Mass Spectrometry & Spectroscopy
Quantitative Transmission Raman Method Development Julia Griffen, Agilent Technologies
‘How much drug is actually inside my drug?’ is an important question drug manufacturers ask on a routine basis. Transmission Raman Spectroscopy (TRS) is an excellent technique to answer this question. TRS can be used to analyse whole intact tablets and capsules in seconds; to generate a bulk quantitative result without sample preparation. This tutorial article is a summary of the method development process.
Transmission Raman Spectroscopy (TRS) is a regulatory approved method for Content Uniformity (CU) testing. Content Uniformity (CU) is a mandatory batch release test for oral solid dose (OSD) forms such as tablets and capsules. It is defi ned by USP <905> Uniformity of Dosage Units [1].
TRS is a fast, bulk, non-destructive technology which presents an effi cient and cost effective complimentary analytical workfl ow in the Pharmaceutical QC laboratory. This technique is not only effective in terms of speed (less than 1 minute per sample) but also effi cient in terms of entirely negating sample preparation solvents and consumables.
Often the route of implementation of spectroscopic methods is not well understood, despite being well referenced and supported by industry guidance [2,3,4,5]. This article sets out the main steps of implementing a spectroscopic technique as a batch release test.
1. Feasibility 2. Calibration 3. Validation 4. Reference measurements & model building 5. Implementation 6. Model maintenance
Although the prominent application of TRS is for CU testing, further quantitative application areas in real time release testing, in-process check, formulation development, crystallinity and polymorph analysis are also possible. The quantitative method development process is analogous between applications.
1. Feasibility
Before a large analytical method development workfl ow is undertaken, it is important to understand if the application will work. Feasibility studies include scanning many different products that may be suitable and looking for a formulation and product form that suits the technology. In general, a formulation with >1% w/w API (active pharmaceutical ingredient) content as a tablet or capsule is suitable. Higher concentrations between 10-40% w/w will be easier as the API contributions in the spectra will be stronger. Other forms of products such as powders, creams and solutions are also viable applications.
Feasibility testing should start with analysis of the fi nal product and pure components to ascertain if good quality spectra can be obtained. It is essential that the API and main excipients are visible in the fi nal product spectra. Clearly defi ned Raman spectra with low levels of fl uorescence are ideal properties for a good candidate. An example is shown in Figure 1, where the API peaks ~700, 1100 and 1600 cm-1 excipient (lactose) peaks around 300-500 cm-1
.
At this stage a small calibration sample set may be built to further investigate the viability. This may be a spiking study of API into a placebo blend; but this is not a complete calibration.
At this stage looking at historical batches of a product to obtain a proper feel for the batch-to-batch spectral variation (if any) is also recommended.
2. Calibration
The next stage is calibration, which involves making samples of different concentration and building a chemometric model. Commonly used chemometric models for bulk quantitation of this nature involve Principal Component Analysis (PCA), Particle Least Squares (PLS), Partial Least Squares Discriminant Analysis (PLS-DA).
are clearly visible, along with the
Figure 1. Example of fi nal product, tablet spectra, compared to pure components and ascertaining if spectral quality is suitable for a quantitative application.
Calibration samples should be made following a design of experiment (DoE) that encapsules the expected process and product variation in factors which are deemed high risk with respect to spectral variability. For example, one may expect API and the main excipients to change in the DoE; but unlikely to expect minor excipients/lubricant or particle size distribution to vary. As part of the calibration design, it’s recommended that a risk-based approach is carried out to ascertain which factors affect the specifi cs of the product being analysed.
A typical calibration will be between 7 to 25 samples compressed into n number of tablets where at least 3 tablets, ideally more, are analysed per sample.
Acquisition parameters will be optimised to achieve satisfactory Raman signal in as shortest time as possible, by altering laser power and scan time.
INTERNATIONAL LABMATE - APRIL 2022
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