What is a near-infrared inspection system, and how does it work?

Near-infrared (NIR) refers to a range of light wavelengths that is slightly outside the visible-light range. An NIR spectrometer is a device that functions in the NIR range much as human eyes do when they perceive visible light, allowing you to see material properties that would otherwise be invisible.

For example, an apple appears red to the human eye because it reflects the red wavelengths back to your eye and absorbs other wavelengths or colors of light. That apple will also reflect and absorb different NIR wavelengths, which the human eye can’t see. Pharmaceutical manufacturers can use this physical principle to quantitatively verify the material properties of their drug products.

NIR Inspection Systems

While NIR inspection systems have traditionally been considered laboratory instruments due to low material throughput, examples of this equipment are now available that can be placed directly in a production line for high-speed drug-product review as a PAT.

Numerous methods are available to capture NIR radiation data. Diode array pulsed spectroscopy and Fourier transform infrared spectroscopy are two examples, but the most viable for production is acousto-optic tunable filter (AOTF) technology. AOTFs have rugged solid-state components and much faster throughput capabilities, which separates them from other NIR options. AOTF technology today can scan at 16,000 wavelengths per second and read up to 10 outputs per second.

The major developments in this technology since its start have been wireless capability, lower operating-voltage requirements, and higher processing speeds. All of these improvements have ultimately led to faster and safer systems.

An NIR inspection system can bolster confidence in a drug product’s quality by verifying parameters that were previously not verifiable. After you have trained an NIR sensor, or spectrometer, to detect certain ratios of NIR spectra that a drug product has absorbed and reflected, it can then detect variations in those ratios as indications of foreign particles in the product that aren’t visible to the human eye, with calculated precision and speed. NIR inspection also can detect inconsistencies in membranes and coatings that control the release of APIs in pharmaceutical tablets.

An example of this can be seen in Figure 1, which shows a trained NIR spectrometer that has verified the spectral signature of acceptable tablet membrane uniformity (in blue) against a tablet tested inline during production (red)

The ability to make swift judgement calls based on NIR’s hard data is particularly important for tooling that is used to manufacture multiple products, because you can verify the material purity of the tooling before use, eliminating cross-contamination and potential costly recalls. NIR inspection can also eliminate the uncertainty of batch testing because it allows you to verify every drug product for material purity autonomously and at production-level speeds.

NIR Safety Requirements

The development of contactless quantitative review of critical process parameters and the changing of processes to use equipment that was once found only in laboratories has required a reevaluation of safety considerations. While developments in NIR technology over the past 30 years have made the process much safer, NIR inspections systems are high-cycling pieces of electronic equipment that require the drug product to be exposed to controlled doses of radiation.

As a result, it’s best to keep the system at a distance from operators and observers to minimize potential contact with hot metal components and protect workers’ eyes and skin. By installing the system correctly in the middle of a production line where operators don’t need to interact with it, manufacturers can address the safety concerns associated with NIR systems while providing higher inspection capacity than ever.

As appeared in December 7, 2020 Tablets & Capsules Solid Dose Digest E-Newsletter www.tabletscapsules.com