TRLFS Studies on luminescence enhancement of U(VI) using oxidants for quencher species in samples

The pulse laser-based method detects photo luminescent emission of U(VI) so that it is highly sensitive for non-isotopic al determination of total uranium concentration. Thus, this method has been used for detection of trace quantity of uranium in the environmental, geological, and bioassay samples. One of widely-used pulse laser-based methods is kinetic phosphorescence analysis (KPA), of which instrument is commercially available. The intensity and lifetime of characteristic phosphorescence at 515-520 nm of hexavalent uranium are measured with an excitation wavelength of 425 nm. Particularly in KPA the use of phosphate-based luminescence enhancing agent (LEA) leading to the formation of uranyl-phosphate complexes extends the luminescence (LM) lifetime of uranium (> ∼ 200 μs) and subsequently the overall luminescence intensity. In KPA, however, an extensive sample pretreatment procedure is required to reduce the luminescence quenching effects of ions and molecules present in samples. During such procedures the uranium species in low oxidation states are also oxidized to hexavalent uranium so that the measurement of the total uranium concentration can be achieved. In general, a series of high temperature dry and wet ashing procedures is implemented prior to the addition of LEA to decompose the interfering substances. The aim of this study is to examine the characteristics of the interfering species exhibiting significant quenching effects and to develop a way of minimizing the time required for the sample pretreatment step particularly for certain oxidizable quencher species. In fact, in a previous study we reported that significant LM quenching effects are observed from those possessing chemical reduction capability such as Fe(II) and cysteine. Under such sample conditions it is shown that the conventional KPA is not applicable due to the short lifetime (< ∼ 1 μs), therefore a time-resolved laser-induced fluorescence spectroscopy (TRLFS) capable of monitoring shorter LM signal should be applied. In this study we examined oxidation reactions of selected strong quenchers including Fe(II), thiols, ascorbate and iodide with various oxidizing agent such as hydrogen peroxide, persulfate, mono persulfate and others. It is found that most of oxidizing agents tested are effective for oxidation of Fe(II) and U(IV) and some for thiols, thus enhancing the LM intensity of U(VI) after oxidation reactions. More importantly, for a few oxidants the oxidation reaction time required for full oxidation of millimolar levels of oxidizable quenchers is shorter than a few hours at room temperature. This implies that such simple room-temperature oxidation method may replace the time-consuming ashing procedure for samples containing oxidizable quenchers. The characteristics of oxidation reactions between the selected oxidants and quenchers were examined in detail including reaction kinetics and temperature effects