Ultraviolet and visible spectrometers have been in general use for the last 35 years and over this period have become the most important analytical instrument in the modern-day laboratory. In many applications, other techniques could be employed but none rival UV-Visible spectrometry for its simplicity, versatility, speed, accuracy and cost-effectiveness. This description outlines the basic principles for those new to UV-Visible spectrometry.
Basic UV-Vis Theory, Concepts and Applications
The radiation from normal hot solids is made up of many wavelengths and the energy emitted at any particular wavelength depends largely on the temperature of the solid and is predictable from probability theory. The curves in Figure 3 show the energy distribution for a tungsten filament at three different temperatures. Such radiation is known as 'black body radiation'. Note how the emitted energy increases with temperature and how the wavelength of maximum energy shifts to shorter wavelengths. More recently it has become common practice to use a variant of this - the tungsten-halogen lamp. The quartz envelope transmits radiation well into the UV region. For the UV region itself the most common source is the deuterium lamp and a UV-Visible spectrometer will usually have both lamp types to cover the entire wavelength range.
Applications of ultraviolet/visible spectroscopy
In research, ultraviolet/visible spectroscopy is used more extensively in assaying than in identification. The trace metal content of an alloy, such as manganese in steel, can be determined by firstly reacting the sample to get the metal into solution as an ion. The ion is then complexed or made to react so that it is in a form that can be measured – eg manganese as the manganate(VII) ion. When the spectrum is recorded, the most useful piece of information is the absorbance because if the absorption coefficient of the chromophore is known the concentration of the solution can be calculated, and hence the mass of the metal in the sample. The same principle can be applied to drug metabolites. Samples are taken from various sites around the body and their solutions are analyzed to determine the amount of drug reaching those parts of the body. A useful feature of this type of analysis is the ability to calculate very small concentrations (of the order 0.0001 mol dm-3) with extreme accuracy. It is important that the absorbance of the solution remains below two for quantitative measurements because of limitations of the instrument and solute-solute interactions that can cause deviations from the Beer Lambert law.