Development of a background electrolyte for the determination of inorganic cations in high ionic strength samples by capillary electrophoresis with indirect UV-absorption detection

In this study, a background electrolyte capable to separate and quantify inorganic cations in high ionic strength samples by UV-absorption indirect detection was designed. In this regard, the four most abundant monovalent and divalent cations in earth crust (K⁺, Na⁺, Ca⁺², Mg⁺²) were selected as model compounds. A group of small carboxylic acids and, several toluidines and pyridines were evaluated as mild strength complexing agents and chromophoric probes, respectively. The optimized background electrolyte was composed of 200 mM 2,4,6-trimethylpyridine as the chromophoric probe, 250 mM lactic acid as the weak complexing agent and pH buffering reagent (adjusted to pH 4.5), and 5% v/v methanol as organic solvent modifier. Based on a minimum number of components, it provided outstanding separation performance in less than 4 min in a wide linear dynamic range (10 - 2500 µg·mL^-1). Performances were contrasted against a reference method based on conductometric detection. Furthermore, studies of separation efficiency and peak shape were carried out at different analyte concentrations in high electric conductivity solutions. The herein developed method demonstrated exceptional features in terms of limits of detection (~10 µg·mL^-1), resolution, speed of analysis, sensitivity and peak capacity in high electric conductivity samples. Moreover, the method was successfully applied to high ionic strength samples such as rock digest, sea water, soy sauce and isotonic drinks.

Development of a micellar electrokinetic capillary chromatography method for the determination of four naphthalenediols in cosmetics and a comparison with a HPLC method

A micellar electrokinetic capillary chromatography (MEKC) method with ultraviolet visible (UV) detection was used for the determination of 1,7-naphthalenediol, 2,3-naphthalenediol, 1,5-naphthalenediol, and 2,7-naphthalenediol in cosmetics. The current method for their determination in various cosmetics is high-performance liquid chromatography (HPLC). Separation conditions affecting the MEKC method were optimized as 20 mM Na₂ B₄ O₇ -50mM SDS, pH 9.8, with 22 kV applied voltage and UV detection at 230 nm. Under optimal conditions, electrophoretic analysis was completed in less than 6 min, with limit of detection (LOD) of 0.070-0.19 μg/mL and limit of quantitation (LOQ) of 0.23-0.63 μg/mL. A good linear relationship (r² > 0.99) was obtained at the range of 0.75-20 μg/mL. Recoveries for the four naphthalenediols in lotion, loose powder, and sun cream are between 91.2-107.2% with relative standard deviation (RSD) less than 4.04%. The method has been successfully applied to the determination of the four naphthalenediols in different kinds of cosmetics. A comparison with HPLC-UV method was also carried out according to the National Standards of the People's Republic of China. The results obtained by MEKC and HPLC methods are comparable, but the proposed MEKC method can help us obtain a much shorter detection time and low cost.

Experimental and numerical analysis of high-resolution injection technique for capillary electrophoresis microchip

This study presents an experimental and numerical investigation on the use of high-resolution injection techniques to deliver sample plugs within a capillary electrophoresis (CE) microchip. The CE microfluidic device was integrated into a U-shaped injection system and an expansion chamber located at the inlet of the separation channel, which can miniize the sample leakage effect and deliver a high-quality sample plug into the separation channel so that the detection performance of the device is enhanced. The proposed 45° U-shaped injection system was investigated using a sample of Rhodamine B dye. Meanwhile, the analysis of the current CE microfluidic chip was studied by considering the separation of Hae III digested ϕx-174 DNA samples. The experimental and numerical results indicate that the included 45° U-shaped injector completely eliminates the sample leakage and an expansion separation channel with an expansion ratio of 2.5 delivers a sample plug with a perfect detection shape and highest concentration intensity, hence enabling an optimal injection and separation performance.

Ceramic capillary electrophoresis chip for the measurement of inorganic ions in water samples

We present a microchip capillary electrophoresis (CE) device build-up in low temperature co-fired ceramics (LTCC) multilayer technology for the analysis of major inorganic ions in water samples in less than 80 s. Contactless conductivity measurement is employed as a robust alternative to direct-contact conductivity detection schemes. The measurement electrodes are placed in a planar way at the top side of the CE chip and are realized by screen printing. Laser-cutting of channel and double-T injector structures is used to minimize irregularities and wall defects, elevating plate numbers per meter up to values of 110,000. Lowest limit of detection is 6 microM. The cost efficient LTCC module is attractive particularly for portable instruments in environmental applications because of its chemical inertness, hermeticity and easy three-dimensional integration capabilities of fluidic, electrical and mechanical components.