Abstract

Laser-Enhanced Ionization (LEI) signals from Na have been investigated by the one-step excitation scheme in flames in the UV region (220-410 nm). This knowledge is of importance for the applicability of the LEI technique since Na often exists as a matrix in a number of samples and gives rise to background signals. In the wavelength region between 220 and 240 nm, photoionization of ground-state atoms is responsible for the LEI signal. Between 240 and approximately 300 nm, the LEI signal originates mainly from wing-excitations of the ordinary 3<i>s</i>-<i>np</i> (<i>n</i> ≥ 5) transitions. Finally, in the wavelength region above the 3<i>s</i>-4<i>p</i> transition (i.e., approximately 360-410 nm), the LEI signal consists mainly of photoionization of the thermally excited 3<i>p</i> state. A theoretical treatment of the various contributions shows good agreement with experiments. It is furthermore found that the LEI sensitivity at 408 nm (the threshold wavelength for photoionization of atoms in the first excited state, the 3<i>p</i> state) is 50 times smaller than the LEI sensitivity at 240 nm (the threshold wavelength for photoionization of ground-state atoms). The selectivity between analyte atoms (Bi and Pb in the present study) and Na is also studied. It is shown that the analyte-versus-Na selectivity decreases considerably as a function of increased laser intensity. It is concluded that there exists an optimum laser intensity for the most efficient detection of a particular element in a certain Na matrix. This optimum intensity, however, is determined by factors which are specific to each experimental setup.

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