition and a secondary titanite glass standard. The (230Th/232Th) data were found to be accurate and reproducible, independent of the instrument setting used, suggesting that corrections made for SEM-Faraday gain and abundance sensitivity were appropriate. However, plasma conditions, laser ablation mode, laser energy and wavelength, and titanite material properties were all found to variably influence the U–Th elemental fractionation and compromise the accuracy of the (230Th/238U) data to different extents. Hot plasma conditions significantly reduce the fractionation between U and Th. A drift in elemental fractionation was observed during single spot analyses using NIR laser ablation and results in errors of up to 29% on the (230Th/238U) data. The magnitude of the drift in the elemental fractionation was different for different laser wavelengths and energies. Ablation using the UV single spot mode was significantly less affected by variable elemental fractionation compared to NIR spot analyses, but precision was limited by lower sample uptake. Scanning mode analyses were not compromised by temporal variation of the U–Th intensity ratios but the degree of elemental fractionation was variable between analyses of different materials (e.g. glass versus minerals). This observation suggests material-dependent differences in U–Th fractionation even for near identical titanite compositions. Analyses of the secondary titanite glass standard TG1 bracketed by TG2 yield the most reproducible and accurate (230Th/238U) data, indicating more adequate correction for elemental fractionation when the calibration standard is matched in terms of material composition and structure.

Archaean and Proterozoic diamond growth from contrasting styles of large-scale magmatism

Precise dating of diamond growth is required to understand the interior workings of the early Earth and the deep carbon cycle. Here we report Sm-Nd isotope data from 26 individual garnet inclusions from 26 harzburgitic diamonds from Venetia, South Africa. Garnet inclusions and host diamonds comprise two compositional suites formed under markedly different conditions and define two isochrons, one Archaean (2.95 Ga) and one Proterozoic (1.15 Ga). The Archaean diamond suite formed from relatively cool fluid-dominated metasomatism during rifting of the southern shelf of the Zimbabwe Craton. The 1.8 billion years younger Proterozoic diamond suite formed by melt-dominated metasomatism related to the 1.1 Ga Umkondo Large Igneous Province. The results demonstrate that resolving the time of diamond growth events requires dating of individual inclusions, and that there was a major change in the magmatic processes responsible for harzburgitic diamond formation beneath Venetia from the Archaean to the Proterozoic.Dating of inclusions within diamonds is used to reconstruct Earth’s geodynamic history. Here, the authors report isotope data on individual garnet inclusions within diamonds from Venetia, South Africa, showing that two suites of diamonds define two isochrons, showing the importance of dating individual inclusions.

“Non-invasive” portable laser ablation sampling of art and archaeological materials with subsequent Sr–Nd isotope analysis by TIMS using 1013 Ω amplifiers

A new integrated trace element and multi-isotope provenancing methodology is presented that uses a portable “non-invasive” pulsed laser ablation sampling technique. Samples are collected on location onto Teflon filters for return to a clean laboratory for low blank (pg) geochemical procedures. Ablation pits approximately 60 or 120 μm in width and depth remove μg amounts of material. Following dissolution, trace element ratios are determined by inductively coupled plasma mass spectrometry and combined Sr–Nd isotopes by thermal ionization mass spectrometry. Use of 1013 Ω resistors allows precise analysis of subnanogram amounts of Sr–Nd isotopes, which coupled with the trace element data, provides highly effective multi-variant discrimination for material provenance and authenticity verification. Monitoring of blank contributions is required.

TIMS analysis of neodymium isotopes in human tooth enamel using 1013 Ω amplifiers

Human provenance studies employing isotope analysis are essential in archaeological and forensic sciences but current applications provide limited spatial resolution. This study reports on the potential of neodymium isotope composition (143Nd/144Nd) to improve human provenancing capabilities. Human tissues contain very low ( 100 pg of Nd, by thermal ionization mass spectrometry (TIMS) using 1013 Ω resistors. Neodymium concentrations in enamel from third molars of modern Dutch residents range between 0.1–21.0 ppb (n = 23). The 143Nd/144Nd values for Amsterdam (0.51204–0.51259, n = 12) and Rotterdam (0.51187–0.51239, n = 8) are significantly different (P value = 0.02), demonstrating the potential of neodymium isotope composition to provide improved spatial resolution. Further assessment of Nd composition in enamel of residents from other geological contexts is required to better understand the human provenance capabilities of neodymium.