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.

Gas chromatography-mass spectrometry with supersonic molecular beams.

The measurements have been conducted in the spectral range from 600 to 2000 A. Integrated oscillator strengths were determined for a number of strong Rydberg transitions above 1200 A. From the photoionization curve the first adiabatic ionization potential was found to be 10.87 plus or minus 0.01 eV. As an aid in interpreting the absorption spectrum, theoretical calculations were made using a single-configuration self-consistent field procedure for the Rydberg states and a model which included mixing between the Rydberg and valence states.

Photoionization and Absorption Spectrum of Formaldehyde in the Vacuum Ultraviolet

The appearance energy of the CH{sub 2}{sup +} fragment from CH{sub 2}CO has been carefully remeasured and fitted by a model curve, producing EA{sub 0}(CH{sub 2}{sup +}/CH{sub 2}CO) = 13743 {+-} 0005 eV This value can be sequentially propagated through selected thermochemical cycles to yield individual values for El(CH{sub 2}), D{sub 0}(H-CH{sub 2}), {Delta}H{sup o}{sub f 0}(CH{sub 2}CO) A set of values with a statistically larger weight is produced by analyzing a local thermochemical network, which combines the present measurement with thirteen other experimental determinations from the literature and encompasses the enthalpies of formation of CH{sub 3}, CH{sub 3}{sup +}, CH{sub 2}, CH{sub 2}{sup +}, and CH{sub 2}CO The recommended simultaneously adjusted thermochemical values are: {Delta}H{sup o}{sub f 0}(CH{sub 3}) = 3586 {+-} 007 kcal/mol (3505 {+-} 007 kcal/mol at 298 K), {Delta}H{sup o}{sub f 0}(CH{sub 3}{sup +}) = 26273 {+-} 006 kcal/mol (26183 {+-} 006 kcal/mol at 298 K), {Delta}H{sup o}{sub f 0}(CH{sub 2}) = 9318 {+-} 020 kcal/mol (9331 {+-} 020 kcal/mol at 298 K), {Delta}H{sup o}{sub f 0}(CH{sub 2}{sup +}) = 33292 {+-} 019 kcal/mol (33304 {+-} 019 kcal/mol at 298 K), {Delta}H{sup o}{sub f 0}(CH{sub 2}CO) = -1110 {+-} 021 kcal/mol (-1185 {+-} 021 kcal/mol atmore » 298 K), as well as D{sub 0}(H{sub 3}C-H) = 10342 {+-} 003 kcal/mol (10499 {+-} 003 kcal/mol at 298 K), D{sub 0}(H{sub 2}C-H) = 10895 {+-} 020 kcal/mol (11035 {+-} 020 kcal/mol at 298 K), D{sub 0}(H{sub 2}C{double_bond}CO) = 7708 {+-} 002 kcal/mol (7873 {+-} 002 kcal/mol at 298 K), El(CH{sub 3}) = 93830 {+-} 00005 eV, and El(CH{sub 2}) = 103962 {+-} 00036 eV These values are in excellent agreement with current and several previous experimental measurements The recommended enthalpy of formation of CH{sub 2} implies that the reaction of singlet methylene with water is essentially thermoneutral (to within {+-}02 kcal/mol) at 0 and 298 K, and slightly endothermic (05 {+-} 02 kcal/mol) at 1000 K« less

Ionization Energy of Methylene Revisited: Improved Values for the Enthalpy of Formation of CH2 and the Bond Dissociation Energy of CH3 via Simultaneous Solution of the Local Thermochemical Network

The photoionization yields of CH3Cl, CH2Cl2, CHCl3, CCl4, and CBr4 have been measured in the 10–13.5 eV energy range. Ionization potentials and fragment appearance potentials derived from a detailed study of the hot band contribution at 300°K have been obtained. Although CCl4+ is apparently unstable, we have found that CBr4+ is stable. Fragmentation of the chlorinated and brominated methanes in the energy range studied appears to be limited to halogen loss, even though hydrogen loss is in certain cases energetically favored. An argument based on the symmetry of the fragments and the transition state producing these fragments is proposed to account for this observation.

Photoionization study of the ionization potentials and fragmentation paths of the chlorinated methanes and carbon tetrabromide

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...

Thermal Vaporization of Biological Nanoparticles: Fragment-Free Vacuum Ultraviolet Photoionization Mass Spectra of Tryptophan, Phenylalanine−Glycine−Glycine, and β-Carotene

The effect of thermal energy on the shape of photoionization‐efficiency curves is analyzed. Several methods for determining thresholds for dethermalized ionization‐efficiency curves are presented. The assumptions and approximations used are explicitly stated for each method. The effect of thermal energy on the photoionization‐efficiency curves of some alkanes is analyzed and the results are in fair agreement with the hypothesis that thermal energy is fully effective in the dissociation of the parent ions.

W. A. Chupka

Papers

Effect of Thermal Energy on Ionization Efficiency Curves of Fragment Ions