Three HPLC columns packed with 3 microm, sub-2 microm, and 2.7 microm Fused-Core (superficially porous) particles were compared in separation performance using two natural product mixtures containing 15 structurally related components. The Ascentis Express C18 column packed with Fused-Core particles showed an 18% increase in column efficiency (theoretical plates), a 76% increase in plate number per meter, a 65% enhancement in separation speed and a 19% increase in back pressure compared to the Atlantis T3 C18 column packed with 3 microm particles. Column lot-to-lot variability for critical pairs in the natural product mixture was observed with both columns, with the Atlantis T3 column exhibiting a higher degree of variability. The Ascentis Express column was also compared with the Acquity BEH column packed with sub-2 microm particles. Although the peak efficiencies obtained by the Ascentis Express column were only about 74% of those obtained by the Acquity BEH column, the 50% lower back pressure and comparable separation speed allowed high-efficiency and high-speed separation to be performed using conventional HPLC instrumentation.

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Illinois 6290 IDonnan dialysis has been shown to be a useful, quantitative method for preconcentration and matrix normalization in the determination of ionic species (1-5). In the method a reproducible fraction of the analyte ions is transported from a sample solution through an ion exchange membrane into a small volume of a relatively high ionic strength receiver solution. Quantification is based on linear calibration curves that are generated with a fixed-time kinetic model. The receiver electrolyte is an ideal matrix for electrochemical methods and is also suitable for many spectroscopic procedures. Both sheet and tubular (6, 7) membranes have been used. The latter is a particularly attractive form because of the high membrane area to receiver volume ratio that is possible. In turn, greater preconcentrations can be obtained.The preconcentration capability of Donnan dialysis also depends upon the rate of ion transport through the membrane. Several limiting steps for the overall flux in Donnan dialysis have been postulated including concentration polarization at the solution-membrane interfaces and mass transport resistance within the membrane (4,8). The influences of the membrane structure and the receiver solution composition on the dialysis rate have also been examined (9-13).Donnan dialysis with a static receiver solution, a magnetic stirrer for sample convection, and sheet membranes yields only low preconcentrations in reasonable times (up to 1 h) when the receiver volume is compatible with the requirements of routine analytical methods, i.e. a few milliliters. Enrichment factors (the ratio of the analyte concentration in the receiver after dialysis to the initial sample concentration) are typically less than 10. High enrichments have been obtained with sheet membranes by using long dialysis times with a continuous flow system (14) and by using receiver volumes of less than 1 mL (15). With tubular membranes, receiver recirculation, and sample convection with a magnetic stirrer enrichment factors of 40-50 have been obtained in 20 min (6).The receiver recirculation method has two drawbacks. The relatively large dead volume of common pumps limits the attainable enrichment factors, and the cost of pumps with the capability to provide a constant flow rate is relatively high. In this paper, two simple and inexpensive methods are described for obtaining high enrichments in short times.EXPERIMENTAL SECTION All chemicals used were reagent grade. The receiver solution was generally a 0.2 M MgS04, 0.5 mM A12(S04)3 mixture that was prepared from a concentrated stock solution which had been purified by electrolysis for at least 1 week at a large mercury pool cathode at -1.3 V vs. SCE. This receiver had been previously shown to be well-suited for the Donnan dialysis of divalent cations (5, 10). All water used with distilled and doubly deionized.The membrane used was Nafion 811 cation exchange tubing (Du Pont Polymer Products, Wilmington, DE) with dry dimensions of 0.64 mm i.d. and 0.89 mm 0.d. Prior to initial use, each ...

A sensor which uses an anion-exchange membrane to enclose a small volume electrolysis cell has been demonstrated to be suitable for nitrate determinations. The three-electrode cell includes a constrained mercury column indicator electrode; filter paper, which is impregnated with a 0.1 M KCI-0.01 M ZrOCI2 electrolyte, serves as the constraining barrier and as the spacer for the thin-layer electrolysis chamber. The anion-exchange membrane sheath permits transfer of nitrate from the sample into the electrolysis chamber by Donnan dialysis and excludes several species which would otherwise interfere.Controlled potential electrolysis at -1.25 V vs. Ag/AgCI provides the sensing current, the value of which is proportional to the sample concentration of nitrate. As a steady-state current is not developed with the present design, a defined current sampling time is used. Linear response over 3 orders-of-magnitude nitrate concentration is obtained. The detection limit using the current at 8 min is 6.7 X 10~® M N03.