The task: With increasing e-cigarette use, the consumption of e-liquids is on the rise, both in their nicotine containing and nicotine free forms. Country by country, different regulatory requirements apply, regarding the type and concentration of the flavorings that can be added. Chemical analysis is required for product quality control as well as for monitoring of regulatory compliance and consumer safety. From an analytical point of view, the e-liquid matrix represents a challenge. Interfering carrier compounds are present in large quantities, while some flavor compounds must be monitored in trace amounts. This means that adequate, efficient sample preparation and extraction technologies are required followed by GC-MS determination. Jim Zhu, application expert at GERSTEL Shanghai, has investigated the matter and developed a highly sensitive and rugged method.
Background: Jim Zhu took on the task of developing a method to monitor the presence and concentration of 108 flavor compounds listed in Chinese standard GP 41700-2022 for e-cigarettes and e-liquids intended for the Chinese market. The specific objective was to determine the compounds in an efficient, sensitive and sustainable manner and finding the optimal extraction technique.
The challenge: "Traditional methods such as solvent extraction (SE), liquid-liquid extraction (LLE) and simultaneous distillation/extraction (SDE) are often used to extract flavor compounds from e-liquids" says the seasoned application expert, reflecting the results of his literature research. However, these methods require significant amounts of potentially toxic solvents, larger sample quantities, and additional concentration steps. In addition, they are time-consuming, labor-intensive and cumbersome. Solid-phase microextraction (SPME) shows only a low sensitivity and a small linear range for the determination. Headspace techniques, on the other hand, suffer from interference by the dominating compounds propylene glycol and glycerin contained in the sample. Jim Zhu found the solution in Stir Bar Sorptive Extraction (SBSE) using the GERSTEL Twister in combination with thermal desorption and GC-MS (AppNote 268).
The approach: SBSE represents a solvent-free, efficient and highly sensitive alternative to classic extraction techniques. SBSE effectively meets the challenge and enables a more accurate determination of flavor compounds in e-liquids. According to Zhu, this modern, environmentally friendly extraction technology is efficient, sensitive and user-friendly in the extraction of flavor compounds.
The extraction: The amount of e-liquid sampled and extracted with the GERSTEL Twister (SBSE) was 50 mg placed in a 20 mL headspace vial with 2-Nonanol (200 mg/kg) added as internal standard for semi-quantitative analysis. A 2 g aliquot of a saturated NaCl water solution was added, followed by a PDMS Twister and the flavor compounds were extracted for 60 minutes at room temperature at 1000 rpm stirring speed. The Twister was then removed with tweezers, rinsed with distilled water, dried with a clean, lint-free disposable paper towel, and transferred to a thermal desorption tube for automated thermal desorption GC-MS analysis (GERSTEL LabWorks platform).
The analysis: Analytes concentrated on the Twister were desorbed in the Thermal Desorption Unit (GERSTEL TDU 2) using a temperature program and cryofocused in the Cooled Injection System (GERSTEL CIS 4), a PTV-type inlet, in the GC-MS (Agilent 7890/5975). Transfer from the CIS 4 to the directly inserted GC separation column (HP-Innowax, 60 m x 0.25 mm ID, 0.25 µm, Agilent Technologies) was performed using a temperature program, which efficiently separated the dominant carrier and nicotine compounds enabling determination of even trace level compounds in the Mass Selective Detector (MSD) using electron impact ionization (EI) and full scan detection (33-400 amu).
The result: According to Jim Zhu, SBSE is a modern, environmentally friendly, solvent-free sample extraction technology. SBSE has proven its efficiency, sensitivity and ease of use in the extraction and determination of flavor compounds in e-liquids. In e-liquid Sample A, 87 volatile flavor compounds were determined, and in Sample B, 105 volatile flavor compounds were determined based on SBSE extraction. The efficiency of SBSE for flavor compound extraction from e-liquids was verified by comparing it with a direct liquid extraction of the samples examined.
Source: GERSTEL AppNote 268