Ambient Mass Spectrometry

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Ambient mass spectrometry methods address the need for rapid analysis with minimal sample preparation. In our laboratory, we use two main approaches to form gas-phase ions at ambient conditions: electrospray- and plasma-based methods, with a number of variants within each of these ionization techniques. Some of the examples in which we apply ambient mass spectrometry methods for a number of applications include:

Secondary Electrospray Ionization-Mass Spectrometry

Electrospray is a widely used ionization technique to bring analytes from the liquid to the gas phase. While this is its primary use in mass spectrometry, electrosprays offer an opportunity to ionize gaseous analytes (i.e. vapors). This variant of electrospray ionization has been dubbed secondary electrospray ionization (SESI), whereby neutral vapors interacting with an electrospray plume of pure solvent are readily ionized and subsequently mass analyzed.

One main advantage of this approach is that it offers the opportunity to monitor gaseous analytes in real time. When SESI is combined with state-of-the-art mass spectrometers, limits of detection in the low parts-per-trillion by volume (pptv) are achievable without any sample pre-concentration. We exploit this feature in our lab in a number of projects requiring fast mass spectrometric analysis of vapors of trace concentrations. 

Schematic drawing and picture of the low flow SESI source  
Schematic drawing and picture of the low flow secondary electrospray ionization source (SESI).

Active Capillary Plasma Ionization

Active capillary plasma ionization directly merges the atmospheric pressure inlet of a mass spectrometer with a small and rigid dielectric barrier discharge (DBDI) based ionization source. This significantly enhances ion transmission into the MS and thereby the sensitivity. The source can be understood as elongation of the inlet capillary of an atmospheric pressure mass spectrometer. A metal capillary inside the glass tube serves as first electrode. The second cooper ring electrode surrounds the glass. If a high AC voltage is applied to the electrodes, a dielectric barrier discharge plasma is formed within the capillary. Although the whole gaseous sample passes the plasma, the ionization process is very soft, yielding mostly MH+ analyte species. As a further advantage the source does not need any additional gas or liquid supplies and can be attached to virtually any existing atmospheric pressure inlet MS interface. The source has proven to be capable of efficiently ionizing a variety of substance classes including chemical warfare agents (CWA), alcohols, PAH, pesticides, and polymers.

Current Scientific Projects

Analysis of exhaled breath. Exhaled breath contains a wealth of biochemical information, whose analysis can be used for a number of purposes. Globally, one of the main goals in our group is to contribute to include breath as an alternative body fluid for untargeted metabolomic studies. For example, in consistency with previous metabolomic studies, we have found that exhaled breath shows temporal fluctuations, probably reflecting circadian changes. In addition, despite this temporal ''noise'', we have found that each individual shows a distinct breathprint stable in time. This is also in agreement with prior work based on urine metabolomics. Ongoing collaborations with the University Hospital Zurich and the Children's University Hospital Zurich are devoted to diagnose non-invasively lung diseases and to monitor drugs in exhaled breath.

Identification of metabolites in exhaled breath. Untargeted metabolomic studies usually result in a pool of statistically significant m/z features which may be useful for diagnosis purposes. However, the chemical identity of these features is rarely determined which results in a huge loss of useful biological information. It is our aim to overcome this lack of information by identifying the metabolites responsible for the different m/z features previously established in breath analysis. To accomplish this goal, we collect and up-concentrate exhaled breath condensate (EBC) samples, in collaboration with the University Hospitals, that are subsequently separated by means of Ultra Performance Liquid Chromatography (UPLC) and analyzed with a LTQ-Orbitrap mass spectrometer. This way, a proper identification is possible thanks to the great capabilities of UPLC-Orbitrap for identifying unknowns by means of exact mass, chromatographic retention times and fragment mass spectra which can be compared to proposed pure reference standards.

Development of a commercial SESI source for preexisting mass spectrometers. Despite its many advantages, SESI-MS is not yet very widespread. One main reason for this is that there are no commercial SESI reaction chambers available which can be easily interfaced with commercially available atmospheric pressure ionization-mass spectrometers (API-MS). We are currently addressing this issue in collaboration with an industrial partner (SEADM, Spain), funded by the European Commission (IAPP-ACID project). As a result, we have developed a SESI ionizer, showing outstanding figures of merit, which can be readily interfaced with major API-MS instruments. This add-on is commercially available and is ideally suited for researchers owning a preexisting API MS, interested in using this technique to analyze vapors.

Staff SESI: Nora NowakMartin GauggAlberto Tejero, Dr. Tobias Bruderer

Staff Plasma: Luzia GyrMario Mirabelli, Dr. Katharina Hubba

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