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Schematic of the experimental setup for collection of infrared spectra with the 3 m multipass gas cell.
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Infrared spectroscopy is commonly applied to the analysis of small gas-phase molecules. One of the limitations of using Fourier transform infrared (FT-IR) spectroscopy for these applications is the time response of long path length gas cells. Hollow waveguides (HW) that transmit in
the mid-infrared spectral range have higher optical efficiencies co...
Context in source publication
Context 1
... (Bruker Optics, Inc., Billerica, MA). A 3 m, 375 mL multipass gas cell (Mars, Gemini Scientific, Inc., Buena Park, CA) was placed in the sample compartment of the FT-IR and in series with a nondispersive infrared (NDIR) CO and a chemiluminescence NO analyzer (ZRH2 and CLD400, respectively, California Analytical Instruments, Inc., Orange, CA) (Fig. 1). The input gas stream was filtered for particulate matter with a 44 mm glass fiber filter with a nominal pore size of 1 lm. All connecting tubing was made from Teflon with an outer diameter of 0.25 in. Gas flow was maintained with a metal bellows vacuum pump (MB-41, Senior Flexonics, Inc., Sharon, MA) and monitored with an in-line ...
Citations
... Generally, a hollow waveguide is defined as a light pipe with the internal surface made from or coated with a dielectric material or metal (e.g., Al 2 O 3 , ZnS/Ag, AgI/Ag, etc.) with a coaxial hollow core enabling radiation propagation by a reflection at the inside walls. 20 Although hollow waveguides were initially designed for delivering high peak power laser light in industrial laser applications and for surgical scenarios, they have proven their value also as highly efficient miniaturized gas cells providing exceedingly low hollow core volumes for probing gas samples (i.e., a few milliliters or less). In the late 1970s, the first HWG was produced by Garmire from two strips of aluminum separated by dielectric spacers. ...
Absorption-based spectroscopy in the mid-infrared (MIR) spectral range (i.e., 2.5–25 μm) is an excellent choice for directly sensing trace gas analytes providing discriminatory molecular information due to inherently specific fundamental vibrational, rovibrational, and rotational transitions. Complimentarily, the miniaturization of optical components has aided the utility of optical sensing techniques in a wide variety of application scenarios that demand compact, portable, easy-to-use, and robust analytical platforms yet providing suitable accuracy, sensitivity, and selectivity. While MIR sensing technologies have clearly benefitted from the development of advanced on-chip light sources such as quantum cascade and interband cascade lasers and equally small MIR detectors, less attention has been paid to the development of modular/tailored waveguide technologies reproducibly and reliably interfacing photons with sample molecules in a compact format. In this context, the first generation of a new type of hollow waveguides gas cells—the so-called substrate-integrated hollow waveguides (iHWG)—with unprecedented compact dimensions published by the research team of Mizaikoff and collaborators has led to a paradigm change in optical transducer technology for gas sensors. Features of iHWGs included an adaptable (i.e., designable) well-defined optical path length via the integration of meandered hollow waveguide structures at virtually any desired dimension and geometry into an otherwise planar substrate, a high degree of robustness, compactness, and cost-effectiveness in fabrication. Moreover, only a few hundred microliters of gas samples are required for analysis, resulting in short sample transient times facilitating a real-time monitoring of gaseous species in virtually any concentration range. In this review, we give an overview of recent advancements and achievements since their introduction eight years ago, focusing on the development of iHWG-based mid-infrared sensor technologies. Highlighted applications ranging from clinical diagnostics to environmental and industrial monitoring scenarios will be contrasted by future trends, challenges, and opportunities for the development of next-generation portable optical gas-sensing platforms that take advantage of a modular and tailorable device design.
... Although such sensors achieved good sensitivity due the extended OPL, the integration of HWG with compact portable sensing systems is hindered by the mechanical flexibility and waveguide length. 26 The result is that the optical HWG-based sensor is governed by the required length of the HWG, affecting the operational footprint of the entire device, therefore limiting the use for applications which requires compactness of the sensor. ...
