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.”

Figure 3: High Speed Chromatogram. Click for larger image.

In fact, when HPLC is used for analysis of dosage forms, the spectrum of the analyte, combined with the retention time, may be used as the “specificity” portion of the analysis. HPLC has also been incorporated (since the 1980s) as a process technique in bioprocessing. Eli Lilly was a pioneer in the field, using LC on-line to monitor the formation and purification of insulin. The earliest process equipment was designed and built by Lilly for this work. While LC is not instantaneous, several minutes is, in effect, real-time when a process (especially a biological-type) takes days. In the 20+ years since its inception, process HPLC has come a long way.

Fluorescence (FL)

While fluorescence spectra resemble vibrational spectra, they are considered electronic because of the mode of their mechanism. Energy (in the form of photons) is absorbed, causing the electrons to become excited and move to a higher level. The electron excitation depends on the incident light (higher light flux causes higher peaks) while the emitted light can be strongly influenced by the matrix and temperature. Emitted photons may be reabsorbed or the excited electrons may interact with solvent at higher temperatures and not emit a photon at all. However, unlike the very simple UV and visible spectra, fluorescence spectra are “rich.” That is, they contain many more peaks and give more structural information than are seen in either visible or ultraviolet spectra. Since the mechanism of fluorescing is “multidirectional,” the detectors are placed perpendicular to the incident light. Thus, unlike UV/Vis, small absorbances (followed by emission) and not the total ouput of the light source are seen against a “dark” background. This allows for detection of trace amounts of analyte.

Simple Fluorescence (On-line)

Where applicable, these monitors (grating or filter) can give molecular information with little interference from the solvent. Since the emission spectra of a fluorescing molecule has a specific spectrum and is quite sensitive (low analyte concentration), a number of reactions may be followed in detail within complex systems.

HPLC

Highly sensitive, fluorescence monitors may be scanning or simple filter types. These are, simply, smaller samplesized versions of an on-line monitor. With the small sample size used in analytical chromatography, fluorescence detectors may be placed in tandem with UV detectors. Typical analytes include salicylic acid and ergot alkaloids; both present in small to minute levels. Alkaloids are potent and may be present in low levels, while salicylic acid is an impurity in aspirin and must be quantified in all dosage forms containing aspirin (production and stability samples) down to parts per million levels.

Light-Induced Fluorescence (LIF)

Initially the “L” in LIF stood for laser, but lately, laser diodes have become popular as light sources due to their high-intensity, monochromatic light. In cases where lower concentrations of drug substance are present (less than 1% relative), LIF outperforms NIR and Raman. One such case would be in blend uniformity of a mixture, prior to either compression or granulation.

Figure 4: LIF Monitor on  Tablet Press. Click for larger image.

Figure 4 shows an LIF probe attached directly to a tableting machine, monitoring 100% of the tablets produced. While LIF has many of the limitations of normal fluorescence, the higher light flux may induce fluorescence in materials not normally considered candidates for fluorescence analysis. Other than NIR, LIF is one of the few spectral techniques usable on powder mixtures.

Laser-Induced Breakdown Spectroscopy (LIBS)

LIBS has been around for years, but isn’t often mentioned alongside “common” PAT tools. It consists of a finely focused laser that strikes, for example, a tablet core or coated tablet, causing vaporization at the point of contact. The light emitted is studied for metals and a number of other elements, such as nitrogen and halides. The time at which the light is measured gives elemental or molecular information. The point of laser contact is hit with subsequent pulses, causing the beam to, in essence, drill a small hole into the tablet, extruded material, or whatever the target. The emission spectra gathered provide a depth-profile of various elements at that point. The three dimensional pattern formed is shown in Figure 5.

Figure 5: LIBS Profile on Tablet. Click for larger image.

By repeating the operation at adjacent points on the sample, the analyst generates a three-dimensional picture of the distribution of, for instance, the magnesium stearate in the tablet or titanium dioxide in the coating. This is obviously a destructive technique and not very fast, but is an excellent tool to show the homogeneity of both a tablet matrix and its coating. Root cause analysis would be one obvious use for LIBS.

II. Vibrational Spectroscopy

All organic molecules are held together by covalent bonds—that is, two adjacent atoms share one, two, or three pairs of electrons in order to bond. Each bond vibrates at a set frequency, which is determined by the masses of the two atoms, the electron-withdrawing or -donating abilities of atoms adjacent to the pair, and factors such as temperature and matrix. The bonds may also move in a wagging or scissoring motion as well as rotate along their vertical axes. Each movement is excited by the proper frequency of energy impinging upon it. The strength of the bond varies according to the elements involved and the nature of adjacent groups. Thus, the chemical nature of a molecule gives a “fingerprint” when all the absorption bands are displayed.

