Conformational distribution of bradykinin [bk+2H]2+ revealed by cold ion spectroscopy coupled with FAIMS

We employ Cold Ion Spectroscopy (CIS) in conjunction with high-Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) to study the peptide bradykinin in its doubly protonated charge state ([bk+2H]2+). Using FAIMS we partially separate the electrosprayed [bk+2H]2+ ions into two conformational families and selectively introduce one of them at a time into a cold ion trap mass spectrometer, where we probe them by UV photofragment spectroscopy. Although the two conformational families have distinct electronic spectra, some cross-conformer contamination can be observed under certain conditions. We demonstrate that this contamination comes from isomerization of ions energized during and/or after their separation and not from incomplete separation of the initially electrosprayed conformations in the FAIMS stage. By varying the injection voltage of the ions into our mass spectrometer we can intentionally induce isomerization to produce what seems to be a gas phase equilibrium distribution of conformers. This distribution is different from the one produced initially by electrospray, indicating that some of the conformers are kinetically trapped and may be related to conformers that are more favored in solution.

Process monitoring of alumina nanoparticle synthesis by inductively coupled RF thermal plasma

Jong-Won Shin

Since the first inductively coupled plasma (ICP) torch was developed in the 60's, RF thermal plasmas have found a variety of applications in material processing such as crystal growth, thermal spray coatings, spheroidization and vaporization of refractory materials. In recent decades, inductively coupled thermal plasmas have been used for the synthesis of high purity nanoparticles, thank to their remarkable advantages such as high energy density, variable operating pressures and low product contamination. The formation of nano-sized particles in thermal plasmas particularly using solid refractory precursor is a complex process. Hereby the injected precursor powders are heated by the high temperature of the thermal plasma. The heated powders are vaporized and decomposed into vapor species. During the following cooling, the vapor species are condensed to nano-sized particles. In this synthesis process, characterized by evaporation-condensation, the properties of the synthesized particles such as particle size, size distribution, phases and morphology are affected by various process parameters. Gas pressure, flow rates of the different torch gases, size of the precursor powders and powder loading in the plasma are considered as the influencing process parameters. In order to optimize and control the process for tailor-made nanoparticle synthesis, it is necessary to understand the effects of the process parameters on plasma properties and on the particle-plasma interactions resulting in final properties of the synthesized powders. In the present work, firstly, the enthalpy probe technique, a well-established plasma diagnostic tool for high pressure thermal plasma, has been applied to characterize the inductively coupled Ar-H2 thermal RF plasma used for alumina nanoparticle synthesis at different operating conditions. The plasma enthalpy has been determined, and the plasma temperature under a given gas pressure could subsequently be calculated assuming local thermodynamic equilibrium (LTE), once the gas composition has been determined by mass spectrometry. The gas velocity of the plasma jet could also be obtained by using the Bernoulli equation and stagnation pressure. Secondly, the influence of flow rates of central gas and precursor feed rates on the Al2O3 precursor evaporation has been investigated by process monitoring. Optical emission spectroscopy (OES) and laser light extinction (LE) measurements have been carried out to monitor in-situ the injection of the alumina precursors and to obtain information on the ongoing precursor evaporation. The emission line intensities of the aluminum vapor were used to determine the dependence of process parameters on precursor evaporation. Furthermore, the laser light extinction was used to calculate the number density of non plasma-treated powders, from which the number fraction of evaporated powders could be deduced. The size and morphology of the synthesized particles have been characterized ex-situ. Finally, the influence of quenching conditions on the condensation and formation of the nano-sized alumina particle has been studied by process analysis of the gas phase reactions of alumina. Optical emission spectroscopic was performed at different axial positions to monitor the gas phase reactions of the vaporized particles. Axial profiles of the emission line intensities of the main vapor species of alumina, Al(g) and AlO(g), were compared with the results of the enthalpy probe measurements and the thermal decomposition reactions of alumina found in literature. The results allow explanation of the alumina reactions in the thermal plasma as well as gives indications for the optimal axial position of the quenching gas injection. In addition to the determination of the optimal quenching position, the influence of the flow rates of quenching gas on the nanoparticle formation has been investigated. The synthesis of the submicron alumina using vapor-condensation principle predominantly leads to various thermodynamically metastable crystal structures called transition alumina. Therefore, the occurrence of different transition aluminas is discussed with regards to the flow rates of the quenching gas. The results presented in this thesis show that the synthesis process of the nanoparticle can be described on the basis of experimental results and basic information on the powder materials. The explanation of the influence of process parameters on powder evaporation and nanoparticle formation shows the usefulness of the in-situ plasma process monitoring, and the large potential for optimizing and controlling the nanopowder synthesis process in an inductively coupled RF thermal plasma.

