In Situ Analysis of Trace Elements in Quartz Using Laser Ablation Inductively Coupled Plasma Mass Spectrometry

Abstract

In situ micro analysis of ultra trace element composition of quartz using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) allows rapid screening of lattice bound impurities of potential high-purity quartz resources and samples for petrological research without the need to remove solid and liquid inclusions by expensive dressing techniques prior to chemical analysis. Information of the analysed trace element content can not only be used to determine the economic quality of quartz but also the conditions of quartz crystallisation and the origin of the quartz-forming fluids and melts. The main purpose of this paper is to describe an efficient and precise analytical method for estimating the concentrations of lattice-bound trace elements in quartz. The best choice of instrument is considered to be a double focusing sector field inductively coupled plasma mass spectrometry ICP-MS that provides high sensitivity and a mass resolution high enough to separate K from its interferences. The ICP-MS should be coupled to a 193 nm excimer laser, a femto second laser or a similar ablation system. The following elements are included in the standard analytical protocol applied at the Geological Survey of Norway (NGU): Al, B, Be, Ca, Cr, Fe, Ga, Ge, K, Li, Mg, Mn, Na, P, Rb, Sb, Sr, Ti and Zn. Any element with isotopes that can be ionised in an Ar plasma can easily be included if suitable reference materials are available. External calibration was done using the international reference materials NIST SRM 610, 612, 614, 616 and 1830 from the National Institute of Standards and Technology (NIST), BCS 313/1 from the Bureau of Analysed Samples (BAS) and the certified reference material “pure substance No. 1” silicon dioxide SiO₂ from the Federal Institute for Material Research and Testing, Berlin, Germany (BAM). To improve the lower limit of quantification and analytical uncertainty at low concentrations, it is important to have calibration curves with well defined intercepts. This can be achieved by the use of certified standards, with trace element concentrations lower than the quantification limit. Even better is to use a standard where the analyte is not present, however in laserablation matrixmatcing is usually important and a standard blank is usually not avilable. In this work, BAM no. 1 SiO₂ is the reference material used whith the lowest consentration of the analytes. Because of the absence of a SiO₂ blank, the BAM no. 1 SiO₂ is used for the estimation of detection limits. Detection limits for most of the elements are between 1 and 0.02 μg g⁻¹. Analysis time and laser spot size are adjusted to the size of the quartz crystal and the thickness (~300 μm) of the polished thick section. The size of the ablation raster is commonly 100 × 350 μm with a depth of 10–20 μm applying the analysis time of less than 1 min.