Laser-induced fluorescence: Limits to the remote detection of hydrogen ion, aluminum, and dissolved organic matter
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Laser-induced fluorescence (LIF) of dissolved organic matter (DOM) in lake water is, to a certain extent, indicative of water quality. DOM fluorescence intensity varies directly with connentration of the fluorescing organic matter; however, the intensity and spectral distribution of fluorescence are also affected by other physical and chemical factors. The chemical factors include concentrations of trace metals, hydrogen ion, and the composition of the DOM itself. An empirical study was conducted seeking specific relationships between the intensity and spectral distribution of DOM fluorescence and the concentration of aluminum and hydrogen ion in lake water. Water samples from 49 lakes in northern Michigan and northern Wisconsin were collected and their fluorescence emission spectra analyzed with respect to the lake water chemistry. Trends that had been apparent in previous measurements using prepared samples were less visible in the lake water data. Observed changes in the fluorescence emission can be produced by one of two general mechanisms: quenching and precipitation. Quenching of DOM fluorescence - a result of cation reactions - is seen as a reduction of the total fluorescence intensity and a shift of the overall emission spectrum to shorter wavelengths. In contrast, preferential precipitation of the high molecular weight DOM by cation reactions appears to shift DOM fluorescence to the longer wavelengths. These nonspecific effects make it difficult, if not impossible, to predict the complicated relationships between fluorescence emission and individual water chemistry parameters. By itself, a simple fluorescence emission spectrum of DOM is not a sufficiently accurate indicator of hydrogen ion or aluminum concentration; however, fluorescence parameters correlate well with dissolved organic carbon (DOC) And with the organic aluminum per unit dissolved organic carbon (Al0 / DOC). DOM fluorescence emission spectra should also be useful as a survey tool in conjunction with limited water chemistry or as a means of monitoring change in a lake whose initial chemistry and fluorescence properties have been defined. Lastly, by augmenting fluorescence emission data with other remote sensing data, e.g., fluorescence excitation spectra, fluorescence decay rates, and Raman spectra, a more detailed chemical analysis should be possible.