Specialising in custom-designed, precision scientific instruments, built, programmed and calibrated to the most exacting standards. The range includes precision dataloging barographs, with built-in statistical analysis, Barographic Transient Event Recorders and computer-interfaced detectors and sensors for environmental monitoring & process control.
A site dedicated to scientific techniques, experimental methods, & investigative tools for the inventor, researcher and laboratory pioneer. Articles on glassblowing, electronics, metalcasting, magnetic measurements with new material added continually. Check it out! www.drkfs.net
click on any item in the list for its wikipedia entry if available.
Irrespective of the procedure used for sampling, the device needs to be constructed of materials that are transparent to IR light of the appropriate wavelength range. A list of those substances that can be used as windows for infrared sample cells is given table 2.
Sodium chloride is soluble in water and slightly soluble in alcohol so these solvents cannot be used to dissolve the sample. Mechanically sodium chloride is fairly strong and can be easily polished. potassium bromide is similar to sodium chloride but is hygroscopic and also slightly soluble in ether . Calcium fluoride, is insoluble in water and is resistant to many acids and bases and does not fog. It is mechanically strong and, thus, can be used at elevated pressures. Barium chloride is thermally and mechanically susceptible, insoluble in water but soluble in acids and ammonium chloride.
Table 2. Some Materials that are Transparent to Infrared Light
Potassium chloride, having similar properties to sodium chloride, is less soluble but hygroscopic.
Cesium bromide is hygroscopic and soluble in water and acids.
Cesium iodide is hygroscopic and soluble in water and alcohol.
A simple infrared cell can consist of two gaskets on either side of each window that are separated appropriately to provide the required sample volume between them. The sample can be dissolved in an appropriate solvent or, alternatively, the sample may be spread between two IR transparent plates pressed together so the sample exists as a thin film. This technique can only be satisfactorily employed if the sample is relatively involatile. A Spectrum obtained in this manner for hexylamine is shown in figure 12.
This sampling procedure can be very rapid. If a solvent is used, it must be chosen so that the sample is adequately soluble and should be non-polar and have limited groups that will provide IR absorption or interact with the sample. In most cases the Spectrum of the solvent must be subtracted from the Spectrum of the sample solution to obtain the Spectrum of the sample alone. There are three methods of obtaining spectra from solid samples and they are, dispersion in pressed Halide Disks, dispersion in mulls and as films.
Halide Disks Samples
To obtain a sample dispersed in a halide disk, a few milligrams of the sample is mixed with about 150 mg of dry halide powder and ground in a mortar. The mixture is then pressed under vacuum into a disk in an IR disk press.
The pressure employed is usually about 1.6 x 105 kgcm-2. At this pressure the material is sintered and a clear transparent disk should be produced. The most commonly used alkali halide employed for this purpose is potassium bromide, which, as shown in table 2, is transparent in the mid-infra red region.
To prepare a mull, the sample is first ground to a fine powder in a pestle and mortar and then about 50 mg is suspended in about 20 μl of a mulling agent. The suspension is further ground into a smooth paste. One of the more common mulling agents is liquid paraffin sold under the name of Nujol . To obtain suitable spectra the proportions of sample to Nujol may need to be adjusted and the particle size of the sample must be sufficiently small. An example of a the Spectrum of potassium benzoate obtained by subtracting the Spectrum of Nujol from the Spectrum of potassium benzoate plus Nujol is shown in figure 13.
Film samples are particularly useful for the examination of polymers . They can be either formed by deposition from a solvent or by melting the samples and pressing between two plates. The film is then pealed from the plates and supported appropriately in the infrared light path. The film can also be formed on an infrared window using a similar procedure. If the sample melts at an appropriate temperature and remains stable, it can be hot-pressed by means of a hydraulic press.
Vapour and Gas Samples
Gas samples are often of interest in pollution studies and spectra can be obtained from such samples by employing a gas sample cell similar to that shown in figure 14. The cell is gold plated and polished internally to reduce light loss and increase absorption of the infrared light. The windows were constructed from silver chloride (horn silver ) to provide mechanical strength and rigidity, and also to be relatively impervious to water vapour. This type of cell was used in the early gas chromatography and in IR spectroscopy tandem systems. Another type of gas cell that can provide very high sensitivity and, thus, spectra from very small amounts of sample is the light pipe. This device was originally designed to provide on line spectra from peaks eluted from a gas chromatography capillary column where the amount of solute eluted may be less than one microgram.
