Should You Consider FTIR Testing Techniques For VOC Measurements?
On May 14, 1999, the U.S. Environmental Protection Agency approved use of Test Method 320 for measurement of vapor phase organic and inorganic emissions by extractive Fourier transform infrared (FTIR) spectroscopy. Since this approval was published concurrently with the Portland Cement Industry NESHAP Standard, the method approval may have gone unnoticed by some. Although FTIR has been used in laboratories for materials analysis for over 70 years, regulatory agencies, industry sources, and others are now giving serious consideration to using FTIR testing for compliance testing and other suitable environmental applications.
What is it? Basically, one performs infrared absorption spectroscopy by directing an infrared beam through a sample to a detector. Some of the infrared radiation is absorbed by the sample and some of it passes through the sample. For most molecules, the absorbance occurs in a characteristic and reproducible pattern. A compound can be uniquely identified from this pattern or "spectrum" – much like a molecular fingerprint. Like a fingerprint, no two unique molecular structures produce the same infrared spectrum. A simplified scheme of the normal FTIR instrumental process is as shown:
The compounds are identified by comparing the measured spectra to "reference spectra" which are developed in a laboratory using standard samples at known concentrations. (Reference spectra of over 100 Hazardous Air Pollutants (HAPS) are available from the EPA). Similarly, concentrations of the compounds are measured by comparing sample band intensities with reference band intensities. Thus, with a PC and modern software algorithms, the FTIR can provide accurate, quantitative analysis in virtually real time.
What are the advantages of FTIR? Proponents of FTIR claim several advantages of these techniques. Among them, realtime analysis,
reduced field measurement costs, improved data quality, and practical measurements of previously impractical-to-measure emissions are some of the most commonly mentioned. Also, since the measured spectra are stored in the memory of the computer, unknown compounds can be identified at a later date by comparing measured spectra to reference spectra that become available later.
What are some of the interferences that can affect FTIR? Water vapor and carbon dioxide can interfere with FTIR analysis, and both are commonly present in emissions sources. However, choosing an alternate analytical region can minimize or avoid the spectral interference. For example, carbon dioxide interferes with benzene analysis near wave number 670; however, benzene can also be measured near wave number 3000 – albeit with less sensitivity.
Analyte spiking is used to determine accuracy of the method and to identify interferences. Other quality control and pre-test procedures are described in the "Protocol for the Use of Extractive Fourier Transform Infrared (FTIR) Spectrometry in Analyses of Gaseous Emissions from Stationary Sources" (EPA Protocol). The EPA Protocol and other information on FTIR are available for download at the EPA web site [www.epa.gov].
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