Ion structure determinations and ion—molecule reactions by double quadrupole mass spectrometry

The QQ mass spectrometer is shown to be applicable to ion structure determination via collision-induced dissociations of mass-selected ions. The instrument can be scanned so as to record the products of dissociation as well as those of ion—molecule association reactions. The dissociations correspond to those observed at high kinetic energy in mass-analyzed ion kinetic energy spectrometers and the association reactions show parallels with reactions seen in ion cyclotron resonance spectroscopy and in high-pressure mass spectrometry     

Negative and positive secondary ion mass spectra of organic acids

Organic compounds can be ionized by sputtering the solid sample. The resulting negative and positive secondary ions provide mass spectra which characterize both the molecular weights and the structures of the compounds. Ionization occurs either by direct ejection of charged species from the solid into vacuum or by electron or proton transfer. The sputtered secondary ions dissociate unimolecularly to give fragment ions. These reactions are identical to those which occur when the secondary ions are independently generated by chemical ionization, selected by mass and dissociated in a high energy gas phase collision. The negative ion SIMS spectra show molecular ions (M^?.) or (M-H)^? ions as the dominant high mass species together with fragments due to decarboxylation, dehydration and losses of other simple molecules. Stronger acids show larger (M-H)^?/M^?.abundance ratios. The positive ion spectra are complementary and also useful in characterizing molecular structures. Attachment of cations to organic molecules (cationization) occurs much more readily than anion attachment and this makes negative SIMS spectra simpler than these positive ion counterparts.     

Determination of ammonia, ethanol and acetic acid in solution using membrane introduction mass spectrometry

Methods have been developed to allow applications of membrane introduction mass spectrometry (MIMS) to monitor solution phase components of fermentation broths using electron ionization. The solutions are transported by flow injection analysis (FIA) through a direct insertion membrane probe, fitted with a silicone membrane in the sheet configuration. Analytes of interest pass through the membrane and are ionized by electron implant ionization. The compounds monitored are ammonia, acetic acid, and ethanol, with ammonia being detected as the monochloramine derivative which is generated at pH 10 upon addition of hypochlorite. Quantitation is achieved using external standard solutions. The dynamic range for the quantification of ammonia is 2-8000 ppm, and for ethanol and acetic acid 10-1000 ppm. This method provides rapid detection of analytes of interest, on-line monitoring capabilities, and the advantage of electron ionization. The introduction of samples into the mass spectrometer is achieved readily and automatically, the response time is a few seconds, and there are no memory effects.     

Clustering of nucleosides in the presence of alkali metals: Biologically relevant quartets of guanosine,deoxyguanosine and uridine observed by ESI-MS/MS

Electrospray ionization (ESI) mass spectra of nucleosides, recorded in the presence of alkali metals, display alkali metal ion-bound quartets and other clusters that may have implications for understanding non-covalent interactions in DNA and RNA. The tetramers of guanosine and deoxyguanosine and also their metaclusters (clusters of clusters), cationized by alkali metals, were observed as unusually abundant magic number clusters. The observation of these species in the gas phase parallels previous condensed-phase studies, which show that guanine derivatives can form quartets and metaclusters of quartets in solution in the presence of metal cations. This parallel behavior and also internal evidence suggest that bonding in the guanosine tetramers involves the bases rather than the sugar units. The nucleobases thymine and uracil are known to form magic number pentameric adducts with K+, Cs+ and NH4+ in the gas phase. In sharp contrast, we now show that the nucleosides uridine and deoxythymidine do not form the pentameric clusters characteristic of the corresponding bases. More subtle effects of the sugars are evident in the fact that adenosine and cytidine form numerous higher order clusters with alkali metals, whereas deoxyadenosine and deoxycytidine show no clustering. It is suggested that hydrogen bonding between the bases in the tetramers of dG and rG are the dominant interactions in the clusters, hence changing the ribose group to deoxyribose (and vice versa) generally has little effect. However, the additional hydroxyl group of RNA nucleosides enhances the non-selective formation of higher-order aggregates for adenosine and cytidine and results in the lack of highly stable magic number clusters. Some clusters are the result of aggregation in the course of ionization (ESI) whereas others appear to be intrinsic to the solution being examined.     

Building mass spectrometers and a philosophy of research

Development of the techniques of ion kinetic energy spectrometry and mass-analyzed ion kinetic energy spectrometry is described. The extension of these concepts to the method of tandem mass spectrometry for direct mixture analysis is traced, and a rationale for the construction of hybrid mass spectrometers is presented. Collisions of polyatomic ions with surfaces are discussed as an outgrowth of gaseous collisions. An attempt is made to describe a philosophy of research that guided the construction of a dozen mass spectrometers and the exploration of organic ion chemistry in as many contexts.     

Determination of pentachlorophenol by negative ion chemical ionization with membrane introduction mass spectrometry

Pentachlorophenol (PCP) was used as a model compound to explore the potential of desorption chemical ionization (DCI) in the determination of polychlorinated pesticides using membrane introduction mass spectrometry (MIMS). A direct insertion membrane probe was modified so that a chemical ionization plasma could be established at the membrane surface. Using selected ion monitoring (SIM) in a tandem triple quadrupole mass spectrometer with isobutane chemical ionization (CI), the PCP detection limit under positive chemical ionization is 20 ppb whereas negative CI gives detection limits in the low ppb range. This performance is achieved without any pre-treatment or derivatization of the sample. Negative ion CI gives a signal that is linear over a concentration range of 2-1000 ppb. Comparison of data obtained with low ppb samples of 2,4,6-trichlorophenol, 2,3,4,6-tetrachlorophenol and pentachlorophenol suggests that the sensitivity of this analytical procedure increases with increase in the number of electronegative substituents in the molecule.     

