The Internal Energy of Neutral Ethylene Glycol Molecules Created in the Laser Vaporization of Aerosol Particles

The Distant Double Bond Determines the Fate of the Carboxylic Group in the Dissociative Photoionization of Oleic Acid

The valence threshold photoionization of oleic acid has been studied using synchrotron VUV radiation and imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. An oleic acid aerosol beam was impacted on a copper thermodesorber, heated to 130 °C, to evaporate the particles quantitatively. Upon threshold photoionization, oleic acid produces the intact parent ion first, followed by dehydration at higher energies. Starting at ca. 10 eV, a large number of fragment ions slowly rise suggesting several fragmentation coordinates with quasi-degenerate activation energies. However, water loss is the dominant low-energy dissociation channel, and it is shown to be closely related to the unsaturated carbon chain. In the lowest-barrier process, one of the four allylic hydrogen atoms is transferred to the carboxyl group to form the leaving water molecule and a cyclic ketone fragment ion. A statistical model to analyze the breakdown diagram and measured rate constants yields a 0 K appearance energy of 9.77 eV, which can be compared with the density functional theory result of 9.19 eV. Alternative H-transfer steps yielding a terminal C=O group are ruled out based on energetics and kinetics arguments. Some of the previous photoionization mass spectrometric studies also reported 2 amu and 26 amu loss fragment ions, corresponding to hydrogen and acetylene loss. We could not identify such peaks in the mass spectrum of oleic acid.

VUV photodynamics and chiral asymmetry in the photoionization of gas phase alanine enantiomers

The valence shell photoionization of the simplest proteinaceous chiral amino acid, alanine, is investigated over the vacuum ultraviolet region from its ionization threshold up to 18 eV. Tunable and variable polarization synchrotron radiation was coupled to a double imaging photoelectron/photoion coincidence (i(2)PEPICO) spectrometer to produce mass-selected threshold photoelectron spectra and derive the state-selected fragmentation channels. The photoelectron circular dichroism (PECD), an orbital-sensitive, conformer-dependent chiroptical effect, was also recorded at various photon energies and compared to continuum multiple scattering calculations. Two complementary vaporization methods-aerosol thermodesorption and a resistively heated sample oven coupled to an adiabatic expansion-were applied to promote pure enantiomers of alanine into the gas phase, yielding neutral alanine with different internal energy distributions. A comparison of the photoelectron spectroscopy, fragmentation, and dichroism measured for each of the vaporization methods was rationalized in terms of internal energy and conformer populations and supported by theoretical calculations. The analytical potential of the so-called PECD-PICO detection technique-where the electron spectroscopy and circular dichroism can be obtained as a function of mass and ion translational energy-is underlined and applied to characterize the origin of the various species found in the experimental mass spectra. Finally, the PECD findings are discussed within an astrochemical context, and possible implications regarding the origin of biomolecular asymmetry are identified.

Desorption dynamics, internal energies, and imaging of organic molecules from surfaces with laser desorption and vacuum ultraviolet (VUV) photoionization

There is enormous interest in visualizing the chemical composition of organic material that comprises our world. A convenient method to obtain molecular information with high spatial resolution is imaging mass spectrometry. However, the internal energy deposited within molecules upon transfer to the gas phase from a surface can lead to increased fragmentation and to complications in analysis of mass spectra. Here it is shown that in laser desorption with postionization by tunable vacuum ultraviolet (VUV) radiation, the internal energy gained during laser desorption leads to minimal fragmentation of DNA bases. The internal temperature of laser-desorbed triacontane molecules approaches 670 K, whereas the internal temperature of thymine is 800 K. A synchrotron-based VUV postionization technique for determining translational temperatures reveals that biomolecules have translational temperatures in the range of 216-346 K. The observed low translational temperatures as well as their decrease with increased desorption laser power is explained by collisional cooling. An example of imaging mass spectrometry on an organic polymer by using laser-desorption VUV postionization shows 5 μm feature details while using a 30 μm laser spot size and 7 ns pulse duration. Applications of laser-desorption postionization to the analysis of cellulose, lignin, and humic acids are briefly discussed.

