Condensed Phase Membrane Introduction Mass Spectrometry with Direct Electron Ionization: On-line Measurement of PAHs in Complex Aqueous Samples

A State-of-the-Science Review of Alcoholic Beverages and Polycyclic Aromatic Hydrocarbons

BACKGROUND

The association between alcohol and certain cancers is well established, yet beyond ethanol and its metabolite acetaldehyde, little is known about the presence of other carcinogenic compounds in alcoholic beverages, including polycyclic aromatic hydrocarbons (PAHs), such as benzo[a]pyrene (a Group I carcinogen).

OBJECTIVES

We summarized the published literature on PAH levels in alcoholic beverages to identify potential gaps in knowledge to inform future research.

METHODS

Medline and Scopus were searched for primary research published from January 1966 to November 2023 that quantified PAH levels among various types of alcoholic beverages, including whisky, rum, brandy, gin, vodka, wine, and beer. Studies that were not primary literature were excluded; only studies that quantified PAH content in the specified alcoholic beverages were included.

RESULTS

Ten studies published from 1966 to 2019 met the criteria for review. Other than beverage type, no publication reported selection criteria for their samples of tested alcohol products. Studies used a variety of analytical methods to detect PAHs. Of the 10 studies, 7 were published after 2000, and 6 assessed < 20 products. Of the studies, 7 examined spirits; 3, beer; and 4, wines. Benzo[a]pyrene was most prevalent among spirit products, particularly whisky, with values generally exceeding acceptable levels for drinking water. Some beer and wine products also contained PAHs, albeit at lower levels and less frequently than spirit products.

DISCUSSION

PAHs are found in some alcohol products and appear to vary by beverage type. However, there is an incomplete understanding of their presence and levels among large, representative samples from the range of currently available alcohol products. Addressing this gap could improve understanding of alcohol-cancer relationships and may have important implications for public health and the regulation of alcohol products. In addition, novel methods, such as direct mass spectroscopy, may facilitate more thorough testing of samples to further investigate this relationship. https://doi.org/10.1289/EHP13506.

Condensed Phase Membrane Introduction Mass Spectrometry: A Direct Alternative to Fully Exploit the Mass Spectrometry Potential in Environmental Sample Analysis

Membrane introduction mass spectrometry (MIMS) is a direct mass spectrometry technique used to monitor online chemical systems or quickly quantify trace levels of different groups of compounds in complex matrices without extensive sample preparation steps and chromatographic separation. MIMS utilizes a thin, semi-permeable, and selective membrane that directly connects the sample and the mass spectrometer. The analytes in the sample are pre-concentrated by the membrane depending on their physicochemical properties and directly transferred, using different acceptor phases (gas, liquid or vacuum) to the mass spectrometer. Condensed phase (CP) MIMS use a liquid as a medium, extending the range to new applications to less-volatile compounds that are challenging or unsuitable to gas-phase MIMS. It directly allows the rapid quantification of selected compounds in complex matrices, the online monitoring of chemical reactions (in real-time), as well as in situ measurements. CP-MIMS has expanded beyond the measurement of several organic compounds because of the use of different types of liquid acceptor phases, geometries, dimensions, and mass spectrometers. This review surveys advancements of CP-MIMS and its applications to several molecules and matrices over the past 15 years.

A Direct Mass Spectrometry Method for the Rapid Analysis of Ubiquitous Tire-Derived Toxin N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine Quinone (6-PPDQ)

The oxidative transformation product of a common tire preservative, identified as N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ), has recently been found to contribute to "urban runoff mortality syndrome" in Coho salmon at nanogram per liter levels. Given the number of fish-bearing streams with multiple stormwater inputs, large-scale campaigns to identify 6-PPDQ sources and evaluate mitigation strategies will require sensitive, high-throughput analytical methods. We report the development and optimization of a direct sampling tandem mass spectrometry method for semiquantitative 6-PPDQ determinations using a thin polydimethylsiloxane membrane immersion probe. The method requires no sample cleanup steps or chromatographic separations, even in complex, heterogeneous samples. Quantitation is achieved by the method of standard additions, with a detection limit of 8 ng/L and a duty cycle of 15 min/sample. High-throughput screening provides semiquantitative concentrations with similar sensitivity and a full analytical duty cycle of 2.5 min/sample. Preliminary data and performance metrics are reported for 6-PPDQ present in representative environmental and stormwater samples. The method is readily adapted for real-time process monitoring, demonstrated by following the dissolution of 6-PPDQ from tire fragments and subsequent removal in response to added sorbents.

The history of electron ionization in LC-MS, from the early days to modern technologies: A review

This review article traces the history of the use of liquid chromatography coupled with mass spectrometry (LC-MS) using electron ionization (EI) from the first attempts up to the present day. At the time of the first efforts to couple LC to MS, 70 eV EI was the most common ionization technique, typically used in gas chromatography-mass spectrometry (GC-MS) and providing highly reproducible mass spectra that could be collated in libraries. Therefore, it was obvious to transport this dominant approach to the early LC-MS coupling attempts. The use of LC coupled to EI-MS is challenging mainly due to restrictions related to high-vacuum and high-temperature conditions required for the operation of EI and the need to remove the eluent carrying the analyte before entering the ion source. The authors will take readers through a journey of about 50 years, showing how through the succession of different attempts it has been possible to successfully couple LC with EI-MS, which in principle appear to be incompatible.

