Dual UV‐absorbing background electrolytes for simultaneous separation and detection of small cations and anions by capillary zone electrophoresis

Abstract

The simultaneous separation and detection of small cations and anions by capillary zone electrophoresis (CZE) with indirect ultraviolet (UV) detection was successfully demonstrated in a background electrolyte (BGE) containing two UV-absorbing components. Benzylamine, imidazole, benzenesulfonic acid, sulfosalicylic acid, and pyromellitic acid were tested as the components of the BGE. The success of the simultaneous separation of the cations and anions is dependent upon the proper selection of the electrolyte components and control of the migration of the ions towards the detector. High pH is beneficial to the detection of anionic analytes but not to the separation of cationic analytes because of large electroosmotic flow produced under this condition. The upper pH limit of the working pH range is confined by the pKa value of the cationic component of the BGE. The influence of pH and total electrolyte concentration on the electroosmatic flow (EOF) counteracted each other. This counteraction effect imposes an upper limit on the change of total electrolyte concentration at certain pH. It was found that the EOF should be larger by at least 10 x 10(-5) cm2V(-1)s(-1) than the electrophoretic mobilities of the anions so that the anions could be detected on the cathodic side within reasonable times and with good peak shapes. In the imidazole-sulfosalicylic acid BGE, the detection limits (signal to noise, S/N = 3) for the cations and anions ranged from 100 to 900 ppb. In the benzylamine-pyromellitic acid BGE, K+, Na+, Li+, CH3 COO-, HPO4(2-), F-, ClO3-, ClO4-, NO3-, NO2-, Cl- and SO4(2-) were separated within twelve minutes. The strategies for selection of the electrolyte components of the binary BGE were also discussed.

Enhancement of detection sensitivity for indirect photometric detection of anions and cations in capillary electrophoresis

“…Xiong and Li [120] investigated the use of benzylamine and imidazole (cationic probes) and benzenesulfonic acid, sulfosalicylic acid and pyromellitic acid (anionic probes) in BGEs for simultaneous detection of anions and cations. Sample was injected hydrostatically at the anodic end of the capillary and ions were transported past the detection window by a high-velocity EOF.…”

A Theoretical Analysis of the Influence of Electroosmosis on the Effective Ionic Mobility in Capillary Zone Electrophoresis

“…This makes it possible to detect both cations and anions at the cathodic side of the capillary. Xiong et al (21) have developed such a method by using a low ionic strength background electrolyte solution containing a cationic and anionic component both capable of absorbing UV light at 210 nm, so that indirect UV detection can be applied. Some of their experimental results will be analyzed here.…”

“…Let us now analyze the experimental results of Xiong et al (21) using the values for the surface density of silanol groups and the corresponding pK a values estimated from the analysis of the results reported in ref 16 (Figure 3), that is, N = 4.3 × 10 -7 mol•m 2 , f A = 0.25; K a,A = 1.0 × 10 -4 mol•L (pK a,A = 4) and K a,B = 3.2 × 10 -6 mol•L (pK a,B = 5.5). The protonated base BH + and dissociated acid A nin the background electrolyte function as the light absorbing components with respect to the indirect UV detection of the cations and anions, respectively.…”

Simultaneous separation of anions and cations by capillary electrophoresis with high magnitude, reversed electroosmotic flow

“…Clearly, the BGE must be selected carefully in order to allow successful separation and detection of anions and cations in a single run. Some examples of the two probe, dual-end injection approach include (where the cationic probe is listed first and the anion probe second) imidazolium/nitrate [2], copper(II)ethylenediamine/nitrate [2], copper(II)-ethylenediamine/ chromate [1], 4-aminopyridinium/chromate [3], imidazolium/sulfosalicylate [19], benzylammonium/pyromellitate [20], 1,2-dimethylimidazolium/trimellitate [20], imidazolium/thiocyanate [12], and dimethyldiphenylphosphonium/trimesate [12].…”