Charles A. Lucy

Improved resolution of inorganic anions in capillary electrophoresis by modification of the reversed electroosmotic flow and the anion mobility with mixed surfactants

Abstract The standard method for analysis of inorganic anions by capillary electrophoresis involves adding tetradecyltrimethylammonium bromide (TTAB) to the buffer to reverse the electroosmotic flow (EOF). The resolution achieved using this procedure is greatly improved by adding the zwitterionic surfactant, coco amidopropylhydroxydimethylsulfobetaine (CAS U) to lower the magnitude of the reversed EOF and alter the anion mobilities. In a mixed surfactant system, varying the ratio of TTAB to CAS U allows monotonic alteration of the EOF from fully reversed (TTAB alone) to near zero (CAS U alone). The total surfactant concentration (if greater than the critical micelle concentration) and buffer pH have minimal effect on the EOF. In addition, the anion mobilities can be altered to a minor degree by varying the ratio of TTAB to CAS U, which contributes to the improved anion resolution. The effect on the EOF of other surfactant systems involving CAS U and other cationic or anionic surfactants is also studied.

Determination of surfactant concentration using micellar enhanced fluorescence and flow injection titration.

Flow injection titration was used for the determination of anionic, cationic, nonionic and zwitterionic surfactants. The procedure was based on the micellar-enhanced fluorescence of 1,8-anilino-naphthalene sulfonate (ANS). Samples were injected into a carrier stream of phosphate buffer and 1.0 mol l(-1) NaCl. The sample then passed through a mixing chamber which generated the exponential peak shape needed for the titration as well as diluted the sample in the carrier stream to control the pH and ionic strength of the sample. The peak width was linearly related to the logarithm of the surfactant concentration. The minimum detectable concentration was governed by the critical micelle concentration for anionic, zwitterionic and nonionic surfactants, but below the critical micelle concentration for cationic surfactants. The linear range extended for approximately 1.5 orders of magnitude. Reproducibility ranged from 12% at the lower end of the calibration range to 1.1% at higher concentrations. For SDS recoveries of 82-108% were achieved in matrices as concentrated as 1 mol l(-1) in NaCl or Na(2)SO(4).

Factors affecting selectivity of inorganic anions in capillary electrophoresis.

Capillary zone electrophoretic separations of inorganic anions are largely governed by the intrinsic (infinite dilution) mobility of the anion. This in turn is a function of the hydrodynamic friction caused by the size of the ion and the dielectric friction caused by the charge density of the anion re-orienting the surrounding solvent. The influence of these factors on the mobility of anions is examined in both water and nonaqueous solvents. The influence of other experimental parameters, such as ionic strength, ion association, electroosmotic flow modifier concentration, and the addition of complexing agents such as polymeric cations, cyclodextrins, crown ethers and cryptands are also reviewed. From this discussion, some rules of thumb as to when different approaches will be most effective are drawn.

Recent advances in ion chromatography: A perspective

Abstract To properly ascertain what are the key advances in a field one must have a vision of where the field is going in the future. Generally such a view is myopic and focuses largely on ones own field of study. To give a clearer view of where ion chromatography is now and where it is going in the future, Herb Laitinens ‘Seven Ages of an Analytical Method’ was applied to ion chromatography. Currently, ion chromatography lies in the sixth age, wherein the method is a standard procedure. Recent advances that open new areas of application for ion chromatography are noted as these pull the technique back into the fifth age. However, ion chromatography also teeters on the brink of the seventh age of an analytical instrument, in which a technique of greater convenience, selectivity and sensitivity (i.e., capillary electrophoresis?) replaces it. Thus many of the current advances in ion chromatography are improvements relative to capillary electrophoresis. While often overlooked, probably the most notable improvements in ion chromatography in recent years have been in its convenience. Such improvements are essential for a technique in its sixth age. For if the procedure is to remain a standard method, analysts must be confident and comfortable in its use.

Separation of positional and structural isomers by cyclodextrin-mediated capillary zone electrophoresis

Previously derived models for optimization of cyclodextrin (CD)-mediated capillary zone electrophoresis (CZE) referred only to the separations of enantiomers. These models assume that the mobility of the inclusion complexes of the two solutes are equal (i.e., μACD = μBCD). With other types of solutes, such as positional and structural isomers, this assumption is not valid (i.e., μACD ≠ μBCD). In this work, the effectiveness of the model of Wren and Rowe, which was developed for enantiometric separations, is evaluated for cyclodextrin-mediated CZE of other types of solutes. Experimental data is obtained for the α-cyclodextrin-mediated separation of positional and structural isomers, modelled by nitrophenols and phenylbutyric acids, respectively. It was found that the mobilities of the inclusion complexes of the isomers differed from one another (μACD ≠ μBCD) and that the complex mobility did not correlate with the solute mobility, the formation constant or the “quality of fit”. Despite the complex mobilities for the positional and structural isomers not being equal, the Wren and Rowe model is nonetheless effective for predicting the optimum α-cyclodextrin concentration. Only when the formation constants for two isomers are approximately equal (KACD ≈ KBCD) does the optimum α-cyclodextrin concentration differ from that predicted.

