Infrared Spectra, 571, Other Substances

Fingerprint region (Section 13 20) The region 1400-625 cm of an infrared spectrum This region is less character istic of functional groups than others but varies so much from one molecule to another that it can be used to deter mine whether two substances are identical or not Fischer esterification (Sections 15 8 and 19 14) Acid cat alyzed ester formation between an alcohol and a carboxylic acid... 

In addition to transfer techniques in quantitative analyses, such as polarography, titrimetry, spectroscopy and other analytical methods used after separation by TLC, the in situ optical measurement is the most widely employed technique for quantitative determinations. In most cases UV-absorption is used, while coloured substances can be determined by absorption measurement in the visible range of the spectrum. Fluorescent substances are preferably determined by fluorescence measurement. Infrared absorption techniques are not used in routine analysis up to this date. 

Applications of Quantitative Infrared Spectroscopy Infrared spectrophotometry offers the potential for determining an unusually large number of substances because nearly all molecular species absorb in the infrared region. Moreover, the uniqueness of an infrared spectrum provides a degree of specificity that is matched or exceeded by relatively few other analytical methods. This specificity has particular application to the analysis of mixtures of closely related organic compounds. 

The absorption bands of water vapor and carbon dioxide are always present in the infrared spectrum of the expired breath because both these substances strongly absorb infrared radiation. On the other hand, oxygen, nitrogen, and the inert gases do not absorb infrared radiation and therefore cannot be detected— an advantage for the methods to be discussed. 

Most molecules have many more vibrational modes than the ones shown in this section for a single CH2 group. Some involve relatively simple structural units, others a substantial fraction of the atoms in a molecule. Thus the infrared spectrum of each compound is unique, and superimposability of their IR spectra is convincing evidence that two substances are the same. 

Temperature can be measured from heat transfer by conduction, convection, or radiation. Household thermometers use either the expansion of metals or other substances or the increase in resistance with temperature. Thermocouples measure the electromotive force generated by temperature difference. Pyrometers measure infrared radiation from a heat source. Spectroscopic thermometry compares the spectrum of radiation against a blackbody spectrum. Temperature-sensitive paints and liquid crystals change intensity of radiation in certain wavelengths with temperature. 

In addition to these problems in applying the single-band method to quantitative infrared spectroscopic analysis, the single-band method is not suitable for determining the molar ratios of two or more substances existing in a sample. The single-band method, which depends only on the selected key band, does not utilize all the other bands in the observed infrared spectrum. Thus, it is reasonable to seek an alternative method that makes the optimum use of an entire infrared absorption spectrum for quantitative analysis. 

In contrast to infrared spectrometry there is no decrease in relative sensitivity in the lower energy region of the spectrum, and since no solvent is required, no part of the spectrum contains solvent absorptions. Oil samples contaminated with sand, sediment, and other solid substances have been analysed directly, after being placed between 0.5 mm 23-reflection crystals. Crude oils, which were relatively uncontaminated and needed less sensitivity, were smeared on a 2 mm 5-reflection crystal. The technique has been used to differentiate between crude oils from natural marine seepage, and accidental leaks from a drilling platform. The technique overcomes some of the faults of infrared spectroscopy, but is still affected by weathering and contamination of samples by other organic matter. The absorption bands shown in Table 9.1 are important in petroleum product identification. 

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