A Spectroscopy Guide for PAT

Process analytical technology (PAT) and Quality by Design (QbD) have engendered an era of scientific growth and experimentation within the pharmaceutical industry. As these initiatives have blossomed, FDA has maintained that PAT and QbD are not so much about equipment as they are about using “best scientific judgment” to plan for and produce a quality product.

But to make good judgments, one needs quality data. Physical parameters never before measured for raw and process materials are now being recognized as essential to PAT and QbD: flowability, crushability, surface area, porosity, morphology and particle size distribution of excipients and APIs. Standard physical testing, while accurate, is slow and labor-intensive. Means are needed to make rapid, multivariate measurements on solid materials. This suggests spectroscopy.

This article will focus on the latest spectroscopic methodologies used in PAT/QbD: what they are, how they work, what they tell us, and where they are best implemented. For purposes of simplification, they will be divided into two main categories: electronic and vibrational.

I. Electronic Spectroscopy

The manner in which light is absorbed is at the electronic level, wherein a quantum of energy is absorbed by an electron (or pair of electrons) and one electron is elevated to a higher level (quantum) of energy. It rapidly drops to its previous state, emitting a photon of lower energy than the exiting photon. In absorption spectroscopy, the energy absorbed is used to qualitatively (what was the wavelength absorbed or emitted?) and quantitatively (how much light was absorbed or emitted?) identify the analyte in question. Figure 1 shows the energy levels associated with spectroscopy. The lower set of levels is involved with electronic spectroscopy and shows the types of transitions which occur in electronic spectra generation.

Figure 1: Energy Levels and Interactions. Click for larger image

The most common type of spectrometer used in analysis is in the UV/Vis (Ultraviolet/Visible) spectral range. Measured in wavelength, we usually accept UV as 190-325 nm (nanometers), or in the frequency range 52,632-30,770 cm-1 (wavenumbers). Wavelength is more commonly used. The visible range is from 325 to approximately 750 nm (30,770-13,333 cm–¹). Since the two ranges are commonly integrated in laboratory instruments, the term UV/ Vis is often applied.

In emission spectroscopy, the sample is excited with a high-energy source, such as a laser or plasma torch, causing the electrons to become excited (and leave the atom in some cases). When the atom regains its electrons in the “resting” state, it does so through the emission of energy. This energy (light) is specific to the atoms being excited and the total emitted energy proportional to the number of atoms present.

Methods/Equipment/Applications

Some of the more commonly used electronic absorbance techniques/equipment available and used in PAT/QbD include:

UV/Vis (Ultraviolet/Visible)

Probably the oldest used analytical instrumentation, these spectrometers have been staples in the laboratory for many decades. The basic equation used in analytical spectroscopy, Beer’s law, which relates path length, concentration and wavelength, is derived from visible spectral work generations ago. Inexpensive and rugged, these tools have been somewhat overlooked as “sexier” instruments have been added to the analytical toolbox. Nonetheless, as the best “cost per measuring point” tools, they should be considered wherever there is a need for versatile detectors. Two distinct processes may be distinguished: moving liquid through a pipeline or a bulk solution in a tank or drum.

On-line UV/Vis

Because of the simplicity of the components, a UV/Vis monitor, where applicable, is the lowest “per point” measurement in an industrial setting. Small, compact units may be used in-line, while fiber optics may be placed with the majority of the unit outside any hazardous areas. (Figure 2 shows the spectrum of a hydrogenation reaction system over time.)

Figure 2: UV Monitoring of a Reaction. Click for larger image.

The monitoring task may be as simple as following the concentration of one or more components in a liquid mix or a pH change of moieties to as complex as following a reaction in real-time. UV and visible tend to be less dependent on temperature and, to a limited extent, solvent type and pH than a vibrational spectrum tool.

HPLC

While not usually recognized as spectrometers, high performance liquid chromatography (HPLC) detectors are many times more abundant than “conventional” spectrometers in any lab setting. Either grating or filter-based, they are used extensively in the pharmaceutical industry. Early models were simpler and based on the wavelengths of deuterium or mercury (254 nm or 280 nm), as most organic compounds generally absorbed in one or both regions. As models became sophisticated, gratings and diode arrays became commonplace and wavelength speci city—and, indeed, entire spectral coverage—became possible. Figure 3 shows a spectrum from a diode array LC detector; note the sensitivity and speed of elution. In a biological setting, seven minutes is considered “real-time.”