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Portfolio: Monitoring Projects
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Development
of AOTF Multi-Element Metal CEM for Compliance Monitoring
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Innovative Technology
Summary Report
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TTP#:
CH1-7-C233; Tech ID: 1564
Project Overview
The goal of this project is to provide a system capable of monitoring
elemental emissions from thermal waste treatment facilities. The
purpose of this monitoring system is to provide documentation of
regulatory compliance with regard to toxic metal or radioactive
emissions. In order to achieve this goal, we have developed a compact,
high-resolution spectrometer based on Acousto-Optic Tunable Filter
(AOTF) and high-resolution echelle grating technologies. The spectrometer
is combined with an air-plasma Inductively Coupled Plasma-Atomic
Emission Spectrometry (ICPAES) system developed by collaborators
at Mississippi State University (MSU). The combined system is capable
of compliance monitoring of heavy metal (e.g., Resource Conservation
and Recovery Act [RCRA] metals) and actinide (e.g., alpha emitters)
contaminants in stack emissions. In Fiscal Year (FY) 2000,
innovations include 1) adaptation of the echelle spectrometer
system for use as a Hg vapor monitor with simultaneous background
and interference monitoring, and 2) application of the existing
AOTF-echelle spectrometer system to other emission-based Continuous
Emission Monitoring (CEM) techniques. In addition, the reduced-pressure
ICP CEM that was developed and tested with Diagnostic Instrumentation
and Analysis Laboratory (DIAL) in FY 1999 will be modified
and improved based on results of these initial tests. These innovations
will improve the speed and reliability for compliance with Environmental
Protection Agency (EPA) Maximum Achievable Control Technology (MACT)
CEM regulations at thermal waste treatment facilities. The spectrometer
has an advanced user interface capable of easy adaptation to atomic
or molecular emission spectrometric applications such as hot cell
and nuclear fuel analysis and processing, field analyses in complex
matrices, or passive remote sensing. Additional applications are
being sought.
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Technology
Description
This project involves the development of a high-resolution,
solid-state, compact spectrometer for monitoring spectral emission
from an air, inductively coupled plasma, atomic emission spectrometer
(air-ICP-AES) system. This fieldable spectrometer provides the
resolution and sensitivity of a 1.0- to 1.5-m spectrometer in a
package that is less than one-tenth the usual size and weight. The
system consists of a 0.38-m echelle-grating spectrometer with an
acousto-optic tunable filter (AOTF) performing grating-order selection.
An array detector, either a linear photodiode array or a rectangular
charged coupled device (CCD) array, detects the dispersed emission.
The AOTF is a quartz crystal device that selects a narrow band (~1
nm) of emitted light and rotates its polarization by 90 degrees.
When placed between crossed polarizers, only the selected wavelength
band is transmitted to the echelle grating. The AOTF wavelength
is tuned by changing an applied radio frequency. The AOTF allows
extremely rapid sequential or simultaneous selection of wavelengths
with no moving parts. The wavelength-switching rate is limited to
several milliseconds by the electronics and the speed of the acoustic
wave in the quartz crystal.
Compared to more conventional echelle-grating spectrometers that
use cross-dispersion gratings or prisms, this spectrometer provides
advantages in reduced size, simplified optical components, reduced
off-axis aberrations and light losses, minimized exposure of optical
components to excessive ultraviolet (UV) source radiation, and smaller
and less expensive detector requirements. Compared to tunable-grating
spectrometers with comparable resolution, this detection system
is smaller, lighter, provides more rapid wavelength tuning, and
is more flexible than direct-reader spectrometers that require moving
the detector components to change selected lines.
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