Anal. Chem. 2003, 75, 2140-2146
Applications of Attenuated Total Reflection Infrared Spectroscopic Imaging to Pharmaceutical Formulations K. L. Andrew Chan,† Stephen V. Hammond,‡ and Sergei G. Kazarian*,†
Department of Chemical Engineering and Chemical Technology, Imperial College London, South Kensington Campus, London, SW7 2AZ, U.K., and Pfizer, Sandwich, Kent, U.K.
This paper demonstrates an approach to obtain chemical images of pharmaceutical tablets using attenuated total reflection infrared (ATR-IR) spectroscopy. FT-IR images with different fields of view and spatial resolution have been obtained using a combination of different ATR accessories. FT-IR imaging with the diamond ATR accessory and micro-ATR imaging technique have been compared. With the diamond ATR imaging accessory, compaction to a tablet can be performed and the chemical image measured in situ. It has been found that the diamond ATR imaging accessory gives information on the overall distribution of different components in a tablet while the micro-ATR imaging technique provides a closer look at the tablet with 4-µm spatial resolution. Lowconcentration components down to 0.5% have been detected by the micro-ATR method. Both experimental and commercial systems are studied in this paper. To better understand the manufacturing processes used for pharmaceutical formulations and have the ability to effectively solve issues if they arise, it is important to know the distribution of different ingredients in a tablet matrix. The distribution of different components affects a number of physical and chemical properties of the tablet, such as hardness and robustness, adhesion to the tablet punch, and dissolution properties of the tablet. Many methods have been employed to study the distribution of different components in a tablet. Spectroscopic techniques such as Raman scattering, near-infrared (NIR), and mid-infrared spectroscopy can all be used as nondestructive measurements: no stain or any other labeling additive is needed prior to measurement. NIR imaging1-3 has been used to study the distribution of different ingredients in a relatively large area of a tablet. Unfortunately, the spatial resolution achieved (∼20-100 µm) was larger than the particle size of most ingredients. Thus, this method does not allow the study of tablets with the spatial * Corresponding author. E-mail:
[email protected]. Phone: 44-207594-5574. Fax: 44-207-594-5604. † Imperial College London. ‡ Pfizer. (1) Koehler, F. W., IV; Lee, E.; Kidder, L. H.; Lewis, E. N. Spectrosc. Eur. 2002, 14 (3), 12. (2) Clarke, F. C.; Jamieson, M. J.; Clark, D. A.; Hammond, S. V.; Jee, R. D.; Moffat, A. C. Anal. Chem. 2001, 73, 2213. (3) Sekulic, S. S.; Ward, H. W.; Brannegan, D. R.; Stanley, E. D.; Evans, C. L.; Sciavolino, S. T.; Hailey, P. A.; Aldridge, P. K. Anal. Chem. 1996, 68, 509.
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resolution of a few micrometers. Raman spectroscopy can achieve a spatial resolution down to 1 µm.4,5 However, some very commonly used excipients, such as cellulose, fluoresce under laser illumination that is required to achieve such high spatial resolution. Nevertheless, some advantages of the Raman imaging approach to pharmaceuticals have recently been demonstrated.5 FT-IR imaging has emerged as an additional powerful tool to characterize the distribution of different chemicals in heterogeneous materials. The inherently rich information contained in the mid-infrared spectrum allows one to identify different components from their chemical structure. Applications of FT-IR imaging have been discussed by many authors.6-13 The potential of applications of FT-IR imaging to polymeric materials of industrial interest was illustrated recently.9,12,14 FT-IR images can be obtained by measuring infrared spectra at different areas of the sample in a grid pattern, and the variation of the intensity of a specific absorption band that represents a particular component can be plotted as a map. Conventionally, this is achieved by performing a point-by-point mapping with an aperture and a computer-controlled motorized stage. However, this is a very time-consuming process. With the introduction of the focal plane array (FPA) detector, images based on the distribution of IR absorption bands can be collected within a matter of a few minutes or even seconds. With the development of multivariate analysis such as principal component analysis (PCA), it is possible to study complex systems with a large number of components. This is especially useful when dealing with drug (4) Keen, I.; Rintoul, L.; Fredericks, P. M. Appl. Spectrosc. 2001, 55, 984. (5) Schaeberle, M. D.; Morris, H. R.; Turner, J. F.; Treado, P. J. Anal. Chem 1999, 71, 175A. (6) Koenig, J. L. Microspectroscopic Imaging of Polymers; American Chemical Society: Washington, DC, 1998; p 411. (7) Sommer, A. J.; Tisinger, L. G.; Marcott, C.; Story, G. M. Appl. Spectrosc. 