Energy-resolved Mass Spectra Research Articles

Elimination of difluorocarbene from the molecular ions of α,α,α‐Trifluorocresols with a fluorine atom migration in an ion trap mass spectrometer

The elimination of difluorocarbene (CF2) from the molecular ions of m-α,α,α-trifluorocresol (MW 162, 1) and p-α, α, α-trifluorocresol (MW 162, 2) upon electron impact have been investigated using a quadrupole ion trap mass spectrometer. This reaction involves a fluorine (F) atom migration from the trifluoromethyl group to the benzene ring. In the case of 1, an F atom migrates via a four-membered ring transition state to give rise to the molecular ion of p-fluorophenol (MW 112, 5) and a small amount of o-fluorophenol (MW 112, 3). On the other hand, in the case of 2, an F atom migrates to the ipso position of the benzene ring via a three-membered ring transition state to give rise to 5+. These conclusions were confirmed by the comparison of the energy-resolved mass spectra (collision-induced dissociation (CID) spectra versus collision energy) of the m/z 112 ions from 1+ and 2+ with those of reference compounds, o-, m- and p-fluorophenol (3–5). Copyright © 1999 John Wiley & Sons, Ltd.

Elimination of difluorocarbene from the molecular ions of alpha, alpha,alpha-trifluorocresols with a fluorine atom migration in an ion trap mass spectrometer.

The elimination of difluorocarbene (CF(2)) from the molecular ions of m-alpha,alpha,alpha-trifluorocresol (MW 162, 1) and p-alpha, alpha, alpha-trifluorocresol (MW 162, 2) upon electron impact have been investigated using a quadrupole ion trap mass spectrometer. This reaction involves a fluorine (F) atom migration from the trifluoromethyl group to the benzene ring. In the case of 1, an F atom migrates via a four-membered ring transition state to give rise to the molecular ion of p-fluorophenol (MW 112, 5) and a small amount of o-fluorophenol (MW 112, 3). On the other hand, in the case of 2, an F atom migrates to the ipso position of the benzene ring via a three-membered ring transition state to give rise to 5(+). These conclusions were confirmed by the comparison of the energy-resolved mass spectra (collision-induced dissociation (CID) spectra versus collision energy) of the m/z 112 ions from 1(+) and 2(+) with those of reference compounds, o-, m- and p-fluorophenol (3-5).

Energy-resolved mass spectrometry: a comparison of quadrupole cell and cone-voltage collision-induced dissociation.

Collision-induced dissociation (CID) can be effected in the interface region between atmospheric pressure ionization sources and single quadrupole mass analyzers. By varying the electric field in these cone-voltage CID experiments energy-resolved mass spectra can be obtained leading to breakdown graphs exploring the energy evolution of the fragmentation pathways. The breakdown graphs obtained from these cone-voltage CID studies are very comparable to those obtained by varying the collision energy in the quadrupole collision cell of a BEqQ mass spectrometer. The comparison has been made for the protonated peptides H-Leu-Gly-Gly-OH, H-Gly-Leu-Gly-OH, H-Gly-Gly-Leu-OH and Leu-enkephalin

Relative Dissociation Energies of Protonated Peptides by Electrospray Ionization/Surface-Induced Dissociation

Relative dissociation energies (RDEs) are obtained for the major fragment ions produced by electrospray ionization/surface-induced dissociation of singly protonated triglycine, tetraglycine, leucine enkephalin, and leucine enkephalin arginine. A previously described data analysis method (Lim, H.; et al. J. Phys. Chem. B 1998, 102, 4753) is employed to analyze the energy-resolved mass spectra by subtracting out the distribution of energy transferred to the surface, integrating over the distribution of the incident ion energy, and taking into account the precursor ion initial internal energy and kinetic energy distributions. These variables are optimized by anchoring the RDE for the lowest energy fragment of a given precursor ion to its literature values and then using these optimized parameters to obtain the other RDEs. The RDEs of the four major fragments of triglycine vary from 2.4 eV for the b(2) fragment ion to 6.0 eV for the a(2) ion. The RDEs of the four major fragments of tetraglycine vary from 3.2 eV for the y(2) ion to 5.7 eV for the a(2) ion. The leucine enkephalin RDEs range from 1.1 eV for the b(4) ion to 2.1 eV for the b(2) ion. The leucine enkephalin arginine RDEs all lay between 2.5 and 3.5 eV. The overall trend of fragmentation order for all peptides is (y(n), b(n)) < a(n) and is consistent with the results from other experiments. The peptide RDEs presented here are only as accurate as the literature values to which they are anchored. Determination of absolute dissociation energies from SID data will require further refinement of the data analysis method.

Energy-resolved collision-induced dissociation atmospheric pressure chemical ionization mass spectrometry of constitutional and stereo steroid isomers

Energy-resolved mass spectra were recorded by controlled up-front collision-induced dissociation (CID) or conventional CID tandem mass spectrometry. Differentiation between constitutional isomers and stereoisomers of hydroxysteroids was studied using atmospheric pressure chemical ionization interfaced to a triple quadrupole mass spectrometer. Relative abundances of fragment ions from mainly water loss were used. © 1998 John Wiley & Sons, Ltd.

