Development and Optimization of an External-Membrane Introduction Photoionization Mass Spectrometer for the Fast Analysis of (Polycyclic)Aromatic Compounds in Environmental and Process Waters

Abstract

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-...

External trap‐and‐release membrane inlet for photoionization mass spectrometry: Towards fast direct analysis of aromatic pollutants in aquatic systems

“…For this, tap water was contaminated with diesel as well as heavy fuel oil by suspending 2 μL of each fuel in 1.0 L of water. The results reflect the basic findings for an external sheet membrane setup described in Gehm et al 46 : the most prominent compounds for water contaminated with diesel fuel are alkylated benzenes and indanes, whereas naphthalene and phenanthrene and their alkylated derivatives are the most abundant species for heavy fuel oil in water. Furthermore, in direct comparison with an external sheet membrane setup, 46 the utilization of T&R extends the spectrum of detectable analytes to higher molecular weights, while the LODs can be improved at the same time.…”

“…In the OEC approach, steady‐state conditions are reached in approximately 1.5 and 2 min of sampling time for toluene and p ‐xylene, respectively. These response times, however, are considerably longer than those of the sheet‐membrane assembly presented in a former study 46 and can be attributed to increased thickness of the hollow‐fiber membrane. Nevertheless, both compounds show similar MIMS behavior, due to similarities in their vapor pressures.…”

“…The homebuilt laser photoionization mass spectrometry (PIMS) system has been described previously 46 . Briefly, the TOF mass spectrometer (Stefan Kaesdorf GmbH, Munich, Germany) comprises a two‐stage Wiley/McLaren‐type ion extraction, a two‐stage ion‐mirror and a multichannel plate detector.…”

“…Furthermore, in many cases the transfer line into the mass spectrometer can act as a security device restricting the water inflow in the case of membrane rupture. Previously, we demonstrated the applicability of EMIs using sheet membranes for fast and continuous determination of various AHs using REMPI‐TOFMS 46 . The published results therein indicate that for effective determination of higher boiling point compounds, stimulated desorption is of crucial importance.…”

Determination of Relative Ionization Cross Sections for Resonance Enhanced Multiphoton Ionization of Polycyclic Aromatic Hydrocarbons

“…Our presented approach is primarily devoted to take a step towards the complicated semi-quantification of online REMPI-TOFMS experiments for environmental and process monitoring. For online gas phase REMPI-MS measurements [24][25][26] or direct REMPI-MS analysis of liquid samples with either membrane inlet [27][28][29] or direct inlet techniques [30], a native or isotope labeled internal standard (e.g., toluene or D3-toluene) can be added to semi-quantify the polycyclic aromatic compounds via relPICS. Additionally, for REMPI-based analytical techniques, which do not allow a straight forward internal standardization approach, such as laser desorption REMPI-MS online analysis of PAHs on a single aerosol particle [31], the relPICS allow the determination of the pattern of the aromatic compounds.…”