The development of sensitive analytical techniques for the realtime detection of aromatic (AH) and polycyclic aromatic hydrocarbons (PAH) is of high importance due to their impact on human health and the environment. A promising approach, regarding to direct determination of (P)AHs in aqueous samples, is resonance-enhanced multiphoton ionization (REMPI) coupled to external-membrane introduction mass spectrometry (eMIMS). In eMIMS, analytes are extracted from the water phase into the gas phase, which is supplied to the MS by using an external semipermeable membrane setup. As a result, no laborious enrichment techniques are needed. With REMPI, ions are formed by the subsequent absorption of two photons via an excited molecular state. The unique ionization scheme of REMPI provides selective and sensitive detection of (P)AHs. When combining the capabilities of REMPI and MIMS, direct measurements of subµg/L concentrations of small (polycyclic)aromatic compounds are feasible. In this study, we present an external sheet membrane probe (ESMP) for the determination of selected (polycyclic)aromatic species in water samples by using REMPI time-of-flight mass spectrometry (REMPI-TOFMS). This inlet design shows promising results with respect to the direct analysis of (P)AHs in aquatic environments. With this early-stage system, concentrations down to tens of ng/L for selected small (polycyclic)aromatic compounds are accessible in minutes without any sample preparation.
INTRODUCTIONFast and sensitive analysis of aromatic (AH) and polycyclic aromatic hydrocarbons (PAH) in aqueous matrices is vitally important due to their high impact on environmental and human health 1,2 . The low concentrations of AHs and PAHs in environmental bodies of water complicate their monitoring 3 . Although available chromatographic offline techniques offer clear advantages in respect to sensitivity [4][5][6][7] , various laborious enrichment techniques (e.g. purge-and-trap, solid-phase extraction, liquid-liquid extraction) have to be applied to increase the concentrations to a detectable level [8][9][10] . For this reason, the development of online techniques moves into focus. A simplified analytical method for PAHs and similar compounds in complex matrices has been developed by Mirabelli et al. by using online solid-phase microextraction (SPME) coupled to atmosphericpressure photo-ionization mass spectrometry (APPI-MS). They were able to reach limit-of-detections (LOD) down to 0.1 ng/L for PAHs. 11 Nevertheless, for this approach, analytes in the water samples have to be extracted and pre-concentrated by SPME before the SPME-fibers are separately introduced into the inlet system of the MS. On the other hand, a powerful analytical technique, for online determination of PAHs in gaseous mixtures, is reso-48 nance-enhanced multiphoton ionization (REMPI) coupled to 49 time-of-flight mass spectrometry (TOFMS) 12-14 . In REMPI 50 ions are formed by the subsequent absorption of at least two 51 photons including real molecular electronic states. Fundamen-...