Direct detection of inorganic ions and underivatized amino acids in seconds using high-speed capillary electrophoresis coupled with back-scatter interferometry

Microscale quantitation of heavy metals by back-scattering interferometry in conjunction with photothermal effect using a single laser beam

A microanalytical technique based on the photothermal effect in conjunction with back-scattering interferometry (BSI) using a single laser beam was developed for quantitative detection of heavy metals. After the chromogenic reaction of an analyte in a capillary tube, the photothermal effect induced by irradiation with the same laser beam leads to a change of the refractive index of the solution, which can be "quantified" using the BSI technique. For prove-of-concept, Cu(II) was chosen as the trial analyte, for which the solution changes to purplish through reacting with the chromogenic reagent; a single laser beam of 532 nm was adapted for both inducing the photothermal effect and realizing BSI detection. With as little as 1.0 μL solution, a limit of detection (LOD) of 0.10 mg/L for Cu(II) was achieved. In addition, the versatility of the technique was demonstrated by detecting other two heavy metal ions, Fe(II) and Cr(VI), with limits of detection of 0.06 mg/L and 0.04 mg/L, respectively. The demonstrated detection sensitivity, application versatility, and instrumentation simplicity of this new technique promises it as a practical tool for environmental monitoring and beyond.

Investigation of induced electroosmotic flow in small-scale capillary electrophoresis devices: Strategies for control and reversal

Electroosmotic flow (EOF) is the bulk flow of solution in a capillary or microchannel induced by an applied electric potential. For capillary and microchip electrophoresis, the EOF enables analysis of both cations and anions in one separation and can be varied to modify separation speed and resolution. The EOF arises from an electrical double layer at the capillary wall and is normally controlled through the pH and ionic strength of the background buffer or with the use of additives. Understanding and controlling the electrical double layer is therefore critical for maintaining acceptable repeatability during method development. Surprisingly, in fused silica capillaries at low pH, studies observe an EOF even though the capillary surface should be neutralized. Previous work has suggested the presence of an "induced electroosmotic flow" from radial electric fields generated across the capillary wall due to the separation voltage and grounded components external to the capillary. Using thin-wall (15 µm) fused silica separation capillaries to facilitate the study of radial fields, we show that the EOF mobility depends on both the separation voltage and the location of external grounds. This is consistent with the induced EOF model, in which radial electric fields embed positive charges at the capillary walls to create an electrical double layer. The magnitude of the effect is characterized and shown to have long-range influences that are difficult to completely null by moving grounded components away from the separation capillary. Instead, active EOF control using externally applied potentials or a passive approach using a negative separation voltage are discussed as two possible methods for controlling the induced EOF. Both methods can reverse the EOF and improve the resolution and peak efficiency in amino acid separations.

Photothermal Backscatter Interferometry for Enhanced Detection in Capillary Electrophoresis

Refractive index (RI) detection using backscatter interferometry (BSI) enables universal detection in capillary electrophoresis (CE). BSI detection is a versatile on-capillary approach that is easily integrated with capillary or microfluidic channels, straightforward to miniaturize, and inexpensive. The focused BSI light source can also double as the excitation source for fluorescence, enabling simultaneous universal (BSI) and specific (fluorescence) signals from the same detection volume. To improve BSI detection and expand orthogonal content, we integrate photothermal absorption with BSI detection. Nonradiative relaxation of an excited analyte releases heat into the surroundings, which modifies both the local RI and conductivity (viscosity) of the analyte zone. We recently showed that the BSI signal is sensitive to both RI and conductivity, which makes photothermal absorption a promising route to signal enhancement. Here, we use coaxially delivered BSI and photothermal absorption beams to characterize BSI, photothermal BSI, and fluorescence detection using the separation of test samples. We show that photothermal absorption leads to 3 orders of magnitude improvement in BSI detection limits at the powers studied and provides new opportunities for studying binding interactions with CE.