Light pipes were introduced by Wilks and Brown in 1964  and consist of tubes of circular or rectangular cross-section with highly reflecting internal surfaces that are usually produced by gold plating. A diagram of the optical system of the Perkin Elmer instrument, which may be considered typical for a light pipe gas sampler, is shown in figure 15.
The light source is moved back, so that the focus, which normally coincides with the entrance of the light pipe, is transferred to the exit of the light pipe, but the process is not completely efficient. Internal reflections at the walls of the light pipe results in the 'apparent path length' being increased by about 33%. Many modern GC/IR systems employ Light Pipes to augment the IR signal.
A diagram of a light pipe originally designed by the Perkin Elmer Corporation is shown in figure 16. The light pipe itself is 120 mm long, 1 mm I.D and coated internally with gold. The oven surrounding the light pipe, can be operated up to 350⁄C, and is carefully designed to eliminate any cold spots on the tube, which might allow solute condensation (the elimination of any possibility of sample condensation is extremely important). In normal use for gas sampling, the sample is passed into the inlet tube and out of the exit tube in the normal manner. When used as GC/IR interface the capillary column is led into the interface through a heated tube right up to the actual light pipe. Concentric to the column, and through the same heated tube is fed a stream of scavenging gas that carries the solute through the IR light pipe. This maintains the integrity of the separation at the expense of some solute dilution and consequent slight loss of sensitivity. If the solute bands were not swept out by the scavenging gas, the solute peaks from the column would accumulate in the IR light pipe, and as a consequence, several solutes would be detected and measured simultaneously, and resolution would be lost.
It is claimed that satisfactory spectra can be obtained from a 1-5 ng of material in the light pipe. This claim is supported by the Spectrum shown in Figure 17.
The Spectrum from 10 ng of material has sufficient IR absorption data to allow the identity of the solute to be confirmed by comparison with the library Spectrum shown below the sample Spectrum. Clearly, the system can be used to, either aid in structure elucidation, or confirm compound identity.
About the Author
RAYMOND PETER WILLIAM SCOTT was born on June 20 1924 in Erith, Kent, UK. He studied at the University of London, obtaining his B.Sc. degree in 1946 and his D.Sc. degree in 1960. After spending more than a decade at Benzole Producers, Ltd. Where he became head of the Physical Chemistry Laboratory, he moved to Unilever Research Laboratories as Manager of their Physical Chemistry department. In 1969 he became Director of Physical Chemistry at Hoffmann-La Roche, Nutley, NJ, U.S.A. and subsequently accepted the position of Director of the Applied Research Department at the Perkin-Elmer Corporation, Norwalk, CT, U.S.A.
In 1986 he became an independent consultant and was appointed Visiting Professor at Georgetown
University, Washington, DC, U.S.A. and at Berkbeck College of the University of London; in 1986 he retired but continues to write technical books dealing with various aspects of physical chemistry and physical chemical techniques. Dr. Scott has authored or co-authored over 200 peer reviewed scientific papers and authored, co-authored or edited over thirty books on various aspects of physical and analytical chemistry. Dr. Scott was a founding member of the British chromatography Society and received the American Chemical society Award in chromatography (1977), the M. S. Tswett chromatography Medal (1978), the Tswett chromatography Medal U.S.S.R., (1979), the A. J. P. Martin chromatography Award (1982) and the Royal Society of Chemistry Award in Analysis and Instrumentation (1988).
Dr. Scott’s activities in gas chromatography started at the inception of the technique, inventing the Heat of Combustion Detector (the precursor of the Flame Ionization Detector), pioneered work on high sensitivity detectors, high efficiency columns and presented fundamental treatments of the relationship between the theory and practice of the technique. He established the viability of the moving bed continuous preparative gas chromatography, examined both theoretically and experimentally those factors that controlled dispersion in packed beds and helped establish the gas chromatograph as a process monitoring instrument. Dr. Scott took and active part in the renaissance of liquid chromatography, was involved in the development of high performance liquid chromatography and invented the wire transport detector. He invented the liquid chromatography mass spectrometry transport interface, introduced micro-bore liquid chromatography columns and used them to provide columns of 750,000 theoretical plates and liquid chromatography separations in less than a second. Dr. Scott has always been a “hands-on” scientist with a remarkable record of accomplishments in chromatography ranging from hardware design to the development of fundamental theory. He has never shied away from questioning “conventional wisdom” and his original approach to problems has often produced significant breakthroughs.