Electrokinetic supercharging with a system‐induced terminator and an optimized capillary versus electrode configuration for parts‐per‐trillion detection of rare‐earth elements in CZE

Abstract

A further improvement of electrokinetic supercharging (EKS) methodology has been proposed, with the objective to enhance the sensitivity of the conventional CZE-UV method down to a single-digit part per trillion (ppt) level. The advanced EKS procedure is based on a novel phenomenon displaying the formation of a zone with an increased concentration of the hydrogen ion, capable to perform the function of a terminator, behind the sample zone upon electrokinetic injection. In combination with a visualizing co-ion of BGE, protonated 4-methylbenzylamine, acting as the leading ion, such system-induced terminator a effected the transient ITP state to efficiently concentrate cationic analytes prior to CZE. Furthermore, to amass more analyte ions within the effective electric field at the injection stage, a standard sample vial was replaced with an elongated vial that allowed the sample volume to be increased from 500 to 900 μL. Alongside, this replacement made the upright distance between the electrode and the capillary tips prolonged to 40.0 mm to achieve high-efficiency electrokinetic injection. The computer simulation was used for profiling analyte concentration, pH, and field strength in order to delineate formation of the terminator during sample injection. The proposed preconcentration strategy afforded an enrichment factor of 80,000 and thereby the LODs of rare-earth metal ions at the ppt level, e.g. 0.04 nM (6.7 ng/L) for erbium(III).

Simultaneous determination of rare earth elements in ore and anti-corrosion coating samples using a portable capillary electrophoresis instrument with contactless conductivity detection

“…␣-Hydroxyisobutyrate (HIB − ) from HIBA is by far the most frequently used ligand for separation of REEs with CE [18,20,22,23,26,32]. This reagent was therefore included into the BGE for the separation of REEs with CE-C 4 D. The effect of the concentration of HIBA on peak separation and peak heights is shown in Fig.…”

“…For separation of REEs by CE, normally acidic conditions have been employed to prevent any possible precipitation of metallic ions from the samples during electrophoresis [18,20,22,23,26,32]. From our experience, acidic BGEs would offer good baseline stability and reproducibility of migration times in many cases when the capillary is equilibrated with the BGE over a long period (many h) prior to CE-C 4 D analyses and flushed with the BGE between successive runs [37,38].A good CE-C 4 D performance for REEs separation using a BGE at low pH is expected as i) the electrophoretic mobilities of REEs are sufficiently high to ensure their arrival at the detection position without having recourse to a high electro osmotic flow (EOF) and ii) there is no adsorption of REEs (inorganic cations) to the capillary's internal wall.…”

“…Indeed, the simplest format of the technique, namely capillary zone electrophoresis (CZE) has been reported for the determination of REEs in different matrices, using UV-vis (UV-vis) detection [16][17][18][19][20][21][22][23], laser induced fluorescence (LIF) detection [24,25], mass-spectrometric detection [26] or detection by ICP-MS [27,28]. Other modes of CE have also been used for this purpose, notably micellar electrokinetic chromatography (MEKC) [29][30][31], electrokinetic supercharging (ESK) [32] and isotachophoresis (ITP) [33]. Complexation with suitable UV/Vis or fluorescence absorbing ligands was needed to enable optical detection of the REEs.…”

Recent progress in analytical capillary isotachophoresis

“…LODs ranged between 6.7 and 18.7 ng/L (BGE = LE: 10–70 mM NH 4 HCO 3 , pH 9.2; TE: 8 mM CAPS). Xu et al optimized large sample volume injection with an increased volume of sample vial, common wire electrode replaced by a ring electrode, and stirring of the sample solution. With these alterations, more analyte ions were accumulated during electrokinetic injection and then maintained as stacked zones due to tITP.…”

Simultaneous Determination of Pyridine-Triphenylborane Anti-Fouling Agent and Its Degradation Products in Paint-Waste Samples Using Capillary Zone Electrophoresis with Field-Amplified Sample Injection

“…However, CE has a low concentration sensitivity from minute sample volumes (at the nL level), and a short optical path length (typically 25 -100 μm) available with UV detection. 1 Different types of in-line sample concentration techniques have been proposed to enhance the CE sensitivity: large volume sample stacking, 2 large-volume sample stacking with an electroosmotic flow (EOF) pump (LVSEP), 3,4 sweeping, [5][6][7] dynamic pH junction, 8,9 field-amplified sample injection (FASI), [10][11][12][13][14][15] two-end field amplified sample injection (TE-FASI), 16 acetonitrile (ACN)-mediated stacking, 17 isotachophoresis (ITP) and transient ITP, [18][19][20] electrokinetic supercharging (EKS), [21][22][23] counter-flow electrokinetic supercharging (CF-EKS), 24 pressure-assisted injection techniques, 25,26 and simultaneous electrokinetic and hydrodynamic injection (SEHI). 27,28 These procedures are useful, but some were somewhat complicated in their application to real samples.…”