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Evaluation of Presence or Absence of α-Tocopherol and α-Tocopheryl Acetate in Various E-Liquids

Data publikacji: 20 Apr 2021
Tom & Zeszyt: Tom 30 (2021) - Zeszyt 1 (March 2021)
Zakres stron: 44 - 49
Otrzymano: 21 Oct 2020
Przyjęty: 11 Feb 2021
Informacje o czasopiśmie
License
Format
Czasopismo
eISSN
2719-9509
Pierwsze wydanie
01 Jan 1992
Częstotliwość wydawania
4 razy w roku
Języki
Angielski
INTRODUCTION

The presence of α-tocopheryl acetate in e-liquids is linked to EVALI (E-cigarette or Vaping product use-Associated Lung Injuries) (1, 2). Typical e-liquids containing flavors and nicotine have a glycerin/propylene glycol carrier and are highly unlikely to contain α-tocopherol or α-tocopheryl acetate. However, e-liquids containing cannabidiol (CBD) or even tetrahydrocannabinol (THC) may also contain α-tocopherol or α-tocopheryl acetate for enhancing the solubility of CBD or of THC which are lipophilic compounds. For this reason it was of interest to develop an analytical procedure for the analysis of α-tocopherol and of α-tocopheryl acetate in e-liquids.

Various procedures for the analysis of tocopherols are reported in the literature (3,4,5,6,7,8,9,10,11,12,13,14,15). These methods include high performance liquid chromatography (HPLC) analysis with fluorescence detection (FD) (3), HPLC with ultra violet (UV) detection at various wavelengths (4,5,6), detection using tandem mass spectrometry (MS/MS) (10,11,12), as well as gas chromatography coupled with mass spectrometry (GC/MS) analysis (9). Depending on the matrix, and the desired sensitivity of the analysis, a specific procedure must be selected. However, the available methods were not found adequate for the analysis using both UV and MS/MS detection. The advantage of UV detection is its excellent reproducibility but it may suffer from poor selectivity. On the other hand, MS/MS detection is highly selective, but is frequently affected by comparatively larger reproducibility errors. The combination of the two techniques offers both good reproducibility and selectivity of the analysis. A technique for the analysis of α-tocopherol or α-tocopheryl acetate in tobacco and cigarette smoke using both UV and MS/MS detection with the elimination of potential interferences was recently reported in the literature (16). This method has been applied on 37 e-liquids, three of them containing cannabidiol (CBD), and the rest only nicotine. A summary description of the method and the results obtained on the e-liquid samples are reported in this study.

EXPERIMENTAL
Materials

α-Tocopherol, α-tocopheryl acetate, 2,5-bis(5-tert-butyl-benzoxazol-2-yl)thiophene (Uvitex-OB), and 2,6-di-tert-butyl-4-methylphenol (BHT) as well as acetonitrile, formic acid, iso-propanol, and methanol were purchased from Sigma-Aldrich (St. Louis, MO, USA). E-liquids were commercially available and were purchased from the U.S. market in the period from 2018–2020. The GC vials were 2-mL with screw top caps with PTFE lined septa.

Instrumentation

Two HPLC systems were used for the analysis. A1260–1290 HPLC system from Agilent (Wilmington, DE, USA) was used for UV detection. This HPLC system consisted of a binary pump, autosampler, column thermostatted compartment, and UV detector (diode array). The peak integration was performed with OpenLab ChemStation Edition. The other system was an Agilent 1200 HPLC binary system that consisted of a binary pump, an autosampler with cooling capability, and a column thermostatted compartment coupled with an API-5000 triple quadrupole mass spectrometer (AB Sciex, Framingham, MA, USA) controlled using Analyst 1.6.2 software.

The peak integration was performed with MultiQuant 3.0.1 software. Both HPLC systems were equipped with a Zorbax Eclipse XDB-C18 column, 4.6 × 250 mm, 5 μm, also from Agilent. An ultrasonic bath 5510 from Branson (Danbury, CT, USA) from Thermo Fisher Scientific was also used.

Sample preparation

Prior the analysis the e-liquids were kept in a freezer at −20 °C. For the analysis, 100 mg (±10 mg) of e-liquid was precisely weighed in a 2-mL GC vial. 900 μL of methanol containing 560.4 μg/mL BHT, and 20 μL internal standard (I.S.) was added to the vial. BHT was added as an antioxidant. The stock concentration of the Uvitex-OB that was used as I.S. was 523.5 μg/mL in methanol. The sample was submitted for analysis by HPLC-UV and if a peak was detected at the corresponding retention time for α-tocopherol or of α-tocopheryl acetate, the same sample was submitted for analysis by HPLC-MS/MS.

