Transmission Electron Microscopy of Pharmaceutical Materials
MARK D. EDDLESTON,1 ERICA G. BITHELL,2 WILLIAM JONES1
1
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom
2
Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ,
United Kingdom
Received 12 February 2010; revised 16 April 2010; accepted 19 April 2010
Published online 2 June 2010 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jps.22220
ABSTRACT: Transmission electron microscopy (TEM) and its facility for electron diffraction has
long been a key technique in materials science. Its use for characterization of pharmaceutical
samples has, however, been very limited, largely due to the difficulties associated with the
preparation of appropriately thin samples, as well as issues with sample damage caused by the
electron beam. In this overview, we describe straightforward approaches for overcoming these
issues which have enabled us to characterize a variety of pharmaceutical compounds, including
theophylline, paracetamol and aspirin, and also pharmaceutical salts and cocrystals. A range of
relevant information about these compounds is derived including morphology, polymorph
identification, mapping of crystal habit to crystal structure and crystal defect characterization.
With theophylline, we identify crystals of   impurity  polymorphic phases in samples that appear
from powder X-ray diffraction to be monophasic, and observe that crystal growth behavior of
samples prepared from nitromethane is significantly different to that of samples prepared from
methanol. The existence of imperfections, such as dislocations, is also established and these are
shown to be likely sites at which fracturing occurs when the crystals are stressed. The results
demonstrate that various issues associated with pharmaceutical form development might
usefully be addressed using TEM. ß 2010 Wiley-Liss, Inc. and the American Pharmacists Association
J Pharm Sci 99:4072 4083, 2010
Keywords: microscopy; polymorphism; solid state; cocrystals; crystal defects; crystallogra-
phy; crystals; materials science; morphology; nanoparticles
INTRODUCTION Transmission electron microscopy (TEM) has long
been used in materials science as a powerful
Characterization of crystal form is an important issue analytical tool.8 13 Its application to pharmaceutical
in pharmaceutical materials science.1,2 The existence materials, however, has been very limited. Some
of polymorphs, for example, is considered to be a key examples include imaging crystals of the compounds
concern,3 5 as is the stability of a chosen form to dipyridamole14 and taxol,15 and defects in liquid
various processing (and storage) conditions, for crystals of fenoprofen,16 and an electron diffraction
example, milling and tableting.6 The recent emer- study on roxifiban,17 but the full potential of the
gence of pharmaceutical cocrystals as alternatives to technique has yet to be fully appreciated.
salts and amorphous forms is recognized,7 but raises There are two major reasons for the underdevelop-
questions during development of the importance that ment of TEM in this field. The first relates to sample
will need to be attached to the purity of the phases preparation: because of the strong interaction of the
produced, for example, components of the target electron beam with the sample18 it is required that
cocrystal present as   impurity  phases or the possible the specimens be very thin ( 500 nm even for light,
generation of small amounts of a different cocrystal organic compounds).19 Methods have been developed,
stoichiometry beyond the usual detection limits of however, for organic molecular crystals and as we
routine analytical methods such as powder X-ray demonstrate these can be used to prepare appropriate
diffraction (PXRD). pharmaceutical materials. Even when this is not
possible (i.e., when the analysis is of a specific
material such as milled crystals), we demonstrate
that useful information can still be obtained. The
Correspondence to: William Jones (Telephone: þ44-1223-
second deterrent is the inherent susceptibility of an
336468; Fax: þ44-1223-762829; E-mail: [e-mail])
organic material to electron beam damage.19 For
Journal of Pharmaceutical Sciences, Vol. 99, 4072 4083 (2010)
ß 2010 Wiley-Liss, Inc. and the American Pharmacists Association certain types of analysis, such as a full defect
4072 JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 9, SEPTEMBER 2010
TEM OF PHARMACEUTICAL MATERIALS 4073
characterization, this will certainly limit the applica- matched to calculated values for known structures,
tion of TEM. Nevertheless, we demonstrate that giving both the composition of the sample and the
sufficient data can be obtained for identification of zone axis of the diffraction pattern. The experimental
crystalline imperfections. diffraction pattern was then compared with a
In this overview, we illustrate various instances in simulated diffraction pattern of the given zone axis
which TEM has provided useful information on to ensure a match using CrystalMaker SingleCrystal
crystal habit, polymorph identification, defect content v1.3 software from crystal structures published in the
and the analysis of ground samples obtained using Cambridge Structural Database (CSD). In practice,
the recently developed approach of liquid assisted some diffraction patterns, from high index zone axes,
grinding.20,21 We use theophylline, paracetamol and could not be unambiguously indexed. If a known
ranitidine hydrochloride as examples along with a 2:1 crystal form had no reported crystal structure,
caffeine/oxalic acid cocrystal. Given that the sample is obtained electron diffraction patterns were compared
held under high vacuum during analysis, TEM is not with a PXRD trace of the form by plotting reflections
likely to be generally suitable for hydrated or solvated on a 2u scale.