Direct mass spectrometric analysis of naphthenic acids and polycyclic aromatic hydrocarbons in waters impacted by diluted bitumen and conventional crude oil

Crude oil spills have well-documented, deleterious impacts on the hydrosphere. In addition to macroscopic effects on wildlife and waterscapes, several classes of petroleum derived compounds, such as naphthenic acids (NAs) and polycyclic aromatic hydrocarbons (PAHs), may be released into the water and present aquatic contamination hazards. The concentrations of these contaminants may be affected by both oil type and water chemistry. We characterize the concentrations of NAs and PAHs in natural and constructed waters, spanning a range of pH and salinity, and directly compare the influence of diluted bitumen (DB) and conventional crude (CC) oil, using condensed-phase membrane introduction mass spectrometry (CP-MIMS) as a direct sampling, on-line technique. The concentration and isomer class profiles of classical NAs in the aqueous phase were assessed using electrospray ionization in negative-ion mode as [M-H]^- whereas PAH concentrations were monitored using liquid electron ionization (LEI) in positive-ion mode as [M^+•]. NA concentrations (0.03-25 ppm) were highly pH-dependent, and an order of magnitude greater in water samples contaminated with DB than CC. Conversely, concentrations of naphthalene (10-130 ppb) and alkyl-naphthalenes (10-90 ppb) were three to four-fold higher in water samples exposed to CC. We demonstrate that naturally occurring dissolved organic matter does not bias results from the membrane sampling approach employed, and that DB and CC contaminated waters can be differentiated using principal component analysis of the NA isomer class distribution in both constructed and natural waters. Finally, we describe the first demonstration of the concurrent analysis of trace NAs and PAHs in the same water sample by controlling perm-selectivity at the membrane and the ionization mode of the mass spectrometer. The techniques employed here for trace analysis of petroleum derived compounds in water can be applied to rapid screening and real-time monitoring of contamination and remediation processes.

Mass Spectrometry Based Approach for Organic Synthesis Monitoring

Current mass spectrometry-based methodologies for synthetic organic reaction monitoring largely use electrospray ionization (ESI), or other related atmospheric pressure ionization-based approaches. Monitoring of complex, heterogeneous systems may be problematic because of sampling hardware limitations, and many relevant analytes (neutrals) exhibit poor ESI performance. An alternative monitoring strategy addressing this significant impasse is condensed phase membrane introduction mass spectrometry using liquid electron ionization (CP-MIMS-LEI). In CP-MIMS, a semipermeable silicone membrane selects hydrophobic neutral analytes, rejecting particulates and charged chemical components. Analytes partition through the membrane, and are then transported to the LEI interface for sequential nebulization, vaporization, and ionization. CP-MIMS and LEI are both ideal for continuous monitoring applications of hydrophobic neutral molecules. We demonstrate quantitative reaction monitoring of harsh, complex reaction mixtures (alkaline, acidic, heterogeneous) in protic and aprotic organic solvents. Also presented are solvent-membrane compatibility investigations and, in situ quantitative monitoring of catalytic oxidation and alkylation reactions.

Evaluation of a liquid electron ionization liquid chromatography-mass spectrometry interface

Liquid Electron Ionization (LEI), is an innovative liquid chromatography-mass spectrometry (LC-MS) interface that converts liquid HPLC eluent to the gas-phase in a mass spectrometer equipped with an electron ionization (EI) source. LEI extends the electronic spectra libraries access to liquid chromatography, providing a powerful tool in the untargeted approacssh. Negligible matrix effects allow accurate quantitative information. The purpose of this research was to evaluate the main aspects concerning the interfacing process. These fundamental studies were necessary to understand the mechanism of LEI in details, and improve the interfacing process, especially regarding robustness and sensitivity. Hardware components were installed to prevent analytes precipitation, reduce thermal decomposition of sensitive compounds, and to stabilize the nano-flow delivery with different mobile-phase compositions. Particular attention was devoted to insulating the heated vaporization area from the LC part of the system. Experiments were performed to optimize the interface inner capillary dimensions, and other operative parameters, including temperature, gas and liquid flow rates. Test compounds of environmental interest were selected based on molecular weight, thermal stability, volatility, and polarity. Robustness was evaluated with a set of replicated injections and calibration experiments using a soil matrix as a test sample. MRM detection limits in the low-picogram range were obtained for five pesticides belonging to different classes in a soil sample. High-quality electron ionization mass spectra of a mixture of pesticides were also obtained.

Atmospheric Pressure Vaporization Mechanism for Coupling a Liquid Phase with Electron Ionization Mass Spectrometry

A novel liquid chromatography-mass spectrometry (LC-MS) interfacing concept is presented and discussed. The new interface, called liquid-EI (LEI), is based on electron ionization (EI) but, differently from any previous attempt, the vaporization of solutes and mobile phase takes place at atmospheric pressure into a specifically designed region, called "vaporization microchannel", before entering the high-vacuum ion source. The interface is completely independent from the rest of the instrumentation and can be adapted to any gas chromatography/mass spectrometry (GC/MS) system, as an add-on for a rapid LC-MS conversion. Pressure drop and temperature gradient between LC and MS were considered to enhance the analyte response and reduce band broadening and/or solute carryovers. A fused silica liner, placed inside the vaporization microchannel, acts as an inert vaporization surface speeding up the gas-phase conversion of large molecules while lessening possible memory effects. The liner is easily replaceable for a quick and extremely simple interface maintenance. Proof of concept and detailed description of the interface are here presented.