Studies of consecutive reactions of quinones in a reversed geometry mass spectrometer

Consecutive reactions involving the ejection of molecules of carbon monoxide from quinone systems have been studied. Using a reversed geometry double focusing mass spectrometer, the individual steps of a consecutive reaction may be separated into different field free regions. The structure of ions formed in the ion source may be compared with those formed from the fragmentation of metastable ions in a field free region by studying differences in the pattern of translational energies released when they break. In the case of anthraquinone it is found that two structures exist for the [M? CO]^+˙ ion. Critical energy measurements strengthen this argument. Furthermore, ¹⁸O labelling has been used to distinguish between the loss of carbon monoxide from different positions on the molecular ion of 1-hydroxyanthraquinone.     

Ion current surface for a double focusing mass spectrometer of reversed geometry

Ion currents obtained for fragmentations in a mass spectrometer of reversed geometry have been represented as three-dimensional surfaces, which are termed ‘ion current surfaces’. In addition to ion current, useful dimensions of the ion current surfaces are a parameter ln(|ρ|) where ρ = (V₀/V)(E/E₀) and a parameter ln(|m*|) where m* = (V₀/V)m_B. Each ion reaction occurring in the source and flight path gives rise to a pattern of peaks and ridges on the surface. If the major features of the surface corresponding to each ion reaction are projected onto the zero ion current plane, they are disposed in a characteristic trapezoidal pattern. All simple and linked modes of scanning correspond to cross-sections of the surfaces, and representing the scanning modes in this light facilitates the identification of ‘cross-talk’.     

Calculation of mass spectra of some organic compounds undergoing two consecutive reactions from the molecular ions

The molecular ions of phenyl benzoate, methyl benzoate, t-butylbenzene, phthalic anhydride, p-dimethoxybenzene, o-dimethoxybenzene and acetophenone each undergo two consecutive reactions ([M]^+.→[A]⁺→[B]⁺). These unimolecular reactions are treated within the framework of the quasi-equilibrium theory (QET); a simple modification of the original equation relating the rate constant (k) with the internal energy (E) was used to calculate mass spectra of the above compounds from 12 to 20 eV. Good agreement between calculated and observed spectra is obtained.     

Ion/molecule reactions in a miniature RIT mass spectrometer

Ion/molecule reactions were explored in a newly developed miniature mass spectrometer fitted with a rectilinear ion trap (RIT) mass analyzer. The tandem mass spectrometry performance of this instrument is demonstrated using collision induced dissociation (CID) and ion/molecule reactions. The latter includes Eberlin transacetalization reactions and electrophilic additions. Selective detection of the chemical warfare simulant dimethyl methyl phosphonate (DMMP) was achieved through selective Eberlin reactions of its characteristic phosphonium fragment ion CH3OP(+)(O)CH3 (m/z 93), with 1,4-dioxane or 1,3-dioxolane. Efficient adduct formation as a result of electrophilic attack by the phosphonium ion on various nucleophilic reagents, including 1,1,3,3-tetramethyl urea, methanesulfonic acid methyl ester, dimethyl sulfoxide and methyl salicylate, was also observed using the RIT device. The product ions of these reactions were analyzed using CID and the characteristic fragmentation patterns of the ionic addition products were recorded using multiple-stage experiments in the miniature RIT instrument. This study clearly demonstrates that a small, home-built, miniature RIT mass spectrometer can be used to perform analytically useful ion/molecule reactions and also that instruments like this have the potential to provide a portable platform for in situ detection of organophosphorus esters and related compounds with high specificity using tandem mass spectrometry.     

Scanning a triple quadrupole mass spectrometer for doubly charged ions

Charge exchange reactions within a triple quadrupole mass spectrometer characterize doubly charged ions formed in the ion source. Two methods have been developed for identifying the singly charged ions formed from doubly charged ions by charge exchange in the collision quadrupole. The first is based on the characteristically high kinetic energy-to-charge ratios of the products of charge exchange; this property can be used to separate these ions from all other singly charged ions. This retarding potential method is analogous to procedures for recording doubly charged ion mass spectra using sector instruments. The second method is based on the fact that, although mass remains constant in the charge exchange reaction, the change in mass-to-charge ratio can be followed. A charge exchange linked scan, predicated on changes in charge rather than mass, but otherwise analogous to neutral loss/gain scans, is described. Information on the structure of doubly charged ions can be obtained by recording the fragmentation products of dissociative charge exchange. The utility of the charge exchange linked scan for the selective identification of polynuclear aromatic compounds in a complex mixture is described. The methods given can be generalized to cover other charge permutation reactions.     

