Positive Chemical Ionization Mass Spectra Research Articles

Development of a Method to Identify Keto Acids in Ozonated Fulvic Acid Solutions

A gas chromatographic-mass spectrometric method, combining an O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine–diazomethane double derivatization, was developed to identify major keto acids in ozonated fulvic acid solutions. Three keto acids, glyoxylic, pyruvic, and ketomalonic acid, four aldehydes and two analytical artifacts were identified. The detailed mass spectra, obtained in both electron impact and positive chemical ionization modes, and their fragment assignments are presented. Owing to the similarity of their electron impact mass spectra, electron impact mass spectra along with positive chemical ionization mass spectra are needed to identify these keto acids reliably. The effects of the formation of keto acids on water quality are also discussed. © 1997 by John Wiley & Sons, Ltd.

The presence of free D-serine in rat brain

Free amino acid enantiomers in adult rat brain extracts were analysed as their N,O-pentafluoropropionyl isopropyl derivatives by gas chromatography on a capillary column of Chirasil-L-Val. A peak X, which exhibited the same retention time as the N,O-pentafluoropropionyl isopropyl derivative of authentic D-serine, was detected in the brain extracts. Electron impact and positive chemical ionization mass spectra of the peak X of the brain extracts were identical to those of authentic D-serine. The concentration of free D-serine and the ratio of D-serine/total serine in the brain were estimated to be 0.27 and 0.23, υmol/g of wet weight, respectively. These data provide the first evidence that substantial quantities of free D-serine are present in mammalian brain tissues.

Gas phase ion molecule reactions of η 6-arenetricarbonylchromium compounds with propene, benzene and toluene

Positive chemical ionization mass spectra of arenetricarbonylchromium compounds with propene, benzene and toluene, at about 0.5 torr, are reported. Protonation, charge exchange, fragmentation and CO substitution processes occur for all these complexes. The abundances of the [M + H] + and substituted ions strongly depend on the reagent gas and, for any given gas, on the coordinated arene. The effects of the nature of the reagent gas and of the coordinated arene on the ion abundances are discussed and reaction mechanisms suggested.

Positive chemical ionization mass spectra of (?6-arene) tricarbonylchromium complexes with ammonia and isobutane

The positive chemical ionization mass spectra, with ammonia and isobutane at 0.5 torr of (η6-arene)Cr(CO)3, where arene is C6H5COMe, C6H5COEt, C6H5COC3H7, C6H5COCMe3, 2-MeC6H4COC3H7, C6H5COOMe, C6H5CH2 COEt, C6H5Me and 1,3,5-Me3C6H3 are reported. All these compounds exhibit [M + H]+ as base peak, with isobutane. In the presence of ammonia, [M + NH4]+ is the most abundant ion, when the arene is a ketone or an ester and the addition site is suggested to be the ligand. Conversely [M + H]+ is the base peak when arene is toluene and mesitylene and the protonation site is likely to be the metal atom. In the presence of ammonia, ions such as [M — 3 CO + NH3]+ and [M — 3 CO + 2 NH3]+ are also observed and their abundances strongly depend on the pressure of the reagent gas.

Negative ions

During the last few years, improvements in ionization techniques have revived interest in negative ion mass spectrometry and its applications. Ion sources that operate at pressures of up to 1 Torr (133 Pa) produce high concentrations of low energy electrons which may react directly with suitable sample molecules to form negative molecular and fragment ions. Alternatively, a reagent gas may be used to generate negative chemical ionization mass spectra which, in general, exhibit less fragmentation than many positive chemical ionization mass spectra. The sensitivity of the electron attachment processes may be very high for compounds containing electronegative elements and it may be enhanced for other compounds by making use of huorinated derivatives. Under chemical ionization conditions, reagent ions which react as Bransted bases frequently give [M —H] - ions in great abundance. Use of selected ion monitoring leads to sub-picogram detection levels in favourable cases. The gas phase ion chemistries of such ions as O -. and O 2 -. exhibit several unusual types of reaction which reveal further structural and mechanistic information, examples of which are discussed.