Recent work reported in the literature has shown new evidence of a
sputtered ion mechanism for associative
ion surface reactions occurring during surface induced dissociation
(SID) tandem mass spectrometry (MS/MS). In
the context of a sputtered ion mechanism, we have studied selected
routes to the formation of C7H7
+
during the
low-energy collision (ca. 30 eV) of benzene molecular ions with
surfaces covered with a hydrocarbon overlayer. A
key approach utilized in this work is the use of gas phase ion molecule
chemistry to model reactive ion surface
collisions. Benzene, 2H6-labeled benzene,
and 13C6-labeled benzene have been examined by
SID and collision activated
dissociation (CAD). The CAD experiments have been used to
investigate the fragmentation of hydrocarbon adducts
of benzene and labeled benzene formed by methane and isobutane chemical
ionization (CI) as models of those ions
which may be formed via a sputtered ion mechanism during SID. In
addition, the thermodynamics of many of the
possible sputtered ion routes to the formation of
C7H7
+ have been compared using
experimental heats of formation
as well as total energies and zero-point vibrational energies (ZPVE)
obtained by ab initio (MP2 6-31G*//HF 6-31G*)
calculations. While there are several seemingly viable sputtered
ion routes to the formation of
C7H7
+ during SID of
benzene molecular ions, the combination of thermodynamic considerations
and experimental results suggest that a
likely sputtered ion route involves the reaction of neutralized benzene
ions with C3H5
+ ions (sputtered
from the
hydrocarbon overlayer) followed by the loss of ethene.
Membrane Introduction mass spectrometry with flow Injection analysis sampling has been utilized for on-line monitoring of the major products and the volatile metabolites of fermentation by the Bacillus polymyxa and Klebsiella oxytoca organisms. A flow Injection sampling system was used to rapidly deliver fermentation broth or an external standard to the mass spectrometer. Analyte Introduction occurred via a direct Insertion membrane probe In which the aqueous solutions flowed past a membrane located within the Ion source of the mass spectrometer. For both organisms, concentrations of the liquid-phase products, acetic acid, acetoln, 2,3-butanediol, and ethanol, were monitored as a function of time after permeation through the membrane and ionization by chemical Ionization. Tandem mass spectrometry confirmed that these measurements were made without Interference. Off-line gas chromatography was utilized to test the accuracy of these measurements, and excellent agreement was found. Dissolved oxygen In the fermentation broth was also monitored, and carbon dioxide and oxygen were followed In the offgases. The 02 and C02 measurements were compared with other common measurement techniques (galvanic 02 electrode, Infrared absorption, and paramagnetic 02 analysis), and very good agreement was found. The use of tandem mass spectrometry has allowed the detection of additional compounds that were previously not known to be present In measurable amounts.
The dynamic response of the membrane probe in flow injection/membrane introduction mass spectrometry has been studied theoretkaily. A mathematical model is formulated to obtain analyte concentratton profiles in the flow chamber and indde the membrane and to calculate the analyte flux across the membrane. For ideal input perturbations such as block and step inputs, analytkai soluHons are p r o w for nonlinear input perturbations an efficient and robust computational procedure for obtalning numerical solutions is presented to account for mixing of the analyte sample in the flow conduit before entering the flow chamber. The effect of soma system parameters and operational variables on the dynamic behavior of the probe is investigated. This information should help In the design of new membrane probes and in the selection of new membranes.
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