Recent advances in enhancing the sensitivity of electrophoresis and electrochromatography in capillaries and microchips (2016-2018)

One of the most cited limitations of capillary and microchip electrophoresis is the poor sensitivity. This review continues to update this series of biannual reviews, first published in Electrophoresis in 2007, on developments in the field of online/in-line concentration methods in capillaries and microchips, covering the period July 2016-June 2018. It includes developments in the field of stacking, covering all methods from field-amplified sample stacking and large-volume sample stacking, through to isotachophoresis, dynamic pH junction, and sweeping. Attention is also given to online or in-line extraction methods that have been used for electrophoresis.

Amplification-free detection of DNA in a paper-based microfluidic device using electroosmotically balanced isotachophoresis

We present a novel microfluidic paper-based analytical device (μPAD) which utilizes the native high electroosmotic flow (EOF) in nitrocellulose to achieve stationary isotachophoresis (ITP) focusing. This approach decouples sample accumulation from the length of the channel, resulting in significant focusing over short channel lengths. We provide a brief theory for EOF-balanced ITP focusing under continuous injection from a depleting reservoir and present the design of a short (7 mm) paper-based microfluidic channel, which allows a 200 μL sample to be processed in approximately 6 min, resulting in a 20 000-fold increase in concentration - a full order of magnitude improvement compared to previous paper-based ITP devices. We show the stability of the assay over longer (40 min) durations of time, and using Morpholino probes, we present the applicability of the device for amplification-free detection of nucleic acids, with a limit-of-detection (LoD) of 5 pM in 10 min. Finally, we utilize the small footprint of the channel and show a multiplexed platform in which 12 assays operate in parallel in a 24-well plate format.

Determination of inorganic anions in saliva by electroosmotic flow controlled counterflow isotachophoretic stacking under field-amplified sample injection

Under a strong counter-electroosmotic flow, five salivary inorganic anions, bromide, iodide, nitrite, nitrate and thiocyanate were determined by field-amplified sample injection in combination with isotachophoretic stacking. Separation and concentration conditions were investigated. A terminating electrolyte, 5mM borate, was added in the sample. Under the optimized conditions, Br(-), I(-) and SCN(-) were concentrated online using 150mM HCl-Tris buffer at pH 7.8 in a bare fused capillary, providing more than ten thousand of sensitivity enrichment compared with normal injections. The relative standard deviations (RSDs, n=5) were less than 1% in migration times, 8% in peak areas. Using direct UV detection at 200nm and 226nm, the limits of detection (LODs, S/N=3) were of 0.002-0.01μM. Unfortunately, NO2(-) and NO3(-) could be observed in purified or deionized water. Therefore, a low dilution factor was applied to saliva samples. Due to the matrix effect, samples were injected in a shorter time, and standard addition method was applied to determine all the five inorganic anions in saliva. The RSDs of the migration times and peak areas were in a range of 0.2-0.4% and 3.0-4.0%, respectively. The LODs were 0.2-2.0μM. The salivary levels of the anions obtained were in accord with the reference data. The external standard method can not be adapted to real samples due to biases caused by electrokinetic injection and errors from high dilutions.

Toward 10,000-fold sensitivity improvement of oligosaccharides in capillary electrophoresis using large-volume sample stacking with an electroosmotic flow pump combined with field-amplified sample injection

A combination of two online sample concentration techniques, large-volume sample stacking with an electroosmotic flow pump (LVSEP) and field-amplified sample injection (FASI), was investigated in CE to achieve highly sensitive oligosaccharide analysis. In CE with LVSEP-FASI, analytes injected throughout the capillary were concentrated on the basis of LVSEP, followed by an electrokinetic introduction of concentrated analytes from the inlet vial by the FASI mechanism. After switching the inlet vial solution from the sample to running buffer, the concentrated analytes were then separated by CZE. In the present LVSEP-FASI-CZE, pressure was applied to the capillary inlet until the inlet vial solution was exchanged. The applied pressure generated a counterflow against the EOF. It kept the stacked sample zone within the capillary, minimizing loss of concentrated analytes. Fluorescein was first analyzed by LVSEP-FASI-CZE to optimize preconcentration condition. Up to 110 000-fold sensitivity increase was obtained with 200 μL of sample, compared to normal CZE with sample injection of 0.3 psi for 3 s (ca. 1.7 nL). From the results, the pressure application improved the efficiency of the FASI-mode concentration significantly at total concentration time longer than 10 min. In the analysis of maltoheptaose, a 10 000-fold sensitivity increase was achieved, which is the highest concentration efficiency ever reported in CE of oligosaccharides. The relative standard deviations of the detection time and peak height were 2.4 and 11%, respectively. In the analysis of glucose oligomer, up to 8600-fold sensitivity increases were achieved without reducing the separation performance of conventional CZE.

