Trace element analysis by EPMA in geosciences: detection limit, precision and accuracy

Abstract

Abstract. Use of the electron probe microanalyser (EPMA) for trace element analysis has increased over the last decade, mainly because of improved stability of spectrometers and the electron column when operated at high probe current; development of new large-area crystal monochromators and ultra-high count rate spectrometers; full integration of energy-dispersive / wavelength-dispersive X-ray spectrometry (EDS/WDS) signals; and the development of powerful software packages. For phases that are stable under a dense electron beam, the detection limit and precision can be decreased to the ppm level by using high acceleration voltage and beam current combined with long counting time. Data on 10 elements (Na, Al, P, Ca, Ti, Cr, Mn, Co, Ni, Zn) in olivine obtained on a JEOL JXA-8230 microprobe with tungsten filament show that the detection limit decreases proportionally to the square root of counting time and probe current. For all elements equal or heavier than phosphorus (Z = 15), the detection limit decreases with increasing accelerating voltage. The analytical precision for minor and trace elements analysed in olivine at 25 kV accelerating voltage and 900 nA beam current is 4 -18 ppm (2 standard deviations of repeated measurements of the olivine reference sample) and is similar to the detection limit of corresponding elements. To analyse trace elements accurately requires careful estimation of background, and consideration of sample damage under the beam and secondary fluorescence from phase boundaries. The development and use of matrix reference samples with well-characterised trace elements of interest is important for monitoring and improving of the accuracy. An evaluation of the accuracy of trace element analyses in olivine has been made by comparing EPMA data for new reference samples with data obtained by different in-situ and bulk analytical methods in six different laboratories worldwide. For all elements, the measured concentrations in the olivine reference sample were found to be identical (within internal precision) to reference values, suggesting that achieved precision and accuracy are similar. The spatial resolution of EPMA in a silicate matrix, even at very extreme conditions (accelerating voltage 25 kV), does not exceed 7 -8 µm and thus is still better than laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) or secondary ion mass spectrometry (SIMS) of similar precision. These make the electron microprobe an indispensable method with applications in experimental petrology, geochemistry and cosmochemistry. IntroductionThere is no rigorous definition of a trace element, and typically accepted definitions depend on the field of science. For example, in analytical chemistry, a trace element is one whose concentration is less than 100 parts per million (ppm) or less than 100 micrograms per gram; while in geochemistry, a trace element has a concentration less than 1000 ppm or 0.1 wt% of a rock's mass and substitutes for major elements in the rock-forming minerals. Reed [1] defines a...

New Olivine Reference Material for In Situ Microanalysis

“…) and ISTerre internal XEN olivine (Batanova et al . , ) were run as unknowns three times after every batch of 30–40 measurements, in order to monitor potential instrumental drift and to estimate accuracy and precision. Additionally, ten grains were analysed using a 5 × 5 grid with a step from 100 to 300 μm.…”

“…The ZAF correction procedure was applied to correct for matrix compositional effects. San Carlos olivine (USNM111312-44 (SCOL), Jarosewich et al 1980) and ISTerre internal XEN olivine (Batanova et al 2015(Batanova et al , 2018 were run as unknowns three times after every batch of 30-40 measurements, in order to monitor potential instrumental drift and to estimate accuracy and precision. Additionally, ten grains were analysed using a 5 9 5 grid with a step from 100 to 300 lm.…”

Fingerprints of Kamafugite-Like Magmas in Mesozoic Lamproites of the Aldan Shield: Evidence from Olivine and Olivine-Hosted Inclusions

“…Sobolev institute of Geology and Mineralogy SB RAS (IGM SB RAS) (Novosibirsk, Russia) Olivine was analyzed both in polished thin sections and in epoxy or indium mounts with mineral separates. To obtain an extensive chemical dataset, we employed both high-precision electron microprobe methods [17,38] and laser ablation inductive-coupled plasma mass spectrometry (ICP-MS). High-precision electron probe microanalysis (EPMA) was performed on a JEOL JXA-8230 at IGM SB RAS) and ISTerre (Grenoble, France) at accelerating voltage 20 kV, probe current 900 nA and beam diameter 2 µm.…”

Geochemical Investigations of Fe-Si-Mn Oxyhydroxides Deposits in Wocan Hydrothermal Field on the Slow-Spreading Carlsberg Ridge, Indian Ocean: Constraints on Their Types and Origin

“…In-situ geochemical investigations (using Electron Probe Microanalysis (EPMA) are essential tools in geochemical studies [27,28]. The improved methodology on spectrometers and efficient operations of the electron column at high probe current and acceleration voltage have increased the detection limit and precision of EPMA analysis to ppm levels [29]. References [15,30] The relative abundance of Fe-oxyhydroxide grains from~250 to 260 counts (Table 2) shows up tõ 54% and~67% abundance on the Wocan-1 and Wocan-2 samples.…”