UNEXPECTED CHARACTERISTICS OF RHODAMINE WT AS A GROUNDWATER TRACER

Fluorescein, eosine, and rhodamine WT (and other rhodamine dyes) are xanthene based. They all fluoresce with a similar stokes shift, allowing for simultaneous examination of environmental samples with a synchronously scanning spectrofluorometer. Different functional groups on the base structure for these dyes control the wavelength of fluorescence. Most rhodamine dyes have two alkylated-amine groups at different locations on the base structure. Their location and that of other functional groups control the fluorescence wavelength of the dye. Removal of the alkylated-amine functional groups is referred to as “deaminoalkylation” and will shift the fluorescence to lower wavelengths. Deaminoalkylation of rhodaine WT (DAR WT) appears to have two possible degradation products with different fluorescence wavelengths. One of these can interfere with the analysis or be mistaken for fluorescein, and the other has similar consequences for eosine. The formation of DAR WT has been observed to form in as short a time as one month in groundwater tracing studies, but does not occur in every use of rhodamine WT. DAR WT has not been observed in laboratory standards. In some cases the rhodamine WT and the DAR WT have been recovered at a sample point simultaneously and in other cases only the DAR WT has been observed. DAR WT has been recoverable at least 12 years after the original injection of the parent dye and appears to be very stable after it has formed. The presence of DAR WT in a sample that is being analyzed with a filter fluorometer could easily give a false positive for fluorescein due to its similar fluorescence peak wavelength. Misidentification of DAR WT may have been common with early spectrofluorometers. Reexamination of past groundwater tracing results with current equipment and an understanding of the existence of DAR WT has resulted in recovery of lost rhodamine WT traces and corrected apparently successful traces improperly ascribed to fluorescein. Similar changes in dye characteristics have been observed by groundwater tracing practitioners in Europe for rhodamine B and likely occurs for the other rhodamine dyes.