Ozone is an oxidizing molecule used for disinfecting a wide variety of environments, such as in dental clinics, and has most recently been promoted as a sanitizing agent to prevent coronavirus transmission. The easy access to ozone-generating sources also enables their ubiquitous use. However, exposure to ozone may seriously affect human health by amplifying or inducing respiratory diseases and distress syndromes and has been associated with premature deaths from other diseases. In this scenario, miniaturized, low-cost, and portable optical sensors based on the absorption signature of ozone in the ultraviolet (UV) range of the electromagnetic spectrum are an innovative approach for providing real-time monitoring of gaseous ozone, ensuring the safety of indoor and workplace environments. In this paper, a miniaturized ozone sensor based on the absorption signature of ozone at deep-UV frequencies was developed by integration of so-called substrate-integrated hollow waveguides (iHWG) with a miniaturized ultraviolet lamp and a fiber-optic USB-connected spectrophotometer. The innovative concept of iHWGs facilitates unprecedented compact dimensions with a high degree of flexibility in the optical design of the actual photon absorption path. The proposed device rapidly responded to the presence of ozone (<1 min) and revealed a suitable linearity (r 2 > 0.99) in the evaluated concentration range. The limit of detection was determined at 29.4 ppbv, which renders the device suitable for measurements in the threshold range of the main regulatory agencies. Given the adaptability and modularity of this platform, we anticipate the application of this innovative concept to be equally suitable for the in situ and real-time analysis of other relevant gases providing suitable UV absorption signatures.
... Despite this compensation, the intrinsic low absorption strength of the gases in the NIR region may still hinder the ultra-high sensitivity gas detection. More recently, with the development of MIR optics, MIR fiber gas sensing has attracted much attention and has shown great potential for high-sensitivity measurement [64][65][66][67]80,94,100,[104][105][106][107][108]. Table 1. ...
... Several attempts have been made for gas detections with different fibers and principles in the MIR region [64][65][66][67]94,100,[105][106][107][108]. In these studies, hollow silica waveguides (HSWs), HC-PBGFs, and HC-ARFs have been used as gas cells. ...
... In these studies, hollow silica waveguides (HSWs), HC-PBGFs, and HC-ARFs have been used as gas cells. HSWs consist of a silica tube with bore diameters ranging from 250 μm [106] up to 2100 μm [107], which means that it is difficult to be compatible with traditional fiber systems. Besides, they suffer from significant transmission losses and bend losses when coiled, and thus the output power is lower compared with other gas cells [100]. ...
Fiber gas sensing techniques have been applied for a wide range of industrial applications. In this paper, the basic fiber gas sensing principles and the development of different fibers have been introduced. In various specialty fibers, hollow-core photonic crystal fibers (HC-PCFs) can overcome the fundamental limits of solid fibers and have attracted intense interest recently. Here, we focus on the review of HC-PCF gas sensing, including the light-guiding mechanisms of HC-PCFs, various sensing configurations, microfabrication approaches, and recent research advances including the mid-infrared gas sensors via hollow core anti-resonant fibers. This review gives a detailed and deep understanding of HC-PCF gas sensors and will promote more practical applications of HC-PCFs in the near future.
... Sensing in the evanescent field, e.g. using attenuated total reflection (ATR) fibers or waveguides with small diameter such as silver halide fibers, can potentially overcome these limitations [8][9][10]. For example, hollow core waveguides allow for sensitivity enhancement of up to 60% per meter optical path length compared to multi-pass gas cells [11]. In particular, the great advances in the field of integrated optics of the past decades hold promise to achieve the functionality and advantages of conventional IR spectroscopic chemical sensing, but at a drastically reduced foot-print and lower cost due to CMOS-fabrication [12]. ...
Sensitivity of evanescent wave sensing of gaseous species can be vastly increased by enrichment materials that locally concentrate the analyte on the sensor. Here, we investigate functionalized mesoporous silica films as versatile enrichment layer for sensing volatile organic compounds (VOCs) from gas-phase. Attenuated total reflection (ATR) crystals were coated with silica films of different pore sizes and their capability to enrich three different aromatic hydrocarbons from a vapor stream was studied by means of Fourier Transform infrared (FTIR) spectroscopy. Thereby, single-digit ppmv limits of detection (LOD) were achieved with an effective path length of only 6.3 µm. The selectivity introduced by the functionalization of the silica films effectively minimized interferences of water vapor, which gave access to the spectral fingerprint region between 1550 and 1450 cm-1. This allowed to discriminate and quantify toluene, p-xylene and 1,2,4-trimethylbenzene in multicomponent mixtures at high humidity. Fast response and regeneration times and enrichment factors up to 32 000 showcase the high potential of this material for evanescent wave sensing.
