Pulsed supersonic beams with nucleobases

The dissolution of the primary nucleobases in supercritical fluids has been investigated using pulsed molecular beam mass spectrometry. Due to the low critical temperatures of ethylene and carbon dioxide, their adiabatic jet expansion permits transferring thermally sensitive solutes into the gas phase. This feature is particularly attractive for pharmaceutical and biomedical applications. In this study, adenine, guanine, cytosine, thymine, and uracil have been dissolved in supercritical ethylene with a few percent of ethanol as cosolvent. At source temperatures of 313?K, these solutions have been expanded from supercritical pressures into high vacuum using a customized pulsed nozzle. A mass spectrometer was used to monitor the relative amounts of solute, solvent, and cosolvent in the supersonic beam. The results suggest a paramount influence of the cosolvent.     

Characterization of supersonic beams of polyatomic molecules

Supersonic free jets and beams have been characterized in terms of the terminal Mach number, velocity spread, translational temperature and the average value of specific heat ratio (γ) pertinent to supersonic expansion. It has been found that for a polyatomic molecule, the average value of γ, used to describe the flow, depends upon the stagnation conditions. Vibrational relaxation of SF₆ as a representative of polyatomic molecules has been investigated using the “sudden freeze” model of Knuth and the relaxation rate equation model of Quah. Though the “sudden freeze” model predicts the experimental terminal vibrational temperature of Luijks et al., it overestimates cooling close to the nozzle exit. Quah's model shows that the vibrational relaxation cross section (σ_v) increases with decrease in temperature, contrary to the prediction of the Landau—Teller relaxation mechanism. This has been attributed to enhanced efficiency of vibrational relaxation at lower temperature due to facile exchange of internal energy with translational energy through the formation of quasi-resonant and van der Waals complexes.     

Generation of intense secondary pulsed molecular beams of high kinetic energy controllable by a powerful IR laser

A method for generation of intense secondary pulsed molecular beams and beams of radicals of high kinetic energy controllable by a powerful IR laser is described. A pressure shock (shock wave) is used as a source of secondary beams. The pressure shock is formed in interaction between an intense pulsed supersonic molecular beam (or flow) and a solid surface. The characteristics of the secondary beams were studied. Their intensities and the degree of gas cooling in them were shown to be comparable with the corresponding characteristics of the unperturbed primary beam. The acceleration of molecules in the secondary beam is achieved due to vibrational excitation of them by high-power IR laser pulse in the pressure shock and subsequent vibrational to translational (V–T) relaxation, which occurs when a gas expands through the orifice into a vacuum. Intense [10²⁰ molecules/(sr s)] beams of SF₆ and CF₃I molecules with kinetic energies approximately equal to 1.5 and 1.2 eV, respectively, were generated in the absence of carrier gases. The SF₆ molecular beams with kinetic energies approximately from 2.5 to 2.7 eV with carrier gases H₂, He and CH₄ (SF₆/carriergas=1/10) were obtained. The possibility of generation of intense beams of cold radicals by this method is demonstrated. The intense beams of cold and accelerated CF₃ radicals were generated when the CF₃I molecules in the shock were dissociated by high-power CO₂ laser radiation. The spectral and energetic characteristics of acceleration of SF₆ and CF₃I molecules in the secondary beams were studied. The optimal conditions were found for obtaining high-energy molecules.     

