IR gas cells

In this IR sampling technique, a thermogravimetric (TG) analyzer is interfaced to an IR spectrophotometer so that the evolved gas from the sample/TG furnace is directed to an IR gas cell. This IR sampling technique lends itself to the identification and quantitation of residual solvent content for a pharmaceutical solid [17], and also to the investigation of pharmaceutical pseudopolymorphs. 

Fig. 7 Thermogravimetric weight loss curve (A) and subsequent IR spectra measured at the designated temperatures (B) 70°C, (C) 95°C, (D) 120°C, (E) 160°C, and (F) 180°C. A slight lag time exists between the TG weight loss and IR spectral acquisition due to the evolved gas being carried into the IR gas cell by the He carrier gas. Each IR spectrum is plotted on the same absolute intensity scale (Abs. units).

As each component exits the chromatographic column, it is channeled into an infrared (IR) gas cell and the component s IR spectrum obtained. A thermal conductivity detector (TCD) (see Chapter 13) can be used to determine when a component is emerging from the column. The TCD detector does not destroy the sample and none of the common gases used in GC have IR spectra, and thus do not interfere with the spectrum of eluting components. Half-peak height is a common time to obtain the spectrum of that component and the setup for detection and obtaining the spectrum can thus be automated [6,13],... 

Another approach that has been reported is GC linked to IR spectroscopy linked to MS. The gas phase components from a GC analysis can easily be directed into an IR gas cell for analysis. Note that the carrier gases used in GC will not generally interfere with or be detected by IR spectroscopy and thus... 

Figure 16.28 Hyphenated TGA-FTIR, showing the heated transfer line and heated IR gas cell. (Courtesy of Netzsch Instruments, Inc., Paoli, PA.)...

The two IR interfaces in common use are the light pipe [8] and so-called matrix isolation [9]. In the former method, the column effluent is passed through a heated IR gas cell (light pipe), and in the latter, it is condensed and frozen into a matrix suitable for analysis by IR [10]. 

In this method, gas was extracted from the bottle using a gas-tight syringe and injected into a pre-vacuumed infrared (IR) gas cell. The spectrum obtained was then analysed according the standard pattern for hydrogen sulphide (Figure 63). 

Co concentration was determined by spectrophotometer (Varian Cary 500) at 692 nm wave length, with the sample diluted with a 9 mol/L concentrated HCl solution. NO content in gas phase was obtained by an on-line Fourier transform infrared spectrometer (Nicolet E.S.P. 460 FT-IR) equipped with a gas cell and a quantitative package, Quant Pad. 

The use of IR pulse technique was reported for the first time around the year 2000 in order to study a catalytic reaction by transient mode [126-131], A little amount of reactant can be quickly added on the continuous flow using an injection loop and then introduce a transient perturbation to the system. Figure 4.10 illustrates the experimental system used for transient pulse reaction. It generally consists in (1) the gas flow system with mass flow controllers, (2) the six-ports valve with the injection loop, (3) the in situ IR reactor cell with self-supporting catalyst wafer, (4) the analysis section with a FTIR spectrometer for recording spectra of adsorbed species and (5) a quadruple MS for the gas analysis of reactants and products. 

The soil atmosphere can also be passed into a long-path IR gas sampling cell. Because of the long path length, gases that might otherwise not be detectable are measured and identified. This should not be confused with IR used in remote sensing, which is covered in Section 14.14. 

Ehnholt et al.8 produced a broad paper covering raw materials, and in-process and final-product measurements. While the uses are primarily in the food industry, the rancidity was often caused by microorganisms. One case involved off-flavor materials being produced in drying and curing ovens. Marker compounds (concomitant) released during the breakdown process (of saturated and unsaturated compounds) were nonenal, decenal, and octenone for the unsaturated aldehydes and ketones, and nonanal, decanal, and octanone for the saturated molecules. A 10-m folded path gas cell was used with an FT-IR for measurements down to 1 Lig/m3. 

If the sample is in the gas or vapor phase and is measured either at high or moderate concentrations then either IR or NIR can be used (Raman is generally not applicable). However, if a trace gas apphcation is involved then IR is traditionally more applicable. Gas cells with extended path lengths up to 20m are readily available for measurements in the ppb levels. 

Infrared spectra may be obtained for gases, liquids, or solids. The spectra of gases or low-boiling liquids may be obtained by expansion of the sample into an evacuated cell. Gas cells are available in lengths of a few centimeters to 40 m. The sampling area of a standard IR spectrophotometer will not accommodate cells much longer than 10 cm long paths are achieved by multiple reflection optics. 

Cells with a minimum optical path of a few centimetres are used for gay analysis (Fig. 10.17). If the absorbance under these conditions is too weak, a cell using mirrors to increase the path length can be used. Cells with an optical path of several metres represent a complex and cluttered arrangement. Filliform gas cells with a volume of a few tens of microlitres (1 = 10 cm with diameter < 1 mm) are used for the hyphenated technique of GC-IR. These cells use reflecting side walls. 

Figure 10.17—Cells used for mid IR spectroscopy, a) Gas cell with direct optical path (reproduced with permission of Wilmad) b) expanded view of a liquid cell with a fixed path-length (from Eurolabo document).

As CO is highly toxic, and causes respiratory problems, its elimination is quite important Industrially, CO elimination is required for C02 laser application, and the production of pure hydrogen for fuel cells. Catalysis testing of the reaction has been performed, independently, with an IR gas-sensor by Hoffmann et al. and with a combustible CO gas sensor by Yamada et al. [25, 26]. In both groups, C02 produced by CO oxidation was quantified by an IR gas-sensor. 

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]...

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