New methodical and instrumental developments in laser ablation inductively coupled plasma mass spectrometry

摘要

Many tasks in bulk analysis, micro analysis and depth profile analysis can be solved advantageously by laser ablation inductively coupled plasma mass spectrometry (Laser ICP-MS) in particular, when both the chemical and elemental distributions in the sample are to be determined. However, the analyst has to take into account that the analytical precision and accuracy of the Laser ICP-MS is influenced decisively by signal standardization, the homogeneity of the samples as well as calibration standards and the mass-spectrometric measuring mode, which is usually sequential when performed with scanning mass spectrometers such as quadrupol-or sector-based instruments. Using the ablated mass as standard, an excellent level of the analytical precision and accuracy (relative standard deviation R.S.D. < 0.5%) has been obtained for homogeneous sample materials such as alloys. For inhomogeneous samples, such as pressed pellets, a statistical test is described, which is based upon the auto-correlation function to characterize the sample inhomogeneity. The application of the test allows us to calculate the representative mass for the quantitative analysis at previously defined analytical precision. In the instrumental part of the paper a new type of an ICP-time-of-flight (TOF) mass spectrometer-is described, constructed and built up in our laboratory. For fast signal counting an application-specific integrated circuit (ASIC) was developed, which permits a time resolution of 1 ns. The analytical performance of the TOF when used in combination with an ICP is demosntrated in terms of resolution, ion extraction rate, detection limits and dynamic range. The determination of ~(39)K~+ and ~(40)Ca~+ at trace level can be realized in a cool plasma condition (high central gas fow) only with a small interference by ~(40)Ar~+. Detection limits of 23 elements were measured with typical values in the lower nanograms per liter range. The ion extraction rates, measured for a sample mass of 1 ng in terms of counts per second divided by the relative isotope abundance, are one order of magnitude higher than those obtained with a quadrupol-based instrument.