Multipass gas cell

The flow response of a gas cell is an important parameter for sensing applications as it defines the actual response time of the system. The theoretical aspects of determining the time constants from the decay component of the response curve are straightforward. The time constants of the gas cell are calculated from the concentration decay curves (Fig. 6). For example, the time constants for CO are 1.23 and 12.4 s for the HWG and the multipass gas cell,... 

Fig. 6 Flow response decay curves for FT-IR spectroscopy with a F1WG gas cell (dotted line) and FT-IR spectroscopy with a 3-m multipass gas cell (dashed line) [43]...

A comparison of the IR spectra of sidestream cigarette smoke with the 3-m multipass gas cell and the 55-cm HWG is shown in Fig. 16. 

Fig. 16 IR spectra of sidestream cigarette smoke recorded in a a HWG cell and b a 3-m multipass gas cell [65]...

The quantitative determination of small concentrations of organic solvents in a working place atmosphere by IR spectroscopy has been reported by Schacke et al. (1972). The measurements were carried out using a multipass gas cell with a maximum pathlength of 10 m. Such cells allow the determination of compounds in the ppm or even ppb region. 

Spectra of compounds with vapor pressure down to 0.001 Torr can be obtained using multipass gas cells. This technique has proved very useful for the detection of atmospheric impurities or trace components in waste and combustion gases. 

Another important application of infrared gas analysis is for trace analysis, such as for the analysis of a dilute mixture (in the ppm range) or an environmental specimen. In such cases, individual analytes are measured from the high ppb levels to the lO s or lOO s of ppm. For such analyses, extended path lengths are required, and typically multipass gas cells from 1 to 20 m in path length are used. One very specific application is an open-path measurement for ambient air monitoring in manufacturing plants or in toxic waste sites in which no cell is used. Instead, a source and interferometer combination are focused on a remote detection system with the aid of special telescope optics. In such cases, several hundreds of meters of effective path length are used. 

Figure 11.2. Position of the images on the field mirror of a multipass gas cell. (Reproduced from [6], by permission of Academic Press copyright 1979.)...

Most extractive measurements are made with the use of multipass gas cells with a pathlength of 20 or 40 m. The construction of these cells was described in Section 11.2. Some of these cells are reasonably small and may be mounted in the sample compartment of the spectrometer. Typical detection limits are on the order of 100 parts per billion by volume (ppbv). 

Fig. 7 Wavenumber versus sensitivity per nominal pathlength for a 3-m multipass and b HWG gas cell...

Figure 18 shows the real-time traces of the CO concentration for smoke runs of the reference cigarette IM16 using the 3-m multipass and HWG gas cells. Evident in both plots are the breaks between the three cigarettes smoked during the run and the minute but noticeable increases in concentration due to puffing. The improvement of the temporal resolution due to the HWG is... 

Fig. 17 Carbon monoxide concentration traces for IM16 reference cigarette smoke runs a with 3-m multipass and b HWG gas cells (dashed line nondispersive IR (NDIR) analyzer, solid line FT-IR spectrometer using partial least-squares calibrations) [43]...

The above standard mixtures contained in cylinders are supplemented by several gas measurement facilities which can provide dynamic calibrations of gas mixtures and of gas monitoring instruments. These include an on-line facility which injects gas dynamically into a passivated multipass optical gas cell, where the gas concentration is certified spectroscopically. Some of the gas mixtures which can be certified by these dynamic blending facilities are given in Table 2. 

A sketch of a gas cell with multipass optics in the so-called White arrangement is given in Fig. 6.2. It comprises three spherical mirrors, which can be adjusted for the desired number of passes. Four passes (as shown in Fig. 6.2(a)) is the minimum number of passes in such an arrangement. The achievable maximum number of passes depends on the reflectivity of the mirrors and on the quality of the incoming beam. The latter is partly defined by the size of the IR source. Commer-... 

Figure 1 Examples of gas cells for mid-infrared transmission measurements (A) photograph of a multiple-pass gas cell ( Long Path Miniceir), with a high path-to-volume (530 ml) ratio. Allows paths from 1.2 m (eight passes) to 7.2 m (48 passes) (B) schematic of a multipass cell with transfer optics for use in a center-focus sample compartment (C) and (D) photographs of Pyrex and stainless steel bodied 10-cm pathlength cells, respectively. ((A and B) Reproduced by kind permission of Infrared Analysis, Inc., Anaheim CA, USA. (C and D) Reproduced by kind permission of Specac Ltd., Orpington, Kent, UK.)...

The key component of an LOAS instrument is the spectrophone. Its construction depends critically on the state of aggregation of the sample. The SP for gas samples consists of a gas cell with input and output windows for laser radiation and a microphone, fixed on an inside or outside wall of the cell. As in the case of MAS with SDLs. the sensitivity can be improved by the use of multipass SPs. Plane capacitor or electronic microphones are the most widely used. The sensitive element of these microphones is a thin (1-10 pm) elastic membrane made of Mylar. Teflon, or metallic foil. The membrane serves as one electrode of a dielectric capacitor (membranes made of dielectric materials have metallic coatings). Thus, the acoustic vibrations of a gas mixture in the cell can be directly converted to an electrical signal. Commercial microphones with sensitivities of 5-50mV/Pa are utilized in routine applications of LOAS specially constructed and optimized ones are used for ultrasensitive analyses. 

Experimental limitations initially limited the types of molecular systems that could be studied by TRIR spectroscopy. The main obstacles were the lack of readily tunable intense IR sources and sensitive fast IR detectors. Early TRIR work focused on gas phase studies because long pathlengths and/or multipass cells could be used without interference from solvent IR bands. Pimentel and co-workers first developed a rapid scan dispersive IR spectrometer (using a carbon arc broadband IR source) with time and spectral resolution on the order of 10 ps and 1 cm , respectively, and reported the gas phase IR spectra of a number of fundamental organic intermediates (e.g., CH3, CD3, and Cp2). Subsequent gas phase approaches with improved time and spectral resolution took advantage of pulsed IR sources. 

In the third stage the 708 nm light is converted to 6.02 pm via third Stokes Raman shift in a high pressure hydrogen cell. The 0.5 mJ, 7 ns long pulses are injected into a multipass cavity inside the gas target in order to effectively illuminate the muon stop region. 

Aim multipass cell for i.r. spectroscopy, utilizing two parallel concave mirrors, has allowed path lengths up to 150 m to be achieved. For a study of collision-induced simultaneous transitions in binary gas mixtures, a 2m sample cell has been constructed that allows pressure variation up to 1500 bar. A cell has been designed for pressures up to lOKbar and temperature variation over the range 10—300 A Pfund-type cell has been constructed for i.r. spectroscopy with... 

Unfortunately, experimental limitations have severely restricted types of molecular systems that could be studied by TRIR spectroscopy in the past. The main obstacles have been the lack of readily tunable intense IR sources and sensitive fast IR detectors. Early TRIR work focused on gas phase studies because long path lengths and/or multipass cells [3] could be used without inter-... 

---