Analytical capillary isotachophoresis after 50 years of development: Recent progress 2014-2016.

This review brings a survey of papers on analytical ITP published since 2014 until the first quarter of 2016. The 50th anniversary of ITP as a modern analytical method offers the opportunity to present a brief view on its beginnings and to discuss the present state of the art from the viewpoint of the history of its development. Reviewed papers from the field of theory and principles confirm the continuing importance of computer simulations in the discovery of new and unexpected phenomena. The strongly developing field of instrumentation and techniques shows novel channel methodologies including use of porous media and new on-chip assays, where ITP is often included in a preseparative or even preparative function. A number of new analytical applications are reported, with ITP appearing almost exclusively in combination with other principles and methods.

Recent progress of sample stacking in capillary electrophoresis (2014-2016).

The term "sample stacking" comprises a relatively broad spectrum of techniques that already form an almost inherent part of the methodology of CZE. Their principles are different but the effect is the same: concentration of a diluted analyte into a narrow zone and considerable increase of the method sensitivity. This review brings a survey of papers on electrophoretic sample stacking published approximately since the second quarter of 2014 till the first quarter of 2016. It is organized according to the principles of the stacking methods and includes chapters aimed at the concentration adjustment principle (Kohlrausch stacking), techniques based on pH changes, micellar methods, and other stacking techniques. Not reviewed are papers on transient ITP that are covered by another review in this issue.

Recent progress in analytical capillary isotachophoresis.

This paper brings a survey of papers on analytical capillary ITP published since 2012 until the first quarter of 2014. These papers are ranged according to their nature, the techniques used, and the instrumentation employed. The sequence of the related chapter titles is as follows: Theory and simulations, techniques and instrumentation, single-column and column-switching applications of ITP, ITP in microfluidic systems, on-line ITP-CZE and transient ITP (tITP) techniques and applications. The review shows the position of analytical capillary ITP among contemporary separation techniques and implies the potential future trends.

Contemporary sample stacking in analytical electrophoresis.

This contribution is a methodological review of the publications about the topic from the last 2 years. Therefore, it is primarily organized according to the methods and procedures used in surveyed papers and the origin and type of sample and specification of analytes form the secondary structure. The introductory part about navigation in the architecture of stacking brings a brief characterization of the various stacking methods, with the description of mutual links to each other and important differences among them. The main body of the article brings a survey of publications organized according to main principles of stacking and then according to the origin and type of the sample. Provided that the paper cited gave explicitly the relevant data, information about the BGE(s) used, procedure, detector employed, and reached LOD and/or concentration effect is given. The papers where the procedure used is a combination of diverse fragments and parts of various stacking techniques are mentioned in a special section on combined techniques. The concluding remarks in the final part of the review evaluate present state of art and the trends of sample stacking in CE.

Fine-tuning of electromembrane extraction selectivity using 18-crown-6 ethers as supported liquid membrane modifiers.

Selectivity of electromembrane extractions (EMEs) was fine-tuned by modifications of supported liquid membrane (SLM) composition using additions of various 18-crown-6 ethers into 1-ethyl-2-nitrobenzene. Gradually increased transfer of K(+) , the cation that perfectly fits the cavity of 18-crown-6 ethers, was observed for EMEs across SLMs modified with increasing concentrations of 18-crown-6 ethers. A SLM containing 1% w/v of dibenzo-18-crown-6 in 1-ethyl-2-nitrobenzene exhibited excellent selectivity for EMEs of K(+) . The established host-guest interactions between crown ether cavities in the SLM and potassium ions in donor solution ensured their almost exhaustive transfer into acceptor solution (extraction recovery ∼92%) within 30 min of EME at 50 V. Other inorganic cations were not transferred across the SLM (Ca(2+) and Mg(2+) ) or were transferred negligibly (NH4 (+) , Na(+) ; extraction recovery < 2%) and had only subtle effect on EMEs of K(+) . The high selectivity of the tailor-made SLM holds a great promise for future applications in EMEs since the range of similar selective modifiers is very broad and may be applied in various fields of analytical chemistry.

