Analyzing small samples with high efficiency: capillary batch injection–capillary electrophoresis–mass spectrometry

Assays to Estimate the Binding Affinity of Aptamers

Aptamers have become coming-of-age molecular recognition elements in both diagnostic and therapeutic applications. Generated by SELEX, the 'quality control' of aptamers, which involves the validation of their binding affinity against their respective targets is pivotal to ascertain their potency prior to use in any downstream assays or applications. Several aptamers have been isolated thus far, however, the usage of inappropriate validation assays renders some of these aptamers dubitable in terms of their binding capabilities. Driven by this need, we provide an up-to-date critical review of the various strategies used to determine the aptamer-target binding affinity with the aim of providing researchers a better comprehension of the different analytical approaches in respect to the molecular properties of aptamers and their intended targets. The techniques reported have been classified as label-based techniques such as fluorescence intensity, fluorescence anisotropy, filter-binding assays, gel shift assays, ELISA; and label-free techniques such as UV-Vis spectroscopy, circular dichroism, isothermal titration calorimetry, native electrospray ionization-mass spectrometry, quartz crystal microbalance, surface plasmon resonance, NECEEM, backscattering interferometry, capillary electrophoresis, HPLC, and nanoparticle aggregation assays. Hybrid strategies combining the characteristics of both categories such as microscale thermophoresis have been also additionally emphasized. The fundamental principles, complexity, benefits, and challenges under each technique are elaborated in detail.

Characterization of linearly coupled capillaries with various inner diameters in the context of capillary electrophoresis

As a result of continuous instrumental progress, capillary electrophoresis has become an established separation technique. However, the choice of the suitable capillary inner diameter is sometimes difficult due to different instrumental requirements concerning injection, separation, or detection. To overcome this problem, we assembled two capillaries with different inner diameters, meaning that the inner diameter of the capillary at the injection side was different from that at the detection side. Since this was a rather uncommon approach, we focused on the associated effects in this proof-of-concept study. For the experiments, a non-aqueous model system was used, consisting of an acetonitrile-based background electrolyte and the two ferrocene derivates, ferrocenemethanol and decamethylferrocene. Using capillary flow injection analysis hyphenated to capacitively coupled contactless conductivity detection, it could be shown that fragmented capillaries of the same inner diameter had slightly lower volume flow rates than non-fragmented capillaries. With non-aqueous capillary electrophoresis hyphenated to UV detection, it was found that the coupling of capillaries with different inner diameter had a much stronger effect on the capillary electrophoresis flow than combinations with the same inner diameter. Additionally, if the inner diameter of the second capillary was larger than the inner diameter of the first capillary, a higher theoretical plate number and an increased sensitivity were found. Furthermore, it was found that there was no significant peak tailing introduced by the coupling.

Graphic abstract

Capillary Electrophoresis with Capacitively Coupled Contactless Conductivity Detection for Quantitative Analysis of Dried Blood Spots with Unknown Blood Volume

Blood volume in dried blood spot (DBS) analysis is assumed to be constant for DBS punches with a fixed area. However, blood volume in the punch is dependent on several factors associated with the blood composition and is preferentially normalized by off-line analysis for quantitative purposes. Instead of using external instrumentation, we present an all-in-one approach for the simultaneous determination of exact blood volume in the DBS punch and the quantitation of target analytes. A DBS is eluted with 500 μL of elution solvent in a sample vial, and the eluate is directly subjected to an automated analysis by capillary electrophoresis with capacitively coupled contactless conductivity detection (CE-C⁴D). The capillary blood volume in the eluate is calculated from the concentrations of the inorganic blood constituents (K⁺, Na⁺, or Cl^-) determined by CE-C⁴D, which are linearly proportional to the blood volume originally sampled onto the DBS card. Alternatively, conductivity of the DBS eluate can be used for the blood volume determination by using C⁴D in a nonseparation flow-through mode. The methods are suitable for the determination of blood volume in unknown DBS samples by punching out the entire DBS or by subpunching a small section of a large DBS with variations of the true vs the determined volume ≤5.5%. Practical suitability was demonstrated by the simultaneous CE-C⁴D determination of K⁺ and Na⁺ (for DBS volume calculation) and amino acids (target analytes) in unknown DBS samples. Quantitative analysis of selected amino acids (related to inborn metabolic disorders) in the unknown DBS was compared with a standard analytical procedure using wet-blood chemistry, and an excellent fit was obtained. The use of CE-C⁴D represents an important milestone in quantitative DBS analysis since the detection technique is universal, and the separation technique enables the determination of cations and/or anions and the use of multiple detectors, which further enhance selectivity/sensitivity of the analysis and the range of detectable analytes.

