DART-TOFMS measurements The DART ion source was operated in positive ion mode with helium as the ionizing medium at a flow rate of 2.7 L/min. The gas beam was heated to 300°C, discharge needle voltage set to 3000 V, perforated and grid electrode voltages were+150 V and +250 V, respectively. Accurate mass spectra were acquired in a range of m/z 50–500 employing 0.2 s recording interval; the peaks voltage value was set to 1000 V. A solution containing a mixture of poly(ethylene glycol) PEG 600 and 200 was introduced at the end of each sample analysis to compensate any mass drift. The examined samples were introduced automatically with the use of an AutoDart sampler and Dip-it™ tips. Following steps were involved: (i) sampling tip immersed into the sample; (ii) placing of tip in front of the DART gun exit close to the ion source – mass spectrometer axis; (iii) sampling tip disposed. Five replicate measurements were carried out on examined samples.