Advantages and disadvantages of the NMR technique

The first limitation in using NMR for food authentication (and the most prohibitive one) is the cost of the equipment. A new 500 MHz NMR spectrometer might cost 7-8 times as much as a new HPLC/UV-DAD system. The second limiting reason is the relatively low sensitivity of NMR compared to other techniques such as HPLC or GC. However, the versatility of the technique means that the initial high cost may well be overridden by a number of advantages that other techniques may not provide. The first of them is obviously the power of structural elucidation of the technique. Food authentication research is constantly looking for techniques that can identify marker compounds to permit the detection of adulteration or testify to the quality of a high price food product. The second advantage is that NMR is probably the best non-targeted technique to use for the screening of food extracts: all the main metabolites (fatty, amino and organic acids, sugars, aromatic compounds) can be detected in a single spectrum with minimal and non-destructive sample preparation (this is the third advantage). The recording can also be completed in less than an hour. This allows the possibility of analysing a large number of samples. With the detection of several tens of compounds and the structural elucidation power of NMR, a new marker compound is more likely to be identified. The sensitivity of NMR is good enough to observe the main primary and secondary metabolites in a simple whole food extract.

For the detection of minor components, larger amounts of the starting material can be extracted initially and then fractionation steps can be added to the sample preparation in order to concentrate certain types of compound.27'47 Also, high field NMR instruments allow more compounds to be detected since they provide improved sensitivity and signal dispersion, and the introduction of cryo-probes will give significantly increased sensitivity on existing instruments. Another advantage of using NMR is the fact that it is quantitative. A number of analyses have been carried out on oils and wine using this attribute.7,23,48-49

The measurements are easily repeatable and reproducible over the long term. The instrumental parameters that need careful attention from the analyst and that may vary from one data recording session to another are the tuning and the resolution. A difference (even small) in tuning will affect the intensity of all peaks in the spectra. In order to compare quantitatively sets of spectra recorded at different times, all the spectra can be normalised to the reference peak provided that a fixed amount of internal standard is added to all the samples. The problems of repeatability and reproducibility of chromatographic techniques that are caused by the ageing of columns (affecting separation performances and changing retention times), temperature fluctuations, etc., are not applicable to NMR. The disturbances that can occur are the small shifts of the NMR peaks caused by differences in pH in the NMR spectra of food extracts or the broadening of certain signals due to chemical exchange of protonated and non-protonated species and metabolite-metal interactions (complexation with Ca++ or Mg++ ions). To overcome the variable pH problem, it is possible to manually adjust the pH to the same value for all samples by adding drops of NaOH or HCl to the mixture with a micropipette while monitoring the pH with a microelectrode. Phosphate buffers in D2O can also be used as solvent12,47 but they may not be strong enough to control the pH of some samples when used in a methanol/water or similar mixed solvent. EDTA is effective in reducing line broadening of citric acid signals when this is due to metal complexation.

The other solution to the problem of chemical shift displacements is to treat the NMR signals after recording. Vogels et al50 proposed a program called Partial Linear Fit (PLF) that lines up peaks affected by small displacements. As a matter of fact, it has been possible to create macros in Matlab software (partly derived from the PLF algorithm) that enable the lining up of NMR peaks (Defernez51) but these procedures currently require considerable operator input and it has not yet proved possible to automate them. The consequences for chemometric analysis of ignoring changes of lineshape and chemical shift displacements between spectra have been explored (Defernez and Colquhoun52).

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