Cation-exchange with reversed-phase

Despite these difficulties, there are a number of approaches to automate multidimensional chromatographic separations and to couple them to mass spectrometry. Column switching that combines, for example, cation exchange with reversed-phase HPLC, followed by on-line ESI mass spectrometry, has been... 

Proteins from albumin/immunoglobulin-depleted CSF (see Fig. 4) were digested with trypsin and the resultant peptides fractionated by two rounds of chromatography using cation exchange and reversed phase columns. 

Sample purification and concentration used double-mechanism (cation exchange and reversed phase) solid phase extraction cartridges. The extracts from 5 mL of urine were dried, redissolved with 100 /aL of running buffer, and injected, to achieve a detection limit of 100 ng/mL. For peak identification, not only the retention times were used, but also the on-line-recorded UV spectra of the peaks, which were compared to computer-stored models. 

Cleanup and concentration of both polar and nonpolar anticoccidial drugs from coextracted substances have mainly been accomplished with polar sorbents, such as silica (379, 391, 396, 398, 399, 402, 407, 410, 411), alumina (227, 257, 403, 404, 406, 412), or Florisil (388), since they provide high recovery of the analytes. However, in many cases, cation exchange sorbents (409), reversed-phase sorbents (405), or combinations of silica with reversed-phase sorbents (395, 400), alumina with Sephadex LH-20 (406), or alumina with Sephadex LH-20 and silica (397, 401) sorbents have been reported to be a powerful approach for the isolation and/or cleanup of these compounds. It is significant to note that cleanup systems using alumina are not suitable for isolation of lasalocid residues since they are irreversibly bound to the sorbent. 

The ion-exchange constant does vary from one pair of ions to another therefore selectivity can be controlled by appropriate choice of the counter ion. However, selectivity is more conveniently manipulated by controlling the pH of the mobile phase and taking advantage of differences in the values of the analytes to be separated. In contrast with reversed-phase liquid chromatography (see section 3.6.2.1) the retention of weak acids and weak bases will reach a maximum when the compounds are in their ionized forms. Zwitterionic compounds such as amino acids, peptides and proteins can be separated on anion exchangers or cation exchangers. 

In contrast to conventional cation exchangers, a reversed elution order is observed with crown ether phases, which is mainly determined by the size ratio between crown ether ring and alkali metal ion. Due to the high affinity of poly(benzo-15-crown-5) toward potassium and rubidium ions, these are more strongly retained than lithium, sodium, and cesium ions, respectively. However, the complexing properties of crown ethers also depend on the counter ion being employed. Thus, in potassium salts, for example, an increase in retention in the order KC1 < KBr < KI is observed with an increasing size of the counter ion. 

Size exclusion, reversed-phase, paired ion reverse-phase, and ion exchange (cation and anion) HPLC have been used for selenium speciation. Size exclusion, also used in combination with affinity chromatography, has been used for the determination of selenoproteins and studies of interactions of selenium with proteins in the body. Ion-exchange and reversed-phase HPLC are often used for separation... 

In the last decade, a large number of sorbents were made commercially available, with a large diversity in composition (polymeric structures for instance, that enhance sorbent compatibility to samples with more extreme pH values) and in functional groups (for anion and cation exchange coupled with reversed phase characteristics, originating mix-mode sorbents). 

---