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Laser-Enhanced Ionization Spectroscopy for Ultratrace Mercury DetectionEPA Grant Number: U914941
Title: Laser-Enhanced Ionization Spectroscopy for Ultratrace Mercury Detection
Investigators: Clevenger, Wendy L.
Institution: University of Florida
EPA Project Officer: Jones, Brandon
Project Period: January 1, 1996 through February 12, 1998
Project Amount: $102,000
RFA: STAR Graduate Fellowships (1996) RFA Text | Recipients Lists
Research Category: Academic Fellowships , Engineering and Environmental Chemistry , Fellowship - Analytical Chemistry
The objective of this research project is to determine and use an analytical technique that is capable of detecting trace amounts of mercury, as mercury contamination remains an important environmental problem.
In this work, a novel type of mercury resonance ionization detector is described, which involves the optical emission detection of laser-enhanced ionization in a buffer gas. It is based on the measurement of the emission of excited buffer gas atoms, which are created by interactions with electrons in a strong electric field.
The first part of this research project will involve observing the rates of ionization for different conditions. The experimental setup is as follows: three dye lasers (1 and 2, Model Scanmate 1, Lambda Physik, Acton, MA; 3, DL II, Molectron Corporation, Santa Clara, CA) are pumped by an excimer laser (Model LPX-240i, Lambda Physik, Acton, MA) operated with XeC1 ( = 308 nm). The first dye laser is tuned to 253.7 nm, the second to 435.8 nm, and the third is tunable to more than 450-584 nm. The laser beams are directed into the quartz cell, which is saturated (at room temperature) with mercury vapor mixed with varying pressures of buffer gas. A pair of electrodes are inserted in the cell; each electrode is connected through a capacitor of 1.3 nF to the high-voltage pulse generator. This generator, which was constructed inhouse, is used to produce two bipolar pulses of variable duration from 20 to 170 ns with a voltage up to 20 kV; the time delay of the pulses with respect to the trigger can be varied from 0 to 2 µs, and the separation between the pulses can range from 0 to 11 µs. Both electrical and optical signals can be detected; the electrical detection system consists of a home-built charge-sensitive preamplifier (capable of detecting 1,500 electrons per pulse) and a digital oscilloscope (Tektronix TDS 620A), and the optical detection system consists of a photomultiplier tube (Hamamatsu 1P28) masked with an orange (OS14) glass filter and the oscilloscope.
The second part of this research project will involve observing the rate of ionization as a function of temperature. A T-shaped quartz cell with a pair of electrodes (one nickel plane, one tungsten wire) in each side of the T will be constructed. The signals at each set of electrodes will be observed simultaneously, as the temperature varies at one set, while remaining unaffected at the other. The effect of these different temperatures will be observed for the first time, and again, an optimal choice will be made for the ultratrace analysis. These experimental results will be compared to a theoretical model using a density matrix calculation program for a four-level atomic system.