On‐line redox derivatization liquid chromatography using double separation columns and one derivatization unit

Abstract

A new on-line redox derivatization technique using double separation columns and one redox derivatization unit was presented for enhancement of separation selectivity of HPLC. This on-line redox derivatization HPLC system consisted of two separation columns and one redox derivatization unit placed between them. The redox reaction proceeds in the derivatization unit so that an analyte compound migrates as its original form in the first column, while as its oxidized or reduced form in the second column. The retention of the analytes is controlled by the lengths of the two separation columns in this system. We adopted a small column packed with porous graphitic carbon (PGC) as a redox derivatization unit and two C18 silica columns treated with hexadecyltrimethylammonium chloride as separation columns. The redox activity of PGC and the efficiency of the on-line redox derivatization HPLC system for enhancement of separation selectivity were investigated using EDTA complexes of some metal ions. Original untreated PGC and PGC treated with hydrogen peroxide completely oxidized Co(II)-EDTA and converted it to Co(III)-EDTA, while the other metal complexes eluted as their original oxidation states throughout the system. Selective separation and determination of cobalt in a reference copper alloy by the developed method were demonstrated.

Determination of Trace Amount of Cobalt in a Steel Sample by Two-dimensional On-line Redox Derivatization Liquid Chromatography

“…[1][2][3][4] As the charges on the trivalent and divalent metal-edta complexes are typically -1 and -2 respectively, the former metal complexes usually are eluted faster than the latter ones in anionexchange chromatography. Therefore it should not be difficult to separate cobalt from iron and chromium after they have been converted to their edta complexes because the latter two metal elements form trivalent complexes with edta, Fe(III)-edta and Cr(III)-edta, while cobalt does divalent one, Co(II)-edta, under the complexation conditions described in Sec.…”

“…1,3,4) We have thus used in this study a short column (4.6 mm i.d. × 35 mm) packed with PGC particles (3.5 μm) treated with 5.0 × 10 -4 mol/L aqueous hydrogen peroxide solution as the on-line derivatization unit.…”

“…1,2) This method utilizes two separation columns and one derivatization unit placed between them. The analyte compound is derivatized in the derivatization unit after elution from the first column and migrates in the second column as the derivatized chemical form different from the original form.…”

“…Consequently, when the two separation columns containing the same packing material are used, the retention factor, k, of an analyte in this on-line derivatization HPLC system is given by where k 1 and k 2 are the retention factors of the original and derivatized forms of the analyte, and L 1 and L 2 denote the lengths of the first and the second separation columns, respectively. Equation (1) indicates that the retention of the analyte can be controlled by the lengths of the two separation columns; the difference between k 1 and k 2 should be as large as possible since the k value can be changed between k 1 and k 2 depending on the ratio of L 1 to L 2 . If the analyte would be eluted from the first column together with some coexsisting components in the sample matrix, the derivatized form of the analyte can be separated from them during elution in the second column because the retention factor of the derivatized form is different from that of the original form of the analyte.…”

“…So far we have found out that porous graphitic carbon (PGC) shows catalytic redox activity, which can be modified by treating PGC with a suitable oxidizing or reducing agent. 1,3,4) For example, PGC oxidizes ethylenediaminetetraacetate (edta) complex of Co(II) (Co(II)-edta) to Co(III)-edta if it has been treated with hydrogen peroxide solution, while it reduces Co(III)-edta to Co(II)-edta when PGC treated with sodium sulfite solution is used. Employing a short column packed with PGC particles as the derivatization unit, we demonstrated that selective separation and determination of cobalt in a copper alloy can be achieved by on-line redox derivatization HPLC.…”