Electrolyte system strategies for anionic ITP with ESI-MS detection. 3. The ITP spacer technique in moving-boundary systems and configurations with two self-maintained ITP subsystems.

This contribution is the third part of the project on strategies used in the selection and tuning of electrolyte systems for anionic ITP with ESI-MS detection. The strategy presented here is based on the creation of self-maintained ITP subsystems in moving-boundary systems and describes two new principal approaches offering physical separation of analyte zones from their common ITP stack and/or simultaneous selective stacking of two different analyte groups. Both strategic directions are based on extending the number of components forming the electrolyte system by adding a third suitable anion. The first method is the application of the spacer technique to moving-boundary anionic ITP systems, the second method is a technique utilizing a moving-boundary ITP system in which two ITP subsystems exist and move with mutually different velocities. It is essential for ESI detection that both methods can be based on electrolyte systems containing only several simple chemicals, such as simple volatile organic acids (formic and acetic) and their ammonium salts. The properties of both techniques are defined theoretically and discussed from the viewpoint of their applicability to trace analysis by ITP-ESI-MS. Examples of system design for selected model separations of preservatives and pharmaceuticals illustrate the validity of the theoretical model and application potential of the proposed techniques by both computer simulations and experiments. Both new methods enhance the application range of ITP-MS and may be beneficial particularly for complex multicomponent samples or for analytes with identical molecular mass.

Effects of selected operational parameters on efficacy and selectivity of electromembrane extraction. Chlorophenols as model analytes.

Effects of organic solvent type, pH value, and composition of donor/acceptor solution on the efficacy of electromembrane extraction (EME) were examined. For the first time, a comprehensive quantitative study, based also on measurements of electric charge passed through the EME system, was carried out, which demonstrates that apart from the pH value, also the nature of counter-ions in donor and acceptor solution plays a significant role in the electrically induced transfer of charged analytes across supported liquid membranes (SLMs). The EME transfer of model analytes correlated well with electrophoretic mobilities of inorganic cations, which were added to acceptor solutions during their alkalization with alkali metal hydroxides, and were highest for counter-cations with highest mobilities. Up to a 53-fold improvement of extraction efficiency was achieved for EMEs using optimized composition of donor (alkalized with KOH to pH 7) and acceptor (10 mM CsOH, pH 12) solutions. Six chlorophenols (CPs) were selected as model analytes due to the wide range of pH values that are required for their ionization and due to their high environmental relevance; quantitative measurements were carried out by CE with UV detection. Extraction recoveries of the six CPs ranged between 14 and 25% for 5 min EMEs at 150 V and 750 rpm across SLMs impregnated with 1-ethyl-2-nitrobenzene. Calibration curves were strictly linear (r(2) ≥ 0.999) in 0.01-10 µg/mL range, repeatability values of peak areas were between 0.7 and 5.6% and LODs for standard solutions and environmental samples were better than 5 ng/mL.

Recent progress in analytical capillary isotachophoresis.

This review brings a survey of the literature on analytical isotachophoresis (ITP) from the years 2010-2012. It confirms the fact that ITP alone is not used for analyses frequently but that its online combinations with other methods are of paramount importance. This review shows that the inherent features of the technique and first of all its concentrating ability are still unique for reaching high sensitivity and efficient sample cleanup in analytical applications. The part devoted to theory is mostly represented by computer simulations and confirms the power and significance of this approach. The section oriented at instrumentation and techniques shows the advantages of ITP in column combinations and microchip techniques. The chapter reviewing the applications is categorized according to the techniques applied, viz., column switching, on line ITP-CZE and on-chip analyses. The final part of the review is devoted to the nearly omnipresent electrophoresis principle of transient isotachophoresis, and to the advantages that it may offer for detection and sampling. In all parts, the significance of the operational conditions is also considered and where possible, the electrolyte system is explicitly presented.

A simple sample pretreatment device with supported liquid membrane for direct injection of untreated body fluids and in-line coupling to a commercial CE instrument.

