Concentration-gradient elution

The analyzer uses a heated, high-pressure (up to 4000 psi) anion exchange column, concentration gradient elution with an aqueous acetate buffer for separation and transport of the constituents of the sample mixture, and a recording photometer for detection and quantification of the separated constituents (Fig. 6). Earlier models of this analyzer were housed in standard 24 X 24 X 63 in. cabinets (Fig. 7) however, miniaturized versions with capillary separation columns are now being used (Fig. 8). An anion exchange resin produced by Bio-Rad Laboratories (Aminex A-27) in the size range of 10-15 ju has been found to be satisfactory. 

Efficient separation of rare earths by the HPLC-ICP-AES system has been obtained using a linear concentration gradient elution method and ammonium lactate as the mobile phase (Figure 138). Detection limits for... 

The expected difficulty of a separation also needs to be considered. For an easy separation, a shorter column may be appropriate to obtain a faster analysis. Sample ions that are strongly retained by the column may require a more concentrated eluent in order to avoid overly long retention times. A complex sample may require a programmed increase in eluent concentration (gradient elution) to adequately resolve the sample peaks. 

The acceptable separation of Am(III) and Cm(III) by countercurrent chromatography (CCC) was achieved using both isocratic elution, and a new approach to the creation of the reagent concentration gradient in the stationary phase [1]. This way allows reduce the experiment length. 

Sodium trimetaphosphate was used as an eluting agent for the removal of heavy metals such as Pb, Cd, Co, Cu, Fe, Ni, Zn and Cr from aqueous solutions. Distribution coefficients of these elements have been determined regarding five different concentrations of sodium trimeta phosphate (3T0 M 5T0 M 0.01 M 0.05 M 0.1 M) on this resin. By considering these distribution coefficients, the separation of heavy metals has been performed using a concentration gradient of 3T0 - 5T0 M sodium trimetaphosphate. Qualitative and quantitative determinations were realized by ICP-AES. 

It has been seen that this resin has also some important advantages over the other resins in the literature like high total ion exchange capacity, easy synthesis, lower cost, simple regeneration. Furthermore, very good sepai ations were obtained using a concentration gradient of elution. In these elutions, very low concentrations of sodium trimetaphosphate were used. As a result, the resin synthesized can be used as an adsorbent for the effective removal of Pb, Cd, Co, Cu, Fe, Ni, Zn and Cr from aqueous solutions. 

The conceptual basis for understanding the connection between isocratic and gradient elution is well established and is called "linear solvent strength theory".22 27 Linear solvent strength theory proposes that, for a given solute, mobile phase, and column, if one measures the retention time of an analyte at two organic component concentrations, it will be possible to predict the retention time with any other mobile phase composition. The k value that would be observed in pure water, kw, is related to the actual k by the relationship... 

The limit of detection for this instrument is about 10 pg/ ml for polystyrene in 2-butanone,163 which is close to two orders of magnitude higher than that of the deflection-type DRI. Moreover, the response of the ELSD is linear over only two decades in concentration.163 The ELSD is a useful backup detector when the DRI or UV detectors are not appropriate, e.g., when the UV absorbance or RI change is a function of copolymer composition as well as concentration or in gradient elution systems where changes in solvent composition cause drift in baselines of the UV and DRI detectors. Compounds about as volatile as the solvent are poorly detected by ELSD. 

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