Ion chromatography is a form of liquid chromatography where retention is predominantly controlled by ionic interactions between the ions of the solute and counter ions that are situated in, or on, the stationary phase. For example, to separate organic acids, it is the negatively charges acid ions that need to be selectively retained. It follows that the stationary phase must contain immobilized positively charged cations as counter ions to interact with the acid ions to retain them. Conversely, to separate cations, the stationary phase must contain immobilized anions as counter ions with which the cations can interact. Ion exchange stationary phases are available in mainly two forms. One form (probably the most popular) consists of cross-linked polystyrene polymer beads of an appropriate size which has been suitably treated to link ionic groups to the surface. The other form is obtained by chemically bonding ionic groups to silica gel by a process similar to that used to produce bonded phases. These materials are called ion exchange media, a term which has given rise to the term ion exchange chromatography as an alternative to ion chromatography. Ionic substances can also be adsorbed on the surface of a reverse phase media and act as an adsorbed ion exchanger. The mobile phase is made to contain a small percentage of a soluble organic ionic material (e.g. tetrabutyl ammonium dihydrogen phosphate or n-octyl sulphonate). These substances are adsorbed onto the surface by dispersive interactions between the alkyl groups of the agent and those of the bonded phase and act as counterions. In general ion chromatography is one of the more difficult types of liquid chromatography to exlpoit and is most often used for analysis of anions for which there are no other rapid analytical methods.
Author: RPW Scott
Book:Gas Chromatography - Tandem Techniques
Section:GC-Tandem GC-IR
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
GC-Tandem GC-IR
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Applications Liquid-Chromatography Ionic-Interaction
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
Principles Applications Liquid-Chromatography Ionic-Interaction
Author: RPW Scott
Book:The Mechanism of Chromatographic Retention
Section:Retention Chromatographic-Interactions Ionic
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
Retention Chromatographic-Interactions Ionic
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Selectivity Ionic
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
Principles Selectivity Ionic
Author: RPW Scott
Book:Liquid Chromatography
Section:HPLC Electrical-Conductivity
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
HPLC Electrical-Conductivity
Author: RPW Scott
Book:Gas Chromatography - Tandem Techniques
Section:GC-Tandem Quadrapole-Mass-Spectrometer Ion-Trap-Detector
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.
 
GC-Tandem Quadrapole-Mass-Spectrometer Ion-Trap-Detector