A simple sample pretreatment device was developed employing extractions across supported liquid membranes (SLMs) and in-line coupling to a commercial CE instrument. The device consisted of two polypropylene conical units interspaced with a polypropylene planar SLM, which were impregnated with 1-ethyl-2-nitrobenzene. The two units and the SLM were pressed against each other, donor unit was filled with 40 µL of an untreated body fluid and acceptor unit with 40 µL of DI water. The device was then placed into conventional CE vial fitted with a soft spring, which was depressed during injection into CE capillary and ensured that the SLM was not ruptured. Position of separation capillary injection end and high-voltage electrode in the CE instrument was optimized in order to ensure efficient injection of pretreated body fluids. The device can be easily assembled/disassembled and SLMs can be replaced after each extraction thus minimizing sample carry-over, avoiding tedious SLM regeneration, and reducing total pretreatment time and costs. The pretreatment device was examined by direct injection of human urine and serum spiked with nortriptyline, haloperidol, and loperamide. The basic drugs were diffusionaly transported across the SLM within 10 min and were injected into the separation capillary directly from the SLM surface in the acceptor unit, whereas matrix components were retained by the SLM. The in-line SLM-CE method showed good repeatability of peak areas (3.8-11.0%) and migration times (below 1.4%), linear relationship (r(2) = 0.990-0.999), and low LODs (12-100 µg/L).

Electrolyte system strategies for anionic isotachophoresis with electrospray-ionization mass-spectrometric detection. 2. Isotachophoresis in moving-boundary systems.

This contribution is the second part of the project on strategies used in the selection of electrolyte systems for anionic ITP with ESI-mass spectrometric detection. It presents ITP as a powerful tool for selective stacking of anionic analytes, performed in a nonconventional way in moving-boundary systems where two co-anions are present in both the leading and terminating zones. The theoretical background is given to substantiate the conditions for the existence and migration of ITP boundaries in moving-boundary systems and stacking of analytes at these boundaries. The practical aspects of the theory are shown in form of stacking-window diagrams that bring immediate information about which analytes are stacked in a given system. The presented theory and strategy are illustrated and verified on the example of analysis of a model mixture of salicylic acid, ibuprofen and diclofenac, and comparison of regular and free-acid ITP with moving-boundary ITP systems formed by formic and propionic acids and ammonium as counterion.

Electrolyte system strategies for anionic isotachophoresis with electrospray-ionization mass-spectrometric detection. 1. Regular isotachophoresis and free-acid isotachophoresis.

The subject of this work is the definition of a simple model based on general ITP theory that allows describing and predicting the behavior of ITP systems compatible with ESI-MS detection. The model is exemplified by anionic ITP of weak acids that represent an interesting potential application field of ITP-ESI-MS. Suitable ESI-compatible electrolyte systems of very simple composition are proposed including a special free-acid ITP arrangement. The properties of these systems are discussed using illustrative diagrams of their stacking windows. The use of anionic ITP-ESI-MS in negative-ion ESI mode is reported for the first time and its suitability for sensitive trace analysis is demonstrated. The presented ITP-ESI-MS application example comprises a free-acid ITP system formed of formic and propionic acids and direct injection analysis of ibuprofen and diclofenac in waters with quantitation limits of the order 10(-10) M.

Advanced electrolyte tuning and selectivity enhancement for highly sensitive analysis of cations by capillary ITP-ESI MS.

In this contribution we present an innovative way to easy, fast, and highly sensitive analyses by CE with ESI-MS detection. The new method is designed to be applied to ESI-compatible electrolytes (e.g. ammonium acetate) and offers advanced tuning of selectivity conditions within a wide range of analyte mobilities. We use a full capillary ITP format to provide powerful on-line analyte stacking at the ITP boundary all the way to detection and introduce the model of extended ITP where a controlled concentration of the leading ion is added to the terminating zone. Such systems preserve all properties of an ITP system and the velocity of the stacking ITP boundary can be tuned by the composition of both the leading and terminating zone. In this way, the system properties can be controlled flexibly and the mobility window of stacked analytes can be tailored to actual needs. The presented theory and the newly defined concept of zone-related boundary mobility allow easy assessment of system selectivity using simple diagrams. We demonstrate the model and its potential on the example of simple acidic cationic systems composed of only two substances (ammonium and acetate) including the example of thiabendazole analysis with a detection limit of 10(-10) M (20 ng/L) and its determination in orange juice by direct sampling after filtration, selective stacking by a tuned extended ITP system, and ESI-MS detection.