Gas chromatography-mass spectrometry with supersonic molecular beams

Gas chromatography-mass spectrometry (GC-MS) with supersonic molecular beams (SMBs) (also named Supersonic GC-MS) is based on GC and MS interface with SMBs and on the electron ionization (EI) of vibrationally cold analytes in the SMBs (cold EI) in a fly-through ion source. This ion source is inherently inert and further characterized by fast response and vacuum background filtration capability. The same ion source offers three modes of ionization including cold EI, classical EI and cluster chemical ionization (CI). Cold EI, as a main mode, provides enhanced molecular ions combined with an effective library sample identification, which is supplemented and complemented by a powerful isotope abundance analysis method and software. The range of low-volatility and thermally labile compounds amenable for analysis is significantly increased owing to the use of the contact-free, fly-through ion source and the ability to lower sample elution temperatures through the use of high column carrier gas flow rates. Effective, fast GC-MS is enabled particularly owing to the possible use of high column flow rates and improved system selectivity in view of the enhancement of the molecular ion. This fast GC-MS with SMB can be further improved via the added selectivity of MS-MS, which by itself benefits from the enhancement of the molecular ion, the most suitable parent ion for MS-MS. Supersonic GC-MS is characterized by low limits of detection (LOD), and its sensitivity is superior to that of standard GC-MS, particularly for samples that are hard for analysis. The GC separation of the Supersonic GC-MS can be improved with pulsed flow modulation (PFM) GC x GC-MS. Electron ionization LC-MS with SMB can also be combined with the Supersonic GC-MS, with fast and easy switching between these two modes of operation.     

Relaxation and spectroscopy of CaCl in seeded supersonic beams

The rotational and vibrational relaxation of a calcium-monochloride beam, seeded in He, Ar and Kr has been investigated by laser-induced fluorescence. The rotational distributions cannot be described by a single temperature. A two-parameter formula is given which represents the measured distributions and which is characterized by local temperatures from 30 to 200 K depending on the rotational states and the expansion conditions with source temperatures of T₀ = 1320 K and T₀ = 1520 K. The effective cooling increases from He to Ar as carrier gases. In contrast, the vibrational distributions could be represented by Boltzmann distributions with temperatures of 1400 to 589 K. The order of vibrational cooling goes from Ar < He < Kr. Parts of the CaCl X-B spectrum have been recorded with high resolution. The attempt to identify the measured lines reveals that the best molecular constants for this system do not reproduce all lines of Ca³⁵Cl and especially not those of Ca³⁷Cl within the needed accuracy. The spin-rotational constant of the B²Σ state is determined to γ₀₀ = ?0.06513 (9) with a 2σ uncertainty.     

Deceleration of supersonic beams using inhomogeneous electric and magnetic fields

Methods for the production of cold atomic and molecular samples relying on the deceleration of pulsed supersonic beams are described and a review of the corresponding literature is presented. These methods include multistage Stark deceleration, multistage Zeeman deceleration, and Rydberg-Stark deceleration. Recent applications of the cold samples produced with these techniques are summarized.     

Liquid chromatography mass spectrometry with supersonic molecular beams

A new approach for liquid chromatography mass spectrometry (LC-MS) is described, based on achieving soft thermal vaporization followed by supersonic expansion and direct sample compound ionization, while in a supersonic molecular beam (SMB). The soft molecular vaporization step utilizes spray formation that is continued by fast thermal vaporization inside a channel supersonic nozzle, followed by ultrafast supercooling in a supersonic expansion. The short time (several microseconds) spent by the vaporized compound in the heated nozzle prior to its expansion cooling may result in incomplete vibrational equilibrium and thus reduced degree of dissociation. In addition, even if vibrational equilibrium at the nozzle temperature is obtained, the sample compounds have significantly reduced time for their dissociation, which is thus further minimized (kinetic consideration). As soon as the molecules expand and form a SMB, they are supercooled and any further dissociation is avoided. While in the SMB, the sample molecules can be ionized either by electron ionization as described in this paper or by hyperthermal surface ionization. The major goal of this method is to obtain high quality library searchable electron ionization mass spectra, for a broad range of thermally labile compounds, with higher sensitivity than that achievable by particle beam LC-MS. The soft thermal vaporization nozzle is described and mass spectral results with corticosterone are demonstrated. The potential advantageous features of this new method are discussed.     

Isotopically selective two-photon ionization of aniline in supersonic beams

Tunable laser two-photon ionization of aniline cooled in supersonic expansions combined with TOF mass spectrometry reveal an isotopic shift of the vibronic origin at 2938 Å (ππ¹; ¹B₂←¹A₁ transition). The shift (+4.6 cm^?1) is smaller than the rotational bandwidth and would be unobservable by laser-induced fluorescence.     