A new green approach to L-histidine and β-alanine analysis in dietary supplements using rapid and simple contactless conductivity detection integrated with high-resolution glass-microchip electrophoresis

The analysis of dietary supplements is far less regulated than pharmaceuticals, leading to potential quality issues. Considering their positive effect, many athletes consume supplements containing L-histidine and β-alanine. A new microfluidic method for the determination of L-histidine and β-alanine in dietary supplement formulations has been developed. For the first time, capacitively coupled contactless conductivity detection was employed for the microchip electrophoresis of amino acids in real samples. A linear relationship between detector response and concentration was observed in the range of 10-100 µmol L-1 for L-histidine (R2 = 0.9968) and β-alanine (R2 = 0.9954), while achieved limits of detection (3 × S/N ratio) were 4.2 µmol L-1 and 5.2 µmol L-1, respectively. The accuracy of the method was confirmed using recovery experiments as well as CE-UV-VIS and HPLC-UV-VIS techniques. The developed method allows unambiguous identification of amino acids in native form without chemical derivatization and with the possibility of simultaneous analysis of amino acids with metal cations.

Liquid phase detection in the miniature scale. Microfluidic and capillary scale measurement and separation systems. A tutorial review

Microfluidic and capillary devices are increasingly being used in analytical applications while their overall size keeps decreasing. Detection sensitivity for these microdevices gains more importance as device sizes and consequently, sample volumes, decrease. This paper reviews optical, electrochemical, electrical, and mass spectrometric detection methods that are applicable to capillary scale and microfluidic devices, with brief introduction to the principles in each case. Much of this is considered in the context of separations. We do consider theoretical aspects of separations by open tubular liquid chromatography, arguably the most potentially fertile area of separations that has been left fallow largely because of lack of scale-appropriate detection methods. We also examine the theoretical basis of zone electrophoretic separations. Optical detection methods discussed include UV/Vis absorbance, fluorescence, chemiluminescence and refractometry. Amperometry is essentially the only electrochemical detection method used in microsystems. Suppressed conductance and especially contactless conductivity (admittance) detection are in wide use for the detection of ionic analytes. Microfluidic devices, integrated to various mass spectrometers, including ESI-MS, APCI-MS, and MALDI-MS are discussed. We consider the advantages and disadvantages of each detection method and compare the best reported limits of detection in as uniform a format as the available information allows. While this review pays more attention to recent developments, our primary focus has been on the novelty and ingenuity of the approach, regardless of when it was first proposed, as long as it can be potentially relevant to miniature platforms.

Electric field-enhanced backscatter interferometry detection for capillary electrophoresis

Backscatter interferometry (BSI) is a refractive index (RI) detection method that is easily integrated with capillary electrophoresis (CE) and is capable of detecting species ranging from inorganic ions to proteins without additional labels or contrast agents. The BSI signal changes linearly with the square of the separation voltage which has been used to quantify sample injection, but has not been explored as a potential signal enhancement mechanism in CE. Here we develop a mathematical model that predicts a signal enhancement at high field strengths, where the BSI signal is dominated by the voltage dependent mechanism. This is confirmed in both simulation and experiment, which show that the analyte peak area grows linearly with separation voltage at high field strengths. This effect can be exploited by adjusting the background electrolyte (BGE) to increase the conductivity difference between the BGE and analyte zones, which is shown to improve BSI performance. We also show that this approach has utility in small bore capillaries where larger separation fields can be applied before excess Joule heating degrades the separation. Unlike other optical detection methods that generally degrade as the optical pathlength is reduced, the BSI signal-to-noise can improve in small bore capillaries as the larger separation fields enhance the signal.

