Multiple emission line analysis for improved isotopic determination of uranium — a computer simulation study

Forty-three atomic emission lines for 235U and 238U were compiled for computer simulation of isotopic analysis using laser induced breakdown spectroscopy (LIBS). The spectral line profile was assumed to be Lorentzian in shape and the magnitude of three common types of noises (detector-read, photon-shot and flicker) were experimentally determined and incorporated into the simulation. Precision and root mean square error of prediction (RMSEP) for isotopic analysis of a single U line were simulated, and it was found that analytical performance (precision) primarily depended on the signal-to-background ratio (SBR) and net intensity of the emission line, rather than on the magnitude of isotopic splitting (IS), when partial least squares (PLS) was used for calibration. This is because PLS multivariate calibration can be performed correctly even when the spectra are only partially resolved, which in turn relaxes the requirement on having IS larger than the spectral resolution. The analytical performance was found to improve with multiple-line analysis. Depending on the criteria (e.g., SBR, net intensity, magnitude of IS, or best single-line performance) used in sorting the spectral lines into the multiline pool, improvement factors ranging from 2 × to 9 × were obtained. The absolute uncertainty of isotopic analysis is practically constant and independent of isotopic abundance, which makes experimental estimation of the detection limit in isotopic analysis straightforward because one can experimentally measure this uncertainty with one arbitrary and conveniently chosen isotopic standard and then estimate the detection limit through simple extrapolation.