Molecular Cluster Ions Research Articles

Preparation of a rhodium catalyst from rhodium trichloride on a flat, conducting alumina support studied with static secondary ion mass spectrometry and monochromatic X-ray photoelectron spectroscopy

A rhodium catalyst has been prepared by electrostatic adsorption of RhCl3-derived species in aqueous solution on a model support, consisting of a 4–5 nm thick layer of aluminum oxide on an aluminum foil. The conversion of the rhodium precursor species into metallic rhodium has been studied by monochromatic XPS and static SIMS. Freshly prepared catalysts contain adsorbed Rh-complexes with only one chloro ligand; this is explained by a mechanism in which chloro ligands of the initially adsorbed complex, of the form [RhCln(OH)4-n(H2O)2]−, are displaced by surface OH groups. Analysis of molecular secondary cluster ions of the type RhCl− shows that the Rh-Cl species decompose at reduction temperatures below 200 °C, whereas reduction temperatures well in excess of 200 °C are needed to remove chlorine from the alumina support.

Differentiation of Aroclors using linear discrimination for environmental samples analyzed by electron capture negative ion chemical ionization mass spectrometry

Environmental samples suspected of containing polychlorinated biphenyls (PCBs) and analyzed by Environmental Protection Agency (EPA) methods frequently contain non-PCB components, such as phinalates, polyaromatic hydrocarbons, or organochlorine pesticides. The presence of these interferences can often obscure the gas chromatographic (GC) patterns and cause problems in differentiating the aroclor types by visual inspection. Because EPA methods require the identification of Aroclor types in order to determine which Aroclor standard should be used for the quantitation calibration, and possibly, to trace the source of PCB occurrences, electron capture negative ion chemical ionization (ECNICI) mass spectrometry was used to provide additional parameters for discriminating PCB congeners from the interferences. In this study, eight Aroclors (i.e., 1016, 1232, 1242, 1248, 1254, 1260, 1262, and 1268) were analyzed by ECNICI for a range of concentration levels. Using selective ion display, the ion abundances of the prominent peaks in the isotopic clusters of molecular ions were measured for eight PCB homologs (Cl) within their GC retention time windows for each sample. Corresponding relative ion abundances from eight Aroclor standards were used as classification training sets. These training sets were used to develop a classification algorithm using standard linear discriminant analysis to classify the PCBs present inmore » an unknown sample(s) as a specific Aroclor. This technique employed a sequential application of two sets of linear discriminant functions to successfully identify known EPA quality control and environmental samples. However, classifications of Aroclor mixtures were less successful. 26 refs., 4 figs., 4 tabs.« less

Magic cluster ion distributions: (NO) 3 + (N2O)n and (NO) 3 + (CO2)n

Supersonic jet expansions of mixtures of nitric oxide with either nitrous oxide or carbon dioxide have been investigated over a wide range of relative concentrations. Mixed molecular cluster ions of the form (NO) m + (N2O)n and (NO) m + (CO2)n are detected following non-resonant two-photon ionization. Over a wide range of intermediate concentrations, the cluster ion distributions (NO) 3 + (N2O)n and (NO) 3 + (CO2)n with n≤30 are significantly more intense than clusters containing other numbers of nitric oxide molecules. The extra abundance of these species is attributed to their especially stable structures and several possible forms are discussed. An intriguing possibility involves a stable cyclic nitric oxide trimer (or ion) when combined with nitrous oxide or carbon dioxide clusters.

Enhanced production of unprotonated hydrogen-bonded cluster ions

A novel technique is described which may be employed in mass spectrometry for the efficient production of unprotonated hydrogen-bonded cluster ions. Using the pick-up technique, clusters of the species to be investigated are embedded in large Ar x host clusters and carried by them to the ion source. The argon atoms constituting the substrate cluster are evaporated upon ionization and argon-free cluster ions can be obtained. Owing to the efficient transfer of the excess energy from the excited molecular cluster ion to the substrate cluster, the commonly encountered protonation reaction is Considerably quenched, thus leading to the production of unprotonated species. The new technique is demonstrated for the generation of (CH 3F) + n and (CH 3OH) + n cluster ions with n = 2 or 3.