Thermal vaporization of biological nanoparticles: fragment-free vacuum ultraviolet photoionization mass spectra of tryptophan, phenylalanine-glycine-glycine, and beta-carotene

A simple, new way to introduce fragile biomolecules into the gas phase via thermal vaporization of nanoparticles is described. The general utility of this technique for the study of biomolecules is demonstrated by coupling this source to tunable synchrotron vacuum ultraviolet radiation. Fragment-free photoionization mass spectra of tryptophan, phenylalanine-glycine-glycine, and beta-carotene are detected with signal-to-noise ratios exceeding 100. The 8.0 eV photoionization mass spectrum of tryptophan nanoparticles vaporized at 373 K is dominated by a single parent ion peak that exhibits a 20-fold enhancement over the methylene indole fragment ion. The degree of dissociative photoionization of tryptophan can be precisely controlled either by the thermal energy imparted into the neutral tryptophan molecule or by the energy of the ionizing photon. The results reveal how approximately 0.5 eV changes in internal energy affect both the photoionization mass spectrum of tryptophan and the appearance energy of the daughter ion fragments. This method allows the ionization energies of glycine (9.3 +/- 0.1 eV), tryptophan (7.3 +/- 0.2 eV), phenylalanine (8.6 +/- 0.1 eV), phenylalanine-glycine-glycine (9.1 +/- 0.1 eV), and beta-carotene (<7.0 eV) molecules to be determined directly from the photoionization efficiency spectra.

How to make big molecules fly out of liquid water: applications, features and physics of laser assisted liquid phase dispersion mass spectrometry

Applications, features, and mechanistic details of laser assisted liquid phase dispersion mass spectrometry are highlighted and discussed. It has been used in the past to directly isolate charged molecular aggregates from the liquid phase and to determine their molecular weight employing sensitive time-of-flight mass spectrometry. The liquid matrix in this MALDI (matrix assisted laser desorption and ionization) type approach consists of a 10 microm diameter free liquid filament in vacuum (or a free droplet) which is excited with a focused infrared laser pulse tuned to match the absorption frequency of the OH-stretch vibration of bulk water near 2.8 microm. Due to these features we will refer to the approach as free liquid matrix assisted laser dispersion of ions or ionic aggregates (IR-FL-MALDI), although also LILBID ("laser induced liquid beam (bead) desorption and ionization") has been proposed early as a descriptive acronym for the technique and may be used alternatively. Low-charge-state macromolecular adducts are isolated in the gas phase from solution via a yet poorly characterized mechanism which sensitively depends upon the laser intensity and wavelength, and after the gentle liquid-to-vacuum transfer the aggregates are analyzed via time-of-flight (TOF) mass spectrometry (MS). Possible mechanisms for the isolation and charging of biomolecules directly from liquid solution are discussed in the present contribution. Recent technical advances such as minimizing the sample consumption, strategies for high throughput mass spectrometry, and coupling of liquid beam MS with HPLC will be highlighted as well. An interesting feature of IR-FL-MALDI is what we call the linear response, i.e., a surprising linearity of the gas phase mass signal on the solution concentration over many orders of magnitude for a large number of biomolecular systems as well as ions. Due to these features the approach may be regarded as a true solution probing spectroscopy, which enables elegant biokinetic studies. Several experiments in which time resolved IR-FL-MALDI-MS has recently been employed successfully are given. A particular highlight is the possibility to quantitatively detect oxidation states in solution, which clearly distinguishes the present approach from other established MS source concepts. Due to the good matrix tolerance also proteins in complex mixtures can be monitored quantitatively.

Product Screening of Fast Reactions in IR-Laser-Heated Liquid Water Filaments in a Vacuum by Mass Spectrometry

In the present article a novel approach for rapid product screening of fast reactions in IR-laser-heated liquid microbeams in a vacuum is highlighted. From absorbed energies, a shock wave analysis, high-speed laser stroboscopy, and thermodynamic data of high-temperature water the enthalpy, temperature, density, pressure, and the reaction time window for the hot water filament could be characterized. The experimental conditions (30 kbar, 1750 K, density approximately 1 g/cm3) present during the lifetime of the filament (20-30 ns) were extreme and provided a unique environment for high-temperature water chemistry. For the probe of the reaction products liquid beam desorption mass spectrometry was employed. A decisive feature of the technique is that ionic species, as well as neutral products and intermediates may be detected (neutrals as protonated aggregates) via time-of-flight mass spectrometry without any additional ionization laser. After the explosive disintegration of the superheated beam, high-temperature water reactions are efficiently quenched via expansion and evaporative cooling. For first exploratory experiments for chemistry in ultrahigh-temperature, -pressure and -density water, we have chosen resorcinol as a benchmark system, simple enough and well studied in high-temperature water environments much below 1000 K. Contrary to oxidation reactions usually present under less extreme and dense supercritical conditions, we have observed hydration and little H-atom abstraction during the narrow time window of the experiment. Small amounts of radicals but no ionic intermediates other than simple proton adducts were detected. The experimental findings are discussed in terms of the energetic and dense environment and the small time window for reaction, and they provide firm evidence for additional thermal reaction channels in extreme molecular environments.

