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ScienceDirect Physics Procedia 73 (2015) 163 – 167
4th h International Confereence Photon nics and Info formation Optics, O PhIO O 2015, 28-330 January 2015 2
Ionizattion of thhe nitroaaromaticc compou unds in an a ion m mobility sp pectromeeter withh an ion source s bbased on porous silicon uunder lasser irradiiation. Igor Maartynov, Yury Y Kuzish hchin*, Dm mitriy Dovzzhenko, Geenadii Kotk tkovskii, Alexander A Chistyakov National N Researchh Nuclear Univerrsity MEPhI (Mosscow Engineeringg Physics Institutte), Kashirskoye shosse s 31, Moscoow, 115409, Russia
Absttract Now wadays surface aassisted laser desorption/ioniz d zation is widelyy used in differrent analytical methods. m Somee of the most in nterest meth hods are based oon laser irradiaation of nanostru uctured surfacees, porous silico on (pSi) in partticular. This meethod already proved p itselff in mass spectrrometry due to the combinatio on of high sensiitivity and possibility of investtigation of smalll molecules beecause of th he absence of thhe influence of a substrate on the t ion signal. IIn this work wee present summ marized results oof our investigaations dediccated to the usee of the surface assisted laser desorption/ioniz d zation on pSi in n ion mobility spectrometry (IM MS), which is one o of the most m promisingg analytical methods m in the area of fast ddetection of low w concentrations of organic molecules. We W use trinittrotoluene (TNT) as a substaance to be inveestigated. Obtaained results sh how that TNT ionization mecchanism under laser irrad diation is compllicated and relaates both to thee electron emisssion process from the pSi surrface and subseequent ion-moleecular reacttions in gas phaase and to the suurface proton transfer as well. © Published by by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license © 2015 20 015The TheAuthors. Authorrs. Published y Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer--review under rresponsibility of the National Research R Nucleear University MEPhI M (Moscow w Engineering PPhysics Institutte). Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) Keyw words: IMS; pSi; D DIOS; ionizationn; TNT; detection.
1. In ntroduction In n the last decaade methods of o highly sensiitive express-aanalysis of org ganic compou unds are of intterest, especially in the area a of securitty and defensee. Among maany of them oone could be distinguished d for its compacctness [Kotko ovskii
* Corresponding C autthor. Tel.: +7-9166-00-84-38. E-mail E address:
[email protected]
1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) doi:10.1016/j.phpro.2015.09.147
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et al. a (2011)], fasst analysis [Kotkovskii et al. a (2010), (20011)], simpliciity and low deetection threshhold [Kotkovsskii et al. (2010), ( (2011)]. Analysis of o the time-resolved ion sppectrum of an analyte in a constant electtric field lies in the basee of ion mobillity spectromeetry (IMS). In I order to obttain analyte ioon formation different d ionizzation method ds could be ussed. Our groupp specializes on o the imp provement of laser ionizatioon for explosives detectionn. This metho od has high seensitivity, seleectivity, absen nce of frag gmentation annd flexibility of engineerin ng solutions [[Kotkovskii et al. (2009), (2010), (20111); Martynov et al. (2011), Veber et al. (2010)]. metry [Alimp Nowadays N lasser desorption//ionization on n porous silicoon (DIOS) is of interest in mass-spectrom piev et al. (2001), ( Budim mir et al. (20077), Go et al. (2 2003), Li & L Lipson (2013),, Northen et all. (2007), Shenn et al. (2001)), Wei et al. a (1999)]. D DIOS is one of o the most seensitive methood and has po ossibility to investigate com ompounds with low mollecular mass [Alimpiev et al. (2001), Go G et al. (20033), Shen et all. (2001)] at the t same timee. In this stud dy we pressent summarizzed data of ouur investigatio ons dedicated to the possibilities of usin ng DIOS in IM MS. Trinitroto oluene (TN NT) has been used as an analyte. a TNT is a typical nitroaromaticc compound and its detecttion is of praactical inteerest. Data recceived demonsstrates that TN NT laser ionizzation mechan nism from pSi surface is ver ery complicateed and relaates both to thhe electron em mission from pSi p surface, aand subsequen nt ion-molecullar reactions iin gas phase and a to the surface protonn transfer as well. w 2. Experimental E l Lateral L ion m mobility specttrometer desccribed in [M Martynov et al. (2011)] haas been usedd for ion speectrum regiistration. Ion ssource consistts from two ellectrodes sepaarated with dieelectric (fig. 1). 1 Inside the iion source theere are outp put channel foor buffer gas and input chaannels for sam mple probe flo ow, laser beam m and pSi sam mple. This typ pe ion sourrce has a posssibility to work w in two reegimes: contiinuous flow of o analyte vap pors and regiime of prelim minary app plying of the annalyzed substtance on the pSi surface. In I the first caase vapors of the substancee analyzed weere pumped near the surfacce of the pSi sample, whicch had been n irradiated w with laser. In the second case analyzed substance weere applied to o the pSi samp mple outside th he ion sourrce and then w were put in thee ion source and a irradiated w with laser. Spectrometer S construction provides a possibility p of using differeent buffer gass, purified airr or pure nitrrogen. Sorp ption of analyyzed substancce on the pSi surface was ccarried out in a special cuvette in air atm mosphere or in n pure nitrogen [Kuzishchin et al. (20015)]. Laser L irradiatiion of pSi sam mples were caarried out withh 4th harmonic of YAG:Nd d3+-laser with 6 ns pulse du uration and d 10 Hz repetiition rate. Fluuence of laserr pulses variedd in the rangee of 50-50 mJJ/cm2, which ccorresponded to the inteensities from 11·107 to 3·107 W/cm2.
