The GC/MS and HPLC methods for determination of cannabinoids were validated according to FDA validation guidelines [ 33 ]. The assays showed to be robust, selective, reproducible, accurate and sensitive. Intraday and interday precision and accuracy (bias) data are summarized in Supporting Information S1 and S2 Tables.

The contents of cannabinoids in THC- and CBD-type cannabis used for validation of vaporizers, determined by GC/MS and HPLC, are summarized in Supporting Information S3 Table , whereas typical GC/MS and HPLC profiles are shown in Figs 1 and 2 , respectively. HPLC analysis revealed that 90.4% of THC tot was present as THCA-A in the THC-type cannabis, whereas in the CBD-type cannabis CBDA was accounting for 85.8% of CBD tot . The concentrations of THC tot and CBD tot determined with HPLC were about 20% higher compared to those obtained with GC/MS, which can be explained by the incomplete thermal decarboxylation of acidic cannabinoids in the GC injector as described by Dussy and colleagues [ 34 ]. The content of CBN as a degradation product of THC was low in the THC-type and no CBN was detected in the CBD-type cannabis.

In vitro validation of vaporizers

The recovery data, i.e. the cannabinoid concentrations measured in the sample fractions (vapor, residue, vaporizer parts) of the 5 vaporizers tested are summarized in Table 1 and Fig 3. The data of the connection tube are not included in Fig 3 as only traces (< 1%) or no cannabinoids were detected. Fig 4 depicts typical GC/MS profiles of vapor (top graphs) and residue (bottom graphs) obtained with THC-type (A) and CBD-type cannabis (B).

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larger image TIFF original image Download: Fig 3. Percentage of total cannabinoids found in the vapor, residue and device parts of Volcano Medic®, Plenty Vaporizer®, Arizer Solo®, DaVinci Vaporizer®, and Vape-or-Smoke™. Depicted are the mean values + 1 SD. Bars, left to right: THC from THC-type cannabis; THC from CBD-type cannabis; CBD from CBD-type cannabis; THC standard; CBD standard. https://doi.org/10.1371/journal.pone.0147286.g003

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larger image TIFF original image Download: Fig 4. GC/MS-SIM profiles of the vapor (top graphs) and residue (bottom graphs) after vaporization of THC-type cannabis (A) and CBD-type cannabis (B) using the DaVinci Vaporizer®. The target ion m/z 231 of CBD and THC is plotted on the left side of the dotted line at 15.6 min, whereas m/z 295 as the target ion of CBN is depicted on the right side of the dotted line. Abbreviations as in Fig 1. https://doi.org/10.1371/journal.pone.0147286.g004

Using cannabis (both types) and Volcano Medic® more than 58% of the THC tot was found in the vapor, whereas the recovery of CBD tot was with 51.4% slightly lower. On the other hand, the recovery of THC and CBD was with 64.0 and 73.3%, respectively, higher compared to cannabis when vaporizing cannabinoid standards, whereas the amount of cannabinoids remaining in the residue was considerably lower. The reason may be that the cannabinoid standards were in the sample chamber directly applied onto the metal liquid pad designed for liquids, facilitating the vaporization of standards. RSD values of THC tot and CBD tot in vapor samples were 10 to 13% and 3.6 to 10%, respectively. When vaporizing cannabinoid standards, the recovery of CBD was higher than that of THC. THC and CBD left in the residue after vaporization was ≤ 15% and < 3% for cannabis and cannabinoid standards, respectively. The variability of the cannabinoids left in the residue was dependent on the percentage of total cannabinoids found in these samples. Consistently, RSD values were highest and up to 94.2% for the cannabinoid standards with the lowest content remaining in the residue. Only a minor percentage of ≤ 3.6% of total cannabinoids was deposited onto the device parts (i.e. sample chamber, connection valve and mouthpiece). The cannabinoids adsorbed on the balloon were not measured, which may be one explanation why the sum of all fractions was found to be between 64.6 and 76.5% with RSD being < 11.4%.

With 50.7 to 66.8% the recoveries in the vapor of Plenty Vaporizer® were similar to those of Volcano Medic®. For this inhalation device, no differences were observed between cannabis and cannabinoid standards. The RSD values were significantly lower than those of Volcano Medic®, indicating a reliable and quantitative release of the cannabinoids from the sample matrix. With 1.1 to 2.1% (cannabis) and 0.4% (cannabinoids) remarkably less cannabinoids were left in the residue after vaporization with Plenty Vaporizer® compared to Volcano Medic®. However, a considerable proportion of total cannabinoids, ranging from 18.7 to 25.7%, was found in the device parts. Some vapor condensation (RSD 6.1–19.4%) resulting in deposition and loss of cannabinoids occurs in the long metal cooling tube with lamellae designed to cool the vapor before inhalation. The sum of cannabinoids found in all fractions was with 89.6%– 92.8% highest for THC vaporized from cannabis.

