Calibration of laser-ablation ICP-MS. Can we use synthetic standards with pneumatic nebulization?

Abstract

An approach for the determination of trace element concentrations in high purity metals, using an inductively coupled plasma mass spectrometer (ICP-MS) with a laser-ablation system for direct solid sample introduction after calibration with nebulized liquid standards was made. Due to the inherent differences in the rate of sample introduction with laser-ablation and pneumatic nebulization, a matrix element must be used as an internal standard. This is problematical for elements that have no isotope with a relative abundance of less than 0.1 %, since the ion signals would be too high for direct measurement, and reduction of the ablation rate would compromise the sensitivity for trace elements. Due to the high stability of ICP-unit and mass filter of the instrument used, it was found that the tail of a mass-peak of the matrix element could be used as an internal standard. Therefore, a position at –0.5 amu from the matrix-isotope (e.g. 62.5Cu in copper samples) was used for internal standardization. The standard deviation of this signal in a period of 2.5 h was 3.6% RSD with no notable drift when the laser ablation was used for sample introduction. The calibration of the matrix-element by nebulizing liquid standards showed that the ion signal measured on the peak-tail is directly proportional to the element concentration in the ICP. This indicates that the peak shape is not only stable, but also independent of the peak height. The advantages of this method lie in the easy preparation of calibration standards for quantitative measurements with a laser-ablation system and access to homogeneous standards for materials, that are difficult to homogenize in the solid state. The calibration of the traces is performed relatively to a fixed concentration of the matrix element. Calibrations were carried out for trace concentrations in high purity copper and good recoveries were obtained for high-purity reference standards.