Modification of a Quadrupole/Orbitrap/Linear Quadrupole Ion Trap Tribrid Mass Spectrometer for Diagnostic Gas-Phase Ion-Molecule Reactions.

Tandem mass spectrometry based on diagnostic gas-phase ion-molecule reactions represents a robust method for functional group identification in unknown compounds. To date, most of these reactions have been studied using unit-resolution instruments, such as linear quadrupole ion traps and triple quadrupoles, which cannot be used to obtain elemental composition information for the species of interest. In this study, a high-resolution mass spectrometer, a quadrupole/orbitrap/linear quadrupole ion trap tribrid, was modified by installing a portable reagent inlet system to obtain high-resolution data for ion-molecule reactions. Examination of a previously published test system, the reaction between protonated 1,1'-sulfonyldiimizadole with 2-methoxypropene, demonstrated the ability to perform ion-molecule reactions on the modified tribrid mass spectrometer. High-resolution data were obtained for ion-molecule reactions of three isobaric ions (protonated glycylalanine, protonated glutamine, and protonated lysine) with diethylmethoxyborane. On the basis of these data, the isobaric ions can be differentiated based on both their measured accurate mass as well as the different product ions they generated upon the ion-molecule reactions. In a different experiment, analyte ions were subjected to collision-induced dissociation (CID), and the structures of the resulting fragment ions were examined via diagnostic ion-molecule reactions. This experiment allows for the functional group interrogation of fragment ions and can be used to improve the understanding of the structures of fragment ions generated in the gas phase.

Nonmetal-to-Metal Transition of Magnesia Supported Au Clusters Affects the Ultrafast Dissociation Dynamics of Adsorbed CH3Br Molecules.

The detection of intermediate species and the correlation of their ultrafast dynamics with the morphology and electronic structure of a surface is crucial to fully understand and control heterogeneous photoinduced and photocatalytic reactions. In this work, the ultrafast photodissociation dynamics of CH3Br molecules adsorbed on variable-size Au clusters on MgO/Mo(100) is investigated by monitoring the CH3+ transient evolution using a pump-probe technique in conjunction with surface mass spectrometry. Furthermore, extreme-UV photoemission spectroscopy in combination with theoretical calculations is employed to study the electronic structure of the Au clusters on MgO/Mo(100). Changes in the ultrafast dynamics of the CH3+ fragment are correlated with the electronic structure of Au as it evolves from monomers to small nonmetallic clusters to larger nanoparticles with a metallic character. This work provides a new avenue to a detailed understanding of how surface-photoinduced chemical reactions are influenced by the composition and electronic structure of the surface.

Bacterial growth monitored by two-dimensional tandem mass spectrometry.

The growth of the bacterium E. coli was monitored by targeting the phospholipid constituents through desorption electrospray ionization and characterizing individual sets of isomers by recording the full 2D MS/MS data domain in a single scan of a modified quadrupole ion trap mass spectrometer. The experiments tested the applicability of the new instrumental capabilities which include sample interrogation at the molecular level for multiple components at speeds of <10 seconds/sample. The major lipids observed were phosphatidylethanolamines and phosphatidylglycerols and the growth experiment showed fatty acid chain modification from alkene to cyclopropyl groups over time. Notably, these novel MS scans were also performed using desorption electrospray ionization (DESI) to quickly sample complex mixtures without pre-separation. This demonstration experiment has implications for the value of ambient ionization mass spectrometry for monitoring biological systems on physiologically relevant timescales.

Immediate and sensitive detection of sporulated Bacillus subtilis by microwave release and tandem mass spectrometry of dipicolinic acid.

Spore lysis of Bacillus species is achieved by brief (1 min) microwave irradiation while tandem mass spectrometry (MS/MS) allows identification of the characteristic spore marker, dipicolinic acid. This rapid measurement, made on 105-108 spores, has significant implications for biothreat recognition.

Atmospheric Pressure Drift Tube Ion Mobility Spectrometry Coupled with Two-Dimensional Tandem Mass Spectrometry.

Atmospheric pressure drift tube ion mobility was coupled with two-dimensional tandem mass spectrometry (2D MS/MS) in a linear ion trap to simultaneously collect ion mobility and the entire MS/MS data domain. Utilizing ion intensities from precursor ion and neutral loss scan lines, ion mobility spectra of multiple compounds with particular functional groups were acquired in a single experiment. Functional group-specific ion mobility spectra were demonstrated for a standard mixture of lipids. Additionally, ion mobility was used to separate isobaric ions prior to 2D MS/MS. The combination of these two methods offers improvements for the analysis of complex mixtures.

