Measurement of Vapor Phase Organic and Inorganic Emissions by Extractive Fourier Transform Infrared (FTIR) Spectroscopy – USEPA Method 320

USEPA Method 320 is used for the analysis of vapor phase organic or inorganic compounds which absorb energy in the mid-infrared spectral region, about 400 to 4000 cm-1 (25 to 2.5 µ m).  This method is used to determine compound-specific concentrations in a multi-component vapor phase sample, which is contained in a closed-path gas cell.  Spectra of samples are collected using double beam infrared absorption spectroscopy.  A computer program is used to analyze spectra and report compound concentrations.
 
The analytical range is determined by the instrumental design and the composition of the gas stream.  For practical purposes there is no upper limit to the range because the pathlength may be reduced or the sample may be diluted.  The lower detection range depends on:

• the absorption coefficient of the compound in the analytical frequency region;
• the spectral resolution;
• the interferometer sampling time;
• the detector sensitivity and response;
• the absorption pathlength.

The practical lower quantification range is usually higher than the instrument sensitivity allows and is dependent upon:

• the presence of interfering species in the exhaust gas including water (H₂O), carbon dioxide (CO₂), and sulfur dioxide (SO₂);
• analyte losses in the sampling system;
• the optical alignment of the gas cell and transfer optics;
• the quality of the reflective surfaces in the cell (cell throughput).

Potential interferences include:

• analytical interferences
• background intereferences
• spectral interferences
• sampling system interferences