Infrared (IR) spectroscopy has been an established benchtop laboratory analytical technique for many years. It identifies and quantitates compounds through the use of their infrared absorption spectra. Click here for a typical IR spectrum. Another use of the infrared spectra is found with recently developed video cameras. These cameras use infrared absorption to image the absorbing compounds on a video tape. The image appears as a cloud on the video, but the instrument does not identify or quantitate the individual compounds.

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In the environmental field, field-portable IR instruments have found use in measuring air contaminants in an open-path configuration, volatile organic chemicals in water, and total petroleum hydrocarbon content in soils and water.

Open-path Fourier transform IR (OP-FTIR) provides multiple, rapid scanning of contaminants in air, which allows near real-time analysis. Applications for which FTIR is suitable include fence-line or site perimeter monitoring, worker exposure monitoring, emission rate assessment, air impact measurement during emergency removals, air impact evaluation during remedial actions, vapor suppression technique evaluation, accidental release early warning systems, and industrial facility monitoring. In October 1996, EPA issued Toxic Compendium Method TO-16, which recognizes open-path FTIR as an ambient air monitoring method. This document was updated in 1999. The third edition of the EPA FT-IR Open-Path Monitoring Guidance Document (TR-4423-99-03) was issued in 1999.

While not as common as open path, IR has been used to measure volatile and some semi-volatile organic chemicals in water using a headspace technique. The method has been verified by EPA's Environmental Technology Verification (ETV) program.

More commonly, field-portable IR is used as a screening tool for determining hydrocarbon contamination in soil and water. This technique has been verified by the EPA Site Innovative Technology Evaluation (SITE) program.

IR imaging video cameras are used to detect leaks from process equipment and storage tanks at refinery and chemical plants, as well as from natural gas pipelines. These cameras produce an image of escaping gases but do not identify the gas components or quantitate them. Video examples:

• TexAQS II Remote Sensing VOC Project, Select Raw Footage: Houston Ship Channel, July 11-13, 2005 (Running time 7 minutes, 32 seconds)
• IR Camera View of Gas Pump (Running time 9 seconds)

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IR analysis, including FTIR, relies on passing a collimated beam of infrared light through or onto the sample or atmosphere containing contaminants. The contaminant (analyte) absorbs the infrared light at frequencies specific to the functional groups present in the molecule. Thus, each contaminant has characteristic infrared spectra that can be used to identify the compound. Also, the amplitude of absorbance is proportional to the analyte concentration. Note that not all contaminants absorb in the infrared range. Prominent examples would be noble gases, vapor-phase metals, and homonuclear diatomics (e.g., O₂, N₂, Cl₂).

The open-path FTIR system can be likened to a "particle counter" that sums up the total amount of energy a target chemical absorbs between the FTIR sending unit and the detector. The FTIR itself cannot distinguish where along the beampath the burden of the concentration lies, nor can it distinguish between a narrow concentrated plume or a broad diluted plume that is contained within its beampath. The deployment of the system, which is discussed below, can be configured to allow for the detection of hotspots and estimation of the total contaminant flux leaving a site.

Concentrations reported by these open-path IR systems can be reported as either "path averaged" or "path integrated." A path-averaged concentration is computed by dividing the measured amount of the chemical by the path distance. Typically, path-averaged concentrations are reported in units of ppm or ppb. A path-integrated concentration does not average the measured chemical over the pathlength and is typically reported as ppm-meters or ppb-meters. FTIR can provide detection limits in the low ppb range for many compounds. See open path FTIR for examples.

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FTIR instruments can be made to operate in passive or active mode. In the passive mode, the instrument relies on an external infrared energy source (e.g., the sun or hot emissions from a stack) and is deployed in a "point and shoot" manner. In the active mode, the instrument has its own infrared source.

A typical active FTIR system consists of an IR source, Michelson interferometer, helium-neon laser for beam alignment, collimating optics, transmitting and receiving telescopes, detector, and depending upon the deployment configuration, retroreflectors. The effective pathlength for most systems is about 500 meters. This provides long path coverage.

