Transient Isotachophoretic−Electrophoretic Separations of Lanthanides with Indirect Laser-Induced Fluorescence Detection

Isotachophoresis: Theory and Microfluidic Applications

Isotachophoresis (ITP) is a versatile electrophoretic technique that can be used for sample preconcentration, separation, purification, and mixing, and to control and accelerate chemical reactions. Although the basic technique is nearly a century old and widely used, there is a persistent need for an easily approachable, succinct, and rigorous review of ITP theory and analysis. This is important because the interest and adoption of the technique has grown over the last two decades, especially with its implementation in microfluidics and integration with on-chip chemical and biochemical assays. We here provide a review of ITP theory starting from physicochemical first-principles, including conservation of species, conservation of current, approximation of charge neutrality, pH equilibrium of weak electrolytes, and so-called regulating functions that govern transport dynamics, with a strong emphasis on steady and unsteady transport. We combine these generally applicable (to all types of ITP) theoretical discussions with applications of ITP in the field of microfluidic systems, particularly on-chip biochemical analyses. Our discussion includes principles that govern the ITP focusing of weak and strong electrolytes; ITP dynamics in peak and plateau modes; a review of simulation tools, experimental tools, and detection methods; applications of ITP for on-chip separations and trace analyte manipulation; and design considerations and challenges for microfluidic ITP systems. We conclude with remarks on possible future research directions. The intent of this review is to help make ITP analysis and design principles more accessible to the scientific and engineering communities and to provide a rigorous basis for the increased adoption of ITP in microfluidics.

Chromatographic Techniques for Rare Earth Elements Analysis

The present capability of rare earth element (REE) analysis has been achieved by the development of two instrumental techniques. The efficiency of spectroscopic methods was extraordinarily improved for the detection and determination of REE traces in various materials. On the other hand, the determination of REEs very often depends on the preconcentration and separation of REEs, and chromatographic techniques are very powerful tools for the separation of REEs. By coupling with sensitive detectors, many ambitious analytical tasks can be fulfilled.

Liquid chromatography is the most widely used technique. Different combinations of stationary phases and mobile phases could be used in ion exchange chromatography, ion chromatography, ion-pair reverse-phase chromatography and some other techniques. The application of gas chromatography is limited because only volatile compounds of REEs can be separated. Thin-layer and paper chromatography are techniques that cannot be directly coupled with suitable detectors, which limit their applications. For special demands, separations can be performed by capillary electrophoresis, which has very high separation efficiency.

Capillary electrophoresis coupled with in-column fiber-optic laser-induced fluorescence detection for the rapid separation of neodymium

In this study, in-column fiber-optic (ICFO) laser-induced fluorescence (LIF) detection technique is coupled with capillary electrophoresis (CE) for the rapid separation of neodymium for the first time. The effects of buffer concentration, buffer pH, and separation voltage on the CE behaviors, including electrophoretic efficiency and detection sensitivity, are investigated in detail. Under the optimal condition determined in this study (15 mM borate buffer, pH 10.50, separation voltage 24 kV), neodymium could be separated effectively from the neighboring lanthanides (praseodymium and samarium) within several minutes, and the limit of detection for neodymium is estimated to be at the ppt level. The ICFO-LIF-CE system assembled in this study exhibits unique performance characteristics such as low cost and flexibility. Meanwhile, the separation efficiency and detection sensitivity of the assembled CE system are comparable to or somewhat better than those obtained in the previous traditional CE systems, indicating the potential of the assembled CE system for practical applications in the fields of spent nuclear fuel analysis, nuclear waste disposal/treatment, and nuclear forensics.

