A Low Cost Time-Resolved Raman Spectroscopic Sensing System Enabling Fluorescence Rejection

This paper describes a novel, compact, fiber-coupled, time-resolved Raman spectroscopy system that takes advantage of recent developments in diode laser and data acquisition technology to exploit the natural temporal separation between Raman and fluorescence phenomena and thereby limits the influence of fluorescence on Raman observations. The unit has been designed to be particularly low cost and is intended to provide the foundation for a wide range of in-line or fieldable sensing devices that can enhance the potential and affordability of in situ chemical analyses. The system operating principles, design, and performance are discussed along with its advantages and tradeoffs relative to traditional continuous wave (CW) Raman techniques. The system relies on a 6.4 kHz repetition rate 900 ps pulsed diode laser operating in the visible wavelength range (532 nm) to enhance the quality or Raman observations relative to CW and infrared systems, particularly for analytes examined in the presence of fluorophores. Time-resolved photon counting, achieved through a combination of off-the-shelf and custom hardware and software, limits the influence of fluorescence on Raman observations under pulsed excitation. The paper presents examples of the quality of Raman signatures that can be obtained with the system for a variety of compounds such as trichloroethylene, benzene, an aqueous nitrate solution, and olive oil. Further, the paper demonstrates an approximately 15-fold improvement in signal-to-noise ratio when comparing long- and short-gated time-resolved photon counting acquisition scenarios for a neat benzene sample doped with rhodamine 6G at a concentration of 1 x 10(-4) M. The system's versatility and effectiveness in the assessment of complex mixtures representative of industrial or field settings is demonstrated through analysis of a gasoline sample. Additional discussion outlines how efficient signal averaging over extended observation periods can enable low concentration chemical analyses, particularly relevant in field settings.