Cation exchange, conditions

From Fig. 10, peptides were, of course, eluted from the RPC column (top) in order of increasing hydrophobicity. Under characteristic cation-exchange conditions (middle) [the presence of 10% (v/v) acetonitrile helps to eliminate unwanted hydrophobic interactions between solutes and the column matrix (see earlier)], the four peptides were very poorly resolved, as expected given the identical net charge on the peptides. Interestingly, the low concentration of acetonitrile (10%) has already induced hydrophilic interactions with the matrix in that the elution order is already opposite to that of RPC (the most hydrophobic peptide was eluted first and the most hydrophilic last). In contrast, under HILIC/CEC conditions (bottom), the elution order remains the same but the peptides are now well resolved. Clearly, to effect a separation of these peptides on the cation-exchange column, an increased concentration (80%, v/v) of acetonitrile was required in the mobile phase in order to promote hydrophilic interactions with the column matrix to complement the ionic interactions. 

Weak and strong acid-type resins are for removal of cations and are called cation exchangers. Weak and strong base resins remove anions and are called anion exchangers. In addition to these four resin types, there are specialty resins used in appHcations where higher specificity for certain ions under challenging conditions is a critical factor. 

The acryHc weak base resias are syathesized from copolymers similar to those used for the manufacture of weak acid cation-exchange resias. For example, uader appropriate temperature and pressure conditions, a weak acid resia reacts with a polyfuactioaal amine, such as dimethylaminopropylamine [109-55-7] (7) to give a weak base resia with a tertiary amine fuactioaaHty. 

SG- PVSA composite film obtained in the presence of Tween 20 possesed the best cation exchange characteristics. The maximum soi ption capacity of SG-PVSA composite film obtained under optimal condition was, mol/g 6,2-10 for Fe (Phen), 2,3T0 for Phodamine 6G and 2,5T0 for protons. The optimal content of PVSA in sol was 4%. 

Cation exchange resin. Conditioned before use by successive washing with water, EtOH and water, and taken through two H -Na -H cycles by successive treatment with M NaOH, water and M HCl then washed with water until neutral. 

Figure 12.22 SFC-GC analysis of aromatic fraction of a gasoline fuel, (a) SFC trace (b) GC ttace of the aromatic cut. SFC conditions four columns (4.6 mm i.d.) in series (silica, silver-loaded silica, cation-exchange silica, amino-silica) 50 °C 2850 psi CO2 mobile phase at 2.5 niL/min FID detection. GC conditions methyl silicone column (50 m X 0.2 mm i.d.) injector split ratio, 80 1 injector temperature, 250 °C earner gas helium temperature programmed, — 50 °C (8 min) to 320 °C at a rate of 5 °C/min FID detection. Reprinted from Journal of Liquid Chromatography, 5, P. A. Peaden and M. L. Lee, Supercritical fluid chromatography methods and principles , pp. 179-221, 1987, by courtesy of Marcel Dekker Inc.

If the experimental conditions are such that the equilibrium is completely displaced from left to right the cation C+ is completely fixed on the cation exchanger. If the solution contains several cations (C+, D+, and E + ) the exchanger may show different affinities for them, thus making separations possible. A typical example is the displacement of sodium ions in a sulphonate resin by calcium ions ... 

Fig. 11a. Fractionation of (/) chymotrypsinogen A, (2) cytochrome C and (3) lysozyme on strong cation exchangers, a) Support Fractogel TSK 650(s)SP (conventional type) sample, 1 mg each flow rate, 1 ml/min column size, 150 x 10 mm T.D. Solvent A = 0.02 mol/1 phosphate, pH 6.0 solvent B = A + 1 mol/1 NaCl gradient, 0-10 min, 0% B 10-70 min, 0-100% B. b) Support Fractogel EMD 650(s)SO( — (tentacle type) conditions as in (a) [78]...

Where hardness removal is required, the simplest pretreatment method for smaller, lower pressure boiler plants (below 200-300 psig) is to use a cation-exchange softener. This removes the calcium and magnesium at source and converts the bulk of temporary hardness salts into sodium bicarbonate (NaHC03), which decomposes to form sodium carbonate (soda ash) but does not scale under normal boiler operating conditions. 

For an individual pesticide (e.g., carbendazim) or a limited class of pesticides (e.g., carbamates, benzoylphenylureas, pyrethroids ), it may be possible to optimize the SPE conditions so that the pesticide(s) are selectively retained on the cartridge. A wash step can be introduced to elute the matrix selectively, thus producing an extremely clean extract. For example, when cleaning up sample extracts for carbendazim analysis, a cation-exchange (SCX) cartridge may be used and the pH carefully controlled to retain the carbendazim firmly, whilst the co-extractives are washed to waste. The carbendazim residues can then be eluted from the column by adjusting the pH. 

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