The highest recoveries were obtained with Arizer Solo®, namely 67.5–82.7%. No differences were seen between THC and CBD. Except for CBD vaporized from cannabis (RSD = 11.6%) the variability was small with RSD values typically being < 8%. Only a minor portion of ≤ 3% of the cannabinoids remained in the residue after vaporization of cannabis and no cannabinoids were found in the piece of cotton used to vaporize the cannabinoid standards. Additional 9.8%– 13.6% of THC tot and CBD tot were adsorbed onto the glass tube and the sample chamber of the device, resulting in an overall recovery of 79.0–98.1%. The sum of the cannabinoids found in the various fractions was highest for THC vaporized from cannabis and reached 88.9% in CBD-type and 98.1% in THC-type cannabis. The relatively simple design of the device consisting of smooth surfaces and inert materials such as metal and glass may be responsible for the high yield.

The percentage of cannabinoids found in the vapor of the DaVinci Vaporizer® varied between 48.5 to 56.7% and 37.3 to 47.2% for cannabis and cannabinoid standards, respectively. The oil can delivered with the DaVinci Vaporizer® was used to vaporize the cannabinoid standards dissolved in MeOH. The insertion of the oil can into the sample chamber changes the design of the device as the oil can has to be heated passively by the sample chamber. This may be the reason for the lower recoveries obtained with the cannabinoid standards. Consistently, the variability was smaller with cannabis (RSD ≤ 11%) and bigger with the standards (< 22%). Less than 5% of the THC tot and CBD tot remained in the residue after vaporization of cannabis or standards. In case of cannabis, up to 12.9% of the THC and CBD was adsorbed onto the device parts. With 15.5–20.3% a higher proportion of THC and CBD was remaining on the device parts after vaporization of cannabinoid standards. This is a further indication that the insertion of the oil can into the sample chamber is causing a change in the heat distribution and/or temperature, therefore hindering the vaporization process. With ≤ 1.3%, only small amounts of cannabinoids were found in the connection tube. The overall recovery as the sum of all fractions was lower than that obtained with the other vaporizers, being between 56.7% and 71.5%. This may be the result of a limited sealing of the sample compartment.

The most important difference between the Vape-or-Smoke™ and all other vaporizers tested is that the Vape-or-Smoke™ is not electrically heated but with a butane gas flame. Consequently, the temperature cannot be adjusted or controlled. Therefore, it is difficult to maintain stable experimental conditions, which can be seen in the high variability of all measurements. 45.9% to 55.9% of the total cannabinoids present in cannabis were recovered from the vapor with RSD values between 9.7% and 37.2%. Only 27 to 28% of the cannabinoid standards were found in the vapor, which is most probably due to the experimental setup. As for the Arizer Solo®, the standards were vaporized from a piece of cotton introduced into the sample chamber. The lower part of the cotton turned brown during vaporization and isolated probably the upper part of the pellet, resulting in an incomplete vaporization of the cannabinoids. Consistently, a considerable amount of 44.1 to 46.8% of the cannabinoid standards remained in the residue. In case of cannabis, 14.4 to 16.2% of total cannabinoids were found in the residue. Again, the variability was with RSD values up to 152.7% extremely high, illustrating the difficulties in maintaining stable vaporization conditions. Overall, 70.3 to 85.4% of the total cannabinoids were found in all fractions together and no differences were found between the test materials.

As a consequence of the high vaporization temperature the concentration of CBN in vapor, formed by oxidation of THC, is slightly increased compared to cannabis (see Figs 1 and 4A). When vaporizing cannabis, an important issue is the efficient and quantitative decarboxylation of acidic cannabinoids, usually predominant in fresh cannabis, as neutral THC and CBD are considered to be pharmacologically active. In contrast to GC/MS, where derivatization (e.g. silylation) is required to prevent thermal decarboxylation of acidic cannabinoids, HPLC allows the direct analysis of acidic and neutral cannabinoids. The total decarboxylation rates achieved with the various vaporizers are summarized in Table 2. Excellent and reliable decarboxylation of THCA-A and CBDA was observed with all electrically-driven vaporizers, which allowed an exact control of the temperature. With ≥ 99.8% Plenty Vaporizer® and DaVinci Vaporizer® showed the highest decarboxylation efficiencies. With 97.3 to 98.7%, similar decarboxylation rates were found with Volcano Medic® and Arizer Solo®, respectively. Only in case of CBD a slightly lower decarboxylation efficiency was found when vaporized with Volcano Medic®. On the other hand, Vape-or-Smoke™ operated with a butane gas flame showed lower decarboxylation of acidic cannabinoids with the respective rates being 87.7 to 93.3%. In addition, the variability of decarboxylation was generally found to be larger with this device with RSD values up to 12.7%, reflecting the difficulties in maintaining a constant temperature. Fig 5 with the corresponding HPLC profiles shows that in the THC-type cannabis THCA-A is almost completely converted to THC (A). The same holds true for CBD-type cannabis (B), i.e. the decarboxylation of CBDA to CBD.