Separation, of ions

A typical flow modifler is a surfactant with a hydrocarbon chain of 14 or 16 carbon atoms and a quaternary ammonium group at one end of the chain. A 1-5 mM concentration of this surfactant in the electrolyte coats the capillary wall with surfactant micelles. These give the wall a positive charge and result in a strong EOF in the same direction as the electrophoretic migration of the analyte anions. 

UV-Vis spectroscopy is the most common detector technique used in CE. 

A requirement of an anion used for indirect detection is that its mobility must match that of the sample ions as closely as possible. If the mobilities do not match reasonably well, peaks may be fronted or tailed. Since the mobilities of sample ions will differ, the mobility of the visualization reagent is matched as closely as possible to the mobility of the middle sample ions. 

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In general terms, the main function of the magnetic/electric-sector section of the hybrid is to be able to resolve m/z values differing by only a few parts per million. Such accuracy allows highly accurate measurement of m/z values and therefore affords excellent elemental compositions of ions if these are molecular ions, the resulting compositions are in fact molecular formulae, which is the usual MS mode. Apart from accurate mass measurement, full mass spectra can also be obtained. The high-resolution separation of ions also allows ions having only small mass differences to be carefully selected for MS/MS studies. 

Separation of ions according to their m/z values can be effected by magnetic and/or electric fields used as mass analyzers, which are described in Chapters 24 through 27. However, apart from measurement of m/z values, there is often a need to be able to transmit ions as efficiently as possible from one part of a mass spectrometer to another without any mass separation. 

The Z-trajectory ensures excellent separation of ions from neutral molecules at atmospheric pressure. In line-of-sight or conventional electrospray sources, the skimmer is soon blocked by ions and molecules sticking around the edges of the orifice. In Z-spray sources, the final skimmer, being set off to one side, is not subjected to this buildup of material. 

This focusing action gives an ion beam, in which the m/z values can be measured so accurately that the resolution of a magnetic/electric-sector instrument (separation of ions of different m/z values) is measured as a few parts per million, compared to the more modest few parts per thousand in, say, a quadmpole or ion-trap instrument. 

Analytically, the inclusion phenomenon has been used in chromatography both for the separation of ions and molecules, in Hquid and gas phase (1,79,170,171). Peralkylated cyclodextrins enjoy high popularity as the active component of hplc and gc stationary phases efficient in the optical separation of chiral compounds (57,172). Chromatographic isotope separations have also been shown to occur with the help of Werner clathrates and crown complexes (79,173). 

Anyway, it is clear that the fmdings obtained in these artificial transport systems do contribute to the understanding of biological phenomena and point the way to possible practical applications, such as the separation of ions. Accordingly, the development of synthetic ionophores which possess high selectivity for specific cations is expected to gain importance in the future. 

The solvation (hydration) and desolvation of ions is important to the gelation process in AB cement chemistry. The large dipole moment of ion-pairs causes them to interact with polar molecules, including those of the solvent. This interaction can be appreciable. Much depends on whether the solvent molecule or molecules can intrude themselves between the two ions of the ion-pair. Thus, hydration states can affect the magnitude of the interaction. The process leading to separation of ions by solvent molecules was perceived by Winstein et al. (1954) and Grunwald (1954). 

Ion-exchange chromatography (lEC) is used mainly for the separation of ions and easily ionized substances (e.g., substances that form ions by pH manipulation or complexation) in which one of the principal contributions to retention is the electrostatic attraction between mobile phase ions, both sa le and eluent, for immobilized ion centers of opposite charge in the stationary phase. The sample ions are separated based on differences in their relative affinity for the stationary phase ion centers compared to that of the mobile phase counterions in a dynamic exchange system, in which sample ions and eluent ions interact with multiple stationary phase ion centers as they pass through the column. Ion-... 

IEC continues to have numerous applications to the detection and quantification of various inorganic ions.1 1 This is particularly true in water analysis.5-14 Inorganic ions in a variety of other sample types, such as food and beverages,1518 rocks,19-23 biological fluids, (blood, urine, etc.),24-31 pharmaceutical substances,32 33 concentrated acids,34 alcohols,35 and cleanroom air36 have also been analyzed by IEC. IEC has also been employed in isotopic separation of ions,37 including the production of radioisotopes for therapeutic purposes.3839 Typical IEC sample matrices are complex, and may contain substances that interfere with measurement of the ion(s) of interest. The low detection limits required for many IEC separations demand simple extraction procedures and small volumes to avoid over-dilution. Careful choice and manipulation of the eluent(s) may be needed to achieve the desired specificity, especially when multiple ions are to be determined in a single sample. 

