Determination of fluorine in urine and tap water by laser-excited molecular fluorescence spectrometry in a graphite tube furnace with front-surface illumination

Abstract

Fluorine was determined in urine and tap water using laser-excited molecular fluorescence spectrometry (LEMOFS) with an unmodified atomic absorption graphite tube furnace. Molecular fluorescence from magnesium fluoride (excited at 268.94 nm, detected at 358.82 nm) was collected using front-surface illumination and detection. A frequency doubled excimer pumped dye laser operating at 500 Hz was used for excitation. This is the first report of the use of front-surface illumination for molecular fluorescence. After optimization of the chemical, furnace and laser conditions, the detection limit of 0.3 pg of fluorine (as fluoride) was from two to six orders of magnitude better than other methods commonly used for the determination of fluorine, and two orders of magnitude more sensitive than the best previously reported LEMOFS detection limit. The linear dynamic range of the technique was five orders of magnitude. Significant interferences from other ions (Na⁺, H⁺, Cl^–, Br^–) were observed. The sensitivity for the determination of fluorine in a freeze-dried urine standard reference material, and in tap water, by LEMOFS was sufficiently high to allow the samples of be diluted by a factor of 100 to remove the interferences. Good agreement with certified values was obtained, with an analytical precision of 7–11%.