Simplified transient isotachophoresis/capillary gel electrophoresis method for highly sensitive analysis of polymerase chain reaction samples on a microchip with laser-induced fluorescence detection

We present a sensitive, simple and robust on-chip transient isotachophoresis/capillary gel electrophoresis (tITP/CGE) method for the analysis of polymerase chain reaction (PCR) samples. Using chloride ions in the PCR buffer and N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) in the background electrolyte, respectively, as the leading and terminating electrolytes, the tITP preconcentration was coupled with CGE separation with double-T shaped channel network. The tITP/CGE separation was carried out with a single running buffer. The separation process involved only two steps that were performed continuously with the sequential switching of four voltage outputs. The tITP/CGE method showed an analysis time and a separation efficiency comparable to those of standard CGE, while the signal intensity was enhanced by factors of over 20. The limit of detection of the chip-based tITP/CGE method was estimated to be 1.1 ng/mL of DNA in 1x PCR buffer using confocal fluorescence detection following 473 nm laser excitation.

Direct fluorometric detection of paramagnetic and heavy metal ions at sub-amol level using an aromatic polyaminocarboxylate by CZE: Combination of pre- and on-capillary complexation technique

The low sensitivity of simple CZE for detecting metal ions is a long-standing problem even when an LIF detection system is employed. We have successfully achieved an ultrasensitive CE-LIF using a simple CZE mode (typical detection limit: 10(-11)-10(-10) mol/dm(3)). Both the design of a newly synthesized ligand and the combination of a precapillary derivatizing technique with an on-capillary ternary complexing technique have enabled us to achieve this extremely low LOD and high resolution of large metal complexes. The direct fluorescent detection of the paramagnetic metal ions was achieved for the first time despite their intrinsic fluorescent quenching nature. The fluorescent ligand (L) consists of a polyaminocarboxylate chelating moiety, a strongly emissive fluorescein moiety and a spacer connecting the two portions. The migration behavior of various metal-L complexes was investigated. The resolution among the complexes was improved by the introduction of a ternary complex equilibrium of the kinetically stable mother complexes with OH(-) ion. The analytical potential of our simple system was examined, and it was proved that the system was one of the most sensitive methods without the need for any preconcentration process.

Selective ultratrace detection of Al(III) and Ga(III) complexed with a calcein isomer by capillary zone electrophoresis with laser-induced fluorescence detection

A highly sensitive and selective capillary zone electrophoresis with laser-induced fluorescence detection (CZE-LIF) for Al(3+) and Ga(3+) was developed using a calcein 4',5'-isomer as a pre-capillary complexing agent. Direct fluorescent detection without an agent added to the carrier buffer provided the notable detection limits of 3.9x10(-10) and 4.6x10(-10)moldm(-3) for Al and Ga, respectively. By applying this system successfully to a river water sample, its potential for practical applications was ascertained. A comparison of three aromatic polyaminocarboxylate ligands related to the coordination structure and the distance between the fluorophore and the chelating moiety, revealed some interesting factors regarding their selectivity.

Direct fluorescence detection of ultratrace lanthanide(iii) ions complexed with aromatic polyaminocarboxylate, avoiding quenching of ligand-centered emission, using capillary zone electrophoresis with a ternary complexing technique

The ultratrace level detection and the separation of lanthanide ions (Ln3+) were achieved using capillary zone electrophoresis with laser induced fluorescent detection (CZE-LIF) using an aromatic polyaminocarboxylate ligand synthesized in our previous work. The ligand forms kinetically stable Ln complexes at the pre-capillary derivatizing step. It effectively avoids quenching processes of the ligand-centered fluorescence through complexation with Ln3+ without paramagnetic and heavy atom effects because of the distance between the chelating and the antenna moieties. During the on-capillary separation step, the mother Ln complexes competitively form ternary complexes with the auxiliary ligands, iminodiacetate and citrate, which provide different mobilities for each of the Ln3+ complexes. The emissively labeled Ln3+ complexes were efficiently separated, based on the ternary complex equilibrium. Since the carrier buffer employed was free from emissive ligands, a high signal to noise ratio was obtained. A lower detection limit of 9.1 x 10(-11) mol dm(-3) (15.6 ng dm(-3), 0.46 attomole as an amount basis) was successfully achieved typically for Lu3+ with a simple CZE mode. We propose a combination of a pre-capillary and an on-capillary complexing technique as a method that provides both high sensitivity and high resolution.