Aerodynamical acceleration and rotational-vibrational temperatures in seeded supersonic molecular beams

The time-of-flight (TOF) technique was used to study the aerodynamical acceleration in seeded supersonic molecular beams of heavy molecules and light seeding gas. We also studied the correlation between the degree of aerodynamic acceleration achieved, and rotational-vibrational temperatures as measured using the laser-induced fluorescence (LIF) technique. The velocity slip (difference) between helium and hydrogen carrier gases and iodine and aniline heavy molecules was determined in free-jet expansion by TOF measurements and compared with rotational temperatures measured by LIF. The helium translational temperature was found to be abnormally high and dependent on teh heavy-molecule concentration, even at concentration as low as 400 ppm. In the case of iodine it was found that the rotational degrees of freedom were equilibrated with the helium or hydrogen seeding gas translational and slip temperatures, although this temperature was more than an order of magnitude higher than theoretical predictions obtained for the pure-gas expansion. In aniline, the rotational temperature is found to be higher than the gas-dynamic temperature and rotational relaxation is incomplete. The heavy-molecule kinetic energy increases linearly with the light-gas pressure up to = 50% of its maximum available kinetic energy. The possibility of making accurate heavy-molecule kinetic-energy measurements using the LIF technique is discussed. It is claimed that the existence of velocity slip can largely effect theoretical calculations concerning vibrational and rotational relaxation in seeded supersonic beams.     

Electron impact mass spectrometry of alkanes in supersonic molecular beams

The electron impact mass spectrometry of straight chain alkanes C₈H₁₈-C₄₀H₈₂, squalane, methylstearate, 1-chlorohexadecane, 1-bromohexadecane, and dioctylphthalate was studied by sampling them with supersonic molecular beams. A fly-through Brink-type electron impact ion source was used, utilizing a vacuum background ion filtration technique based on differences between the kinetic energy of the supersonic beam species and that of thermal molecules. The 70-eV electron impact mass spectra of all the alkanes were characterized by a pronounced or dominant molecular weight peak together with all the fragment ions normally exhibited by the standard thermal 70-eV EI mass spectra. In contrast, the NIST library of most of these molecules did not show any molecular weight peak. By eliminating tile intramolecular thermal vibrational energy we gained control over the degree of molecular ion fragmentation by the electron energy. At an electron energy of 18 eV the molecular ion dissociation was further reduced considerably, with only a small absolute reduction in the peak height by less than a factor of 2. The effect of vibrational cooling increased with the molecular size and number of atoms. Pronounced differences were observed between the mass spectra of the straight chain triacontane and its branched isomer squalane. Similar mass spectra of octacosane (C₂₈H₅₈) achieved with 70-eV EI in a supersonic molecular beam were obtained with a magnetic sector mass spectrometer by using an electron energy of 14 eV and an ion source temperature of 150 °C. However, this ion source temperature precluded the gas chromatography-mass spectrometry (GC-MS) of octacosane. The GC-MS of alkanes was studied with an ion trap gas chromatograph-mass spectrometer at an ion source temperature of 230 °C. Thermal peak tailing was observed for C₂₀H₄₂ and heavier alkanes, whereas for C₂₈H₅₈ and heavier alkanes the severe peak tailing made quantitative GC-MS impractical. In contrast, no peak tailing existed even with C₄₀H₈₂ for GC-MS in supersonic molecular beams. The minimum detected amount of eicosane (C₂₀, H₄₂) was shown to be 60 fg. This was demonstrated by using single ion monitoring with the quadrupole mass analyzer tuned to the molecular weight peak of 282 u. The coupling of electron impact mass spectrometry in supersonic molecular beams with hyperthermal surface ionization and a fast GC-MS inlet is briefly discussed.     

