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The neutralisation and reionisation of mass-selected positive ions by inert gas atoms
InternationaC Journal of Mass Spectrometry and Ion Processes, 64 (1985) 245-250 Elsevier Science Publishers B-V., Amsterdam - Printed in The Netherlands
245
Short Communication THE NFJJTRALISATION AND REIONISATION POSITIVE IONS BY INkRT GAS ATOb
OF MASS-SELECTED
JOHAN K. TERLOUW, WILLEM M. ICIESKAMP Analytical Chemistry Laboratory, University of Utrecht, 3522 AD Utrecht (The Netherlands) JOHN L. HOLMES, ALEXANDER
A. MOMMERS
Department of Chemistry, University of Ottawa, Ottawa, Ont. KIN 9B4 (Canada) PETER C. BURGERS Department de Chimie, Universit& Naval, Q&bee, P.Q. GlK 7P4 (Canada) (Received 28 !kptember 1984)
Recent experiments in these laboratories [l-4] have shown how the structures of neutral fragments formed in the dissociative ionisation of organic compounds can be ascertained by means of their collisionally induced dissociative ionisation mass spectra. Two experimental methods, using a VG-Analytical ZAB-2F mass spectrometer of reversed geometry and having a collision gas cell situated in the second field-free region, were describe!. The first involved reversal of the ion beam by means of a high electric potential applied to the gas cell [1,2] while the second [3] used an ion beam deflector electrode placed in front of and close to the collision cell. Thus only neutral products of metastable fragmentations of mass selected ions entered the cell and their high translational energy ensured their collision-induced ionisation by the target gas, usually He. It was observed [4], particularly at high target gas pressures, that some of the incident ion beam was neutralised by charge exchange with target gas which had diffused into the region between the electromagnet and the collision cell. Neutralisation phenomena not associated with the target gas were also reported. Neutralisation by charge exchange in conventional collisionally induced dissociation mass spectra has been identified by Laramk et al. [5]. Recently, Danis et al. [6] have generated neutral species from mass-selected ions of known structure by an elegant but experimentally difficult method. The ions collide with metal vapour target atoms, selected so that resonant charge exchange conditions are approached. The intention of the experi0168-1176/85,‘$03.30
8 1985 Elsevier Science Publishers B.V.
ments was to produce a good flux of high velocity neutral species having low internal energies and hence a useful experimental lifetime. The reaction chamber for the metal vapour preceded an ion beam deflector electrode and a conventional collision cell in which ionisation and dissociation of the neutral species took place. It was concluded that neutralisation rather than fragmentation was the predominant event when the ions met the metal atoms. In the present paper, it will be shown that collisions between mass-selected ions and Xe as target gas in a conventional gas cell can also produce their stable neutral counterparts, in excellent yield and without appreciable contributions from collisionally generated neutral fragments. EXPERIMENTAL
The observations were made using a standard VCL4nalytical ZAB-2F double-focusing mass spectrometer of reversed geometry (B/E) [7] with the following modifications. A second, electrically insulated collision gas cell (cell I) was placed ca. 10 cm in front of the standard collision gas cell (cell II) which is situated in the second field-free region of the mass spectrometer 171. An ion beam deflector electrode [3] was placed midway between the two cells. Thus, all ions exiting the .first cell could be deflected away and prevented from entering the second cell. In the majority of experiments, cell I contained Xe at a measured pressure of 7 X IO-’ Torr (note that the ion gauge which recorded this pressure is situated above the diffusion pump between cell II and the electric sector [7]). Cell II contained He at a similarly measured pressure of 2 X 10m7 Torr. Mass-selected ions having 8 kV translational energy interacted with Xe in cell I. Ions leaving cell I were prevented from entering cell II by means of the deflector electrode at a potential of + 1.5 kV. Neutral products from cell I collided with He in cell II and the resulting positive ions were energy (mass) analysed by the-electric sector. Compounds were of research grade and used without further purification. RESULTS
AND DISCUSSION
Figure 1 shows the neutralisation-reionisation mass spectra (NRMS) of the molecular ions of acetone (A) and its enol (B); the latter was generated via elimination of C,H, from ionised 2-hexanone. These spectra are very similar to those reported by Danis et al [6] who used Zn vapour in the neutralisation step. The ionisation energy (IE) of Zn = 9.4 eV [8] is not far removed from those of both acetone, 9.7 eV [8], and its enol, 8.5 eV [9]. Although IE(Xe) = 12.1 eV [8], the endothermic charge exchange processes yield neutral fluxes and subsequently collisionally derived positive ion cur-
247
(A)
(B)
Fig. 1. NR mass spectra of (A) acetone and (B) its enol from neutralization corresponding ions with Xe and reionization with He.