Layer-resolved ultrafast extreme ultraviolet measurement of hole transport in a Ni-TiO2-Si photoanode.

Metal oxide semiconductor junctions are central to most electronic and optoelectronic devices, but ultrafast measurements of carrier transport have been limited to device-average measurements. Here, charge transport and recombination kinetics in each layer of a Ni-TiO2-Si junction is measured using the element specificity of broadband extreme ultraviolet (XUV) ultrafast pulses. After silicon photoexcitation, holes are inferred to transport from Si to Ni ballistically in ~100 fs, resulting in characteristic spectral shifts in the XUV edges. Meanwhile, the electrons remain on Si. After picoseconds, the transient hole population on Ni is observed to back-diffuse through the TiO2, shifting the Ti spectrum to a higher oxidation state, followed by electron-hole recombination at the Si-TiO2 interface and in the Si bulk. Electrical properties, such as the hole diffusion constant in TiO2 and the initial hole mobility in Si, are fit from these transient spectra and match well with values reported previously.

Fourier Transform-Ion Mobility Linear Ion Trap Mass Spectrometer Using Frequency Encoding for Recognition of Related Compounds in a Single Acquisition.

Fourier transform-ion mobility spectrometry is implemented by coupling a 3D-printed drift tube ion mobility spectrometer, operated at atmospheric pressure, to a linear ion trap mass spectrometer. FT-IMS separations are demonstrated for tetraalkylammonium salts, explosives, fentanyls, and amphetamines. Mobility resolving powers of up to 17 are measured for the tetraalkylammonium cations. When ions are fragmented in the FT-IMS mode, the product ions maintain the frequency and amplitude relationships established during the mobility measurement. Therefore, precursors and product ion relationships can be identified through the mobility information. Using in-source activation for nonspecific fragmentation of all precursors, functional group families of precursors and product ions are identified in a single acquisition. The identity of the precursor ion is not known a priori, but the m/z values for both precursors and product ions are measured.

Ion Manipulation in Open Air Using 3D-Printed Electrodes.

Ambient ionization techniques provide a way to sample materials via creation of ions in the air. However, transferring and focusing of these ions is typically done in the reduced pressure environment of the mass spectrometer. Spray-based ambient ionization sources require relatively large distances between the source and mass spectrometer inlet for effective desolvation, resulting in a small fraction of the ions being collected. To increase the efficiency of ion transfer from atmosphere to vacuum, 3D-printed focusing devices made of conductive carbon nanotube doped polymers have been designed and evaluated for ion focusing in air. Three main classes of electrodes are considered: (i) conic section electrodes (conical, ellipsoidal, and cylindrical), (ii) simple conductive and non-conductive apertures, and (iii) electrodes with complex geometries (straight, chicane, and curved). Simulations of ion trajectories performed using the statistical diffusion simulation (SDS) model in SIMION showed a measure of agreement with experiment. Cross-sectional images of ion beams were captured using an ion detecting charge-coupled device (IonCCD). After optimization, the best arrangements of electrodes were coupled to an Agilent Ultivo triple quadrupole to record mass spectra. Observations suggest that electrode geometry strongly influences ion trajectories in air. Non-conductive electrodes also assisted in focusing, due to charge buildup from ion deposition. We also observed minimal spreading of the ion packet after exiting the focusing electrodes indicating that atmospheric collisions do not reduce collimation of the beam. The study suggests that high pressures need not be viewed as a hindrance to ion transport, but as a potentially useful force.

Reaction Acceleration in Electrospray Droplets: Size, Distance, and Surfactant Effects.

Phenylhydrazone formation from isatin is used to examine the effects on the reaction rate of (i) electrospray emitter distance from the mass spectrometer (MS) inlet, (ii) emitter tip diameter, and (iii) presence of surfactant. Reaction rates are characterized through measurement of conversion ratios. It is found that there is an increase in the conversion ratio as (i) the electrospray source is moved further from the inlet of the mass spectrometer, (ii) smaller sprayer diameters are used, and (iii) when surfactants are present. Each of these experimental operations is associated with an increase in reaction rate and with a decrease in average droplet sizes. The size measurements are made using super resolution microscopy from the "splash" on a collector surface produced by a fluorescent marker sprayed using conditions similar to those used for the reaction mixture. This measurement showed that droplets undergo significant evaporation as a function of distance of flight, thereby increasing their surface to volume ratios. Similarly, the effect of nanoelectrospray emitter size on conversion ratio is also found to be associated with changes in droplet size for which a 4 to 10 times increase in reaction rate is seen using tip diameters ranging from 20 µm down to 1 µm. Finally, the effects of surfactants in producing smaller droplets with corresponding large increases in reaction rate are demonstrated by splash microscopy. These findings point to reaction acceleration being strongly associated with reactions at the surfaces of microdroplets.

