Chemical imaging using NIR spectroscopy can obtain data from rounded surfaces, powders, granules, beads, and intact capsules.
"With NIR spectroscopy, we can look at beads in a capsule without having to take a capsule apart, it would be very difficult
to do that with the mid-IR or Raman," explains Linda Kidder.
"NIR, mid-IR, and Raman can tell formulators whether an API is co-localizing with a particular excipient," adds Kidder. "[Formulators
can determine] whether the API has a tendency to associate or aggregate with a particular excipient, which may be affecting
the tablet's dissolution properties. You can't get that [information] with HPLC and you can't get it with single-point spectroscopy.
That is absolutely unique to imaging."
Another common application of NIR, mid-IR, and Raman imaging is mapping thicknesses of coatings and (for Raman) of pigments
(see Figure 2). This analysis can be performed two ways. First, imaging can be a primary technique; that is, no correlation
with other data is necessary. The tablet is cut in half and the cross section is imaged. The coating is seen as a series of
pixels and pixel size variation relates to coating thickness. "We can generate an image through the use of color and show
where the thin spots on the tablet are. Because it's imaging, you can actually tell that the coating is 100 μm. And by looking
at hundreds of locations, you can obtain a statistical representation of the coating distribution—the mean value—and a standard
deviation in that value," explains Kidder.
Figure 2: An RGB (red/green/blue) composite generated from a Raman Chemical Image of a controlled release system. The image
was taken on a "Falcon II Raman chemical imaging system (ChemImage). The green and red areas of the image indicate chemically
different layers (or coatings) on a bead (indicated by in blue). The goal of the project was to study the uniformity of thickness
of multiple controlled-release coatings. This image clearly indicates that the coatings are not uniform in thickness across
the bead's surface.
It is also possible to collect data from the surface of a tablet without first taking a cross section. A tablet is placed
on its bottom or its top, and the analyst looks down on the surface. The variation in intensity of the vibrational band associated
with the coating across the surface of the tablet provides a statistical variation of the coating thickness. This is a secondary
analytical technique, and therefore obtaining absolute (rather than relative) values of the coating thickness requires a correlation
with real distance measurements.
For NIR techniques, there has been some confusion between the terms "chemical imaging" and "mapping" because both provide
an image and generate the same data. The difference is in how an image is produced and data are collected. "Chemical imaging
uses a two-dimensional detector, so you are taking pictures, whereas mapping or 'fast mapping' uses either linear arrays or
collects a line of spectra at one time to build up an image while the sample is moving on a stage," explains Kidder. In chemical
imaging, the sample does not move—a picture is taken over a series of wavelengths—whereas mapping uses an interferometer,
which is based on a moving mirror. This differentiation is also found in both Raman and mid-IR, where both mapping and imaging
Because NIR mapping is based on an interferometer, sample surfaces must be very flat to collect good data. In contrast, "the
depth of focus for NIR chemical imaging is very forgiving," says Kidder. "We can look at rounded sample and get reasonably
good data from the whole sample" (see Figure 3). In addition, Malvern has developed an NIR-based imaging system that has no
moving parts. As a stable system, it can move from an R&D setting into QA/QC setting or manufacturing setting and collect
data. Mapping systems based on an interferometer have moving sample stages and the instrument stability and therefore spectral
quality may be compromised by the vibrations found in a manufacturing or QA/QC environment.
Figure 3: NIR chemical image reveals granule distribution in tablet (wavelength = 2060 nm) IMAGE COURTESY OF MALVERN INSTRUMENTS