Conventional spectrophotometers use a diffraction grating to achieve spectral dispersion. Fourier transform optics use the collimated source radiation that passes into a Michelson interferometer (which consists of a beam splitter and stationary and moving mirrors) to achieve the spectral dispersion. The beam splitter transmits half of the incident radiation from the source to a moving mirror; the other half is reflected to the stationary mirror. The moving mirror affects the relative path length of the two beams, thus introducing a phase difference. The amplitudes of the waves are combined at the beam splitter to form an interferogram signal, and the resulting encoded beam is directed through the area to be tested for contamination and onto a detector. The detector digitizes the signal whence it is sent to a computer to have a Fourier transform applied to it. The result of the Fourier transform is an infrared spectrum.

There are two classes of detectors used when longer than 1.2 micrometer (µm) wavelengths are being considered. Otherwise, the same detection methods for ultraviolet and visible radiation are used.

Two types of detectors—pyroelectric and photoconductive—are in common use. Pyroelectric detectors are made of thin crystals constructed from materials such as deuterated tryglycine sulfate (DTGS) or lithium tantalate. These detectors are sensitive to changes in temperature caused by the infrared radiation and operate at room temperatures. Pyroelectric detectors generally are not used in open-path spectroscopy for trace compounds because of their elevated detection limits.

Photoconductive detectors are semiconductors that show an increase in electrical activity when exposed to infrared radiation. They have a rapid response time and high sensitivity. While the most commonly used detector is constructed from mercury, cadmium, and telluride (MCT) and requires cryogenic cooling (usually with liquid nitrogen), detectors constructed from indium antimonide (InSb) are also in use and may be better suited for some applications.

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Active open-path FTIR is typically used in a fixed location. It can be deployed in a monostatic or bistatic configuration. In the monostatic configuration, the transmitting unit is placed on one side of the area to be measured and the receiving unit is placed on the other. This deployment has some measurement efficiencies over bistatic configuration but generally limits the activity to ground-level, straight-line measurement. In the bistatic configuration, the sending and receiving units are co-located and the light is transmitted across the field of measurement to a retroreflector that bounces it back to the receiving unit. In both of these configurations the instrument is measuring only what crosses its line of sight; contaminants that are released at a height above this line of sight may be missed by the instrument.

Bistatic configurations can be used to perform horizontal and vertical radial plume mapping, which is used to locate hotspots and estimate total emission flux from a facility. If an area to be measured does not contain many above-ground obstructions, it can be gridded and retroreflectors placed in each grid square. The average concentrations from the instrument to each retroreflector can be subtracted from each other to determine sector contributions, or the values can be computer-manipulated to yield a contour map. A large increase between retroreflectors is interpreted as a hotspot area. The hotspot area is then gridded on much smaller squares in a repetitive process until the source is located. See EPA 2005 for an example site.

To obtain potential hotspot data and total flux leaving a facility, a series of retroreflectors is placed along the downwind side of the site at different distances and heights. If the total plume falls within the height of the vertical retroreflectors, an estimated flux can be calculated using wind speed measurements taken at the same time. The accuracy of the wind speed measurements can be improved by using Doppler radar. In all measurements, background concentration levels will need to be subtracted. See EPA 2004 for an example site.

More complete information on how OP-FTIR works.

Passive open-path FTIR relies on a difference in temperature between the emitting target compounds and background. The instrument can be deployed in two modes: in a stationary point and shoot mode or mounted to a vehicle or aircraft. The Swedish government has sponsored the development of a vehicle-mounted, passive FTIR (solar occultation flux) method to monitor fugitive emissions at refineries and other chemical plants. Measurements are made by placing the suspected plume area between the instrument and the sun. Detection limits are generally higher with passive instruments than with active ones, but the range is longer (up to several kilometers).

Infrared sensing video cameras record the differences in absorption of specific infrared wavelengths in their field of vision. The presence of chemicals that absorb at the specified wavelengths is made visible as a clouded area. The camera has been very effective in locating large hydrocarbon releases from places that are difficult to access. It does not detect all chemicals, nor does it speciate or quantify them. Releases under 500 ppm are generally not detectable. The camera can be combined with a passive FTIR system to provide visual indication of the origin of a release as well as its chemical composition and concentrations. Examples of IR camera use.

Infrared detectors for oil in water or soil are small benchtop instruments that require an electrical supply (AC or 12 volt battery). Petroleum hydrocarbons are extracted from a soil or water sample using proprietary solvents, generally a chlorofluorocarbon or methylene chloride. The extract is then either filtered (soil) or decanted (water) into a sample holder that is placed in the instrument for analysis. The readout is in ppm of total petroleum hydrocarbons (TPH).

