Uniting single-point spectroscopy and digital imaging, chemical imaging is slowly being adapted for the analysis of tablets and capsules. Advanced from traditional optical microscopy, which uses a material's refractive index among other material properties as the basis for generating image contrast, chemical imaging instead uses the underlying spectroscopy associated with the materials being analyzed at each spatial location as a means to produce image contrast. Multivariate analysis and chemometric software then translate this chemical image information into useful quantitative data.
The motivation for adapting chemical imaging lies primarily in the changing nature of today's dosage forms. "Definitely the business is moving in the direction of more complicated formulations, and the standard tools don't provide the information needed to characterize these products," says Linda Kidder, product manager, Chemical Imaging Systems, at Malvern Instruments, Inc. (Columbia, MD). "The single-point spectroscopies just can't do it, and HPLC [high-performance liquid chromatography] just can't do it. This is where the need is growing, and most of the companies working on advanced formulation, really understand this need."Applications
The advantage of chemical imaging in solid-dosage form analysis is the ability determine the distribution and size of the active pharmaceutical ingredients (APIs) and excipients. The information is then correlated to optimize process operations and understand how ingredients interact within the tablet. "This information aids in quality control, trying to help understand how things dissolve, and their lifetimes with respect to shelf lives," says Richard Bormett, PhD, business manager at Renishaw (Hoffman Estates, IL). "Tableting processing can change the crystal form of the API, and being able to map where the API is and discriminate between its polymorphic forms is an important tool."
Previous methods of viewing the distribution didn't offer the quality that analysts needed. The time required to collect the data was long, making it not feasible, and high spatial-resolution mapping could take days. As Bormett observes, "If you looked at a whole tablet at a time, the spatial resolution of the probe became poorer and poorer so you had to look at larger and larger sections. If an API reached 5 to 1 microns, it wasn't always easy to see or determine its distribution."
Variations. The primary distinction between chemical imaging systems is the type of spectroscopic technique. Within the pharmaceutical industry, numerous spectroscopies have advanced, including Raman, mid-infrared (mid-IR), near-infrared (NIR) absorption–reflectance, UV–vis absorption–reflectance and luminescence. Each have been demonstrated useful for many applications.