Development of a near-infrared tunable diode laser absorption spectrometer for trace moisture measurements in helium gas

Abstract

There is a demand for water vapor or moisture measurement in gas mixture at trace level from industries like combustion studies, semiconductor industry, environmental studies, cryogenic services, etc. There are few principles, direct or indirect, to measure moisture—each having individual limitations. Among all, the chilled mirror hygrometer is very accurate but response time is poor; therefore, it is suitable for calibration purposes not for online measurement. An instrument based on near-infrared absorption spectroscopy has been developed and discussed in this article. In this instrument, the laser is modulated with both sinusoidal and slower linear scan, split into two beams: an intense one is passed only through air and the other one through air and the absorption cell of 50 cm length. The balanced detection technique is adopted between the two beams called the reference and the measurement channels, respectively, and the difference signal between the two undergoes the 2-f lock-in detection. This method is good for elimination of moisture present in air and any common-mode noise present in both the channels and as well determines the splitting ratio keeping the absorption cell in vacuum. This method also eliminates the 1-f component present in the signal, which causes asymmetry in the two wings of the 2-f absorption spectra. For computation of absorption spectra, it is found that the Hitran database is sufficient to predict for air as the base gas but Exomol should be used in the cases where helium is the base gas. Simulation and experiment both were performed at different concentration and pressures and the results are found in good agreement for the central spectrum with little difference in wings’ peak. The sensitivity of this instrument achieved at 100 mb pressure in the absorption cell for helium as the base gas with refresh rate of 625 Hz is ∼17 ppmv and ∼4 ppmv for direct and 2-f measurement, respectively. The applicability of the instrument for air as the base gas is also shown in the paper.

QCM Sensor Arrays, Electroanalytical Techniques and NIR Spectroscopy Coupled to Multivariate Analysis for Quality Assessment of Food Products, Raw Materials, Ingredients and Foodborne Pathogen Detection: Challenges and Breakthroughs

“…Importantly, the recently reported approach showed that recovery of parameters in a multi-wavelength time-resolved data can be achieved without prior instrument calibration [47]. Other notable NIR innovations in instrumental methods for spectral analysis during the review period includes a dual detection channel instrument for NIR time-resolved diffuse optical spectroscopy [48], functional NIR spectroscopy (fNIRS) instruments for high-density diffuse optical tomography [49,50], and other NIR-based sensors [51][52][53][54][55].…”

“…Electroanalytical techniques including electronic nose [59], electronic tongue [60,61], and biosensors [62] have notably gained attention for quality control assessment due to their sensitivity and portability. Some of these sensors are nanomaterial-based [50,[62][63][64], carbon electrode [65,66], lab-on-a-disc (LOD) [67], paper-based multiplexed [68], and electrochemical aptamer-based (E-Aptasensor) [69] sensors. The real-world applications of electrochemical biosensors for the quality assessment of food products and for the detection of foodborne pathogens are provided in Section 13 of this review.…”

Water Vapor Concentration Measurements in High Purity Gases by Means of Comb Assisted Cavity Ring Downspectroscopy

“…Fourier Transform Infrared spectroscopy (FTIR) is a well-established technique in the field, enabling detection of multiple species, including water vapor. However, the use of more selective and powerful near-infrared laser-based techniques, such as Tunable Diode Laser Spectroscopy (TDLAS) [12][13][14][15] and Cavity Ring-Down Spectroscopy (CRDS) [16][17][18][19][20][21][22][23][24], has increased significantly within the last decades, leading to the development (and commercialization in some cases) of several dedicated moisture analyzers. The interest in CRDS has enormously increased due to the possibility of using continuous wave (cw) diode laser sources, which are small, can operate at ambient temperature, and can guarantee high repetition rates.…”

Near-infrared imaging of water vapour in air

“…The wavelength range used in these techniques, particularly for concentration measurement, is 2500-2800 nm (equivalent to 4000-3571 cm −1 in wavenumber) because of the strong absorption peaks of gaseous water molecules derived from their vibration and rotation. For the measurement of WV concentration, absorption peaks of around 1400 nm (v 1 + v 3 band) [9][10][11][12] and 1900 nm (v 2 + v 3 band) [13] in the near-infrared (NIR) region have also been used, although the strength of the two bands is smaller than that of 2500-2800 nm, mainly because more useful light sources such as laser diodes and detectors, particularly for 1400 nm, are less expensive. In order to apply this light absorption-based method to open-path imaging, an optical beam scanning or expansion device is required, as well as a counter or reflection arrangement of the laser source and detector with a mirror and/or screen [1-4, 14, 15].…”