of the
rents of intensity ca. 1000 times that which in our equipment would require signal averaging (compare ref. [6]), see Fig. 1. Thus it can be concluded that for these mass-selected ions, near-resonant charge exchange is not necessary for obtaining NRMS of good intensity. The energy deficit may be made up from the ions’ translational energy or possibly in part from their internal energies which are limited by their first dissociation thresholds. Comparing the present results with those of Danis et al. [6], the most significant differences are the higher relative yields of molecular ions, 16% of the total NRMS ion current for acetone and 24% for the enol (ketone 5%, enol 8%
248
[6]). The larger m/z 58 for the enol is consistent with the relative stabilities of the molecular ions [lo], not their neutral species [9] and Danis et al. [6] concluded that the observations indicated that most of the fragmentation occurred after reionisation. However, in principle, the interaction with Xe in Cell I can produce neutral species formed by collisionally induced decomposition of the incoming ions. The relative importance of neutralisation vs. fragmentation depends markedly on the inert gas used in cell I. For example, the molecular ion of 1-propanol m/z 60, [CH&H&H,OH]+‘, in a Xe NRMS experiment yields m/z 31 as base peak, as in the normal mass spectrum. With He in cell I, m/z 31 becomes a minor peak (ca. 20% of pase peak) and the spectrum is now dominated by m/z 29 ([C,H,]+ and [HCO]) and m/z 27, [C2H3]+. These hydrocarbon fragments are produced from collisionally generated ethyl radicals [2], the neutral counterpart of m/z 31, being the base peak in the collisional activation mass P-WHl+, spectrum of [CHJH,CH,OH]+-. These observations are in keeping with the charge exchange experiments of Laram& et al. [5] who concluded that for the inert gases Ar and He, the latter was much more efficient than the former as a target gas for collision-induced fragmentation. Resonant electron exchange with the target gas does not appear to be a strict criterion for the generation of an NRMS of good intensity and wl$ch cyntains
ions+ of the p!eselected
mass. For example,
NRMS
of [CH,CO],
[CH,OH], [COOH], [CH,], [CH,OH]+- and [H&O]+‘, whose ionisation energies range from ca. 7-11 eV, were all readily observed and details will be published elsewhere. To illustrate the high sensitivity of the present experiments, Fig. 2 shows the NR mass spectrum (without signal averaging) of [C,H$’ ions metadably generated from [CH$H&H,OH]+ and transmitted from the first field-free region by selecting the ions at m/z 29.4 ( = 422/60). This spectrum is very closely similar to that obtained from ion-source-generated [C,H,]+‘. Such sensitivity also permits the observation of doubly charged ions generated from encounters between neutral species. Danis et al. [6] were unable to decide whether the neutral ylid [CH,ClH] or its conventional isomer CH,Cl was generated in the Zn atom neutralization of m/z 52 [H,,C,37Cl] +- ions from slCH,COOH. The latter have been shown to contain the ylid ion [CH,ClH], which can readily be distinguished from [CH,Cl]+’ by their charge stripping mass spectra [11,12]: the ylid ion generates an intense [CH27C1H]++ signal and weaker peaks at m/z 25.5 and 25 in contrast to [CH,Cl]+- which produces only m/z 25.5 and 25. The NR mass spectrum of the m/z 52 ions from [ClCH,COOH]+’ is closely similar to both the collisional activation, mass spectrum and the NRMS of [CH337C1]+-; moreover, the doubly charged ion at m/z 26, characteristic of the ylid ion, is absent.
249
27
1:
Fig. 2. NR mass spectrum of metastably generated [C,H,]+’
ions from [CH,CH,CH,OH]+‘.
Neutralization-reionization using Xe as the charge exchange gas produces mass spectra similar to, but much more intense than those reported [6] using metal vapour as the target gas. These experiments clearly have great potential for exploring the behaviour of beams of fast neutral species and may also provide an additional method for the investigation of the structures of ions. ACKNOWLEDGEMENTS
.
