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IPT 37 2011
9/6/11
10:31
Page 52
Laboratory Technology
X-Ray-Based Pharma Analysis
X-ray-based analytical techniques offer a number of advantages over other
commonly used methods. IPT Editor Pam Barnacal looks at the options
available, how they are used and the features that will raise their level of
acceptance by pharma labs.
Accurate and reliable analytical procedures underpin
most activities in the pharmaceutical industry, and Xray-based techniques can form the basis of many of
these procedures. Advances in areas such as speed of
detection and analytical software have extended the
scope of X-ray techniques, and today they play a role in
a wide range of pharma functions – from R&D to
formulation of the finished product. This article
reviews the two main types of X-ray technique – X-ray
diffraction (XRD) and X-ray fluorescence (XRF) – and
looks at the ways in which they are employed in
pharma analysis.
X-RAY DIFFRACTION
X-ray diffraction (XRD) can be used to determine the
atomic structure of a substance and is based on observing
the scattered intensity of an X-ray beam hitting a sample.
As a group, XRD-based analytical methods comprise
several non-destructive techniques that can be
used to reveal information about the crystallographic
structure, chemical composition and physical properties
of a substance.
X-ray powder diffraction (XRPD) is a well-established
technique for characterising pharmaceutical compounds.
As the name implies, the sample is usually in a powder
form, although the method can also be used for studying
polycrystalline solids, particles in liquid suspensions or
droplets in aerosol formulations. The XRPD pattern of a
pharma substance is used to determine its crystal structure,
although in practice identifying the lattice type and
dimensions of the crystal unit cell is usually sufficient.
Once all the peaks in the diffraction pattern generated by
pure drug have been recorded, the drug can be uniquely
identified or ‘finger-printed’.
In drug discovery, XPRD can be used for the fingerprinting of new chemical entities (NCEs). This can play
an important role when patenting a new drug and can also
prove useful for charaterising alternative forms when the
patents on the original drug are due to expire. In drug
formulation, XRPD patterns can be used to identify
different drug polymorphs, as well as to measure the
degree of crystallinity of a drug which can be an important
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factor in determining its processing behaviour as well as its
pharmacological profile. Being non-destructive, XRPD
can provide definitive information about the formation of
polymorphs or hydrates under a variety of different
enviromental conditions; this is useful where high
temperatures and/or humidity during transport or storage
may adversely affect the properties of a drug. The
technique is also an ideal tool for excipient selection as
careful evaluation of drug-excipient compatibility is
necessary to ensure a constant level of drug release and
bioavailability, as well as to avoid stability problems later
on in the formulation process. Quantitative analysis
using XRPD is also useful for developing optimal
pharma formulations.
During manufacturing, XRPD plays a role in monitoring
and improving production efficiency, as well as controlling
the quality of the final product. The technique can also be
used to detect and quantify polymorphic contamination
and to identify crystallographic changes in the final dosage
form. It has a particular value in analysis of the
proportions of individual active ingredients in a finished
product, together with the amounts of any crystalline or
amorphous excipients used. Using a method known as
transmission geometry, it is even possible to analyse tablets
through a blister pack.
Apart from XRPD, other XRD-based techniques include
single-crystal XRD and small angle X-ray scattering
(SAXS). Single-crystal XRD is the oldest and most precise
method of X-ray crystallography and involves a beam of
X-rays striking a single crystal to produce scattered beams.
It is used to solve the complete structure of crystalline
materials, ranging from simple inorganic solids to complex
macromolecules such as proteins.
SAXS measurements are typically concerned with
scattering angles of less than one degree, and are
technically very challenging because of the small angular
separation of the very intense direct beam and the
scattered beam. The technique is commonly used for
probing large-scale structures such as biological
macromolecules, including proteins, nucleic acids and
so on.
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X-RAY FLUORESCENCE
XRF spectrometry is an analytical technique used for the
elemental and chemical analysis of solid, powdered and
liquid samples. It is capable of measuring elements from
beryllium (Be) to uranium (U) and beyond, at trace levels
often below one part per million and up to 100 per cent.