Direct sample injection from a syringe needle into a separation capillary

An automatic micro-injector was developed for electrophoretic analysis of a microlitre amount of clinical samples, enabling injection of the sample from a Hamilton syringe. The outlet of the syringe needle is located directly opposite the inlet of the separation capillary at a defined distance of the order of hundreds of μm in the injection space. During the injection, the background electrolyte is forced out by air from this space and a drop of the sample is forced out of the syringe by a micro-pump so that it is caught at the entrance to the capillary. From the drop the sample is injected into the capillary by applying a negative pressure pulse or simply by spontaneous injection. The injection space is then filled with background electrolyte, which washes away excess sample and separation is commenced. The injector was tested in electrophoretic separation of a model sample with equimolar concentrations of 100 μM NH₄⁺, K⁺, Na⁺, Mg²⁺ and Li⁺ in a short capillary with total/effective length of 16.5/11.5 cm. The repeatability of the migration time and peak area expressed as the RSD value is 2% and 4%, respectively. The practical applicability of the injector was verified on the determination of the antiparasitic pentamidine in 10 μL of rat plasma. Electrophoretic separation of pentamidine was performed in 100 mM of acetic acid/NaOH at pH 4.55, the sample consumption per analysis is 125 nL, the separation time is 45 s and the attained LOQ using contactless conductivity detection is 8 μM.

Dual detection for non-aqueous capillary electrophoresis combining contactless conductivity detection and mass spectrometry

Coupling of two detectors is a powerful tool to enhance the overall analytical performance generating complementary information and overcoming the limitations of the single detectors. In this work, capacitively coupled contactless conductivity detection (C⁴D) and electrospray ionization mass spectrometry (ESI-MS) were coupled in conjunction with non-aqueous capillary electrophoresis (NACE). Non-aqueous electrolytes are highly compatible with ESI due to their volatility. Moreover, they exhibit low background conductivity, which is essential for the detection with C⁴D. A NACE-C⁴D-MS method was developed using an acetonitrile buffer containing 2 M HAc and 4 mM NH₄Ac as background electrolyte. The influence of the inner diameter of the separation capillary on the C⁴D was studied and taken into account. A capillary with 50 µm inner diameter was found to be best suited. The complementarity of the two detectors was shown by determining a sample mixture containing choline, thiamine, nitrate, and chloride as well as bromide and acetylcholine as internal standards. The C⁴D was the detector of choice for the inorganic ions, which were not detectable with the MS whereas the MS had much lower limits of detections for the organic biomolecules. The method was applied on an extract of a food supplement containing the model analytes.

Detection of contaminants in water supply: A review on state-of-the-art monitoring technologies and their applications

Water monitoring technologies are widely used for contaminants detection in wide variety of water ecology applications such as water treatment plant and water distribution system. A tremendous amount of research has been conducted over the past decades to develop robust and efficient techniques of contaminants detection with minimum operating cost and energy. Recent developments in spectroscopic techniques and biosensor approach have improved the detection sensitivities, quantitatively and qualitatively. The availability of in-situ measurements and multiple detection analyses has expanded the water monitoring applications in various advanced techniques including successful establishment in hand-held sensing devices which improves portability in real-time basis for the detection of contaminant, such as microorganisms, pesticides, heavy metal ions, inorganic and organic components. This paper intends to review the developments in water quality monitoring technologies for the detection of biological and chemical contaminants in accordance with instrumental limitations. Particularly, this review focuses on the most recently developed techniques for water contaminant detection applications. Several recommendations and prospective views on the developments in water quality assessments will also be included.

Micro-injector for capillary electrophoresis

A novel micro-injector for capillary electrophoresis for the handling of samples with volumes down to as little as 300 nL was designed and built in our laboratory for analyses in which the available volume is a limitation. The sample is placed into a small cavity located directly in front of the separation capillary, and the injection is then carried out automatically by controlled pressurization of the chamber with compressed air. The system also allows automated flushing of the injection chamber as well as of the capillary. In a trial with a capillary electrophoresis system with contactless conductivity detector, employing a capillary of 25 μm diameter, the results showed good stability of migration times and peak areas. To illustrate the technique, the fast separation of five inorganic cations (Na(+) , K(+) , NH4 (+) , Ca(2+) , and Mg(2+) ) was set up. This could be achieved in less than 3 min, with good limits of detection (10 μM) and linear ranges (between about 10 and 1000 μM). The system was demonstrated for the determination of the inorganic cations in porewater samples of a lake sediment core.

Two-dimensional separation of ionic species by hyphenation of capillary ion chromatography × capillary electrophoresis-mass spectrometry

The separation of complex mixtures such as biological or environmental samples requires high peak capacities, which cannot be established with a single separation technique. Therefore, multidimensional systems are in demand. In this work, we present the hyphenation of the two most important (orthogonal) techniques in ion analysis, namely, ion chromatography (IC) and capillary electrophoresis (CE), in combination with mass spectrometry. A modulator was developed ensuring a well-controlled coupling of IC and CE separations. Proof-of-concept measurements were performed using a model system consisting of nucleotides and cyclic nucleotides. The data are presented in a multidimensional contour plot. Analyte stacking in the CE separation could be exploited on the basis of the fact that the suppressed IC effluent is pure water.