Separation with Ionic Eluents

In this type of separation the analyte cations compete with the eluent cation for ion-exchange sites and move down the eolumn at different rates. The ionic eluent selected depends on the cations to be separated, the type of separation column and on the detector. In many cases an aqueous solution of a strong acid such as hydrochloric, sulfuric or methanesulfonic acid is a satisfactory eluent. Sample cations commonly separated include the following alkali metal ions (Li, Na+, K , Rb, Cs ), ammonium, magnesium, alkaline earths (Ca, Sr +, Ba ), and various organic amine and alkano-lamine cations. Most other metal cations are separated with a weakly complexing eluent. 

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Separations of metal cations with ionic eluents has been limited mostly to the alkali metals, ammonium, magnesium(II), calcium(Il), strontium(ll) and barium(ll). Separations of other metal cations are usually performed with eluents that complex the sample cations to varying degrees (see Section 7.4). Some organic cations have also been separated with ionic eluents, although this appears to be an under-utilized area of cation chromatography. 

As already discussed, chromatography on silica or alumina stationary phases nowadays should be restricted to the separations of organic soluble compounds. Of course, also ionic and other water-soluble substances have been and will be separated with such phases. But in order to obtain highly efficient separations (i.e., elution of solutes without tailing) the eluent composition has to be selected and adjusted carefully. Such polar components are better separated by applying tended phases or ion exchange or ion pair chromatography. 

The separator column separates the analytes, and the suppressor replaces the Ionic eluent with a nonionic species. 

The eluent is rather acidic in most protein separations. With silica-based packings, the reason for the selection of this condition is related to the surface silanols which are weakly acidic groups. Above pH 3.5-4, these groups are dissociated and ionic interactions will occur with basic sample compounds. These interactions can be reduced simply using acidic mobile phases that prevent ionization of the silanols. Another reason is the stability of the silica which is higher in acidic media72). 

Selectivity in the separation of ionic solutes may be varied by changing either the pH of the mobile phase or the nature or concentration of the displacing ions. The pH of the eluent affects not only the ionic form in which the eluent exists but also the form of the solute and the functional group on the ion-exchange resin. The nature and concentration of the displacing ion will determine the ease with which solute ions are displaced the more concentrated the competing ion in the eluent, the more effectively it will displace solute ions from the stationary phase. 

The development of the first CE-MS was prompted by the early reports on electrospray ionization (ESI-MS) by Fenn and co-workers in the mid-1980s [1], when it was recognized that CE would provide an optimal flow rate of polar and ionic species to the ESI source. In this initial CE-MS report, a metal coating on the tip of the CE capillary made contact with a metal sheath capillary to which the ESI voltage was applied [5]. In this way, the sheath capillary acted as both the CE cathode, closing the CE electrical circuit, and the ESI source (emitter). Ideally, the interface between CE and MS should maintain separation efficiency and resolution, be sensitive, precise, linear in response, maintain electrical continuity across the separation capillary so as to define the CE field gradient, be able to cope with all eluents presented by the CE separation step, and be able to provide efficient ionization from low flow rates for mass analysis. 

A,codeine,morphine,dihydrocodeine,quinine, quinidine,caffeine, theophylline, ajmaline Various basic drugs Separation of basic drugs on silica gel with non-aqueous ionic eluents Spherisorb S5W Silica 250x4.9 Me0H-hexane(85 15)contai ni ng o.iox hcio4 66 ... 

Buffered eluents are important for the separation of ionic solutes because they maintain a constant desired pH. Very often, only a defined and constant pH (therefore a buffer) will allow the desired selectivity and reproducibility. So far, so good. Sometimes, you adopt a method with 5 mM salt in the mobile phase, while the next method uses 100 mM or even 150 mM solutions. Sometimes you simply do not know, as in the following example Weigh out jr mg of salt X and add y ml of acid Y or base Z How critical is the ionic strength and how does the ionic. strength influence the separation and column lifetime ... 

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