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Target analytes for FTIR systems include volatile and semi-volatile organics, ammonia, carbon disulfide, carbon monoxide, hydrogen chloride, hydrogen cyanide, hydrogen fluoride, hydrogen sulfide, nitric acid, nitric oxide, nitrogen dioxide, ozone, phosphine, silane, sulfur dioxide, and sulfur hexafluoride.

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Performance specs include information on detection limits, and calibration.

Detection Limits

Field-portable IR: 10-100 ppm (TPH in soil); 0.5-10ppm (TPH in water)

OP-FTIR: low ppm to low ppb (air) depending on the chemical, atmospheric conditions, path length, detector used, and whether the system is passive or active.

IR camera: hundreds of ppm. Note: This instrument does not provide quantitative results.

Calibration

The open-path FTIR monitor is not calibrated in the classical sense. That is, a sample of known concentration is not presented to the instrument for measurement (EPA 1999). Open-path FTIR calibration is therefore primarily a verification of instrument response. To conduct instrument performance verification, a target compound is analyzed to evaluate accuracy. Instrument functionality and precision can also be demonstrated by obtaining spectra for cells containing surrogates.

In addition, FTIR spectroscopy involves a single beam, so a background spectrum is required. Before collecting data, background spectra are collected to be subtracted subsequently from analytical spectra. For open-path investigations, it is important to obtain background readings under meteorological conditions similar to those of the contaminant measurements.

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• An FTIR system consistently scans the infrared spectrum in fractions of a second throughout its optical range. Very useful where fast, repetitive scanning is needed.
• The system simultaneously measures all wavelengths. Scans are added. The signal is N times stronger, noise is N1/2 as great, and therefore the signal-to-noise advantage is N1/2.
• There are no slits or gratings, thus energy throughput is high and more energy is at the detector where it is needed most.
• Near real-time data collection and reporting can be achieved.
• Archived data can be re-analyzed for new compounds.
• The generation of a path-integrated concentration yields contaminant information along the entire path length and not just at a single point, so there is less chance of missing a plume.
• Compound speciation of any compound with an IR absorbance can be obtained.
• No sample collection, handling, or preparation is necessary.
• FTIR provides cost effectiveness versus multiple discrete sampling points with separate analysis.
• The system can be used to calculate the total flux of contaminants escaping from a facility.
• FTIR can be used to locate discrete emission hotspots at a facility/landfill.

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The minimum detection limits are influenced by factors such as water vapor, carbon dioxide concentrations, pathlength, and chemical interferences. The signal can be reduced in several ways: beam divergence; atmospheric absorption due to water and scattering of the IR source from particulates; misalignment due to operator error, wind, or temperature; and beam blocks by pedestrians, vehicles, and buildings.

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FTIR costs vary significantly. Instrument design and accessories affect instrument prices. The capital cost of the FTIR is $100,000 or more. The cost of doing a week-long study, including quality assurance (QA) and a report, is on the order of $50,000. Long-term (one year or more) FTIR fence-line monitoring costs around $250,000. If sites are in the same general area, the cost per site should be lower (EPA 2006).

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Verification of the performance of site characterization and field analytical technologies is conducted through a variety of programs. Evaluation and verification reports from EPA's SITE Measuring and Monitoring Program and ETV Program are provided below.

Innova AirTech Instruments Type 1312 Multi-gas Monitor was verified for measurement of chlorinated volatile organic compounds in water. The verification documents available consist of a verification report and verification statement.

The EPA ETV Program has tested two OP-FTIR instruments.

Ail Systems Inc. - RAM 2000 Fourier Transform Infrared Open-Path Monitor
Verification Statement
Verification Report

Spectrex, Inc. - SafEye 227 Infrared Open-Path Monitor
Verification Statement
Verification Report

The ETV Program has tested one field-portable infrared monitor for measurement of chlorinated volatile organic compounds in the headspace of water samples.

Innova AirTech Instruments Type 1312 Multi-Gas Monitor
Verification Statement
Verification Report

The EPA SITE program has tested one field-portable infrared monitor for measurement of TPH in soil. The analyzer can also measure TPH in water.

Horiba Instruments Incorporated OCMA-350 Oil Content Analyzer
Verification Statement

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IR Cameras. Texas Commission on Environmental Quality. 2013.

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