Abstract: Six involatile organic acids, including DL-tartrate, malic acid, lactic acid, citric acid, acetic acid, and benzoic acid, in e-liquid were determined by the method shown by the title. An aliquot (0.1 g) of sample was taken, and 0.05 mL of 1 g · L⁻¹ mixed isotope internal standard solution of benzoic acid-d₅ and succinic acid-d₆, and 10 mL of 10% (volume fraction) methanol solution containing 0.1% (volume fraction, the same below) formic acid were added. After covering by a seal and shaken for 20 min by vortex, the solution was diluted 20 times by 0.005 g · L⁻¹ mixed isotope internal standard solution, and passed through an organic phase filter membrane. The filtrate was determined by ultra-high performance liquid chromatography-tandem mass spectrometry. Six involatile organic acids were separated by gradient elution on Waters HSS T3 column with mixed solutions of 5 mmol · L⁻¹ ammonium formate solution (adjusting its acidity to pH 2.5 with formic acid) and methanol solution containing 0.1% formic acid at different volume ratios as mobile phase, ionized in ESI⁻ mode, detected in MRM mode, and quantified by the internal standard method. It was shown that linear relationships between values of the mass concentration ratios and peak area ratios of six involatile organic acids to isotopic internal standards were kept in definite ranges, with detection limits in the range of 0.01-0.22 mg · g⁻¹. Test for recovery was made according to the standard addition method, giving recoveries in the range of 90.4%-106%, and RSDs (n=6) of the determined values obtained from intra-day and inter-day precision tests were found in the ranges of 0.60%-4.9% and 1.5%-7.9%, respectively. The method was used for the analysis of 30 actual samples, and six involatile organic acids were detected to varying degrees, but the detected amounts did not exceed the maximum allowable amount limit of GB 41700—2022.