... Consequently, HWGs facilitate the development of miniaturized MIR gas sensing systems providing extended and well-defined optical path lengths with high optical throughput, while maintaining fast response times (<1 min) due to the small transient gas sample volume. The utility and adaptability of HWGs coupled to FTIR spectrometers operating in the MIR spectral range for various gas sensing applications has extensively been described in literature [20][21][22] . ...
Following the Kyoto protocol, all signatory countries must provide an annual inventory of greenhouse-gas emission including N2O. This fact associated with the wide variety of sources for N2O emissions requires appropriate sensor technologies facilitating in-situ monitoring, compact dimensions, ease of operation, and sufficient sensitivity for addressing such emission scenarios. In this contribution, we therefore describe an innovative portable mid-infrared chemical sensor system for quantifying gaseous N2O via coupling a substrate-integrated hollow waveguide (iHWG) simultaneously serving as highly miniaturized mid-infrared photon conduit and gas cell to a custom-made preconcentrator. N2O was collected onto a solid sorbent material packed into the preconcentrator unit, and then released via thermal desorption into the iHWG-MIR sensor utilizing a compact Fourier transform infrared (FTIR) spectrometer for molecularly selective spectroscopic detection with a limit of detection (LOD) at 5 ppbv. Highlighting the device flexibility in terms of sampling time, flow-rate, and iHWG design facilitates tailoring the developed preconcentrator-iHWG device towards a wide variety of application scenarios ranging from soil and aquatic emission monitoring and drone- or unmanned aerial vehicle (UAV)-mounted monitoring systems to clinical/medical analysis scenarios.
... HWGs are generally described as light pipes made from dielectric materials with a coaxial hollow core coated on the inside wall with IR-reflective metal layers, enabling radiation propagation by reflection. 45 If gaseous samples are injected into the hollow core, the HWG may simultaneously act as a highly miniaturized gas cell providing several meters of optical pathlength yet demanding only minute vapor phase sample volumes (i.e., several hundreds of microliters). In addition, serving as an optical waveguide, radiation is propagated at low attenuation losses, enabling extended absorption path lengths. ...
... Although the combination of compact FT-IR spectrometers with iHWGs provides for a broad accessible spectral window, again the combination with QCLs (and more recently ICLs) facilitates enhanced signal-tonoise ratios (S/N)-and therefore lower limits of detection in tunable diode laser absorption spectroscopy (TDLAS) configurations-along with substantial device miniaturization (34)(35)(36)(37)(38). Early sensor designs based in part on photonic band gap hollow waveguides (39)were used for the detection of volatile organic compounds (VOCs) including CO, CO 2 , CH 4 , C 2 H 4 , xylenes, and NO X species (39)(40)(41)(42)(43)(44), with the latter of particular interest in catalysis research (45). These early designs required elongated hollow fibers for obtaining suitable optical pathlengths facilitating trace gas detection. ...
Mid-infrared (MIR, 3-20 μm) sensor platforms are increasingly adopted in chemical and biological analysis, and they are applied in areas ranging from process monitoring to medical diagnostics. Because of the inherent access to molecule-specific fingerprints via well-pronounced fundamental vibrational, rotational, and rotovibrational transitions, quantitative information at parts-per-million to parts-per-billion concentration levels and beyond is achievable in solids, liquids, and gases. In particular, the combination of quantum cascade lasers (QCLs) with correspondingly tailored waveguide technologies serving as optical transducers - thin-film waveguides for liquid-solid phase analysis, and substrate-integrated hollow waveguides for gaseous samples - facilitates the use of miniaturizable and integrated optical chemical and biological sensors and diagnostics for applications such as exhaled breath analysis, food safety, and environmental monitoring.
... 13,14 In comparison to conventional leaky-mode fiberoptic HWGs, iHWGs are a competitive technology in terms of sensitivity (e.g., for CO and CO 2 ). 14,15 Conventional HWG devices usually rely on tubes drawn from glass, silica or sapphire, and must be mechanically supported along their physical length. The main advantages of the iHWGs therefore include significantly increased mechanical robustness, compact dimensions, and cost-effectiveness during production. ...
