Interpreting the differences between infrared spectra requires a good understanding of the technique. For "routine" verification, where infrared is used to confirm the identity of a raw material or material produced from a validated manufacturing process, warning letters issued by the FDA have highlighted fundamental failures in meaningful use of infrared due to poor understanding of the technique (4, 5).
Material identification is a fundamental strength of Fourier Transform Infrared (FT-IR) analysis and the technique can be used for many applications. Because quantitative analysis can be performed by infrared, measurement of the amount of material present is possible (6). To identify gaseous species in counterfeit drugs, hyphenated techniques such as Thermogravimetric (TG)-IR may be used. For drug distribution in a tablet, however, which is the property of a manufacturing process, infrared can be used. The United States Pharmacopoeia (USP) general chapter on "Spectrophotometric Identification Tests" <197> states: "The IR absorption spectrum of a substance, compared with that obtained concomitantly for the corresponding USP reference standard, provides perhaps the most conclusive evidence of the identity of the substance that can be realized from any single test" (7). The strength of this statement has given rise to much interest in FT-IR identification within the pharmaceutical industry.Although instrumentation and software for infrared spectroscopy have continued to evolve, the loss of specialist spectroscopy knowledge in companies is a concern. Coupled with confusion over what is required by different pharmacopoeias, this lack of understanding has led to regulatory compliance issues. According to FDA warning letters: "This practice is unacceptable and raises serious concerns regarding the integrity and reliability of the laboratory analysis conducted by your firm." (8)
With sufficient experience and reference texts such as Bellamy, and Colthup et al. (9–11), an infrared spectrum can be interpreted to identify which functional groups are present in the compound under examination. In order to identify a material, however, its infrared spectrum must be compared to one previously recorded.
Compliance with the respective national pharmacopoeia is a requirement when supplying pharmaceuticals in a country. There are at least 42 national pharmacopoeias (12), all of which assume that identification is performed by Chemical Reference Material or Certified Reference Spectrum. Some pharmacopoeias contain infrared reference spectra that can be used for "pharmacopoeial-compliant" identification. The European Pharmacopoeia (EP) supports the use of reference spectra, but does not include them, while the USP does not support identification by reference spectrum, making the clear statement in General Chapter <197> that the corresponding USP reference standard must be used and recorded "concomitantly" (7) Warning letters issued by the FDA include citations for not using USP reference materials, with one recent letter highlighting that the use of a reference spectrum from the Chinese Pharmacopoeia is unacceptable (13,14). Each pharmacopoeia requires the use of chemical reference materials from that particular pharmacopoeia.
When a reference spectrum is used or generated within a laboratory, there are two essential requirements that must be satisfied for reduced compliance risk when using infrared. Firstly, the spectrum of the reference material should be of the appropriate quality—this should be verified and documented. The reference spectrum will be used to compare batches of material so it is essential that it is a good quality spectrum. The second requirement is that where an external reference spectrum is used for identification, it should be justified and documented by a supervisor or laboratory manager so that both the spectrum source and the appropriate use of the spectrum are approved by a qualified person. The first point is significant because traditional techniques of sample preparation for solids, such as the preparation of alkali halide disks (USP <197K>) or mineral oil mulls (USP<197M>) (7), are variable, and the quality of the infrared spectrum obtained will depend on the skill and experience of the analyst.
One technology that has had a huge impact on sample preparation for infrared is high-pressure attenuated total reflectance (ATR) technology, which enables good-quality infrared spectra to be recorded on a wide range of materials in a short space of time. Additionally, for solid powders, the variations in spectra due to grinding or the application of high pressure are removed because quality spectra can be obtained directly by pressing the powder into contact with the ATR crystal. The increased productivity and consistency of this approach—the same technique can be applied to a range of material types—has transformed sample preparation. These advantages are highly applicable when checking for counterfeit drugs, as larger numbers of samples can be screened effectively. The USP actively supports the use of the newer ATR technology: USP <851> states: "A particularly common technique used for IR reflectance measurements is termed attenuated total reflectance (ATR)."(15) USP <197> adds: "The ATR <197A> and the <197E> techniques can be used as alternative methods for <197K>, <197M>, <197F>, and <197S> where testing is performed qualitatively and the Reference Spectra are similarly obtained." (7)
However, this is not the case for the other pharmacopoeias. Of those reviewed in this article, the USP is the only one that documents the equivalency of ATR.