The possible presence of other sugars in e-liquids

Most of the disaccharides are built of monomers such as glucose, galactose and fructose. This means that they will generate the same pseudomolecular and fragmentation ions during MS/MS. In fact, only the difference in the retention times between different sugars allows their identification. The samples of e-liquids were tested for the presence of two disaccharides (lactose and maltose) and two common monosaccharides (fructose and glucose) in order to exclude their possible coelution with sucrose. The mixture containing each sugar at 200 ng/mL was analysed under the conditions described in “HPLC conditions”. The chromatogram presenting the separation of standards is shown in Fig. 2. Under the proposed HPLC conditions, sucrose is baseline-separated from other sugars; hence, they will not interfere with its quantitative determination. None of the 37 samples analysed were found to contain sugars other than sucrose.

Fig. 2 Chromatogram of a mixture of analytes: fructose, glucose, sucrose, maltose, lactose and raffinose (each at 200 ng/mL) detected by negative electrospray ionization tandem mass spectrometry in HILIC mode Full size image

Within-laboratory validation

Analytical figures of merit

A six-point calibration curve was constructed using raffinose as an internal standard, and each calibration solution (see “Preparation of standards and calibration solutions”) was analysed in triplicate. The curve was linear in the range of concentrations studied. The LOD was calculated with the equation LOD = 3.3S b /a, where S b is the standard deviation of the intercept and a is the slope of the calibration curve. The limit of quantitation was calculated as three times the LOD. The LOD obtained (0.73 μg/g) is similar to [17, 18] or even lower [19, 20] than the values reported by others. Within-day precision was estimated by replicate (n = 6) analysis of samples fortified at three concentrations (10, 20 and 30 μg/g) on 1 day. Data obtained during the within-day precision investigation were also used to assess the trueness of the method. Intermediate (between-day) precision was verified by analysing the single fortified solution (20 μg/g) for three consecutive days. Again, each analysis was performed six times (n = 6).

As can be seen from Table 2, the recovery values at all spiking levels are close to 100 %, which means that no matrix effects or bias was observed. This allows the use of external calibration instead of a matrix-matched approach. The method also performs well in terms of precision. In no case was the coefficient of variation greater than 2.5 %.

Table 2 Determination of sucrose in fortified e-liquid samples: calibration parameters, trueness and repeatability data Full size table

Analysis of real samples

All samples were prepared according to the protocol described in “Sample preparation and preparation of fortified samples”. Samples were chosen from among the most manufacturers and the popular brands available on the market. The content of sucrose in the samples analysed is presented in Table 3.

Table 3 Concentration of sucrose in e-liquids for electronic cigarettes: analysis of real samples Full size table