Separation of Organic Cations

In acidic solution organic amines can be separated by CE as the protonated cations. If necessary, some methanol or acetonitrile may be mixed with the aqueous BGE to enhance the solubility. For example, amino acids are zwitterions throughout much of the pH range but they have a net positive charge at very low pH values. The CE sepa- 

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As in anion chromatography, the IC separation of organic cations has long been known, or at least suspected, that its mechanism involved more than simple ion exchange. Hoffman and co-workers [9,10] have shown that two mechanisms occur in such cases ion exchange and hydrophobic interaction between the sample cations and the resin matrix. For example, these authors showed that the slopes of the linear plot of log k vs. carbon number for protonated amine cations decrease going from 30 % acetonitrile (70 % water) to 70 % acetonitrile in the eluent. This is due to lower hydro-phobic interaction in the 70 % acetonitrile. 

These results suggest that a successful separation of organic cations by IC depends on differences in hydrophobic attraction between the solute ions and the ion exchanger as well as on differences in electrostatic attraction. Incorporation of an organic solvent in the eluent will increase the solubility of samples containing organic solutes. However, it is usually better to work with a mixed organic-aqueous eluent rather than... 

Evaluations of an integrated adsorption system were also conducted. In this system, by varying the pH conditions, the dissolved organics (model compounds) are separated into fractions by isolation onto Amberlite XAD-8, AG MP-50 cation-exchange resin, and graphitized carbon black. The procedure is based on the separation of organic solutes into hydrophobic and hydrophilic neutral, acidic, and basic fractions. 

Similarly, the counterion in the mobile phase during run can affect the partition ratios. Figure 9.9 shows an example of a separation of organic acids at pH 4.4 in which the only variable was the cation counterion— K+ in Figure 9.9a and Na+ in Figure 9.9b. Because K+ is more strongly held than Na+, and it competes with the analytes for ionic sites on the resin, the overall retention times in Figure 9.9a are shorter. A secondary effect in this case is the alteration in the elution order. 

The extraction of various components, in particular from systems used for extraction separation of metal cations, can be accompanied by the formation of a foam, if there is a surfactant present either in the aqueous or in the organic phase. However, the formation of a foam in either phases, is an undesirable event here. A small amount of the foam can lead to... 

Much of the selectivity in separating mixtures of organic cations and anions has been shown to come from differences in adsorption rather than from differences in ion exchange selectivity [1,2]. However, adsorption and subsequent desorption is apt to be a slower process than ion exchange and is therefore to be avoided as much as possible in ion chromatography. 

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. 

Two other types of organic cations did show enough difference in their retention factors for practical separations. Separation of aniline, /V-methylaniline and N,N-dimethylaniline in 100 % methanol is shown in Fig. 7.10. Significant differences in k values of octylamine, dioctylamine and trioctylamine were observed in methanol, ethanol and 2-propanol. 

The minimum uncertainty for simple matrices is achieved with use of electrochemical sensors, especially for the assay of organic cations and organic and inorganic anions in food and clinical analysis. The selectivity and sensitivity of these sensors are adequate to detect numerous pharmaceutical products, without any prior separation. The ability of electrochemical sensors to determine continuously the activity of an ion in solution has made their use possible in in vitro and in vivo dissolution tests of drugs. 

Most environmental samples require some pretreatment, usually in the form of filtration and centrifugation, before these are injected via a sample loop. lEC separation of organic acids is achieved on cation exchange columns which are characterized by their particle diameter, substrate crosslinking, ion exchange capacity, type of functional group, and hydrophobicity. The most commonly used columns are comprised of fully sulfonated, crosslinked divinylbenzene/polystyrene copolymers (e.g., HPICE-AS6 or -ASl, Dionex). However, a comparison smdy found other columns to be equally as effective for organic acid separation. Unmodified silica gel columns... 

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