What are the unanswered (and unasked) questions in ion analysis

Abstract This article “free-associates” through a few of the questions that remain unanswered and unasked in ion analysis. Some of the questions nosed into include: What limits precision and accuracy in ion chromatography and capillary electrophoresis?; What is the cause of nonlinear calibrations curves in suppressed ion chromatography?; Why do we use ion chromatography?; What is meant by speciation in ion chromatography?; What self-imposed limits are restricting our application of ion chromatography?; and What is the relationship between ion chromatography and capillary electrophoresis for ion analysis?.

Displacement post-column detection reagents based on the fluorescent magnesium 8-hydroxyquinoline-5-sulfonic acid complex

Abstract This non-specific fluorescent reagent is based on the displacement mechanism, wherein a metal ion displaces Mg 2+ from the Mg(CDTA) 2− complex (CDTA = cyclohexylenedinitrilotetraacetic acid). The liberated Mg 2+ then reacts with 8-hydroxyquinoline-5-sulfonic acid (HQS) to form an intensely fluorescent complex. For simultaneous addition of Mg(CDTA) 2− and HQS 2− , the detector response is governed by the stabilities of both the CDTA and the HQS complexes of Mg 2+ and the displacing metal. Under these conditions, alkaline earth metals respond positively, but no response is observed for transition metals such as Cu 2+ , Ni 2+ and Co 2+ . For the sequential addition of Mg(CDTA) 2− followed by HQS 2− , the detector response is governed by the relative stabilities of only the CDTA complexes of Mg 2+ and displacing metal, and positive signals are observed for Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Mn 2+ , Cu 2+ , Ni 2+ and Co 2+ . The response for Mn 2+ was linear from 25 to 2500 picomoles. The reagent pH affects the background intensity and the displacement kinetics, and to a lesser extent the fluorescence intensity.

A simple means of generating pH gradients in capillary zone electrophoresis

Abstract Injection of a “pulse” of a secondary pH buffer just prior to the injection of analyte provides a mild pH gradient within the primary pH buffer. Separations of weakly acid or basic analytes are easily optimized using this gradient. A model explaining the impact of the pulse of secondary buffer is proposed. Experiments verify that the strength of the pH gradient depends on the effective width of the pulse, the difference in mobility of the analyte in the two buffers and on the difference in mobility between the analyte and the pulse of secondary buffer. Sixteen chlorinated phenols are baseline separated in 25 min using a 45-mM phosphate-15 mM tetraborate buffer (adjusted to pH 7.0) when 2.6% of the capillary is filled with 22.5 mM phosphate-7.5 mM tetraborate (adjusted to pH 10.0).

Ion chromatographic determination of chelating ligands based on the postcolumn formation of ternary fluorescent complexes

The formation of the lutetium, trans-1,2-diaminocyclohexane-N,N,N,N-tetraacetic acid, and 8-hydroxyquinoline-5sulfonic acid (Lu-CDTA-HQS) fluorescent ternary complex is used to determine chelating ligands. The ligands are complexed with Lu 3+ and separated on an anion-exchange column. The eluent mixes with 1 mM CDTA-HQS postcolumn reagent. CDTA displaces the analyte ligand from its lutetium complex to form the more stable Lu-CDTA complex, which further complexes with HQS to form the ternary fluorescent complex. The mechanism and kinetics of this postcolumn reaction are discussed. The scheme of sequential addition of CDTA-HQS at low pH and followed by NaOH enables detection of triethylenetetramine (Trien), tetraethylenepentamine (Tetren), nitrilotriacetic acid (NTA), ethylene glycol-bis(β-aminoethylether) N,N,N,N,-tetraacetic acid (EGTA), ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) and CDTA. The ligands of NTA, EGTA, EDTA, DTPA, and CDTA were separated by ion chromatography and detected with postcolumn addition of CDTA-HQS (pH 2.8) and then NaOH. The detection limits obtained were l.lo -7 M (ca. 1 ng) for NTA, 2.5.lo -8 M (ca. 0.5 ng) for EGTA, 2.5.lo -8 M (ca. 0.5 ng) for EDTA, 5.lo -8 M (ca. I ng) for DTPA, and 5.lo -8 M (ca. I ng) for CDTA. The response linear range for NTA was from 100 ng to 2.5 ng ; for EGTA from 750 ng to 1 ng ; for EDTA from 750 ng to 1 ng ; for DTPA from 5000 ng to 1 ng ; and for CDTA from 5000 ng to 2 ng. The existence of ten-fold excess alkaline earth metal and transition metal ions did not interfere with the determination of these chelating ligands after a metal-exchange sample pretreatment step was included in the procedure. The recovery rates were greater than 80%.

Co-current chromatography: A new mode of liquid-liquid chromatography

Abstract When two immiscible solvents are forced to flow simultaneously through narrow tubing, the resultant flow consists of alternating discrete segments of each phase. One solvent, the continuous phase, forms a stationary wetting film along the walls of the tubing. If this film is sufficiently thick, a difference in the velocities will develop between the two phases which can be used for chemical separation. This differential velocity forms the basis for a new mode of liquid-liquid chromatography termed “co-current chromatography”. Equations describing the retention and band-broadening behavior in co-current chromatography are derived and experimentally verified.