TOF-SIMS Characterization and Imaging of Controlled-Release Drug

Anal. Chem. 2000, 72, 5625-5638

TOF-SIMS Characterization and Imaging of Controlled-Release Drug Delivery Systems Anna M. Belu,*,† Martyn C. Davies,‡ J. Mike Newton,§ and Nikin Patel|

Physical Electronics, 6509 Flying Cloud Drive, Eden Prairie, Minnesota 55344, School of Pharmaceutical Sciences, University of Nottingham, Nottingham NG7 2RD U.K., School of Pharmacy, University of London, 29/39 Brunswick Square, London WC1N 1AX U.K., and Molecular Profiles Ltd., University Park, Nottingham NG7 2RD U.K.

Time-of flight secondary ion mass spectrometry (TOFSIMS) was used for the analysis of multilayer drug beads that serve as controlled-release drug delivery systems. TOF-SIMS analysis of a cross section of each bead system allowed molecular chemical information to be gained from all of the layers simultaneously, in situ. The integrity of each of the layers was evaluated through imaging of specific ion species for the drug, excipient, and coating materials. The three beads in this study each showed a unique distribution of ingredients. Images of the parent molecular ion for each drug (theophylline, paracetamol, prednisolone) showed their distribution ranged from micrometer-sized particles in one bead cross section to almost homogeneous in another bead cross section. The chemical composition of each of the layers in the beads was evaluated through mass spectrometry; the ingredients did not always match the manufacturer’s specification. In addition, many common drug bead ingredients were analyzed as pure substances, providing TOF-SIMS reference spectra of these materials for the first time. The use of controlled-release drug delivery systems has increased in popularity in the past decade. Many advantages associated with controlled-release drug delivery systems include sustained drug effects with reduced side effects, increased patient compliance, and reduced fluctuations of plasma drug concentration.1,2 One popular controlled-release method is the reservoir controlled-release system in which a drug core is enclosed with one or more polymeric layers. The release of drug depends on the physicochemical properties of the polymers used to coat the core particle. Often, a number of different polymer layers are used to form a multilayer system with each layer designed for a specific purpose, for example, to release the drug at a specific rate or at a specific pH. In designing a controlled-release drug delivery system, the coating system must be easy to manufacture, must * Corresponding author: (e-mail) [email protected]; (fax) 952-828-6449. † Physical Electronics. ‡ University of Nottingham. § University of London. | Molecular Profiles Ltd. (1) Park, H.; Park, K. Polymers in Pharmaceutical Products. In Polymers of Biological and Biomedical Significance; American Chemical Society: Washington, DC, 1994; pp 1-15. (2) Mathiowitz, E. The Encyclopedia of Contolled Drug Delivery; Wiley: New York, 1999. 10.1021/ac000450+ CCC: $19.00 Published on Web 10/21/2000

© 2000 American Chemical Society

Figure 1. Description of the chemical composition of the layers of the three drug bead systems analyzed by TOF-SIMS.

Figure 2. Schematic of TOF-SIMS analysis of a partial area of a bead cross section. (A) A total ion image and a total area spectrum are obtained during data acquisition. (B) Data analysis allows spectral information to be extracted from specific areas of interest, as well as images to be created from specific ion signals.

be stable over an extended shelf life, and must deliver the appropriate drug dosage to the target area over the desired time period. Since the fabrication of controlled-release systems can be technically difficult compared with conventional dosage forms, evaluation and analysis of the final product is vital. Furthermore, drug bead formulations are often patented, and characterization is necessary for defending against infringement and counterfeiting. Analytical Chemistry, Vol. 72, No. 22, November 15, 2000 5625

Table 1. Characteristic TOF-SIMS Peaks for Drug Molecules

In practice, dissolution studies have the capability to monitor the rate of drug release; however, any unwanted effects observed by these methods cannot be readily associated with specific defects in the structure or composition of the controlled-release system. To overcome this, an analytical technique is required that can probe the interior or probe cross sections of the device for the detection and distribution of various ingredients. Vibrational spectroscopy techniques such as Raman microprobe and FT-IR microscopy are useful for chemical characterization; however, these techniques have limited spatial resolution for analyzing the distribution of ingredients or excipient within a system. Time-offlight secondary ion mass spectrometry (TOF-SIMS) is a powerful method for characterizing cross sections of drug dosage forms, allowing imaging with high spatial resolution and spectroscopy for molecular chemical identification.3-6 5626

