Discovery and Formulation - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 216

9.3 Quantitative Raman Approaches in API
Characterisation
A more challenging analysis is the quantitative analysis of polymorphic mix-
tures. Thermal techniques (e.g. thermogravimetric analysis (TGA) and DSC)
and PXRD are commonly used to determine polymorphic form in drug sub-
stance with limits of detection as low as 1% w/w. However these techniques
often lack sensitivity in the presence of excipients. PXRD typically has a limit
of detection of approximately 5% w/w in formulated product and is unsuitable
for samples containing large amounts of excipients [18]. Solid state NMR has
a number of advantages. It can routinely determine polymorphic form with a
limit of detection lower than 1% w/w in drug substance and it is suitable for
use with formulated product. However, the sensitivity is greatly reduced in
samples containing large amounts of excipients. This technique is expensive,
requires specialist equipment and operators and significant sample prepara-
tion. It is also extremely time consuming with some bulk samples taking up
to 24 h to analyse.
In this application, Raman spectroscopy is gaining acceptance as a pow-
erful tool for pharmaceutical analysis and has a number of advantages over
both FT-IR and non-spectroscopic techniques. Raman requires no sample
handling or preparation and may be collected non-invasively from within a
sealed container [20]. It is non-destructive, fast and can assess both drug sub-
stance and formulated product. In contrast to FT-IR, Raman spectra may be
obtained in the presence of moisture [17] and the Raman spectra of most
excipients are significantly less intense than that of the API, thus giving
Raman an inherent selectivity for the API of interest [22]. As well as use in
the qualitative determination of unknowns, proof of structure and confirma-
tion of identity, Raman spectroscopy is being increasingly used quantitatively
to measure API concentrations and determine compositions of polymorphic
forms [17, 18, 20, 22-27]. Using modern chemometric approaches it is possible
to build robust quantitative methods for polymorph or impurity identifica-
tion using FT-Raman. Depending on the material and quality of standards
prepared detection limits of below 2% can be readily achieved [28]. Similarly,
spectral pre-processing techniques such as taking derivatives and orthogonal
signal correction (OSC) have been successfully used to remove backgrounds
and sampling variations [17, 29-31]. Unsupervised techniques such as princi-
pal component analysis (PCA) and supervised techniques such as partial least
squares (PLS) have been used in combination with vibrational spectroscopy
and have demonstrated an increased ability to achieve precise quantification
in pharmaceutical systems [17].
Raman is not immune from experimental di culties. One of the major
problems associated with using Raman spectroscopy to analyse pharmaceu-
tical products is fluorescence [32]. This arises from the emission of a pho-
ton during relaxation to the electronic ground state and may be caused by
the drug itself, a low-level impurity, excipients in the formulated product or