J.K.T., J.L.H. and A.A.M. acknowledge a collaborative research award from the NATO Scientific Affairs Division, J.L.H. and P.C.B. thank the Natural Sciences and Engineering Research Council of Canada for continuing financial support and P.C.B. similarly thanks the Mink&e de l’Education du Quebec, (programme F.C.A.C.). J.K.T. and A.A.M. thank Dr. R.H. Bateman (VG Analytical Ltd.) for discussions about the design of the second collision gas cell. REFERENCES PC. Burgers, J.L. Holmes, A.A. Mommers and J.K. Terlouw, Chem. Phys. Lett., 102 (1983) 1. P.C. Burgers, J.L. Holmes, A.A. Mommers, J.E. Szulejko and J.K. Terlouw, Org. Mass Spectrom., 19 (1984) 442. J.L. Holmes and A.A. Mommers, Org. Mass Spectrom., 19 (1984) 460.
250 4 5 6 7 8 9 10 11 12
R. Clair, J.L. Holmes, A.A. Mommers and P.C. Burgers, Org. Mass Spectrom., 20 (1985). J.A. Laram~, D. Cameron and R.G. Cooks, J. Am. Chem. Sot., 103 (1981) 12. P-0. Danis, C. Wesdemiotis and F.W. McLafferty, J. Am. Chem. Sot., 105 (1983) 7454. R.P. Morgan, J.H. Beynon, R.H. Bateman and B.N. Green, Int. J. Mass Spectram. Ion Phys., 28 (1973) 171. R.D. Levin and S.G. Lias, Ionisation Potential and Appearance Potential Measurements, NSRDS-NBS 71, U.S. Dept. Commer., Washington, DC, 1982. J.L. Holmes and F.P. Lossing, J. Am. Chem. Sot., 104 (1982) 2684. J.L. Holmes and F.P. Lossing, J. Am. Chem. Sot., 102 (1980) 1591. J.K. Terlouw, W. Heerma, G. Dijkstra, J.L. Holmes and P.C. Burgers, Int. J. Mass Spectrom. Ion Phys., 47 (1983) 147. F. Maquin, D. Stahl, A. Sawaryn, P. von R. Schleyer, W. Koch, G. Frenking and H. Schwarz, Chem. Commun., (1984) 504.
245
Short Communication THE NFJJTRALISATION AND REIONISATION POSITIVE IONS BY INkRT GAS ATOb
OF MASS-SELECTED
JOHAN K. TERLOUW, WILLEM M. ICIESKAMP Analytical Chemistry Laboratory, University of Utrecht, 3522 AD Utrecht (The Netherlands) JOHN L. HOLMES, ALEXANDER
A. MOMMERS
Department of Chemistry, University of Ottawa, Ottawa, Ont. KIN 9B4 (Canada) PETER C. BURGERS Department de Chimie, Universit& Naval, Q&bee, P.Q. GlK 7P4 (Canada) (Received 28 !kptember 1984)
Recent experiments in these laboratories [l-4] have shown how the structures of neutral fragments formed in the dissociative ionisation of organic compounds can be ascertained by means of their collisionally induced dissociative ionisation mass spectra. Two experimental methods, using a VG-Analytical ZAB-2F mass spectrometer of reversed geometry and having a collision gas cell situated in the second field-free region, were describe!. The first involved reversal of the ion beam by means of a high electric potential applied to the gas cell [1,2] while the second [3] used an ion beam deflector electrode placed in front of and close to the collision cell. Thus only neutral products of metastable fragmentations of mass selected ions entered the cell and their high translational energy ensured their collision-induced ionisation by the target gas, usually He. It was observed [4], particularly at high target gas pressures, that some of the incident ion beam was neutralised by charge exchange with target gas which had diffused into the region between the electromagnet and the collision cell. Neutralisation phenomena not associated with the target gas were also reported. Neutralisation by charge exchange in conventional collisionally induced dissociation mass spectra has been identified by Laramk et al. [5]. Recently, Danis et al. [6] have generated neutral species from mass-selected ions of known structure by an elegant but experimentally difficult method. The ions collide with metal vapour target atoms, selected so that resonant charge exchange conditions are approached. The intention of the experi0168-1176/85,‘$03.30
8 1985 Elsevier Science Publishers B.V.