Chemical Reactions in Molecular Cluster Ions

The promotion and observation of chemical reactions in molecular cluster ions is discussed in terms of results from three recent experimental studies. The presence of “magic numbers” in cluster ions of the type Arn · X+, where X is either a reactant or fragment ion, provides evidence of molecules occupying sites on or close to the surface of a cluster. Results from experiments on excitation transfer are used to discuss the possibility that a single chemical reaction in an ion Y+, could be promoted through the selective excitation of a host cluster of the form XnY+. Finally, results are present from a new experiment designed to study laser-induced chemical reactions in mass-selected cluster ions. Preliminary data are given for photoinduced reactions in cluster ions of 1,3-butadiene.

Reactions and properties of clusters

The elucidation from a molecular point of view of the differences and similarities in the properties and reactivity of matter in the gaseous compared to the condensed state is a subject of considerable current interest. One of the promising approaches to this problem is to utilize mass spectrometry in conjunction with laser spectroscopy and fast-flow reaction devices to investigate the changing properties, structure and reactivity of clusters as a function of the degree of solvation under well-controlled conditions. In this regard, an investigation of molecular cluster ions has provided considerable new insight into the basic mechanisms of ion reactions within a cluster, and this paper reviews some of the recent advances in cluster production, the origin of magic numbers and relationship to cluster ion stabilities, and solvation effects on reactions. There have been some notable advances in the production of large cluster ions under thermal reaction conditions, enabling a systematic study of the influence of solvation on reactions to be carried out. These and other new studies of magic numbers have traced their origin to the thermochemical stability of cluster ions. There are several classes of reaction where solvation has a notable influence on reactivity. A particularly interesting example comes from recent studies of the reactions of the hydroxyl anion with CO 2 and SO 2, studied as a function of the degree of hydration of OH −. Both reactions are highly exothermic, yet the differences in reactivity are dramatic. In the case of SO 2, the reaction occurs at near the collision rate. By contrast, CO 2 reactivity plummets dramatically for clusters having more than four water molecules. The slow rate is in accord with observations in the liquid phase.

Advances in flowing afterglow and selected-ion flow tube techniques

New developments in flowing afterglow and selected-ion flow tube (SIFT) techniques are briefly reviewed. Particular emphasis is given to the new chemical and physical information that can be obtained with use of the tandem flowing afterglow-triple quadrupole apparatus developed in the author's laboratory. Several outstanding recent achievements in the design and utilization of flowing afterglow and SIFT instruments in other laboratories are briefly highlighted that illustrate the power and flexibility of flow-tube-based methods. These include isotope tracer experiments with the tandem flowing afterglow-SIFT instrument in Boulder, studies of large molecular cluster ions with the variable temperature facility at Penn State, and gas-phase metal ion reactions with the laser ablation/fast flow reactor in Madison. Recent applications of the flowing afterglow-triple quadrupole instrument in our laboratory have made use of collision-induced dissociation (CID) as a tool for synthesizing novel ions and for obtaining new thermo-chemical information from threshold energy measurements. Collision-induced decar☐ylation of organic car☐ylate ions provides access to a variety of unusual and highly basic carbanions that cannot be generated with conventional ion sources. The formation and properties of saturated alkyl ions and studies of gas-phase reactions of the methyl anion are briefly described. We have developed a new method for carrying out “preparative CID” in a flowing afterglow with use of a mini-drift tube; some recent applications of this new ion source are presented. Measurement of CID thresholds for simple cleavage reactions of thermalized ions can provide accurate measures of bond strengths, gas-phase acidities and basicities, and heats of formation for ions and reactive neutral species. Applications of this approach in the thermochemical characterization of carbenes, benzynes and biradicals are described. Future prospects for the continued development of flow reactor-based methods are also briefly discussed.

Metallo-Carbohedrenes [M 8 C 12 + (M = V, Zr, Hf, and Ti)]: A Class of Stable Molecular Cluster Ions

Findings of magic peaks corresponding to M(8)C(12)(+) (M = V, Zr, and Hf) formed from reactions of the respective metals with various small hydrocarbons, in conjunction with recent findings for the titanium system, establish metallo-carbohedrenes as a stable general class of molecular cluster ions. A dodecahedral structure of T(h) point symmetry accounts for the stability of these ionic clusters.

