Reversed Electrode Polarity Stacking Mode Research Articles

A high-efficiency capillary electrophoresis-based method for characterizing the sizes of Au nanoparticles

In this paper, we present a highly efficient capillary electrophoresis (CE)-based method for characterizing the sizes of Au nanoparticles (NPs). We used the reversed electrode polarity stacking mode (REPSM) of CE for on-line enhancement of the detection and separation of Au NPs. Under the on-line enhancement separation conditions [buffer: SDS (70 mM) and 3-cyclohexylamino-1-propanesulfonic acid (10 mM) at pH 10.0; applied voltage: 20 kV; using the REPSM for sample concentration], the detection sensitivity toward the Au NPs improved by up to 260-fold. A linear relationship ( R 2 = 0.994) existed between the electrophoretic mobilities and the sizes of the Au NPs within the range of diameters from 5.3 to 59.9 nm; the relative standard deviations of the electrophoretic mobilities for these NPs were below 0.6%. When using these conditions to analyze Au NPs produced through seed-assisted synthesis, a good correlation existed between the sizes obtained using CE and those provided by transmission electron microscopy. Furthermore, the analyses performed using CE were rapid (<4 min) and required very low sample volumes (several nanoliters), yet they proceeded with high sensitivity. These findings confirm that CE is an efficient tool for characterizing the sizes of NPs.

On-line enhancement and separation of nanoparticles using capillary electrophoresis

We describe a rapid, simple, and highly efficient capillary electrophoresis (CE)-based method for the analysis of nanoparticles (NPs). In this study, we used the reversed electrode polarity stacking mode (REPSM) of CE to assess the feasibility of enhancing the detection of Au NPs and Au/Ag NPs, optimizing parameters such as the length of time for which the REPSM was applied, the concentrations of the buffer and the sodium dodecylsulfate (SDS) surfactant, and the pH. Under the optimized on-line enhancement conditions [buffer: SDS (40 mM) and 3-cyclohexylamino-1-propanesulfonic acid (CAPS; 10 mM) at pH 10.0; applied voltage: 20 kV; REPSM applied for 24 s], the detection limits of the Au NPs and Au/Ag NPs increased by ca. 30- and 140-fold, respectively. In addition, when the NPs were subjected to on-line enhancement and separation by CE using diode array detection (DAD), this approach allowed chemical characterization of the NP species. Our results suggest that such CE analyses will be useful for accelerating the rates of fabrication and characterization of future nanomaterials.

MEKC combined with SPE and sample stacking for multiple analysis of pesticides in water samples at the ng/L level

In this work, a new multiresidue analytical method based on MEKC with UV detection combined with SPE as off-line preconcentration strategy, and reversed-electrode polarity stacking mode (REPSM) as on-line stacking procedure, has been developed for the monitoring of 12 pesticides (carbendazim, pirimicarb, metalaxyl, pyrimethanil, procymidone, nuarimol, azoxystrobin, tebufenozide, fenarimol, benalaxyl, penconazole, and tetradifon) that are currently being used in the Canary Islands (Spain). The optimized MEKC buffer, consisting of 100 mM sodium tetraborate and 30 mM SDS at pH 8.5 with 6% v/v 1-propanol, provided baseline resolution of the 12 pesticides in less than 20 min. The developed method was applied to the analysis of mineral, stagnant, and tap water samples. The proposed SPE-REPSM-MEKC-UV method showed high extraction efficiencies with detection limits (LODs) at the low ng/L level providing LOD values down to 64 ng/L for these real samples.

Determination of pesticides in wine using micellar electrokinetic chromatography with UV detection and sample stacking

In this work, the analysis of a group of four fungicides (pyrimethanil, nuarimol, procymidone and cyprodinil) and one insecticide (pirimicarb) by micellar electrokinetic chromatography (MEKC) with UV detection using the on-line preconcentration strategy called reversed electrode polarity stacking mode (REPSM) is proposed. After optimisation, an adequate separation electrolyte for the separation and stacking of these pesticides was obtained which consisted of 100 mM borate, 60 mM sodium dodecyl sulphate (SDS), at pH 9.0 and 2% 2-propanol. The use of this running buffer together with the REPSM preconcentration method provided limits of detection (LODs) between 38.3 and 241 μg/L. In order to apply the developed methodology for the analysis of these pesticides in wine samples, several off-line preconcentration strategies (mainly, solid-phase extraction, SPE, and solid-phase microextraction, SPME) were tested. Although the use of a SPE procedure, optimized in this work for water samples, using Oasis HLB cartridges, provided mean recovery values between 79 and 100% for spiked water samples, it could not be applied to the extraction of these pesticides from wine samples due to high interference from the sample matrix. However, the use of a SPME procedure using polydimethylsiloxane/divynilbenzene (PDMS/DVB) fibers allowed the selective extraction of four of the five pesticides which could be perfectly determined. The final combination of the off-line SPME and on-line REPSM preconcentration strategies allowed obtaining LODs between 17.6 and 32.3 μg/L.