Generation and detection of alkyl peroxy radicals in a supersonic jet expansion

Alkyl peroxy radicals are synthesized in a supersonic jet expansion by the initial production of alkyl radicals and subsequent reaction with molecular oxygen. Parent ions CH3OO+/CD3OO+ are observed employing vacuum ultraviolet (VUV) single photon ionizationtime-of-flight mass spectroscopy (TOFMS). Employing infrared (IR) + VUV photofragmentation detected spectroscopy, rotationally resolved infrared spectra of jet-cooled CH3OO and CD3OO radicals are recorded for the A 2A' <-- X 2A" transition by scanning the IR laser frequency while monitoring the CH3 + and CD3 + ion signals generated by the VUV laser. The band origins of the A 2A'<--X 2A" transition for CH3OO and CD3OO are identified at 7381 and 7371 cm(-1), respectively. Rotational simulation for the CH3OO and CD3OO 0(0) 0 transitions of A<--X yields a rotational temperature for these radicals of approximately 30 K. With the aid of ab initio calculations, two and five vibrational modes for the A 2A' excited electronic state are assigned for CH3OO and CD3OO radicals, respectively. Both experimental and theoretical results suggest that the ground electronic state of the ions of ethyl and propyl peroxy radicals are not stable although their ionization energies (IE) are less than 10.5 eV. The C2H5OO+/C3H7OO+ cations can readily decompose to C2H5 +/C3H7 + and O2. This is partially responsible for the inability of IR+VUV photofragmentation spectroscopy to detect the near IR A<--X electronic transition for these radicals.     

Laser desorption fast gas chromatography–Mass spectrometry in supersonic molecular beams

A novel method for fast analysis is presented. It is based on laser desorption injection followed by fast gas chromatography-mass spectrometry (GC-MS) in supersonic molecular beams. The sample was placed in an open air or purged laser desorption compartment, held at atmospheric pressure and near room temperature conditions. Desorption was performed with a XeCl Excimer pulsed laser with pulse energy of typically 3 mJ on the surface. About 20 pulses at 50 Hz were applied for sample injection, resulting in about 0.4 s injection time and one or a few micrograms sample vapor or small particles. The laser desorbed sample was further thermally vaporized at a heated frit glass filter located at the fast GC inlet. Ultrafast GC separation and quantification was achieved with a 50-cm-long megabore column operated with a high carrier gas flow rate of up to 240 mL/min. The high carrier gas flow rate provided effective and efficient entrainment of the laser desorbed species in the sweeping gas. Following the fast GC separation, the sample was analyzed by mass spectrometry in supersonic molecular beams. Both electron ionization and hyperthermal surface ionization were employed for enhanced selectivity and sensitivity. Typical laser desorption analysis time was under 10 s. The laser desorption fast GC-MS was studied and demonstrated with the following sample/matrices combinations, all without sample preparation or extraction: (a) traces of dioctylphthalate plasticizer oil on stainless steel surface and the efficiency of its cleaning; (b) the detection of methylparathion and aldicarb pesticides on orange leaves; (c) water surface analysis for the presence of methylparathion pesticide; (d) caffeine analysis in regular and decaffeinated coffee powder; (e) paracetamol and codeine drug analysis in pain relieving drug tablets; (f) caffeine trace analysis in raw urine; (g) blood analysis for the presence of 1 ppm lidocaine drug. The features and advantages of the laser desorption fast GC-MS are demonstrated and discussed.     

Stopping paramagnetic supersonic beams: the advantage of a co-moving magnetic trap decelerator

The long standing goal of chemical physics is finding a convenient method to create slow and cold beams intense enough to observe chemical reactions in the temperature range of a few Kelvin. We present an extensive numerical analysis of our moving magnetic trap decelerator showing that a 3D confinement throughout the deceleration process enables deceleration of almost all paramagnetic particles within the original supersonic expansion to stopping velocities. We show that the phase space region containing the decelerating species is larger by two orders of magnitude as compared to other available deceleration methods.     

Generation of harmonic radiation by helium atoms in intense laser pulses

Measurements of natural radiative lifetimes in atomic tellurium in the 5p ³7p configuration are reported. Two-step laser excitation was applied on tellurium vapour in a cell that was differentially heated and pumped through a capillary. The decay of the fluorescence signal was recorded at different vapour pressures. The following four radiative lifetime values were obtained: 7p ⁵ P ₁: 128(11) ns, 7p ⁵ P ₂: 99(8) ns, 7p ³ P ₁: 82(10) ns and 7p ³ P ₂: 124(8) ns.     

