Eluent concentration, high, separations

Atteberry and Boyd have separated TcO and ReO on Dowex-2 resin using a mixture of 0.1 M (NH j SO and 0.1 M (NH j SCN as eluent. The two elution peaks are partially resolved, but the cross contamination is rather high. Much better results are obtained if perchlorate is used as the elutriant . Because of some peculiarities of commercial resins, the highest practically attainable separation factors of TcO and ReO are l(f -10 It has been found that the volumes of the eluents consumed in the elution of pertechnetate and perrhenate are inversely proportional to the perchlorate concentration. The separation on Dowex 1-X4 is described in Fig. 6. Partial reduction of TcO or ReO reduces the efficiency of the chromatographic separation since the elution peaks frequently become blurred . 

The suppressor column obviously will eventually become depleted and will have to be regenerated (with HCl for the cation exchanger and with NaOH for the anion exchanger). The suppressor column is usually a small-volume bed of a high-capacity resin to minimize band spreading in the colunm. Since microgram or smaller quantities of analytes are usually separated, a low-capacity analytical column is used coupled with relatively low eluent concentration (1 to 10 mM). 

When considering the salt load of eluents, new developments in column technology provide columns with high separation efficiency even at low (buffer) salt concentrations (e.g., Michalke et al. 2001). For pH stability during separation, the sample should be prepared in the respective LC buffer. Sometimes it may be necessary to dilute the sample in this buffer when separation is compromised... 

The authors found that Co(II) and Zn(II) could not be separated on the bonded-phase column. In addition, only low eluent concentrations could be used with the low capacity bonded-phase column, and this proved to be important when large concentrations of acid samples were injected. Separation of Co(II) and Ni(II), or Cu(II) and Ni(II), could not be achieved for samples with high levels of Ni(II) using the resin-based column and a citrate eluent. Tartrate eluents improved the separations, and the authors eventually used tartrate eluents for the assays. An important outcome of this study was the realization that either the gradient or the complexing agent could be changed in order to minimize interferences between the seven cations and the main constituents of those materials analyzed. 

HOPC uses a column packed with porous materials that have a pore diameter close to a dimension of the solvated polymer to separate. A concentrated solution of the polymer is injected into the solvent-imbibed column by a high-pressure liquid pump until the polymer is detected at the column outlet. The injection is then switched to the pure solvent, and the eluent is fractionated. A schematic of an HOPC system is illustrated in Fig. 23.1. A large volume injection of a concentrated solution makes HOPC different from conventional SEC. 

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