Some influence is also exerted by “nearby” atoms and bonds both internal and external to the molecule in question. This causes shifts in what would be a “pure” spectrum of the chemical. These shifts may be used to determine environmental factors, such as degree of crystallization, polymorphic form, and solvation effects. The intramolecular influences on the mid-infrared (MIR) spectral region are indirectly affected (crystal structure, hydrogen bonding, etc.). The same forces affect nearinfrared (NIR) and are accentuated by the fact that the overtones in NIR are mainly hydrogen atoms, strongly influenced by hydrogen bonding. In the case of farinfrared, the influences are more intermolecular and are mostly only important at interfaces and boundaries between phases.

Methods/Equipment/Applications

Because, in a typical organic molecular spectrum, there are more potential “unique” modes of vibration (and stretching and rotation) than found in electronic signatures, it is easy to understand why it has become a staple in almost every laboratory in the world. Vibrational spectroscopy became popular during World War II. While the near-infrared spectral range was discovered first (circa 1800), mid-range infrared came to the fore because it was extensively used in the synthetic rubber industry. Because of its long service, it tops the list of equipment/methodologies:

Mid-Range Infrared (MIR)

Originally done with a salt prism, the MIR technique moved almost entirely to gratings by the 1960s. In the early 1980s, interferometric or Fourier Transform (FT-IR) instruments became the new standard in industry. Fast, quiet and simple to operate, the latest models are beginning to appear in process applications. The spectral bands arise from non-symmetric bonds (i.e., separation of electron density) and, while dominated by vibrational bands, the spectra also contain bending and rotational bands.

MIR is excellent for speciation and is found in every synthetic organic laboratory. That is, the structure of molecules is easily determined in a pure material from the distinctive spectrum of each molecule. In mixtures, the resolution available to IR allows specification of analytes within the mix. One minor difficulty is that MIR radiation is strongly absorbed by the analytes and solvents. So, in order to use MIR in situ, sample viewing is usually performed by devices based on attenuated total reflection, or ATR. ATR is essentially a surface technique where the IR radiation is guided along a crystal and “sees” a thin layer of the sample (either powder or liquid).

MIR is used in liquid chemical reaction vessels and more and more in biological processes. The ability to follow reactions such as fermentation allows biopharmaceutical operations to respond more quickly in either batch-fed operations or simply to quench a reaction in a timely fashion and not rely merely on timeconsuming, biological assays.

Near-Infrared (NIR)

Developed by the US Department of Agriculture in the late 1950s, NIR has a history of being used for samples where the matrix is left intact—to determine, for example, protein content of wheat, starch in corn, or oil in soy. Since the most common mode of operation was diffuse reflection, it seemed ideal for the powders, granules and tablets in pharmaceutical manufacturing. While work was done on apples and melons decades ago (USDA), new, sensitive detectors allow for quality transmission spectra through solid-dosage forms.

With its long history in process manufacturing, NIR has the largest variety of hardware and software available for PAT (as well as the most applications published). Explosion-proof NIR instruments, capable of working in any environment, are traditionally lab instruments enclosed in containment chambers.

Figure 6: NIR on a Fluid Bed Dryer. Click for larger image.

Figure 6 shows the NIR viewing a fluid bed dryer. Information about amount of solvents, particle size, and other factors may all be gathered in this manner. Often, it is as important to know where the water molecules lie (hydrate, surface) as how much water is present.

Because the NIR spectra are influenced by hydrogen bonding in the chemical matrix, various physical data may be determined as well as chemical data. NIR spectra vary with particle size (of the same chemical) as baseline offsets, increasing in apparent absorbance with increasing particle size. The physical interaction of optical isomers is apparent in many molecules, due to the intermolecular hydrogen bonding and van der Waal’s repulsions between them. These are most apparent when racemic mixtures are formed. Polymorphic forms of the same substance exhibit spectral differences for the same reasons.