Analytical Chemistry, Vol. 72, No. 22, November 15, 2000

TOF-SIMS has been employed for the characterization of a range of materials including electronic, metallic, salt, organometallic, organic, and polymeric substances.7 During recent years, the growth and establishment of TOF-SIMS has been through the characterization of organic and polymeric materials, due to its unique capabilities for molecular information from insulating materials. Some common biomedical polymers and film coatings have been studied using TOF-SIMS spectroscopy, and (3) Van Vaeck, L.; Adriaens, A.; Gijbels, R. Mass Spectrom. Rev. 1999, 18, 1-47. (4) Adriaens, A.; Van Vaeck, L.; Adams, F. Mass Spectrom. Rev. 1999, 18, 48-81. (5) Odom, R. W. Appl. Spectrosc. Rev. 1994, 29 (1), 67-116. (6) Benninghoven, A. Angew. Chem., Int. Ed. Engl. 1994, 33, 1023. (7) Vickerman, J. C.; Briggs, D.; Henderson, A. The Static SIMS Library; Surface Spectra Ltd.: Manchester U.K., 1999.

Figure 3. (+)TOF-SIMS spectra of drugs theophylline, paracetamol, and prednisolone. P identifies the parent molecular ion, * marks characteristic molecular signals for the drug, and s represents contamination (silicon oil).

Figure 4. (+)TOF-SIMS spectra of polysaccharide filler materials. Characteristic peaks are labeled with *.


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Figure 5. (+)TOF-SIMS spectra of filler materials. Characteristic peaks are labeled with *.

Figure 6. (+)TOF-SIMS spectra of coating materials. Characteristic peaks are labeled with *. 5628


Table 2. Characteristic TOF-SIMS Peaks for Drug Bead Filler Materials

the results have provided chemical characterization and reference spectra of the pure materials.8-11 Most recently, researchers are (8) Davies, M. C.; Wilding, I. R.; Short, R. D.; Melia, C. D.; Rowe, R. C. Int. J. Pharm. 1990, 62, 97-103. (9) Davies, M. C.; Short, R. D.; Khan, M. A.; Watts, J. F.; Brown, A.; Eccles, A. J.; Humphrey, P.; Vickerman, J. C.; Vert, M. Surf. Interface Anal. 1989, 14 (3), 115-120. (10) Hearn, M. J.; Ratner, B. D.; Briggs, D. Macromolecules 1988, 21, 29502959.

applying TOF-SIMS in the biological area for the study of cells and tissue.12-14 For the analysis of drug delivery systems, only a few studies to date have employed TOF-SIMS.15 The first TOF-SIMS (11) Hearn, M. J.; Briggs, D. Surf. Interface Anal. 1988, 11, 198-213. (12) Pacholski, M.; Cannon, Jr., D.; Ewing, A. G.; Winograd, N. Rapid Commun. Mass Spectrom. 1998, 12, 1232-1253. (13) John, C. M.; Odom, R. W. Int. J. Mass Spectrom. Ion Processes 1997, 161 (1-3), 47-67.


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Table 3. Characteristic TOF-SIMS Peaks for Drug Pellet Coating Materials

study of a drug delivery system employed the technique for surface analysis of a drug-containing particles.16 The surfaces of cellulose beads (average diameter of 1.1 mm) were mapped to localize the drug theophylline. The images of fragment ions and molecular ions specific for the drug molecule were useful for demonstrating an even drug distribution over the bead surface. More recently, TOF-SIMS has been used to study a polymer film drug delivery system. TOF-SIMS was employed to determine the localization of a peptide drug dispersed in a medicinal patch of hydroxypropyl cellulose (HPC).17 The distribution and ultimate bioavailability of leuprolide along the surfaces was evaluated by imaging the protonated molecular ions as well as specific fragment (14) Todd, P. J.; McMahon, J. M.; Short, R. T.; McCandlish, C. A. Anal. Chem. 1997, 69 (17), 529A. (15) Davies, M. C.; Shakesheff, K. M.; Roberts, C. J.; Tendler, S. J. B.; Bryan, S. R.; Patel, N. Characterization of Delivery Systems Using SIMS Analysis. In The Encyclopedia of Controlled Drug Delivery; Mathiowitz, E., Ed.; Wiley: New York, 1999; pp 269-275. (16) Davies, M. C.; Brown, A.; Newton, J. M.; Chapman, S. R. Surf. Interface Anal. 1988, 11, 591. (17) John, C. M.; Odom, R. W.; Salvati, L.; Annapragada, A.; Fu Lu, M. Y. Anal. Chem. 1995, 67, 3871-3878.

