First analytical results using a multi-ion counting system of a spark source mass spectrometer

Isotope dilution measurements of geological reference materials are described using a newly developed multi-ion counting (MIC) system for a spark source mass spectrometer. Compared with the conventional photoplate detection system, there are considerably improved analytical features of the MIC system, especially the high sensitivity which leads to short measuring times of 1 min to 1 h for trace element analysis on the μg/g — ng/g level.     

Reactions in the source of a mass spectrometer: The mass spectrum of 1-chlorobenzotriazole

The mass spectrum of 1-chlorobenzotriazole contains ions derived from benzotriazole, a product of an exchange reaction with water in the source of the mass spectrometer. Relatively intense ions derived from iodine, iodine monobromide and other inter-halogen compounds are also observed. These species are produced by oxidation of involatile halide coatings in the source region by 1-chlorobenzotriazole. It is suggested that anomalous peaks produced by oxidation-reduction reactions in the source may be expected in the spectrum of any organic oxidant.     

Non-thermal internal energy distribution of ions observed in an electrospray source interfaced with a sector mass spectrometer

The internal energy distribution P(E(int)) of ions emitted in an electrospray (ESI) source interfaced with a sector mass spectrometer is evaluated by using the experimental survival yield (SY) method including the kinetic shift. This method is based on the relationship between the degree of fragmentation of an ion and its amount of internal energy and uses benzylpyridinium cations due to their simple fragmentation scheme. Quantum chemical calculations are performed, namely at G3(MP2)//B3LYP and QCISD/MP2 levels of theory. The results show that the internal energy distribution of the ions emitted in the ESI source interfaced with a sector analyzer is very narrow. The MassKinetics software is used to confirm these observations. The P(E(int)) is the parameter that allows to fit the experimental SY of each substituted benzylpyridinium cation with theoretical mass spectra generated by the MassKinetics software. The resulting internal energy distributions are similar to the ones obtained with the experimental SY method. This indicates that in the present experimental conditions, P(E(int)) cannot be compared with a 'thermal-like' Boltzmann distribution. In addition, it appears that with the sector analyzer, increasing the collision energy in the first pumping stage of the ESI source does not correspond to a warm-up of the produced ions.     

Photodissociation of high molecular weight peptides and proteins in a two-stage linear time-of-flight mass spectrometer

A two-stage linear time-of-flight mass spectrometer is used to investigate the requirements for performance of laser photodissociation of peptide and protein ions. Results are presented that demonstrate that desorption and dissociation laser pulses can be synchronized to irradiate ions that travel at high velocities down the drift tube of a time-of-flight mass spectrometer. For example, 193-nm photodissociation of bovine insulin and doubly charged lysozyme is demonstrated, and laser power studies suggest that dissociation is initiated by the absorption of a single 193-nm photon. These results are encouraging because they suggest that laser photodissociation of high molecular weight proteins can lead to fragmentation on time scales compatible with time-of-flight mass spectrometry.     

Variation of relative ion abundances with accelerator potential in the mass spectrometer: The importance of source penetration

Calculations have been carried out, using the Quasi-Equilibrium Theory of Mass Spectra to show that a decrease in penetration of the accelerating field intol the ion source of a mass spectrometer may significantly increase the number of primary daughter ions [A]⁺ produced from a molecular ion [M]⁺ in the source. The calculations show that this effect appears to be more important than the decay of [M]⁺ between source and collector in implementing an increase in [A]⁺/[M]⁺ at the collector with decreasing accelerator potential.     

Metastable decay of peptide ions on a Fourier transform mass spectrometer equipped with an external ion source

Metastable decay rates of two peptides, RPPGFSPF and PKPQQFFGLM, were determined from ions produced in an external matrix-assisted laser desorption/ionization source with a Fourier transform mass spectrometer. An isolation and subtraction method that gives difference spectra was employed to monitor the product formation and metastable decays. The dependence of metastable decay rates on laser fluences and matrixes was demonstrated.     

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.     

Improving the analytical performance of ion mobility spectrometer using a non-radioactive electron source

For the ionization of gas mixtures, several ionization sources can be coupled to an ion mobility spectrometer. Radioactive sources, e.g. beta radiators like ⁶³Ni and ³H, are the most commonly used ionization sources. However, due to legal restrictions radioactive ionization sources are not applicable in certain applications. Non-radioactive alternatives are corona discharge ionization sources or photoionization sources. However, using an electron gun allows regulation of ion production rate, ionization time and recombination time by simply changing the operating parameters, which can be utilized to enhance the analytical performance of ion mobility spectrometers. In this work, the impact of an ionization source parameter variation on the ion mobility spectrum is demonstrated. Increasing the ion production rate, the amount of the generated ions increases leading to higher signal intensity while the noise remains constant. Thus, the signal to noise ratio can be increased, leading to better limits of detection. In a next step, the ion production rate is kept constant while the influence of ionization time on the ion mobility spectrum is investigated. It is shown, that varying the ionization time allows the determination of the reaction rate constants as additional information to the ion mobility. Furthermore, we show the prevention of discrimination processes by using short ionization times combined with an increased ion production rate. Thus, the limit of detection for benzene in presence of toluene is improved. Additionally, it is shown that using ion-ion recombination leads to the detection of the ion species with the highest proton affinity at higher recombination times while the low proton affine ions already recombined. Thus, the measurement of the ion mobility spectra at a defined recombination time allows a suppression of disturbing low proton affine substances.     