Collision-induced fragmentation of small (CO 2) +n clusters

An apparatus for investigating the energy dependence of collision-induced fragmentation of internally cold, mass-selected molecular cluster ions is described. First results on the collisional energy dependence of the cross sections for fragmentation of small (CO) + n clusters in collisions with Ar atoms are presented. Interesting structure is observed in the cross sections and the results indicate that the collision occurs between the Ar atom and one CO 2 molecule within the cluster. This leads to either vibrational excitation of a loosely bound CO 2 monomer which then leaves the cluster or excitation of the entire cluster to a dissociative state.

Direct ejection of clusters from nonmetallic solids during laser vaporization.

We have demonstrated that laser vaporization of nonmetallic solids can produce atomic or molecular clusters and cluster ions by direct ejection of fragments from the sample surface. The spectra of cluster ions ([MX${]}_{\mathit{n}}$${\mathit{M}}^{+}$ and [MX${]}_{\mathit{n}}$${\mathit{X}}^{\mathrm{\ensuremath{-}}}$, n=1--40) formed by vaporizing mixed alkali-halide powder samples include virtually no alloyed species, indicating that the observed clusters do not grow from molecular vapor but are instead laser sputtered from the solid in a nonthermal shattering process. In carbon, large clusters result from partial evaporation of ultrafine particles on the sample.

Mass spectral confirmation of chlorinated and brominated diphenylethers in human adipose tissues

This study has resulted in the detection and confirmation of polyhalogenated diphenylethers (PHDPEs) in human adipose tissues. The PHDPEs are commercially produced as brominated fire retardants and are known contaminants in some other commercial products, e.g., pentachlorophenol. The samples analyzed were selected from composites of the fiscal year 1987 National Human Adipose Tissue Survey (FY87 NHATS) repository. Identification of the PHDPEs was based on comparison of full scan mass spectra of the samples to the available standards, application of SIM techniques to compare theoretical ion ratios to observed ion ratios for characteristic ions, and measurement of fragment losses from the molecular ion clusters. Estimated concentrations were achieved through comparison of responses from the analysis of a PCDPE standard solution, the results of the analysis of two commercial fire retardants and the analysis of a decabromodiphenylether standard. All samples were found to contain detectable levels of the PHDPEs. The response profiles or patterns for both the chlorinated and brominated diphenylethers were consistent across all samples.

High-sensitivity FAB-MS strategies for O-GlcNAc characterization

In this paper we report the first application of fast atom bombardment mass spectrometry (FAB-MS) to O-linked N-acetylglucosamine (O-GlcNAc)-bearing glycopeptides. Using N-acetylgalactosamine (GalNAc)- and Gal-GalNAc-containing glycopeptides (isolated from Tn glycophorin and desialylated normal glycophorin, respectively) as readily available model compounds, rapid and sensitive derivatization/FAB-MS strategies applicable to serine/threonine-rich glycopeptides have been devised. Peptides and glycopeptides were propionylated in a 1 min reaction using a mixture of trifluoroacetic anhydride and propionic acid, and the product mixtures were analysed directly by FAB-MS. Glycopeptides and peptides rich in hydroxylated residues afforded characteristic clusters of molecular ions at high sensitivity. Additional sensitivity enhancement was achieved by prior esterification of carboxyl groups. These methods were used in a study of O-GlcNAc glycopeptides produced by purified O-GlcNAc transferase addition of GlcNAc to the synthetic peptides YSDSPSTST and YSGSPSTST in which Y is tyrosine, S is serine, D is aspartic acid, P is proline, T is threonine and G is glycine. The propionyl derivatives afforded high-quality spectra which unequivocally showed that the majority of the glycopeptides were substituted with a single GlcNAc residue. Low pmol quantities of material gave detectable signals. The propionylation/FAB-MS procedure has been combined with gas-phase sequencing strategies and shows promise for defining the sites of glycosylation of O-GlcNAc glycopeptides that are available in limited quantities.

