When regulators evaluate an emissions abatement system for environmental compliance, they first look at Destruction and Removal Efficiency (DRE). DRE quantifies how effectively the abatement system, such as a thermal oxidizer or scrubber, reduces hazardous air pollutant concentrations between the inlet and the stack outlet. However, the calculated DRE percentage carries weight only if inlet and outlet concentration measurements are analytically sound. A Fourier Transform Infrared (FTIR) gas analyzer applies broadband infrared spectroscopy to measure how individual molecules absorb specific wavelengths of light, generating distinct spectral fingerprints for each compound present. With direct compound-level quantification, FTIR eliminates reliance on bulk hydrocarbon surrogates and anchors DRE in verified molecular chemistry, not procedural assumption.
Optimized FTIR Configurations for Inlet and Outlet Monitoring
Measurements for DRE involve two distinct analytical conditions, high-concentration inlet streams and trace-level outlet emissions. Many facilities therefore deploy two FTIR gas analyzers optimized for their respective concentration ranges. A short-pathlength FTIR gas analyzer measures the inlet gas stream where pollutant concentrations can reach percent levels. For the outlet, a high-sensitivity system such as the ASTG Model FT2-FTIR measures residual emissions at trace concentrations. Using dedicated instruments allows each analyzer to operate within its optimal absorbance range while maintaining the linear response required for accurate DRE calculations. This analytical capability is grounded in the Beer-Lambert law, which defines the linear relationship between infrared absorbance and concentration. Because FTIR spectroscopy quantifies molecular absorption directly, it sustains linearity across a wide concentration range when appropriate pathlengths are used.
Short gas cells accommodate high-concentration inlet streams without detector saturation; longer-path FTIR analyzers provide the sensitivity required to quantify trace-level outlet emissions. Conventional sensors, including flame ionization detectors and non-dispersive infrared analyzers, may saturate at elevated concentrations or lose sensitivity at trace levels, introducing measurement uncertainty at one end of the range. FTIR analyzers overcome these limitations by pairing molecular absorption spectroscopy with optical pathlengths matched to the expected concentration levels, enabling reliable measurement of both high-concentration inlet gases and trace-level outlet residuals.
DRE is calculated by subtracting the outlet concentration from the inlet concentration and dividing that difference by the inlet value. As the outlet concentration approaches zero relative to the inlet, the calculated DRE becomes increasingly sensitive to small analytical deviations. Even minor bias or signal noise at very low outlet levels can shift the reported percentage. Reliable measurement at both high inlet concentrations and trace-level outlet concentrations is thus necessary to control error propagation and ensure the reported DRE reflects actual pollutant destruction instead of analytical variability.
Elimination of Proxy Measurements via Speciation
An FTIR gas analyzer identifies and quantifies individual compounds using molecular spectroscopy. Each molecule absorbs infrared energy at characteristic frequencies, producing a unique spectral signature that enables simultaneous measurement of benzene, toluene, formaldehyde, methanol, and other volatile organic compounds within a single analysis. In contrast, traditional Total Hydrocarbon (THC) analyzers generate a bulk carbon response. They indicate overall hydrocarbon concentration but do not distinguish which specific compounds are present, destroyed, or formed during treatment. Non-regulated gases, such as methane, can elevate a THC signal, artificially lowering apparent DRE even when the target hazardous air pollutant has been effectively removed. As a result, DRE derived from bulk hydrocarbon data can misrepresent actual treatment performance. Changes in total carbon concentration do not necessarily correspond to the destruction of the regulated compound specified in a permit or test protocol. FTIR removes that disconnect by anchoring the calculation to the concentration of the target hazardous air pollutant itself. The reported DRE thus reflects compound-specific destruction efficiency, aligned directly with regulatory intent.
Hot and Wet Sampling and the Prevention of Bias
Sample conditioning can directly influence reported DRE values. Many volatile organic compounds and acid gases are water-soluble or prone to condensation as temperature decreases. Cold-dry sampling systems that use chillers remove moisture from the gas stream, but they can also take away soluble or condensable pollutants before the sample reaches the FTIR gas analyzer. FTIR gas analyzers configured for hot and wet operation avoid this bias by maintaining the probe, heated transfer line, and gas cell within its extractive sampling system at temperatures of approximately 180°C or higher. Keeping the entire sample path above the condensation point preserves the vapor-phase composition of the stack gas during transport and analysis.
If water-soluble or high-boiling compounds are partially removed in the course of sample conditioning, the measured outlet concentration is artificially reduced, inflating the calculated DRE. To prevent such a measurement bias, an FTIR gas analyzer equipped with a heated extractive sampling system limits this loss mechanism and ensures that reported outlet values reflect actual residual emissions, not artifacts introduced through sample preparation.
Spectral Defensibility and Data Validation
Each measurement generated by an FTIR gas analyzer produces both a raw interferogram and a processed infrared spectrum, forming a traceable analytical record rather than a single reported value. The ability to retain and reanalyze full spectral data becomes vital when DRE results are subject to scrutiny. Unlike single-channel sensors that offer only concentration numbers, FTIR retains the full spectral dataset, enabling a retrospective review of compound identity, spectral interferences, and quantification parameters. Should an unexpected concentration spike appear during a DRE test, the stored spectrum can be reexamined to determine whether the event reflects a genuine process deviation of an analytical artifact. Together, spectral archiving and method-based validation establish a defensible evidentiary record for DRE reporting under audit and regulatory oversight.
Precision Engineering with the ASTG Model FT2-FTIR
The ASTG Model FT2-FTIR was developed specifically for demanding DRE environments where trace-level outlet emissions must be measured with high analytical confidence. In many DRE test configurations, a short-cell FTIR analyzer measures the high-concentration inlet stream, and the FT2-FTIR quantifies the low-level residual gases at the outlet. The high-sensitivity optical design of the FT2-FTIR supports accurate trace-level detection, while its heated extractive sampling system preserves sample integrity at elevated temperatures. Combined with full spectral archiving and EPA Method 320 compliant validation capabilities, the system produces defensible data for DRE compliance. Speak with ASTG to gather more information about the FT2-FTIR.