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the space between the first and second electrodes and a burst of electrons (over a period of about a microsecond) is allowed to produce ions. An extraction potential (E) is then applied for another short time period which, as those further from the second electrode will experience a greater force than those closer to the second electrode, will result in the ions being focused.     After focusing, an accelerating potential (V) is applied for a much shorter period than that used for ion production (ca 100 nsec) so that all the ions in the source are accelerated virtually simultaneously. The ions then pass through the third electrode into the drift zone and are eventually collected by the sensor electrode. The time of flight mass spectrometer is not employed extensively in gas chromatography/mass spectroscopy combination systems as it is more commonly used to examine high molecular weight materials Many analysts that use GC/Mass Spectrometer combined systems are neither

the interacting surface would be virtually pure 2-propyl alcohol. Whether the interaction is by sorption or displacement is difficult to determine. It is likely that the early peaks interacted by sorption and the late peaks possibly by displacement. Ionic Interaction Chromatography Ionic interaction chromatography, or ion chromatography as it is usually called, is typically carried out employing ion exchange resins as the stationary phase. There are some silica based ion exchange materials available, but the bonded silicas tend to be unstable in the presence of high salt concentrations and at extremes of pH. As a consequence, they have very limited areas of application. Alternatively, the polystyrene divinyl benzene cross-linked polymers, are extremely stable to a wide range of salt concentrations and can function well within the pH range of 2.0 to 12.0. An obvious application area for ion exchange chromatography is in the separation

employed as a retentive mechanism in GC. Ionic interactions, however, are the dominant retentive mechanism in ion exchange chromatography which is widely used in analytical chemistry. The stationary phase usually consists of a cross-linked polystyrene resin to which ionic materials have been chemically bonded. They are formed in the shape of tiny spheres that are packed into a column in the usual way. The mobile phase usually carries a buffer that is set at a pH that allows the solutes and the ion exchange resin to be ionized and thus the charged groups are available for mutual interaction. Both anion and cation exchange resins are available to separate the complementary ions of a solute molecule. In addition, both anion and cation exchange resins are available as both strong and weak ion exchangers. An example of a separation achieved by ionic interactions employing strong and weak ion exchangers is shown in figure 6. Courtesy of the TosoHaas Corporation   1

Separations Based on Ionic Interactions Ionic materials are not volatile under the conditions normally employed in GC, so, ionic interactions cannot be exploited in GC stationary phases to control retention. However, they are very important in LC, and ion exchange chromatography (the name given to LC separations that employ ionic interactions to control retention) is widely used to analyze ion mixtures. The use of ionic interactions to separate some alkali and alkaline earth cations is shown in figure 16.   Courtesy of Whatman Inc. Figure 16 The Separation of Cations by Ion-Exchange Chromatography The column used was IonPacCS12 (a proprietary cation exchange column) and the mobile phase was a 20nM solution of methanesulfonic acid in water. The flow rate was 1 ml/min. and 25ml of sample was injected. The separation almost exclusively involved

it entered the detector. The reverse phase will remove all organic material by adsorption due to the strong dispersive forces that will occur between the hydrocarbon chains of the reverse phase and the methyl group of the methanesulphonic acid. The ion suppression column eventually saturates and require regeneration by desorbing the methane sulphonic acid with a strong dispersive solvent that is miscible with water such as acetonitrile. This technique of ion suppression is frequently used in ion exchange chromatography when using the electrical conductivity detector. A wide variety of different types of ion suppression columns are available but it should be pointed out that, any suppresser system introduced between the column and the detector, will� cause some degree band spreading and consequently reduce the resolving power of the system. It follows, that the connecting tubes and suppression column itself must be very carefully designed to eliminate or reduce this dispersion to an

The Ion Trap Detector   The ion trap detector is a modified form of the quadrapole mass spectrometer, but was designed more specifically as a chromatography detector than for use as a combined gas chromatography/mass spectrometry instrument for structure elucidation and solute identification. The electrode orientation of the quadrapole ion trap mass spectrometer is shown in figure 28.