Zdena Malá

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.

Contemporary sample stacking in CE

Sample stacking techniques remain an important tool for enhancement of the selectivity and sensitivity of analyses in contemporary CZE. This contribution reviews new knowledge on this topic published since 2006. It is organized according to the operational principles used, which include concentration adjustment, application of a pH step, MEKC and sweeping, and transient ITP. Techniques combining several of these principles and comparative studies are also included.

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: CE and CEC

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).

Recent progress in analytical capillary ITP

ITP is an electrophoretic technique that has been attracting constant attention for many years due to its pronounced capability to concentrate trace analytes by several orders of magnitude. In practice, it is used predominantly in capillary format, where the capillaries used have id ranging between 0.02 and 0.8 mm. The volumes of the samples introduced may be up to several tens of microliters and trace analytes diluted in such a volume are concentrated into zones having volumes in the range of picoliters. Moreover, it offers simultaneously efficient separation of the analytes. That is why ITP retains its important position in many current multistage and multidimensional separation schemes where it is used always as the starting step that brings preseparation and concentration of sample components. This article links up previous reviews on the topic and summarizes the progress of analytical capillary ITP since 2006; 90 reviewed papers include theory and methodological novelties as well as analytical applications. Papers using ITP as part of multistage separation schemes are also included.

Recent progress in analytical capillary isotachophoresis: CE and CEC

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.

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.

System effects in sample self-stacking CZE : Single analyte peak splitting of salt-containing samples

In CZE one often gets more peaks than the number of sample components. In practice the additional peaks are often left unexplained or assigned to unidentified impurities or system peaks although cases exist when one analyte forms two or more regular distinct zones. One source of such effects are samples with high salt content that are generally assumed to bring higher sensitivity due to the sample self‐stacking mechanism. The subject of this contribution is the theoretical and experimental investigation of the electromigration behavior of salt‐containing samples. It is shown that they can exhibit splitting of the analyte zone into mutually independent parts detectable as well‐developed distinct peaks. Theory based on velocity diagrams and computer simulations reveals that these effects originate in the transient phase of separation where electromigration dispersion profiles and sharp boundaries are formed and evolve. During this, the sample may induce parallel existence of several transient sharp boundaries (including system boundaries) that are simultaneously capable of stacking an analyte. Their electromigration is convergent and depending on whether they merge before the analyte destacks from them, permanent or transient double or multiple peaks are formed. Presented examples of anionic and cationic systems show good agreement with theory. The appearance of multiple peaks can be very variable, ranging from double or triple peaks to a major peak with a minor peak quite apart. Knowledge of the peak‐splitting mechanism allows both to identify its presence in a given BGE and sample and to find effective remedy.

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.

Electromigration behaviour of DNA molecules at the free electrolyte-polymer solution interface

The velocity at which DNA molecules migrate across an interface between a free solution of an electrolyte and a sieving medium was investigated in capillaries. A model was proposed where the DNA molecules are supposed to be stacked at the interface and then, due to consecutive conformational changes, their velocities increase and reach the value of their effective electrophoretic mobilities in the polymer solution filling the rest of a capillary. This behaviour was shown by measuring the electromigration injection bias and by extrapolation of the migration times in capillaries of different lengths to the zero migration path. The experiments showed an extraordinarily high size-based separation selectivity of the electromigration across the interface, which seems to offer a potential for successful high resolution separation of DNA fragments in capillaries as short as several centimetres.