Cannabis sativa L. is the world’s most recognizable, notorious, and controversial plant known since the ancient times for its medicinal and textile uses, an emblematic example of a multi-purpose crop [1, 2]. It is also by far the most widely cultivated, trafficked, and abused illicit drug .
A widely adopted standard approach to uncertainty evaluation is the Guide to the Expression of Measurement Uncertainty (GUM) , using a bottom-up approach. On the other hand, it is possible to carry out collaborative studies on standard test methods, and measurement uncertainty evaluation is achieved using precision and trueness estimates (top-down approach) .
Marijuana has been considered a leading drug of abuse and has been seriously criminalized, with enormous law enforcement costs and social upheaval . It is currently included in Schedule I of the United Nation (UN) Single Convention on Narcotics Drugs (1961)  and only recently it was removed from Schedule IV, the most restrictive . In the last decade, decriminalization of Cannabis for industrial and medicinal uses and even recreational marijuana has occurred, or it is occurring in many jurisdictions as the result of sociological, philosophical, political, and legal developments . A limit of 0.3% of THC content (on a dry inflorescences weight basis) was established by Small et al. (1976)  and adopted in many countries as a criterion to distinguish cultivars that can be legally cultivated under license from those considered to have a too high drug potential . Some jurisdictions have increased this limit for legal cultivars up to 1.0% [1, 10, 11].
We determined the measurement uncertainty, which characterizes the dispersion of the values reasonably attributed to the analytes, by both the bottom-up approach  and the top-down one .
The name “hemp” or “industrial hemp” designates fiber and oilseed cultivars of C. sativa with very limited content of Delta-9-tetrahydrocannabinol (Δ9-THC, or simply THC). Conversely, “marijuana” is the name used for the drug kind of plant, containing a high level of THC. THC and CBD (cannabidiol) are the plant phytocannabinoids of most importance. THC is the principal intoxicant and psychotropic constituent, while CBD, devoid of psychotropic effects and known to possess several pharmacological properties, is instead the principal cannabinoid of hemp . These compounds, as other cannabinoids, exist in the fresh plant mostly in the form of carboxylic acids, THCA and CBDA, possessing several pharmacological properties but no psychotropic activity . These acids undergo decarboxylation into their neutral counterparts under the influence of light, time (such as prolonged storage), alkaline conditions, or high temperature (smoked or cooked marijuana) following the reaction shown in Fig. 1.
The authors would like to acknowledge the pharmacy Farmacia Tundo Dr. Alfredo (Alliste, Italy) for the useful and fruitful discussions and argumentations on hemp and cannabinoids.
Heatmap built with the identified cannabinoids. Color-coding consists of shades of red and blue, where higher intensity of red stands for very high concentration and higher intensity of blue stands for very low concentration. The samples are shown in colors at the top of the heatmap, while cannabinoids are reported on each row.
Authentic samples were obtained by diluting 100 μL of hemp seed oil with 395 μL of 2-propanol and 5 μL of IS working solution.
Conflict of Interest Statement
The same consideration could be made for the acidic precursor THCA (Supplementary Figure S13), which shows a fragmentation spectrum in positive mode similar to that of CBDA to the point that they could be easily mistaken. Conversely, the fragmentation of THCA in negative mode shows only a major peak at m/z 313.2173 (45%) corresponding to the loss of CO2 to generate the “neutral” derivative THC. The loss of water leads to a very small fragment 339.1962 (5%), which is probably more unstable that the corresponding species obtained with CBDA. The dihydropyran ring probably confers different chemical properties and reactivity to the whole molecule. Moreover, the acidic species elutes after the neutral counterpart, opposite to the case of CBDA/CBD.
CBC has a fragmentation pattern in positive mode very similar to THC so that they are quite undistinguishable ( Figure 6A ). In negative mode ( Figure 6B ), it is possible to discriminate CBC from THC by the ionic abundance of the fragments. Like THC, the molecular ion [M–H] – 313.2171 is the base peak, but unlike THC it generates a higher product ion 245.1544 (25%) deriving from the loss of one isoprene unit. The other two product ions, 191.1068 (55%) and 179.1068 (35%), are higher in CBG than THC, where they are below 10%.
Lastly, a semi-quantification was carried out in order to provide approximate concentrations of the identified cannabinoids, since absolute quantification is applicable only to level 1 cannabinoids, for which authentic standards are available. Absolute quantification of cannabinoids from level 2 to 4 1 is not viable without appropriate analytical ploys. Hence, the concentrations of level 1 cannabinoids (CBDA, THCA, CBGA, CBD, Δ 9 -THC, CBC, CBDV, CBN and CBG) were calculated by external calibration of authentic standards analyzed in the same LC-MS conditions. The linear equations for these cannabinoids are reported in the Supplementary Material. For level 2 cannabinoids, for which analytical standards were not available, we employed the calibration curve of the cannabinoid standard with the closest structural similarity. For those acid cannabinoids with no structural similarity, the calibration curve was set as the average ion response obtained for the same concentration for all the available acid cannabinoid standards. The same was applied to level 2 neutral cannabinoids, though leaving CBDV and CBN out as they displayed completely different ion responses most likely due to shorter alkyl chain and additional aromatization, respectively. The results of the semi-quantification are reported in Table 2 .
Hemp seed oil is well known for its nutraceutical, cosmetic and pharmaceutical properties due to a perfectly balanced content of omega 3 and omega 6 polyunsaturated fatty acids. Its importance for human health is reflected by the success on the market of organic goods in recent years. However, it is of utmost importance to consider that its healthy properties are strictly related to its chemical composition, which varies depending not only on the manufacturing method, but also on the hemp variety employed. In the present work, we analyzed the chemical profile of ten commercially available organic hemp seed oils. Their cannabinoid profile was evaluated by a liquid chromatography method coupled to high-resolution mass spectrometry. Besides tetrahydrocannabinol and cannabidiol, other 30 cannabinoids were identified for the first time in hemp seed oil. The results obtained were processed according to an untargeted metabolomics approach. The multivariate statistical analysis showed highly significant differences in the chemical composition and, in particular, in the cannabinoid content of the hemp oils under investigation.