The atmospheric pressure Penning ionization was applied to the analysis of atmospheric-pressure ambient gaseous samples. The metastable rare gas atoms (Rg*) were produced by a negative-mode corona discharge of reagent rare gas (Rg). The rare gas containing Rg* in the discharge cell flowed out into the ion source cell through a nozzle with an inner diameter of 1 mm as a jet stream. The jet stream acts as a gas-jet pump and the pressure of the ion source cell becomes somewhat lower than the atmospheric pressure. The sample gas at ambient atmospheric pressure was sucked into the reduced-pressure ion source cell through the stainless steel tube (10 cm in length, 1 mm in diameter). The sample gases introduced in the ion source cell are ionized by Penning ionization. This technique was found to be applicable to various gaseous samples such as aromatic compounds, oxygen- and nitrogen-containing compounds, etc. However, no ion signal could be detected for aliphatic hydrocarbons. This may be due to the much smaller ionization cross sections for these compounds than N2 and O2 molecules which are introduced into the ion source together with sample gases.
A method for determining the peroxide value (POV) of oxidized vegetable oil using electron ionization mass spectrometry (EI-MS) was developed. The range of POV measured by the method developed in this study is 0-100 meq/kg, which is a matter of interest for those involved in the vegetable oil industry. This method consists of three steps: the hydroperoxides in the sample oxidize triphenylphosphine (TPP), which produces triphenylphosphine oxide (TPPO); the TPPO concentration in the reaction solution is determined by EI-MS; and the POV of the sample is obtained from the calibration curve, which correlates the POV from the iodometric method with the TPPO concentration. The oxidation reaction of TPP was obtained by mixing TPP, oxidized vegetable oil, 3,5-di-t-butyl-4-hydroxy toluene (antioxidant), and the mixed solvent of chloroform and methanol in a test tube. The test tube was tightly sealed and then stored in the dark at 60°C for 60 min. The resultant solution was poured into a test tube for the EI and ionized using an ionization energy of 20 eV. The ion strength of the TPPO dehydride molecule, by which the TPPO concentration was obtained, was determined using the maximum value of the selected ion current chromatogram peak. The calibration curve was prepared from the POV obtained from the iodometric method and the TPPO concentration obtained by this method, using a moderately oxidized vegetable oil. For samples of air-oxidized cotton seed oil, olive oil, soybean oil, and safflower oil, the POV obtained from the TPPO concentration using the calibration curve showed good agreement with that obtained by the iodometric method.
Photoionization (PI) with vacuum ultraviolet (VUV) light provides an efficient and fragmentation-free method for the soft ionization of gaseous compounds. A photoionization source with a deuterium (D2) lamp for PI was installed in a vacuum chamber in the ionization chamber of a mass spectrometer. The PI mass spectrometry (PIMS) was applied to the evolved gas analysis (EGA) system for the volatilization of organic solvents and thermal decomposition of polymers in an inert gas atmosphere. The PI mass spectrum obtained was satisfactorily characterized by only the parent ions with no contribution as a result of fragmentation during ionization. The results suggested that EGA-PIMS was an especially powerful and desirable in situ thermal analysis method for substances which evolve organic gases simultaneously and sequentially. The combination of a VUV-light source with a quadropole-mass spectrometer in EGA is described briefly, and the results compared with EGA using conventional electron impact ionization mass spectrometry are presented.
We describe a detailed assignment strategy for B-type fragments of neutral lactooligosaccharides derived from negatively charged precursor ions in matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS). The post-source decay (PSD) ions produced by the loss of 180 u were assigned to the B-type fragments. The existing analyses of a series of lactooligosaccharides, as summarized in a previous report [T. Yamagaki, et al., J. Am. Soc. Mass Spectrom., 17, 67 (2006)], assisted us in the correct assignments. A clarification of B-type fragment assignments was essential, since we considered that the generation of B-type fragments from negatively charged precursor ions was typical of MALDI PSD, as opposed to the products of electrospray ionization MS low-energy collision induced dissociation.
The unified atomic mass unit (unit symbol: u) is a non-SI unit of mass defined as one twelfth the mass of a single 12C atom in its ground state. This definition was agreed upon by both the International Union of Pure and Applied Physics and the International Union of Pure and Applied Chemistry in early 1960s to resolve a longstanding difference between two scales of atomic mass unit. The term “atomic mass unit” (unit symbol: amu) has been used to a unit of mass defined as one sixteenth the mass of a single atom 16O [m(16O)=16 amu] in physics, or as one sixteenth the isotope-averaged atomic mass (equivalent to the atomic weight) of oxygen [Ar(O)=16 amu] in chemistry. It is a common mistake to use the deprecated term “atomic mass unit” and the deprecated unit symbol “amu” for the unit of mass defined as one twelfth the mass of single atom 12C.