Elimination of Spectral Interference Using Two-Step Excitation Laser Enhanced Ionization

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Abstract
Spectral interferences from matrix elements in solutions are investigated utilizing Laser Enhanced Ionization (LEI) in flames. Mechanisms which influence the analytical selectivity are discussed and it is shown that the selectivity can be significantly increased by a proper choise of experimental parameters. This is exemplified by determination of low concentrations of Mg in a matrix of Na. The spectral interference, occurring in one-step excitation LEI, is diminished by a factor of 70 when utilizing two-step excitation LEI. The spectral resolution is further improved to a total factor of 1300 by an optimized choice of laser light intensities without any significant loss in sensitivity. It is pointed out that an efficient correction for signal contributions from interfering substances can be made in two-step excitation LEI without scanning the laser wavelengths. Background signals emanating from NO molecules inherent to the flame gases are also detected and virtually eliminated. The very high sensitivity and selectivity achieved by two-step excitation LEI shows that the method is widely applicable for trace element analysis.