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

Abstract

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.