Separation, detection, and quantitation

The HPLC separation was isocratic using as solvent a mixture of 15% isopropanol in acetonitrile. The injection volume for UV detection was 20 μL. The detection by UV absorption was performed at 210 nm. The flow rate was 1.5 mL/min. For the MS/MS detection electrospray ionisation (ESI) technique has been used with multiple reaction monitoring (MRM) detection in positive mode. The mobile phase for the MS/MS analysis was the same as for the UV analysis but the acetonitrile contained 0.1% formic acid, and because the flow rate of 1.5 mL/min was too high for MS/MS system the effluent from the chromatographic column was split and only 0.5 mL/min were introduced into the mass spectrometer. Also, the injection volume was 10 μL. The transitions utilized for the MS/MS detection were as follows: for α-tocopherol 431.4 → 165.1, for α-tocopheryl acetate 473.4 → 165.1, and for Uvitex-OB (I.S.) 431.5 → 415.3. The experimental conditions for the MS/MS detection were the same as described in reference (16).

Quantitation has been performed using the same procedure as described in reference (16), using a set of 10 standards with concentrations between 98.63 μg/mL and 0.19 μg/mL for α-tocopherol and between 96.47 μg/mL and 0.19 μg/mL for α-tocopheryl acetate. The calibration was linear for both UV detection and for MS/MS detection, with equations of the form: Y=aX+b {\rm{Y}} = {\rm{aX}} + {\rm{b}}

For the UV detection, the values for Y were the concentrations of the analyte while the values for X were the peak areas necessary for the calculation of the concentrations of the unknown samples. The peak areas of the internal standard were used in case of UV detection only to monitor the reproducibility of each analysis. The coefficients a and b for the calibrations in case of UV detection are indicated in Table 1.

Coefficients and R2 for the calibration curves Y = a X + b used in quantitation with UV detection.

Compound Detection type a b R2
α-Tocopherol UV 0.022401 −0.05633 1.00
α-Tocopheryl acetate UV 0.022655 −0.05204 1.00

For the quantitation using the MS/MS data, an equation of the same form as given by expression [1] was used, but in this case Y remained the concentrations of the analyte while the values for X were the peak areas of the analyte normalized by the area of the internal standard. The stability of the MS/MS detector is lower than that of the UV detector, and the normalization generated more precise values. The coefficients a and b for the calibrations in case of MS/MS detection are indicated in Table 2.

Coefficients and R2 for the calibration curves Y = a X + b used in quantitation with MS/MS detection.

Compound Detection type a b R2
α-Tocopherol MS/MS 34.2270 −1.00506 0.9997
α-Tocopheryl acetate MS/MS 10.7472 −1.3916 0.9997
Brief method validation

Both LC-UV and LC-MS/MS methods for α-tocopherol and α-tocopheryl acetate analysis can be considered as having good specificity. For the UV detection, retention time is basically the only criterion for selectivity, but for the MS/MS detection, besides the retention time the masses of the ion for Q1 (the parent ion) and the ion for Q3 (the daughter ion) are also important.

Since the e-liquids are miscible with methanol, no analyte recovery was performed in this study. The limits of quantitation (LOQ) for both α-tocopherol and α-tocopheryl acetate was taken as equal to the concentration of the lowest calibration standard, and because the samples are prepared using a 1/10 dilution in methanol, the LOQ per g of sample was 10 times lower (LOQ = 1.9 μg/g sample). The solutions of both α-tocopherol and α-tocopheryl acetate in the presence of BHT are stable up to two weeks when stored at −5 °C.

RESULTS AND DISCUSSION
E-liquids containing nicotine

A variety of e-liquids containing nicotine were evaluated for the potential presence of α-tocopherol and/or α-tocopheryl acetate. The list of these e-liquids is given in Table 3. The table also indicates (when available) the nicotine level and content of glycerin and propylene glycol (PG). None of the e-liquids listed in Table 3 were found to contain either α-tocopherol or α-tocopheryl acetate at levels above limits of detection (LOD) of 0.6 μg/g.

List of e-liquids containing nicotine evaluated for the presence of α-tocopherol and/or α-tocopheryl acetate (nicotine level, and content of glycerin and PG indicated when available).