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Figure 7. Images of the entire bead system cross section (800 × 800 µm2, negative ion mode). In the total ion image (left), the outer shell can be visually distinguished from the inner layer. The cross section has a lot of topography which can be observed by the absence of signal in the center of the bead. The distribution of the prednisolone drug (right image, prednisolone parent ion m/z 543) is observed only in the inner layer and has a size variation of clusters up to 20 µm. Color is based on a log scale. Distance scale bar ) 100 µm.

Figure 8. Top image: total ion image of the full bead cross section (800 × 800 µm2). Bottom image: a closer view of the layers of the sample (250 × 250 µm2). To evaluate chemical composition, positive ion spectra were extracted from the outer coating (green) and inner layer (pink) of the drug bead. The spectrum of the outer coating correlates well with a spectrum of ethylcellulose. The inner layer shows significant peaks for the prednisolone drug, lactose, and at m/z 130 (unidentified).

peaks of the peptide in cross sections of HPC. The distribution was found to be dependent on the peptide/polymer blend formulation. In this study, we report on the use of TOF-SIMS for analysis of multilayer drug beads that serve as controlled-release drug delivery systems. A typical bead is composed of multilayers of active drug, fillers, additives, and coating materials. A detailed analysis of three systems is provided that reflects the range of different types of formulations employed in the pharmaceutical industry (Figure 1). We report on the potential of TOF-SIMS spectroscopy for the molecular identification of such components within the different domains of the dosage form in the solid state and the significant role for TOF-SIMS imaging in furnishing chemical maps of complex heterogeneous delivery systems. EXPERIMENTAL SECTION Materials and Methods. The model drugs utilized in this study were (1) prednisolone sodium metasulfobenzoate, referred to as prednisolone in this paper (Hoechst Marion Rousel), (2) theophylline BP (BASF U.K. Ltd.), and (3) paracetamol (Becpharm, Harlow, U.K.). The beads were prepared to contain the drug plus microcrystalline cellulose (FMC Corp., Cork, Ireland) and, depending on the beads, lactose BP fine grade, barium sulfate (Chemische Werb, GmbH) and glyceryl monostearate (Hicks Milton Weynes, U.K.). The film coatings constituted of, for bead 1, a mixture of 1 part amylose dispersion (AIRIC, Norwich, U.K.) and 4 parts Surelease (Colorcon, Dartford, U.K.); for bead 2, an aqueous dispersion of Eudragit L30D (Rohm Pharma GmbH, Darmstadt, Germany); and for bead 3, a

3% solution of ethylcellulose (Ethocel R, Dow Chemical Co., Midland, MI) in 95% ethanol containing 17.5% poly(vinylpyrrolidone) (BASF). The three bead systems were prepared by the process of extrusion/spheronization, details of which were described by Yuen et al.18 The coating was achieved with fluid bed coating using a bottom spray technique. The thickness of coat applied varied with the different bead systems. TOF-SIMS Analysis. TOF-SIMS data acquisition was performed using a TRIFT II instrument (Physical Electronics, Eden Prairie, MN).19 For high mass resolution spectroscopy, the instrument employed a 15-keV Ga+ ion source. The 600-pA dc primary ion beam was pulsed at 11-kHz frequency with a pulse width of 12 ns. During high spatial resolution imaging, the instrument employed the Ga+ source at 25 keV with 20-ns pulses. A low-energy electron beam was used for charge compensation. The intact beads were directly analyzed to gain chemical information on the outer coating. To analyze the layers within the beads, cross sections of the beads were prepared. Each bead was secured to a silicon substrate using an epoxy resin glue. The bead was glued only at the bottom with no glue present near the area of cross sectioning, which prevented smearing of glue across the cross-sectioned surface. The bead was sectioned in half with a sharp blade, and the exposed surface was then analyzed. To generate reference spectra, the polymer, drug, and excipient (18) Yuen K. H.; Deshmukh A. A.; Newton J. M.; Short M.; Melchor R. Int. J. Pharm. 1993, 97 (1-3), 61-77. (19) Schueler, B. Microsc. Microanal. Microstruct. 1992, 3, 119-139.


