By AZoNano
Table of Contents
Introduction
Working Principle of AFM Raman Setup
Characterization of Pharmaceutical Sample Using Raman AFM
Technique
Conclusion
About NT-MDT
Introduction
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.
Conclusion
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.
About NT-MDT
NT-MDT
has 550 employees, including Ph.D. scientists, many of whom are leaders
in their field. The company has more than 600 installations in 39
countries, and has been operating in the APM market for more than 15
years, achieving worldwide distribution of their devices. NT-MDT's
clients include Universities and colleges, laboratories, governments,
research centers and scientific companies of all sizes in the
nanotechnology field.
This information has been sourced, reviewed and
adapted from materials provided by NT-MDT Co.
For more information on this source, please visit NT-MDT Co.