Reduction of Spectral Interferences from Na in Laser-Enhanced Ionization Spectrometry by Laser Pre-Ionization

Abstract

Pre-ionization of Na atoms prior to the analysis of other analyte atoms using the Laser-Enhanced Ionization (LEI) technique in flames has been investigated in order to reduce the influence of spectral interferences (in this case of Na) to the measured LEI signal. A large fraction of the Na atoms present in the flame was ionized with the use of one laser system. A short time later (400 µs) another laser system was aimed at a position 4 mm above the first laser in the flame, thus probing the part of the flame in which the concentration of (neutral) Na atoms had been significantly reduced. Different excitation schemes were used for the preionization process. It was found that the degree of depletion of Na was 13% when the atoms were first excited to the 3 p state by 589-nm light and then photoionized by focused pump laser light (308 nm). When the atoms were excited in two resonant steps (3 s-3 p-7 d), 80% of all the Na atoms could be depleted. With all three lasers illuminating the atoms, finally, 83% of the Na atoms were depleted. (If laser-enhanced chemical processes can be neglected within the duration of the laser pulse, which is most probable in the case of ns pulse duration, the degree of depletion is equivalent to the degree of ionization.) The second laser system was then used for studies of the LEI signal from other elements in the same manner as above. It was then found that this Na pre-ionization did not interfere in any way with the detection of other elements which were detected 400 μs later (4 mm higher in the flame). This was illustrated by a measurement of Mg in a Na matrix in which the Mg signal recovery was 100% while the Na signal was decreased by a factor of 5 (resulting in a corresponding increase in selectivity).