On-column refractive-index detection based on retroreflected beam interference for capillary electrophoresis

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.

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

High speed capillary electrophoresis (HSCE) combined with refractive index (RI) detection is developed for the rapid separation and detection of inorganic ions and amino acids. A mixture of three inorganic ions (K⁺, Na⁺, Li⁺) and eight amino acids (Lys, Arg, Ala, Gly, Val, Thr, Trp, Asp) are detected using back scatter interferometry (BSI), without the need for chemical modifications or contrast. A thin-walled separation capillary (50 μm i.d. by 80 μm o.d.) helps mitigate Joule heating at the high field strengths required for rapid separations. This, combined with a short 8 cm length-to-detector (10 cm total length), enables separations on the seconds time scale. Using a background electrolyte (BGE) of 4 M acetic acid (pH 1.6) and a field strength of 900 V cm^-1, all 11 analytes are separated in less than 40 s. Moreover, peaks in the BSI signal arising from the sample injection and EOF, enable electrophoretic mobilities to readily be obtained from apparent mobilities. This leads to excellent repeatability, with analyte electrophoretic mobilities varying from 0.39 to 1.56 % RSD over eight consecutive separations. The universal detection of inorganic ions and amino acids without prior chemical modification or additives in the BGE is an advantage of refractive index detection. A disadvantage arises from modest detection limits. Here, however, we show that submicromolar detection is possible with careful thermostatting of the thin separation capillary. A series of electropherograms are used to quantify arginine concentrations from 700 nM to 500 μM, using 50 μM Li⁺ as an internal standard. The resulting calibration curve leads to a calculated LOD of 376 nM and a LOQ of 1.76 μM. Diagnostically relevant amino acid panels are also separated, illustrating the potential for future applications in neurodegenerative and metabolic disease diagnostics. HSCE combined with BSI detection, therefore, is shown to be a rapid, sensitive, and universal approach for analyzing sample mixtures.

Silicon Photonic Microring Resonator Arrays as a Universal Detector for Capillary Electrophoresis

Electrophoretic separations conventionally rely on chromogenic, fluorogenic, or redox properties for analyte detection that, in many instances, involve chemical modification of samples prior to analysis. For analytes natively lacking chemical signatures, refractive index-based measurements are appealing as a method to detect these molecules without pretreatment. Microring resonators are a type of whispering gallery mode sensor capable of detecting bulk changes in refractive index. Here, we demonstrate the use of silicon photonic microring resonator arrays as a postcolumn detector for capillary electrophoresis. In this approach, we establish the universal detection capabilities of microrings through calibration with analytes lacking unique spectral signatures. Separations of small molecule mixtures are demonstrated using capillary zone electrophoresis. For these separations, the microring resonators maintain a linear response over several orders of magnitude in concentration for three candidate small molecules. Successful separation of three sugars with direct detection is also demonstrated. We further present the successful separation and detection of three model proteins, exemplifying the promise of microring resonators arrays as a biocompatible detector for capillary electrophoresis. Additionally, the spatially offset, array-based nature of the sensing platform enables real-time analysis of analyte mobility and performance characterization-a combination that is not typically provided using single-point detectors.

Wavelength Modulated Back-Scatter Interferometry for Universal, On-Column Refractive Index Detection in Picoliter Volumes

Wavelength-modulated back scatter interferometry (M-BSI) is shown to improve the detection metrics for refractive index (RI) sensing in microseparations. In M-BSI, the output of a tunable diode laser is focused into the detection zone of a separation channel as the excitation wavelength is rapidly modulated. This spatially modulates the observed interference pattern, which is measured in the backscattered direction. Phase-sensitive detection using a split photodiode detector aligned on one fringe of the interference pattern is used to monitor RI changes as analytes are separated. Using sucrose standards, we report a detection limit of 700 μg/L in a 75 μm i.d. capillary at the 3σ level, corresponding to a detection volume of 90 pL. To validate the approach for electrophoretic separations, Na⁺ and Li⁺ were separated and detected with M-BSI and indirect-UV absorbance on the same capillary. A 4 mg/L NaCl and LiCl mixture leads to comparable separation efficiencies in the two detection schemes, with better signal-to-noise in the M-BSI detection, but less baseline stability. The latter arises in part from Joule heating, which influences RI measurements through the thermo-optic properties of the run buffer. To reduce this effect, a 25 μm i.d. capillary combined with active temperature control was used to detect the separation of sucrose, glucose, and lactose with M-BSI. The lack of suitable UV chromophores makes these analytes challenging to detect directly in ultrasmall volumes. Using a 55 mM NaOH run buffer, M-BSI is shown to detect the separation of a mixture of 174 mg/L sucrose, 97 mg/L glucose, and 172 mg/L lactose in a 15 pL detection volume. The universal on-column detection in ultrasmall volumes adds new capabilities for microanalysis platforms, while potentially reducing the footprint and costs of these systems.

