A calibration-free scanned wavelength modulation spectroscopy scheme requiring minimal laser characterization is presented. Species concentration and temperature are retrieved simultaneously from a single fit to a group of 2f/1f-WMS lineshapes acquired in one laser scan. The fitting algorithm includes a novel method to obtain the phase shift between laser intensity and wavelength modulation, and allows for a wavelength-dependent modulation amplitude. The scheme is demonstrated by detection of H(2)O concentration and temperature in atmospheric, premixed CH(4)/air flat flames using a sensor operating near 1.4 µm. The detection sensitivity for H(2)O at 2000 K was 4 × 10(-5) cm(-1) Hz(-1/2), and temperature was determined with a precision of 10 K and absolute accuracy of ~50 K. A parametric study of the dependence of H(2)O and temperature on distance to the burner and total fuel mass flow rate shows good agreement with 1D simulations.

Potassium (K) is an important element related to ash and fine-particle formation in biomass combustion processes. In situ measurements of gaseous atomic potassium, K(g), using robust optical absorption techniques can provide valuable insight into the K chemistry. However, for typical parts per billion K(g) concentrations in biomass flames and reactor gases, the product of atomic line strength and absorption path length can give rise to such high absorbance that the sample becomes opaque around the transition line center. We present a tunable diode laser atomic absorption spectroscopy (TDLAAS) methodology that enables accurate, calibration-free species quantification even under optically thick conditions, given that Beer-Lambert's law is valid. Analyte concentration and collisional line shape broadening are simultaneously determined by a least-squares fit of simulated to measured absorption profiles. Method validation measurements of K(g) concentrations in saturated potassium hydroxide vapor in the temperature range 950-1200 K showed excellent agreement with equilibrium calculations, and a dynamic range from 40 pptv cm to 40 ppmv cm. The applicability of the compact TDLAAS sensor is demonstrated by real-time detection of K(g) concentrations close to biomass pellets during atmospheric combustion in a laboratory reactor.

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We report on an interband cascade laser (ICL)-absorption spectrometer for absolute, calibration-free, atmospheric CO amount fraction measurements, addressing direct traceability of the results. The system combines first-principles direct tunable diode laser absorption spectroscopy (dTDLAS) with a metrological validation. Using a multipath cell with 76 m path length, our detection limit is 0.5 nmol/mol at Δt=14  s. The system is highly linear (slope: 0.999±0.008) in the amount fraction range of 0.1-1000 μmol/mol and thus is interesting for industrial as well as environmental applications. The sensor repeatability at 300 nmol/mol is 0.06 nmol/mol (with Δt=10  min). The sensor's absolute response is in excellent agreement with the gravimetric values of a set of primary gas standards used to test the sensor accuracy. The relative expanded uncertainty (k=2) of the measured CO amount fraction is 2.8%. Due to this performance and the calibration-free approach, the spectrometer may be used as an optical transfer standard (OTS) if gas standards are for whatever reason not available or applicable, e.g., for airborne instruments. Our dTDLAS approach has shown excellent stability and accuracy in H₂O detection [Appl. Phys. B116, 883 (2014)APPCDL0721-726910.1007/s00340-014-5775-4] even when compared to primary standards. We therefore deduce that the ICL spectrometer (after its adaptation to field conditions, similar to our H₂O spectrometers) has good potential to meet the 2 nmol/mol compatibility goal stated by the World Meteorological Organization for atmospheric CO measurements, and serve as an OTS which does not need frequent calibrations using reference gases.