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Figure 9. TOF-SIMS images of bead system 1 (250 × 250 µm2). The prednisolone drug is present as clusters in the inner layer up to 20 µm in size. The lactose is present in similar clusters as the prednisolone but also is evident in additional clusters. Unexpected components are also present in the inner layer, as evidenced by the signal at m/z 130; this species is present as smaller clusters (up to 10 µm) and is in regions different from the prednisolone and lactose. The m/z 59 signal represents ethylcellulose in the Surelease outer coating, which is ∼50 µm thick.

standards were also analyzed. The standards were analyzed as received; these powdered materials were deposited on doublesided sticky tape mounted on a Si substrate. TOF-SIMS data analysis can be done during acquisition (in real time) or postacquisition using the collected “raw data stream”. If the analyst a priori knows the peaks of interest in the mass spectrum or the area of interest in the image, data analysis can be done in real time. If, however, the sample is unknown, postacquisition (or off-line) analysis is insightful. When the data are collected as a raw data stream, a mass spectrum is collected at every pixel of the primary ion beam raster (256 × 256 pixels). As shown in Figure 2, the TOF-SIMS information gained is a total area spectrum (spectra summed from all pixels) and a total ion image (labeled A). The postacquisition mode of data analysis uses the collected raw data stream and allows images of specific ions to be generated, as well as spectra from specific regions of interest (labeled B). The resulting images allow the distribution of species of interest to be evaluated. The resulting spectra allow the chemistry of the selected regions to be identified. Oftentimes the power of this mode of analysis is realized by iterating in a comparative mode between spectra and images. For example, the spectrum extracted from a region of interest could reveal an unexpected species in a mixture. Then, by generating an image of the unexpected species, its distribution can be identified, and sometimes it is evident in areas other than the selected region of interest. 5632 Analytical Chemistry, Vol. 72, No. 22, November 15, 2000

RESULTS Reference Spectra. To allow the interpretation of the TOFSIMS data from the sectioned beads, it is first necessary to examine the reference spectra for the drugs, excipients, and coating materials. The components of the drug beads were analyzed as pure materials by TOF-SIMS spectroscopy. Although reference spectra for numerous chemicals and materials have been organized into a number of libraries (distributed by the instrument manufacturers and through ref 7), only a small portion of these materials are common to the pharmaceutical industry. The TOFSIMS spectra of these materials are briefly described in the following section by identifying characteristic signals and their corresponding chemical formulas. Drugs. The positive ion TOF-SIMS spectra of each of the drugs in this study are shown in Figure 3. All of the drug molecules give parent molecular ion signals representative of the intact molecule and fragment signals at lower mass values. Table 1 lists the characteristic peaks that were identified for each drug and their corresponding chemical formulas, for both positive and negative ion species. The combination of peaks, or “fingerprint”, for each drug is useful in spectroscopy for unambiguous identification of the molecule, especially when it is present in a mixture of species (such as in drug bead formulations). As shall be describe later, these characteristic peaks can also be used to map the distribution of the drug in a delivery system. The distribution of a molecular species can be inferred by mapping the intensity of a specific peak in the area of analysis, as long

Figure 10. TOF-SIMS images of bead system 2 (250 × 250 µm2). The distribution of the drug theophylline is fairly homogeneous throughout the core. The lactose is observed to segregate to the outer edge of the inner layer (interfacial region). Na and ethylene glycol are also concentrated in the interfacial region. The image of m/z 219 shows it is located in the inner layer in small clusters that contrast with the distribution of the drug. The peak does not correspond to any of the known bead ingredients but often is observed in the spectra of Irganox additives.

