Fiber-Optic Quantum Counter Probe for Incident Excitation Energy Correction in Fluorescence Measurements

Abstract

Excitation-power correction of fluorescence spectra is essential for meaningful interpretation of analytical data. In particular, fluorescence measurements obtained using optical fibers require special consideration due to the possibility of variations in the light intensity delivered to the sample or the incident excitation energy. A quantum counting fiber-optic probe has been developed to simultaneously and independently collect fluorescence spectra of both analytes and a quantum counter. The probe consists of excitation and emission fibers angled to maximize distal cone overlap and a third fiber positioned at a greater angle to allow excitation of rhodamine B, which is affixed to its distal end with a polymer. The fluorescence of the quantum counter affixed to the fiber was linear with incident 266 nm excitation energy over the measured range 1.2 × 1014 photons per exposure to 1.2 × 1015 photons per exposure at 266 nm. The corrected fluorescence signals were immune to adverse condition experiments such as laser power reduction and launch fiber–laser beam misalignment, while the uncorrected signals suffered greatly. The concentration calibration generated using the fiberoptic quantum counter method showed an improvement in the correlation coefficient, r2 = 0.950, compared with the calibration generated using uncorrected data, r2 = 0.906. Different types of adverse conditions were tested including power attenuation, power increase, and fiber launch misalignment; the prediction errors were between −87% and 366% for the uncorrected data compared with prediction errors between −9% and 15% when using the fiber-optic quantum counter method.