materials. Scanning electron microscopy (SEM) images were
obtained with a JEOL JSM-5510LV instrument.
Samples were prepared on a sticky carbon sample
EXPERIMENTAL mount placed on a brass SEM stub and sputter coated
with platinum to reduce charging during analysis.
All chemicals were purchased from Sigma Aldrich Polarized light microscopy (PLM) was performed on a
and used as received. Solution cooling, solvent Leica DM1000 instrument with a polarising filter.
evaporation, crystallization from the melt, crystal Powder X-ray diffraction analysis was performed on a
growth on a water surface and grinding were used to Philips X Pert Diffractometer with Cu Ka radiation at
prepare samples for TEM analysis. Solution cooling a wavelength of 1.5406 Å and data collected between 3
samples were prepared by cooling solutions to below and 508 2u at ambient temperature.
the saturation temperature to induce precipitation.
The resulting crystals were then pipetted onto a TEM
sample support grid. Solution evaporation specimens RESULTS AND DISCUSSION
were prepared by evaporative precipitation of crystals
directly onto TEM grids. Melt crystallization samples Figure 1 displays electron diffraction patterns
were heated until liquid and then spread thinly over a obtained for a sample of theophylline prepared by
TEM grid. Crystallization on a water surface was cooling a solution of theophylline in nitromethane.
performed by dissolving compounds in a water The patterns show loss of sample crystallinity caused
immiscible solvent such as p-xylene. A few drops of by exposure to the electron beam. The direction of
these solutions were pipetted onto a water surface view, however, remained down the same zone axis of
and allowed to evaporate slowly. Grinding experi- the crystal throughout the duration of the experi-
ments were conducted on approximately 200 mg of ment, demonstrating that the electron beam induced
sample in 10 mL stainless-steel containers with two little sample movement or tilting. Though beam
stainless-steel balls of 7 mm diameter. The grinding damage is unavoidable during TEM analysis, it was
was carried out in a Retsch MM200 mixer mill, found that the rate could be reduced sufficiently to
operating at a frequency of 30 Hz. For liquid-assisted allow specimens to be characterized. This was
grinding experiments, 20 mL of nitromethane was achieved primarily by reducing the flux of electrons
also added into the grinding container. Less aggres- through the sample during analysis. A liquid nitrogen
sive grinding conditions were used for some samples. cooled sample holder was used to further increase the
In these cases, crystals were lightly crushed between stability of particularly beam sensitive compounds
two glass slides. such as aspirin.
Transmission electron microscopy characterization Figure 2 (theophylline) illustrates the clear advan-
was performed at room temperature in a Philips tages that the high magnifications available when
CM30 instrument operating at 300 kV (unless stated) imaging with TEM, and the ability to generate
and data were collected on photographic films, which diffraction data, give this technique over optical
were scanned in order to generate digital images. A microscopy and SEM. The small crystallites are
double tilt sample holder was used and the samples visible in the optical micrograph, but no detailed
supported on holey-carbon films on 400 mesh copper information can be obtained. In the SEM image the
grids. Electron diffraction patterns were indexed by presence of small, overlapping, triangular crystals is
comparison with known crystal structures. The clear, but no structural or compositional information
positions of reflections in experimental diffraction is possible. The TEM image also shows the triangular
patterns were measured, converted to d-spacings and morphology of the theophylline crystals. In addition,
DOI 10.1002/jps JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 9, SEPTEMBER 2010
4074 EDDLESTON, BITHELL, AND JONES
dark lines running across the crystals are bend
contours and signify regions where Bragg diffraction
conditions are met. Disruption to bend contours as
they cross the sample reveals the presence of defects
running in the [010] crystal direction. Many of the
bend contours cross, in the region marked with a
circle, to form a bend contour pole. The associated
zone axis electron diffraction pattern, taken from the
circled region, was used to determine the crystal
phase.19 The pattern can be indexed on the basis of
the form II orthorhombic unit cell reported by
Ebisuzaki et al.22 (CSD ref. BAPLOT01, Pna21,
a ź 24.612 Å, b ź 3.8302 Å, c ź 8.5010 Å) and corre-
sponds to a view down the <100> axis of the crystal
(the indexing of selected reflections is shown, as is a
simulated diffraction pattern of the <100> zone axis).