Infrared photodissociation spectroscopy of electrosprayed ions in a Fourier transform mass spectrometer

Previous gas-phase methods for infrared photodissociation spectroscopy (IRPD) require sample volatility. Our method instead uses electrospray ionization to introduce even large nonvolatile molecules into a Fourier transform mass spectrometer, where extended (>15 s) ion storage makes possible high sensitivity spectral measurements with an OPO laser over a range of 3050-3800 cm(-1). The spectra of 22 gaseous proton-bound amino acid complexes are generally correlated with the H-stretching frequencies established for O-H and N-H functional groups in solution. For theoretical structure predictions of the Gly2H+ and N-acylated Asp2H+ dimers, IRPD spectra clearly differentiate between the predicted lowest energy conformers. In contrast to solution, in the gas phase the glycine zwitterion is approximately 20 kcal/mol less stable than the neutral; however, glycine is clearly zwitterionic in the gaseous GlyLysH+ dimer. The level of theory is inadequate for the larger Lys2H+ dimer, as all low energy predicted structures have free carboxyl O-H groups, in contrast to the IR spectrum. IRPD appears to be a promising new technique for providing unique information on a broad range of biomolecular and other gaseous ions, especially on noncovalent bonding involving O-H and N-H groups.     

A new linked scan for reversed geometry mass spectrometers

A new linked scan is described by which fragmentations occurring in the second field free region of a two sector instrument can be monitored. The scan can be used only if the first stage allows selection of ions according to their masses. The magnetic sector and electric sector are scanned in unison so that the product B²E is maintained constant. A spectrum of all parents of a preselected daughter ion is obtained and the resolution depends only on the mass resolution of the magnetic sector.     

Surface-induced dissociation of molecular ions in a quadrupole ion trap mass spectrometer

A method is reported by which surface-induced dissociation is used to activate ions stored in a quadrupole ion trap mass spectrometer. The method employs a short (< 5 μs), fast-rising (< 20-ns rise time), high voltage direct current (dc) pulse, which is applied to the endcaps of a standard Paul-type quadrupole ion trap. This is in contrast to the application of an alternating current (ac) signal normally used to resonantly excite and dissociate ions in the trap. The effect of the dc pulse is to cause the ions rapidly to become unstable in the radial direction and subsequently to collide with the ring electrode. Sufficient internal energy is acquired in this collision to cause high energy fragmentations of relatively intractable molecular ions such as pyrene and benzene. The dissociations of limonene are used to demonstrate that high energy demand processes increase in relative importance in the dc pulse experiment compared with the usual resonance excitation method used to cause activation. The fragments are scanned out of the ion trap using the conventional mass-selective instability scan mode. Simulations of ion motion in the trap provide evidence that surface collisions occur at kinetic energies in the range of tens to several hundred electronvolts. The experiments also demonstrate that production of fragment ions is sensitive to the phase of the main radiofrequency drive voltage at the point when the dc is initiated.     

Ion-molecule reactions studied at non-zero scattering angles in a mass spectrometer

A method is described by which ion-molecule reactions which involve non-zero scattering angles can be studied in a double-focusing mass spectrometer. Reactions in which there is no alteration in the charge on the reactant ion, while the target species is ionized or excited, have been examined at relative translational energies of 1-10 keV. The method allows the measurement of kinetic energy losses.Interference due to the main beam of non-reacting or elastically scattered ions is minimized relative to the zero scattering angle case. In addition to applications for the determination of ionization potentials and other quantities of thermochemical interest, it is suggested that the study of collisional excitation might be facilitated.     