Recent advances of transient isotachophoresis-capillary electrophoresis in the analysis of small ions from high-conductivity matrices

Since its invasion into the field of small ion analysis, quantitation of trace ionic analytes has always been a challenging task for CE, especially when dealing with highly saline samples. This review summarizes the method's progress and significant developments in the area due to its combination with on-line preconcentration by transient ITP (tITP). Principles of tITP stacking in high-conductivity solutions and operational preconcentration modes are considered. The most important application areas covered in this review include the analysis of seawater and biological fluids (urine, serum, etc.). Examples cited in this review demonstrate that tITP-CE is now on a certain way of becoming a recognized technique when the determination of trace ionic species in loaded matrices is the aim.

Applicability of Capillary Electrophoresis to the Analysis of Trace Rare Earth Elements in Geological Samples

This study developed a methodology to analyze trace rare earth elements (REEs) in geological materials by capillary electrophoresis (CE). Changed from dilute HNO3 into a water medium by heating, REE ions are detectable at approximately 2 ng mL(-1). In the presence of coexisting elements from geological samples, REE separations were carried out. After sample fusion with Na2O2 and interference separation with ammonium pyrrolidinedithiocarbamate chelate, REE analytes were coprecipitated with Mg(OH)2 at pH 8.5, and then prepared into a water medium for CE determination. Using the standard addition method, this protocol was validated by analyses with better than 5% precision. This method was applied to geological materials; the REE results are in consistence with their certified values. With electrokinetic injection, internal standard (IS) selected among lanthanides is a prerequisite of high-quality REE data. An approach was proposed to derive the IS content for further correcting its contribution from unknown samples.

Positively Charged Sol−Gel Coatings for On-Line Preconcentration of Amino Acids in Capillary Electrophoresis

A novel on-line method is presented for the extraction and preconcentration of amino acids using a sol-gel-coated column coupled to a conventional UV/visible detector. The presented approach does not require any additional modification of the commercially available standard CE instrument. Extraction, stacking, and focusing techniques were used in the preconcentration procedures. Sol-gel coatings were created by using N-octadecyldimethyl[3-(trimethoxysilyl)propyl]ammonium chloride (C18-TMS) in the coating sol solutions. Due to the presence of a positively charged quaternary ammonium moiety in C18-TMS, the resulting sol-gel coating carried a positive charge. For extraction, the pH of the samples was properly adjusted to impart a net negative charge to amino acids. A long plug of the sample was then passed through the sol-gel-coated capillary to facilitate extraction via electrostatic interaction between the positively charged sol-gel coating and the negatively charged amino acid molecules. Focusing of the extracted amino acids was accomplished through desorption of the extracted amino acid molecules carried out by local pH change. Two different methods are described. Both methods showed excellent extraction and preconcentration effects. Preconcentration results obtained on sol-gel-coated columns were compared with the CZE analysis performed on bare fused-silica columns with traditional sample injections. The described procedure provided a 150,000-fold enrichment effect for alanine. The two methods provided acceptable repeatability in terms of both peak height and migration time.

Analysis of Metal Ions by Sweeping via Dynamic Complexation and Cation-Selective Exhaustive Injection in Capillary Electrophoresis

To improve the detection sensitivity of metal ions in capillary zone electrophoresis (CZE), a novel method that combines complex formation and on-line sample preconcentration by sweeping was developed. Sweeping is defined as the picking and accumulating of analytes by a carrier in the background solution, with which they have considerable affinity. In this sweeping method, using ethylenediaminetetraacetic acid as carrier, dynamic complexation to form a UV-absorbing chelate and on-line preconcentration occur simultaneously during a run. The technique was validated in terms of the limit of detection, reproducibility, and sensitivity enhancement. Detection responses of some divalent metal ions, in terms of peak heights, were improved from 60- to 180-fold, relative to conventional CZE which employed precapillary complexation. The limits of detection were in the range of (1.8-23.4) x 10(-8) M. This method was applied to the analysis of trace metal ions in factory wastewater. Furthermore, sweeping in conjunction with sample stacking accompanying electrokinetic injection, cation-selective exhaustive injection (CSEI-sweeping), was also examined. Up to 140 000-fold improvement in detector responses for some divalent and trivalent metal ions was realized by CSEI-sweeping. The limits of detection were in the range (2.4-25.2) x 10(-11) M.