Single diffraction patterns are often enough to
uniquely identify the phase, but if required, a series
of diffraction patterns at different sample tilts could
be obtained as further confirmation. Furthermore, by
moving around the specimen and collecting a series of
diffraction patterns from different crystals it is
possible to assess the phase purity of the sample.
The identification of different polymorphs of para-
cetamol is shown in Figure 3. Paracetamol has been
reported to have three polymorphic phases,23 of which
form I is the most stable,24 though there has also been
interest in form II due to its superior compaction
properties.25 Samples of forms I and II were prepared
from the melt using procedures described by Di
Martino et al.26 Diffraction patterns from these two
samples were indexed against the unit cells of both
form I and form II reported by Haisa et al.27 (CSD ref.
HXACAN01, P21/a, a ź 12.93 Å, b ź 9.40 Å, c ź 7.10 Å,
b ź 115.98) and Nichols et al.28 (CSD ref. HXACAN08,
Pbca, a ź 17.1657 Å, b ź 17.7773 Å, c ź 7.212 Å),
respectively. The diffraction pattern from the sample
of form I matched the <010> zone axis of this form,
and was not a match for any of the zone axes of form II
of paracetamol. The simulated electron diffraction
pattern of the <010> zone axis is shown for
comparison. Likewise, the diffraction pattern from
the sample of form II corresponded to a view down the
<001> zone axis and did not match any of the zone
axes of form I. Again, the simulated electron
diffraction pattern of this zone axis is shown for
comparison.
As shown in Figure 2, the area of sample used to
generate an electron diffraction pattern is typically
less than 10 mm2 in our experiments. Polymorph
identification by TEM therefore requires just a single
micro-crystal, and in favorable cases can be successful
Figure 1. (a c) <100> zone axis diffraction pattern of
on the nanometre scale, making it a significantly
form II of theophylline after approximately 2, 20, and
more sensitive technique than PXRD.
40 min, respectively, showing loss of crystallinity over time.
In Figure 4 the use of diffraction pattern indexing
for the identification of the crystalline phase of a
pharmaceutical salt (ranitidine hydrochloride) and a
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TEM OF PHARMACEUTICAL MATERIALS 4075
Figure 2. (a) Polarized light microscopy image of theophylline crystals prepared on a
copper sample support grid. (b) Scanning electron microscopy image of triangular
theophylline crystals. (c) TEM image of overlapping triangular crystals of form II of
theophylline. The dark lines running across the crystals are bend contours. (d) Electron
diffraction pattern from the circled region in image (c). (e) Simulated electron diffraction
pattern of the <100> zone axis of form II of theophylline (CSD ref. BAPLOT01).22
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4076 EDDLESTON, BITHELL, AND JONES
Figure 3. Selected area diffraction of two polymorphs of paracetamol. (a) <010> zone
axis electron diffraction pattern of form I of paracetamol. (b) Simulated electron
diffraction pattern of the <010> zone axis of form I of paracetamol (CSD ref. HXA-
CAN01).27 (c) <001> zone axis electron diffraction pattern of form II of paracetamol. The
010 reflection should be systematically absent, but is seen in the diffraction pattern due
to multiple scattering. This effect is commonly observed with electron diffraction, unlike
X-ray diffraction, due to the stronger interaction of the electrons with specimens. (d)
Simulated electron diffraction pattern of the <001> zone axis of form II of paracetamol
(CSD ref. HXACAN08).28
cocrystal (2:1 caffeine/oxalic acid) is shown. Both et al.30 (CSD ref GANXUP, P21/c, a ź 4.41430 Å,
specimens were prepared by careful crushing of large b ź 14.7701 Å, c ź 15.9119 Å, b ź 96.48508) and corre-
crystals between two glass slides. The ranitidine sponds to a view down the <110> axis of the crystal.
