Mass spectroscopy electron ionization

Nitrobenzene in aqueous samples may be analyzed by GC and GC/MS techniques. Wastewaters and solid and hazardous wastes may be analyzed by EPA Methods 609 (GC) or 625 and 8250 or 8270, respectively, using GC/MS (U.S. EPA 1992, 1997). The characteristic ions to identify nitrobenzene by mass spectroscopy (electron-impact ionization) are 77, 123, and 65. The GC techniques involve the use of FID and NPD (in the nitrogen mode) as detectors. The former is less sensitive than the latter. 

Mass spectroscopy is a useful technique for the characterization of dendrimers because it can be used to determine relative molar mass. Also, from the fragmentation pattern, the details of the monomer assembly in the branches can be confirmed. A variety of mass spectroscopic techniques have been used for this, including electron impact, fast atom bombardment and matrix-assisted laser desorption ionization (MALDI) mass spectroscopy. 

Peaks are analyzed separately by their retention times, absorption, and fluorescence properties. RCCs show absorbance maxima near A.500 and 316 nm. For FCCs, UV-Vis specna show two prominent bands near 361 and 320 mn and a luminescence maximum at 436 mn and NCCs show UV-Vis spectra with absorbance maxima near 320 and 210 nm. Nevertheless, as none of these approaches is suitable for elucidating structures, it is necessary to apply additional MS and NMR analyses to fully characterize snuctural features. Electron spray ionization (ESI) and high-resolution EAB mass spectroscopy have been applied to elucidate the molecular formulae of colorless compounds. ... 

A detailed description of sources used in atmospheric pressure ionization by electrospray or chemical ionization has been compiled.2 Atmospheric pressure has been used in a wide array of applications with electron impact, chemical ionization, pressure spray ionization (ionization when the electrode is below the threshold for corona discharge), electrospray ionization, and sonic spray ionization.3 Interferences potentially include overlap of ions of about the same mass-charge ratio, mobile-phase components, formation of adducts such as alkali metal ions, and suppression of ionization by substances more easily ionized than the analyte.4 A number of applications of mass spectroscopy are given in subsequent chapters. However, this section will serve as a brief synopsis, focusing on key techniques. 

Pyrolysis mass spectroscopy was conducted with a Hewlett-Packard model 5985B gas chromatograph/quadrupole mass spectrometer, operated at sslO- Torr and 70eV electron-impact ionization energy. Samples were introduced into the mass spectrometer via a glass lined direct insertion probe (DIP). The samples were decomposed in the DIP to a nominal temperature of 300°C at a heating rate of 30°C/min. 

MS, chemical ionization-MS, and sometimes GC/infrared spectroscopy (IR) have been used with GC/MS to obtain structural information. Examples of the use of GC/ MS for identifying new DBFs include the recent identification of iodo-acids. The iodo-acids were discovered in drinking water treated with chloramination through the use of full-scan GC/MS on the methylated extracts. Empirical formula information for both the molecular ions and the fragment ions was obtained by high-resolution electron ionization (EI)-MS, and the spectra were interpreted to yield tentative identifications of five new iodo-acids (iodoacetic acid, bromoiodoacetic acid, ( )-3-bromo-3-iodopropenoic acid, (Z)-3-bromo-3-iodopropenoic acid, and )-2-iodo-3-methylbutenedioic acid). Structural assignments were then confirmed by the match of mass spectra and GC retention times to authentic chemical standards, several of which had to be synthesized. 

Pleil JD, Oliver KD, McClenny WA. 1988. Ambient air analyses using nonspecific flame ionization and electron capture detection compared to specific detection by mass spectroscopy. JAPCA 38 1006-1010. 

Chemists use an instrument called a mass spectrometer to measure the relative abundance of isotopes. There are different kinds of mass spectrometers, but the basic idea is to measure the mass of a substance by applying a force. The response to this force depends on the object s mass—think of Newton s second law, where acceleration equals force divided by mass. In the case of mass spectroscopy, the substances to be measured are first ionized—they are made into charged particles called ions by stripping electrons. A magnetic field deflects the motion of an ion, and the deflection depends on the ion s mass, most of which is due to the protons and neutrons in the nucleus. The technique separates different isotopes and measures their abundance in a given sample. 

When exposed to sufficient energy, a molecule may lose an electron to form a cation-radical, which then may undergo fragmentation of bonds. These processes make mass spectroscopy (ms) a useful tool for structure proof. Very small concentrations of the parent molecules (RS), in the vapor state, are ionized by a beam of energetic electrons (e ),... 

Experimental Techniques A absorption CIMS = chemical ionization mass spectroscopy CK = competitive kinetics DF discharge flow EPR = electron paramagnetic resonance FP = flash photolysis FT = flow tube FTIR Fourier transform intra-red GC = gas chromatography, UF = laser induced fluorescence LMR = laser magnetic resonance MS = mass spectroscopy PLP = pulsed laser photolysis SC = smog chamber SP = steady (continuous) photolysis UVF = ultraviolet flourescence spectroscopy... 

Until this point, results appear to be rather divergent, possibly because these studies were performed on SMA dissolved in various solvents and solvation might modify the distribution of electrons in the Si-C-N unit. In order to circumvent this problem, gas phase studies and theoretical approaches were developed. In the gas phase (ion cyclotron resonance mass spectroscopy), trimethylsilylmethyldimethylamine was reported to be more basic than the analogous neopentyldimethylamine from proton affinity measurements based upon proton affinity of ammonia (201.0 kcal/mol) 227.1 and 225.8 kcal/mol respectively.32 Conversely, the ionization potential of MSMA indicates a basicity lower than that of its carbon analog and the authors emphasize the fact that this result is opposite... 

Combined electron ionization mass spectroscopy (EIMS) and matrix isolation FTIR spectroscopic data on vacuum pyrolysis of 1,1-dimethyl-l-germa-3-thietane assisted by theoretical calculations provide a reasonable foundation for mechanistic interpretation of its thermal decomposition (see Section 2.21.6.1, Equation 7) <1998JA5005>. 

---