# A Narrative Review of Metabolomic Insights into Olive Oil's Nutritional Value
## 3 Techniques Used for Metabolite Profiling and Evolution
Ionization methods that are also used in LC-MS, such as electron and chemical ionization, can be applied. There are several methods for sample pretreatment, including solid-phase microextraction (SPME), dynamic headspace sorptive extraction combined with thermal desorption, purge and trap extraction, and solvent-assisted flavor evaporation. SPME can be performed through headspace or liquid sampling, with the former being the most common method for analyzing olive oil metabolites because it allows a valuable level of automation and repeatability. GC-MS excels in overcoming the challenges commonly associated with LC-MS, such as matrix effects and ion suppression arising from co-eluting compounds, and it is able to provide superior chromatographic resolution. Nonetheless, this technique has a significant limitation in that it is primarily applicable for the separation and analysis of low-molecular-weight compounds [76]. CE-MS allows the profiling of polar ionized and ionizable metabolites and offers several advantages, such as the possibility of utilizing small samples and decreased reagent volumes, minimal or no organic solvent consumption, and the use of simple fused silica capillaries instead of expensive LC columns. Therefore, this study presents a sustainable and cost-effective analytical technique [54]. NMR spectroscopy is an analytical technique based on the principles of nuclear spin physics. This technique exploits the inherent magnetic properties of the atomic nuclei. In fact, it operates on the assumption that either nuclei with an odd mass number (number of protons + number of electrons) or nuclei with an even mass number but an odd atomic number generate a tiny magnetic field. They are characterized by the presence of a property called nuclear spin, a form of angular momentum, and a nuclear spin quantum number (I). For atoms whose nuclei have odd mass numbers, I assumes half-integer values such as 1/2 or 3/2, whereas for atoms with even mass numbers and odd atomic numbers, I takesinteger values. These atoms generate a magnetic field that interacts with the significant external magnetic field created by the NMR instrument, allowing researchers to study these atoms and their properties. Furthermore, atoms with I = 1/2, such as hydrogen (1H) or carbon-13 (13C), result in clear signals in the NMR spectrum, which are often used in food science. NMR can be either 1D or 2D, with the latter being a better option for complex mixtures [81]. In the context of olive oil, 1H NMR has proven to be a valuable tool for understanding the details of lipid classes as well as the composition of fatty acids and several bioactive minor compounds and the levels of unsaturation, whereas 13C NMR has provided distinctive insights into the arrangement of fatty acids on the glycerol moiety and the stereochemical aspects of unsaturation. In general, no sample pretreatment is required for these methods. However, their effectiveness is not consistently high, particularly when attempting to quantify certain minor compounds such as mono-or diacylglycerols [82]. The evolution of metabolite profiling techniques has been instrumental in improving our ability to characterize a complex mixture of compounds found in olive oil and has provided valuable insights into the factors that contribute to its unique flavor and nutritional properties. Table 1 provides an overview of the commonly employed methods for analyzing the metabolites present in olive oil. This table systematically outlines each method along with its respective advantages and disadvantages.