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A commercial PTR-TOF-MS has been optimized in order to allow the measurement of individual organic nitrates in the atmosphere. This has been accomplished by shifting the distribution between different ionizing analytes, H₃O⁺∕ H₃O⁺(H₂O)_<i>n</i> or NO⁺∕ NO₂⁺. The proposed approach has been proven to be appropriate for the online detection of individual alkyl nitrates and functionalized nitrates. It has been shown that hydroxyl and ketonitrates have a high affinity towards NO⁺, leading to the formation of an adduct that allows the easy identification of the organic nitrate (R) from the R–NO⁺ ion signal. The recorded sensitivities for both ionization modes correspond to detection limits of tens of ppt min⁻¹ in the case of hydroxy- and ketonitrates. Alkyl nitrates exhibit a moderate affinity towards NO⁺ ionization leading to detection units of few hundreds of ppt and the highest sensitivity in H₃O⁺ mode was obtained for the water adducts signals. However, this method exhibits much lower capabilities for the detection of peroxyacetyl nitrates with detection limits in the ppb range.