Whitepaper Chemical Characterization of Diverse Pharmaceutical Samples by Confocal Raman Microscopy WITec GmbH, Lise-Meitner-Str. 6, 89081 Ulm, Germany, www.witec.de Introduction The development and production of drug delivery systems requires efficient and reliable control mechanisms to ensure the quality of the final products. These products can vary widely in composition and application. Therefore analyzing methods that provide both comprehensive chemical characterization and the flexibility to adjust the method to the investigated specimen are preferred in pharmaceutical research. Confocal Raman Microscopy (CRM) is a well-established and widely-used spectroscopic method for the investigation of the chemical composition of a sample. In pharmaceutics, CRM can be used to probe the distribution of components within formulations, to characterize homogeneity of pharmaceutical samples, to determine the state of drug substances and excipients and to characterize contaminants and foreign particulates. The information obtained by CRM is also extremely useful for drug substance design, for the development of solid and liquid formulations, as a tool for process analytics and for patent infringement and counterfeit analysis [1-3]. Being a non-destructive method there is the opportunity to combine CRM with other imaging techniques such as Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) or fluorescence analysis. CRM is furthermore an extremely flexible technique which can be adjusted to changing requirements. This whitepaper will demonstrated how high-resolution, large-area Raman images, topographic and 3D Raman images, and ultra-fast Raman images of various pharmaceutical samples can be acquired using state-of-the-art confocal Raman microscopes optimized for speed, sensitivity and resolution. The Principle of Confocal Raman Microscopy The Raman effect is based on light interacting with the chemical bonds of a sample. Due to vibrations in the chemical bonds this interaction causes a specific energy shift in the back scattered light which appear in a unique Raman spectrum that can be detected. The confocal Raman Imaging technique combines Raman spectroscopy with a confocal microscope. Thus the spatial distribution of the chemical components within the sample are imaged. Additional sample characteristics such as the relative amount of a specific component, stress and strain states, or crystallinity can be further analyzed. High-resolution confocal Raman microscopes acquire the information of a complete Raman spectrum at every image pixel and achieve a lateral resolution at the diffraction limit (λ/2 of the excitation wavelength). A confocal microscope setup is furthermore characterized by an excellent depth resolution and features the generation of 3D Raman images and depth profiles. Page 1
The Raman Spectrum Raman spectroscopy is used to identify materials and chemical compounds by their unique Raman spectrum. In addition to the chemical characterization, many other properties can be determined through the Raman spectra. Example Raman Spectrum of Cellulose Materials and Methods The Confocal Raman Microscope A WITec alpha 300 R Raman Microscope (Fig. 1a) was used to study the chemical composition of various pharmaceutical samples. The laser light was guided to the microscope with a single-mode optical fiber (Fig. 1b). These single-mode fibers are designed to carry only a single transverse mode (TEM 00) which can be focused to a diffraction-limited spot and therefore act as a perfect point light source. This light is focused onto the sample using a dichroic beam-splitter that reflects the exciting laser beam, but is fully transparent for the frequency-shifted Raman light. The sample is scanned with a piezo-electric scan table with capacitive feedback correction for high resolution. The Raman scattered light is collected with the same objective and is focused into the core of another optical fiber that is connected to a spectrometer with a CCD camera. Only the core of the fiber guides the light and therefore acts as a pinhole for confocal microscopy. The fiber core also doubles as an entrance slit for the spectrometer, so that no additional optical components are necessary between microscope and spectrometer. The light is dispersed inside the spectrometer and the spectra are acquired with an ultra-sensitive, back-illuminated CCD camera. At every image pixel a complete Raman spectrum is acquired and used to extract the relevant chemical information from the sample. Page 2
