Determination of Ultratrace-Level Fluorescent Tracer Concentrations in Environmental Samples Using a Combination of HPLC Separation and Laser-Excited Fluorescence Multiwavelength Emission Detection: Application to Testing of Geothermal Well Brines

Lowering the limits of detection for fluorescent tracers in environmental samples can reduce the cost and environmental impact of tracer testing and allow a wider variety of tracer dyes to be used. Detecting dyes in environmental water samples at the sub-part-pertrillion level raises significant challenges in the sensitivity and selectivity of measurement. In the present work, we address this issue by developing a high-sensitivity multiwavelength fluorescence detector for high-performance liquid chromatography (HPLC)-separated samples. The fluorescence flow-cell utilizes fiber-optic coupling of laser excitation and the collected emission, which is dispersed in a short spectrograph and detected with a cooled charge-coupled device (CCD). The HPLC separation step not only resolves the target tracer from fluorescent impurities in the sample but also transfers the dye molecules into a solution of reproducible composition which provides a constant Raman scattering background against which the tracer fluorescence spectrum may be detected. The combination of emission-wavelength and elution-time measurement provides a multidimensional data set that improves selectivity for detecting a tracer. For analysis of the data, partial least-squares modeling is tested, along with two methods that do not require prior knowledge of interfering species: rank annihilation and self-modeling curve resolution. The method is developed and tested on fluorescein standards, and a detection limit for fluorescein of 40 ppq (4.0 × 10 −14 g/mL) is predicted. This capability was tested on geothermal well samples, in which ∼40 ppq fluorescein could be detected in the presence of 15-fold greater fluorescence from an unknown interferent.