<p>Herein, we report on the development of a compact proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) for airborne measurements of volatile organic compounds (VOCs). The new instrument resolves isobaric ions with a mass resolving power (m/Δm) of &nbsp;1000, provides accurate m/z measurements (Δm &lt; 3 mDa), records full mass spectra at 1 Hz and thus overcomes some of the major analytical deficiencies of quadrupole-MS-based airborne instruments. 1 Hz detection limits for biogenic VOCs (isoprene, α total monoterpenes), aromatic VOCs (benzene, toluene, xylenes) and ketones (acetone, methyl ethyl ketone) range from 0.05 to 0.12 ppbV, making the instrument well-suited for fast measurements of abundant VOCs in the continental boundary layer. The instrument detects and quantifies VOCs in locally confined plumes (&lt; 1 km), which improves our capability of characterizing emission sources and atmospheric processing within plumes. A deployment during the NASA 2013 DISCOVER-AQ mission generated high vertical- and horizontal-resolution in situ data of VOCs and ammonia for the validation of satellite retrievals and chemistry transport models.</p>

Using recent developments in proton transfer reaction mass spectrometry, proof-of-principle investigations are reported here to illustrate the capabilities of detecting solid explosives in real-time. Two proton transfer reaction time-of-flight mass spectrometers (Ionicon Analytik) have been used in this study. One has an enhanced mass resolution (m/Δm up to 8000) and high sensitivity (∼50 cps/ppbv). The second has enhanced sensitivity (∼250 cps/ppbv) whilst still retaining high resolution capabilities (m/Δm up to 2000). Both of these instruments have been successfully used to identify solid explosives (RDX, TNT, HMX, PETN and Semtex A) by analyzing the headspace above small quantities of samples at room temperature and from trace quantities not visible to the naked eye placed on surfaces. For the trace measurements a simple pre-concentration and thermal desorption technique was devised and used. Importantly, we demonstrate the unambiguous identification of threat agents in complex chemical environments, where multiple threat agents and interferents may be present, thereby eliminating false positives. This is of considerable benefit to security and for the fight against terrorism.

Here we present proof-of-principle investigations on a novel inlet system for proton-transfer-reaction mass spectrometry (PTR-MS) that allows for the analysis of trace compounds dissolved in water. The PTR-MS technique offers many advantages, such as real-time analysis, online quantification, no need for sample preparation, very low detection limits, etc.; however it requires gas phase samples and therefore liquid samples cannot be investigated directly. Attempts to measure trace compounds in water that have been made so far are mainly headspace analysis above the water surface and membrane inlet setups, which both are well suitable for certain applications, but also suffer from significant disadvantages. The direct aqueous injection (DAI) technique which we will discuss here turns out to be an ideal solution for the analysis of liquid samples with PTR-MS. We show that we can detect trace compounds in water over several orders of magnitude down to a concentration level of about 100 pptw, while only consuming about 100 μl of the sample. The response time of the setup is about 20 s and can therefore definitely be called “online”. Moreover the method is applicable to the analysis of all substances and not limited by the permeability of a membrane.

Proton-transfer-reaction mass spectrometry (PTR-MS) developed about 10 years ago is used today in a wide range of scientific and technical fields allowing real-time on-line measurements of volatile organic compounds in air with a high sensitivity and a fast response time. Most instruments employed so far use quadrupole filters to analyze product ions generated in the reaction drift tube. Due to the low mass resolution of the quadrupoles used this has the disadvantage that identification of trace gases under study is not unambiguous. Here we report the development of a new version of PTR-MS instruments using a time-of-flight mass spectrometer, which is capable of measuring VOCs at ultra-low concentrations (as low as a few pptv) under high mass resolution (as high as 6000 m/Δm in the V-mode) with a mass range of beyond 100 000 amu. This instrument was constructed by interfacing the well characterized and recently improved Ionicon hollow cathode ion source and drift tube section with a Tofwerk orthogonal acceleration reflectron time-of-flight mass spectrometer. We will first discuss the set-up of this new PTR-TOF-MS mass spectrometer instrument, its performance (with a sensitivity of several tens of cps/ppbv) and finally give some examples concerning urban air measurements where sensitivity, detection limit and mass resolution is essential to obtain relevant data.

Proton-transfer-reaction mass-spectrometry (PTR-MS) developed in the 1990s is used today in a wide range of scientific and technical fields. PTR-MS allows for real-time, online determination of absolute concentrations of volatile (organic) compounds (VOCs) in air with high sensitivity (into the low pptv range) and a fast response time (in the 40–100 ms time regime). Most PTR-MS instruments employed so far use an ion source consisting of a hollow cathode (HC) discharge in water vapour which provides an intense source of proton donor H3O+ ions. As the use of other ions, e.g. NO+ and O2+, can be useful for the identification of VOCs and for the detection of VOCs with proton affinities (PA) below that of H2O, selected ion flow tube mass spectrometry (SIFT-MS) with mass selected ions has been applied in these instances. SIFT-MS suffers, however, from at least two orders lower reagent ion counts rates and therefore SIFT-MS suffers from lower sensitivity than PTR-MS. Here we report the development of a PTR-MS instrument using a modified HC ion source and drift tube design, which allows for the easy and fast switching between H3O+, NO+ and O2+ ions produced in high purity and in large quantities in this source. This instrument is capable of measuring low concentrations (with detection limits approaching the ppqv regime) of VOCs using any of the three reagent ions investigated in this study. Therefore this instrument combines the advantages of the PTR-MS technology (the superior sensitivity) with those of SIFT-MS (detection of VOCs with PAs smaller than that of the water molecule and the capability to distinguish between isomeric compounds). We will first discuss the setup of this new PTR+SRI-MS mass spectrometer instrument, its performance for aromates, aldehydes and ketones (with a sensitivity of up to nearly 1000 cps/ppbv and a detection limit of about several 100 ppqv) and finally give some examples concerning the ability to distinguish structural isomeric compounds.

A recently developed technique, Proton-Transfer-Reaction Mass-Spectrometry (PTR-MS), is reviewed based on applications on coffee. PTR-MS is a sensitive and fast method for on-line trace gas analysis. It consists of a specially designed chemical ionization cell, where headspace gas is continuously introduced and volatile organic compounds ionized by proton-transfer from H3O+. Protonated compounds are then mass analyzed in a quadrupole mass filter. First a description of the method will be given, with emphasis on the ionization mechanism. We then discuss a series of experiments that allow mass spectral intensities to be related to chemical compounds. Finally, two applications on coffee are discussed.