Selective capillary electrophoresis separation of mono and divalent cations within a high-surface area-to-volume ratio multi-lumen capillary

Highlights

• New selective cationic separation using multi lumen capillary in CE is presented.

• Significant wall ion-exchange interactions are observed for the divalent cations.

• Mono and divalent cations were completely separated with less than 4 min.

• This method was successfully applied for the drinking water and soil extracts.

Abstract

In this study, the separation of inorganic mono and divalent cations using multi-lumen silica capillaries (MLCs) of 126 channels, each with either 4 or 8 μm inner diameter, was investigated using capillary electrophoresis and on-capillary capacitively coupled contactless conductivity detection (CE-C⁴D). MLCs provided sufficiently high surface area-to-volume ratios to generate significant wall ion-exchange interactions for the divalent cations, which significantly affected resultant selectivity, whereas monovalent cations were predominantly separated by simple electrophoresis. The resultant hybrid selectivity was seen for both 4 and 8 μm channel multi-lumen capillaries, without any preconditioning or capillary wall modification. Remarkably, the electrophoretic mobilities for the divalent cations Mg²⁺ and Ca²⁺ were reduced 7.5 times compared to those determined using a single channel open tubular capillary of 50 μm i.d., providing much improved selectivity. Apparent electrophoretic mobilities of divalent cations increased as the concentration of BGE increased, while those of monovalent cations decreased parallel to electroosmotic mobility. These results show the electrostatic interaction between the divalent cations and the silica wall. At least, this specific separation of mono- and divalent cations were clearly observed with a mixture standards solution of less than 200 μmol L⁻¹. Using a MLC with 126 × 8 μm i.d. channels and 49.1 cm in length, together with a 20 mmol L⁻¹ MES/His BGE, containing 2 mmol L⁻¹ 18-crown-6, monovalent cations (NH₄⁺, K⁺ Na⁺ and Li⁺) and divalent cations (Ca²⁺ and Mg²⁺) could be completely separated within 4 min. For monovalent cations, on-capillary detection using C⁴D provided calibration curve (0–200 μmol L⁻¹) correlation coefficients in the range R² = 0.995–0.999, and limits of detection of 2.2–6.6 μmol L⁻¹. Relative standard deviations for migration times were less than 0.6%, and recoveries ranged from the 93.8%–105.4%. The new method was applied to the separation and quantitative determination of monovalent and divalent cations in drinking waters and soil extracts.

Introduction

Since first being demonstrated as an analytical technique for separation and determination of inorganic ionic species in 1990 [1], capillary electrophoresis (CE) has maintained its popularity, and research on theory, separation modes, instrumentation, and applications continues to be reported [2]. Ion chromatography remains the gold standard approach for separation of inorganic ions in general, although CE represents a complimentary technique, which can offer orthogonal selectivity and matrix tolerance. In terms of detection, the on-capillary ultra violet (UV) absorption detector remains the most commonly applied detection approach in CE, even though the majority of common inorganic ions do not have sufficient absorptivity to be detectable at low concentration. In 1998, the capacitively coupled contactless conductivity detector (C⁴D) was introduced as an alternative detector for CE [3,4], providing universal on-capillary detection for small organic and inorganic ions, without having to resort to sample derivatisation or indirect methods [5]. As a result, over the past twenty years there has been a significant number of CE-C⁴D based methods reported, determining inorganic and organic ion species in foodstuffs, beverages, fine chemicals and pharmaceuticals, industrial brines and solvents, natural waters and wastewaters, and other environmental samples, a recent review of which has been prepared by Kubáň and Hauser [6].

The type of capillary used in CE has not varied a great deal over the years, with the standard single channel open tubular-fused silica capillary (OT-FSC) of i.d. between 25 and 150 μm, with a polyimide coating, are typically used being the accepted physical format. Exceptions include specialty capillaries, modified to provide specific advantages, typically related to detection, such as those with expanded zones or bubble cells for extended path length detection, or those with UV-transparent coatings etc [[7], [8], [9]].

To approach optimal separation efficiency using CE, zone broadening arising/resulting from various factors, such as Joule heating, longitudinal diffusion, injection plug length, laminar flow, solute wall adsorption, electrodispersion, and detector cell size, should be controlled [9,10]. Taking longitudinal diffusion as the major cause of band broadening, then separation efficiency, expressed as plate height (H), is of course directly related to field strength (E).

