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Selective Chemical Ionization of Nitrogen and Sulfur Heterocycles in Petroleum Fractions by Ion Trap Mass Spectrometry

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Selective Chemical Ionization of Nitrogen and Sulfur Heterocycles in Petroleum Fractions by Ion Trap Mass Spectrometry View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Selective Chemical Ionization of Nitrogen and Sulfur Heterocycles in Petroleum Fractions by Ion Trap Mass Spectrometry C. S. Creaser*, F. Krokos, and K. E. O’Neill Shod of Chemical Sciences, University of East Anglia, Norwich, United Kingdom M. J. C. Smith and P. G. McDowell BP Research, Sunbury-on-Thames, Middlesex, United Kingdom A procedure is reported for the selective ammonia chemical ionization of some nitrogen and sulfur heterocycles in petroleum fractions using ion trap mass spectrometry (ITMS). The ion trap scan routine is designed to optimize the population of ammonium reagent ions and eject from the trap (by radio frequency/direct current isolation) electron ionization products formed during reagent ion formation prior to ionization of the sample. The ITMS procedure is compared with standard ion trap detector and conventional quadrupole ammonia chemi- cal ionization for the determination of nitrogen and sulfur heterocycles in gas oil and kerosine samples. Greatly enhanced selectivity is shown for the ITMS procedure by sup res- sion of competing charge-exchange processes. (1 Am Sot Mass Spectrum 1993, 4, 322-326 ‘; he removal of organic compounds containing reagent ion NH;, is formed via the reactions [5] nitrogen and sulfur is an important part of the T petroleum-refining process 111. Their presence in NH, ‘1 NH;’ (1) petroleum distillates results in the formation of envi- NH, f NH:‘+ NH; + NH; (2) ronmental pollutants (SO,, NO,) during combustion, and their reactivity can lead to catalyst poisoning Whereas NH: is unreactive toward a wide range of in refinery processes. The presence of heterocyclic hydrocarbons, the NH;’ ion (ionization potential of nitrogen compounds influences the storage stability of ammonia, 10.16 eV [6]) undergoes charge-exchange fuels. Alkyl indoles, in particular, have been linked reactions with aromatic hydrocarbons. Continuous to sediment formation in diesel fuels [2]. Analytical production of NH: in the ion source of a conventional methods are required to identify the nitrogen and mass spectrometer, together with restrictions on the sulfur compounds in petroleum distillates so that the ammonia pressure, limits the selectivity achievable in most efficient means may be devised for their removal. ionization of basic nitrogen and sulfur compounds by Characterization of minor heteroatomic components proton transfer from NH:. of complex hydrocarbon mixtures is a demanding task It has been shown that ion-trapping devices (quad- (see, for example, ref 3). Chemical ionization (CI) pro- rupole ion trap and ion cyclotron resonance [ICR] mass vides a means of reducing the potential for interfer- spectrometers) can generate CI spectra free from con- ence from the hydrocarbon matrix; by careful choice of tamination by ions formed by electron-impact ioniza- a reagent ion, selectivity may be achieved in the ion- tion [7-91. Furthermore, ion-trapping devices offer ization of target compound types [4]; however, the CI advantages over conventional instruments for highly reagent plasma formed in the ion source of a conven- selective CI because they operate efficiently in a pulsed tional mass spectrometer usually contains a variety of mode, enabling a temporal separation of the reagent ions, and selectivity is limited by the occurrence of ion formation process and the CI reaction. Continuous side reactions between the sample matrix and impurity production of unwanted ions can therefore be avoided. ions in the plasma. In ammonia CI, one of the sim- They also provide facilities for selective storage of pler reagent systems in this respect, the proton donor ions [lo] and enable purification of the reagent ion for subsequent, more selective ionization of target compounds [ll]. *Present address and address for reprint requests:C. 5. Creaser, By virtue of its high resolution and accurate mass Department of Chemistry&Physics, Nottingham Trent University, Clifton Larw, Nottingham NGll 8NS, UK. measurement capabilities, Fourier transform ICR mass D 1993 American Society for Mass Spectrometry Received July 15, 1992 1044-0305/93/$6.00 Revised October 28,lYYZ Accepted October 28,lYYZ J Am SC Mass Spectrom 1993, 4, 322-326 SELECTIVE Cl OF N- AND SHETEROCYCLES 323 spectrometry (lCR/MS) provides a powerful means of 91 identifying minor components of complex mixtures, 100% particularly when combined with selective ionization methods [ 71; however, the required instrumentation, which incorporates a superconducting magnet and fast signal-processing electronics, is costly. Quadrupole ion trap mass spectrometers, on the other hand, require comparatively simple instrumentation, and current 67 instruments offer sensitivity advantages. The present work was conducted to examine the I, Ill. ! I . I selective ionization of nitrogen and sulfur compounds I . 