Substrate-integrated hollow waveguides (iHWG) represent an innovative generation of photon conduits, which can simultaneously serve as highly miniaturized gas cells with low sample volume. In this communication, we introduce a novel concept for analyzing the performance of catalysts via infrared gas phase analysis based on iHWGs. Due to rapid gas exchange and sample transient times within the iHWG, compositional changes of a continuous gas stream after interaction with a catalyst assembly can be monitored with high time resolution.
... Young et al.[22]used an external cavity QCL (EC-QCL) with a hollow waveguide gas cell for multianalyte sensing over the spectral range of 7.71–8.20 μm with detection limits at the parts per billion (ppb) level. Thompson et al.[23]used an in-house-developed, 2100 μm bore diameter hollow waveguide with an FTIR for spectroscopy of CO and measured an order of magnitude improvement in response time when compared with a 3 m multipass cell. Chen et al.[24]describe a system incorporating a vertical cavity surface emitting laser (VCSEL) (1.68 μm and 2.37 μm) and a hollow waveguide gas cell with a bore diameter of 750 μm. ...
Hollow silica waveguides (HSWs) are used to produce long path length, low-volume gas cells, and are demonstrated with quantum cascade laser spectroscopy. Absorption measurements are made using the intrapulse technique, which allows measurements to be made across a single laser pulse. Simultaneous laser light and gas coupling is achieved through the modification of commercially available gas fittings with low dead volume. Three HSW gas cell configurations with different path lengths and internal diameters are analyzed and compared with a 30 m path length astigmatic Herriott cell. Limit of detection measurements are made for the gas cells using methane at a wavelength 7.82 μm. The lowest limit of detection was provided by HSW with a bore diameter of 1000 μm and a path length of 5 m and was measured to be 0.26 ppm, with a noise equivalent absorbance of 4.1 × 10 − 4 . The long-term stability of the HSW and Herriott cells is compared through analysis of the Allan–Werle variance of data collected over a 24 h period. The response times of the HSW and Herriott cells are measured to be 0.8 s and 36 s, respectively.
... Their main advantages include the high (laser) power thresholds, low insertion losses, robustness, and small beam divergence [6]. Some examples of their usage include the determination of volatile organics in field environments [7], the determination of 12 CO 2 and 13 CO 2 ratios in exhaled mouse breath [8, 9], the quantification of carbon monoxide in sidestream cigarette smoke[10], and as a detector in gas chromatography [11]. However, since any organic molecule absorbs in the mid-infrared region, mixtures of gases frequently reveal highly convoluted and/or overlapping absorption features rendering the direct quantification of selected target analytes difficult [12, 13]. ...
The use of chemometrics in order to improve the molecular selectivity of infrared (IR) spectra has been evaluated using classic least squares (CLS), partial least squares (PLS), science-based calibration (SBC), and multivariate curve resolution-alternate least squares (MCR-ALS) techniques for improving the discriminatory and quantitative performance of infrared hollow waveguide gas sensors. Spectra of mixtures of isobutylene, methane, carbon dioxide, butane, and cyclopropane were recorded, analyzed, and validated for optimizing the prediction of associated concentrations. PLS, CLS, and SBC provided equivalent results in the absence of interferences. After addition of the spectral characteristics of water by humidifying the sample mixtures, CLS and SBC results were similar to those obtained by PLS only if the water spectrum was included in the calibration model. In the presence of an unknown interferant, CLS revealed errors up to six times higher than those obtained by PLS. However, SBC provided similar results compared to PLS by adding a measured noise matrix to the model. Using MCR-ALS provided an excellent estimation of the spectra of the unknown interference. Furthermore, this method also provided a qualitative and quantitative estimation of the components of an unknown set of samples. In summary, using the most suitable chemometrics approach could improve the selectivity and quality of the calibration model derived for a sensor system, and may avoid the need to analyze expensive calibration data sets. The results obtained in the present study demonstrated that (1) if all sample components of the system are known, CLS provides a sufficiently accurate solution; (2) the selection between PLS and SBC methods depends on whether it is easier to measure a calibration data set or a noise matrix; and (3) MCR-ALS appears to be the most suitable method for detecting interferences within a sample. However, the latter approach requires the most extensive calculations and may thus result in limited temporal resolution, if the concentration of a component should be continuously monitored.
