In situ element analysis of spodumenes by fs-LA-ICPMS with non-matrix-matched calibration: Signal beat and accuracy

Abstract

Abstract In this work, in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analysis with non-matrix-matched calibration is proposed for determining the composition of spodumene samples. Accuracy of the quantification in dependence on instrument operating conditions and the signal beat effect on quantification accuracy have been investigated in detail. Using NIST SRM 610 as reference material for external calibration, the mass fractions of major, minor and trace elements (Li, Si, Al, Na, Mg, Mn, Fe, Ga, and others) were determined using femtosecond-LA-ICPMS and compared with nanosecond-LA-ICPMS. Helium (He) was employed as the carrier gas in the ablation cell, and the aerosols produced under hole drilling ablation condition are then transported to a quadrupole ICPMS by a mixture of He and Ar as the make-up gas. Results show that the carrier gas flow rate had a substantial effect on both transient signal structure and LA-ICPMS quantification. With different He flow rates, both synchronized and non-synchronized acquisition with and without aliasing in the transient signals recorded were compared. Synchronized acquisition settings could attenuate the oscillations in the transient signals substantially but showed large variability (up to 18% RSD for Li2O) among individual acquisitions. Non-synchronized acquisition, with and without visible aliasing, on the other hand, was producing reproducible results without systematic biases when averaging ion signals from a sufficient number of scans (>100) for quantification. For the latter it was found that non-aliasing methods allowed for a direct identification of heterogeneities or inclusions in the transient signals and these were thus preferred for analysis. The accuracy of the analyses with non-matrix matched calibration was evaluated by comparing the matrix Al2O3/SiO2 mass fraction ratios with those obtained by electron probe microanalysis (EPMA). It was found that operation with only 0.2\xa0L/min of He achieved the best agreement, while higher gas flow rates caused an increase in Al2O3/SiO2 and Li2O/SiO2 mass fraction ratios. This is considered to be a result of differences in ionization efficiency within the ICP caused by the higher abundance of easily ionized elements in the spodumenes compared to the calibration standard. By selecting appropriate acquisition settings for sequential MS detection and robust operating conditions, the repeatability of the analyses could be improved to values less than 1% for the Li2O mass fractions determined. When using the proposed settings for the analysis of 11 spodumene samples of different origin, the Al2O3/SiO2 mass fraction ratios agreed within −2% to +4% to those obtained by EPMA. These results confirm that spodumenes can be accurately analyzed in situ by LA-ICPMS without matrix-matched calibration standards. While matrix elements of the studied spodumenes were all in a comparable range, minor and trace elements showed substantial intra- and inter-sample variabilities.