20 40 60 80 100 120 140 160 in petroleum distillates bv ammonia CI. The perfor- 1 I A L mance of ion trap and conventional quadrupole mass spectrometry was compared. In particular, the capabil- 91 ity of the low-resolution quadrupole ion trap instru- 100% ment to characterize low levels of hetcroatomics in complex hydrocarbon matrices was investigated. 68 I f Experimental Ion trap CI experiments were carried out on a Finnigan-MAT (San Jose, CA) research quadrupole ion trap mass spectrometer, fitted with a Varian 3400 gas III I I. I chromatograph operated using quadrupole ion trap ,‘,.,I, I’,‘,‘,‘,‘,.,‘,‘,., mass spectrometry (ITMS) or standard ion trap detec- 20 40 60 80 100 120 140 160 tor @ID) software. The helium pressure in the mani- fold was 1.5 x lo-” torr, and the ammonia reagent gas 68 pressure was maintained at 2.8 x lo-” torr above the lOO%- background pressure, measured at the vacuum hous- ing ion gauge. Pyrrole (I, Figure 1, bottom) in toluene (1 + 9) (high-performance liquid chromatography grade; Fisons, Loughborough, UK) was introduced via a liquid reservoir. Gas oil (light gas oil/light cycle oil from a UK refinery) and kerosinc (from a Far East refinery) samples (diluted 1 + 9 in toluene) were intro- duced by gas chromatography (GC) using a 50-m X I I 0.25-mm inside-diameter (id) RSL-200 BP column ,.,“‘,‘l.,“‘,“‘1’1’,“‘, (Alltech, UK). The GC conditions were as follows: 20 40 60 80 100 120 140 160 injector temperature, 250 “C; transfer line, 240 “C; 1 PL Figure 1. Mass spectra of pyrmle in toluene (top) EI spectrum, splitless injection (split flow at 20 mL/min). The helium (middle) CI spectrum, and (bottom) CI spectrum using rf/dc isolation of NH:. head pressure was 12 psi. The column oven tcmpera- ture was 50 “C for 1 min, raised to 100 “C at 15 “C/min, then to 250 “C at 10 “C/min, and held for 20 min. Results and Discussion Quadrupole CI mass spectrometry (CIMS) analyses were carried out on a VG Trio-3 quadrupole mass The ion trap scan routine used for the selective CI of spectrometer combined with a Hewlett-Packard nitrogen and sulfur heterocycles is shown in Figure 2. (Avondale, PA) HP5890 gas chromatograph fitted with The reagent ion-formation step in the ion trap ammo- a 25-m x 0.22 mm-id CPSil-5CB column (Chrompak, nia Cl was designed to create a large population of UK) using the same temperature program. ICR/MS NH: reagent ions (m/z 18) while minimizing the experiments were conducted on a Nicolet Fourier population of NH:’ ions (m/z 17) and electron ioniza- transform mass spectrometry 2000 dual-cell instru- tion (El) products. These ions are formed during the ment, Oil samples (between 2 and 10 mg) were intro- initial ionization of ammonia and sample present in duced via a Brunfeldt all-glass heated inlet without the trap. Ammonium ions are produced in part B of separation. Internal standards (&methyl quinoline; 2,6- the scan routine (Figure 2) in which NH:. ions, formed dimethylquinoline; Z-phenylquinoline; acridine; and from the reagent gas by EI, are held in the ion trap for 1,2,3,4,5,6,7,8-octahydroacridine) were added for mass a period of up to 120 ms to react with ammonia to give calibration and approximate quantification. The NH: (Eqs 1 and 2). To establish the optimum condi- ammonia CI experimental sequence for the ICR/MS tions for NH: reagent ion formation, the abundances has been described previously [7]. of NH:’ and NH: were measured at various reagent- 324 CREASER ET AL. J Am Sot Mass Spectrom 1993,4,322-326 - reaction times, and the results for an ammonia partial pressure of 2.8 X low5 torr are shown in Figure 3a. The abundance of NH:’ (m/z 17) is initially high but rapidly decreases at longer reaction times as more F NH: ions are formed. There is an optimum reaction time between 20 and 60 rns, where the NH: : NH; ratio is greater than ZOO:1. At longer reaction times, E the NH: signal decreases because of space-charge c / effects and increased scattering. A reaction time of 0 D 20 ms was chosen for further studies. A I I I I I Reagent ion formation is followed by reaction with sample molecules present in the trap (Figure 2, D) The Figure 2. ITMS scan routine for the selective ammonia CT effect of increasing the Cl time for the reaction of NH: of nitrogen and sulfur heterocycles (not to scale): A, ionization- with pyrrole (I) in a toluene matrix (diluted 1 + 9) was (0.2 ms); B, formation of reagent ions (20 ms); C, isolation Iof observed by monitoring the [M + H]+ (m/z 68) prod- reagent ions with dc applied voltage (-14 V for 1 ms); D, uct ion abundance (Eq 3): formation of sample ions (180 ms); E, selection of low-mass cutoff for start of acquisition (50 u); F, analytical scan (50-300 u).
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