Sample stacking for the analysis of penicillins by microemulsion electrokinetic capillary chromatography

We present a method for determining eight penicillin antibiotics using microemulsion electrokinetic chromatography (MEEKC). We studied how the composition of the microemulsion affected separation by modifying such parameters as the surfactant or the addition of organic solvents. The best microemulsion system consisted of 0.5% ethyl acetate, 1.2% 1-butanol, 2% Brij 35, 10% 2-butanol and 86.3% 10 mM borate buffer at pH 10. We studied the suitability of this microemulsion composition for analyzing a commercial drug. To improve the sensitivity of the method, we used the stacking technique reversed electrode polarity stacking mode (REPSM), which increased the detection limits by about 40-fold.

Strategies for Analyzing Cephalosporins by Microemulsion Electrokinetic Chromatography

We studied microemulsion electrokinetic chromatography (MEEKC) for the analysis of cefazolin, cefoperazone and cephalexin. We compared the separation of the cephalosporins obtained by MEEKC at a high pH and at a low pH. Good separations of the compounds were achieved within 9 min by both methods. To reduce the analysis time, we used the "short-end" injection technique. We also present two on-line preconcentrations of the analytes by sample stacking, reversed electrode polarity stacking mode (REPSM) (at high-pH) and stacking with reverse migrating pseudostationary phase (SRMP) (at low-pH). We also report parameters for validation such as linearity and limit of detection. Our results show that the developed methods are suitable for analyzing these three cephalosporins.

Sample stacking for the analysis of eight penicillin antibiotics by micellar electrokinetic capillary chromatography.

We studied the use of micellar electrokinetic capillary chromatography for separating eight penicillins. The method consists of (i) an electrophoretic separation based on micellar electrokinetic capillary chromatography, which uses sodium dodecyl sulfate (SDS) as surfactant; (ii) a sample stacking technique called reverse electrode polarity stacking mode (REPSM); and (iii) direct UV detection. The background electrolyte that gave complete separation contained 20 mM sodium borate buffer and 60 mM SDS. The sensitivity of the method was improved by an enrichment step that used on-column stacking. The limits of detection were at the microg.L(-1) level for the penicillins and did not detract from the peak resolution.

On-line concentration in micellar electrokinetic chromatography for triazine determination in water samples: evaluation of three different stacking modes

Three different stacking modes for the on-line concentration of seven selected triazines (desisopropylatrazine, desethylatrazine, simazine, hydroxyatrazine, atrazine, propazine and prometryn) by micellar electrokinetic chromatography (MEKC) were evaluated, namely the normal stacking mode, field enhancement sample injection and reverse electrode polarity stacking mode (REPSM). The best results were obtained using REPSM at –20 kV for 90 s. Concentration factors from 4 to 10 were achieved and the detection limits varied within the range 3.3–8.5 μg l−1. This methodology was combined with a solid-phase extraction (SPE) procedure to allow the determination of these analytes at trace levels. The SPE-REPSM-MEKC method developed was validated with a certified reference material of pesticides in drinking water (CRM 606) and was applied to the determination of the selected triazines in drinking waters at the level required by the European Union (0.1 μg l−1).

On-line concentration of neutral analytes for micellar electrokinetic chromatography: II. Reversed electrode polarity stacking mode

Basic conditions for the on-line neutral sample concentration by reversed electrode polarity stacking mode for micellar electrokinetic chromatography are developed theoretically and experimentally verified. Stacking is mainly dependent on analyte retention factors and nature of the pseudostationary phases. More than an order of magnitude improvement in concentration detection limit is achieved for resorcinol using high molecular mass surfactants.