ChemInform Abstract: The Production and Spectroscopy of Molecular Ions Isolated in Solid Neon

Studies of the molecular spectra of small polyatomic molecular ions are stlll in therr infancy. The availability of survey spectra would facilitate the search for individual vibrational and vibronic bands of gas-phase ion species using sophisticated, highly sensitive laser techniques. Deeectable concentrations not only of simpee ions such as CO+2 and H+2 but also of dimer cations and anions such as O+4 and O-4 have been stabilized in solid neon when the parett molecule is codeposited with a beam of excited neon atoms. The vibrational fundamentals heretofore observed for the simpee ions isolated in solid neon lie very close to the positions of the corresponding gas-phase band centers. Molecular spectroscopic data are not yet available for the gas-phase dimer ions. Therefore, it is difficutt to estimate how closely the neon-matrix vibrational frequencies here reported for a numbrr of dimer ions correspond to the gas-phase band centers. Some guidance is available from comparison of the argon-matrix spectra of small molecules hydrogen-bonded to HF with the gasphaee spectra of theee hydrogen-bonded species [55]. In theee studies, the effect of the argon matrix is to enhance the apparent hydrogen-bond strength; the HF stretching frequency is lowered by 1 to 5% from the gas-phase value, and the absorptions contributed by the flexing of the HF with respect to the other molecule are raised by 5% or more. Since neon matrices are generally less perturbing than argon matrices, the deviation from the gas-phase frequencies shoudd be somewhat less in neon-matrix studies. In the present experiments, only the high frequency stretching fundamentals have been observed, suggesting that matrix shifss shoudd amoutt to less than 3 or 4%. Therefore, matrix isolation studies such as theee promise to provide a valuable new tool for the detection and spectroscopic characterization of small molecular ions and cluster ions.

New cluster ions in the FD- and FAB-mass spectra of π-allylpalladium chloride complexes

The field desorption (FD) and fast atom bombardment (FAB) mass spectra of steroidal and terpenoid π-allylpalladium chloride complexes have been recorded. The molecular ions and cluster ions (those containing two or more metal atoms) with lower intensity than the molecular ions have been observed under field desorption conditions. On the other hand the FABMS spectra showed the presence of ( M − Cl) + ions and the cluster ions ( M + M/2 − Cl) +. Cluster ions for less volatile and unstable π-allylpalladium chloride complexes are reported here for the first time.

Determination of the primary structure of peptides using fast atom bombardment mass spectrometry

The determination of the amino acid composition and sequence of a peptide, using both conventional and tandem fast atom bombardment mass spectrometry, is presented. First a list of potential empirical formulae is generated using the accurate mass and isotopic peak intensity ratios from the molecular ion cluster. The formulae are mathematically analyzed to ensure that they correspond to linear peptides composed of 19 common amino acids. Then using the ions below m/z 160 Da, which are characteristic of the amino acid content of the peptide, the formulae are decomposed into possible amino acid compositions. From each composition sequences are generated and their predicted fragment ions are compared to ions present in the mass spectrum. A score based on the intensities and the occurrence of consecutive sequence fragments is calculated. Finally sequences with the ten highest scores are retained. Analyses performed on typical peptides with molecular weights below 1500 Da indicate that identification can generally be achieved using this approach.

Spectroscopy and Structure of Aromatic?Rare Gas Cluster Ions

Ionic clusters consisting of a polyatomic ion surrounded by a few 'solvent' atoms or molecules, provide a connecting link between the isolated gas phase ion and the ion solvated in a condensed medium. Analysis of the vibrational structure associated with the motion of the cluster atoms can reveal details concerning the intermolecular potential. However, for large polyatomic ions, information concerning the cluster vibrational motion has been difficult to obtain using conventional spectroscopic methods. We have developed a new combination of the previously available techniques of supersonic cooling, resonance-enhanced multi photon ionisation, timeof- flight mass spectroscopy, in concert with one-photon photodissociation spectroscopy. This new technique takes advantage of the facile predissociation of an electronically excited cluster and affords us a method of studying the previously unmeasurable vibrational structure associated with the motion of a molecular cluster ion. Using this technique we have obtained vibrationally resolved photodissociation spectra of a number of aromatic-rare gas cluster ions. Analysis of their vibrational structure permits structural details of the cluster cation to be deduced.

Mass spectrometry analyses of recombinant hirudins (7 kDa).