3D printed cartridge for high-speed capillary electrophoresis with sheath liquid thermostatting and contactless conductivity detection

The high-speed capillary electrophoresis (HSCE) method is a technique that utilizes a high electric field strength applied through a short capillary to reduce the time required for sample separation. However, the increased electric field strength may result in pronounced Joule heating effects. To address this, we describe a 3D-printed cartridge with integrated contactless conductivity detection (C⁴D) head and a sheath liquid channel. The C⁴D electrodes and Faraday shield layers are fabricated by casting Wood's metal in chambers inside the cartridge. Effective thermostatting of the short capillary is achieved by flowing Fluorinert liquid, which provides better heat dissipation compared to airflow. A HSCE device is created by using the cartridge and a modified slotted-vial array sample-introduction approach. Analytes are introduced through electrokinetic injection. With the help of sheath liquid thermostatting, background electrolyte concentration can be increased to several hundred mM, resulting in improved sample stacking and peak resolutions. Additionally, the baseline signal is flattened. Typical cations such as NH₄⁺, K⁺, Na⁺, Mg²⁺, Li⁺, and Ca²⁺ can be separated within 22 s with an applied field strength of 1200 V/cm. The limit of detection ranges from 2.5 to 4.6 μM with a relative standard deviation of migration times of 1.1-1.2% (n = 17). The method has been applied to detect cations in drinking water and black tea leaching for drink safety testing, and to identify explosive anions in paper swabs. Samples can be directly injected without the need for dilution.

Sample plug induced peak splitting in capillary electrophoresis studied using dual backscattered interferometry and fluorescence detection

The appearance of unexpected peaks in capillary electrophoresis (CE) is common and can lengthen the time of method development as assay conditions and experimental parameters are varied to understand and mitigate the effects of the additional peaks. Additional peaks can arise when a single-analyte zone is split into multiple zones. Understanding the underlying mechanism of these phenomena, recognizing conditions that favor its presence, and knowing how to confirm and eliminate the effect are important for efficient method optimization. In this study, we examine how the overlap of analyte zones with the sample plug can lead to peak splitting. This is explored experimentally using dual detection CE, which enables both the sample plug and analyte zones to be independently and simultaneously measured from the same detection volume. Simulations performed via COMSOL Multiphysics confirm the origin of the splitting and help guide experiments to reduce and eliminate the effect. Our findings show that this peak splitting mechanism can arise in separations of both small and large molecules but is, especially, prevalent in separations of slowly migrating macromolecules. This effect is also more prevalent when using a short length-to-detector, as is commonly found in microfluidic applications. A simple diffusion-less model is introduced to develop strategies for reducing peak splitting that avoids modifying the apparatus, such as by lengthening the separation length, which can be difficult. Decreasing the sample plug length and slowing the electroosmotic flow can both reduce this effect, which is confirmed experimentally.

Dual detection high-speed capillary electrophoresis for simultaneous serum protein analysis and immunoassays

Serum protein electrophoresis (SPE) separates serum proteins into bands whose shape and amplitude can alert clinicians to a range of disorders. This is followed by more specific immunoassays to quantify important antigens and confirm a diagnosis. Here we develop a high-speed capillary electrophoresis (HSCE) platform capable of simultaneous SPE and immunoassay measurements. A single laser excitation source is focused into the detection zone of the capillary to measure both refractive index (SPE) and fluorescence signals (immunoassays). The refractive index signal measures characteristic SPE profiles for human serum separated in 100 mM boric acid (pH 10), 100 mM arginine (pH 11), and 20 mM CHES (pH 10). For the immunoassay, the fluorescence electropherograms reveal that CHES provides the optimal buffer for measuring the immunocomplex and separating it from the free antigen. Immunoassays in CHES yield a LOD of 23 nM and a LOQ of 70 nM for the detection of fluorescein. The high pH reduces protein adsorption but reduces antibody affinity. Preliminary studies carried out in 50 mM barbital at pH 8 show improved stability of the immunocomplex and better separation for immunoassay quantification. Further optimization will open new capabilities for measuring orthogonal diagnostic signals in seconds with HSCE.