Recent advances in enhancing the sensitivity of electrophoresis and electrochromatography in capillaries and microchips (2010-2012)

CE has been alive for over two decades now, yet its sensitivity is still regarded as being inferior to that of more traditional methods of separation such as HPLC. As such, it is unsurprising that overcoming this issue still generates much scientific interest. This review continues to update this series of reviews, first published in Electrophoresis in 2007, with updates published in 2009 and 2011 and covers material published through to June 2012. It includes developments in the field of stacking, covering all methods from field amplified sample stacking and large volume sample stacking, through to isotachophoresis, dynamic pH junction and sweeping. Attention is also given to online or inline extraction methods that have been used for electrophoresis.

Preconcentration and detection of the phosphorylated forms of cardiac troponin I in a cascade microchip by cationic isotachophoresis

This paper describes the detection of a cardiac biomarker, cardiac troponin I (cTnI), spiked into depleted human serum using cationic isotachophoresis (ITP) in a 3.9 cm long poly(methyl methacrylate) (PMMA) microfluidic channel. The microfluidic chip incorporates a 100× cross-sectional area reduction, including a 10× depth reduction and a 10× width reduction, to increase sensitivity during ITP. The cross-sectional area reductions in combination with ITP allowed visualization of lower concentrations of fluorescently labeled cTnI. ITP was performed in both "peak mode" and "plateau mode" and the final concentrations obtained were linear with initial cTnI concentration. We were able to detect and quantify cTnI at initial concentrations as low as 46 ng mL(-1) in the presence of human serum proteins and obtain cTnI concentrations factors as high as ~ 9000. In addition, preliminary ITP experiments including both labeled cTnI and labeled protein kinase A (PKA) phosphorylated cTnI were performed to visualize ITP migration of different phosphorylated forms of cTnI. The different phosphorylated states of cTnI formed distinct ITP zones between the leading and terminating electrolytes. To our knowledge, this is the first attempt at using ITP in a cascade microchip to quantify cTnI in human serum and detect different phosphorylated forms.

Recent advances in enhancing the sensitivity of electrophoresis and electrochromatography in capillaries and microchips (2008-2010)

Capillary electrophoresis has been alive for over two decades now; yet, its sensitivity is still regarded as being inferior to that of more traditional methods of separation such as HPLC. As such, it is unsurprising that overcoming this issue still generates much scientific interest. This review continues to update this series of reviews, first published in Electrophoresis in 2007, with an update published in 2009 and covers material published through to June 2010. It includes developments in the fields of stacking, covering all methods from field-amplified sample stacking and large volume sample stacking, through to ITP, dynamic pH junction and sweeping. Attention is also given to on-line or in-line extraction methods that have been used for electrophoresis.

Gradient counterflow electrophoresis methods for bioanalysis

CE has evolved as one of the most efficient separation techniques for a wide range of analytes, from small molecules to large proteins. Modern microdevices facilitate integration of multiple sample-handling steps, from preparation to separation and detection, and often rely on CE for separations. However, CE frequently requires complex geometries for performing sample injections and maintaining zone profiles across long separation lengths in microdevices. Two novel methods for performing electrophoretic separations, gradient elution moving boundary electrophoresis (GEMBE) and gradient elution isotachophoresis (GEITP), have been developed to simplify microcolumn operations. Both techniques use variable hydrodynamic counterflow and continuous sample injection to perform analyses in short, simple microcolumns. These properties result in instruments and microdevices that have minimal ‘real-world’ interfaces and reduced footprints. Additionally, GEITP is a rapid enrichment technique that addresses sensitivity issues in CE and microchips.

Electrophoretic exclusion for the selective transport of small molecules

A novel method capable of differentiating and concentrating small molecules in bulk solution termed "electrophoretic exclusion" is described and experimentally investigated. In this technique, the hydrodynamic flow of the system is countered by the electrophoretic velocity to prevent a species from entering into the channel. The separation can be controlled by changing the flow rate or applied electric field in order to exclude certain species selectively while allowing others to pass through the capillary. Proof of principle studies employed a flow injection regime of the method and examined the exclusion of Methyl Violet dye in the presence of a neutral species. Methyl Violet was concentrated almost 40 times the background concentration in 30 s using 6 kV. Additionally, a threshold voltage necessary for exclusion was determined. The establishment of a threshold voltage enabled the differentiation of two similar cationic species: Methyl Green and Neutral Red.

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.