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Infrared gas analyzer is a spectroscopic instrument that quantifies gas concentration by measuring selective absorption of infrared radiation along a defined optical path and converting the attenuated signal to mole fraction using calibrated transfer functions. In its non-dispersive form a broad-band or narrow-band IR source, an optical path or cell, and a thermopile or pyroelectric detector with interference filters or gas-correlation optics isolate target rovibrational bands in the mid-IR; optical layouts range from single-pass tubes to Herriott/White multipass cells with gold-plated or blackened interiors to set effective path length and stray-light rejection. Dispersive and transform variants use gratings or a Michelson interferometer (FTIR) with a broadband source and a wideband detector to derive spectra for multi-component analysis with chemometric fitting. Photoacoustic IR analyzers transduce absorption to pressure waves detected by microphones or quartz tuning forks for compact cells and high pressure. Measurement fidelity depends on source stability, detector noise and bandwidth, wavelength selectivity and line-shape treatment, temperature and pressure control of the sample, water vapor handling, and suppression of optical etalons and baseline drift; performance is characterized by limit of detection at a stated averaging time, T₉₀ response, linear dynamic range, span and zero drift, cross-interference, and long-term Allan deviation. The instrument stack combines an IR emitter (incandescent filament, micro-hotplate MEMS, Globar, or IR LED), beam conditioning and apertures, a gas interaction region with controlled pressure, temperature, and flow, IR-transparent windows or seals where required, and a detector matched to the spectral band (thermopile, pyroelectric, InGaAs, InAsSb, or HgCdTe depending on wavelength). Sample conditioning adds particulate filtration, water knock-out or drying where matrix effects are critical, corrosion-resistant wetted parts for acid gases, and heated lines and cells for hot-wet streams. Electronics include low-noise transimpedance stages, synchronous detection for chopped or modulated beams, high-resolution ADCs, and embedded processors executing temperature/pressure compensation and spectral or harmonic fitting; calibration metadata, span factors, and diagnostics are stored in nonvolatile memory with event logs for traceability. Manufacture proceeds through opto-mechanical and gas-path integration followed by electronic assembly and calibration. Optical benches are machined and aligned to maintain source-detector symmetry and path length; multipass cells are assembled with precision mirror spacing, blackened baffles, and helium leak testing; gas manifolds and cells are passivated to minimize adsorption memory for polar species and fitted with calibrated pressure and temperature sensors. Sources are bonded and aged for radiance stability; detectors are mounted with thermal management and, where applicable, optical windows and cold shields. Control and acquisition boards are populated by SMT, coupled to precision current drivers and synchronous demodulators, and verified for noise and linearity. Factory calibration uses certified gas mixtures across multiple points to establish zero, span, linearity, and pressure-broadening compensation; environmental qualification applies temperature and humidity cycling, vibration and shock, and contamination challenges with water and hydrocarbons to quantify drift, recovery, and matrix cross-sensitivities. Applications include combustion and process control in boilers, furnaces, and reformers; continuous emissions monitoring for CO, CO₂, NO, SO₂, H₂O, NH₃, HCl, and N₂O on stacks and ducts; automotive and catalyst development benches for exhaust species; ambient and indoor air quality monitoring; semiconductor bulk and specialty-gas surveillance for CO₂ and moisture where IR is applicable; agricultural and ecosystem flux measurements using fast-response CO₂/H₂O IRGAs in eddy-covariance towers; medical breath and metabolic gas analysis; and safety monitoring in enclosed spaces for CO and CO₂. Selection of architecture—NDIR single-gas, FTIR multi-gas, or photoacoustic—is determined by target species spectra, expected concentrations and interferences, response-time requirements, and the thermal and chemical environment of the measurement point.
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