Time-Resolved Micro Liquid Desorption Mass Spectrometry: Mechanism, Features, and Kinetic Applications

Liquid water beam desorption mass spectrometry is an intriguing technique to isolate charged molecular aggregates directly from the liquid phase and to analyze them employing sensitive mass spectrometry. The liquid phase in this approach consists of a 10 µm diameter free liquid filament in vacuum which is irradiated by a focussed infrared laser pulse resonant with the OH-stretch vibration of bulk water. Depending upon the laser wavelength, charged (e.g. protonated) macromolecules are isolated from solution through a still poorly characterized mechanism. After the gentle liquid-to-vacuum transfer the low-charge-state aggregates are analyzed using time-of-flight mass spectrometry. A recent variant of the technique uses high performance liquid chromatography valves for local liquid injections of samples in the liquid carrier beam, which enables very low sample consumption and high speed sample analysis. In this review we summarize recent work to characterize the ‘desorption’ or ion isolation mechanism in this type of experiment. A decisive and interesting feature of micro liquid beam desorption mass spectrometry is that — under certain conditions — the gas-phase mass signal for a large number of small as well as supramolecular systems displays a surprisingly linear response on the solution concentration over many orders of magnitude, even for mixtures and complex body fluids. This feature and the all-liquid state nature of the technique makes this technique a solution-type spectroscopy that enables real kinetic studies involving (bio)polymers in solution without the need for internal standards. Two applications of the technique monitoring enzyme digestion of proteins and protein aggregation of an amyloid model system are highlighted, both displaying its potential for monitoring biokinetics in solution.

Laser Ablation of Imidazolium Based Ionic Liquids

Time-of-flight mass spectrometry has been used to investigate the IR ablation of several ionic liquid imidazolium salts of the form R(1)R(2)Iium X (R(1) = methyl; R(2) = methyl, ethyl, butyl, and hexyl; X = Cl(-), NO(3)(-), and CH(3)SO(4)(-)). The ablated ionic species were analyzed by time-of-flight mass spectrometry using pulsed extraction, and neutral species were detected using vacuum UV photoionization at 10.5 eV. The results demonstrate that at least 99% of the ablated material is removed in the form of nano- or microdroplets consisting of intact ionic liquid. Approximately 1% is ejected as imidazole molecules (R(1)R(2)Im) produced through the elimination of HCl, and about 0.1% of the material is ejected in the form of single salt molecules of R(1)R(2)Iium X. A chemical thermometer was used to measure the internal temperature (475 +/- 25 K) of the ablated vapor plume.

Application of Infrared Laser Desorption Vacuum-UV Single-Photon Ionization Mass Spectrometry for Analysis of Organic Compounds from Particulate Matter Filter Samples

A new built instrument suitable for laser desorption-single photon ionization time-of-flight mass spectrometry (LD-SPI-TOFMS) with use of Vacuum-UV photons with a wavelength of 118 nm was used for the analysis of organic compounds. Fragmentation-free analysis of a variety of substances was achieved for desorption experiments with pure compounds desorbed from quartz glass filters applying low desorption energies. It was further found that the rate of fragmentation is strongly dependent on the desorption energy. Matrix effects were investigated by desorption experiments utilizing soot spiked with several organic compounds.The characteristics of the desorption process are assessed in more detail and the impact on the analysis of ambient particulate matter (PM) samples on filters are discussed. First results obtained from the application of the technique to the analysis of organic compounds from ambient PM are presented. Furthermore, possibilities of future developments of the method, in particular for analysis of ambient PM, are discussed.

On-Line Analysis of Organic Components in Fine and Ultrafine Particles by Photoionization Aerosol Mass Spectrometry

A new method, photoionization aerosol mass spectrometry (PIAMS), is described for real-time analysis of organic components in airborne particles below approximately 300 nm in diameter. Particles are focused through an aerodynamic lens assembly into the mass spectrometer where they are collected on a probe in the source region. After a sufficient amount of sample has been collected, the probe is irradiated with a pulsed infrared laser beam to vaporize organic components, which are then softly ionized with coherent vacuum ultraviolet radiation at 118 nm (10.5 eV). Since the photon energy is close to the ionization energies of most organic compounds, fragmentation is minimized. Both aliphatic and aromatic compounds of atmospheric relevance are detected and quantified in the low- to midpicogram range. The photoionization signal intensity increases linearly with the amount of material sampled and is independent of particle size. The fragmentation induced by laser desorption is greater than that observed with thermal vaporization, suggesting that the internal energy imparted by the former is greater. Although some molecular fragmentation is observed, mass spectra from common sources of ambient organic aerosol are distinguishable and consistent with previous off-line measurements by gas chromatography/mass spectrometry. These results illustrate the potential of PIAMS for molecular characterization of organic aerosols in ambient and smog chamber measurements.