On-line flow injection analysis of volatile organic compounds in seawater by membrane introduction mass spectrometry

The combination of flow injection analysis with membrane introduction mass spectrometry for analysis of volatile organic compounds (VOCs) in seawater is examined and is compared to measurements made in water. Membrane introduction mass spectrometry is performed using a benchtop ion trap mass spectrometer, and characterization of various aspects of the flow injection and ion trap combination for the analysis of volatile organic compounds (including anthropogenic halocarbons) in seawater is carried out. The analyte responses are shown to be linear over several orders of magnitude (e.g. for methylene chloride), independent of seawater pH (e.g. for chlorobenzene) and independent of matrix effects for the VOCs studied. A comparison of the performance of a microporous (Teflon) membrane with that of an amorphous silicone membrane is made, and the former is shown to provide lower detection limits which are in the parts-per-trillion range (300 ppt for chlorobenzene, 190 ppt for trans-1,2-dichloroethene). The microporous membrane provides faster response times by a factor of four to five for relatively more polar compounds, such as chlorobenzene. An analysis of a seven-component mixture demonstrates the ability of this on-line combination to allow multicomponent analysis of mixtures of some complexity.     

Differentiation and quantitation of isomeric dipeptides by low-energy dissociation of copper(II)-bound complexes

Application of the kinetic method based on the dissociation of transition metal centered cluster ions is extended from chiral analysis (Tao, W. A.; Zhang, D.; Nikolaev, E. N.; Cooks, R. G. J. Am. Chem. Soc. 2000, 122, 10598) to quantitative analysis of isomeric mixtures, including those with Leu/Ile substitutions. Copper(II)-bound complexes of pairs of peptide isomers are generated by electrospray ionization mass spectrometry and the trimeric complex [CuII(ref)2(A) - H]+ (analyte A, a mixture of isomeric peptides; reference compound ref, usually a peptide) is caused to undergo collisional dissociation. Competitive loss of the neutral reference compound or the neutral analyte yields two ionic products and the ratio of rates of the two competitive dissociations, viz. the product ion branching ratio R is shown to depend strongly on the regiochemistry of the analyte in the precursor [CuII(A)(ref)2 - H]+ complex ion. Calibration curves are constructed by relating the branching ratio measured by the kinetic method, to the isomeric composition of the mixture to allow rapid quantitative isomer analysis.     

A model for energy transfer in inelastic molecular collisions applicable at steady state or non-steady state and for an arbitrary distribution of collision energies

A new model for energy exchange between translational and internal degrees of freedom in atom-molecule collisions has been developed. It is suitable for both steady state conditions (e.g., a large number of collisions with thermal kinetic energies) and non-steady state conditions with an arbitrary distribution of collision energies (e.g., single high-energy collisions). In particular, it does not require that the collision energies be characterized by a quasi-thermal distribution, but nevertheless it is capable of producing a Boltzmann distribution of internal energies with the correct internal temperature under quasi-thermal conditions. The energy exchange is described by a transfer probability density that depends on the initial relative kinetic energy, the internal energy of the molecule, and the amount of energy transferred. The probability density for collisions that lead to excitation is assumed to decrease exponentially with the amount of transferred energy. The probability density for de-excitation is obtained from microscopic reversibility. The model has been implemented in the ion trap simulation program ITSIM and coupled with an Rice-Rampsberger-Kassel-Marcus (RRKM) algorithm to describe the unimolecular dissociation of populations of ions. Monte Carlo simulations of collisional energy transfer are presented. The model is validated for non-steady state conditions and for steady state conditions, and the effect of the kinetic energy dependence of the collision cross-section on internal temperature is discussed. Applications of the model to the problem of chemical mass shifts in RF ion trap mass spectrometry are shown.     

Recognition and quantification of binary and ternary mixtures of isomeric peptides by the kinetic method: metal ion and ligand effects on the dissociation of metal-bound complexes

The kinetic method is applied to differentiate and quantify mixtures of isomeric tripeptides based on the competitive dissociations of divalent metal ion-bound clusters in an ion trap mass spectrometer. This methodology is extended further to determine compositions of ternary mixtures of the isomers Gly-Gly-Ala (GGA), Ala-Gly-Gly (AGG), and Gly-Ala-Gly (GAG). This procedure also allows to perform chiral quantification of a ternary mixture of optical isomers. The divalent metal ion Ca(II) is particularly appropriate for isomeric distinction and quantification of the isobaric tripeptides Gly-Gly-Leu/Gly-Gly-Ile (GGL/GGI). Among the first-row transition metal ions, Cu(II) yields remarkably effective isomeric differentiation for both the isobaric tripeptides, GGI/GGL using GAG as the reference ligand, and the positional isomers GAG/GGA using GGI as the reference ligand. This is probably due to agostic bonding: alpha-agostic bonding occurs between Cu(II) and GAG and beta-agostic bonding between Cu(II) and GGI, each produces large but different steric effects on the stability of the Cu(II)-bound dimeric clusters. These data form the basis for possible future quantitative analyses of mixtures of larger peptides such as are generated, for example, in combinatorial synthesis of peptides and peptide mimics.