Reliable Characterization of Organic & Pharmaceutical Compounds with High Resolution Monochromated EEL Spectroscopy

Abstract

Organic and biological compounds (especially those related to the pharmaceutical industry) have always been of great interest for researchers due to their importance for the development of new drugs to diagnose, cure, treat or prevent disease. As many new API (active pharmaceutical ingredients) and their polymorphs are in nanocrystalline or in amorphous form blended with amorphous polymeric matrix (known as amorphous solid dispersion-ASD), their structural identification and characterization at nm scale with conventional X-Ray/Raman/IR techniques becomes difficult. During any API synthesis/production or in the formulated drug product, impurities must be identified and characterized. Electron energy loss spectroscopy (EELS) at high energy resolution by transmission electron microscope (TEM) is expected to be a promising technique to screen and identify the different (organic) compounds used in a typical pharmaceutical or biological system and to detect any impurities present, if any, during the synthesis or formulation process. In this work, we propose the use of monochromated TEM-EELS, to analyze selected peptides and organic compounds and their polymorphs. In order to validate EELS for fingerprinting (in low loss/optical region) and by further correlation with advanced DFT, simulations were utilized.

Keywords: EELS; TEM; amorphous; monochromator; organic; pharmaceutical; plasmon loss.

Figure 1

Scanned area and EDS mapping of TH-15 (up) and TH-27 (down) peptide compound shows presence of presence of C, N, S, O (from the molecule) and F (impurities). Cu signal is homogeneously distributed and is an artifact due the TEM sample grid being made of copper. Top left corner image shows the high annular dark field image (HAADF) of the observed area.

Figure 2

Low loss electron energy loss spectroscopy (EELS) for various organic small molecules showing characteristic (π–π* transition peaks) in low loss region, for (a) beta-cyclodextrin with no characteristic signal; (b) hexacarboxy cyclohexane ~8 eV; (c) tannin ~4.5, ~4.9, ~7.2 eV; (d) peptide TH_15~3.8, ~6.9 eV, (e) peptide TH_27~6.5 eV peak was observed.

Figure 3

High resolution low loss EELS spectra from piroxicam form 1 (monoclinic crystal structure) and form 2 (triclinic crystal structure).

Figure 4

Complex dielectric function of monoclinic form 1 (left) and triclinic form 2 (right) phases of piroxicam. The dotted line corresponded to the imaginary part of the CDF and the dashed line to the real part. The insets corresponded to a zoom of the zero-crossing energies, $\Re$(ε) = 0.

Figure 5

Comparison between calculated EELS spectra (solid line) and experimental EELS data (dotted line), for phases form 1 and form 2.

Figure 6

Comparison of the interband transitions observed on theoretical (left) and experimental data (right) for both forms of piroxicam.