No. Brand Flavor Nicotine mg/g Glycerin % (w/w) PG % (w/w)
1 Kilo Dewberry Cream 8.57 61.72 29.53
2 Frequency Liquid Boom Box 2.35 73.27 21.75
3 ZVR Lemon Cheesecake 5.25 54.70 32.42
4 OKVMI Haute Mocha 5.64 48.09 23.42
5 CRFT Gravel Pit 3.22 53.10 40.18
6 Noca Elixirs Bezoar 2.28 78.72 13.49
7 Cosmic Fog The Shocker 2.25 73.55 15.50
8 Clancy's Riptide 9.28 69.97 21.96
9 AlfaLiquid Noble Leaf 2.56 49.80 47.54
10 Virgin Vapor Best Damn Tobacco 11.76 86.90
11 Series 8 Strawnilla 48.40 43.65
12 Five Pawns Mixology Edition 2.45 53.97 36.63
13 Vype Aniseed 10.83 50.78 30.88
14 BRV Colonel's Custard 2.43 49.83 42.07
15 Taffy Man TR4 BLU 0.17 77.93 18.06
16 Bad Drip Bad Blood 2.24 69.47 20.69
17 Henley Brighton Peach 1.86 57.14 36.46
18 Cuttwood Boss Reserve 2.11 65.14 24.27
19 Kings Crest Duchess 8.97 77.00 11.30
20 Haus Berry 22.06 25.33 59.93
21 Haus Ocean Mist 22.55 23.20 59.56
22 VaporFi ? 10.91 52.79 41.88
23 X2O Natural Tobacco 17.77 60.93 35.39
24 Dewwy Bobba 4.60 72.24 20.72
25 Njoy Para Mour ? 54.25 38.58
26 ALD Group Mango (1) 50 mg/mL a ? ?
27 ALD Group Mint (1) 50 mg/mL a ? ?
28 ALD Group Crème (1) 50 mg/mL a ? ?
29 ALD Group Mango (2) 50 mg/mL a ? ?
30 ALD Group Mint (2) 50 mg/mL a ? ?
31 ALD Group Crème (2) 50 mg/mL a ? ?
32 ALD Group Fruit Medley 50 mg/mL a ? ?
33 Leap GO Mint 50 mg/mL a ? ?
34 JUUL Classic Tobacco 50 mg/mL a ? ?

50 mg/mL as reported

? indicates: presence but level not reported

– indicates: presence unknown

E-liquids containing cannabidiol (CBD)

In addition to the e-liquids containing nicotine, three e-liquids containing cannabidiol (CBD) were also evaluated. The list of these three e-liquids is given in Table 4.

List of e-liquids containing CBD evaluated for the presence of α-tocopherol and/or α-tocopheryl acetate.

No. Brand Flavor CBD mg/mL Glycerin % (w/w) PG % (w/w)
1 Korent Fresh mint 10.0 ? ?
2 Kore Organic Blueberry mist ? ? ?
3 Quanta Bubble mint 1.75/serv.

? indicates: presence but at unknown level

/serv. indicates: “per serving”

– indicates: not reported

The samples “Korent” and “Kore Organic” did not contain any α-tocopherol or α-tocopheryl acetate. However, in the LC-UV trace of the “Quanta” sample, a peak eluted at 11.904 min, a retention time very close to the retention time of α-tocopherol. No peak corresponded to the retention time of α-tocopheryl acetate. The HPLC chromatogram with UV detection for the “Quanta” sample is shown in Figure 1.

The retention time of the suspected α-tocopherol peak in the “Quanta” sample does not match exactly the retention time for α-tocopherol, and the peak shape is far from Gaussian. For this reason, it was suspected that an interference is responsible for the peak at 11.73 min in Figure 1.

Figure 1

The HPLC chromatogram with UV detection for the “Quanta” sample.

However, the sample was also analyzed by LC-MS/MS procedure. The extracted ion trace for α-tocopherol in the LC-MS/MS chromatogram for the “Quanta” sample is shown in Figure 2.

Figure 2

The ion trace for α-tocopherol in the LC-MS/MS chromatogram for the “Quanta” sample.

The result from Figure 2 indicated that α-tocopherol is indeed present in the “Quanta” sample. The quantitation based on the LC-MS/MS trace indicated that the level of α-tocopherol in the sample is 10.2 μg/g (RSD% = 3.71%). The calculation based on the area count from the UV trace indicated a different, significantly higher value, indicating that the peak in the UV trace has an interference.

