The main aim for the work on this thesis was to find a fast and sensitive method for qualitative and quantitative determination of the naphthenic acids (NAs) called the ARN acid family in crude oil. As there are three main components in the ARN acid family with quite similar molecular mass, separation of the acids with respect to the hydrophobicity, was essential to obtain reliable determination of all three acids. The sample preparation developed in this work consisted of liquid-liquid extraction of 10 mL crude oil with 20 mL methanol (MeOH):triethyl amine (TEA) (99.9:0.1). 500 nl of the MeOH phase was loaded on a 0.3 mm I.D. x 10 cm 3.5 μm Kromasil C18, 100 Å column at a flow rate of 5 μL/min. A water- MeOH mobile phase (MP) gradient containing 0.1 % TEA was also developed and used during the entire work on this thesis. The time for one injection, including reconditioning of the column was 35 min. The ARN acids were separated and detected down to 4.36 ppm with negative electrospray (ESI) on a time of flight mass spectrometer (TOF MS). However more work is needed to get control of the method, as the validation work with an internal standard gave poor results.