Fig. 1. The scheme of ion moobility spectromeeter ion source.
Igor Martynov et al. / Physics Procedia 73 (2015) 163 – 167
3. Results R and diiscussion We W focused inn our investigaations on the understandingg of pSi influence on the mechanism m off the TNT neg gative ion formation. Inn our previouus studies wee’ve investigaated ion form mation in [Veeber et al. (22010)] the case of own that in tthe gas phasee negative ion formation ggoes through h ionmulttistage laser iionization. It has been sho moleecular reactionns. In generall, neutral mollecules of the medium invo olved in the io onization proccess despite of o the neuttral TNT moleecules and eleectrons formed d due to the prrobe ionizatio on [Daum et al. a (2002)]. A As a result, ion n type depeends on the composition of the spectrrometer gaseoous medium. In oxygen containing c attmosphere, aiir for exam mple, (ɌɇɌ-ɇ ɇ) – ions weree observed. While W in nitroogen atmosph here ɌɇɌ- ion ns were obserrved [Veber et al. (201 10)]. These exxperimental reesults agree well w with the results obtain ned using ion n sources of ddifferent typess and prov ves common cconcept about the mechanism m of negativee TNT ions forrmation in a gas g phase. In n the case of surface assistted laser ionizzation on pSi ion formation n process sign nificantly diffe fers due to thee new posssible ionizatioon mechanism m, namely lasser-stimulatedd proton transsfer from TN NT molecule tto the pSi su urface [Dov vzhenko et al. (2014), Kuzzishchin et al. (2015)]. It iss worth noticiing that in thee case of usinng monocrystaalline silicon as a substrrate this effecct is missing. We W have show wn in [Dovzh henko et al. (2 2014), Kuzishhchin et al. (20 015)] nsfer mechaniism prevailed d in the case of o preliminaryy sorption of TNT that TNT ionizatiion through thhe proton tran T moleculees desorb from m the pSi surfface in neutraal state and geet involved in n ionmoleecules. Howevver, part of TNT moleecular reactionns. Illustrationns of these pro ocesses are preesented on Fig g. 2.
Fig. 2. Mechanisms of TNT T negative ionn formation underr laser irradiation n on pSi.
In n order to reveeal all mechannisms listed an nd to evaluatee their impact on the overall signal of TN NT ions we used an apprroach based onn the investiggation of the in nfluence of sppectrometer gas g medium co omposition onn the ion type. It is obviious that surfaace proton traansfer does no ot depend on the composittion of spectro ometer gas m medium. In con ntrast ionizzation mechannism based onn the desorptio on of neutral m molecules and d subsequent ion-molecular i r reactions stro ongly depeends on the coomposition off the gas mediium. The pressence or absen nce of oxygen n molecules inn it will lead to t the diffeerent types of formed ions. General resultts are presenteed on Fig. 3.
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Fig. F 3. Influence oof gas medium chhange on the ion signal of negativee TNT ions (left) under irradiation n of pSi with preliiminary sorbed TNT T moleccules, (right) – in the case of TNT vapors been pum mped near the pSi surface.
