Sheelagh Halsey
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Nearinfrared (NIR) spectroscopy is a wellknown tool for identifying raw materials in the pharmaceutical industry. It is a
very sensitive, nondestructive technique using overtones and combination bands of fundamental vibrations derived from the
mid-infrared. Spectra can be used to distinguish closely related materials such as API polymorphs and excipient analogues.
Analysis time is only a few seconds and samples can be analysed in disposable glass vials or directly with a fibre optic probe.
However, NIR spectroscopy can be used for more than just raw material identification. With current industry trends using PAT
and Quality by Design (QbD) initiatives, building knowledge and reducing risk in production processes are becoming increasingly
important. Many companies are focusing research on unit operations such as blending and tableting, but there is much to be
gained by revisiting the quality of raw materials entering the process.
Figure 1: NIR spectra of lactose polymorphs.
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Raw materials can now be identified and tested for manufacturing suitability. Most unit operations are physical processes.
Physical properties of raw materials affect the manufacturing process when used in an inflexible recipe, resulting in poor
quality products. Changes in particle size or polymorphism will influence flow properties and moisture uptake, which in turn
affect blend and compression behaviour. The result could be poor content uniformity. NIR spectra are sensitive to these physical
changes, as well as chemical identification. Figure 1 shows NIR spectra of lactose polymorphs generated using an FT-NIR analyser
(Antaris II; Thermo Scientific, UK). These spectra show that the hydrated form has a marked peak at around 5150 cm-1 (1940 nm), indicating chemical sensitivity to water. The spectra also demonstrate the sensitivity of the technique to morphology
changes. The amorphous form shows fewer features than the crystalline form. For example, many sharp peaks can be seen in the
4000–4500 cm–1 (2200–2500 nm) region in the crystalline forms, whereas the amorphous form shows little detail. These spectral changes allow
the material to be identified as lactose, but also to be qualified as the correct morphology required for a particular process.
Figure 2: NIR spectra of cellulose of different particles.
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Figure 2 shows the NIR spectra of microcrystalline cellulose of three different particle sizes. Reflectance spectra exhibit
baseline shifts because of different light scattering from the samples. This leads to larger particle size samples having
a higher baseline than smaller particle sizes. Hence, the 180-μ cellulose sample has a higher absorbance than the 50-μ sample.
Furthermore, the reflectance and scattering is not constant across the whole NIR spectrum. Shorter wavelengths (higher frequency)
light penetrates more and scatters less than longer wavelength (lower frequency) light. This gives rise to larger baseline
shifts at shorter wavenumbers (longer wavelengths) than at higher wavenumbers (shorter wavelengths). A more comprehensive
explanation of diffuse reflectance theory can be found in reference 1.
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