Assessment of the triacylglycerol fraction of olive oil by 1D-NMR spectroscopy: exploring the usefulness of DEPT tool on the peak assignments of 13C NMR spectra

Abstract

Nuclear magnetic resonance (NMR) spectroscopy has been extensively used to date for the analysis of olive oil samples and constitutes a valuable tool particularly for the study of the triacylglycerol fraction, which is the major fraction of olive oils. In this report, we demonstrate the usefulness of one-dimensional NMR spectroscopy (1 H and 13 C NMR) for the nondestructive analysis of the triacylglycerol fraction of olive oils. To achieve this goal, 13 C spectral editing technique known as distortionless enhancement by polarization transfer (DEPT), namely 13 C NMR DEPT 45, 13 C NMR DEPT 90, 13 C NMR DEPT 135 was used for the analysis of the triacylglycerol fraction of the olive oil sample owing to the simplification on peak assignments of 13 C spectra. The suitability and usefulness of those DEPT experiments using different angle values in only one pulse sequence will be explored on the interpretation of 13 C NMR spectra. In overall, DEPT tool improves the sensitivity of the analysis due to the enhancement in the signal-to-noise ratio, which is important for this type of analysis contributing to the characterization of olive oils.

Seeking a Fast Screening Method of the Varietal Origin of Olive Oil: The Usefulness of an NMR-Based Approach

“…The pulse sequences used in these experiments were installed in the Bruker TOPSPIN 3.2 pl 6 suite. The free induction decay (FID) acquisition parameters for the standard single pulse test were as follows: (1) (zg30) in 1 H NMR: spectral width (SW) = 20.64 ppm, relaxation delay (D1) = 1 s, dummy scans (DS) = 2, number of scans (NS) = 16, acquisition time (AQ) = 4.089 s and receiver gain (RG) = 10, giving a total run time of 1 min and 32 s; and (2) (zgpg30) at 13 C NMR DEPTSP 45: spectral width (SW) = 238.89 ppm, relaxation delay (D1) = 2 s, dummy scans (DS) = 4, number of scans (NS) = 256, acquisition time (AQ) = 1.3631 s and receiver gain (RG) = 2050, giving a total running time of 14 min and 46 s [ 15 ]. The 1 H NMR spectra were processed using exponential multiplication, with a line-broadening factor of 0.3 Hz and a fifth-order polynomial baseline correction (ABSG = 5).…”

“…After comparing the effectiveness of both the DEPT 45 and DEPT 90 methods (which are simple and quick, with good sensitivity), we selected the DEPT 45 method due to the larger number of data points ( 13 C signals) compared to the former method (which gives only tertiary 13 C signals). The 13 C NMR DEPT 45 spectra of the olive oils under study present 38 characteristic resonances ( Figure S2), which are listed with their respective chemical shifts and the corresponding assignments of the carbon signals in Table 2 (detailed assignments were given in a previous publication [15]). The DEPT 45 spectrum of the triglyceride fraction of olive oils shows 13 C resonances that can be grouped into three defined spectral regions: unsaturated carbons, ranging from 130.20 to 127.92 ppm; glycerol carbons, ranging from 68.91 to 62.10 ppm, and aliphatic carbons, ranging from 34.20 to 14.09 ppm [21,25].…”

“…Indeed, NMR is a robust quantitative analytical technique used to obtain “high-throughput” spectroscopic and structural information on a wide range of molecular compounds [ 11 , 12 , 13 , 14 ]. The main advantages of NMR analysis are that (i) it can be used without using specific standards, (ii) it provides structural information, (iii) it is a non-destructive method, (iii) one can analyze simultaneously various components in a mixture without pre-isolation and (4) it requires a relatively short analysis time compared to other conventional analytical techniques (such as GC and HPLC), and the preparation of the sample is very simple and rapid [ 15 ]. Due to the higher sensitivity and lower relaxation times of proton nuclei, 1 H NMR spectroscopy is the most widely used method for most NMR applications [ 16 ].…”

“…The principal limitation of 1 H NMR spectroscopy is the complex nature of the 1 H NMR spectrum, which contains several overlapping peaks due to the presence of different multiplet patterns that arise from spin coupling of the different protons, which are condensed into a very narrow spectral window (~15 ppm). Another limitation of 1 H NMR spectroscopy is that it is not possible to see the positional distribution of the fatty acids in the glycerol backbone [ 15 ].…”

“…Moreover, in our previous publication, we showed the power and potential of DEPT editing techniques for assessing variations in the compositions of different olive oils [ 15 ]. Variation in the peak heights of the carbons present in the triacylglycerol fraction was used as the means to determine variability within olive oil samples.…”