A separation-free method for simultaneous determination of and sunset yellow in different abundance by integrating RBI and detectors

A novel method for simultaneous determination of sucrose and sunset yellow in retail soft drink without separation by combination of retro-reflected beam interference-based refractive index (RBI) and thermal lens (TL) detections was developed. The method proposed here successfully realized individual measurements of sucrose and sunset yellow in one single sample, while their concentrations were thousands of times different in the sample. Under optimized conditions, a separation-free process was carried out so that sucrose and sunset yellow were quantified at the same time. Limits of detection (LODs) of sucrose and sunset yellow were 0. 21 mg ml-1 and 0.23 μg ml-1 and the relative standard deviation (RSDs) were 3.9% and 3.07%. The developed method was successfully applied for the simultaneous analysis of sucrose and sunset yellow in retail soft drink. The soft drink containing 123.6 ± 2.6 mg ml-1 sucrose and 46.6 ± 1.8 μg ml-1 sunset yellow have been simultaneously measured without any separation procedure.

Simultaneous coaxial thermal lens spectroscopy and retro-reflected beam interference detection for capillary electrophoresis

A novel optical scheme, which accomplished simultaneous coaxial thermal lens spectroscopy and retro-reflected beam interference detection for capillary electrophoresis, has been described. By a special design, an adjustable pump laser waist relative to the probe laser waist was implemented, while some key elements for both detection modes were optimized. In either detection modes, certain preponderance compared with former reports was indicated. With both coaxial thermal lens spectroscopy and retro-reflected beam interference detection, the reported detection scheme combined high sensitivity and universal property for capillary electrophoresis detection.

Integrated Refractive Index Optical Ring Resonator Detector for Capillary Electrophoresis

We developed a novel miniaturized and multiplexed, on-capillary, refractive index (RI) detector using liquid core optical ring resonators (LCORRs) for future development of capillary electrophoresis (CE) devices. The LCORR employs a glass capillary with a diameter of approximately 100 mum and a wall thickness of a few micrometers. The circular cross section of the capillary forms a ring resonator along which the light circulates in the form of the whispering gallery modes (WGMs). The WGM has an evanescent field extending into the capillary core and responds to the RI change due to the analyte conducted in the capillary, thus permitting label-free measurement. The resonating nature of the WGM enables repetitive light-analyte interaction, significantly enhancing the LCORR sensitivity. This LCORR architecture achieves dual use of the capillary as a sensor head and a CE fluidic channel, allowing for integrated, multiplexed, and noninvasive on-capillary detection at any location along the capillary. In this work, we used electro-osmotic flow and glycerol as a model system to demonstrate the fluid transport capability of the LCORRs. In addition, we performed flow speed measurement on the LCORR to demonstrate its flow analysis capability. Finally, using the LCORR's label-free sensing mechanism, we accurately deduced the analyte concentration in real time at a given point on the capillary. A sensitivity of 20 nm/RIU (refractive index units) was observed, leading to an RI detection limit of 10-6 RIU. The LCORR marries photonic technology with microfluidics and enables rapid on-capillary sample analysis and flow profile monitoring. The investigation in this regard will open a door to novel high-throughput CE devices and lab-on-a-chip sensors in the future.

Analysis of capillary interferometry for measuring refractive indices of minute samples

A method of measuring the refractive indices of minute samples by analyzing capillary interferometry is introduced. With the interference theory of light, the intensity distribution of an interference fringe pattern formed by a cylindrical tube of a capillary is obtained, and the influence of some parameters on the fringes are discussed. The measurement accuracy and its relative problems are analyzed.

Laser-based non-fluorescence detection techniques for liquid separation systems

Over the last two decades, the possibility to use lasers for detection purposes in column liquid chromatography (LC) and capillary electrophoresis (CE) received much attention in the analytical chemistry literature. Most attention has been devoted to laser-induced fluorescence. The present review covers developments on non-fluorescence techniques for LC and CE. The techniques considered are thermal lens spectrometry, photoacoustic detection, refractive index detection including refractive index backscattering, Raman spectroscopy and degenerate four-wave mixing (a special mode of transientholographic spectroscopy). The paper starts with an outline of the characteristics of lasers; it ends with an overall evaluation and a discussion of the perspectives of the techniques dealt with.

Detection in capillary electrophoresis

A review of the four major, on-line, capillary electrophoresis (CE) detection modalities is presented. It is shown that each detection method, fluorescence, absorbance (conventional and nonconventional), electrochemical and refractive index, have distinct advantages and limitations when applied to analysis in a CE format. Various aspects of CE detection are considered and a perspective regarding the applicability of the technique is provided. It is shown that because of widely varying detection limits (ranging from single molecule to 10(-5) M) and detection scheme complexity, the particular application should dictate the selection of detection methodology in CE.