Contemporary sample stacking in analytical electrophoresis.

Sample stacking is a term denoting a multifarious class of methods and their names that are used daily in CE for online concentration of diluted samples to enhance separation efficiency and sensitivity of analyses. The essence of these methods is that analytes present at low concentrations in a large injected sample zone are concentrated into a short and sharp zone (stack) in the separation capillary. Then the stacked analytes are separated and detected. Regardless of the diversity of the stacking electromigration methods, one can distinguish four main principles that form the bases of nearly all of them: (i) Kohlrausch adjustment of concentrations, (ii) pH step, (iii) micellar methods, and (iv) transient ITP. This contribution is a continuation of our previous reviews on the topic and brings an overview of papers published during 2010-2012 and relevant to the mentioned principles (except the last one which is covered by another review in this issue).

Electrokinetic injection across supported liquid membranes: new sample pretreatment technique for online coupling to capillary electrophoresis. Direct analysis of perchlorate in biological samples.

A simple and sensitive method for quantifying perchlorate in biological samples using CE and capacitively coupled contactless conductivity detection was developed. An online combination of a supported liquid membrane, an inert polypropylene membrane impregnated with 1-hexanol, and electrokinetic injection of perchlorate across the supported liquid membrane directly into the separation capillary reduced the need for laborious sample pretreatment procedures, resulting in a cheap and rapid method with low LODs capability. Baseline separation of perchlorate and other anions in biological samples was achieved in background electrolyte solution consisting of 15 mM nicotinic acid and 1 mM 3-(N,N-dimethylmyristylammonio)propanesulfonate at pH 3.3. The analytical method showed excellent parameters in terms of reproducibility; RSD values for peak areas and corrected migration times at a spiked concentration of 100 µg/L of perchlorate were below 10 and 0.4%, respectively. Linear calibration curves were obtained for perchlorate in the concentration range 10-1000 µg/L (r(2) >0.999) with LODs between 2 and 5 µg/L for human urine, breast milk, serum, cow's milk, and red wine. Recoveries at 25 µg/L of perchlorate were between 97 and 106% for all biological samples. The low LODs rivaling those of presently used analytical methods support the use of this method for quantification of perchlorate in biological samples in the future.

Electrophoresis today and tomorrow: Helping biologists' dreams come true.

Intensive research and development of electrophoresis methodology and instrumentation during past decades has resulted in unique methods widely implemented in bioanalysis. While two-dimensional electrophoresis and denaturing polyacrylamide gel electrophoresis in sodium dodecylsulfate are still the most frequently used electrophoretic methods applied to analyses of proteins, new miniaturized capillary and microfluidic versions of electromigrational methods have been developed. High-throughput electrophoretic instruments with hundreds of capillaries for parallel separations and laser-induced fluorescence detection of labeled DNA strands have been of key importance for the scientific and commercial success of the Human Genome Project. Another powerful method, capillary isoelectric focusing with pressurized and pH-driven mobilization, provides efficient separations and on-line sensitive detection of proteins, bacteria and viruses. Electrophoretic microfluidic devices can integrate single-cell injection, cell lysis, separation of its components and fluorescence or mass spectrometry detection. These miniaturized devices also proved the capability of single-molecule detection.

Recent advances in CE-MS: Synergy of wet chemistry and instrumentation innovations.

CE with MS detection is a hyphenated technique which greatly improves the ability of CE to deal with real samples, especially with those coming from biology and medicine, where the target analytes are present as trace amounts in very complex matrices. CE-MS is now almost a routine technique performed on commercially available instruments. It faces currently a tremendous development of the technique itself as well as of its wide application area. Great interest in CE-MS is reflected in the scientific literature by many original research articles and also by numerous reviews. The review presented here has a general scope and belongs to a series of regularly published reviews on the topic. It covers the literature from the last 2 years, since January 2008 till June 2010. It brings a critical selection of related literature sorted into groups reflecting the main topics of actual scientific interest: (i) innovations in CE-ESI-MS, (ii) use of alternative interfaces, and (iii) ways to enhance sensitivity. Special attention is paid to novel electrolyte systems amenable to CE-MS including nonvolatile BGEs, to advanced CE separation principles such as MEKC, MEEKC, chiral CE, and to the use of preconcentration techniques.

