Microreactors have gained increasing attention in their application toward continuous micro flow synthesis. An unsolved problem of continuous flow synthesis is the lack of techniques for continuous product purification. Herein, we present a micro free-flow electrophoresis device and accompanying setup that enables the continuous separation and purification of unlabeled organic synthesis products. The system is applied to the separation and purification of triarylmethanes. For imaging of the unlabeled analytes on-chip a novel setup for large area (3.6 cm) deep ultra violet excitation fluorescence detection was developed. Suitable separation conditions based on low conductivity electrophoresis buffers were devised to purify the product. With the optimized conditions, starting materials and product of the synthesis were well separated (R > 1.2). The separation was found to be very stable with relative standard deviations of the peak positions smaller than 3.5% over 15 min. The stable conditions enabled collection of the separated compounds, and purity of the product fraction was confirmed using capillary electrophoresis and mass spectrometry. This result demonstrates the great potential of free-flow electrophoresis as a technique for product purification or continuous clean-up in flow synthesis. Graphical Abstract Micro free-flow electrophoresis (μFFE) allows continuous separation and purification of small organic synthesis products. Enabled by a novel deep-UV imaging setup starting materials and product of a recently developed synthesis for triarylmethanes could be purified. Thereby demonstrating the potential of μFFE as continuous purification technique for micro flow synthesis.
The continuous separation mechanism of micro free-flow electrophoresis (μFFE) is a straightforward, suitable tool for microscale purification of reaction mixtures. However, aqueous separation buffers and organic reaction solvents limit the applicability of this promising combination. Herein, we have explored nonaqueous micro free-flow electrophoresis for this purpose and present its suitability for a continuous workup of organic reactions performed in acetonitrile. After successful nonaqueous FFE separation of organic dyes, the approach was applied to continuously recover the photocatalyst [Ru(bpy)3]2+ from a homogeneous, acetonitrile-based reaction mixture. This approach opens up possibilities for further downstream processing of purified products and is also attractive for recycling of precious catalyst species.
Capillary electrophoresis‐mass spectrometry often lacks sufficient limits of detection for trace substances in the environment due to its low loadability. To overcome this problem, we conducted a feasibility study for column‐coupling isotachophoresis to capillary electrophoresis‐mass spectrometry. The first dimension isotachophoresis preconcentrated the analytes. The column‐coupling of both dimensions was achieved by a hybrid capillary microfluidic chip setup. Reliable analyte transfer by voltage switching was enabled by an in‐chip capacitively coupled contactless conductivity detector placed around the channel of the common section between two T‐shaped crossings in the chip connecting both dimensions. This eliminated the need to calculate the moment of analyte transfer. A commercial capillary electrophoresis‐mass spectrometry instrument with easily installable adaptations operated the setup. Prior to coupling isotachophoresis with capillary zone electrophoresis‐mass spectrometry, both dimensions were optimized individually by simulations and verified experimentally. Both dimensions were able to stack/separate all degradation products of glyphosate, the most important herbicide applied worldwide. The first dimension isotachophoresis also removed phosphate, which is a critical matrix component in many environmental samples. Enrichment and separation of glyphosate and its main degradation product aminomethylphosphonic acid by the two‐dimensional setup provided an excellent limit of detection of 150 pM (25 ng/L) for glyphosate.
This work introduces new hardware configurations for a capacitively coupled contactless conductivity detector (C 4 D) based on capacitance-to-digital conversion (CDC) technology for CE. The aim was to improve sensitivity, handling, price, and portability of CDC-based C 4 D detectors (CDCD) to reach LODs similar to classic C 4 Ds with more sophisticated electric circuits. To achieve this, a systematic study on the CDCDs was carried out including a direct comparison to already established C 4 D setups. Instrumental setups differing in electrode lengths, measurement modes, and amplification of excitation voltages were investigated to achieve LODs for alkali metal ions of 4 to 12 μM, similar to LODs obtained by classic C 4 D setups. Lowest LODs were achieved for a setup with two 10 mm electrodes at a distance of 0.2 mm and an excitation voltage of 24 V. The detection head was exceptionally lightweight with only 2.6 g and covered only 20 mm of the capillary on total. This allowed the use of multiple detectors along the separation path to enable spatial tracking of analytes during separation. The entirely battery-powered detector assembly weighs less than 200 g, and the data are transmitted wirelessly for possible portable applications. The freely accessible hardware and software were optimized for fully automated measurements with real time data plotting and allowed handling multidetector setups. The new developments were applied to quantify the potassium salt of glyphosate in its herbicide formulation.
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