EPFL2006

Electronic and vibrational spectroscopy of cold protonated amino acids in the gas phase

Sébastien Mercier

In this thesis report, we describe a novel home-built instrument designed to study the spectroscopy of biomolecular ions in the gas phase and at low temperature, and we present the first experimental results obtained on protonated aromatic amino acids. The apparatus consists in a tandem mass spectrometer equipped with a nanospray ion source and a cryocooled 22-pole ion trap. The charged species produced by the source traverse a first quadrupole mass filter and mass-selected ions are then accumulated and thermalized in the ion trap, where their vibrational temperature can be lowered to ~ 10 K. Two setups allow us to generate ultraviolet and infrared laser light to spectroscopically probe the trapped ions by photodissociation. The resulting fragment ions are finally released from the trap and mass analyzed by a second quadrupole spectrometer. Photodissociation electronic spectra have been measured for the protonated amino acids tryptophan (TrpH+) and tyrosine (TyrH+), as well as for hydrated complexes of the former. The spectrum of TyrH+ exhibits sharp, fully resolved vibronic transitions, which attests to the low temperature of the ions. In contrast, that of tryptophan shows a broad absorption band that covers several hundreds of wavenumbers. This has been attributed to lifetime broadening related to an ultrafast (< ~ 100 fs) deactivation mechanism of the TrpH+ S1(ππ*) excited state. This phenomenon is caused by a strong coupling between this ππ* state and a nearby dissociative state of πσ* character mainly localized on the charged ammonium group. Solvation of TrpH+ by only two water molecules is sufficient to significantly destabilize the σ* orbitals and reduce that coupling, which results in a longer excited-state lifetime and a fully resolved electronic spectrum. These interpretations are supported by ab initio and density functional theory (DFT) calculations. Infrared-ultraviolet double resonance spectroscopic techniques have been successfully implemented to identify the different conformers of TyrH+ and doubly hydrated protonated tryptophan (TrpH+·W2) by measuring conformer-specific vibrational and electronic spectra for each of them. By comparison with the results of DFT geometry and harmonic frequency calculations, we could confidently determine the structures of the TyrH+ conformers and suggest possible assignments for those of TrpH+·W2. Two important stabilizing interactions could be inferred from these results: (i) the attraction between the charged ammonium group and the π-electron cloud of the aromatic ring; (ii) the hydrogen-bonding interaction of a donor water molecule to the acceptor indole ring of TrpH+·W2.

EPFL2008

Spectroscopic studies of cold, gas-phase biomolecular ions

Thomas Rizzo

While the marriage of mass spectrometry and laser spectroscopy is not new, developments over the last few years in this relationship have opened up new horizons for the spectroscopic study of biological molecules. The combination of electrospray ionization for producing large biological molecules in the gas phase together with cooled ion traps and multiple- resonance laser schemes are allowing spectroscopic investigation of individual conformations of peptides with more than a dozen amino acids. Highly resolved infrared spectra of single conformations of such species provide important benchmarks for testing the accuracy of theoretical calculations. This review presents a number of techniques employed in our laboratory and in others for measuring the spectroscopy of cold, gas-phase protonated peptides. We show examples that demonstrate the power of these techniques and evaluate their extension to still larger biological molecules.