Improving quantitative 13C NMR performance by an adiabatic scheme

Abstract

NMR is a primary method of measurement that provides quantitative traceable results without the need for a standard of the same measurand. Despite the advantages of the 1H nucleus, such as high sensitivity, short relaxation times and widespread distribution in organic compounds, some molecules present only labile H atoms, or the 1H peaks of a sample may overlap with impurities. With the CHORAD scheme – a combination of the pulse sequence CHORUS and adiabatic modulation for 1H decoupling – this study has shown that 13C NMR can be used for quantitative analyses with a bias smaller than 2% and expanded uncertainty of 0.7% (for a confidence level of 95%). Since 13C is found with significant non-statistic isotope fractionation, all the peaks of the sample and internal standard should be integrated and their bulk 13C abundances obtained by isotope ratio mass spectrometry (IRMS) must be considered to meet quantitative requirements. Future experimental work may be performed to test this procedure for complex matrix samples. This approach also proved to be suitable for studies of site-specific 13C distribution and led to better heteronuclear decoupling when compared with the use of adiabatic decoupling with ordinary hard 90° pulses, a combination extensively reported in the literature over the last decade. An examination of the molecules studied in this work and others from the literature revealed an overall trend in the 13C intramolecular profile – hydrogen-containing positions are often 13C-depleted. A potential contribution of 1JCH coupling to these results is discussed.