To evaluate the quality of an olive oil, the positive sensory attributes (fruitiness) and negative ones (defects) are important, linked to the identification and quantification of volatile molecules, with a methodology known as panel tests.
While executing the chemical-physical analyses of an oil, is relatively simple, the organoleptic evaluation still presents some drawbacks despite the tasters of Panel certificates are adequately trained and follow the guidelines detailed in the current official method.
The large number of samples that the Panel must analyze and the fact that it is a time-consuming methodology, this can limit the execution of repetitions precisely in case of contradictory classifications. For these reasons, the creation of predictive models, based on the use of very high sensitivity instrumental methods, for the evaluation of the sensorial quality of olive oil, could be a “useful subsidy”, in order to reduce the number of samples to be evaluated by the sensory panels and to be of support in cases of contradictory assessments. Furthermore, in some countries, few exist Panel accredited.
Over time, a great deal of experience on volatile molecules has been accumulated, starting from the 60s, with the research of the physicist Earl W. McDaniel, some of whose books on the subject are famous, followed by the research of Cohen MJ so much so that in 1969 he filed a patent on a system of gas chromatography (GC) and mass spectrometry, the so-called “plasma chromatography”.
This method, in addition to being cheap and fast, used a portable instrument which it did not require an expensive vacuum system, such as classical mass spectrometry (MS). At the time, its use was limited in the field of gas measurement.
In the 80s this plasma chromatography has been defined “Ion mobility spectrometry” (STI) and used as a detector for chromatographs.
Thanks to its portability, robustness and suitability for field assessment, the IMS is used in the detection of illegal drugs, in the military industry chemical-bacteriological weapons (such as nerve gases, blistering or asphyxiating agents), of explosives, particularly in airports, especially after the attacks of 9 September, but also to volatile substances coming from the petrochemical industry or, for industrial purposes, in the detection of harmful substances in the air or organophosphorus pesticides.
With such an endowment it would have been possible to understand the circumstances of the poisoning of Sergei and Yulia Skripal directly to the crime scene, without waiting for laboratory analyses, allowing the distinction of a use of fourth generation chemical weapons (such as nerve agents of the Novichok class) or of bacteria, fungi and viruses based on their fingerprints, unique within a given environment.
The IMS has such a molecular sensitivity that it can detect explosives inside letters, parcels, blocks of different shapes and sizes, in a few seconds and can be used, even for detection inside human luggage.
Today it is used in the study of food products such asbalsamic vinegar, The tomato puree, cheese, honey, coffee, wine, the freshness of fish and eggs, in the control of the quality of raw materials, storage conditions, flavor stability and detection of off-flavor, in the control of production processes, For the 'product authentication, in adulterations and in deterioration of foods, finally also in medical field in metabolic profiling of human breathing.
In the pharmaceutical industry IMS is used in cleaning validations, demonstrating that reaction vessels are clean enough to proceed with the next batch of a pharmaceutical product or in the analysis of biological materials, in particular in proteomics, in metabolomics, i.e. in the field of foodomics.
In real commercial applications, Snyder AP (in Third International Workshop on Ion Mobility Spectrometry; NASA. Johnson Space Center, Houston: US) used a tool GC-IMS portable to quickly identify the freshness of fish, which shows that this technology can replace the traditional FDA method of determining freshness through smell (1).
In the current state of knowledge, the GCxIMS Library Search software has a index of over 80.000 compounds which can be further extended, using IMS-drift times, to achieve maximum certainty for unknown molecules present in the samples.
La GC-IMS is used as an analytical choice in alternative to gas chromatography-mass spectrometry (GC-MS), especially as a powerful separation technique for the detection of volatile, trace components in foods and among these also in olive oil with a result closer to the real situation.
GC-IMS, as an alternative technique to GC-MS, also features a better sensitivity (LOQ limits of quantification vary between 0,08 and 0,8 µg/g compared to 0,2 and 2,1 µg/g in HS-GC-MS mass spectrometry) (2).
Among other things, laboratory techniques, based on various principles that imitate olfactory and gustatory perceptions, have the drawback of requiring a pretreatment of the sample, beyond the fact that they are non-portable instruments, instead in the GC-IMS, the sample is taken directly from the headspace (HS - Head Space) and fed into a gas chromatograph-spectrometer. Here a double separation of the volatile components takes place, the first in the chromatographic column (GC), the second in the ion mobility spectrometer (IMS).
So this method applied to headspace molecules (HS-GC-IMS), it arises as an alternative to the evaluation of volatile molecules with solid phase microextraction in the headspace (HS-SPME, HeadSpace Solid-Phase MicroExtraction) coupled to GC-MS.
La HS-GC-IMS arises straddling human sensory analysis and instrumental analysis thanks to the two dimensions of separation of volatile molecules, to its extreme sensitivity (minimum detection limit is at the billionth and billionth of a gram level), to informative precision making the technique precise, sensitive and a powerful means suitable for determining sophistication ed adulterations of a food but also to evaluate, among other things, its "shelf life".
The principle on which the HS-GC-IMS is based is the separation of gaseous ions due to their mobility which depends on their size, shape and charge, in the presence of a weak electric field.
After chromatographic separation of the gaseous molecules, they enter the IMS spectrometer, consisting of a segmented drift tube, along which an electric field is applied.
The tube carries, at one end, a ionization source (which uses isotopes like tritium 3H, among other things it is the most frequent beta particle generator, or nickel 63Ni or americium 241Am) and, at the other end, a detector.
According to the EU directive, the exemption limit for the total activity of tritium has been set at 1 GBq therefore, the use of this low radiation source (activity of 300 MBq) is not subject, among other things, to authorization .
A flow of an inert gas (nitrogen) is circulated in the tube, in countercurrent, to facilitate the separation of the ions during their "flight", as electrically charged molecules; consequently, different ions reach the detector, through the “drift region”, at different times (the so-called, drift times).
La “ion mobility” can be defined as their drift velocity per unit electric field intensity.
Due to the balance between the acceleration due to the electric field and the deceleration due to the collision with the gas molecules (nitrogen), transported in countercurrent, the ions move with a constant speed towards the detector.
Depending on mass characteristic, of carica and structure, the ions are separated in the drift tube and reach the detector with different drift times.
Therefore, each volatile component is characterized by two parameters: “retention time” (determined by gas chromatography, CG) e “drift time” (determined by spectrometry, IMS), the latter defined as the time it takes for the ions to travel the distance between the ion shutter and the detector (a Faraday plate) that records them, too for quantitative purposes.
The IMS technique is somewhat similar to the “time-of-flight mass spectrometry” technique, however the latter must operate under high vacuum conditions, whereas IMS operates at atmospheric pressure (3).
(1 – continued)
Bibliography
Yang X. et al. 2023 Acta Chromatographica; 35:1, 35-45. DOI: https://doi.org/10.1556/1326.2022.01005
García-Nicolás M. et al. Foods.; 9(9): 1288. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7555980/
Yang X. et al. 2022, Acta Chromatographica. 35:1; 35–45. https://akjournals.com/view/journals/1326/35/1/article-p35.xml
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