Fast and accurate identification of contaminants can be the most important and most stressful task for a solid state scientist and microscopist. And yet, it is a lot of fun! The more challenging the identification, the more fun. At least, that’s my opinion. Microscopy is particularly well suited for the identification of particle contaminants. The microscope techniques include optical, polarized light, scanning electron microscopy along with microspectroscopy. BTW the fun bit is a secret.
Example 1 Fibers
Fibers are among the top 3 contaminants that I have encountered. I use a combination of optical crystallography, IR and Raman microscopy, and SEM/EDS to identify fibers. In this case, there are three fiber types two of which are identified as below.
Example 2 Metal in Pharmaceutical Tablet
Example 3 Oil on Pharmaceutical Tablet
Another very common contaminant with pharmaceutical tablets is lubrication oil. The tableting machinery is lubricated with food grade oil. The oil can contaminate tablets through a variety of different mechanisms. Oil contamination is quite distinctive but also quite hard to prove. IR is helpful but not definitive. The absorbances are hardly distinctive and nearly always have similarities to tablet components (see below). The conclusion of oil contamination is based on appearance, IR consistency, and the lack of anything else! Hardly satisfactory but generally the best that we can do.
Summary
Microscopy is the best technique for the identification of contaminants. It is, however, the skill of the analyst that determines the quality of the identification more than the sophistication of the instruments. That skill can only be acquired through practice and study.