A corona discharge atmospheric pressure chemical ionization source with selective NO + formation and its application for monoaromatic VOC detection Martin Sabo and ˇ Stefan Matejˇ c´ ık * We have developed a new type of corona discharge (CD) for atmospheric pressure chemical ionization (APCI) for application in ion mobility spectrometry (IMS) as well as in mass spectrometry (MS). While the other CD-APCI sources are able to generate H 3 O + $(H 2 O) n as the major reactant ions in N 2 or in zero air, the present CD-APCI source has the ability to generate up to 84% NO + $(H 2 O) n reactant ions in zero air. The change of the working gas from zero air to N 2 allows us to change the major reactant ions from NO + $(H 2 O) n to H 3 O + $(H 2 O) n . In this paper we present the description of the new CD-APCI and discuss the processes associated with the NO + formation. The selective formation of NO + $(H 2 O) n reactant ions offers chemical ionization based on these ions which can be of great advantage for some classes of chemicals. We demonstrate here a significant increase in the sensitivity of the IMS-MS instrument for monoaromatic volatile organic compound (VOC) detection upon NO + $(H 2 O) n chemical ionization. Introduction In the last few decades there was a rapid development in the eld of atmospheric pressure ionization sources for different mass spectrometric techniques. The desorption electrospray ionization (DESI), 1,2 direct analysis in real time (DART), 3 atmo- spheric pressure-matrix assisted laser desorption ionization (AP-MALDI) 4 and CD 5–7 were developed for the surface analysis. For liquid samples electrospray ionization (ESI) 8 was developed by Fenn in the 1980s and later direct electrospray probe ioni- zation (DEPI) 9 techniques were developed. In the eld of volatile or semi-volatile compounds, different ion sources based on atmospheric pressure chemical ionization (APCI) 11,12 were developed followed by the secondary electrospray ionization (SESI) 10 technique. The development of a new atmospheric pressure ionization source is also of great importance in the eld of IMS. 13,14 The gradual expansion of these techniques has led to the use of IMS instruments in many applications, 14,15 either as stand- alone instruments or in combination with other analytical techniques. 16,17 Besides the widely used radioactive ionization sources 18 there exist many others that have been successfully imple- mented to IMS like CD, 19–21 glow discharge, 22 low-temperature plasma ionization (LTPI), 23 atmospheric pressure photoioniza- tion (APPI), 24 pulsed electron source, 25 ESI 26,27 and SESI. 10 In order to increase the analytical power and the selectivity of IMS, instruments with multiple ionization sources were devel- oped. 28,29 The applications of different ionization sources in the IMS technique are well described in the review article by Guharay et al. 30 The CD ionization sources in IMS were developed as alter- natives to widely used radioactive ion sources based on Ni 63 . The implementation of the reverse gas ow mode to the nega- tive CD resulted in the ability of these ionization sources to generate similar negative reactant ions like the radioactive ionization sources. 20,31 It is generally accepted that in the case of positive polarity CD the primary ions in N 2 or in air are effi- ciently converted to the reactant ions H 3 O + $(H 2 O) n and with a small admixture of NO + $(H 2 O) n . 32,33 Thus the reactant ions from CD are identical to those produced from radioactive ion sour- ces. 34 It was demonstrated in our recent publication that NO + (H 2 O) n reactant ions are more effective for the detection of the monoaromatic volatile organic compounds (VOCs) like benzene, toluene and m-xylene (BTX) 35 (for extensive studies on the mechanism and kinetics of chemical ionisation processes by NO + ,H 3 O + and O 2 + of different VOCs (aldehydes, ketones, carboxylic acids, esters, alcohols.) please see the review by Smith and ˇ Spanˇ el and references therein). 36 Therefore it is of great importance for IMS and MS techniques to develop a simple ion source, which is able to generate selectively or at least strongly enhanced NO + ions in air. The CD ionization source presented in this paper has the ability to produce both H 3 O + $(H 2 O) n and NO + $(H 2 O) n . Due to different chemical ionization mechanisms of H 3 O + $(H 2 O) n and NO + $(H 2 O) n reactant ions, 36 we are able to change the selectivity and the detection efficiency of IMS Comenius University, Faculty of Mathematics, Physics and Informatics, Department of Experimental Physics, Mlynska dolina F2 842 48 Bratislava, Slovakia. E-mail: matejcik@fmph.uniba.sk; Fax: +421-2-65429-980 Cite this: Analyst, 2013, 138, 6907 Received 14th May 2013 Accepted 9th September 2013 DOI: 10.1039/c3an00964e www.rsc.org/analyst This journal is ª The Royal Society of Chemistry 2013 Analyst, 2013, 138, 6907–6912 | 6907 Analyst PAPER Published on 09 September 2013. Downloaded by UNIVERSITAETSBIBLIOTHEK INNSBRUCK on 22/10/2013 07:59:13. View Article Online View Journal | View Issue