Laser desorption in transmission geometry inside a Fourier-transform ion cyclotron resonance mass spectrometer

We report here the first application of laser desorption (LD) in transmission geometry (backside irradiation of the sample through a transparent support) inside a Fourier-transform ion cyclotron resonance mass spectrometer (FT-ICR). A probe-mounted fiber optic assembly was used to simplify the implementation of this LD technique. This setup requires little or no instrument modifications, has minimum maintenance requirements, and is relatively inexpensive to build. The performance of the probe was tested by determining the molecular weight of a commercial polystyrene standard from its matrix-assisted laser desorption/ionization (MALDI) spectrum. The measured average molecular weight is comparable to that obtained for the same sample by MALDI in the conventional top-illumination arrangement (reflection geometry) and by the manufacturer of the sample by gel permeation chromatography. The average velocities measured for ions evaporated by transmission mode LD of several neat samples are about half the velocity of those obtained by using the reflection geometry. Therefore, transmission mode irradiation of the sample holds promise to desorb ions that are easier to trap in an ICR cell. An oscillating capillary nebulizer was adapted for the deposition of analytes to improve sampling reproducibility.     

Metal-metal chelate exchange reactions in the ion source of a mass spectrometer

In the mass spectra of several transition metal chelates with diethyldithiocarbamate as a ligand, intense peaks are present which could be assigned to extraneous metal (particularly nickel) containing ions. Using an Au-Rh coated ion source, evidence was obtained for the occurrence of exchange reactions in the region of the filament assembly.     

Size-exclusion chromatography with organic carbon detection using a mass spectrometer

A novel organic carbon detector for size-exclusion chromatography (SEC) is described. The instrument uses the conventional UV-persulfate oxidation method to convert organic carbon to CO(2), which is then detected using a mass spectrometer. This system, using the mass spectrometer, had lower limits of detection (LOD) and limits of quantification (LOQ) than a previously described system using a Fourier transform infrared (FTIR) spectroscopy 'lightpipe' detector (i.e. when quantification was based on calibration using phthalate standards). When used to analyse natural organic matter (NOM) in water, it also had a superior signal-to-noise ratio to the previously described system. The use of a mass spectrometer to detect organic carbon (as CO(2)) enables the possibility of further characterisation of NOM by measuring the stable carbon isotope ratios of the various molecular size fractions of organic carbon, as obtained by SEC.     

Energy-dependent dissociation of benzylpyridinium ions in an ion-trap mass spectrometer

Benzylpyridinium ions, generated via electrospray ionization of dilute solutions of their salts in acetonitrile/water, are probed by collisional activation in an ion-trap mass spectrometer. From the breakdown diagrams obtained, phenomenological appearance energies of the fragment ions are derived. Comparison of the appearance energies with calculated reaction endothermicities shows a reasonably good correlation for this particular class of compounds. In addition, the data indirectly indicate that at threshold the dissociation of almost all of the benzylpyridinium ions under study leads to the corresponding benzylium ions, rather than the tropylium isomers. Substituent effects on the fragmentation for a series of benzylpyridinium ions demonstrate that neither mass effects nor differences in density of states seriously affect the energetics derived from the ion-trap experiments.     

Studies on thermokinetics of consecutive first-order reactions

The transformation equation for the thermokinetics of consecutive first-order reactions has been deduced, and a thermokinetic research method of irreversible consecutive first-order reactions, which can be used to determine the rate constants of two steps simultaneously, is proposed. The method was validated and its theoretical basis was verified by the experimental results.     

Conformer selection of protein ions by ion mobility in a triple quadrupole mass spectrometer

Electrospray mass spectra of multiply charged protein molecules show two distinct charge state distributions proposed to correspond to a more highly charged, open conformational form and a lower charged, folded form. Elastic collisions carried out in the radiofrequency-only collision cell of a triple quadrupole mass spectrometer have dramatic effects on the appearance of the mass spectra. The different cross sectional areas of the conformers allow preferential selection of one charge state distribution over the other on the basis of ion mobility. Preferential selection is dependent on the nature and pressure of the target gas as well as the nature of the protein. In the case of positively charged horse heart apomyoglobin (MW 16,951 da), a high charge state distribution centered around (M + 20H)²⁰⁺ predominates at low target gas pressures and a second distribution centered around (M + 10H)¹⁰⁺ predominates at high target gas pressures. Bimodal distributions are observed at intermediate pressures and, remarkably, charge states between the two distributions are not effectively populated under most of the conditions examined. Hard sphere collision calculations show large differences in collision frequencies and in the corresponding kinetic energy losses for the two conformational states and they demonstrate that the observed charge state selectivity can be explained through elastic collisions.