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Laser-Enhanced Ionization Spectroscopy for Ultratrace Mercury Detection
EPA Grant Number: U914941Title: Laser-Enhanced Ionization Spectroscopy for Ultratrace Mercury Detection
Investigators: Clevenger, Wendy L.
Institution: University of Florida
EPA Project Officer: Thompson, Delores
Project Period: January 1, 1996 through February 12, 1998
Project Amount: $102,000
RFA: STAR Graduate Fellowships (1996)
Research Category: Academic Fellowships , Engineering and Environmental Chemistry , Fellowship - Analytical Chemistry
Description:
Objective: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.
Approach: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.
Supplemental Keywords:fellowship, mercury, laser-enhanced ionization, LEI, mercury detection, mercury contamination, temperature, ultratrace analysis. , Water, Scientific Discipline, Analytical Chemistry, Mercury, Chemistry, Environmental Chemistry, ultrfine analysis, mercury detection, laser based studies, mercury resonance ionization detector