Semi-permanent surfactant coatings for inorganic anion analysis in capillary electrophoresis

Capillary electrophoretic separations of inorganic anions are performed using a capillary coated with a mixture of the cationic surfactant didodecyldimethylammonium bromide (DDAB) and the zwitterionic surfactant 1,2-dilauroyl-sn-phosphatidylcholine (DLPC). These double-chained surfactants form semi-permanent coatings on the capillary wall, which allows the excess surfactant to be removed from the buffer prior to separation. Interactions between surfactant aggregates in the buffer and analyte anions are thus eliminated. The electroosmotic flow (EOF) can be altered from fully reversed (100% DDAB) to near zero (100% DLPC) using different ratios of DDAB and DLPC. Controlling the EOF allows for improved resolution of the anions while maintaining a rapid, co-EOF separation, free from analyte-surfactant additive interactions.

Determination of zwitterionic and cationic surfactants by high-performance liquid chromatography with chemiluminescenscent nitrogen detection

The use of chemiluminescent nitrogen specific detection (CLND) combined with an HPLC separation allows for the identification and quantification of cationic and zwitterionic surfactants. The CLND provides equimolar responses, based on the amount of nitrogen within any compound. This allows for the detection of any nitrogen containing surfactant. Reversed-phase separation methods using cyano columns are developed for cationic and zwitterionic (sulfobetaine) surfactant mixtures. The limits of detection for these surfactants are in the single micromolar range (1 ng N). A linear response was obtained (R2=0.9981) between 50 microM and 5 mM. The methodology was then applied to the determination of an industrial zwitterionic surfactant, Rewoteric AM CAS U [coco(amidopropyl)hydroxyldimethylsulfobetaine].

Ultra-fast HPLC separation of common anions using a monolithic stationary phase

In this work, a monolithic column was used to perform ultrafast separations of common inorganic anions in as little as 15 seconds. Separations were performed using ion-interaction chromatography with tetrabutylammonium-phthalate as the ion-interaction reagent and were monitored using either direct conductivity or indirect absorbance detection. Detection limits for direct conductivity were in the low ppm range, whereas those for indirect absorbance detection were up to an order of magnitude higher. The reproducibility was 0.4% and 2% RSD for retention time and peak area, respectively, for a one minute separation, and 2.8% and 3-15%, respectively, for the 15 second separation. The proposed method was validated versus standard ion chromatography with suppressed conductivity detection for the analysis of an industrial water sample.

Phospholipid bilayer coatings for the separation of proteins in capillary electrophoresis

The double-chained, zwitterionic phospholipid 1,2-dilauroyl-sn-phosphatidylcholine (DLPC, C12) was investigated for its use as a wall coating for the prevention of protein adsorption in capillary electrophoresis. DLPC forms a semipermanent coating at the capillary wall, which allows excess phospholipid to be removed from the capillary prior to electrophoretic separation. A DLPC-coated capillary allowed for the separation of both cationic and anionic proteins with efficiencies as high as 1.4 million plates/m. Migration time reproducibility was less than 1.3% RSD from run to run and less than 4.0% RSD from day to day. Protein recovery was as high as 93%. Cationic and anionic proteins could be separated over a pH range of 3-10, all yielding good efficiencies (N up to 1 million plates/m). The chain length of the phospholipid affected the performance of the wall coating. The C10 analogue of DLPC (DDPC) did not form a coating on the capillary wall while the C14 analogue of DLPC (DMPC) formed a stable coating that prevented protein adsorption to the same extent as its C12 counterpart.

Selective fluorometric detection of polyamines using micellar electrokinetic chromatography with laser-induced fluorescence detection

The polyamines putrescine, cadaverine, spermine and spermidine were separated and quantified by micellar electrokinetic chromatography (MEKC) with laser-induced fluorescence detection. The derivatization reagent, 1-pyrenebutanoic acid succinimidyl ester (PSE), allowed for the selective detection of the polyamines at 490 nm. Multiple labeling of the polyamines with PSE allows the formation of intramolecular excimers that emit at longer wavelengths (450-520 nm) than mono-labeled analytes (360-420 nm). Optimal separation of the labeled polyamines was achieved using a separation buffer consisting of 10 mM phosphate pH 7.2, 30 mM cholate, and 30% acetonitrile. Using these conditions, the four polyamines were separated in under 10 min. Limits of detection for putrescine, cadaverine, spermine and spermidine were 6, 5, 15 and 13 nM, respectively. These are superior or comparable to those previously reported in the literature using fluorescence detection.