It I could be seeen (Fig. 3, left ft), that for TN NT molecules sorbed on thee pSi surface change c in specctrometer gas phase doees not lead to tthe change off (TNT-ɇ) – io on peak magnnitude. Therefo ore, proton traansfer is the m main channel of ion form mation in this case. At the same s time, in nitrogen one small peak appears with io on mobility coorresponding to the TNT T – ions. It is rreasonable to assume that itt corresponds to the molecu ules leaving th he surface in th the neutral statte due to th he thermal deesorption. Furtther these molecules could take part in ion-molecular reactions witth electrons em mitted from m the pSi surfface and form m TNT – ion peak p in nitrogeen atmospherre. Results of the model exxperiment on Fig. F 3 (rig ght) and previoous data [Kuzishchin et al. (2015), Martyynov et al. (20 011)] prove th he correctness of this assum mption. In case c of TNT vvapors being pumped p through the ion souurce without preliminary p so orption negatiive ion signal could be observed. o How wever, ion typpe and peak magnitude m critiically dependss on the oxygen concentratition in spectro ometer gas phase. Hencce laser irradiiated pSi is an a effective eelectron sourcce. These elecctrons triggerr the ion-moleecular which leads to the ionization n of neutral TN NT molecules near the pSi surface. s reacction chain, w 4. Conclusions C In I this study ssummarized reesults of our previous p inve stigations ded dicated to the formation of TNT negative ions on the t surface off porous silicoon irradiated with w laser at tthe atmospheric pressure. It I has been shhown that TN NT ion form mation is a ccomplicated process p relateed both withh electron em mission from the porous ssilicon surface and subsequent ion-m molecular reacctions in the gaas phase and w with surface proton p transferr.
knowledgemeents Ack This T study w was supported by the Ministry of Educcation and Sccience of thee Russian Fedderation, gran nt no. 3.17 718.2014/K annd State task 1718. Refferences Alim mpiev S., Nikiforoov S., Karavanskiii V., Minton T., Sunner J., 2001. On the mechanissm of laser-induce ed desorption–ionnization of organic compounds from m etched silicon and a carbon surfaces. J. Chem. Physs. 115(4), 1891. Budiimir N., Blais J.-C C., Fournier F., Tabet T J.-C., 2007. Desorption/ionizzation on porous silicon mass spec ctrometry (DIOS)) of model cation nized fatty acids. J. Maass Spectrom. 42(1), 42–48. Daum m K.A., Atkinsonn D.A., Ewing R..G., 2002. The role of oxygen in thhe formation of TNT T product ions in ion mobility sspectrometry. Int.. J. Mass Spectrom. 214, 257–67.
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Dovzhenko D.S., Kuzishchin Y.A., Martynov I.L., Eremin I.S., KotkovskiƱ G.E., et al., 2014. The mechanism of laser-stimulated desorption/ionization of nitroaromatic compounds from a nanoporous silicon surface at atmospheric pressure. J. Opt. Technol. 81(8), 435. Go E.P., Prenni J.E., Wei J., Jones A., Hall S.C., et al., 2003. Desorption/ionization on silicon time-of-flight/time-of-flight mass spectrometry. Anal. Chem. 75(10), 2504–6. Kotkovskii G.E., Martynov I.L., Novikova V.V., Chistyakov A.A., 2009. A laser ion-mobility spectrometer. Instruments Exp. Tech. 52(2), 253– 59. Kotkovskii G.E., Sychev A.V., Tugaenko A.V., Chistyakov A.A., 2011. A laser spectrometer of field-asymmetric ion mobility. Instruments Exp. Tech. 54(2), 256–61. Kotkovskii G.E., Tugaenko A.V., Chistyakov A.A., 2010. Negative ion formation in laser ion mobility increment spectrometer. Tech. Phys. Lett. 36(3), 276–78. Kuzishchin Y.A., Martynov I.L., Dovzhenko D.S., Kotkovskii G.E., Chistyakov A.A., 2015. Surface-Assisted Laser Desorption/Ionization of Trinitrotoluene on Porous Silicon under Ambient Conditions. J. Phys. Chem. C. 119(11), 6382–88. Li J., Lipson R.H., 2013. Insights into Desorption Ionization on Silicon (DIOS). J. Phys. Chem. C. 117(51), 27114–19. Martynov I.L., Karavanskii V.A., Kotkovskii G.E., Kuzishchin Y.A., Tsybin A.S, Chistyakov A.A., 2011. Ion mobility spectrometer with ion source based on laser-irradiated porous silicon. Tech. Phys. Lett. 37(1), 15–18. Northen T.R., Woo H.-K., Northen M.T., Nordström A., Uritboonthail W., et al., 2007. High surface area of porous silicon drives desorption of intact molecules. J. Am. Soc. Mass Spectrom. 18(11), 1945–49. Shen Z., Thomas J.J., Averbuj C., Broo K.M., Engelhard M., et al., 2001. Porous Silicon as a Versatile Platform for Laser Desorption/Ionization Mass Spectrometry. Anal. Chem. 73(3), 612–19. Veber A.A., Kotkovskii G.E., Martynov I.L., Chistyakov A.A., 2010. Formation of anions of nitroaromatic compounds in gases during UV laser irradiation. Russ. J. Phys. Chem. B. 4(4), 548–56. Wei J., Buriak J.M., Siuzdak G., 1999. Desorption-ionization mass spectrometry on porous silicon. Nature. 399(6733), 243–46.