Activation Energies for the Fragmentation of Thiophene Ions by Surface-Induced Dissociation

We have improved our previously described method for extracting activation energies of fragmentation for polyatomic ions from surface-induced dissociation (SID) data [Wainhaus, S. B.; et al. J. Am. Chem. Soc. 1997, 119, 4001]. Our method analyzes the energy-resolved mass spectra and the kinetic energy distribution spectra of the parent and fragment ions that scatter off the surface. It extracts the activation energies by integrating over the distribution of the initial ion energy and the energy transferred to the surface, taking into account both the average value and the width of these distributions. The new method gave improved activation energies for SiMe3+ → SiMex+ (x = 0−2) fragmentation at a hexanethiolate-covered gold surface. We then used our data analysis method to analyze the activation energies for the fragmentation of thiophene ions at the hexanethiolate-covered gold surface. The activation energies for the formation of C2H2S+, CHS+, and C3H3+ from C4H4S+ were found to be 4.6 ± 0.7, 6.9 ± 0.7,...

Investigation of the trans effect in the fragmentation of dinuclear platinum complexes by electrospray ionization surface-induced dissociation tandem mass spectrometry.

Cis and trans isomers of two dinuclear platinum complexes, [cis-¿Pt(NH3)2Cl¿2 mu-(NH2(CH2)nNH2)](NO3)2 (1,1/c,c) and [trans-¿Pt(NH3)2Cl¿2 mu-(NH2(CH2)nNH2)](NO3)2 (1,1/t,t), where the diamine was 1,4-butanediamine (n = 4) or 1,6-hexanediamine (n = 6), were studied using electrospray ionization surface-induced dissociation (ESI/SID) tandem mass spectrometry (MS/MS). The same fragment ions were observed for both the cis and trans isomers of each complex (n = 4 or 6), but the relative intensities were dependent on the isomer studied. The ESI/SID data and energy-resolved mass spectra show that the position of the chloride plays a significant role in the fragmentation of these ions. Two major fragmentation pathways were detected for the complexes. The cleavage of the Pt-N bond trans to chloride was the most favorable pathway for both isomers of the complexes following the ion-surface collision. The differences in the ESI/SID spectra between the cis and trans isomers can be explained by the trans effect, namely that the Pt-N bond trans to chloride is the most labile bond.

Energy-resolved mass spectra of ions and neutrals in triode ion plating of Ti(C,N) coatings

Ti(C,N) coatings have been deposited by triode ion plating in a mixture of Ar, N 2 and C 2H 2. The energy spectra of selected ions and neutrals during deposition were measured by an energy-resolved mass spectrometer. The observed energy spectra of ions and neutrals are discussed. The energy spectra of neutral species were measured as the energy spectra of ions produced by crossed-beam electron ionization in the mass spectrometer. Both the energy spectra of Ti + ions and Ti atoms show wide energy spectra, ranging to about 40 eV, Properties of the Ti(C,N) coatings are reported, especially the stress-free lattice parameter and the residual macroscopic stress measured by X-ray diffraction. It is observed that the compressive stress varies from −6 to −21 GPa with the different deposition conditions used. Effects of the total pressure, the substrate voltage and the current in the low voltage arc on the energy spectra of ions and neutrals are discussed.

Determination of Activation Energies for Ion Fragmentation by Surface-Induced Dissociation

We describe here a new method for extracting the activation energy for the formation of any fragment or reactively scattered ion that forms from a parent ion−surface collision. Our model is developed from first principles for collision-induced dissociation in the gas phase and then modified for surface-induced dissociation (SID). This approach is conceptually similar to that used for threshold collision-induced dissociation measurements in that it assumes a similar functional form for the dissociation cross section, it takes into account the partitioning of energy between the projectile and the target, and it deconvolutes these over the kinetic energy distribution of the parent ion beam. The activation energy is extracted by an analysis of the energy-resolved mass spectra and the kinetic energy distribution spectra for the surface-scattered ions. We test our method by determining the activation energies for the formation of the SiMe2+, SiMe+, SiD+ and Si+ fragment ions from the d9-SiMe3+ parent ion scatte...

Ion/surface reactions, surface-induced dissociation and surface modification resulting from hyperthermal collisions of OCNCO +, OCNCS +, (CH 3) 2SiNCO +, and (CH 3) 2SiNCS + with a fluorinated self-assembled monolayer surface

