FLUORESCENCE OF COMMON pH BUFFERS: A POTENTIAL SOURCE OF MISINTERPRETATION OF GROUNDWATER DYE TRACING RESULTS

Buffers for calibration of pH meters are commonly used in water quality investigations, and are typically dyed for easy identification. Fluorospectroscopy analyses show that some pH buffers share fluorescent characteristics with common groundwater tracing dyes, creating a potential source of misinterpretation of the results. Because the dyes can be detected in minute concentrations (often 0.01μg/L), a false positive test is a serious potential problem. In general, fluorescent substances are not kept in laboratories where high precision fluorescence measurements are made.

Dyed Oakton® brand pH buffer solutions were quantitatively evaluated for fluorescence in emission wavelengths of typical dyes used in groundwater tracing investigations. All buffers fluoresced. The blue pH 10 had no fluorescence in a range close to any of the dyes considered. The pH 4 (red) buffer emission spectra had a peak emission wavelength within 6.7 nm of that for sulphorhodamine B, and that for the pH 7 (green) buffer was within 4.0 nm of the peak for fluorescein dyes.

While the resulting concentrations were low enough that accidental contamination are unlikely to result in false positives, they are close enough that some ambiguity could be produced. These buffers thus should be carefully kept away from samples and the instrument, in both the field and lab.