as no other material in the system generates the same peak. Typically, a high-mass peak such as the parent molecular ion is chosen. In this study, peaks at m/z 589+, 181+, and 152+ were used to map the drugs prednisolone, theophylline, and paracetamol, respectively. Excipients. Table 2 describes the characteristic TOF-SIMS peaks for common bead filler materials. A bead core is composed in bulk of filler materials such as microcrystalline cellulose (Avicel PH101), the saccharides lactose and amylose, and/or glycerol monostearate (GMS). The polysaccharides tend not to have distinct high-mass peaks in the TOF-SIMS spectra. Lactose, however, gives a strong signal at m/z 365 (positive ion mode), the molecular ion cationized by Na (Figure 4). In contrast, the positive ion spectrum for amylose appears to have few diagnostic peaks. The positive ion spectrum for the microcystalline cellulose material is shown in Figure 5. Cellulose materials have many fragment peaks in common, but the fingerprint pattern of signal intensities is unique for each type of cellulose. In some beads, additional ingredients are included in the formulation to perform specific functions. Barium sulfate, employed for imaging purposes, gives unique, distinct TOF-SIMS signals. In the positive ion spectrum (Figure 5), the peak at m/z 138 (molecular weight of Ba) is useful for mapping BaSO4. The spectrum of GMS shows several characteristic fragments in the high-mass range (m/z 239, 267, 285, 313, 341, and 359). The peak at m/z 341 represents a large stearate fragment and is useful for mapping GMS. Talc is also an ingredient employed in the Eudragit film coatings in bead formulation 2; it provides a number of diagnostic cations in the

TOF-SIMS spectrum (not shown) including Mg (m/z 24) and Si (m/z 28) (representing magnesium silicate of talc). Film Coatings. A number of commercial formulations based on synthetic polymers have been used as film coating materials to achieve a specific function. In this case, Eudragit L 30 D-55 has been employed, which is composed of 1:1 poly(methacrylic acid-ethyl acrylate). It functions as an enteric coating that dissolves above pH 5.5, which allows the inner components of the drug bead to be released in the small intestine rather than the stomach. The positive ion TOF-SIMS spectrum of this polymer shows a fingerprint pattern of characteristic hydrocarbon signals (Figure 6). In the negative mode (spectra not shown), the methacrylic acid repeat unit signal at m/z 85 is significant and useful for imaging. The characteristic positive and negative TOFSIMS peaks are shown in Table 3. The second film coating employed is based on ethylcellulose, known commercially as Surelease, and contains ammonium hydroxide, coconut oil, and oleic acid to aid film formation and stability. The positive ion spectrum of this polymer is similar to the cellulose materials discussed above with two important exceptions. First, as noted for all the substituted cellulose ethers, diagnostic ions are present for the ethyl substituent as both the intact (CH2CH3+ at m/z 29) and derived from the cleavage from the polysaccharide chain (CH2OCH2CH3+ at m/z 59).4 Second, the peaks at m/z 327, 355, and 383 are most likely due to the fatty acid present in the Surelease formulation. Characterization of Drug-Loaded Beads. Bead System 1. This spheroid bead has a core containing the drug prednisolone, Analytical Chemistry, Vol. 72, No. 22, November 15, 2000

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Figure 11. TOF-SIMS image shows the total ion image of bead system 2. To compare the chemical composition of each of the layers, spectra were extracted from three regions as depicted and are shown below. Peaks labeled with D are characteristic peaks for the drug theoophylline. Peaks labeled with * are discussed in the text.

Avicel, and lactose and is coated with a film coating containing ethylcellulose (Surelease) and amylose. Figure 7 shows images of the entire cross section of one of the prednisolone-loaded beads (800 × 800 µm2, negative ion mode). In the total ion image, the integrity of the layers and the morphology of the bead are clearly evident. The inner layer does show an absence of signal in the center, which is mostly due to the rough topography of the sample (since this region is at a different height from the rest of the bead, it is difficult for the secondary ions to reach the analyzer and detector) and reflects the difficulties in obtaining a flat section for such friable materials. However, the film coating is intact, fully encapsulates the spheroid bead, and can visually be distinguished from the underlying bead core. The image of the prednisolone drug (prednisolone parent ion, m/z 543) shows its distribution is confined to the inner layer and is evident as clusters up to 20 µm in size. The chemical composition of each of the layers can be evaluated through spectroscopy. Regions of interest can be defined within an image and spectra extracted specifically from those regions (in real time or postacquisition). These extracted spectra can then be compared to the reference spectra of the drug and the excipients to identify the composition of the defined regions. The TOF-SIMS spectra were extracted from the film coating and bead core regions using a higher resolution TOF-SIMS image of 5634


