Chemical ionization ammonia

Mass spectra were obtained with the following conditions ES (Electrospray) on anLCTOF MICROIVIASS El (Electronic Impact) and D/CI (Desorption/Chemical Ionization ammonia) on a SSQ7000 FINNIGAN LSIMS (Liquid Secondary Ion Mass Spectrometry) with a SCIEX PERKIN ELMER (Cs 35 KeV 3-nitrobenzyl alcohol matrix). 

Molecular ion Chemical ionization using ammonia as reagent gas establishes the molecular weights of sugar acetates. 

Mass Spectrometry. Mass spectrometry holds great promise for low-level toxin detection. Previous studies employed electron impact (El), desorption chemical ionization (DCI), fast atom bombardment (FAB), and cesium ion liquid secondary ion mass spectrometry (LSIMS) to generate positive or negative ion mass spectra (15-17, 21-23). Firm detection limits have yet to be reported for the brevetoxins. Preliminary results from our laboratory demonstrated that levels as low as 500 ng PbTx-2 or PbTx-3 were detected by using ammonia DCI and scans of 500-1000 amu (unpublished data). We expect significant improvement by manipulation of the DCI conditions and selected monitoring of the molecular ion or the ammonia adduction. 

With the thermospray interface (Figure 4.38(a)), the mobile phase, usually containing an ammonium ethanoate buffer, is passed through a heated probe (350-400°C) into an evacuated source chamber where it forms a supersonically expanding mist of electrically charged droplets. The liquid evaporates to leave charged solid particles which then release molecular ions such as MH+ and, VI by an ammonia chemical ionization (Cl) process. The analyte ions are skimmed off into the mass spectrometer whilst the vaporized solvent is pumped away. An electron beam is also employed to enhance the production of ions by Cl. 

What are the three commonly used chemical ionization reagent gases (methane, isobutane, water, ammonia, methanol, hydrogen, oxygen, nitrogen, etc.). 

Chemical ionization MS of 2-aryl-3-nitro-2//-chromenes and 4-hydroxy-3-nitroflavans have been studied using methane and ammonia as reagent gases102. The behaviour of 2-aryl-3-nitro-2//-chromenes was found to resemble that of aromatic nitro compounds. Thus, the methane spectra are characterized by the MH+ ions, whereas the ammonia... 

The use of ammonia for the protonation of nitroarenes leads frequently to formation of aduct ions, e.g. [M + NH4]+, but not to the protonated species (MH+)112,113. The ammonia chemical ionization spectrum of nitrobenzene shows, in addition to a series of adduct ions, a dominant signal corresponding to the anilinium ion (m/z 94)112114115. Evidence for the isomerization of the [M + NR ]"1" adduct followed by successive loss of NO and OH or NH3 to give ions corresponding to the substitution products, e.g. the anilinium ion, has been given115 see Scheme 41. 

Lawrence, D.L. Accurate Mass Measurement of Positive Ions Produced by Ammonia Chemical Ionization. Rapid Commun. Mass Spectrom. 1990,4,546-549. 

Charrois JW, Arend MW, Froese KL, Hrudey SE (2004) Detecting N-nitrosamines in drinking water at nanogram per liter levels using ammonia positive chemical ionization. Environ Sci Technol 38 4835-4841... 

Aqueous solutions of ammonium sulfate and ammonium bisulfate were deposited on Fluoropore filters, placed in the direct insertion probe, and analyzed in the chemical ionization mode (H2O reagent) gas. The samples were heated from 100°C to 330 C at 15 C/minute. No sample ions were observed under these anlaysis conditions, even when several micrograms of ammonium salts were analyzed. The thermal decomposition of ammonium salts of sulfate has been the subject of many studies. (29,30) Some pathways include sulfuric acid production at one stage of the decomposition while others suggest ammonia, SO2 and SO3 are the products. None of these accurately simulate the conditions (temperature, pressure, gas flow) present in our chemical ionization source. However, no sulfuric acid ions (H3SO4+, etc.) were ob-served... 

A new gas chromatography (GC) method was developed to characterize artemether 28a and its metabolites in body fluids. The extracts were derivatized and then separated on an optimized capillary GC system and identified by chemical ionization MS using ammonia as the reagent gas <1998JCH(B)101>. A sensitive, selective, and reproducible GC-MS-SIM method has also been developed for the determination of artemether 28a and dihydroartemisinin 29a in plasma, using artemisinin 9a as an internal standard <1999JCH(B)251>. 

Agilent GC-MS ChemStation System equipped with capillary gas chromatograph HP 5972,5973, or 6890 series Agilent 6890 series autosampler. The GC-MS system is operated in the positive-ion, chemical-ionization mode the reagent gas is ammonia. The chromatographic parameters of the acquisition method, the sequence of SIM windows, and the ion species being monitored are listed in Tables 3.1.8 and 3.1.9. The total run time is 17.88 min. 

Chemical ionization produces less fragmentation than electron ionization. For chemical ionization, the ionization source is filled with a reagent gas such as methane, isobutane, or ammonia, at a pressure of 1 mbar. Energetic electrons (100-200 eV) convert CH4 into a variety of reactive products ... 

Another conventional ionization technique termed chemical ionization (Cl), utilizes a reagent gas (such as isobutane, methane, ammonia) to form reagent ions (Rif) which can undergo ion-molecule reactions with the compound of interest to form protonated molecules. 

---