Retention mixed-mode

Ion exchange resins based on poly(styrene-divinylbenzene) backbones display mixed mode retention mechanisms. The ion exchange functionality (sulfonic acid or carboxylic acid for cation exchangers and quartemary or primary, secondary, or tertiary amines for anion exchangers) contributes to the ionic mechanism and the backbone polymer to hydrophobic retention. This is exemplified... 

Obviously, mixed-mode retention can be advantageous in some difficult separation problems. Of course, this remark does not imply cancelling the well-established rules in HPLC. 

Fig. 22. Mixed mode retention. Logarithmic retention factor for oligoriboadenylic acids vs. phosphate concentration in the mobile phase, pH 6.3. The figures at the curves indicate the number of adenylyl phosphate residues in the samples. (From Ref. 74) with permission)...

Sun, L.F. Carr, P.W. Mixed-mode retention of peptides on phosphate-modified polybutadiene-coated zirconia. Anal. Chem. 1995, 67, 2517-2523. 

By varying the cross-linker, polyethylenimine ligand loading, and the use of a monoepoxide as a capping agent Kopaciewicz el al. (40) examined the mixed-mode retention effects induced. The cross-linker s chemistry influenced retention selectivity. Since secondary and primary amines are still... 

The linear model [ln(k) vs. is only an approximation. This is especially true for ionizable compounds as they may exhibit a mixed-mode retention. 

Cross, R..F, Ostrowska, E., Muralitharan, H., and Dunshea, F.R. (2000) Mixed Mode Retention and the Use of Competing Acid for the Ag+-HPLC Analysis of Undeiivatized Conjugated Linoleic Acids, J. High Resolut. Chromatogr. 23, 317-323. 

Although Certify is a mixed mode sorbent with C8 and sulfonic acid moieties, the authors rationalized that the hydrophobic retention on this sorbent is more dominant and caused the nonretention of certain drugs during methanol wash. The weak WCX ion exchanger was also excluded for similar reasons. Both the mixed mode strata-X-C and the ion exchange sorbent SCX were found to be most amenable for the derivatization-based GC/MS analysis and both yielded pure extracts. However, the yields were consistently lower with strata-XC than SCX and the authors hypothesized that this was due to the inability of the 5% ammonia/methanol eluent to completely disrupt the hydrophobic and dipolar interactions between the analytes and XC. 

As a consequence of the development of extraction methods for STA based on mixed-mode SPE columns, as well as of the recent introduction of instruments for the automated sample preparation allowing efficient evaporation and derivatization of the extracts, full automation of STA methods based on GC-MS analysis is also available. It needs GC-MS instalments equipped with an HP PrepStation System. The samples directly injected by the PrepStation are analyzed by full scan GC-MS. Using macrocommands, peak identification and reporting of the results are also automated. Each ion of interest is automatically selected, retention time is calculated, and the peak area is determined. All data are checked for interference, peak selection, and baseline determination. 

Fig. 2.6.3. Mixed-mode HPLC-ESI-MS summed ion chromatogram of a sediment extract (32-36 cm depth core slice) showing resolution of NPEOs and NPs. Numbered peaks correspond to NPEOs and [13CeJNPEOs with the indicated number of ethoxy groups (0 = NP, [13C6]NP 1 = NPEOi, [13C6]NPEOi, etc.). Peaks A and B are the internal standards, re-NP and re-NPE03, respectively. (Note the discontinuity at retention time 25.8 min, corresponding to the shift in MS polarity from positive to negative ion mode.)...

A pivotal step in the analytical process is sample preparation. Frequently liquid-liquid extractions (LLEs) are used. Solvents, pH, and multiple back extractions are all manipulated to increase selectivity and decrease unwanted contaminants before injection on the GC system. Solid phase extraction (SPE) is more convenient than it used to be because of an increase in commercially available SPE columns. SPE columns are packed with an inert material that binds the drug of interest, allowing impurities to pass through. As with LEE, solvent choices and pH affect retention and recovery. There are three commercially available types of SPE columns, diatomaceous earth (which uses the same principles as LLE), polystyrene-divinylbenzene copolymer, and mixed mode bonded silica (Franke and de Zeeuw, 1998). 

The CEC phases must be capable of carrying a charge to generate an EOE and appropriate moieties to facilitate the chromatographic processes. Silica-based reversed-phase packing materials have been most widely used in CEC. The use of polymeric and mixed-mode bonded particles has also been reported. Eor the silica-based phases, the carbon chains bonded on the silica surface provide the retention and selectivity for analytes, and the residual silanol groups on the surface of the silica are ionizable and generate the EOF. 

Figure 21, Proposed model of adsorbed chiral selector (A-alkylproline)- Cu(U)-[free amino acid] mixed chelate complex, The lipophilized proline selector is held in position via intercalation of the alkyl chain. Case A the alkyl part of the mixed chelate complex is fixed by hydrophobic interactions with stationary phase (RP-J). Case B the complex formation is stabilized by other types of hydrophobic attraction. Chiral recognition and elution order is therefore not only dependent on the simple and isolatedly viewed chelate complex stability. In general, retention and chiral recognition in chiral LC is based on mixed-mode adsorption/dcsorption processes which act synergisticallv and also antagonistically with respect to the observed chiral resolution and intermolecular complex formation.

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