The main advantages of XRF are that it is non-destructive,
requires little or no sample preparation, can be used
for samples as small as 100 mg and lends itself well
to automation.
In short, X-ray fluorescence involves the bombardment
of a sample with high-energy X-rays, causing the material
to become ionised. If the energy of the radiation is
sufficient, a tightly held inner shell electron is displaced
and outer shell electrons then fall into the vacancy left
behind. In doing so, they normally emit light
(fluorescence) equivalent to the energy difference
between the two states. Since each element has electrons
with more or less unique energy levels, the wavelength of
light emitted is characteristic of the element, and the
intensity of light emitted is proportional to the element’s
concentration. The fluorescent radiation can be analysed
either by using energy-dispersive (ED) analysis to sort
the energies of the photons, or by separating the
wavelengths of the radiation through wavelengthdispersive (WD) analysis.
There are thus two main types of XRF spectrometer –
energy-dispersive (EDXRF) and wavelength-dispersive
(WDXRF) – depending on the method of analysis used.
An EDXRF spectrometer discriminates each specific
fluorescent line, based on the energy of the fluorescent
photon, and includes special electronics and software
modules to ensure that all radiation is properly analysed
in the detector. It provides a lower-cost alternative for
applications where less precision is required.
With a WDXRF spectrometer, the X-ray energies are
separated by means of a diffracting crystal and a detector
that are positioned to measure the energies one after the
other (sequential), or placed in fixed positions to measure
the energies all at the same time (simultaneous). A
sequential XRF spectrometer enables any number and
combination of elements to be measured one after
another. The results of continuous scanning over an
angular range can be plotted as a spectral pattern, from
which the elements present in a sample can be identified.
Individual peak intensities are also measured to
determine element concentrations.
EDXRF spectrometers offer a number of advantages:
they tend to be smaller, faster, simpler in design, less
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expensive and have fewer engineered parts. Also, they can
use miniature X-ray tubes enabling the development of
portable versions. WDXRF spectrometers are, however,
generally perceived to be superior in terms of
performance; they provide better spectral resolution
which can be up to 10 times better for some elements,
excellent light-element performance, low detection
limits, optimal measurement conditions and a high
degree of versatility in terms of applications. The
accuracy and precision of data generated by WDXRF can
be compared with traditional wet chemical methods in
terms of quality, but with the clear advantage of faster
analysis times. This translates into a significant reduction
in the overall costs of analysis.
Being basically an inorganic analytical technique,
traditional applications of XRF have been in areas such
as mining, ceramic and glass manufacturing, metallurgy,
geochemistry, archaeology and forensic science.
Advances in the technology have extended its use to a
range of pharma activities including drug
characterisation, production and quality control, where
it is used to detect and quantify major and minor
elements in final products, active pharmaceutical
ingredients and excipients. XRF can be used to identify
chemical composition, to detect and quantify elemental
impurities and to keep track of quantitative crystalline
phases and polymorphic contents.
CONCLUSION
X-ray techniques are seeing more extensive use within
the pharmaceutical industry. XRPD has been well
established for a number of years, particularly when it
comes to new drug applications and patent filing. XRF
has been less well accepted – largely due to its limits of
sensitivity – but is likely to see increased future demand
as requirements for quality- and authenticity-testing
continue to grow.
In the past, the requirement for an expert operator has
been a major obstacle to wider use in pharma labs, but
this should be overcome with the development of more
user-friendly instrumentation that can be used by
scientists with no specialist knowledge of X-ray analysis.
For the future, both techniques can be expected to
benefit from the development of high-performance,
ultra-fast detectors with improved detection limits and
shorter measurement times, while advances in software
will help deal with the vast amounts of data generated. As
the technology becomes more sophisticated and the
instrumentation more sensitive and user-friendly, X-ray
techniques can be expected to become more common in
pharma R&D and quality control environments.
Innovations in Pharmaceutical Technology