a section of the bead (Figure 8). It is clearly evident from Figure 8 that the spectra reveal very different chemical compositions in the two different areas of the bead. The spectrum of the outer coating shows significant peaks at m/z 29, 59, 116, 125, 143, 155, and 183 (positive ion mode). The entire spectrum, the relative intensity and masses of the signals, correlates well with the reference spectrum of ethylcellulose (see Surelease spectrum, Figure 6). In contrast, the spectrum of the inner layer shows contributions from a number of components. The prednisolone drug is evident by the peak at m/z 589, representative of the parent ion of prednisolone (positive ion mode), and another characteristic signal at m/z 121. The peak at m/z 385 is representative of lactose, one of the major excipients in the speroid bead formulation (molecular ion + Na). A significant signal is also observed at m/z 130, which is unidentified. Since none of the reference spectra show the peak at m/z 130 as significant, this peak suggests the presence of a component in the formulation that has not been revealed. The low-mass range of this spectrum shows mainly hydrocarbon signals, which arise from fragmentation of all of the higher mass components. Avicel is also represented in the lowmass hydrocarbon fragments, since its structure does not generate any unique high-mass fragments. The spectrum of the inner layer of the bead in Figure 8 is qualitative in that it allows the identification of multiple species.

Figure 12. TOF-SIMS images of the outer coating of bead system 2 (60 × 60 µm2). The Mg and Si images indicate that talc is present as clusters ranging in size from 1 to 10 µm.

Quantification of the species by TOF-SIMS is difficult, however. The intensity of a signal in the mass spectrum is dependent on several factors including the surface concentration of the species, primary ion flux, and most importantly the transformation probability (sputter yield and ionization probability) of the species. 20,21 The transformation probability varies over several orders of magnitude for all ions, can change depending upon the matrix of a particular species, and is not known for most molecular species. All of these factors make it difficult to correlate peak intensity to concentration. Nonetheless, relative peak intensities can be compared for samples of similar composition. Furthermore, direct quantification is possible after measuring a controlled set of standards. Once the peaks in a spectrum have been evaluated for chemical information, imaging can be used to gain spatial distribution information. For drug bead characterization, insight can be obtained through the diagnostic ions of the drug and the excipients, by mapping their distributions (Figure 9, 250 × 250 µm2 images). The prednisolone drug and the lactose are known to be in the bead core, and their distributions can be mapped using the characteristic molecular ions identified in the reference spectra. The prednisolone is observed to be present as clusters in the inner layer up to 20 µm in size representing the presence of individual drug crystals rather than a molecular dispersion throughout the core. The lactose is present in regions similar to prednisolone, but also is evident in additional (20) Vickerman, J. C.; Brown, A.; Reed, N. M. Secondary Ion Mass Spectrometry Principles and Applications; Oxford Science Publications: Oxford, 1989. (21) Reed, N. M.; Vickerman, J. C. The Application of Static Secondary Ion Mass Spectrometry (SIMS) to the Surface Analysis of Polymer Materials. In Surface Characterization of Advanced Polymers; Sabbatini, L., Zambonin, P. G., Eds.; VCH: Weinheim, Germany, 1993.

