Measurements of atmospheric volatile organic compounds were performed in the Finnish Boreal forest atmosphere during spring 2003, as part of the project QUEST (Quantification of Aerosol Nucleation in the European Boundary Layer), using a ground-based Chemical Ionization Mass Spectrometer (CIMS) instrument. Based on the study of their hydrate distribution, methanol, acetonitrile, acetaldehyde, dimethyl amine (DMA), ethanol/formic acid, acetone, trimethyl amine (TMA), propanol/acetic acid, isoprene, methyl vinyl ketone (MVK) and metacrolein (MaCR), monoterpenes and monoterpene oxidation product (MTOP) are proposed as candidates for masses 32, 41, 44, 45, 46, 58, 59, 60, 68, 70, 136, and 168amu, respectively. It would be, to our knowledge, the first time DMA, TMA and MTOP are measured with this method. Most compounds show a clear diurnal variation with a maximum in the early night, corresponding to the onset of the noctural inversion and in agreement with independant measurements of CO. Biogenic compounds are highly correlated with each other and the ratio monoterpene/oxidation product shows a typical daily pattern of nightime maxima. However, because isoprene mixing ratios are also maximum during the early night, it is likely that it suffers of the interference from another unidentified biogenic compound. Hence mass 68amu is identified as isoprene+compound X.