Ultrafast time-resolved extreme ultraviolet (XUV) photoelectron spectroscopy of hole transfer in a Zn/n-GaP Schottky junction.

The addition of a metal overlayer to a semiconductor photocatalyst is a frequently used synthetic route to passivate the surface and, via the formation of a Schottky barrier, to enhance catalytic activity of the photocatalyst material. While it is known that Schottky junctions decrease recombination by charge separation, measurements of the depletion region dynamics have remained elusive. Here, we use ultrafast pump-probe transient photoelectron spectroscopy to measure material-specific dynamics of the Zn/n-GaP(100) system. Through photoemission measurements the Schottky barrier height is determined to be 2.1 ± 0.1 eV at 10 monolayers of total Zn deposition. Transient photoemission measurements utilizing a 400 nm pump pulse show that, after excitation, holes are transferred from n-GaP(100) to the Zn overlayer within a few ps, as evidenced by shifts of the Zn 3d and Ga 3d core levels to higher binding energies. Within the timescale of the experiment (130 ps) no carrier recombination is observed in the junction. Furthermore, a long-lived surface photovoltage signal is observed at times >1 ms after photoexcitation. This work further exemplifies the potential of transient extreme ultraviolet photoelectron spectroscopy as a material-specific technique for the study of heterojunctions.

Nonmetal to Metal Transition and Ultrafast Charge Carrier Dynamics of Zn Clusters on p-Si(100) by fs-XUV Photoemission Spectroscopy.

Understanding the electronic structure and charge carrier dynamics of supported clusters is important due to their many potential applications in photochemistry and catalysis. In this investigation, photoemission spectroscopy, in conjunction with femtosecond extreme ultraviolet (XUV) laser pulses, is used to investigate the electronic structure and ultrafast charge carrier dynamics at a Si(100) surface decorated with Zn clusters. Static photoemission spectroscopy is used to investigate the changes in the electronic structure as the dimensionality of the Zn is increased from small clusters composed of a very few atoms to metallic Zn particles. Furthermore, femtosecond optical-pump XUV-probe photoemission spectroscopy is employed to induce a charge transfer from the p-Si(100) substrate to the Zn clusters and to measure in real time the charge trapping at the Zn cluster as well as the subsequent charge relaxation. The ultrafast charge carrier dynamics are also investigated for small clusters and metallic Zn particles. Significant transient charging of the Zn clusters after excitation of the Si(100) substrate by 800 nm light is observed for Zn coverages greater than 0.12 ML Zn, which coincides with the formation of a Schottky barrier at the interface between the Zn particle and the p-Si(100) substrate. The transient signals show that the charge trapping time at the Zn cluster varies with the cluster size, which is rationalized based on the electronic structure of the cluster as well as the band energy alignment at the Zn cluster-Si(100) junction.

Mass Spectrometric and Vibrational Characterization of Reaction Intermediates in [Ru(bpy)(tpy)(H2 O)]2+ Catalyzed Water Oxidation.

Mass spectrometry coupled with an in-line electrochemical electrospray ionization source is used to capture some of the reaction intermediates formed in the [Ru(bpy)(tpy)(H2 O)]2+ (bpy=2,2'-bipyridine, tpy=2,2':6',2"-terpyridine) catalyzed water oxidation reaction. By controlling the applied electrochemical potential, we identified the parent complex, as well as the first two oxidation complexes, identified as [Ru(bpy)(tpy)(OH)]2+ and [Ru(bpy)(tpy)(O)]2+ . The structures of the parent and first oxidation complexes are probed directly in the mass spectrometer by using infrared predissociation spectroscopy of D2 -tagged ions. Comparisons between experimental vibrational spectra and density functional theory calculations confirmed the identity and structure of these two complexes. Moreover, the frequency of the O-H stretching mode in [Ru(bpy)(tpy)(OH)]2+ shows that this complex features a Ru-OH interaction that is more covalent than ionic.

Interaction between ionic liquid cation and water: infrared predissociation study of [bmim](+)·(H2O)n clusters.

