Performance Evaluation of Quantitative Adiabatic 13C NMR Pulse Sequences for Site-Specific Isotopic Measurements

• Published in: Analytical chemistry (Washington) 2010-07, Vol.82 (13), p.5582-5590
• Main Authors: Thibaudeau, Christophe, Remaud, Gérald, Silvestre, Virginie, Akoka, Serge
• Format: Article
• Language: eng ; jpn
• Subjects: Analytical chemistry; Chemistry; Exact sciences and technology; Spectrometric and optical methods
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/ac100478h

2H/1H and 13C/12C site-specific isotope ratios determined by NMR spectroscopy may be used to discriminate pharmaceutically active ingredients based on the synthetic process used in production. Extending the Site-specific Natural Isotope Fractionation NMR (SNIF-NMR) method to 13C is highly beneficial for complex organic molecules when measurements of 2H/1H ratios lead to poorly defined molecular fingerprints. The current NMR methodology to determine 13C/12C site-specific isotope ratios suffers from poor sensitivity and long experimental times. In this work, several NMR pulse sequences based on polarization transfer were evaluated and optimized to measure precise quantitative 13C NMR spectra within a short time. Adiabatic 180° 1H and 13C pulses were incorporated into distortionless enhancement by polarization transfer (DEPT) and refocused insensitive nuclei enhanced by polarization transfer (INEPT) to minimize the influence of 180° pulse imperfections and of off-resonance effects on the precision of the measured 13C peak areas. The adiabatic DEPT sequence was applied to draw up a precise site-specific 13C isotope profile of ibuprofen. A modified heteronuclear cross-polarization (HCP) experiment featuring 1H and 13C spin-locks with adiabatic 180° pulses is also introduced. This sequence enables efficient magnetization transfer across a wide 13C frequency range although not enough for an application in quantitative 13C isotopic analysis.