The use of liquid secondary ion mass spectrometry (LSIMS) in the characterization of related recombinant 7-kDa peptides illustrates the adequacy of average mass measurement by scanning at low resolution. The difficulty in using the high-resolution technique in the case of poor LSIMS sensitive peptides is discussed, as well as the fact that it does not give, for these molecular weights, any real advantage. The average (or chemical) molecular weights of three recombinant hirudin molecules, hirudin variant 2 (rHV2, 6892.4 Da), hirudin variant 2-Lys47 (rHV2-Lys47, 6906.5 Da), and hirudin variant 2-Arg47 (rHV2-Arg47, 6934.5 Da), less than or equal to 10 micrograms each, have been measured with an accuracy less than or equal to 0.3 Da in the narrow-scan mode and less than or equal to 0.5 Da (from the protonated molecular ion) in the wide-scan mode within 10-15 min; this allows easy distinction of the three 65 amino acid proteins, which differ by a single amino acid. These three molecules could also be distinguished from one another in a mixture. Mass spectrometry and limited sequence characterization of several minor, similarly isolated peptides identified them to be N-terminal additions and/or C-terminal deletions of rHV2-Lys47. LSIMS analysis is consistent with there being no covalent dimer of rHV2-Lys47 as a narrow scan of the 7-kDa molecular ion cluster at high resolution shows it not to be a doubly charged ion.

The production and spectroscopy of molecular ions isolated in solid neon

Studies of the molecular spectra of small polyatomic molecular ions are stlll in therr infancy. The availability of survey spectra would facilitate the search for individual vibrational and vibronic bands of gas-phase ion species using sophisticated, highly sensitive laser techniques. Deeectable concentrations not only of simpee ions such as CO+2 and H+2 but also of dimer cations and anions such as O+4 and O-4 have been stabilized in solid neon when the parett molecule is codeposited with a beam of excited neon atoms. The vibrational fundamentals heretofore observed for the simpee ions isolated in solid neon lie very close to the positions of the corresponding gas-phase band centers. Molecular spectroscopic data are not yet available for the gas-phase dimer ions. Therefore, it is difficutt to estimate how closely the neon-matrix vibrational frequencies here reported for a numbrr of dimer ions correspond to the gas-phase band centers. Some guidance is available from comparison of the argon-matrix spectra of small molecules hydrogen-bonded to HF with the gasphaee spectra of theee hydrogen-bonded species [55]. In theee studies, the effect of the argon matrix is to enhance the apparent hydrogen-bond strength; the HF stretching frequency is lowered by 1 to 5% from the gas-phase value, and the absorptions contributed by the flexing of the HF with respect to the other molecule are raised by 5% or more. Since neon matrices are generally less perturbing than argon matrices, the deviation from the gas-phase frequencies shoudd be somewhat less in neon-matrix studies. In the present experiments, only the high frequency stretching fundamentals have been observed, suggesting that matrix shifss shoudd amoutt to less than 3 or 4%. Therefore, matrix isolation studies such as theee promise to provide a valuable new tool for the detection and spectroscopic characterization of small molecular ions and cluster ions.

DYNAMIC SIMS OF SUPERSATURATED SOLUTIONS

In liquid SIMS (or fast atom bombardment mass spectrometry) a liquid sample solution is exposed to an incident keV particle beam and molecular ions are recorded during continuous erosion of the layer. We have investigated this mode of molecular ion formation under dynamic bombarding conditions regarding the effect of concentration, solid particle formation by cristallization of solutes from supersaturated solutions and the formation of a closed solid surface layer prior to the onset of particle bombardment. The experiments were performed with peptides and organic salts applying mass spectrometry and optical microscopy. They showed that molecular ions and/or cluster ions are obtained under dynamic bombarding conditions without accumulation of radiation damage from supersaturated solutions, even in the presence of small precipitated particles and after erosion of thin solid surface layers from the remaining solution. Furthermore it was found that the incident particle beam hampers crystallization of the solute from a supersaturated solution.

Combined SIMS/TPD investigations of UHV-prepared tolyltriazole overlayers on Cu, Ni and Au

The corrosion inhibitor tolyltriazole (TTA) has been investigated by combined secondary ion mass spectrometry (SIMS) and temperature programmed desorption mass spectrometry (TPD). TTA overlayers in the submonolayer to multilayer range are produced by in situ molecular beam exposure of Cu, Ni and Au substrates under UHV conditions. TPD yields information on different binding states and layer thickness. Molecular cluster ions as (2M−2H+Me) −, indicating the formation of TTA molecules in quasipolymeric chains, are emitted from all three substrates. Only for Cu we found a TTA desorption peak at high target temperatures (580 K), correlated to the emission of characteristic secondary ions, mainly (MH) − and (2M−2H+Cu) −. It is obviously the corresponding stable TTA monolayer on Cu that is responsible for the strong corrosion protection effect.