Chemical Imaging (CI)

A technique usually associated with NIR but applicable to Raman and MIR spectroscopy is chemical imaging. (See Pharmaceutical Manufacturing, May 2008, NIR-Based Chemical Imaging.) Where standard spectroscopy measures the “average” properties of a sample, chemical imaging (CI) uses multiple pixel imaging to generate a chemical and special image of the sample. That is, the spectra of individual points on a surface are captured in a two-dimensional matrix. This “hyper-spectral cube” of data can then be analyzed for many statistical properties: spatial distribution of chemical moieties (API and excipients), statistical distribution of particle sizes, homogeneity, and so forth. A “snapshot” of roughly 82,000 pixels, each with a full spectrum, may consume over 25 megabytes of space. Figure 7 shows one ranitidine and one acetaminophen tablet. In the former, the two polymorphic forms of the API and the excipient are highlighted through use of PLS functions, and in the latter, the active and excipients are given false colors by wavelength.

Figure 7: Ranitidine and Acetaminophen Tablets by NIR. Click for larger image.

Far-Infrared or Terahertz

For most of its history, far-infrared was used by crystallographers to measure subtle variations in mor-phology. Based on the rotations of atoms and mainly affected by intra-molecule forces, it is now used to look at interfaces within complex dosage forms. While not an on-line or particularly rapid means of analysis, terahertz can supply information not readily obtainable by other techniques.

While relatively expensive and slower than many other PAT tools, terahertz has a number of unique applications. The source of terahertz absorption is intermolecular, so interfaces between surfaces are the strength of this technique. Figure 8 shows the three interfaces (layer boundaries) of a complex enteric coated tablet; air/tablet surface interface, coating 1/ coating 2 interface, and coating 2/core interface. While limited in application, the data are unique.

Figure 8: Interfaces in Multilayer Tab by Terahertz. Click for larger image.

Raman

The Raman Effect was demonstrated by Sir C.V.Raman, an Indian scientist, in 1938. Using sunlight as a source, Dr. Raman first reported results on organic molecules. Later, when lasers became available, Raman spectroscopy became one more tool for characterization in organic synthesis. While actually not an absorption technique (it is based upon scattering of the light), the information gleaned allows it to be paired with infrared. Whereas mid-range IR has its base in non-symmetric bonds (e.g., C-O, C-H, O-H), Raman has its strongest peaks based on symmetric bonds (e.g., O=C=O, C=C, C=C, N=N, etc.). This makes it a perfect companion for MIR.

Raman is quickly making itself known in PAT. In the past few years it has gone from a laboratory novelty to a hardened process tool. Its ability to ignore water allows for aqueous applications even more sensitive than NIR. Since it is based on scattering (in a clear solution), the light may be focused into the container (i.e., glass) and radiation collected nearby, obviating transmission through a fast-moving, curved container. Certainly, Raman is another technique to consider for both raw material and dosage form identification. Handheld instruments are commercially available and are in use in process and off-campus activities.

One limitation of this technique is the difficulty in absolute quantification. Since the intensity of the incident radiation determines the Raman spectrum and there is no “blank” or zero absorption, the standard I/Io (reflected/incident) ratio cannot be calculated, thus eliminating any Beer’s law calculations. Relative intensities, however, are simple to measure and a process may be followed over time (drying, mixing, etc.). A chemical reaction’s progress is shown in Figure 9, where a Grignard reaction proceeds.

Figure 9: In-process Monitoring of a Reaction by Raman. Click for larger image.

The “Raman shift” or spectrum is followed over time. In a case such as this, the starting materials can be seen to disappear while the product appears. When the process is complete, the Raman spectrum stops changing. At this point, compendial analysis may be run.

Bioprocess Work

While PAT and QbD were originally considered small-molecule concepts, they are rapidly being adopted by biopharmaceutical companies. Some of the typical assay techniques in biopharma are even more time-consuming than the chemical assays in pharma. In-line measurements, while not ubiquitous as yet, are becoming more commonplace. Samples that are drawn for conventional tests risk exposing workers to pathogens and the process to contamination. Thus, time and safety concerns are being addressed with PAT.

Knowing when to perform the conventional assay also becomes critical. Probes have been specially built for bioprocess work, wherein the probe may be automatically withdrawn and cleaned, between readings. Typical analyses are for ammonia, mass, glycerin and sugars. Batch-fed fermentations may have nutrients and products followed in real time.

Working with vendors and other resources, a PAT/QbD group can find a spectrometer for nearly every chemical or physical measurement. It is strongly advised that a process be examined, the parameter needed to be measured determined, and then an instrument company approached to ascertain whether or not that particular measurement is feasible.