Gas Chromatography - Tandem Techniques - The Combination of the Gas Chromatograph with Other Spectroscopic Systems > The Characteristics of UV Absorption > Page 15



The different excitation frequencies can be used in various types of analytical applications some of which are broadly summarized in table 1.


The bands given in table 1, for the most part, comprise all of the analytically useful bands in the ultra violet-visible region. The energy associated with a specific transition, theoretically, should produce a single sharp absorption band.


Table 1 Application Areas for Absorption at Different UV Wavelengths


Adsorption Band- Substance Analytical Application
180-250 nm bands of most aromatic hydrocarbons Excellent for trace analysis and characterization
160-180 nm bands of single olefins Good for trace analysis with an appropriate spectrophotometer
250 nm bands of benzene Reasonable for trace analysis good for characterization
260-290 nm bands of saturated aldehydes and ketones Poor for trace analysis but suitable for characterization
420-520 Useless for most analytical work


However, there may be many similar transitions taking place and each can be perturbed by vibrational energy sources possessed by different bonds and thus produce a combination of individual bands that form an envelope instead of a single sharp absorption peak. In the extreme, a series of transitions may appear as a 'smeared adsorption band right across the UV range, in which case the spectrum is of little use for analytical work. If some fine structure exists then comparison with reference spectra can sometimes result in confirmation of substance identity. Examples of two types of UV spectra are shown in figure 10.