Negative ion chemical ionization (NICI) mass spectra with methane as reagent gas and the ion abundance ratios of the negative to the positive base peak for 51 polycyclic aromatic hydrocarbons and related compounds were measured and evaluated for highly sensitive detection and isomer differentiation. Either (M -HI-, M-' or MH-was the base peak, except for one compound with (M -H2]-' as its base peak. The numbers of compounds with [M -HI-, M-' or MH-as their base peaks were 17, 26 and 7, respectively. Many of the compounds with [ M -HIas the base peak had an aliphatic part in their structure. The average value of NIP (negativelpositive ion abundance ratio at the base peaks) was <1. Many of the compounds with M-' as the base peak had a relatively high electron affinity. A correlation between electron affinities and ion abundances was found. In most cases, the NIP ratios were >1, and even reached 400 in benzo[a]pyrene. Many of the compounds with MH-as their base peaks had a phenyl group, in which cases the NIP ratios were < 1. In the case of compounds with 18 or fewer carbon atoms, in particular, it was easy to distinguish isomers by comparing their NICI mass spectra. The NIP values served as a guideline in sensitive detection. Nine compounds achieved an NIP of 2 50.