Remote Laser-Induced Fluorescence Monitoring of Groundwater Contaminants: Prototype Field Instrument

Abstract

Direct, in situ monitoring of many groundwater contaminants is possible using the Remote Laser-Induced Fluorescence (RLIF) technique.We have designed, laboratory-tested, and field-tested a prototype mobile instrument capable of performing such monitoring. It consists of a portable, computer-controlled fluorimeter with a frequency-quadrupled Nd:YAG laser as a UV excitation source, detachable fiber optic sensor, and a detection module employing two photomultiplier tubes and a dualchannel boxcar averager for signal recovery. In this report, we give a complete description of the hardware and software utilized in the instrument, along with a summary of our field testing procedures and experiences, and an assessment of the technique's current and potential capabilities. 423 Copyright© 1987 by Marcel Dekker, Inc. Downloaded by [McMaster University] at 00:02 16 June 2016 424 KENNY ET AL.

Laser Spectroscopy: Assessment of Research Needs for Laser Technologies Applied to Advanced Spectroscopic Methods

“…Remote monitoring of groundwater contaminants can be$uccessfully tackled with fiber optic-based probes and fluorescence detection, especially when coupled with time resolution (Kenny and Jarvis 1987;Vodacek and Philpot 1987). In this case, naturally fluorescent contaminants can be detected directly; in situ, specific fluorescent-basedoptrodes can be developed; or, finally, a fluorescent tracer can be injected in the water and the motion of the fluorescent plume followed.…”

Evaluation of chemical sensors for in situ ground-water monitoring at the Hanford Site

“…(1 985a) achieved detection limits near or below 1 ppb at a distance of 25 m working with both simulated and actual contaminated ground-water samples. The contaminants and the limits of detection are listed in computer-controlled fluorimeter with a frequency-quadrupled Nd:YAG laser as an ultraviolet (UV) excitation source, a detachable fiber optic sensor, and a detection module containing two photomultiplier tubes and a dual-channel boxcar averager for signal recovery (Kenny et al 1989). This device is illustrated in Figure 4.4.…”

“…A major obstacle to using sophisticated instrumentation is the fact that ambient field conditions are less stable than those n the laboratory. Kenny et al (1989) have have not yet achieved very good conversion of green to UV (0.1 Yo; 10% or better is routinely achievable with laser systems). Although they foresee no problems in achieving the 1-2 mJ per pulse of UV output specified by the laser manufacturer, with the current energies of about 10 pJ, they have had to use shorter fiber lengths (10 m) and higher detection limits (sub-ppm) than in their earlier laboratory work.…”

“…The limit of detection was less than 1 ppb for gasoline and less than 10 ppb for xylenes. In Figure 4.7, a plot of boxcar averager corrected signal versus concentration of phenol is shown (Kenny et al 1989). This phenol curve was generated in the laboratory with the prototype field instrument at an instrumentlanalyte distance of 10 m. The lower curve is the baseline signal; the upper curve is raw data from the fluorescence detector.…”

Contaminant Detection, Identification, and Quantification Using a Microchip Laser Fluorescence Sensor

“…Thus, ultraviolet ͑UV͒ radiation necessary to excite fluorescence in environmental pollutants such as gasoline is generated at the point of contamination, while the infrared diode pump laser remains above ground. This configuration takes advantage of the excellent transmission of infrared energy through fiber optic cable ͑attenuationϽ6 dB/km͒, and minimizes the ultraviolet attenuation ͑ϳ20 dB/ 30 m of fiber͒ and nonlinear effects that characterize conventional fiber-optic-based LIF sys-tems ͑e.g., Pepper et al 2002Pepper et al , 1998Marowsky et al 1998;Gillispie et al 1993;Knowles and Lieberman 1995;Kenny et al 1987͒.…”