Effects of sample surface morphology on laser-induced breakdown spectroscopy

Abstract

Laser-induced breakdown spectroscopy (LIBS) quantitative measurement is hindered by the variability of collected spectra, and the matrix effect of samples is the main reason. The surface geometric morphology of the sample causes spectral variation and can greatly affect the measurement results. To obtain the best quantitative results, it is necessary to consider the effect of the surface geometric morphology of the sample on LIBS measurement. The surface morphology of the sample can be characterized by surface relief and surface roughness and we explore their influence on LIBS. The plasma images recorded with an ICCD show that the plasma morphology is far away and close to the collecting direction when surface relief parameter θ changes from 0° to 20° and from 0° to −20°. The volume of the lower part of plasma decreases with the increase of surface roughness. The characteristic parameters of LIBS spectra including plasma temperature, line intensity, line integral area, and line intensity ratio have monotonic trends with the surface morphology parameter of the sample. When parameter θ changes from −20° to 20° and surface roughness R_a is in the 0.1–0.448 μm range, the data's variation amplitude of line intensity reaches 74% and 48% respectively. We investigate the relationship between θ and the spectral characteristic parameters and find they follow the spatial distribution model of plasma: a cos⁴ θ + b. The relationship between R_a and spectral parameters is nearly linear. The line intensity ratio can reduce partly the effect of the surface morphology, but this is not enough to improve the accuracy of measurement.