The pressure of the desorption nitrogen gas is the key parameter for GIND sampling, because GIND is a geometry-independent device, and the optimization of the angles, distance, etc. is not necessary. Thus, the pressure of nitrogen gas was optimized with
respect to the measured MS signal of oxybenzone (100 ppb) in a day cream sample. The signal intensity ofm/z229 (protonated oxybenzone) increased with elevated pressure of the nitrogen gas for neutral desorption from 1.0 to 2.0 MPa. Further pressure increase (>2.0 MPa) resulted in reduced signal intensities, probably because the analytes were blown away form the extractive ionization region by the high-velocity gas flow without producing sufficient signals. Thus, the pressure of nitrogen gas was selected as 2.0 MPa. As one of the optimized results, the methanol/acetic acid (100:0.1, v:v) solvent was infused at 5L/min as the electrospray solvent and the electrospray voltage was kept as 4.0 kV for all the measurements. The elevated temperature of the heated capillary improves the desolvation process. However, when the capillary temperature was set at more than 300◦C, the thermo-induced dissociation of analyte were observed, resulting in the reduced signal intensity. Note that for different cosmetic samples, the sensitivity of this method for detection of a given sun screener varied along with the matrix (i.e.,the cosmetics itself); however, the highest sensitivity was achievable only at the optimized conditions, which were the same for different analytes present in various cosmetics samples tested.