Recent progress in analytical capillary isotachophoresis.

Capillary ITP is currently used as one of the most important tools for preseparation and preconcentration of trace analytes in complex or diluted samples. This contribution is a continuation of a series of regularly published reviews on the topic and covers the last 2 years. It brings a survey of related literature organized into following sections: theory and methodology, instrumentation and techniques, and applications.

Contemporary sample stacking in analytical electrophoresis.

Sample stacking is of vital importance for analytical CE since it may bring the required sensitivity of analyses. A lot of new relevant papers are published every year and regular surveys seem to be very helpful for experts and practitioners. The contribution presented here is a continuation of a series of regularly published reviews on the topic and covers the last two years. It brings a survey of related literature organized, in accord with the main principle used in the procedure published, in the following mainstream sections: Kohlrausch adjustment of concentrations, pH step, micellar systems and combined techniques. Each part covers literature sorted according to the field of application as, e.g. clinical, pharmaceutical, food, environmental, etc.

Rapid and simple pretreatment of human body fluids using electromembrane extraction across supported liquid membrane for capillary electrophoretic determination of lithium.

Electromembrane extraction was used for simultaneous sample cleanup and preconcentration of lithium from untreated human body fluids. The sample of a body fluid was diluted 100 times with 0.5 mM Tris solution and lithium was extracted by electromigration through a supported liquid membrane composed of 1-octanol into 100 mM acetic acid acceptor solution. Matrix compounds, such as proteins, red blood cells, and other high-molecular-weight compounds were efficiently retained on the supported liquid membrane. The liquid membrane was anchored in pores of a short segment of a polypropylene hollow fiber, which represented a low cost, single use, disposable extraction unit and was discarded after each use. Acceptor solutions were analyzed using capillary electrophoresis with capacitively coupled contactless conductivity detection (CE-C(4) D) and baseline separation of lithium was achieved in a background electrolyte solution consisting of 18 mM L-histidine and 40 mM acetic acid at pH 4.6. Repeatability of the electromembrane extraction-CE-C(4) D method was evaluated for the determination of lithium in standard solutions and real samples and was better than 0.6 and 8.2% for migration times and peak areas, respectively. The concentration limit of detection of 9 nM was achieved. The developed method was applied to the determination of lithium in urine, blood serum, blood plasma, and whole blood at both endogenous and therapeutic concentration levels.

Electromembrane extraction of heavy metal cations followed by capillary electrophoresis with capacitively coupled contactless conductivity detection.

Electromembrane extraction (EME) was used as an off-line sample pre-treatment method for the determination of heavy metal cations in aqueous samples using CE with capacitively coupled contactless conductivity detection (CE-C(4) D). A short segment of porous polypropylene hollow fibre was penetrated with 1-octanol and 0.5% v/v bis(2-ethylhexyl)phosphonic acid and constituted a low cost, single use, disposable supported liquid membrane, which selectively transported and pre-concentrated heavy metal cations into the fibre lumen filled with 100 mM acetic acid acceptor solution. Donor solutions were standard solutions and real samples dissolved in deionized water at neutral pH. At optimized EME conditions (penetration time, 5 s; applied voltage, 75 V; and stirring rate, 750 rpm), 15-42% recoveries of heavy metal cations were achieved for a 5 min extraction time. Repeatability of the EME pre-treatment was examined for six independent EME runs and ranged from 6.6 to 11.1%. Limits of detection for the EME-CE-C(4) D method ranged from 25 to 200 nM, resulting into one to two orders of magnitude improvement compared with CE-C(4) D without sample treatment. The developed EME sample pre-treatment procedure was applied to the analysis of heavy metal cations in tap water and powdered milk samples. Zinc in the real samples was identified and quantified in a background electrolyte solution consisting of 20 mM L-histidine and 30 mM acetic acid at pH 4.95 in about 3 min.