hydrochloride diffraction pattern was indexed on the This diffraction pattern did not match any zone axis of
basis of the form II unit cell reported by Mirmehrabi forms I and II of caffeine or of the alpha or beta
et al.29 (CSD ref TADZAZ03, P21/n, a ź 7.208 Å, polymorphs of oxalic acid.
b ź 12.979 Å, c ź 18.807 Å, b ź 95.068) and corre- The detection of phases additional to those expected
sponds to a view down the <10-1> axis of the crystal. is also an important feature of TEM. The diffraction
This diffraction pattern did not match any zone axis of pattern in Figure 5, obtained for a sample of
form I of ranitidine hydrochloride or of forms I or II of theophylline grown by cooling an ethyl acetate
ranitidine free base. The diffraction pattern of the 2:1 solution, illustrates the detection of a previously
cocrystal of caffeine and oxalic acid was indexed unidentified phase of theophylline. Despite repeated
on the basis of the unit cell reported by Trask attempts to index the pattern on the basis of form II of
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TEM OF PHARMACEUTICAL MATERIALS 4077
Figure 5. Diffraction pattern, from a sample thought to
be solely form II of theophylline, which could not be indexed
to a zone axis of this theophylline polymorph. Several
samples of theophylline have been analyzed to date, but
this diffraction pattern has been observed just once.
compound.33,34 The diffraction pattern from the
unknown phase could then be indexed against these
simulated crystal structures to look for matches.
Figures 6 and 7 show an example of linking the
obtained diffraction and imaging information from
TEM. The PXRD trace of a sample of theophylline
crystallized from methanol, which appears to be
monophasic form II, is shown alongside a simulated
trace of form II of theophylline. During TEM analysis
of this sample, many crystals indexable as form II
Figure 4. Selected area diffraction of a salt and a cocrys-
tal confirming the identity of the phases present. (a) <10-1>
zone axis electron diffraction pattern of form II of ranitidine
hydrochloride. (b) <110> zone axis electron diffraction
pattern of a 2:1 caffeine/oxalic acid cocrystal.
theophylline, no successful indexing was possible.
Likewise, comparison of the diffraction pattern
reflections with PXRD peak positions of other
reported polymorphic forms of theophylline, for which
there is no reported crystal structure (form I, and a
form recently reported by Roy et al.),31,32 did not give
a match. The two possible explanations for this result
are the presence of an unknown impurity phase or the
Figure 6. PXRD trace of a sample of theophylline. (Top)
existence of a previously unknown polymorph.
Simulated pattern of form II of theophylline (CSD ref.
Obtaining such patterns may suggest that further
BAPLOT01).22 (Bottom) Theophylline crystallized from
polymorph screening experiments are necessary. For
methanol. The difference in peak intensities between the
example, crystal structure prediction could be used to
experimental and simulated patterns is believed to result
search for possible low energy crystal forms of the from preferred orientation in the experimental sample.
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4078 EDDLESTON, BITHELL, AND JONES
Figure 8. (a) TEM image of a triangular crystal of theo-
phylline prepared by cooling a nitromethane solution. The
Figure 7. TEM characterization of the same sample of
corresponding <100> zone axis electron diffraction pat-
theophylline as analyzed by PXRD in Figure 6. (a) TEM
tern of theophylline form II is shown as an inset. The
image of lath-shaped crystals of theophylline form II. (b)
direction of most rapid growth for this triangular crystal
TEM image of a crystal which appeared to be significantly
was the [001] direction. (b) TEM image showing a lath-
darker, and therefore thicker, than other crystals of a
shaped crystal of theophylline form II that was crystal-
similar size. The inset is an electron diffraction pattern
lized from methanol. The asterisk marks a region of the
from this crystal, in the correct relative orientation. The
crystal where bend contours are distorted as they pass
diffraction pattern was a match for the polymorph of theo-
across defects running in the [010] direction. The corre-
phylline reported by Roy et al.32
sponding <110> zone axis diffraction pattern is included
as an inset. The ring of diffraction spots is due to reflec-
tions from small crystallites which grew on the amorphous
were observed (Fig. 7). However, the imaging mode
carbon film support during sample preparation. The direc-
also revealed a small number of crystals that were
tion of most rapid growth for this crystal was the [010]
morphologically different to the bulk sample. The
direction.