a) b) Figure 1: a) The confocal Raman microscope alpha300 R is optimized for high-resolution Raman imaging. b) Beam path of the alpha300 R. The Microscope Performance A microscope optimized for speed, sensitivity, and resolution facilitates high-quality Raman imaging. Page 3
TrueSurface Profilometry The patent pending topographic TrueSurface sensor (Fig. 2a) is integrated into the objective turret and can be applied for topographic confocal Raman images. It is particularly useful for samples with rough or inclined surfaces. It uses the principle of chromatic aberration to record the surface topography of a sample (Fig. 2b). Scanning the sample in the x-y-plane reveals a topographic map of the sample. The topographic coordinates from this measurement are used to perfectly follow the sample surface in confocal Raman imaging mode and ensures that the Raman laser is always kept in focus with the sample surface (or at any distance below the surface if required). a) b) Figure 2: TrueSurface Microscopy for topographic confocal Raman imaging. a) The optical TrueSurface sensor is integrated in the objective turret. b) TrueSurface principle. Results High-resolution, large-area confocal Raman Imaging In the first example a pharmaceutical emulsion was investigated. The active pharmaceutical ingredient (API) is dissolved in water. In figure 3a a large-area, high-resolution Raman image of the emulsion is shown. The image scan range is 180 x 180 µm 2 with 2048 x 2048 pixels. At each image pixel a complete Raman spectrum was acquired, therefore the large image is the result of an evaluation of 4,194,304 Raman spectra. The integration time per spectrum was 2 ms and the raw data file size is 12.5 GB. The consecutive zoom-in images (figure 3b and 3c) of the same dataset illustrate the extremely high-resolution of the large-area scan shown in figure 3a. The Raman data were acquired, evaluated and processed with the WITec Project FOUR Software. In the resulting color-coded images the water and API containing phase is presented in blue, whereas with green the oil-matrix is displayed. In addition to the distribution of the known materials, silicone-based impurities could be visualized (red in the images). Volume Scans and 3D Visualizations Volume scans are a valuable tool in providing information about the dimensions of objects or the distribution of a certain compound throughout the sample. The generation of volume scans and 3D images always requires large data sizes. In order to generate 3D images, confocal 2D Raman images of different focal planes are acquired by scanning throughout the sample in the z-direction. The 2D images are then combined into a 3D image stack. To investigate the volume of the impurities of the emulsion from figure 3 in more detail, a 3D scan was performed. For that a volume of 25 x 25 x 20 μm 3 was analyzed with 200 x 200 x 50 pixels. At each image pixel a complete Page 4
Raman spectra with 10 ms integration time per spectrum was generated. The image stack contains the information of a total of 2 million Raman spectra and is based on a raw data file size of 6 GB (Figure 3d). Figure 3: a) Large-area, high-resolution confocal Raman image of a pharmaceutical emulsion. Scan range: 180 x 180 µm 2 ; 2048 x 2048 pixels = 4,194,304 Raman spectra; raw data file size: 12.5 GB. Blue: active pharmaceutical ingredient; Green: Oil; Red: Silicon impurities. b) and c) Consecutive zoom-in images of the same dataset. d) Confocal 3D Raman volume image. The green oil is partially removed in the image to facilitate a better identification of the red impurities. Scan range: 25 x 25 x 20 μm 3, 200 x 200 x 50 pixels = 2,000,000 Raman spectra, Integration time per spectrum: 10 ms, data file size: 6 GB. For a 3D animation of figure 1d go to: www.witec.de/assets/videos/figure1d_emulsion_3d_raman_image_witec.gif Page 5
