Solvents for infrared spectroscopy

With modem sampling techniques, good quantitative infrared analysis with virtually every type of sample is practicable however, liquids are ideal for this purpose, being measured in a liquid cell of fixed thickness, either as 100% sample or diluted with solvent. In this connection it should be taken into account that there are no ideal solvents for infrared spectroscopy [35]. In addition, because absorption bands and path length can be influenced by the temperature of the transmission cell, it is advisable to control the temperature. 

Of the analytical techniques available for process analytical measmements, IR is one of the most versatile, where all physical forms of a sample may be considered - gases, liquids, solids and even mixed phase materials. A wide range of sample interfaces (sampling accessories) have been developed for infrared spectroscopy over the past 20 to 30 years and many of these can be adapted for either near-lme/at-lme production control or on-line process monitoring applications. For continuous on-line measurements applications may be limited to liquids and gases. However, for applications that have human interaction, such as near-line measurements, then all material types can be considered. For continuous measurements sample condition, as it exists within the process, may be an issue and factors such as temperature, pressure, chemical interfer-ants (such as solvents), and particulate matter may need to be addressed. In off-line applications this may be addressed by the way that the sample is handled, but for continuous on-line process applications this has to be accommodated by a sampling system. 

For infrared spectroscopy, this requires proper subtraction of the background moisture and carbon dioxide to get a good approximation of solvent (in the cell). These can then be used to obtain a good approximation of solute A, followed by solute B etc. This process can be done manually by trial and error,or it can be automated for optimal subtraction. The output is a set of n reference spectra. 

Unfortunately it is impossible to estimate these shifts theoretically in any convincing manner, and of course truly reliable interpretations can only be obtained by direct comparison with known spectra of suitable reference compounds containing charged carbon atoms. At the present time infrared spectra of known aliphatic carbonium ions are not available in the literature however, the recent investigations of Baughan and co-workers (141, 142) provided a direct route for obtaining such data. They demonstrated, by cryoscopic and conductivity studies, that stable solutions of aliphatic carbonium ions could be obtained in fused antimony trichloride via ionization of aliphatic chlorides. Conveniently, this solvent is an excellent medium for infrared spectroscopy. In a preliminary study (103) the infrared spectra of the butenyl... 

Certain difficulties arise when using water as a solvent in infrared spectroscopy. The infrared modes of water are very intense and may overlap with the sample modes of interest. This problem may be overcome by substituting water with deuterium oxide (D2O), The infrared modes of D2O occur at different frequencies to those observed for water because of the mass dependence of the vibrational frequency. Table 3.2b lists the characteristic bands observed for both H2O and D2O. 

Figure 8.8. Transparent regions of some common solvents used for infrared spectroscopy. The darkened regions are those in which a 0.1 mm thickness of solvent in an NaCl cell) transmits 30% or less of the incident radiation.

Water, which is usually a poor infrared solvent because it dissolves most cell windows, is an excellent solvent for Raman work because it has few Raman-active frequencies, and these are relatively weak. Very symmetrical, highly Raman-active molecules such as CCI4 are generally poor solvents for Raman spectroscopy. 

The narrower ranges when combined with isotope shifts for and (NO) have been used to distinguish linear and bent nitrosyl complexes, and it was noted that isotope shift differences are more discriminating than isotope frequency ratios. The review also analyses the data for bridging nitrosyl and analyses environmental and solvent effects. Infrared spectroscopy has proved particularly useful for identifying complexes which have structural isomers in the sohd state. For example. 

You should not use certain solvents in the laboratory because of their carcinogenic properties. Benzene, carbon tetrachloride, chloroform, and dioxane are among these solvents. For certain applications, however, notably as solvents for infrared or NMR spectroscopy, there may be no suitable alternative. When it is necessary to use one of these solvents, use safety precautions and refer to the discussions in Techniques 25-28. 

Acetone is infinitely soluble in water and it rapidly absorbs water. Therefore, washing KBr or NaCl plates with acetone will bring water into contact with these salts, which are water soluble. The result will be etching and/or pitting of the plates, making them unusable for infrared spectroscopy. These pressed salt plates should be washed only with solvents that do not contain water. 

Ciampelli and co-workers [2] have developed two methods based on infrared spectroscopy of carbon tetrachloride solutions of polymers at 7.25, 8.65, and 2.32 pm for the analysis of ethylene-propylene copolymers containing greater than 30% propylene. One method can be applied to copolymers soluble in solvents for infrared analysis, the other can be applied to solvent-insoluble polymer films. The absorption band at 7.25 pm due to methyl groups is used in the former case, whereas the ratio of the band at 8.6 pm to the band at 2.32 pm is used in the latter. Infrared spectra of polymers containing 55.5 and 85.5% ethylene are shown in Figure 3.1. 

A suitable extract of the polymer is applied to a thin layer plate and the plate migrated with suitable development solvents to separate the individual polymer additives present. Examination of the plate under an ultraviolet light or the application of spray reagents reveals the separated additives. A second plate is now prepared and the bands of plate which contain the separated additives are removed and each band is separately extracted with a suitable solvent to extract the additives. These extracts are then concentrated and dissolved in suitable spectroscopic solvents for infrared and/or ultra-violet spectroscopy to identify and/or determine each additive. 

Question, What new solvents and sampling techniques are available for infrared spectroscopy ... 

High quahty SAMs of alkyltrichlorosilane derivatives are not simple to produce, mainly because of the need to carefully control the amount of water in solution (126,143,144). Whereas incomplete monolayers are formed in the absence of water (127,128), excess water results in facile polymerization in solution and polysiloxane deposition of the surface (133). Extraction of surface moisture, followed by OTS hydrolysis and subsequent surface adsorption, may be the mechanism of SAM formation (145). A moisture quantity of 0.15 mg/100 mL solvent has been suggested as the optimum condition for the formation of closely packed monolayers. X-ray photoelectron spectroscopy (xps) studies confirm the complete surface reaction of the —SiCl groups, upon the formation of a complete SAM (146). Infrared spectroscopy has been used to provide direct evidence for the hiU hydrolysis of methylchlorosilanes to methylsdanoles at the soHd/gas interface, by surface water on a hydrated siUca (147). 

The acetyl content of cellulose acetate may be calculated by difference from the hydroxyl content, which is usually determined by carbanilation of the ester hydroxy groups in pyridine solvent with phenyl isocyanate [103-71-9J, followed by measurement of uv absorption of the combined carbanilate. Methods for determining cellulose ester hydroxyl content by near-infrared spectroscopy (111) and acid content by nmr spectroscopy (112) and pyrolysis gas chromatography (113) have been reported. 

A critical study has been carried out in order to evaluate the capabilities of Near Infrared spectroscopy for the analysis of commercial pesticide formulations using transmittance measurements. In this sense, it has been evaluated the determination of active ingredients in agrochemical formulations after extraction with an appropriate solvent. 

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