ments was to produce a good flux of high velocity neutral species having low internal energies and hence a useful experimental lifetime. The reaction chamber for the metal vapour preceded an ion beam deflector electrode and a conventional collision cell in which ionisation and dissociation of the neutral species took place. It was concluded that neutralisation rather than fragmentation was the predominant event when the ions met the metal atoms. In the present paper, it will be shown that collisions between mass-selected ions and Xe as target gas in a conventional gas cell can also produce their stable neutral counterparts, in excellent yield and without appreciable contributions from collisionally generated neutral fragments. EXPERIMENTAL
The observations were made using a standard VCL4nalytical ZAB-2F double-focusing mass spectrometer of reversed geometry (B/E) [7] with the following modifications. A second, electrically insulated collision gas cell (cell I) was placed ca. 10 cm in front of the standard collision gas cell (cell II) which is situated in the second field-free region of the mass spectrometer 171. An ion beam deflector electrode [3] was placed midway between the two cells. Thus, all ions exiting the .first cell could be deflected away and prevented from entering the second cell. In the majority of experiments, cell I contained Xe at a measured pressure of 7 X IO-’ Torr (note that the ion gauge which recorded this pressure is situated above the diffusion pump between cell II and the electric sector [7]). Cell II contained He at a similarly measured pressure of 2 X 10m7 Torr. Mass-selected ions having 8 kV translational energy interacted with Xe in cell I. Ions leaving cell I were prevented from entering cell II by means of the deflector electrode at a potential of + 1.5 kV. Neutral products from cell I collided with He in cell II and the resulting positive ions were energy (mass) analysed by the-electric sector. Compounds were of research grade and used without further purification. RESULTS
AND DISCUSSION
Figure 1 shows the neutralisation-reionisation mass spectra (NRMS) of the molecular ions of acetone (A) and its enol (B); the latter was generated via elimination of C,H, from ionised 2-hexanone. These spectra are very similar to those reported by Danis et al [6] who used Zn vapour in the neutralisation step. The ionisation energy (IE) of Zn = 9.4 eV [8] is not far removed from those of both acetone, 9.7 eV [8], and its enol, 8.5 eV [9]. Although IE(Xe) = 12.1 eV [8], the endothermic charge exchange processes yield neutral fluxes and subsequently collisionally derived positive ion cur-
247
(A)
(B)
Fig. 1. NR mass spectra of (A) acetone and (B) its enol from neutralization corresponding ions with Xe and reionization with He.
of the
rents of intensity ca. 1000 times that which in our equipment would require signal averaging (compare ref. [6]), see Fig. 1. Thus it can be concluded that for these mass-selected ions, near-resonant charge exchange is not necessary for obtaining NRMS of good intensity. The energy deficit may be made up from the ions’ translational energy or possibly in part from their internal energies which are limited by their first dissociation thresholds. Comparing the present results with those of Danis et al. [6], the most significant differences are the higher relative yields of molecular ions, 16% of the total NRMS ion current for acetone and 24% for the enol (ketone 5%, enol 8%
248
[6]). The larger m/z 58 for the enol is consistent with the relative stabilities of the molecular ions [lo], not their neutral species [9] and Danis et al. [6] concluded that the observations indicated that most of the fragmentation occurred after reionisation. However, in principle, the interaction with Xe in Cell I can produce neutral species formed by collisionally induced decomposition of the incoming ions. The relative importance of neutralisation vs. fragmentation depends markedly on the inert gas used in cell I. For example, the molecular ion of 1-propanol m/z 60, [CH&H&H,OH]+‘, in a Xe NRMS experiment yields m/z 31 as base peak, as in the normal mass spectrum. With He in cell I, m/z 31 becomes a minor peak (ca. 20% of pase peak) and the spectrum