Topographic Confocal Raman Imaging Although confocal Raman imaging is considered to be a non-destructive method it is common to create a smooth and even sample surface in order to keep the surface in the focal plane while measuring in confocal Raman imaging mode. However, this intervention can lead to misinterpretation of the analytical results and is not required when applying TrueSurface Microscopy prior to the confocal Raman imaging procedure [4]. An example of TrueSurface topographic confocal Raman imaging is presented in Fig. 4. The results were obtained on a commercially available pharmaceutical tablet. The surface topography acquired with TrueSurface Microscopy shows the relief writing on the tablet. The height variation was about 730 µm. Following the topography during confocal Raman image generation maintains the sample s surface in focus over the entire scanned area without Raman signal decrease. The resulting confocal Raman image reveals the distribution of the sample s components. a) b) c) Figure 4: Large-area topographic confocal Raman image scan of a pharmaceutical tablet using TrueSurface Microscopy. a) Topographic TrueSurface profile of the tablet, height variations: 730 µm. b) Confocal Raman image of the tablet acquired by following the topography shown in a). c) Corresponding Raman spectra of the main sample components. Ultrafast Confocal Raman Imaging With the Ultrafast Confocal Raman Imaging option the acquisition time for a single Raman spectrum can be as low as 760 microseconds and 1300 Raman spectra can be acquired per minute. As a confocal Raman image typically consists of tens of thousands of spectra, the option reduces the total acquisition time for a complete image to only a few minutes. For example, a complete hyperspectral image consisting of 250 x 250 pixels = 62,500 Raman spectra can be recorded in less than a minute. The latest spectroscopic EMCCD detector technology combined with the highthroughput optics of a confocal Raman imaging system are the keys to this improvement which can also be advantageous when performing measurements on delicate and precious samples requiring the lowest possible levels of excitation power. Time-resolved investigations of fast dynamic processes can also benefit from the ultrafast spectral acquisition times. A standard, commercially available toothpaste was imaged using the Ultrafast Confocal Raman Imaging option. The scan range was 60x60 μm² and 20x20 μm² respectively at 200 x 200 pixels (40,000 Raman spectra). The acquisition time for a single spectrum was 760 microseconds, resulting in 42 seconds for the complete image. The Raman images in Fig. 5a and 5b show the distribution of the toothpaste's main compounds. Fig. 5c shows the corresponding spectra. b) Page 6
a) c) b) Figure 5: a) Raman image of toothpaste. Scan range: 60x60 μm², 200x200 pixels, 40,000 spectra, 760 microseconds/spectrum, 42 seconds/image. b) Zoom-in of the marked area in Fig 5a, scan range 20x20 μm², 200x200 spectra, 40,000 spectra, acquisition time: 0.76 ms/spectrum, 40 s/image. c) Corresponding Raman spectra. Conclusion The recent developments in confocal Raman microscopy enable remarkable new applications in pharmaceutical research. Its flexibility and adaptability make it an invaluable tool for the investigation of diverse samples. Thus confocal Raman imaging provides a detailed qualitative description of pharmaceutical drug systems and effectively supports their development. References [1] T. Haefele, K. Paulus in: Confocal Raman Microscopy, T. Dieing, O. Hollricher, J. Toporski (Editors), Chapter 8, p. 165, Springer Series in Optical Sciences 158, Berlin, Heidelberg 2010. [2] K. Wormuth in: Confocal Raman Microscopy, T. Dieing, O. Hollricher, J. Toporski (Editors), Chapter 9, p. 203, Springer Series in Optical Sciences 158, Berlin, Heidelberg 2010. [3] L. Franzen, D. Selzer, J. W. Fluhr, U. F. Schaefer, M. Windbergs, Towards drug quantification in human skin with confocal Raman microscopy. European journal of pharmaceutics and biopharmaceutics, 84, 437-444 (2013); published online EpubJun (10.1016/j.ejpb.2012.11.017). [4] B. Kann, M. Windbergs, Chemical imaging of drug delivery systems with structured surfaces-a combined analytical approach of confocal Raman microscopy and optical profilometry. The AAPS journal 15, 505-510 (2013); (10.1208/s12248-013-9457-7). Contact Dr. Sonja Breuninger Technical Marketing & PR Sonja.Breuninger@witec.de WITec GmbH Lise-Meitner-Str. 6 89081 Ulm Germany Tel.: +49 (0) 731 140 70-0 Fax: +49 (0) 731 140 70-200 http://www.witec.de info@witec.de Page 7