iv. Chemical kinetics: UV/visible spectroscopy can be used to monitor the rate of chemical reactions if one of the reactants or products absorbs in a region where no other reactant or product absorbs significantly.

v. Detectors in liquid chromatography instruments: UV/visible detectors are perhaps the most common detectors present in liquid chromatography systems. Modern instruments use photodiode array detectors that can detect the molecules absorbing in different spectral regions (Figure 5.6).

Figure 5.6 Diagram of a photodiode array detector

vi. Determination of melting temperature of DNA: A double stranded DNA molecule can be denatured into the single strands by heating it. Melting temperature, T_m is the temperature at which 50% of the DNA gets denatured into single strands. Denaturation of DNA is accompanied by hyperchromic shift in the absorption spectra in the near UV region. A melting curve (plot between temperature and absorbance at 260 nm ) is plotted and T_m is determined (Figure 5.7).

Figure 5.7 Thermal denaturation of a DNA sample; a plot of absorbance at 260 nm against the temperature allows determination of the melting temperature (T_m).

vii. Microbial growth kinetics: A UV/visible spectrophotometer is routinely used to monitor the growth of microorganisms. The underlying principle behind this, however, is not absorbance but scattering. As the number of microbial cells increase in a culture, they cause more scattering in light. The detector therefore receives less amount of radiation, recording this as absorbance. To distinguish this from actual absorbance, the observed value is referred to as the optical density.