Development of the Sliding Spark Technique for Trace Analytical Spectroscopy of Dielectric Solid Matrices

Abstract

A novel direct solid-state emission spectroanalytical method based on the pulsed sliding spark source has been developed. The technique is characterised by a radiative transient discharge plasma propagating along the surface of a dielectric solid matrix enforced between a pair of electrodes in air at atmospheric pressure, where matrix excitation is driven by plasma-particle interaction based on geometry- and source opto-electric-modulated electron impact excitation. This dissertation embodies the results of the systematic theoretical and experimental study of the sliding spark which aimed at the analytical development of the technique for trace quantitative spectroscopy of the heavy elements embedded in the dielectric matrix surface layers. The optical emission spectrum, when detected and measured in the  range 212–511 nm at 0.05 nm spectral resolution using optical fibre transmission in conjunction with a holographic blazed grating CCD spectrometer, was found to be suitable for simultaneous multi-elemental analysis. Investigation of the spectral characteristics of several heavy metals embedded in a variety of dielectric matrices (a boro-silicate simulate, borax, polyvinyl-alcohol, cellulose, teflon, Al-Zr simulate ceramic powder, simulate and River sediment samples) has led to the realisation of an empirical scheme for optimal identification and selection of the optically thin lines that are suitable for trace quantitative analysis. The utility of the lines depends on the element and sample matrix. Use of PVC as a matrix modifier results in increased sensitivity through formation of volatile halides. Comprehensive characterisation of the measured spectra provides empirical evidence to exploit the sliding spark at fast pulse frequency also as an atom source for combined emission, fluorescence, and absorption spectroscopy applicable to the elucidation of structural and molecular information by temporal gating, time-resolved techniques. Practical approaches found in the search for an appropriate calibration strategy for quantitative analysis include the use of internal standards based on Y and La (added) and Si and C (matrix-derived) spectral lines, which compensate for the differing ablation yield, signal drifts and matrix effects in and between complex matrices. Accurate analytical models have been derived for Mn, Ti, V, Ni, Co, Cu, Cd, Pb, Cr, Al, Fe, Zn, and Hg. A quantification methodology has been developed based on sediment and as model matrix, which combines high sensitivity and satisfactory reproducibility for Mn, Ti, V, Ni, Co, Cu, Cr, Al, Fe, and Zn. Trace quantitative analysis of dielectric solid matrices by sliding spark spectroscopy is realised in the concentration range from several hundred ppb to thousands of ppm depending on the analyte (and the spectral line utilized) and on the calibration strategy adopted for the quantification. The elemental limits of detection vary from several hundred ppb to few tens of ppm depending on the element, analysed matrix, spectral line, and calibration method. Qualitative speciation analysis is possible for Mn, V, Pb, Ti, Cu, and Co. Sliding spark spectroscopy has been validated as a new, simple but robust and versatile technique for the direct trace analysis of complex solid dielectric and refractory matrices with a reproducibility at 12 %, a precision characterised by a confidence interval of (0.5–10) %, and an accuracy by relative efficiency of  0–10 % by the successful analyses of Certified Reference Materials (Stream sediments), sewage sludge, a PVC polymer, an independent XRF laboratory analysis of River Rhine sediment, and by the results of comparative analyses made of the same elements using ICP and XRF (polarised radiation, 3-D Cartesian geometry) techniques.