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Environmental Technology Listing
Title of Listing:
Open-Path Fourier-Transform Infrared Spectrometer
Category:
Characterization Technologies
Subcategory:
 *Air, All Listings
Media:
Soil Gas, Air Particulates, Air Vapors
Contaminants:
VOCs, Non-Halogenated Semovolatiles, Solvents, Organic Acids, Inorganic Cyanides
Web Site:
Email:
Technology Description:

Open-path FTIR spectroscopy is a strictly instrumental method (i.e., no sample handling or wet chemistry required) employing sophisticated software for multicomponent analysis of emission spectra obtained during monitoring. The accuracy of component measurement is limited only by the accuracy of reference spectra preparation. For the open-path FTIR system, this limit is less than 2 percent.

          The methodology relies on measurement of the absorbance of light energy by pollutant molecules within the contaminant plume, measured directly through a representative downwind section of the plume. VOC concentrations are reported as parts-per-million-meters (ppm-m). These concentration values may be converted to path-averaged concentrations in parts-per-million (ppm) by multiplying by the path length (m).

          The open-path FTIR system is manufactured by Environmental Technologies Group, Inc. (ETG). The system consists of an IR source, a Michelson interferometer, a beamsplitter, a helium-neon laser for beam alignment, a collimating telescope, and a mercury-cadmium-telluride (MCT) detector. The detector for this instrument is cooled by means of a closed-cycle Stirling engine. A computer is employed in conjunction with the spectrometer for data analysis and reporting.

          The basic principle of open-path FTIR spectroscopy involves generation of a light beam over a range of wavelengths in the near-IR portion of the spectrum. The beam passes through a parcel of atmosphere in which chemical species absorb IR radiation at characteristic wavelengths. The beam is reflected directly back on itself to the receiver/transmitter via a mirrored corner-cube array (retroreflector). Upon return of the light to the detectors, the spectrum between about 2 and 14 microns is received, inversely Fourier-transformed, and then compared to a library spectrum for each chemical compound of interest so that the compounds present can be identified and quantified. Data analysis is conducted using a PC and a software package which utilizes a component-specific analysis method based on comparison (via a least-squares-fit spectral matching subroutine) to a reference spectra library stored in the system.
Performance Status/
Limitations:

Performance Status:

          During the past five years, ETG Services has used open-path FTIR technology to quantify air emissions at more than 20 CERCLA/RCRA, CAA, and OSHA enforcement-driven regulatory actions

          Limitations:

          Open-path Fourier Transform Infrared (FTIR) technology has some technical limitations that can be properly handled. Minimum detection limits (MDLs) are a product of factors such as water vapor, path length and chemical interferants. All three of the factors are a function of the return signal strength that the detector receives. In general, more signal improves detection limits. The following is a short summary of factors limiting signal strength:

          Reduced signal from beam divergence: Signal strength is greatly reduced as the collimated beam diverges over a path length

          Reduced signal from atmospheric constituents: At greater path lengths the return signal strength is greatly reduced not only from beam divergence, but also from atmospheric absorption of signal due to water and scattering of the IR source energy from particulates passing through the beam path.

          Reduced signal from misalignments: A small angular swing at the FTIR location results in a larger offset at the retroreflector as the beam path distance increases. To maintain alignment at small beam path lengths the tolerances of the tripod head should be greater and the hardware staging locations should be firm. Wind can also cause misalignments. During large temperature changes, misalignments can also arise from the behavior of the metal to which all FTIR and retroreflector components are attached.

          Reduced signal from beam blocks: As the beam path length increases, chances are that unless it is elevated above the path of pedestrians and vehicles, it will periodically be blocked. The beam should be positioned above all possible obstructions.

          These technical limitations are application-specific and are minor when considering the enormous advantages that open-path technology provides over classical sampling (see #14.)
Topics
Analytical Testing, Technologies, Characterization
Additional Topics/Tags/Keywords
Open Path FTIR, Monitor, Gases, VOCs

Organization:
Carala Air Associates (formerly ETG)
Address:
2551 Route 130, Suite A
Phone:
DescriptionNumber
City:
Cranbury
1.
Primary (609) 395-1110
State/Province/Territory:
NJ
2.
Zip/Postal Code:
08512-9575
3.
Country:
United States
4.
Fax:(609) 395-1113
Branch Locations: