Comparison of Reversed Stationary Phases for the Chromatographic Separation of Inorganic Analytes Using Hydrophobic Ion Mobile Phase Additives

Abstract

Journal of Liquid ChromatographyPublication details, including instructions for authors and subscription information:ABSTRACT A1 kyl-modified s i l i c a (RSi) and polystyrenedivinyl benzene (PRP-1) stationary phases a r e compared f o r the chromatographic separation of inorganic analyte anions and cations using hydrophobic ions of opposite charge as mobile phase additives. alkylammonium s a l t s were used f o r anion separations and alkyl sulfonate s a l t s f o r cation separations. Two major e q u i l i b r i a influence the retention of analyte ions on PRP-1. These are: retention of the hydrophobic ion on PRP-1 and an ion exchange s e l e c t i v i t y between the hydrophobic counterion and the analyte ion. a t residual silanol groups, which a c t a s weak cation exchange s i t e s . analyte retention are identified. favorable eluting conditions f o r the separation of inorganic ionic analytes. Of particular i n t e r e s t i s the potential use of PRP-1 and RSi columns f o r the separation of inorganic cations; condit i o n s f o r the separation of a l k a l i metals and a l k a l i n e earths a r e discussed.

Liquid chromatographic separation and indirect detection of inorganic anions using iron(II) 1,10-phenanthroline as a mobile phase additive

“…Since Fe(phen)32+ is divalent this represents 34.4 pequiv/column of anion exchange sites. This is typical of the apparent anion and cation exchange capacities generated when using R4N+ salts (5,9) and RS03" salts (9,11) as mobile phase additives for inorganic analyte anion and cation separations, respectively, and the lower end of exchange ca-pacities used in single column ion chromatography (17). If the mobile phase organic modifier concentration for a given Fe(phen)32+ concentration is increased, the equilibrium amount of the Fe(phen)32+ salt retained on the PRP-1, and subsequently the apparent anion exchange capacity, decreases.…”

“…Also, inspection of the C18 particles indicated that not all the Fe-(phen)32+ salt was removed. Other studies (9) have indicated that free silanol sites within the Zorbax C18 can participate in cation exchange and it is possible that part of the Fe-(phen)32+ is retained as a cation, thus eliminating it as a source for anion exchange. ( 2) PRP-1 being stable throughout the pH range, unlike the C18, allows chromatography at a basic pH.…”

“…When compared to C7S03~Li+ retention on PRP-1 (9) the coverage of the Fe(phen)3S04 for a given mobile phase concentration is greater but the rate at which it increases in surface coverage with increased mobile phase concentration is much less. For example, at 0.0003 M Fe(phen)3S04,27 µ of Fe(phen)3S04 is retained per column while the amount of C7S03"Li+ retained per column is estimated (9) to be about 2.5 µ . From Figure 1A a 10-fold increase in Fe(phen)3S04 concentration increases the amount retained by about 15 µ while for C7S03~Li+ (9) the increase is about 50 µ .…”

“…For example, at 0.0003 M Fe(phen)3S04,27 µ of Fe(phen)3S04 is retained per column while the amount of C7S03"Li+ retained per column is estimated (9) to be about 2.5 µ . From Figure 1A a 10-fold increase in Fe(phen)3S04 concentration increases the amount retained by about 15 µ while for C7S03~Li+ (9) the increase is about 50 µ . This difference is probably due to (1) Fe(phen)32+ has higher retention on PRP-1 due to increased hydrophobicity, (2) Fe-(phen)32+ is larger in size, and (3) since Fe(phen)32+ is a dication, it builds up surface charge in the primary layer at a much greater rate, thus, repelling additional Fe(phen)32+.…”

“…This same strategy has been shown to be applicable to the separation of inorganic analyte ions (5)(6)(7)(8)(9)(10)(11)(12); for example, to separate inorganic anions, a tetraalkylammonium (R4N+) salt can be used as the mobile phase additive (5)(6)(7)(8)(9) while for cations an alkylsulfonate (RS03") salt can be used (9)(10)(11)(12).…”