H = 2D/μ_totE

Where D is the solute diffusion coefficient, μ_tot is the total ionic mobility [11]. However, higher electric field strength will results in the generation of Ohmic heat, more commonly referred to as Joule heating, which can result in loss of radial temperature homogeneity, leading to band broadening [10]. To circumvent this possibility and maintain high separation efficiency, Joule heat must be effectively dissipated, and to do so ideally capillaries with low i.d., and associated high inner surface-area-to-volume (S/V) ratios, should be used. However, sensitivity for on-capillary optical detection is reduced significantly as the optical pathlength is reduced with smaller i.d. capillaries. Where this limitation becomes critical, researchers have the option to select the above mentioned C⁴D, with examples including determination of glycosides [12], saccharides [13,14], amino acids [15], artificial sweeteners [16], and inorganic and organic ions [[17], [18], [19]]. In most of these examples, the best resolution and efficiency (and thus detection limits) were obtained using capillary of 10 μm i.d. or less, with relatively high concentration of background electrolyte (BGE) [13,19].

Practical disadvantages of moving towards smaller i.d. capillaries include sample loading capacity and increased capillary backpressure (particularly for sub 25 μm i.d. capillaries), which can also affect reproducible sample loading volumes. This latter issue also becomes problematic when attempting to flush the capillaries between runs with fresh BGE. Finally, there is the possibility that small i.d. capillaries offering large S/V ratio can result in unexpected (although not necessarily unwanted) interactions between the solutes and the inner surface of capillary, which may affect both efficiency and selectivity.

An interesting alternative approach to try and overcome the above conflicting issues related to small capillary i.d., high backpressure and sample loading capacity, associated with conventional OT-FSC, is the use of multiple parallel channels of small (<10 μm) i.d. within a single capillary. Such capillary formats are now widely available in numerous designs and dimensions as so-called fused-silica photonic crystal fibers (PCFs), also known as micro-structured fibers (MSFs) or multi-lumen capillaries (MLCs). The potential advantage of these fibers and capillaries, relates to the large number of precisely spaced, homogenous and parallel micro-channels, which collectively provide a significant capillary volume for sample loading (high S/V), with associated reduced flow resistance and thus compatibility with higher operational flow rates, when taken in comparison with sub-25 μm i.d. OT-FSCs.

To date, there has been very limited work reported using such MLCs in separation science, and their potential advantages and applications are still being explored [20]. However, recent investigations include wall modified MLCs for the in-capillary extraction of polyaromatic hydrocarbons (PAHs) [21] with subsequent separation using GC-MS [22], the OT-LC separation of 2–3 fluorinated species on fluorosilane wall-modified MLCs [23], as an open tubular enzyme reactor [24], as an on-line solid phase extraction device [25], and as a combined concentrator, separation column and electrospray emitter in capillary-LC-MS [26]. With regard to CE based applications, in 2011, Rogers et al. [27] first reported the application of MLCs with 30–168 channels of 3.8–5.6 μm i.d. to the CE separation of labelled peptides. They observed that compared to single channel capillaries, with equivalent cross-sectional area under the same analytical conditions, Joule heating was suppressed due to the improved heat-dissipation within the MLCs, remarkably resulting in up to 82% improvement in separation efficiency. In a similar study, MLCs with 84 channels of 4.3 μm i.d. were used as the separation capillary in a commercial CE system, showing acceptable reproducibility and asymmetry of test solute peaks (fluorescein), with the MLC demonstrating an improvement in signal strength compared to the conventional single channel capillaries with similar total cross-sectional area [28]. MLCs with 6 channels of 28 μm i.d. have similarly been applied to the separation and detection of the in-house prepared explosive ‘nitro starch’ [29], also applying fluorescence detection. In the only study published to-date combining MLCs with on-capillary C⁴D, a seven-channel capillary, each of 28 μm i.d., was applied to the separation of neurotransmitters and inorganic cations, and compared for sensitivity to conventional single channel capillaries of 25–75 μm i.d. [30]. In this report, similar sensitivity was reported, however the performance and details for the migration of inorganic cations were neither shown nor discussed.

Herein, we investigated the performance of MLCs in the selective CE based separation of inorganic mono- and divalent cations, using on-capillary C⁴D detection. The key unknowns in this study were the impact of the MLC on peak selectivity and efficiency, with the additional question on the impact of the MLC on on-capillary C⁴D sensitivity. Two differing MLCs were investigated, each consisting of 126 parallel channels, one with channels of 8 μm i.d. and the other format with 4 μm parallel channels. With the inherent S/V ratio of these capillary formats, a further question addressed herein, is that of wall interactions and any resultant changes to selectivity, particularly for divalent cations. To determine this selectivity was compared under similar conditions to standard single channel OT-FSCs. Results presented in the following sections show significant wall interactions were observed, effectively presenting as electrochromatographic retention of divalent cations, and delivering unique selectivity for the mono- and divalent cation mixture. To demonstrate application of the developed approach, the MLC based separations were applied to the determination of monovalent cations in several water samples of drinking water, tap water and soil extracts.