Spectroscopy is a class of analytical techniques that measures the interaction of matter and radiation, thereby giving insight into chemical structure and contents. These techniques all provide qualitative data, and some provide significant quantitative data as well. Often referred to as the chemical fingerprints of drugs, the various spectra produced using these techniques elucidate different aspects of drug composition; characteristic absorption or emission peaks correspond to aspects of chemical composition and molecular structure. A chemist can extract detailed chemical and structural information from a spectrum, and an inspector with minimal training can also identify falsified and substandard medicines by comparing the drug spectra to reference materials in drug spectra databases (Kaur et al., 2010). The WHO maintains a digital version of the International Pharmacopoeia with drug quality determination protocols for many common medicines (WHO, 2011). This guide includes a reference infrared spectrum for each drug.
Molecular vibration and rotation energies occur in the infrared regions of the electromagnetic spectrum, and these movements may be observed with infrared, near-infrared, or Raman spectrometers. These techniques are relatively straightforward to use and moderately expensive, and routine comparative applications do not require extensive training. Chemists analyze the absorption peaks in these spectra primarily to identify molecular functional groups; most active pharmaceutical ingredients and some organic excipients and impurities have characteristic spectral peaks or spectral fingerprints that can be used to help identify them.
Infrared spectroscopy The infrared range of the electromagnetic spectrum can be divided into three subregions: the near-infrared, mid-infrared, and far-infrared. The mid-infrared range is the more discerning and commonly used region (Deisingh, 2005). Figure 6-4 shows the different infrared spectra of the antimalarial artemisinin and its derivative, artemether. This comparison can identify the common substitution of artemisinin for more effective and expensive antimalarials (Kaur et al., 2010).
There are several ways to collect infrared spectra, each having advantages and disadvantages. Attenuated total reflectance and Fourier-transformation infrared (FT-IR) is particularly useful for drug quality analyses because it does not require sample preparation, does not destroy the sample, and provides information about the distribution of active ingredients and excipients on the surface of tablets (Martino et al., 2010). A creative application of FT-IR can distinguish between some types of real and falsified packaging. Some manufacturers label their packaging to take advantage of the fact that only inks that absorb in the infrared range will be visible under infrared radiation. In an example from Singapore (see Figure 6-5), an inspector could see only a small amount of writing on a genuine Levitra package under IR radiation but could see all of the text on a falsified package (Lim, 2012).