Ionization mechanisms responsible for the formation of prepeaks and afterpeaks on Ion signals observed In pulsed glow discharges and potential analytical use of pulsed glow-discharge mass spectrometry have been investigated. The formation of a prepeak at the beginning of the pulse Is related to the Ionization efficiency of electrons accelerated across the cathode fall region. The appearance of an afterpeak at the termination of the discharge is associated with the energy, population, and Ionization efficiency of metastable discharge gas atoms. As a result of the differences In pulse profiles, It Is possible to acquire data over specific regions of the pulse that permit discrimination against Interfering signals In the mass spectrum.
Laser radiation (XeCl laser, 308-nanometer wavelength) focused into a cell containing Mo(CO)(6) vapor produced ultrafine particles in the extended waist of the laser beam. Negative ion mass spectrometry revealed molybdenum carbide cluster ions with a stoichiometry MonC4n (n = 1 to 4). The MonC4n(-) (n = 2 to 4) ions are completely unreactive with NH(3), H(2)O, and O(2), suggesting structures in which the molybdenum atoms are unavailable for coordination to additional ligands. Collision-induced dissociation studies of these anions show the loss of MoC(4) units as the main fragmentation pathway. This observation, together with the lack of addition reactions, provides a basis for structures in which a planar cluster of two, three, or four molybdenum atoms is surrounded by, and bonded to, carbon dimers.
The glow discharge (GD) is an old source that is finding many new uses in analytical spectroscopy. Its simplicity of operation, coupled with versatility of application, has led to increasing commercial interest. From an obscure analytical method focusing primarily on metal analysis, the glow discharge has developed into a sophisticated technique suitable for analysis of nonmetals, thin films, semiconductors, insulators, and organic materials. More effort is also being made to understand better the many chemical and physical phenomena that influence the effectiveness of the GD source.The GD is a low-pressure (0.1-10 Torr) plasma composed of two electrodes immersed in a partially ionized noble gas (1). The name arises from the
A series of bromine-containing flame-retardant plastics was used to demonstrate the applicability of pulsed radio frequency glow discharge mass spectrometry to the determination of elements in commercial polymers. The direct analysis of bulk samples produced both atomic and molecular species, allowing elemental identification and molecular characterization. However, the low ion signal intensities hindered quantification. Atomization mechanisms for the plastic samples were studied in detail using scanning electron microscopy and mass spectrometry. Although some thermal desorption was observed, sputter atomization dominated when samples were not subjected to excessive discharge power. The sputter rates of various polymers, as evidenced by sputter weight loss measurements, showed a strong correlation with their ion production capabilities. Sputtering rates were related to the physical and chemical properties inherent to a polymer's composition. Analysis of samples compacted with a silver binder provided intense analyte signals allowing quantitative analysis. Signal stability, measurement accuracy, measurement precision, and detection limits were all assessed.
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