Infrared spectroscopy sampling techniques

Culler, S. R., Diffuse Reflectance Infrared Spectroscopy Sampling Techniques for Qualita-tive/Quantitative Analysis of Solids. In Polymorphism in Pharmaceutical Solids Brittain, H. G., Ed. Marcel Dekker New York 1999, pp. 93. 

Eilert, A.J. 8t Wetzel, D.J., Optics and Sample Handling for Near-Infrared Diffuse Reflection. In Chalmers, J.M. 8c Griffiths, P.R. (eds) Handbook of Vibrational Spectroscopy, Sampling Techniques, Volume 2 John Wiley 8c Sons Chichester, 2002 pp. 1162-1174. 

Simpson was a Bathurst Research Student from 1934 to 1936, following which, with a Travelling Scholarship, she spent 1937 to 1939 undertaking research at the University of Vienna. Her initial research had been in the field of optical rotary dispersion and circular dichroism, but she discovered that she found spectroscopy much more interesting. Before the Second World War, she studied the spectra of small molecules in the vacuum ultraviolet but with the onset of war, she shifted to infrared spectroscopy. This technique was used to analyse samples of enemy fuels for the Air Ministry, in particular to identify the use of synthetic oils in place of natural oil, which, because of the naval blockades, was in very short supply in Germany. 

A. Lee Smith,. Applied Infrared Spectroscopy Fundamentals, Techniques, and Analytical Problem-Solving. New York Wiley, 1979. Comprehensive treatment of IR spectroscopy. Includes history, instrumentation, sampling techniques, qualitative and quantitative applications. 

Infrared Spectroscopy. The technique of infrared spectroscopy is commonly used in the controlled substance and the trace evidence sections of the crime laboratory. This technique can identify illicit drugs present in unknown samples, the type of fiber found at a crime scene or on a person, the polymer present in a paint chip, or the organic compounds present in explosive residues. The evidence is prepared for analysis in several ways, depending on the type of sample. 

Although samples can be rapidly analyzed using infrared spectroscopy, the technique works best for relatively pure samples. If impurities are present in the sample and they also absorb infrared radiation, the resulting spectrum contains contributions from both the sample and the impurities. This can complicate interpretation of the spectrum and subsequent identification of the sample. 

Culler, S. R. (1993) Diffuse reflectance spectroscopy Sampling techniques for qualitative/ quantitative analysis of solids. In Practical Sampling Techniques Techniques for Infrared Aruilysis (P. B. Coleman, Ed.), pp. 93-105. CRC Press, Boca Raton, FL. 

An example of direct examination is Ae examination of the polymer film by infrared or ultra-violet spectroscopy or of Aicker sections of polymer by attenuated total reflectance (ATR) infrared spectroscopy. Such techniques have severe limitations in that, because the additive is in effect heavily diluted with polymer, detection limits are usually well above the low concentration of additive present and Ais method is only applicable if the additive has distinct sharp absorption bands in regions where the polymer itself shows little or no absorption. In-situ spectroscopic techniques are not likely to be of value, then, in the analysis of samples of unknown composition. If known amounts of additive can be incorporated into additive-free polymer, however, these techniques are likely to be extremely useful in Ae study of solvent extraction procedures, and the study of additive ageing processes (ie. the effects of heat, light, sterilization, radiation, etc.), since the rate of disappearance of or decay can be measured directly by the decrease in absorbance of Ae sample at a suitable wave-lengA. 

In many cases, the properties which make polymers attractive may actually make sampling difficult. For example, thermoplastics cannot easily be ground to form a powder for use in infrared, dispersive sampling techniques and many polymers exhibit fluorescence themselves (or the substances introduced to them are fluorescent) which can result in problems when attempting to obtain a Raman spectrum. Raman spectroscopy has two great advantages in that samples often need little, if any, preparation and samples of varying shapes and sizes can be examined. 

Leyden, D. E. Shreedhara Murthy, R. S. Surface-Selective Sampling Techniques in Pourier Transform Infrared Spectroscopy, Spectroscopy 1987, 2(2), 28-36. 

Porro, T. J. Pattacini, S. C. Sample Handling for Mid-Infrared Spectroscopy, Part 11 Specialized Techniques, Spectroscopy 1993,8(8), 39-44. 