2001, 55, 252. (8) Salzer, R.; Steiner, G.; Mantsch, H. H.; Mansfield, J.; Lewis, E. N. Fresenius J. Anal. Chem. 2000, 366, 712. (9) Chalmers, J. M.; Everall, N. J.; Schaeberle, M. D.; Levin, I. W.; Lewis, E. N.; Kidder, L. H.; Wilson, J.; Crocombe, R. Vib. Spectrosc. 2002, 30, 43. (10) Artyushkova, K.; Wall, B.; Koenig, J. L.; Fulghum, J. E. J. Vac. Sci. Technol. A 2001, 19, 2791. (11) Bhargava, R.; Wang, S. Q.; Koenig, J. L. Macromolecules 1999, 32, 2748. (12) Kazarian, S. G.; Higgins, J. S. Chem. Ind. 2002, 10, 21. (13) Kidder, L. H.;. Haka, A. S.; Lewis, E. N. Instrumentation for FT-IR Imaging. In Handbook of Vibrational Spectroscopy; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley & Sons: Chichester, 2002; p 1386. (14) Gupper, A.; Wilhelm, P.; Schmied, M.; Kazarian, S. G.; Chan, K. L. A.; Reussner, J. Appl. Spectrosc. 2002, 56, 1515. 10.1021/ac026456b CCC: $25.00
© 2003 American Chemical Society Published on Web 04/03/2003
formulations because most drug tablets are composed of many different chemicals. An FT-IR image can be obtained in transmission, reflection, or attenuated total reflection (ATR) mode. In transmission mode, the infrared light passes through the sample, while in reflection mode, it is reflected from the surface of the sample. In ATR mode, a high refractive index material (the ATR crystal) is used and the infrared light is totally internally reflected at the interface between a sample and the crystal. At this interface, the electric field of the IR light penetrates into the sample of the lower refractive index as an evanescent wave. This wave decays exponentially with the characteristic depth of penetration, which is a function of the incident angle, refractive indices of the crystal and the sample, and the wavelength. In the sample, the evanescent wave causes an attenuation effect on the incident wave. Hence, by detecting the attenuated radiation, an IR spectrum can be obtained that is usually called the ATR-IR spectrum. The typical path lengths that the evanescent wave travels in the analysis of a pharmaceutical or polymeric sample ranges from 0.2 µm to a few micrometers. This ensures that all measured absorption bands of the sample are on-scale in the middle IR region, thus allowing one to analyze samples regardless their overall thickness. The advantages and limitations of conventional ATR-IR spectroscopy applied to pharmaceuticals have been discussed elsewhere. 15,16 The ATR-IR method is used to collect the FT-IR images in this study, because other methods appear to be challenging for the sample in tablet form. Reflection FT-IR imaging requires a very flat and smooth surface of the sample in order to produce a good signal-to-noise ratio (SNR) within a reasonable acquisition time. Moreover, quantitative studies in reflection mode are rather complicated. Transmission methods have traditionally provided the best SNR, and quantitative studies are relatively straightforward. However, the thickness of the sample in transmission mode is very limited and microtoming of the sample is often required, which is not easy for most pharmaceutical tablets. Transmission diamond anvil cells are also a possibility to generate a desired sample thickness but usually require a very high pressure. However, such high pressures may easily alter the structure of the tablet and hence the actual distribution of different components in the tablet. The ATR-IR method arguably requires the least amount of sample preparation prior to spectroscopic measurement. We17 and other authors7 have recently shown that FT-IR images can be obtained with the different ATR-IR accessories to yield the different images’ size and spatial resolution. A 4-µm spatial resolution (with an imaging area of 250 µm2) was achieved in a previous work17 using a micro-ATR FT-IR objective with germanium crystal as the ATR material and an infrared microscope. A larger size of the FT-IR image can be collected with the macrochamber and an inverted pyramid ATR accessory. However, with the larger ATR crystal, it was found to be difficult to achieve a good contact between the sample and the crystal because the amount of pressure that can be applied to press the sample against the crystal is limited by the hardness of crystal. Having good contact between the sample and the crystal18 is very important in (15) Aldrich, D. S.; Smith, M. A. Appl. Spectrosc. 1999, 34, 275. (16) Clark, D. The analysis of pharmaceutical substances and formulated products by vibrational spectroscopy. In Handbook of Vibrational Spectroscopy; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley & Sons: Chichester, 2002. (17) Chan, K. L. A.; Kazarian, S. G. Appl. Spectrosc. 2003, 57, 381.
