13C spectrum of a mixture of 13 hydrocarbon molecules acquired on a Spinsolve 80 MHz.
NMR is a powerful analytical method widely used to quantify the concentration of chemical compounds present in a sample. Compared to other techniques, NMR offers a number of advantages. The integral of the signals in the NMR spectrum is directly proportional to the concentration of the particular molecule in the full concentration range. Moreover, the signal integrals are independent of the matrix where the analytes are dissolved. Hence, if the concentration of a calibrant (internal or external) is known, the concentration of all components in the mixture can be determined by using simple linear relations. Although this property holds for all NMR active nuclei, quantitative NMR methods are typically based on the detection of 1H signals, as they provide the highest sensitivity. However, when looking at complex mixtures, signals in the 1H spectra may suffer from severe overlapping due to the small chemical shift spreading of this nucleus. In such cases, 13C can be used to improve the signal separation, as this nucleus provides a much larger chemical shift dispersion. The big disadvantage of 13C NMR is its low sensitivity due to the small gyromagnetic ratio and the low natural abundance of this nucleus. To mitigate this limitation, NMR methods transfer polarization from 1H to 13C before the 13C signal is measured. Polarization transfer increases the sensitivity of 13C in an important factor, but not all 13C nuclei are enhanced by the same factor. Hence, the amplitude of the signals within one spectrum cannot be directly compared, as it is typically done in the 1H spectra. Although signal amplitudes cannot be converted to concentrations by using the same reference substance, the linear relationship between signal amplitude and concentration is preserved for each peak. In this way, the analytes of interest can be accurately quantified if their signals are calibrated against external calibration standards containing known concentrations of the same analytes. In this application note, we describe a procedure developed to quantify the individual components present in a hydrocarbon mixture by using 13C NMR. This procedure has been implemented on a Spinsolve 80 MHz benchtop spectrometer. In the petrochemical field, samples are complex mixtures of hydrocarbon molecules that define the physical properties of these liquids. As a proof of concept, we present here an external calibration procedure able to quantify the concentration of 10 model molecules. The performance of the method has been determined by using a set of mixtures prepared with known concentrations.