Infrared spectra of fats and oils are similar regardless of their composition. The principal absorption seen is the carbonyl stretching peak which is virtually identical for all triglyceride oils. The most common appHcation of infrared spectroscopy is the determination of trans fatty acids occurring in a partially hydrogenated fat (58,59). Absorption at 965 - 975 cm is unique to the trans functionaHty. Near infrared spectroscopy has been utilized for simultaneous quantitation of fat, protein, and moisture in grain samples (60). The technique has also been reported to be useful for instmmental determination of iodine value (61). 

Ideally, a mass spectmm contains a molecular ion, corresponding to the molecular mass of the analyte, as well as stmcturaHy significant fragment ions which allow either the direct deterrnination of stmcture or a comparison to Hbraries of spectra of known compounds. Mass spectrometry (ms) is unique in its abiUty to determine direcdy the molecular mass of a sample. Other techniques such as nuclear magnetic resonance (nmr) and infrared spectroscopy give stmctural information from which the molecular mass may be inferred (see Infrared technology and raman spectroscopy Magnetic spin resonance). 

A variety of instmmental techniques may be used to determine mineral content. Typically the coal sample is prepared by low temperature ashing to remove the organic material. Then one or more of the techniques of x-ray diffraction, infrared spectroscopy, differential thermal analysis, electron microscopy, and petrographic analysis may be employed (7). 

There is supporting evidence in the literature for the validity of this method two cases in particular substantiate it. In one, tests were made on plastics heated in the pressure of air. Differential infrared spectroscopy was used to determine the chemical changes at three temperatures, in the functional groups of a TP acrylonitrile, and a variety of TS phenolic plastics. The technique uses a film of un-aged plastic in the reference beam and the aged sample in the sample beam. Thus, the difference between the reference and the aged sample is a measure of the chemical changes. 

Several additional instrumental techniques have also been developed for bacterial characterization. Capillary electrophoresis of bacteria, which requires little sample preparation,42 is possible because most bacteria act as colloidal particles in suspension and can be separated by their electrical charge. Capillary electrophoresis provides information that may be useful for identification. Flow cytometry also can be used to identify and separate individual cells in a mixture.11,42 Infrared spectroscopy has been used to characterize bacteria caught on transparent filters.113 Fourier-transform infrared (FTIR) spectroscopy, with linear discriminant analysis and artificial neural networks, has been adapted for identifying foodbome bacteria25,113 and pathogenic bacteria in the blood.5... 

Capillary electrophoresis has also been combined with other analytical methods like mass spectrometry, NMR, Raman, and infrared spectroscopy in order to combine the separation speed, high resolving power and minimum sample consumption of capillary electrophoresis with the selectivity and structural information provided by the other techniques [6]. 

The carbonyl index is not a standard technique, but is a widely used convenient measurement for comparing the relative extent and rate of oxidation in series of related polymer samples. The carbonyl index is determined using mid-infrared spectroscopy. The method is based on determining the absorbance ratio of a carbonyl (vC = 0) band generated as a consequence of oxidation normalised normally to the intensity of an absorption band in the polymer spectrum that is invariant with respect to polymer oxidation. (In an analogous manner, a hydroxyl index may be determined from a determination of the absorbance intensity of a vOH band normalised against an absorbance band that is invariant to the extent of oxidation.) In the text following, two examples of multi-technique studies of polymer oxidation will be discussed briefly each includes a measure of a carbonyl index. 

A comprehensive review of compositional and failure analysis of polymers, which includes many further examples of analysis of contaminants, inclusions, chemical attack, degradation, etc., was published in 2000 [2], It includes details on methodologies, sampling, and sample preparation, and microscopy, infrared spectroscopy, and thermal analysis techniques. 

Another technique that has been employed for studying certain types of changes in solids is infrared spectroscopy, in which the sample is contained in a cell that can be heated. By monitoring the infrared spectrum at several temperatures, it is possible to follow changes in bonding modes as the sample is heated. This technique is useful for observing phase transitions and isomerizations. When used in combination, techniques such as TGA, DSC, and variable-temperature spectroscopy make it possible to learn a great deal about dynamic processes in solids. 

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