Infrared cell, filling

The construction of two typical infrared cells is shown in Fig. 19.6. Such cells have to be carefully filled by using a syringe or Pasteur pipette to ensure that no air is trapped inside. To prevent evaporation the ports should be plugged with small plastic stoppers once the cell has been filled with the solution. 

The various combustion methods differ primarily in the method of measuring the carbon dioxide generated from the organic carbon. The first really sensitive carbon dioxide detector and the one still most used is the non-dispersive infrared gas analyser. The detecting element senses the difference in absorption of infrared energy between a standard cell filled with a gas with no absorption in the infrared, and a sample cell. Water vapour is the only serious interference, hence the carbon dioxide must be dried before any measurements are made. 

Three large drops of solution will fill the usual sealed infrared cell (Fig. 6). A10% solution of a liquid sample can be approximated by dilution of one drop of the liquid sample with nine of the solvent. Since weights are more difficult to estimate, solid-samples should be weighed to obtain a 10% solution. 

The infrared cell is filled by inclining it slightly and placing about three drops of the solution in the lower hypodermic port with a capillary dropper. The liquid can be seen rising between the salt plates through the window. In the most common sealed cell, the salt plates are spaced 0.1 mm apart. Make sure that the cell is filled past the window and that no air bubbles are present. Then place the Teflon stopper lightly but firmly in the hypodermic port. Be particularly careful not to spill any of the sample on the outside of the cell windows. 

Fig. 1 shows the infrared spectrum of halothane (Ayerst Laboratories Inc. Batch No. 1CKB). The spectrum is that of undiluted halothane in a 0.104 mm. potassium bromide cell vs. a potassium bromide plate. Also, because some of the absorption bands are quite intense, Fig. 1 shows the spectrum of a 4.0 volume percent solution of halothane in carbon disulfide, in a 0.104 mm. potassium bromide cell, vs. a 0.1 mm. cell filled with carbon disulfide. A Beckman Model IR-12 instrument was used. Considering the variety of sample handling techniques used, this spectrum and other published spectra (1-3) are the same. 

Consult your instructor on the proper operation of your instrument. Handle the infrared cell carefully, avoiding contact with water and the fingers. Fill the cell with pure m-xylene and obtain a spectrum on this from 2 to 15 p,m, being sure to record the last peak just before 15 pm (692 cm" )- Each time you run a sample, be sure to check 0% T by placing a card in the sample beam and adjust the pen to 0% T. Empty the cell, rinse and fill with p-xylene, and run a spectrum on this. Repeat for o-xylene. Run spectra on each of the standard mixtures. From the spectra of the pure substances, choose a peak of each isomer to measure. Using the baseline method (see Figure 16.11), measure PqIP for the peak for each compound. Prepare a calibration curve of the ratio of og PJP) J og PolP ri,o and of log(Po/P)para/ log(Po/E)ortho versus concentration for the meta and para isomers, respectively. See Chapter 20 and your CD for spreadsheet preparation using an internal standard. 

The detection system typically consists of a lens to focus the fluorescence, an interference filter to isolate the transition of interest, and an infrared detector element cooled in a dewar. A filter cell filled with a quenching gas is sometimes interposed between the fluorescence cell and the detector to selectively block the A(1)- A(0) fluorescence while passing, for example, the A(2)— A(1) fluorescence. Although any one of several... 

The idea of the experiment is illustrated in Fig.3. The upper part of the figure shows a simple cell filled with H2O. A slow gas flow is maintained at a constant pressure of about 40mTorr. The cell is irradiated by three different laser pulses, which follow each other within less than 50nsec. First an infrared laser at 2.7iJm is fired to vibrationally excite H2O and to prepare a well defined rotational state in the asymmetric stretch mode. The second laser., an excimer laser at 193nm, dissociates the H2O and the third laser analyses the formation of the OH products by LIF. The infrared excitation of H2O is monitored by the photoacoustic method with the microphone shown in the cell. At the short delays and the low pressures the products are analysed collision free. 