Indirect laser-induced fluorescence detection for capillary electrophoresis using a violet diode laser

The violet (415 nm) diode laser is used for indirect laser-induced fluorescence detection in capillary electrophoretic separations of inorganic anions and chemical warfare agent degradation products. Inorganic anions were detected using 8-hydroxypyrene-1,3,6-trisulfonic acid as the indirect probe and achieved submicromolar (40-80 ppb) detection limits in a 2-min separation. The chemical warfare agent degradation products methylphosphonic acid, ethyl methylphosphonate, isopropyl methylphosphonate, and pinacolyl methylphosphonate were detected using the porphyrin tetrakis(4-sulfophenyl)porphine as the indirect probe and achieved detection limits of 0.1 microM (9 ppb), which are 1 order of magnitude better than that achieved using indirect UV detection. Baseline stability achieved with the violet diode laser was excellent, with dynamic reserve (DR) values of > 1000, which are 15 times better than that achieved using an unstabilized HeCd laser.

Recent progress in capillary isotachophoresis

This article is a continuation of previous reviews and summarizes the progress of analytical capillary isotachophoresis in the years 1997-1999. Papers reviewed include theoretical and methodological aspects as well as analytical applications. Included are also papers using isotachophoresis and/or isotachophoretic principles as part of multidimensional separation schemes.

Recent progress in capillary electrophoresis of metal ions

Advances in the fundamental studies and methodology of capillary electrophoresis (CE) as applied to metal ion analysis over the last two years are reviewed, with the objective of providing the interested reader with a state-of-the-art picture of technique's potentialities in the area. In particular, novel strategies for separation selectivity control and CE system innovations designed to enhance the detection sensitivity are described. In addition, a brief overview of the primary metal analytes and samples for which the technique appears to be best suited is given. The current limitations of the technique regarding most of all the implementation for routine use are considered along with the approaches on how they could be addressed. Finally, some pointers as to the likely trends in the future research are discussed.

Sample self-stacking in capillary zone electrophoresis: behavior of samples containing multiple major coionic components

It is a frequent phenomenon in practice that a sample contains bulk levels of more than one coionic component that affect the stacking behavior of minor analytes and in this way also the sensitivity of the method. Here, attention is paid to stacking resulting from the presence of a macrocomponent of leading type that is deteriorated by the presence of another macrocomponent of like charge in the sample. Based on the isotachophoretic model of migration in the initial period of separation, a theoretical approach was elaborated both for strong and weak electrolytes which describes the separation process and finds the conditions that define whether transient isotachophoretic stacking of the analyte takes place or not. It is shown that the crucial parameter is the ratio of the concentrations of macrocomponents migrating in front and behind the analyte of interest. The destacking effect can also be expected when the coion of the background electrolyte is present in the sample. Rules how to cope with effects of destackers present in the sample are given. Theoretical considerations are illustrated by computer simulations and verified experimentally. Examples of antagonistic effects of macrocomponents are demonstrated for model serum samples.

Detection techniques in ion analysis: what are our choices?

Trends in detection techniques for ion analysis by ion-exchange chromatography and capillary zone electrophoresis are reviewed. Special attention is paid to conductivity, UV-Vis absorbance, amperometric and potentiometric detection, mass spectrometry (including inductively coupled plasma MS and atmospheric pressure ionization MS) and post-separation reaction detection. Applications reported within the last few years are summarized.