small deposits rather than a homogenious distribution. Unexpected components are also present in the inner layer, as evidenced by the signal at m/z 130. This species is present as smaller clusters (up to 10 µm) and is in regions different from prednisolone and lactose. Figure 9 also shows the presence of the outer coating of the drug bead composed of Surelease, the ethylcellose material. The most significant, unique ethylcellulose signal is at m/z 59 (attributed to (CH3CH2OCH2) and was used to map the film coating distribution and integrity. The film thickness is ∼50 µm, forms an intact coating around the core, and shows a well-defined interface with the core where there is no indication of intermixing between the core and the polymer film. The minor component of the film is amylose, which provides few prominent diagnostic peaks that distinguish the polysaccharide from the ethylcellulose film. Its reference spectrum shows mainly peaks for low-mass hydrocarbon fragments, which therefore makes it difficult to identify a characteristic peak for mapping its distribution. In addition, the amylose component in the outer layer is minor compared with ethylcellulose (amylose-to-ethylcellulose ratio of 1:3 in formulation). This is reflected in the spectrum pulled from the outer shell which corresponds to mainly to ethylcellulose. Bead System 2. TOF-SIMS images of a second bead system are shown in Figure 10 (partial bead area, 250 × 250 µm2). This system contains an inner core composed of the drug theophylline, Avicel, and lactosee and an outer shell of film coating polymer, Eudragit L 30 D-55. The components of this bead have a completely different distribution compared to the above example. The major difference is the observed homogeneous distribution of the drug (parent molecular ion, m/z 181) throughout the bead core. Theophylline has good water solubility, is likely to be molecularly dispersed throughout the core bead ingredients during the mixing phase prior to spheronization, and hence will be well distributed throughout the beads during production. An interesting feature of this bead system is the surprising observed segregation of lactose to the outer edge of the core. Evaluating the spectra extracted from each of the layers serves to confirm the characteristic ions of known species across the bead cross section (Figure 11). The outer coating of the bead is composed of an Eudragit L 30 D-55 film with talc as a filler. The spectrum extracted from that region (Figure 11c) is dominated by signals for talc rather than the polymer; there is a striking difference to the reference spectrum of Eudragit L 30 D-55 (Eudragit spectrum, Figure 6). The positive spectrum of the coating film is dominated by signals for Mg (m/z 24) and Si (m/z 28), and the negative spectrum also shows signals for SiO2 and SiO3 (m/z 60 and 76). These inorganic ions of talc are much higher in intensity within the SIMS spectra than the characteristic signals for the polymer film and will dominate the spectra even at low solids loading. The images in Figure 12 show a closeup view of the outer coating of the bead. The talc (Mg and Si images) is observed to be present as clusters ranging in size from 1 to 10 µm. In addition, spectra obtained from the outer surface shell of an uncut bead are dominated by peaks representing Mg and Si from talc. The spectrum of the central core region of the bead (Figure 11a) is dominated by signals for the drug theophylline (m/z 181, 165, 122). Furthermore, the spectrum of the inner layer of the Analytical Chemistry, Vol. 72, No. 22, November 15, 2000

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Figure 13. TOF-SIMS images of bead system 3 (300 × 300 µm2). The paracetamol drug is present in a large distinct region (approximately 75 × 150 µm2). Ba from BaSO4 is present in smaller clusters (less than 10 µm), as is Na. The glycerol monostearate appears to be present in all the other regions of the inner layer.

bead shows a signal at m/z 219 that does not correspond to characteristic signals of any of the listed ingredients. This peak is often observed in the spectra of Irganox additives, which may be the source. Such surfactants are often included as wetting agents in pharmaceuticals. The image of m/z 219 shows that it is located in the core region in small clusters that contrast with the homogenious distribution of the drug. An examination of the spectrum (Figure 11b) extracted from the interfacial region of the bead (adjacent to the inner surface of the polymer coating) also yielded interesting results. While the spectrum is similar to the data obtained from the core of the bead, there are a number of important differences. There appears to be an increase in intensity for the lactose signal in this region. However, the m/z 219 is weaker and the signals at m/z 23 (Na) and at m/z 45 (C2H5O) are stronger. The spectrum indicates that the chemical composition of the bead at the interface with the film coating contains a higher concentrations of lactose and Na, and the fragment C2H5O, which is suggestive of an ethylene glycoltype material (see images, Figure 9). Bead System 3. TOF-SIMS images of a third bead system are shown in Figure 13 (partial bead area, 300 × 300 µm2). The bead is composed of the drug paracetamol, GMS, Avicel, and BaSO4 coated with an ethylcellulose film coating. The images show the drug (parent molecular ion at m/z 152), in this case, to be present in large distinct domains (approximately 75 × 150 µm2) representing the intact drug crystals. Such crystals were observed throughout the entire bead dispersed within the excipient formulation. In contrast to the theophylline beads where the drug was homogeneously distributed, the poor water solubility of paracetamol allows the retention of the native drug crystals 5636 Analytical Chemistry, Vol. 72, No. 22, November 15, 2000