associated diffraction pattern recorded from one of
these crystals was a match for the theophylline
polymorph reported by Roy et al.32 This result
demonstrates how TEM can identify phases that
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 9, SEPTEMBER 2010 DOI 10.1002/jps
TEM OF PHARMACEUTICAL MATERIALS 4079
are undetectable by XRPD. It also suggests that TEM and corresponding diffraction patterns of crystals of
could be an important tool for investigating samples theophylline prepared from nitromethane and metha-
which contain a mixture of phases. nol (Fig. 8) demonstrate that the direction of most
A further advantage of the combination of images rapid crystal growth of crystals of theophylline
and diffraction patterns is the ability to map crystal prepared from nitromethane is different to that of
morphology to crystal structure. For example, images crystals prepared from methanol. The triangular
Figure 9. (a) TEM image showing a small region of a triangular crystal of theophyl-
line with dislocations running in the [010] direction. The pair of bend contours marked
with asterisks are unaffected as they cross the dislocations, indicating that the corre-
sponding crystal planes are not bent by the defect. The corresponding diffraction
pattern, of the <100> zone axis, is included as an inset. (b) TEM bright field image
of paracetamol form I crystallized from a melt. The discontinuous nature of the bend
contours as they run across the sample may indicate that micro-twinning has occurred in
the crystals during growth. (c and d) Crystals of theophylline that originally crystallized
with a triangular habit, have fractured along defects that run in the [010] direction to
give smaller, trapezoid-shaped crystals.
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4080 EDDLESTON, BITHELL, AND JONES
Figure 10. Preparation methods for TEM samples. (a) TEM bright field image of a
crystal of aspirin prepared by rapid evaporation of an acetonitrile solution. The sample
was cooled to 1788C during analysis to reduce beam damage. (b) Needle-shaped
crystals of caffeine form I prepared by evaporation of a chloroform solution. Several
of the crystals show signs of beam damage. The crystal marked with an asterisk is
tubular.44 (c) TEM bright field image of a large thin-plate crystal of p-terphenyl,
prepared by growth on a water surface, showing an extended network of defects. (d)
A sample of a 1:1 theophylline/L-malic acid cocrystal prepared by liquid assisted
grinding. The crystallites have habits ranging from thin plate to needle (250 kV).
(e) Polycrystalline diffraction pattern from the same sample.
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TEM OF PHARMACEUTICAL MATERIALS 4081
crystals from nitromethane grew most rapidly in the Jones et al.,48 generated thin foils with thicknesses of
[001] direction, whereas those from methanol grew <500 nm and areas of up to 1 cm2, making them ideal
fastest in the [010] direction. TEM is particularly for TEM analysis. Thin foils could not be obtained for
useful for morphological examination of nanosized the pharmaceutical compounds used in this study,
crystals. but it is believed that this method will prove to be an
The existence of crystal defects has been estab- optimal way of preparing TEM specimens of other
lished for theophylline and paracetamol. Series of pharmaceutical materials.
parallel imperfections running in the [010] crystal- Melt crystallization offers the advantage that
lographic direction, as shown in Figure 9, have been material can be thinly spread over a TEM sample
observed in many crystals of form II of theophylline. grid while liquid, resulting in thin specimens, as
The nature of the interaction of bend contours with shown for paracetamol (Fig. 9).
these defects suggests that they are dislocations. Ball mill grinding was found to reliably generate
Samples of paracetamol form I grown from the melt crystals of sub-micron size, suitably thin to transmit
show possible evidence of micro-twinning. electrons, for which crystal habit information could be
The influence that crystal defects have on sample obtained. However, phase identification for these
behavior was also apparent for theophylline. It was samples proved difficult (but not impossible) as
observed that triangular crystals of theophylline diffraction patterns usually contained contributions
spontaneously changed shape over time. It appears from several crystallites. The pictured crystallites of a
that the crystals fractured in the [010] crystal- 1:1 theophylline/L-malic acid cocrystal,49 prepared by
lographic direction, along the observed dislocations, liquid assisted grinding, have particle sizes ranging
giving smaller, trapezoid-shaped crystals. The frac- from 50 to 1000 nm and morphologies ranging from
turing could also be induced mechanically. needle to thin plate. This result demonstrates the
It is known that defects are sites in crystals at suitability of TEM for the characterization of
which transformations, such as polymorphic transi- pharmaceutical nanomaterials. TEM is already
tions and hydrate formation, are initiated,35 41 and routinely used for imaging liposomal, micellular
that processes routinely used in the manufacture of and polymeric nanoparticles containing encapsulated
pharmaceutical products such as milling and tablet- pharmaceutical compounds.14,50 52
ing introduce defects.42,43 The ability to observe and
characterize crystal defects, as afforded by TEM,
could therefore significantly improve the under- CONCLUDING REMARKS
standing and control of the solid-state behavior of
pharmaceutical compounds. Sample preparation and beam damage issues, widely
Figure 10 compares the nature of the results which considered to prohibit TEM characterization of
can be obtained from specimens prepared by different pharmaceutical samples, can be sufficiently overcome
methods. The amount of information that can be with simple approaches to enable information about a
obtained for samples prepared by solution cooling and range of pharmaceutical materials to be obtained.