Dielectric friction as a mechanism for selectivity alteration in capillary electrophoresis using acetonitrile-water media

The mobilities of a series of aromatic carboxylates and sulfonates, ranging in charge from -1 to -4, were investigated as a function of acetonitrile concentration in the electrophoretic buffer. Absolute mobilities were determined by extrapolation of the effective mobilities to zero ionic strength according to the Pitts' equation. In general, anions of higher charge were more strongly influenced by ionic strength, with similarly charged anions experiencing ionic strength effects that were not significantly different at the 95% confidence level. Furthermore, the relative magnitudes of the Onsager slopes varied with acetonitrile content according to the z/(etaepsilon(1/2)) dependence in the electrophoretic effect of the Pitt's equation. Addition of acetonitrile to the electrophoretic media resulted in changes in the absolute mobilities of the anions. These acetonitrile-induced selectivity alterations were attributed to dielectric friction. As predicted by the Hubbard-Onsager model of dielectric friction, changes in sulfonate mobility were shown to correlate to changes in solvent viscosity (eta), dielectric constant (epsilon), and relaxation time (tau). The combined effects of ionic strength and dielectric friction caused analytes with higher charge-to-size ratios to be slowed to a greater extent upon addition of acetonitrile compared to those with lower charge-to-size. For example, at 75% acetonitrile and 20 mM ionic strength, a migration order reversal occurred between the triply and singly charged sulfonates.

Evaluation of column temperature as a means to alter selectivity in the cation exchange separation of alkali metals, alkaline earth metals and amines

The merits of varying column temperature in a cation exchange separation of alkali metals, alkaline earth metals and amines are considered. Increasing the column temperature (up to 60 degrees C) reduced the retention of all cations, but by varying extents. Consequently, selectivity changes were seen, with reversals in elution order in some cases. To ascertain when temperature is most useful as a separation aid, analytes were classed into three groups according to their temperature behaviour: alkali metals; alkaline earth metals; and amines. Adjusting the column temperature caused selectivity changes between analytes in different groups, but no selectivity changes occurred between analytes in the same group. Further, temperature was compared to the addition of modest amounts of acetonitrile as another means to alter selectivity. The benefits of elevated temperature were not just limited to selectivity changes. Improvements in the efficiencies of all analytes were noted at 60 degrees C. This was especially true for the amines which are severely tailed at ambient temperatures.

Characterization of surfactant coatings in capillary electrophoresis by atomic force microscopy

This paper describes the adsorption mechanisms and aggregation properties of cetyltrimethylammonium bromide (CTAB) and didodecyldimethylammonium bromide (DDAB) surfactants that are used for dynamic coatings in capillary electrophoresis (CE). Atomic force microscopy is used to directly visualize surfactant adsorption on fused silica. It was found that the single-chained surfactant CTAB forms spherical aggregates on silica while the double-chained surfactant DDAB forms a bilayer. Aggregation at the surface occurs at approximately the same surfactant concentration in which EOF reversal is observed in CE. The nearest-neighbor distance between CTAB aggregates varies inversely with buffer pH and becomes constant at the point when the silanol groups are fully ionized. DDAB forms a flat, uniform coating independent of pH. Increasing the buffer ionic strength changes the morphology of the CTAB aggregates from spherical to cylindrical. The change in morphology can alter the surface coverage, which is related to the "normalized" EOF measured in identical buffers. The morphology of a surfactant coating is also shown to affect its ability to inhibit protein adsorption to the capillary wall. Specifically, the full surface coverage provided by DDAB proved superior in a head-to-head comparison with CTAB.

Dielectric friction in capillary electrophoresis: mobility of organic anions in mixed methanol-water media

The mobilities of a series of organic carboxylates and sulfonates, ranging in charge from -1 to -4, were investigated by capillary electrophoresis using buffers containing 0 to 75% (v/v) methanol. Effective mobilities were measured at a series of ionic strengths, and were extrapolated to zero ionic strength using Pitts' equation to yield absolute mobilities. Generally, higher-charged ions were more strongly influenced by ionic strength, as predicted by the Pitts' equation. Some differences in the ionic strength effects for anions of like charge were observed and were consistent with the relaxation effect. The absolute mobilities of anions were altered by the addition of methanol to the buffer. Analytes with higher charge-to-size ratios were slowed to a greater extent than were ions with lower charge-to-size. As a result, dramatic changes in relative mobility were observed, such as a reversal in migration order between anions of -1 and -4 charge at 75% methanol and 20 mM ionic strength. The mobility changes caused by the addition of methanol are attributed to dielectric friction. Mobilities in the methanol-water solutions were found to depend on analyte charge-to-size and solvent dielectric relaxation time (tau) and were inversely dependent upon solvent dielectric constant (epsilon), as predicted by the Hubbard-Onsager mobility model.