CONCLUSIONS

An HPLC separation using UV detection as well as MS/MS detection has been developed previously for the analysis of α-tocopherol and α-tocopheryl acetate. The method has been applied for the quantitation of these compounds in e-liquids. Thirty-four e-liquids containing nicotine were evaluated by the new methods and none of them was found to contain either α-tocopherol or α-tocopheryl acetate.

Three e-liquids containing CBD were analylzed. Two of the liquids did not contain α-tocopherol or α-tocopheryl acetate, but one of the liquids contained a low level of α-tocopherol.

Figure 1

The HPLC chromatogram with UV detection for the “Quanta” sample.
The HPLC chromatogram with UV detection for the “Quanta” sample.

Figure 2

The ion trace for α-tocopherol in the LC-MS/MS chromatogram for the “Quanta” sample.
The ion trace for α-tocopherol in the LC-MS/MS chromatogram for the “Quanta” sample.

List of e-liquids containing nicotine evaluated for the presence of α-tocopherol and/or α-tocopheryl acetate (nicotine level, and content of glycerin and PG indicated when available).

No. Brand Flavor Nicotine mg/g Glycerin % (w/w) PG % (w/w)
1 Kilo Dewberry Cream 8.57 61.72 29.53
2 Frequency Liquid Boom Box 2.35 73.27 21.75
3 ZVR Lemon Cheesecake 5.25 54.70 32.42
4 OKVMI Haute Mocha 5.64 48.09 23.42
5 CRFT Gravel Pit 3.22 53.10 40.18
6 Noca Elixirs Bezoar 2.28 78.72 13.49
7 Cosmic Fog The Shocker 2.25 73.55 15.50
8 Clancy's Riptide 9.28 69.97 21.96
9 AlfaLiquid Noble Leaf 2.56 49.80 47.54
10 Virgin Vapor Best Damn Tobacco 11.76 86.90
11 Series 8 Strawnilla 48.40 43.65
12 Five Pawns Mixology Edition 2.45 53.97 36.63
13 Vype Aniseed 10.83 50.78 30.88
14 BRV Colonel's Custard 2.43 49.83 42.07
15 Taffy Man TR4 BLU 0.17 77.93 18.06
16 Bad Drip Bad Blood 2.24 69.47 20.69
17 Henley Brighton Peach 1.86 57.14 36.46
18 Cuttwood Boss Reserve 2.11 65.14 24.27
19 Kings Crest Duchess 8.97 77.00 11.30
20 Haus Berry 22.06 25.33 59.93
21 Haus Ocean Mist 22.55 23.20 59.56
22 VaporFi ? 10.91 52.79 41.88
23 X2O Natural Tobacco 17.77 60.93 35.39
24 Dewwy Bobba 4.60 72.24 20.72
25 Njoy Para Mour ? 54.25 38.58
26 ALD Group Mango (1) 50 mg/mL a ? ?
27 ALD Group Mint (1) 50 mg/mL a ? ?
28 ALD Group Crème (1) 50 mg/mL a ? ?
29 ALD Group Mango (2) 50 mg/mL a ? ?
30 ALD Group Mint (2) 50 mg/mL a ? ?
31 ALD Group Crème (2) 50 mg/mL a ? ?
32 ALD Group Fruit Medley 50 mg/mL a ? ?
33 Leap GO Mint 50 mg/mL a ? ?
34 JUUL Classic Tobacco 50 mg/mL a ? ?

Coefficients and R2 for the calibration curves Y = a X + b used in quantitation with MS/MS detection.

Compound Detection type a b R2
α-Tocopherol MS/MS 34.2270 −1.00506 0.9997
α-Tocopheryl acetate MS/MS 10.7472 −1.3916 0.9997

Coefficients and R2 for the calibration curves Y = a X + b used in quantitation with UV detection.

Compound Detection type a b R2
α-Tocopherol UV 0.022401 −0.05633 1.00
α-Tocopheryl acetate UV 0.022655 −0.05204 1.00

List of e-liquids containing CBD evaluated for the presence of α-tocopherol and/or α-tocopheryl acetate.

No. Brand Flavor CBD mg/mL Glycerin % (w/w) PG % (w/w)
1 Korent Fresh mint 10.0 ? ?
2 Kore Organic Blueberry mist ? ? ?
3 Quanta Bubble mint 1.75/serv.

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