Internal energy relaxation rates from seeded supersonic beams with a monatomic host gas

Terminal internal temperatures, T_i^∞, are related to relaxation rates of polyatomic seed species in free jet expansions involving monatomic host gases. Comparisons of numerical integration (NI) results with predictions of the often-used sudden freeze model (SFM) show good agreement if freezing is assumed to occur at T_i^∞(SFM) = 0.88 T_i^∞(NI).     

Pulsed flow modulation two-dimensional comprehensive gas chromatography-tandem mass spectrometry with supersonic molecular beams

Pulsed flow modulation (PFM) two-dimensional comprehensive gas chromatography (GC x GC) was combined with quadrupole-based mass spectrometry (MS) via a supersonic molecular beam (SMB) interface using a triple-quadrupole system as the base platform, which enabled tandem mass spectrometry (MS-MS). PFM is a simple GC x GC modulator that does not consume cryogenic gases while providing tunable second GC x GC column injection time for enabling the use of quadrupole-based mass spectrometry regardless its limited scanning speed. The 20-ml/min second column flow rate involved with PFM is handled, splitless, by the SMB interface without affecting the sensitivity. The combinations of PFM GC x GC-MS with SMB and PFM GC x GC-MS-MS with SMB were explored with the analysis of diazinon and permethrin in coriander. PFM GC x GC-MS with SMB is characterized by enhanced molecular ion and tailing-free fast ion source response time. It enables universal pesticide analysis with full scan and data analysis with reconstructed single ion monitoring on the enhanced molecular ion and another prominent high mass fragment ion. The elimination of the third fragment ion used in standard three ions method results in significantly reduced matrix interference. GC x GC-MS with SMB improves the GC separation, and thereby our ability for sample identification using libraries. GC-MS-MS with SMB provides better reduction (elimination) of matrix interference than GC x GC-MS. However, it is a target method, which is not always applicable. GC x GC-MS-MS does not seem to further reduce matrix interferences over GC-MS-MS and unlike GC x GC-MS, it is incompatible with library identification, but it is beneficial to have both GC x GC and MS-MS capabilities in the same system.     

Short-range interactions within molecular complexes formed in supersonic beams: structural effects and chiral discrimination

One- and two-color, mass-selected R2PI spectra of the S1<--S0 transitions in the bare chiral chromophore R-(+)-1-phenyl-1-propanol (R) and its complexes with a variety of alcoholic solvent molecules (solv), namely methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, S-(+)-2-butanol, R-(-)-2-butanol, 1-pentanol, S-(+)-2-pentanol, R-(-)-2-pentanol, and 3-pentanol, were recorded after a supersonic molecular beam expansion. Spectral analysis, coupled with theoretical calculations, indicate that several hydrogen-bonded [R.solv] conformers are present in the beam. The R2PI excitation spectra of [R.solv] are characterized by significant shifts of their band origin relative to that of bare R. The extent and direction of these spectral shifts depend on the structure and configuration of solv and are attributed to different short-range interactions in the ground and excited [R.solv] complexes. Measurement of the binding energies of [R.solv] in their neutral and ionic states points to a subtle balance between attractive (electrostatic and dispersive) and repulsive (steric) forces, which control the spectral features of the complexes and allow enantiomeric discrimination of chiral solv molecules.     

Shape analysis of the velocity distribution in supersonic Ar beams: Comparison between experiment and theory

The shape parameters of the velocity distribution for beam densityF(v) = C($\text{v}\text{α}$)² exp[?($\text{v}\text{?}\text{u}\text{α}$)²]

from a supersonic expansion of Ar are determined both experimentally via time-of-light(TOF) analysis and computationally by solving the Boltzmann equation for a radial flow field via the method of moments. TOF spectra are recorded by means of the detection of metastable Ar atoms. This technique eliminates velocity discrimination in the detection process. Significance test and a sensitivity analysis for the experimental and theoretical results are included. The agreement of measured and calculated shape parameters is very good. Small-angle scattering of particles, travelling with small relative velocity along the same stream line, is still effective beyond the transition region to free molecular flow and continues to modify the distribution function. A primitive model that correlates the shape parameters c₃ and c₄ with the terminal speed ratio and describes their variation with the distance from the nozzlee is developed.