40 CFR § 1065.266 - Fourier transform infrared analyzer.
(a) Application. For engines that run only on natural gas, you may use a Fourier transform infrared (FTIR) analyzer to measure nonmethane hydrocarbon (NMHC) and nonmethane-nonethane hydrocarbon (NMNEHC) for continuous sampling. You may use an FTIR analyzer with any gaseous-fueled engine, including dual-fuel and flexible-fuel engines, to measure CH4 and C2H6, for either batch or continuous sampling (for subtraction from THC).
(b) Component requirements. We recommend that you use an FTIR analyzer that meets the specifications in Table 1 of § 1065.205. Note that your FTIR-based system must meet the linearity verification in § 1065.307. Use appropriate analytical procedures for interpretation of infrared spectra. For example, EPA Test Method 320 (see https://www.epa.gov/emc/method-320-vapor-phase-organic-and-inorganic-emissions-extractive-ftir) and ASTM D6348 (incorporated by reference in § 1065.1010) are considered valid methods for spectral interpretation. You must use heated FTIR analyzers that maintain all surfaces that are exposed to emissions at a temperature of (110 to 202) °C.
(c) Hydrocarbon species for NMHC and NMNEHC additive determination. To determine NMNEHC, measure ethene, ethyne, propane, propene, butane, formaldehyde, acetaldehyde, formic acid, and methanol. To determine NMHC, measure ethane in addition to those same hydrocarbon species. Determine NMHC and NMNEHC as described in § 1065.660(b)(4) and § 1065.660(c)(3).
(d) NMHC and NMNEHC CH4 and C2H6 determination from subtraction of CH4 and C2H6 from THC. Determine CH4 as described in § 1065.660(d)(2) and C2H6 as described § 1065.660(e). Determine NMHC from subtraction of CH4 from THC as described in § 1065.660(b)(3) and NMNEHC from subtraction of CH4 and C2H6 as described § 1065.660(c)(2). Determine CH4 as described in § 1065.660(d)(2) and C2H6 as described § 1065.660(e).
(e) Interference verification. Perform interference verification for FTIR analyzers using the procedures of § 1065.366. Certain interference gases can interfere with FTIR analyzers by causing a response similar to the hydrocarbon species of interest. When running the interference verification for these analyzers, use interference gases as follows:
(1) The interference gases for CH4 are CO2, H2O, and C2H6.
(2) The interference gases for C2H6 are CO2, H2O, and CH4.
(3) The interference gases for other measured hydrocarbon species are CO2, H2O, CH4, and C2H6.