Polycyclic aromatic hydrocarbons (PAHs) are a class of low-polarity environmental contaminants that have severe carcinogenic effects and have drawn worldwide attention. However, there remain challenges for current mass spectrometric ionization techniques in the analysis of low-polarity compounds in small-volume biosamples, such as single cells. In this work, we developed a nanoliter atmospheric pressure photoionization (nano-APPI) source and optimized its parameters for the detection of PAHs in small-volume samples. We evaluated the ionization performance of the source in direct and auxiliary gas-assisted photoionization modes and analyzed different PAH compounds as well as spiked biosamples. By combining the advantages of nano-electrospray ionization (nano-ESI) and atmospheric pressure photoionization (APPI), our newly developed nano-APPI source achieved high sensitivity for the analysis of PAHs down to the fmol level. Compared to conventional atmospheric pressure chemical ionization (APCI), the detection limit of PAHs was increased by 1-2 orders of magnitude. By optimizing various parameters, we achieved highly efficient ionization of PAHs, effective analysis of PAHs in mixed components, and sensitive detection of low-abundance PAHs in single-cell samples. Our optimized nano-APPI source was successfully applied for the sensitive analysis of PAHs in complex biological samples. Based on our study, we believe that nano-APPI holds great promise for toxicological studies on complex biological samples. The present work has implications for improving the detection sensitivity of low-polarity environmental contaminants and advancing the field of MS-based analysis of small-volume biosamples.