during the bead production process. GMS, a matrix component of the bead core, is observed to be distributed throughout the core surrounding the drug crystals, as expected. The BaSO4 is used as an imaging agent for the formulation. Figure 13 shows the image of the distribution of Ba (m/z 138) and infers that BaSO4 is present in discrete small clusters (less than 10 µm) throughout the bead core, reflecting the native particle size of this material. As for the previous bead systems, the chemical composition of regions of interest were evaluated by extracting spectra from those regions (Figure 14). The spectrum extracted from the region of the paracetamol (region 2) is dominated by the paracetamol parent molecular signal. The lower mass range of this spectrum shows a strong signal at m/z 109, representing a paracetamol fragment, as well as typical low-mass hydrocarbon signals (m/z 27, 29, 41, 43, 55). No other significant signals are observed in the spectrum, confirming the region is mainly a paracetamol crystal. The spectrum extracted from the remainder of the core (region 1) is dominated by significant GMS signals in the highmass range (m/z 239, 267, 285, 313, 341, 359). It is interesting to note the low-intensity signals for paracetamol (m/z 152, 109) are also evident in this region probably due to low levels of dissolved drug dispersed in the bead matrix during production. The spectrum is dominated by hydrocarbon fragment signals in the low-mass range (envelope of signals in the range m/z 15-97). Most likely these signals result from fragmentation of both GMS and paracetamol. The spectrum extracted from the region of the BaSO4 (region 3) is dominated by the signal at m/z 138 for Ba. The other peaks in the high-mass range, at m/z 115, 155, 393, and 421, have not been identified; however, they suggest the

Figure 14. TOF-SIMS image showing the total ion image of bead system 3. To compare the chemical composition of each of the layers, spectra were extracted from three regions as depicted and are shown below. Peaks labeled with G are characteristic peaks for GMS. Peaks labeled with D are characteristic of the drug paracetamol.

Figure 15. (+)TOF-SIMS spectrum from outer shell of paracetamol bead system. Characteristic ethylcellulose peaks are labeled with *.

presence of another component in this region. The peaks in the low-mass range representing hydrocarbon fragments suggest the additional component(s) are organic in nature with the composition of CxHyOz. The ethylcellulose outer shell is not shown in the images. To verify the composition of the outer coating, it was analyzed directly. The entire bead was mounted on a Si substrate without cross sectioning. The TOF-SIMS analysis probes only the top 20 Å of the bead and gives a spectrum of the outer shell (Figure 15). The spectrum correlates very well with the ethylcellulose reference spectrum.

CONCLUSION The three beads in this study each had a unique distribution of ingredients. By imaging the molecular ion for each drug, their distribution was shown to range from micrometer-sized particles in one bead cross section to almost homogeneous in another bead cross section. The majority of the ingredients in the bead could be identified by direct analysis of the bead cross section. If the ingredient gives a TOF-SIMS peak that is characteristic of no other ingredient, it is easy to map the distribution of that ingredient. However, if the ingredient gives mainly lowmass peaks (redundant with other ingredients), the ingredient Analytical Chemistry, Vol. 72, No. 22, November 15, 2000

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is best identified from a spectrum extracted from the region of the bead where it is expected to be located. The TOF-SIMS data showed the ingredients of the bead did not match exactly the manufacturer’s specifications. Some ingredients were found to be lacking in the specified layer, and in some layers, additional ingredients were identified by TOF-SIMS spectroscopy. Many common drug bead ingredients were analyzed as pure substances, providing TOF-SIMS reference spectra of these materials for the first time. As a technique, TOF-SIMS allows in situ analysis of drug beads. The ability to image molecular ions with submicrometer spatial resolution makes TOF-SIMS well suited to pharmaceutical analy-

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sis. The integrity of each of the layers can be evaluated through imaging of specific species. Furthermore, TOF-SIMS is a mass spectral technique, which gives structural and molecular information from each of the drug bead layers simultaneously. The chemical composition of each of the layers can then individually be characterized. For the identification of pharmaceutical ingredients, reference spectra are useful.

Received for review April 18, 2000. Accepted August 10, 2000. AC000450+

TOF-SIMS Characterization and Imaging of Controlled-Release Drug

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