<div>Proton-transfer-reaction mass-spectrometry (PTR-MS) developed in the</div> <div>1990s is used today in a wide range of scientific and technical fields.</div> <div>PTR-MS allows for real-time, online determination of absolute concentrations</div> <div>of volatile (organic) compounds (VOCs) in air with high sensitivity</div> <div>(into the low pptv range) and a fast response time (in the 40&ndash;100 ms</div> <div>time regime). Most PTR-MS instruments employed so far use an ion</div> <div>source consisting of a hollow cathode (HC) discharge in water vapour</div> <div>which provides an intense source of proton donor H3O+ ions. As the</div> <div>use of other ions, e.g. NO+ and O2+, can be useful for the identification</div> <div>of \{VOCs\} and for the detection of \{VOCs\} with proton affinities</div> <div>(PA) below that of H2O, selected ion flow tube mass spectrometry</div> <div>(SIFT-MS) with mass selected ions has been applied in these instances.</div> <div>SIFT-MS suffers, however, from at least two orders lower reagent</div> <div>ion counts rates and therefore SIFT-MS suffers from lower sensitivity</div> <div>than PTR-MS. Here we report the development of a PTR-MS instrument</div> <div>using a modified \{HC\} ion source and drift tube design, which allows</div> <div>for the easy and fast switching between H3O+, NO+ and O2+ ions produced</div> <div>in high purity and in large quantities in this source. This instrument</div> <div>is capable of measuring low concentrations (with detection limits</div> <div>approaching the ppqv regime) of \{VOCs\} using any of the three reagent</div> <div>ions investigated in this study. Therefore this instrument combines</div> <div>the advantages of the PTR-MS technology (the superior sensitivity)</div> <div>with those of SIFT-MS (detection of \{VOCs\} with \{PAs\} smaller</div> <div>than that of the water molecule and the capability to distinguish</div> <div>between isomeric compounds). We will first discuss the setup of this</div> <div>new PTR+SRI-MS mass spectrometer instrument, its performance for</div> <div>aromates, aldehydes and ketones (with a sensitivity of up to nearly</div> <div>1000 cps/ppbv and a detection limit of about several 100&amp;#xa0;ppqv)</div> <div>and finally give some examples concerning the ability to distinguish</div> <div>structural isomeric compounds.&nbsp;</div>