Detectors

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Chromatographic methods are routinely automated. A detector that responds to the pollutants' physical or chemical properties analyzes the gas or liquid leaving the column. Detectors can be specific for individual pollutants or classes of pollutants, or nonspecific.

Nonspecific Detectors. Flame ionization, thermal conductivity, and mass spectrometry are common nonspecific detection methods that detect all molecules containing carbon and hydrogen. In mass spectrometry, molecules of a gas are energized in a variety of ways, such as bombardment with electrons or rapid heating, causing them to gain or lose electrons. Because they have different masses and charges, the resulting ions are separated when they pass through magnetic and electric fields. The size and distribution of peaks for ions with different mass-to-charge ratios, known as the mass spectrum, identify the gas and determine its concentration. Gas chromatography coupled with high-resolution mass spectrometry definitively identifies PCBs and is the most accurate way to determine their concentration. Portable gas chro-matograph/mass spectrometers can measure VOCs in soil and water to parts per billion (ppb).

Specific Detectors. Methods that detect classes of pollutants include nitrogen/phosphorous detectors for organophosphate pesticides, thermionic ionization detectors that detect molecules containing NO2, nitro groups, such as dinitrotoluene and electron capture. Electron capture is particularly sensitive to compounds, such as organohalide pesticides that contain the halogen atoms, chlorine, bromine, or fluorine. These atoms strongly attract electrons. The electron capture detector emits electrons that are captured by the halogens atom. The reduction in electric current corresponds to the concentration of pollutant. Chlorinated disinfection by-products, haloacetic acid, and phthalates in drinking water can be separated by gas chromatogra-phy and measured by electron capture. Sulfur hexafluoride, an ozone-depleting gas, can be measured to parts per trillion (ppt) by electron capture. Spectroscopic detection methods including IR, UV, and atomic absorption and emission spectroscopy are unique for specific compounds.

Spectroscopic Detection. The electromagnetic spectrum encompasses all forms of electromagnetic radiation from the most energetic cosmic and gamma rays to the least energetic radio waves. The part of the spectrum that is particularly useful in identifying and measuring pollutants consists of radiation that interacts with the atoms and molecules that make up life on Earth. This includes radiation in the UV, visible, and IR regions.

Atomic Spectra. Atoms of different elements may be thought of as having different arrangements of electrons around the nucleus in increasing energy levels. When metals such as lead, copper, and cadmium are vaporized at high temperatures, some electrons jump to higher energy levels. When the electrons drop to their original levels, the metal atoms emit radiation in a range of wavelengths from IR to UV, including visible light. The colors in fireworks result from such emissions. The wavelengths emitted constitute a unique "fingerprint" for each element and their intensity reflects the metal concentration. Inductively coupled plasma emission spectra (ICP-AES), in which a high-temperature gas or plasma excites metal atoms, are used to identify and quantify heavy metal contamination.

The same spectral fingerprint is obtained from the wavelengths of light that each element absorbs. Trace amounts of certain metals such as mercury and arsenic are more accurately measured from their absorption, rather than their emission spectra.

UV and IR Spectra. Many pollutants can be identified by their UV and IR spectra because all molecules that absorb strongly at specific wavelengths exhibit spectral fingerprints. Pollutants separated by liquid chromatography are often detected by spectroscopy. Gases such as those from vehicle emissions, landfills, industrial manufacturing plants, electric power plants, and hazardous incineration smokestacks can be monitored by spectroscopic methods. Gas and chemical leaks may also be monitored by spectroscopy.

UV Absorption Spectra. Toxic gases such as hydrogen sulfide, ammonia, and styrene can be monitored by their UV absorption spectra. Open path monitors emit UV radiation from a source, such as a bulb containing excited xenon gas, across the area to be monitored. Detectors record the absorbed wavelengths to produce a spectral fingerprint for each gas. Ammonia is often used as a coolant for turbine generators in power plants. It can be monitored for worker safety by its UV spectrum. The EPA has established National Ambient Air Quality standards for the six criteria pollutants: carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide.

open path monitor detection device that employs a beam of light passing through an open space

DIAGRAM OF THE ELECTROMAGNETIC SPECTRUM

DIAGRAM OF THE ELECTROMAGNETIC SPECTRUM

Low Frequency

High Frequency

Low Frequency

Long Wavelength

High Frequency

W V WUUl/VU

Short Wavelength

Satellite instruments monitoring stratospheric ozone generally measure the decrease in intensity in UV solar radiation due to ozone absorption. The total ozone mapping spectrometer on the Earth probe satellite (TOMS/EP) scans back and forth beneath the satellite to detect six individual frequencies of UV light that are scattered by air molecules back through the stratosphere. The more ozone in the stratosphere, the more "backscattered" UV radiation will be absorbed compared to UV radiation directly from the sun.

Some IR open path monitors use a tunable diode laser source in the near IR. The laser emits the specific frequency at which a monitored gas absorbs, so there is no interference from other gases or particles such as rain or snow. Such lasers are widely employed in the telecommunications industry. Pollutants that absorb at specific wavelengths in this range include hydrogen fluoride, an extremely toxic gas used in the aluminum smelting and petroleum industries. Hydrogen fluoride can be monitored to one part per million (ppm) for worker safety by this method.

The greenhouse gases carbon dioxide, nitrous oxide, and methane may also be monitored by IR spectroscopy. Currently, emissions of carbon dioxide from power plants are not generally measured directly but are estimated. However, the amount of carbon dioxide in the atmosphere over Mauna Loa has been measured continuously by IR spectroscopy since 1958. The Mauna Loa Observatory is located on the earth's largest active volcano on the island of Hawaii. It is relatively remote from human activity and changes in carbon dioxide concentration above it are considered a reliable indicator of the trend of carbon dioxide concentration in the troposphere. Data from Mauna

Loa show a 17.4 percent increase in carbon dioxide concentration from 315.98 parts per million (ppm) by volume of dry air in 1959 to 370.9 ppm in 2001.

Remote sensors for vehicle emissions contain units that detect and measure carbon monoxide, carbon dioxide, and hydrocarbons by their IR spectra. Because IR absorption bands from water and other gases found in car exhaust interfere with the IR spectrum of NOx, the sensor also contains a unit that measures NOx from their UV absorption spectra.

Fourier transform IR spectroscopy (FTIR) analyzes the absorption spectrum of a gas mixture to detect as many as twenty gases simultaneously. The technique involves analyzing the spectra mathematically and then comparing the observed fingerprints with calibrated reference spectra stored on the hard drive of the computer to be used for analysis. Reference spectra for more than one hundred compounds are stored, including most of the VOCs considered hazardous by the EPA. Instruments that use UV Fourier transform analysis are now available. The instruments are generally installed at one location, but are portable and can be battery operated for short-term surveys. Multiple gas-monitoring systems are used in a variety of industries, including oil and gas, petrochemical, pulp and paper, food and beverage, public utility, municipal waste, and heavy industrial manufacturing.

absorption spectrum "fingerprint" of a compound generated when it absorbs characteristic light frequencies

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