An ion mobility spectrometer consists of a sample-introduction device a drift tube where ionisation and separation of ions takes place and a detector. Ionisation sources of choice include radioactive sources (e.g. a 63Ni foil), photoionisation methods, corona-spray ionisation, flame ionisation and corona discharge. The most common detection method used to measure the... 

Resolution or resolving power is the ability of a mass spectrometer, and in particular of its analyzer system, to separate ions with different m/z ratios. An example of mass spectra obtained at different resolutions is reported in Figure 2.1 by increasing the resolution the peak shape becomes more and more narrow thus allowing the separation of ions with their m/z values differing in decimals (10 1—10 3). 

Based on the main methods of ion separation, analyzers can be classified into two main groups those based on the separation of ions in space or the separation of ions in time. The former separate ions while they are travelling over a distance (some metres in the case of sector instruments, centimetres in the quadupole) (Table 2.4). In contrast, when separation occurs in time, ions are confined in a small region of space and their separation occurs by varying some parameters (Table 2.4). 

Describe in detail the general mechanism of the separation of ions by electrophoresis. 

Capillary gel electrophoresis (CGE) is a specialised form of CE that employs a fused silica capillary filled with an immobilised and cross-linked polyacrylamide gel. This gel contains a network of pores, which allows separation of ions based on differences in molecular size. CGE was applied to the separation of ionic alkylphenol ethoxy sulfates and... 

The separation of ions according to their m/z ratios is achieved using electric and/or magnetic fields in a number of ways. The trajectories of ions moving in such fields are determined by their m/z values and these can be monitored to ascertain their mass. Double-focusing instruments use the combined effects of electric and magnetic fields to effect separation (Figure 3.19). In a typical instrument, after the ions have been accelerated away from the ion source... 

With this mechanistic scheme, the chemoselectivity of the addition and the formation of rearranged chlorides (but not acetates) have been chosen as criteria to differentiate the ion pair mechanism from the purely ionic one and, on the basis of both criteria, the authors suggest the involvement of a tight ion pair for the addition of ArSCl in AcOH to diene 62 and of solvent separated ion pairs to triene 108. The effects related to the presence of added electrolytes, which favor the formation of rearranged acetates, have been considered in this work127 as evidence that even a larger separation of ions, which should lead to more electrophilic species, is possible. 

MS involves the separation of ions based on their mass-to-charge ratio (m/z). The concept was invented a century ago1 with a dramatic impact on analytical chemistry.2-3 The fundamental principle of MS requires vaporization of the molecules in the gas phase and in ionization. Early ionization methods such as electron impact (El) and chemical ionization (Cl)4-5 were limited to small organic molecules that were volatile and stable to heat and amenable to transfer into high vacuum. Introduction of the fast-atom-bombardment (FAB) method of ionization6... 

An ion that is displaced from the center of the analyzer can be restored to the center of the gap (i.e., compensated) when a DC potential from, for example, 20 to 10 V is superimposed on the separation field. This second potential is called the compensation voltage and will allow an ion to be passed through the DMS drift tube and to a detector. A scan of the compensation voltage provides a measure of all ions in the analyzer and is termed a differential mobiftty spectrum as shown in Figure 10. This method is now understood as a technique for separation of ions based on AK rather than K as seen with conventional drift tubes. 

G.A. Eiceman, E.V. Krylov, N.S. Krylova, E.G. Nazarov and R.A. Miller, Separation of ions from explosives in differential mobility spectrometry by vapor-modified drift gas, Analytical Chemistry 76(17) (2004) 4937-4944. 

This overall process can be considered as composed of two parts (1) separation of ions from the lattice (breaking ion-ion bonds in the lattice), and (2) interaction of the ions with water molecules (hydration). Both processes involve ion-water interaction (Fig. 2.10). During crystal dissolution, the two processes are occurring simultaneously. Thus, we can write for the heat of solvation of a salt... 

Partial pressure measurement devices which are In common use comprise the measurement system proper (the sensor) and the control device required for Its operation. The sensor contains the Ion source, the separation system and the Ion trap. The separation of Ions differing In masses and charges Is often effected by utilizing phenomena which cause the Ions to resonate In electrical and magnetic fields. 

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