Raman microscopy is extensively used in the pharmaceutical sector. This technique enables identification and characterization of functional groups, chemical compounds, molecular conformers, enables authentication of a number of drugs showing the presence of impurities and structural disorder in materials as well as study of stress distributions and temperature effects.
Integration of Raman spectroscopy with Atomic Force Microscopy opens a broad range of new capabilities in imaging and characterization of pharmaceutical products. For instance, AFM topography offers information on the grain size, shape, orientation and distribution. Sophisticated AFM techniques enable high- resolution imaging of a number of physical properties of objects such as friction coefficient, local hardness, surface potential, electrical conductivity and many others. In this application note the integrated Raman AFM is used for a comprehensive study of a pharmaceutical tablet to demonstrate the capabilities of the technique.
Working Principle of AFM Raman Setup
The working principle of the AFM Raman setup is shown in Fig. 1a.
Figure 1. (a) Integrated AFM-Raman instrument and its "focus track” feature. Sample surface always stays in focus due to AFM feedback mechanism. This provides true information about sample chemical composition even for very rough surfaces (b) Standard confocal Raman/fluorescence imaging sample is scanned in X&Y directions; Sample gets out of focus, providing incorrect data about optical properties of the surface.
The atomic force microscope is combined with a high resolution objective in top-illumination geometry. The objective is connected to a confocal Raman/fluorescence microscope. The purpose of the optical part of the setup is to focus excitation laser to a very small spot at the apex of AFM cantilever and to collect the optical signal from a local area on the sample in confocal regime. Raman scattering, fluorescence, Rayleigh scattering and other optical signals can be measured.
While the sample is scanned in X, Y & Z directions, AFM and optical images are obtained simultaneously from exactly the same sample area. Alternatively, it is possible o obtain Raman and AFM images from the same sample area. Here the Raman image is obtained in “standard” configuration without an AFM cantilever.
Characterization of Pharmaceutical Sample Using Raman AFM Technique
As a pharmaceutical sample, an ANADIN EXTRA tablet from Pfizer Consumer Health care Ltd was selected for characterization by the AFM-Raman technique. ANADIN is an analgesic medication comprising Aspirin paracetamol and caffeine. Each tablet contains 200 mg Paracetamol, 300 mg Aspirin and 45 mg caffeine. These active ingredients should be combined uniformly to produce a quality tablet. Homogeneous distribution of components will enhance compression characteristics of tablets, lifetime, hardness, strength, assimilability, biocompatibility and reduce defects and segregation.
It is essential to measure the distribution of the ingredients of the tablet in order to perform impurity testing and monitor the manufacturing process.
Firstly the tablet was sliced in half with a steel razor blade to obtain a flat surface.
The observations are listed below:
- Two characteristic types of Raman Spectra are seen at different places on the tablet as shown in Figure 2. These spectra correspond to Aspirin and Paracetamol
- Caffeine is spread extensively across the tablet, however present only in small discrete areas
- Spectral positions of Raman peaks are different for the different tablet components because of the chemical structure of compounds
- The main peaks of Paracetamol can be identified as the C=O stretch at
- 1651cm-1 NH deformation mode at 1612 cm-1 , HN-C=O bend-stretch at 1559 cm-1 Other Raman modes in the 1370 to 1166 cm-1 spectral region are due to both the N-H bending and C-H deformation vibrations.
- Raman spectra of aspirin have the characteristic bands at 1606 and 1622 cm-1 (shoulder), which can be assigned to the symmetric aromatic ring CC-stretching vibration and CO stretching vibration of the carboxyl group, which can be compared to the axial resolution of the AFM
Figure 2. Raman spectra on the ANADIN Tablet: Aspirin (green color), Paracetamol (red color).
Figure 3, which presents the chemical distribution of Aspirin and Paracetamol shows the difference in the confocal Raman maps taken with and without Focus Track feature in Figures 3a, 3b and 3c, 3d respectively.
Figure 3. Raman mapping: Focus track (a, b) and without Focus track (c, d). Green color corresponds to Aspirin distribution, red color corresponds to Paracetamol distribution. AFM topography image (e), optical image (f). Intensity variations are observed in the marked areas, if Raman measurement is done with and without focus tracking.
The data obtained without Focus Track shows variation of Raman signal intensity because of variations in sample height and chemical composition. Data obtained with the Focus track features shows a precise mapping of the sample composition.
Figure 4 shows Raman maps with Focus Track as well as corresponding AFM images. The Raman maps show a highly accurate spatial distribution of the compounds of the ANADIN tablet free from topography artifacts. Additionally phase separation of Paracetamol and Aspirin compounds were observed. Figure 4a shows an optical image of the tablet with many microcrystalline grains, which are bright and dark areas on the tablet. The smallest particle size of different components is estimated as 1 to 5 microns.
Figure 4. ANADIN tablet measurements with NTEGRA Spectra: (a) optical image of tablet; (b) AFMheight, (c) AFMphase , (d) AFMmag, (e) Raman mapping Paracetamol distribution; (f ) Raman mapping - Aspirin distribution; (g) components on the tablet (red color corresponds to Paracetamol, green color corresponds to Aspirin);
Magnitude and phase of cantilever oscillation recorded during scanning offer complimentary information to the AFM topography. Phase images show grain edges and are not impacted by large-scale height differences enabling clear observation of the fine features of the sample as shown in Figure 4c. Cantilever oscillation amplitude signal offers additional contrast grain edges due to sharp instantaneous changes in the sample height. A highly clear correlation is seen between Raman and AFM images. Characteristic domains on the tablet with increased Paracetamol combinations seen in the Raman images in Figure 4 has corresponding specific features the AFM images.
The integration of Raman Microscopy and Atomic Force Microscopy is a powerful analytical tool for pharmaceutical applications. With the AFM images of tablets, it is possible to obtain information about the grain structure, sample topography, grain boundaries and orientation. In this study the distribution of Aspirin and Paracetamol components in the ANADIN tablet was studied with high spatial resolution and free of any artifacts because of sample roughness. Correlation between sample chemical composition and AFM images is observed.
This information has been sourced, reviewed and adapted from materials provided by NT-MDT Spectrum Instruments.
For more information on this source, please visit NT-MDT Spectrum Instruments.