The infrared predissociation spectra of [bmim](+)·(H2O)n, n = 1-8, in the 2800-3800 cm(-1) region are presented and analyzed with the help of electronic structure calculations. The results show that the water molecules solvate [bmim](+) by predominately interacting with the imidazolium C2-H moiety for the small n = 1 and 2 clusters. This is characterized by a redshifted and relatively intense C2-H stretch. For n≥ 4 clusters, hydrogen-bond interactions between the water molecules drive the formation of ring isomers which interact on top of the imidazolium ring without any direct interaction with the C2-H. The water arrangement in [bmim](+)·(H2O)n is similar to the low energy isomers of neutral water clusters up to the n = 6 cluster. This is not the case for the n = 8 cluster, which has the imidazolium ring disrupting the otherwise preferred cubic water structure. The evolution of the solvation network around [bmim](+) illustrates the competing [bmim](+)-water and water-water interactions.

Characterization of the Oxygen Binding Motif in a Ruthenium Water Oxidation Catalyst by Vibrational Spectroscopy.

For homogeneous mononuclear ruthenium water oxidation catalysts, the Ru-O2 complex plays a crucial role in the rate determining step of the catalytic cycle, but the exact nature of this complex is unclear. Herein, the infrared spectra of the [Ru(tpy)(bpy)(O2)](2+) complex (tpy=2,2':6',2''-terpyridine; bpy=2,2'-bipyridine) are presented. The complex [Ru(tpy)(bpy)(O2)](2+), formed by gas-phase reaction of [Ru(tpy)(bpy)](2+) with molecular O2, was isolated by using mass spectrometry and was directly probed by cryogenic ion IR predissociation spectroscopy. Well-resolved spectral features enable a clear identification of the O-O stretch using (18) O2 substitution. The band frequency and intensity indicate that the O2 moiety binds to the Ru center in a side-on, bidentate manner. Comparisons with DFT calculations highlight the shortcomings of the B3LYP functional in properly depicting the Ru-O2 interaction.

A dual cryogenic ion trap spectrometer for the formation and characterization of solvated ionic clusters.

A new experimental approach is presented in which two separate cryogenic ion traps are used to reproducibly form weakly bound solvent clusters around electrosprayed ions and messenger-tag them for single-photon infrared photodissociation spectroscopy. This approach thus enables the vibrational characterization of ionic clusters comprised of a solvent network around large and non-volatile ions. We demonstrate the capabilities of the instrument by clustering water, methanol, and acetone around a protonated glycylglycine peptide. For water, cluster sizes with greater than twenty solvent molecules around a single ion are readily formed. We further demonstrate that similar water clusters can be formed around ions having a shielded charge center or those that do not readily form hydrogen bonds. Finally, infrared photodissociation spectra of D2-tagged GlyGlyH(+)⋅(H2O)1-4 are presented. They display well-resolved spectral features and comparisons with calculations reveal detailed information on the solvation structures of this prototypical peptide.

Coordination structure and charge transfer in microsolvated transition metal hydroxide clusters [MOH](+)(H2O)1-4.

Infrared vibrational predissociation spectra of transition metal hydroxide clusters, [MOH](+)(H2O)1-4·D2 with M = Mn, Fe, Co, Ni, Cu, and Zn, are presented and analyzed with the aid of density functional theory calculations. For the [MnOH](+), [FeOH](+), [CoOH](+) and [ZnOH](+) species, we find that the first coordination shell contains three water molecules and the four ligands are arranged in a distorted tetrahedral geometry. [CuOH](+) can have either two or three water molecules in the first shell arranged in a planar arrangement, while [NiOH](+) has an octahedral ligand geometry with the first shell likely closed with five water molecules. Upon closure of the first coordination shell, characteristic stretch frequencies of hydrogen-bonded OH in the 2500-3500 cm(-1) region are used to pinpoint the location of the water molecule in the second shell. The relative energetics of different binding sites are found to be metal dependent, dictated by the first-shell coordination geometry and the charge transfer between the hydroxide and the metal center. Finally, the frequency of the hydroxide stretch is found to be sensitive to the vibrational Stark shift induced by the charged metal center, as observed previously for the smaller [MOH](+)(H2O) species. Increasing solvation modulates this frequency by reducing the extent of the charge transfer while elongating the M-OH bond.

Probing the Hydrogen-Bonded Water Network at the Active Site of a Water Oxidation Catalyst: [Ru(bpy)(tpy)(H2O)](2+)·(H2O)(0-4).