Effect of temperature on retention and selectivity in ion chromatography of anions

The temperature dependence of retention of a wide range of inorganic anions is studied on two commercially available ion exchangers (Dionex AS11 and AS14 columns). Anion retention exhibited both exothermic and endothermic behavior, such that varying the temperature from ambient to 60 degrees C produced selectivity changes. The anions displayed three groupings of temperature dependence: weakly retained singly charged anions (e.g., iodate, bromate, nitrite, bromide and nitrate); multiply charged anions (sulfate, oxalate, phosphate and thiosulfate); and strongly retained singly charged anions (iodide, thiocyanate and perchlorate). Temperature was ineffective at changing the selectivity of retention between anions of the same grouping. However, significant selectivity changes, including elution order reversal, could be achieved between anions from different groupings.

Chemiluminescence nitrogen detection in ion chromatography for the determination of nitrogen-containing anions

Chemiluminescence nitrogen detection (CLND) provides equimolar response for nitrogen-containing ions such as nitrate, nitrite, cyanide, ammonium and tetradecyltrimethylammonium. Only azide yields a lower response. Nitrite, azide and nitrate are separated on a Dionex AS11 column using 5 nM NaOH as eluent with a 3 microM (1 ng N) limit of detection. Matrices, such as 1:10 diluted seawater, do not degrade these detection limits. CLND also provides equally sensitive (limit of detection 3 microM, 78 ppb) detection of weak acids such, as cyanide, which yield poor sensitivity with suppressed conductivity detection.

High-sensitivity determination of the degradation products of chemical warfare agents by capillary electrophoresis-indirect UV absorbance detection

Capillary electrophoresis coupled with indirect UV absorbance detection was employed for the determination of the chemical warfare agent degradation products: methylphosphonic acid, ethyl methylphosphonate, isopropyl methylphosphonate, and pinacolyl methylphosphonate. Glutamic acid was used as a buffering agent at its isoelectric point (pH 3.22). In its zwitterionic form, glutamic acid does not act as a competing co-anion in the system, thus providing buffering capacity while maintaining high sensitivity. The indirect probe (phenylphosphonic acid) concentration was lowered to 1 mM from the 10 mM in previous literature studies, further enhancing sensitivity. Detection limits of 2 microM were achieved with hydrodynamic injection and up to 100-fold lower using electrokinetic injection. The increased buffering capacity of this system over previous methods led to migration time reproducibility RSD values of 0.18 to 0.22%. This represents a 10-fold improvement in reproducibility over previous studies with comparable or improved sensitivity.

Indirect laser-induced fluorescence detection for capillary electrophoresis using a violet diode laser

The violet (415 nm) diode laser is used for indirect laser-induced fluorescence detection in capillary electrophoretic separations of inorganic anions and chemical warfare agent degradation products. Inorganic anions were detected using 8-hydroxypyrene-1,3,6-trisulfonic acid as the indirect probe and achieved submicromolar (40-80 ppb) detection limits in a 2-min separation. The chemical warfare agent degradation products methylphosphonic acid, ethyl methylphosphonate, isopropyl methylphosphonate, and pinacolyl methylphosphonate were detected using the porphyrin tetrakis(4-sulfophenyl)porphine as the indirect probe and achieved detection limits of 0.1 microM (9 ppb), which are 1 order of magnitude better than that achieved using indirect UV detection. Baseline stability achieved with the violet diode laser was excellent, with dynamic reserve (DR) values of > 1000, which are 15 times better than that achieved using an unstabilized HeCd laser.

Prediction of electrophoretic mobilities. 4. Multiply charged aromatic carboxylates in capillary zone electrophoresis

The electrophoretic mobilites of aromatic carboxylates and sulfonates at zero ionic strength were correlated with models incorporating both hydrodynamic and dielectric friction. The hydrodynamic friction was predicted using either the Hückel spherical ion model or the Perrin ellipsoidal model. Dielectric friction is the charge-induced drag caused by the reorientation of the solvent dipoles in response to the analyte charge. Based on the Hubbard-Onsager and Zwanzig expressions, the dielectric friction is related to z2/V. Expressions incorporating both the hydrodynamic and dielectric frictional terms successfully predicted infinite-dilution mobilities to within 4.4%. The influence of dielectric friction ranged from 3-8% of the overall drag for singly charged analytes to 39% of the total frictional drag for 1,2,4,5-benzenetetracarboxylate.