Baseline-free Quantitative Absorption Spectroscopy Based on Cepstral Analysis

The accuracy of quantitative absorption spectroscopy depends on correctly distinguishing molecular absorption signatures in a measured transmission spectrum from the varying intensity or "baseline" of the light source. Baseline correction becomes particularly difficult when the measurement involves complex, broadly absorbing molecules or non-ideal transmission effects such as etalons. We demonstrate a technique that eliminates the need to account for the laser intensity in absorption spectroscopy by converting the measured transmission spectrum of a gas sample to a modified form of the time-domain molecular free induction decay (m-FID) using a cepstral analysis technique developed for audio signal processing. Much of the m-FID signal is temporally separated from and independent of the source intensity, and this portion can be fit directly with a model to determine sample gas properties without correcting for the light source intensity. We validate the new approach in several complex absorption spectroscopy scenarios and discuss its limitations. The technique is applicable to spectra obtained with any absorption spectrometer and provides a fast and accurate approach for analyzing complex spectra

Complete reactants-to-products observation of a gas-phase chemical reaction with broad, fast mid-infrared frequency combs

Molecular diagnostics are a primary tool of modern chemistry, enabling researchers to map chemical reaction pathways and rates to better design and control chemical systems. Many chemical reactions are complex and fast, and existing diagnostic approaches provide incomplete information. For example, mass spectrometry is optimized to gather snapshots of the presence of many chemical species, while conventional laser spectroscopy can quantify a single chemical species through time. Here we optimize for multiple objectives by introducing a high-speed and broadband, mid-infrared dual frequency comb absorption spectrometer. The optical bandwidth of >1000 cm-1 covers absorption fingerprints of many species with spectral resolution <0.03 cm-1 to accurately discern their absolute quantities. Key to this advance are 1 GHz pulse repetition rate frequency combs covering the 3-5 um region that enable microsecond tracking of fast chemical process dynamics. We demonstrate this system to quantify the abundances and temperatures of each species in the complete reactants-to-products breakdown of 1,3,5-trioxane, which exhibits a formaldehyde decomposition pathway that is critical to modern low temperature combustion systems. By maximizing the number of observed species and improving the accuracy of temperature and concentration measurements, this spectrometer advances understanding of chemical reaction pathways and rates and opens the door for novel developments such as combining high-speed chemistry with machine learning

Spatially resolved mass flux measurements with dual comb spectroscopy

Providing an accurate, representative sample of mass flux across large open areas for atmospheric studies or the extreme conditions of a hypersonic engine is challenging for traditional intrusive or point-based sensors. Here, we demonstrate that laser absorption spectroscopy with frequency combs can simultaneously measure all of the components of mass flux (velocity, temperature, pressure, and species concentration) with low uncertainty, spatial resolution corresponding to the span of the laser line of sight, and no supplemental sensor readings. The low uncertainty is provided by the broad spectral bandwidth, high resolution, and extremely well-known and controlled frequency axis of stabilized, mode-locked frequency combs. We demonstrate these capabilities in the isolator of a ground-test supersonic propulsion engine at Wright-Patterson Air Force Base. The mass flux measurements are consistent within 3.6% of the facility-level engine air supply values. A vertical scan of the laser beams in the isolator measures the spatially resolved mass flux, which is compared with computational fluid dynamics simulations. A rigorous uncertainty analysis demonstrates a DCS instrument uncertainty of ~0.4%, and total uncertainty (including non-instrument sources) of ~7% for mass flux measurements. These measurements demonstrate DCS as a low-uncertainty mass flux sensor for a variety of applications.Comment: Main Text: 15 pages, 7 figure; Supplement: 6 pages, 4 figures; Submitted to Optic

Angular velocimetry for fluid flows: an optical sensor using structured light and machine learning

Most velocimetry approaches for fluid flows measure linear components of the velocity vector; yet, the angular velocity components, particularly at small scales in turbulent flows, also need to be resolved to study energy transfer and other important flow characteristics. Here, we detail an optical sensor approach to determine a component of the angular velocity vector. This approach uses beams of structured light and a machine learning-based analysis. We discuss the methodology to train the machine learning model and test it in experimentally validated simulations. This approach represents an interesting new direction for fluid flow velocimetry which may be extended to sense other flow parameters by selecting different light structures. &nbsp;</p

Immunodiagnostic confirmation of hydatid disease in patients with a presumptive diagnosis of infection

Information obtained from the routine application of hydatid immunodiagnostic techniques in different clinical situations over a seven-year period is presented. The Immunoelectrophoresis test was used until it was replaced by the simpler, more sensitive and equally specific arc 5 double diffusion (DD5) test. Examination of sera from 1,888 patients with signs and/or symptoms compatible with hydatid disease revealed that the presurgical confirmation of Echinococcus granulosus infection is only obtained by detection of anti-antigen 5 antibodies. The latter were not found in 1,539 presumptive hydatidosis patients whose definitive diagnoses corresponded to other disease conditions. However, false positive latex agglutination test results were obtained in two cases. In all patients whose preoperative serum showed three or more uncharacteristic bands in the absence of anti-antigen 5 antibodies, hydatid cysts were found sur gically. DD5 testing of a fluid sample collected by puncture established its hydatid etiology. Post-operative monitoring of hydatidosis patients demonstrated that persistence of DD5-positivity two years after surgery established the presence of other cysts. Further evidence was obtained in patients with hydatid cysts in intrathoracic, abdominal or other locations associating cyst membrane integrity, antigen release and immunodiagnostic test positivity

Temperature-dependent CO<SUB>2</SUB> line mixing models using dual frequency comb absorption and phase spectroscopy up to 25 bar and 1000 K

International audienceWe perform dual frequency comb laser absorption spectroscopy of CO2 at high pressure and temperature, and use the spectra to test and improve CO2 line mixing models and their temperature dependence. The dual-comb spectrometer spans 6800-7000 cm-1 with 0.0066 cm-1 point spacing, and is coupled to a specialized, high-pressure, high-temperature gas cell providing conditions up to 977 K and 25 bar. We compare our measurements to spectra calculated using an advanced line mixing model for pure CO2 based on the energy-corrected sudden (ECS) approximation. We determine a new set of temperature-dependent ECS model parameters, and show that the new parameters significantly improve the accuracy of the ECS line mixing model over a temperature range spanning 298-977 K. We also compare the measured spectra to a simpler line mixing model developed using the modified exponential gap (MEG) scaling law, and report the temperature and pressure dependence of the MEG model parameters required to scale the model across wide ranges of conditions. Finally, we report high-pressure, ambient-temperature measurements of CO2 spectra in both amplitude (absorption) and phase using the dual-comb spectrometer. We use these measured absorption and phase spectra to assess the performance of the ECS and MEG line mixing models at high densities near room temperature. To the best of our knowledge, these results represent the first study of line mixing using phase spectroscopy. Overall, the results of this study significantly expand line mixing models for CO2 at high pressure and temperature and improve the accuracy and availability of absorption models for harsh conditions encountered in laser-based sensing and planetary science