The background spectrum can be run on an evacuated cell or a cell filled with an infrared transparent gas such as nidogen. 

The primary reference method used for measuring carbon monoxide in the United States is based on nondispersive infrared (NDIR) photometry (1, 2). The principle involved is the preferential absorption of infrared radiation by carbon monoxide. Figure 14-1 is a schematic representation of an NDIR analyzer. The analyzer has a hot filament source of infrared radiation, a chopper, a sample cell, reference cell, and a detector. The reference cell is filled with a non-infrared-absorbing gas, and the sample cell is continuously flushed with ambient air containing an unknown amount of CO. The detector cell is divided into two compartments by a flexible membrane, with each compartment filled with CO. Movement of the membrane causes a change in electrical capacitance in a control circuit whose signal is processed and fed to a recorder. 

In the gas correlation techniques, gas-filled cells mounted on a rotating disk cross the analyzing infrared beam in turn. One correlation cell is filled with a gas that will not absorb infrared light, such as nitrogen (N2). The other cell (or cells) are filled with a high concentration of the gas to be measured. The wavelength range is selected at the absorption band of the gas to be measured by an optical band-pass filter. 

The unique appearance of an infrared spectrum has resulted in the extensive use of infrared spectrometry to characterize such materials as natural products, polymers, detergents, lubricants, fats and resins. It is of particular value to the petroleum and polymer industries, to drug manufacturers and to producers of organic chemicals. Quantitative applications include the quality control of additives in fuel and lubricant blends and to assess the extent of chemical changes in various products due to ageing and use. Non-dispersive infrared analysers are used to monitor gas streams in industrial processes and atmospheric pollution. The instruments are generally portable and robust, consisting only of a radiation source, reference and sample cells and a detector filled with the gas which is to be monitored. 

Uses. Thallium compounds have limited use in industrial applications. The use of thallous sulfate in rodenticides and insecticides has been replaced by other compounds less harm fill to animals (see Insect control technology Pesticides). Thallium sulfide has been used in photoelectric cells (see Photovoltaic cells). A thallium bromide—thallium iodide mixture is used to transmit infrared radiation for signal systems. Thallous oxide is used in the manufacture of glass (qv) that has a high coefficient of refraction. Thallium formate—malonate aqueous solutions (Cletici s solution) have been used in mineral separations. Many thallium compounds have been used as reagents in oiganic synthesis in research laboratories. 

For each series of measurements about 50 g of solvent was transferred quantitatively in the dry box to the cell by pouring it into the dilution bulb this was the minimum amount required to fill the cell bulb. The cell was removed from the dry box, placed in the oil bath, and connected to the bridge. Time was allowed for the attainment of thermal equilibrium then at least three resistance measurements were made at five-min intervals, and the average value was calculated. The cell was removed from the bath and returned to the dry box. Dilute stock solution was quantitatively added to the cell by means of a weighing buret. The contents of the cell were carefully mixed, and the resistance of the solution was measured as before. The procedure just described was repeated several times with the dilute stock solution and then with the concentrated stock solution. About ten concentrations with a hundredfold range were obtained. A portion of the final solution in the cell (the most concentrated solution) was removed, and the infrared spectrum taken no absorption band indicative of traces of water was observed at 3600 cm-1. It was necessary to obtain the densities of... 

Commercial cylindrical quartz cells can be adapted for gas-phase work as illustrated in Fig. 9.18. Such a cell finds use in the near infrared for the determination of overtone vibrational frequencies, and also in visible and ultraviolet spectroscopy. A much less expensive cell which is adequate for most gases may be constructed from Pyrex along the lines of the cell shown in Fig. 9.18. Quartz windows may then be attached by epoxy resin. A cell which is filled from a conventional vacuum line will generally contain mercury vapor which absorbs at 2537 A. Once the origin of this absorption is recognized, it causes little difficulty because of its narrow bandwidth. 

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