is now dominated by m/z 29 ([C,H,]+ and [HCO]) and m/z 27, [C2H3]+. These hydrocarbon fragments are produced from collisionally generated ethyl radicals [2], the neutral counterpart of m/z 31, being the base peak in the collisional activation mass P-WHl+, spectrum of [CHJH,CH,OH]+-. These observations are in keeping with the charge exchange experiments of Laram& et al. [5] who concluded that for the inert gases Ar and He, the latter was much more efficient than the former as a target gas for collision-induced fragmentation. Resonant electron exchange with the target gas does not appear to be a strict criterion for the generation of an NRMS of good intensity and wl$ch cyntains
ions+ of the p!eselected
mass. For example,
NRMS
of [CH,CO],
[CH,OH], [COOH], [CH,], [CH,OH]+- and [H&O]+‘, whose ionisation energies range from ca. 7-11 eV, were all readily observed and details will be published elsewhere. To illustrate the high sensitivity of the present experiments, Fig. 2 shows the NR mass spectrum (without signal averaging) of [C,H$’ ions metadably generated from [CH$H&H,OH]+ and transmitted from the first field-free region by selecting the ions at m/z 29.4 ( = 422/60). This spectrum is very closely similar to that obtained from ion-source-generated [C,H,]+‘. Such sensitivity also permits the observation of doubly charged ions generated from encounters between neutral species. Danis et al. [6] were unable to decide whether the neutral ylid [CH,ClH] or its conventional isomer CH,Cl was generated in the Zn atom neutralization of m/z 52 [H,,C,37Cl] +- ions from slCH,COOH. The latter have been shown to contain the ylid ion [CH,ClH], which can readily be distinguished from [CH,Cl]+’ by their charge stripping mass spectra [11,12]: the ylid ion generates an intense [CH27C1H]++ signal and weaker peaks at m/z 25.5 and 25 in contrast to [CH,Cl]+- which produces only m/z 25.5 and 25. The NR mass spectrum of the m/z 52 ions from [ClCH,COOH]+’ is closely similar to both the collisional activation, mass spectrum and the NRMS of [CH337C1]+-; moreover, the doubly charged ion at m/z 26, characteristic of the ylid ion, is absent.
249
27
1:
Fig. 2. NR mass spectrum of metastably generated [C,H,]+’
ions from [CH,CH,CH,OH]+‘.
Neutralization-reionization using Xe as the charge exchange gas produces mass spectra similar to, but much more intense than those reported [6] using metal vapour as the target gas. These experiments clearly have great potential for exploring the behaviour of beams of fast neutral species and may also provide an additional method for the investigation of the structures of ions. ACKNOWLEDGEMENTS
.
J.K.T., J.L.H. and A.A.M. acknowledge a collaborative research award from the NATO Scientific Affairs Division, J.L.H. and P.C.B. thank the Natural Sciences and Engineering Research Council of Canada for continuing financial support and P.C.B. similarly thanks the Mink&e de l’Education du Quebec, (programme F.C.A.C.). J.K.T. and A.A.M. thank Dr. R.H. Bateman (VG Analytical Ltd.) for discussions about the design of the second collision gas cell. REFERENCES PC. Burgers, J.L. Holmes, A.A. Mommers and J.K. Terlouw, Chem. Phys. Lett., 102 (1983) 1. P.C. Burgers, J.L. Holmes, A.A. Mommers, J.E. Szulejko and J.K. Terlouw, Org. Mass Spectrom., 19 (1984) 442. J.L. Holmes and A.A. Mommers, Org. Mass Spectrom., 19 (1984) 460.
250 4 5 6 7 8 9 10 11 12
R. Clair, J.L. Holmes, A.A. Mommers and P.C. Burgers, Org. Mass Spectrom., 20 (1985). J.A. Laram~, D. Cameron and R.G. Cooks, J. Am. Chem. Sot., 103 (1981) 12. P-0. Danis, C. Wesdemiotis and F.W. McLafferty, J. Am. Chem. Sot., 105 (1983) 7454. R.P. Morgan, J.H. Beynon, R.H. Bateman and B.N. Green, Int. J. Mass Spectram. Ion Phys., 28 (1973) 171. R.D. Levin and S.G. Lias, Ionisation Potential and Appearance Potential Measurements, NSRDS-NBS 71, U.S. Dept. Commer., Washington, DC, 1982. J.L. Holmes and F.P. Lossing, J. Am. Chem. Sot., 104 (1982) 2684. J.L. Holmes and F.P. Lossing, J. Am. Chem. Sot., 102 (1980) 1591. J.K. Terlouw, W. Heerma, G. Dijkstra, J.L. Holmes and P.C. Burgers, Int. J. Mass Spectrom. Ion Phys., 47 (1983) 147. F. Maquin, D. Stahl, A. Sawaryn, P. von R. Schleyer, W. Koch, G. Frenking and H. Schwarz, Chem. Commun., (1984) 504.