The Advantages and Disadvantages of Infrared Spectroscopy

The advantages and disadvantages of infrared spectroscopy as a chemical analysis technique are listed in Table 1.1. These attributes are for the technique generally, regardless of the type or brand of infrared spectrometer being used to measure a spectrum. 

FIGURE 1.6 The infrared spectrum of liquid water. Note the broad peaks around 3500 and 1600 cm .  

By subtracting the spectrum of pure water from the spectrum of a mixture of soap and water, the water peaks can be removed making the soap peaks much easier to see. Spectral subtraction is discussed in detail in Chapter 3. 

One last way of dealing with mixtures is library searching. This technique uses a computer to automate and speed the spectral comparison process. Thousands of spectra can be quickly compared to a mixture spectrum, which will hopefully yield one or more close matches. These matches can then be used to help identify the components in a mixture. Some software packages allow the library spectrum to be subtracted from the unknown spectrum and then the subtraction result can be used in a subsequent search in a technique called subtract and search again. Library searching and subtract aud search again are discussed in detail in Chapter 3. 

Another issue with liquid water is that it dissolves some of the materials used in IR sample preparation. Materials such as KBr and NaCl are transparent in the mid-infrared and are used to make windows and cells to hold infrared samples. These materials are highly water soluble, aud any liquid water present in a sample will damage these cells or windows. There are infrared transparent materials that are not water soluble that can be used (see Chapter 4), but they tend to be more expensive than KBr and NaCl. 

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Recall from above that an FTIR is a specific type of infrared spectrometer. Thus, FTIRs have advantages and disadvantages compared to other types of infrared instruments above and beyond the advantages and disadvantages of infrared spectroscopy generally, which was discussed above. 

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