solution evaporation is determined by crystal thick- Furthermore, identification of defects in samples of
ness. With large or block-shaped crystals the electron theophylline, as described here, could not be achieved
beam is scattered so completely that detailed imaging with any other analytical technique currently used in
is not achievable. However, phase identification is pharmaceutical analysis.
still possible as electron diffraction patterns can be In the wider materials context, TEM has moved
acquired from edges or corners of crystals where they somewhat away from bright/dark field diffraction-
are thinner. Flake or plate crystal habits, as exhibited contrast based techniques towards approaches which
by aspirin crystallized by evaporation from acetoni- yield three dimensional data and/or are directly
trile, can have large regions with suitable thickness interpretable (electron tomography, aberration-cor-
for TEM analysis. Lath or needle-shaped crystals are rected high resolution, scanned probe methods).10,11
often thin enough in at least one dimension for At the present time, the majority of pharmaceutical
imaging as shown for needle-shaped crystals of form I samples are not amenable to these techniques
of caffeine. It is evident that the crystal of caffeine because of the requirement for high signal-noise
marked with an asterisk in the Figure is tubular.44 ratios, and thus high total electron doses. We note
However, the area of view for each crystal is small, here that the long-established diffraction contrast
making defects hard to detect. Suitable crystal techniques13 still have an important role when the
morphologies for TEM can be targeted through choice limitations on the analysis come not from the
of crystallization solvent and/or addition of small instrumentation but from the sample itself.
amounts of impurities.45 47. We have demonstrated that TEM analysis could be
Crystallization of the organic compound p-terphenyl advantageous in the pharmaceutically important
on a water surface, following a method reported by areas of solid-phase identification and patent infrin-
DOI 10.1002/jps JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 9, SEPTEMBER 2010
4082 EDDLESTON, BITHELL, AND JONES
gement, and has the potential to provide a greater 16. Rades T, Mueller-Goymann CC. 1997. Electron and light micro-
scopical investigation of defect structures in mesophases of
understanding of defects, and related reactivity, in
pharmaceutical substances. Colloid Polym Sci 275:1169 1178.
pharmaceutical crystals.
17. Li ZG, Harlow RL, Foris CM, Li H, Ma P, Vickery RD, Maurin
MB, Toby BH. 2002. New applications of electron diffraction in
the pharmaceutical industry: Polymorph determination by using
a combination of electron diffraction and synchrotron X-ray
powder diffraction techniques. Microsc and Microanal 8:134 138.
ACKNOWLEDGMENTS
18. Reimer L, Kohl H. 2008. Transmission electron microscopy:
Physics of image formation. 5th edition. New York: Springer.
M.D.E. and W.J. thank the EPSRC for funding.
19. Jones W, Thomas JM. 1979. Applications of electron micro-
E.G.B. is grateful for support in the form of a Daphne
scopy to organic solid-state chemistry. Progr Solid State Chem
Jackson Fellowship funded by Lucy Cavendish Col- 12:101 124.
20. Trask AV, Jones W. 2005. Crystal engineering of organic
lege, Cambridge, the Thriplow Charitable Trust and
cocrystals by the solid-state grinding approach. Top Curr Chem
the Isaac Newton Trust and is currently supported by
254:41 70.
the ERC. We thank Andrew M.C. Cassidy for supply-
21. Friscic T, Jones W. 2009. Recent advances in understanding the
ing a sample of the 2:1 caffeine/oxalic acid cocrystal.
mechanism of cocrystal formation via grinding. Cryst Growth
Des 9:1621 1637.
22. Ebisuzaki Y, Boyle PD, Smith JA. 1997. Methylxanthines. I.
Anhydrous theophylline. Acta Crystallogr C C53:777 779.
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DOI 10.1002/jps JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 9, SEPTEMBER 2010