Figure 1. Schematic diagram showing the arrangement of the diamond ATR imaging accessory and the sample.
order to obtain a reliable FT-IR image. Pharmaceutical tablets are usually difficult samples for conventional ATR measurements because of their brittleness, hardness, and relatively rough surfaces. Materials that are commonly used with a macro-ATR accessory, e.g., ZnSe and Ge, are too brittle to withstand a high contact pressure with the sample. A large diamond is expensive, and it is not suitable for macro-ATR due to its absorbance at the 1900-2500-cm-1 region. A diamond ATR-IR accessory, which utilizes a smaller diamond crystal, was found particularly useful in spectral measurements of intractable materials such a pharmaceutical tablets. We have recently demonstrated that, using this accessory, it is possible to obtain an FT-IR image of ∼1 mm2 size with a spatial resolution of up to ∼15 µm.17 This opens up opportunities to apply such an accessory in spectroscopic imaging of pharmaceutical formulations. A micro-ATR imaging system using an IR microscope, ATR objective, and FPA detector can only measure a small area, but a combined study with imaging using a diamond ATR accessory offers benefits of high spatial resolution and measurement of relatively large areas. This paper also demonstrates how compaction of pharmaceutical tablets can be studied in situ using a diamond ATR accessory. The opportunity now exists to study how the different parameters, such as the concentration of each ingredient, particle size, granulation parameters, and pressure applied on the tablet, affect the physical behavior of the tablet. To introduce this approach and demonstrate its potential, both a laboratory-manufactured and a commercial pharmaceutical tablet have been studied. EXPERIMENTAL SECTION The laboratory-manufactured tablet is composed of 3.5% (w/w) caffeine (5-100-µm particle size), 35% (w/w) starch (10100-µm particle size) and 61.5% (w/w) hydroxypropylmethylcellulose (HPMC) (20-300-µm particle size). The chemicals were supplied by May and Baker Ltd. and Pfizer. All three components are mechanically mixed for 5 min, and then the mixture is transferred to the surface of the diamond and pressed with a sapphire anvil. The maximum load that can be applied to the sample is ∼730 N, which gives a pressure of ∼2000 bar for a tablet 2 mm in diameter. To simulate the environment of a tablet punch in the actual manufacturing process, an “O” ring is placed on the diamond to prevent the powder from being spread during the compression (see Figure 1). The amount of pressure applied is controlled by a torque wrench. The anvil is slowly pressed against the powder until the required pressure is reached (a mild pressure of ∼200 bar is used in this study). FT-IR images are then measured in situ. The tablet (18) Ekgasit, S.; Padermshoke, A. Appl. Spectrosc. 2001, 55, 1352.
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that is formed on the diamond surface is then removed and transferred to the microscope stage where micro-ATR imaging is performed. The same procedure is applied to a real pharmaceutical sample supplied by Pfizer. Sugar, starch, magnesium stearate, and drug are the four components inside the powder mixture. The general name D is given to represent a proprietary drug. The mixture contains 85% sugar, 13% starch, 1.5% magnesium stearate, and