The infrared spectra of gas-phase mass-selected [Ru(bpy)(tpy)(H2O)](2+)·(H2O)(0-4) clusters (bpy = 2,2'-bipyridine; tpy = 2,2':6,2″-terpyridine) in the OH stretching region are reported. These species are formed by bringing the homogeneous water oxidation catalyst [Ru(bpy)(tpy)(H2O](2+) from solution into the gas phase via electrospray ionization (ESI) and reconstructing the water network at the active site by condensing additional water onto the complex in a cryogenic ion trap. Infrared predissociation spectroscopy is used to probe the structure of these clusters via their distinctive OH stretch frequencies, which are sensitive to the shape and strength of the local hydrogen-bonding network. The analysis of the spectra, aided by electronic structure calculations, highlights the formation of strong hydrogen bonds between the aqua ligand and the solvating water molecules in the first solvation shell. These interactions are found to propagate through the subsequent solvation shells and lead to the stabilization of asymmetric solvation motifs. Electronic structure calculations show that these strong hydrogen bonds are promoted by charge transfer from the H atom of the aqua ligand to the Ru-OH2 bond.

Picosecond Dynamics of Avobenzone in Solution.

Avobenzone, a dibenzoylmethane compound commonly found in sunscreens, can photoisomerize after exposure to near-ultraviolet light. At equilibrium, avobenzone exists as a chelated enol characterized by a strong intramolecular hydrogen bond. Many nanosecond- to microsecond-scale experiments have shown that the photoisomerization involves several nonchelated enol (NCE) isomers and reaction paths, including some that reduce avobenzone's efficacy as a sunscreen. Because some of the NCE isomers are unstable, these experiments do not directly measure their spectroscopic signatures. Here, we report the dynamics of avobenzone on the picosecond time scale. We excite avobenzone at 350 nm and observe the formation and relaxation of new isomers and vibrationally excited species with broadband visible probe pulses and 266 nm probe pulses. Our results show the first direct evidence of two unstable NCE isomers and establish the lifetimes of and the branching ratio between these isomers.

Charge transfer in MOH(H2O)(+) (M = Mn, Fe, Co, Ni, Cu, Zn) complexes revealed by vibrational spectroscopy of mass-selected ions.

Charge transfer between a metal and its ligand is fundamental for the structure and reactivity of a metal complex as it directly dictates the distribution of electron density within the complex. To better understand such charge transfer interactions, we studied the vibrational spectra of mass-selected MOH(H2O)(+) (M = Mn, Fe, Co, Ni, Cu, or Zn) complexes, acquired using cryogenic ion infrared predissociation spectroscopy. We find that there is a partial charge transfer from the hydroxide anion to the metal center for these first-row transition metals, the extent of which is in the order of Mn < Fe < Co < Ni < Cu > Zn, dictated by the 2nd ionization energy of the bare metal. This gradual change across the metal series points to the complexity in the electronic structures of these transition metal complexes. Interestingly, the hydroxide ligand in these complexes can serves as a sensitive in situ probe of this charge transfer. Its vibrational frequency varies by >150 cm(-1) for different metal species, and it is dependent on the electric field produced by the charged metal center. This dramatic vibrational Stark shift is further modulated by the charge present on the hydroxide itself, providing a well-defined relationship between the observed hydroxide frequency and the effective electric field.

Intramolecular hydrogen bonding motifs in deprotonated glycine peptides by cryogenic ion infrared spectroscopy.

The infrared spectra of deprotonated glycine peptides, (Gn-H)(-) with n = 1-4, in the 1200-3500 cm(-1) spectral region are presented. Comparisons between the experimental and calculated spectra reveal the chain length dependent hydrogen bonding motifs that define the geometries of these species. First, an interaction between the terminal carboxylate and the neighboring amide N-H is present in all the peptide structures. This interaction is strong enough to align this amide group in the same plane as the carboxylate. However, we found that the vibrational frequency shift of this hydrogen bonded N-H group is not well reproduced in the calculations. Second, in the longer (G3-H)(-) and (G4-H)(-) species, the peptide chain folds such that the terminal NH2 group also interacts with the carboxylate. Both of these folded structures display an interaction between the terminal NH2 and the neighboring N-H as well. Lastly, an amide-amide interaction is observed in the longest (G4-H)(-) structure. Analysis of the N-H peak positions reveals the interplay among the different hydrogen bonds, especially around the negatively charged carboxylate moiety.