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.     

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.     

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.     

Modifications to an analytical mass spectrometer for the soft-landing experiment

Modifications to a hybrid geometry beam mass spectrometer are described that allow its use for soft-landing of ions onto a stainless steel surface intercepting the ion beam at the site of the intermediate detector. Initial success is documented through the presence of metal ions brought to the surface as part of a complex, metal-containing cluster ion. Further, organic cations have also been soft landed at the collector surface, and electrospray ionization (ESI) and ESI tandem mass spectrometry analysis of the surface wash solution confirm preservation of the original organic structure. Future directions for predictable surface density collection strategies are described.     

The Structures of the [2M–1]+ ions formed from benzene in a high pressure mass spectrometer source

The ionic species [C₁₂H₁₁]⁺ has been formed by ion-molecule reactions in benzene at high pressure (?0.5 Torr). This ion has been shown to be stable for at least 10^?5 s and its structure has been inferred by a study of its unimolecular and collision induced fragmentations in both the first and second field free regions of the ZAB-2F mass spectrometer.     

Processes of hydrogenation of trifluoromethylfullerenes in the mass spectrometer ion source

The processes of hydrogenation of a mixture of trifluoromethylfullerenes was studied in situ by means of positive-and negative-ion mass spectrometry. The effective addition of 1, 5, and 11 hydrogen atoms was revealed. The appearance energies of positive trifluoromethylfullerene ions C ₆₀ (CF₃) _n ⁺ (n = 1–8) and C₆₀(CF₃) _n H⁺ (n = 1, 3, 5, 7) were determined.     

Studies of consecutive reactions of organic ions in a reversed geometry mass spectrometer

Over thirty consecutive reactions of a type m₁⁺ → m₂⁺ → m₃⁺ have been studied in a variety of organic compounds using a reversed geometry mass spectrometer. Two field free regions allow the separation of the two steps that make up the consecutive reaction sequence. Translational energy release measurements are used in the comparison of m₂⁺ ions formed as a result of a high energy process in the ion source with m₂⁺ ions formed as a product of a metastable decomposition. In some cases the structures of such ions have been found to be different. Examples have also been found where consecutive fragmentations of metastable ions do not occur although, when higher energy ions within the source are studied, the two-step reaction does take place. Furthermore, it has been found that a control over ion internal energy may be achieved by selecting portions of a peak due to the fragmentation of a metastable ion. Unimolecular reactions may then be used to study the reactivity of such ‘energy-selected’ ions; collision induced reactions can be used to study the structure of both the reactive and unreactive energy selected ions.     

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.     

Effects of the source gap on transmission efficiency of a quadrupole mass spectrometer

Rationale

Recent trends towards miniature and portable quadrupole mass spectrometry (QMS) entail challenges in instrumental sensitivity, which is influenced by 3D fringe field effects on ion transmission in the Quadrupole Mass Filter (QMF). The relationship of these effects with the gap from the ion source to the QMF entrance (source gap) is significant and little explored. We examine transmission characteristics experimentally and use the results to test the predictive accuracy of a recently developed 3D QMF simulation model. The model is then applied to directly investigate optimal transmission m/z ranges across multiple source gaps.

Methods

A portable single filter quadrupole mass spectrometer is used to analyse transmission characteristics across a range of common gases. We use an experimental approach originally proposed by Ehlert, enhanced with a novel method for absolute calibration of the transmission curve. Custom QMF simulation software employs the boundary element method (BEM) to compute accurate 3D electric fields. This is used to study the effects of the source gap on transmission efficiency.

Results

Experimental findings confirm a centrally peaked transmission curve; simulations correctly predict the optimal transmission location (in m/z) and percentage, and extend the experimental trend. We compare several methods for determining fringe field length, demonstrating how the size of the physical source gap influences both the length and the intensity of the fringe field at the QMF entrance. A complex relationship with ion transmission is revealed in which different source gaps promote optimal transmission at differing m/z ranges.

Conclusions

The presented results map the relationship between the source gap and transmission efficiency for the given instrument, using a simulation method transferrable to other setups. This is of importance to miniature and portable quadrupole mass spectrometers design for specific applications, for the first time enabling the source gap to be tailored for optimal transmission in the desired mass range.

     

A B-TOF mass spectrometer for the analysis of ions with extreme high start-up energies

Weak magnetic deflection is combined with two acceleration stage time-of-flight mass spectrometry and subsequent position-sensitive ion detection. The experimental method, called B-TOF mass spectrometry, is described with respect to its theoretical background and some experimental results. It is demonstrated that the technique has distinct advantages over other approaches, with special respect to the identification and analysis of very highly energetic ions with an initially large energy broadening (up to 1 MeV) and with high charge states (up to 30+). Similar energetic targets are a common case in intense laser-matter interaction processes found during laser ablation, laser-cluster and laser-molecule interaction and fast particle and x-ray generation from laser-heated plasma.