Resonance enhanced multiphoton ionization (REMPI) is a powerful method for the sensitive determination of polycyclic aromatic hydrocarbons (PAHs) in gaseous mixtures via mass spectrometry (MS). In REMPI, ions are produced by the absorption of at least two photons including defined electronic intermediate states. As a result—unlike other laser-based ionization techniques—spectroscopic selectivity is involved into the ionization process. Nevertheless, these wavelength-dependent ionization rates impede the quantification using REMPI. For this purpose, relative photoionization cross sections (relPICS) give an easy-to-use approach to quantify REMPI-MS measurements. Hereby, the ionization behavior of a single compound was compared to that of a reference substance of a given concentration. In this study, relPICS of selected single-core aromatics and PAHs at wavelengths of 266 nm and 248 nm were determined using two different time-of-flight mass spectrometric systems (TOFMS). For PAHs, relPICS were obtained which showed a strong dependence on the applied laser intensity. In contrast, for single-core aromatics, constant values of relPICS were determined. Deviations of relPICS between both TOFMS systems were found for small aromatics (e.g., benzene), which can be assigned to the differences in UV generation in the particular system. However, the relPICS of this study were found to be in good agreement with previous results and can be used for system-independent quantification.
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-...
We developed a novel fast gas chromatography (fastGC) instrument with integrated sampling of volatile organic compounds (VOCs) and detection by single-photon ionisation (SPI) time-of-flight mass spectrometry (TOFMS).
Rationale: Fast and sensitive detection of aromatic hydrocarbons (AHs) in water is of high importance because of their significant impact on human health and the environment. For this, resonance-enhanced multiphoton ionization (REMPI) coupled to trap-and-release membrane-introduction mass spectrometry (T&R-MIMS) offers the possibility of sensitive on-line water analysis with a time resolution of minutes. Methods: REMPI is a versatile tool for sensitive gas-phase analysis, in which AHs are selectively ionized in complex gas mixtures by the subsequent absorption of at least two photons. In T&R-MIMS, selective extraction and enrichment of analytes from water can be achieved using semipermeable membranes. By the subsequent stimulated desorption of enriched compounds, mass spectrometric detection is enabled. Results: We present an external T&R inlet for hollow-fiber membranes coupled to REMPI time-of-flight mass spectrometry, which enables direct and sensitive detection of semi-volatile AHs in water. In laboratory experiments, spiked water samples were analyzed. For the investigated compounds, limits of detection (LODs) in the range 1-47 ng/L were determined. The LODs are approximately one order of magnitude lower than in a previously reported continuous membrane-introduction approach using a planar membrane. Further improvement of LOD may be realized by extending the trapping time and by increasing the release temperature. Furthermore, the system was applied to investigate different fuels suspended in water and real water samples. The obtained data are in good agreement with findings of a former study. Conclusions: In the framework of the present study, we demonstrate the high potential of the combination of REMPI and T&R-MIMS in the form of a newly developed external hollow-fiber membrane inlet. With the developed system, semivolatile AHs can be directly detected down to ng/L levels on a minute time scale. The approach thus may pave the way to future ship application in marine sciences, natural resources exploration or pollutant and hazard detection.
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