Retention Model for Anionic, Neutral, and Cationic Analytes in Reversed-Phase Ion Interaction Chromatography

Extended thermodynamic approach to ion interaction chromatography. A mono- and bivariate strategy to model the influence of ionic strength

Recent breakthroughs in the theory of ion interaction chromatography (IIC) permit new analyses of the dependence of retention on different interdependent factors. The influence of the ionic strength / on the surface potential, the Donnan effect, and salting effects are taken into account to model the chromatographic behaviour of charged analytes in IIC. The most reliable experimental results found in the literature were used to test the retention equations that were developed following both a monovariate (/ changes as the concentration of H, ion interaction reagent, changes) and a bivariate (/ changes because of the simultaneous variation of H and of the background electrolyte concentrations) approach. The present extended thermodynamic model builds on the sound intuition of the electrostatic approach and proves to provide the most successful and exhaustive quantitative explanation of experimental evidence. It is also able to rationalise the less extensive agreement between the pure electrostatic approach predictions and experimental results. The adequacy of the model is supported by physically reliable estimates of the adjustable constant (ion-pair constants, deltaG degrees). Moreover statistical practice demonstrates that all the adjustable parameters (three at most) are statistically significant. A linear, zero crossing function with unit slope is obtained when k(pred) is plotted against k(exp). The mean percent error between k(pred) and k(exp) is 4.5% at most. In the absence of H the present retention equation reduces, as expected, to the relationship that describes the influence of / on the retention behaviour in reversed-phase liquid chromatography.

Determination of sulfur oxides formed during the S(IV) oxidation in the presence of iron

The reaction products (i.e., sulfate (SO4(2-)) and dithionate (S2O6(2-))) of S(IV) oxidation in the presence of iron(III) under different experimental conditions were investigated. Ion-interaction chromatography was used for the separation of sulfate and dithionate using tetrabutylammonium hydroxide (TBAOH) as an ion-pair reagent. The chromatographic method was optimized by varying the composition of the mobile phase (i.e., concentration of TBAOH, acetonitrile and Na2CO3) and by varying the flow rate of the mobile phase. The method was successfully applied to the determination of dithionate formed during the S(IV) oxidation in the presence of Fe(III). In air-saturated solutions sulfate was observed as the only product, while in N2-saturated solutions dithionate was also determined, but it is the minor reaction product and represents about 4% of the total amount of oxidized HSO3- under the studied conditions.

Analysis of complex formation between crown ethers and potassium ion by determining retention factors in reversed-phase high-performance liquid chromatography

Retention factors in reversed-phase high-performance liquid chromatography (RP-HPLC) were utilized for the analysis of complex formation reaction of four crown ethers with potassium ion in water-methanol media. The crown ethers were injected as analytes, potassium ion was present in the eluent, and three types of reversed-phase columns were used. The retention factor of the crown ethers decreased with increasing concentrations of potassium ion in the eluent, which is attributed to the change in the species of the crown ether from a neutral ligand to a positively charged complex. The complex formation constants were determined by analyzing the changes in the retention factor with a non-linear least-squares method.

Carboxylic acids: prediction of retention data from chromatographic and electrophoretic behaviours

A review of the main results reached in the prediction of retention data of carboxylic acids, inferred by their chromatographic and electrophoretic behaviour, is presented. Attention has been focused on the main separation methods used in carboxylic acids analysis, that is ion-exclusion, anion-exchange, reversed-phase (RP) liquid chromatography and capillary electrophoresis. Papers proposing mechanistic models as well as chemometric and multilayer feed-forward neural network analysis of ion chromatography (IC) and RP chromatographic retention data were reviewed. Principal component analysis, PCA, sequential simplex method and simultaneous modelling of response surfaces through simple nonlinear models (not related to equilibria involved in retention) have been considered. Computer simulations for the prediction of retention data have also been discussed. A quick overlook on the prediction of capacity factors of analytes by less common determination methods such as thin-layer, gas chromatography and supercritical fluid chromatography has also been done.