Low-energy (10–90 eV) collisions of the pseudohalogen-containing ions OCNCO + (1), OCNCS + (2), (CH 3) 2SiNCO + (3), and (CH 3) 2SiNCS + (4) with fluorinated self-assembled (F-SAM) monolayer surfaces lead to surface-induced dissociation (SID) and to a variety of ion/surface reactions. The lowest energy fragmentation process in both OC-NCO + and OC-NCS + is C-N bond cleavage but the strength of these bonds is significantly different. They are estimated from surface-induced dissociation data taken as a function of collision energy (energy resolved mass spectra) to be 4 and 3 eV, respectively. The silyl ions, (3) and (4), preferentially fragment by Si-C bond cleavage and dissociate more readily than (1) and (2). Other SID processes also occur by simple cleavage of the various strong bonds in these ions and charge retention by the sulfur in preference to oxygen is evident in the isothiocyanate spectra. The collision energy dependence of the scattered ion spectra display the competition between elastic scattering, dissociation, reaction, and surface sputtering. The extent of sputtering increases with collision energy and is greater in (1) and (2) than it is in the silicon-containing ions (3) and (4), and the implications of this result for the ionization energy of the radicals corresponding to (1)–(4), are addressed. On the other hand, OCNCO + and OCNCS + are less reactive than their silyl counterparts, and data suggest that the observed reactions do not proceed by charge exchange but instead by a direct reaction mechanism. The reaction products in the two pairs of ions (e.g. FCO + from (1), FCS + from (2), vs. SiF + and SiH 2F + from (3) and (4) are notably different, consistent with the differences in the SID behavior and showing that the major reactive site in (3) and (4) is the silicon atom. Comparisons between the isocyanates and isothiocyanates show that the NCS group confers much greater reactivity than the NCO group within each pair of projectile ions. Analogies are found in the ion/surface reactions of the halogens and pseudohalogens, including the apparent displacement of fluorine in the F-SAM by NCO and NCS groups suggested by the scattered ions at m/z 73 (OCNCF +), m/z 64 (NCF + 2) and m/z 45 (NCF +). Evidence that a pseudohalogen group exchanges with a fluorine from the surface is also found in the presence of FCO + and FCS + among the scattered products of collisions of (2) with the fluorocarbon surface. The collision energy dependence of these ion/surface reaction products for the OCNCO + and OCNCS + projectile ions provides evidence for dissociation at the surface followed or accompanied by bond formation. A general mechanism is proposed for the observed ion/surface reactions based on Lewis acid/base rather than redox chemistry and the occurrence of fragmentation at the surface rather than after departure. The silyl ions show ion/surface reactions which are dominated by fluorine abstraction from the surface but they also include such remarkable processes as fluorine-for-methyl substitution, which occurs with the isothiocyanate projectile ion (4). Surface modification of fluorinated self-assembled monolayer surfaces was accomplished by prolonged bombardment with low-energy OCNCS + and (CH 3) 2SiNCS + ions. Evidence is provided for incorporation of methyl, silyl, and NCS groups into the modified surface, although the energetic conditions needed to cause the bond dissociations necessary for ion/surface reactions make the formation of modified surfaces especially difficult in these cases.

Collision spectroscopy in isomer characterization: The case of (E)- and (Z)-p-nitrophenyldiazotert-butyl sulphides

The structural characterization of the E and Z isomers of p-nitrophenyl-diazo tert-butyl sulphide has been achieved by means of different ionization methods (electron impact, fast atom bombardment, positive-ion chemical ionization) and collision experiments performed under different kinetic energy regimes (high- and low-energy collisions, angle-resolved mass spectrometry and energy-resolved mass spectrometry). The two compounds give rise to identical fragmentation patterns. Collision experiments, both at low energy and in the keV range at a scattering angle of 0°, in M+· species obtained by electron impact on the two isomers, do not show any significant differences; the same experiments performed with 8 keV ions at a scattering angle of 2° indicate a clear difference in the absolute abundances of the two main daughter ions. High-energy collisions of MH+ ions obtained by fast atom bombardment lead to different spectra at both 0° and 2° scattering angles, proving that the energy deposition in the preselected species is an important parameter. Low-energy collisions with argon of MH+ ions generated by positive-ion chemical ionization (NH4+) give rise to almost identical energy-resolved mass spectra, whereas the same experiments using helium as target gas lead to a clear distinction between the two isomers.

Surface-induced dissociation of molecular ions

Fragmentation of polyatomic ions upon collision with a metal surface was studied using a modified hybrid mass spectrometer. Daughter spectra of mass-analyzed parentions obtained upon collision with both solid and gaseous targets are compared. As in conventional gas-phase collisions, the extent of fragmentation in surface-induced dissociation (SID) is observed to depend strongly upon the collision energy chosen. Thus, experiments in which the collision energy is varied give energy-resolved mass spectra (ERMS) which correspond to those observed in the analogous gas-phase experiments. While both methods of activation display the same fragment ions, significant differences in the amount of energy deposited are observed between the two methods. At low collision energy, SID gives essentially no dissociation. On the other hand, selection of higher collision energies can result in deposition of even larger amounts of internal energy than are available from the comparable gas-phase collisional processes at either low (eV) or high (keV) energy.

Time- and Energy-Resolved Mass Spectroscopy: The Condensed Vacuum Discharge between Solid Electrodes

A method is described to obtain time- and energy-resolved mass spectra of ions from discharges between solid electrodes in vacuum. The method is used to investigate the triggered condensed discharge. The discharge is found to consist of two distinct phases: a short high-voltage breakdown (spark) is followed by a longer low-voltage discharge (arc). A model is proposed to explain the processes involved.