Hyphenation of microflow chromatography with electrospray ionization mass spectrometry for bioanalytical applications focusing on low molecular weight compounds: A tutorial review

Benefits of miniaturized chromatography with various detection modes, such as increased sensitivity, chromatographic efficiency, and speed, were recognized nearly 50 years ago. Over the past two decades, this approach has experienced rapid growth, driven by the emergence of mass spectrometry applications serving -omics sciences and the need for analyzing minute volumes of precious samples with ever higher sensitivity. While nanoscale liquid chromatography (flow rates <1 μL/min) has gained widespread recognition in proteomics, the adoption of microscale setups (flow rates ranging from 1 to 100 μL/min) for low molecular weight compound applications, including metabolomics, has been surprisingly slow, despite the inherent advantages of the approach. Highly heterogeneous matrices and chemical structures accompanied by a relative lack of options for both selective sample preparation and user-friendly equipment are usually reported as major hindrances. To facilitate the wider implementation of microscale analyses, we present here a comprehensive tutorial encompassing important theoretical and practical considerations. We provide fundamental principles in micro-chromatography and guide the reader through the main elements of a microflow workflow, from LC pumps to ionization devices. Finally, based on both our literature overview and experience, illustrated by some in-house data, we highlight the critical importance of the ionization source design and its careful optimization to achieve significant sensitivity improvement.

Theoretical computation of the band broadening in micro-pillar array columns

We have investigated the possibility to establish a theoretical plate height expression for the band broadening in the most widely used micro-pillar array column format, i.e., a cylindrical pillar array wherein the pillar walls and the channel bottom are coated with a thin layer of meso‑porous material. Assuming isotropic diffusion in the shell-layer, it was found that the vertical diffusive transport along the porous shell-layer covering the pillar walls significantly suppresses the band broadening originating from the vertical migration velocity gradients. As the vertical transport in the shell-layer increases linearly with the retention equilibrium constant K, this leads to an anomalous dependency on the retention factor. Indeed, instead of increasing with k'' and following the classic (1+ak''+bk''2)/(1 + k'')2-dependency governing a classic Taylor-Aris system, the variation of the mobile zone mass transfer resistance term hCm in a 3D pillar array with bottom-wall retention goes through a maximum (resp. factor 1.5 (k''=4) and 2 (k''=16) difference between observed and classic Taylor-Aris behaviour). This effect increases with increasing pillar heights and increasing reduced velocities. Because of this complex k''-dependency, it proves very cumbersome to establish a general plate height equation covering all conditions. Instead, a plate height expression was established that is limited up to k''=4, but remains accurate for higher k''-values for cases where the ratio of pillar height over inter-pillar distance remains below 5. It can however be anticipated the proposed analytical model is only valid in a rather limited range around the presently considered external porosity of ε=0.5.

Phase-separation multiphase flow: preliminary application to analytical chemistry

A two-phase separation mixed solution can undergo phase separation from one phase to two phases (i.e., upper and lower phases) in a batch vessel in response to changes in temperature and/or pressure. This phase separation is reversible. When the mixed solution undergoes a phase change while being fed into a microspace region, a dynamic liquid-liquid interface is formed, leading to a multiphase structure. This flow is called a phase-separation multiphase flow. Annular flow in a microspace, which is one such phase-separation multiphase flow, is interesting and has been applied to chromatography, extraction, reaction fields, and mixing. Here, research papers related to phase-separation multiphase flows-ranging from the discovery of the phenomenon to basic and technical research from the viewpoint of analytical science-are reviewed. In addition, the development of a new separation mode in a high-performance liquid chromatography system based on phase-separation multiphase flow is introduced.

Ultralow flow liquid chromatography and related approaches: A focus on recent bioanalytical applications

Ultralow flow LC employs ultra-narrow bore columns and mid-range pL/min to low nL/min flow rates (i.e., ≤20 nL/min). The separation columns that are used under these conditions are typically 2-30 μm in inner diameter. Ultralow flow LC systems allow for exceptionally high sensitivity and frequently high resolution. There has been an increasing interest in the analysis of scarce biological samples, for example, circulating tumor cells, extracellular vesicles, organelles, and single cells, and ultralow flow LC was efficiently applied to such samples. Hence, advances towards dedicated ultralow flow LC instrumentation, technical approaches, and higher throughput (e.g., tens-to-hundreds of single cells analyzed per day) were recently made. Here, we review the types of ultralow flow LC technology, followed by a discussion of selected representative ultralow flow LC applications, focusing on the progress made in bioanalysis of amount-limited samples during the last 10 years. We also discuss several recently reported high-sensitivity applications utilizing flow rates up to 100 nL/min, which are below commonly used nanoLC flow rates. Finally, we discuss the path forward for future developments of ultralow flow LC.

Dynamic in situ growth of bonded-phase silica nanospheres on silica capillary inner walls for open-tubular liquid chromatography

Silica nanospheres (SNS) were grown on the inner walls of silica capillaries through a dynamic in situ nucleation process to prepare a highly porous and large accessible surface area substrate. The SNS were then functionalized with octadecyl (C18), 3-aminopropyltriethoxysilane (APTES), beta-cyclodextrin (β-CD), and amino groups to develop robust and efficient chromatographic stationary phases. The modified silica capillaries were exploited for open-tubular liquid chromatography (OT-LC) and open-tubular capillary electrochromatography (OT-CEC) applications. The prepared stationary phases were compared to conventional capillaries in terms of separation performance. The synthesis process was optimized, and the bonded-phase stationary phases were characterized by the electron microscopy technique. The effects of different solvents, additives, and functional groups on the geometry and chromatographic resolving power of the SNS were envisaged. The capillaries modified with octadecyl groups were evaluated for the separation of non-steroidal anti-inflammatory drugs, phenones, alkenylbenzenes, and enantiomers of chlorophenoxy herbicides. As an application instance, an SNS-C18-coated capillary was utilized for the separation of alkenylbenzenes from clove extract and protein digest medium, through OT-LC and OT-CEC techniques, respectively. The β-CD functionalized capillary was applied for the OT-CEC separation of a dichlorprop racemic mixture.

Flexible Organic Electronic Ion Pump for Flow-Free Phytohormone Delivery into Vasculature of Intact Plants

Plant vasculature transports molecules that play a crucial role in plant signaling including systemic responses and acclimation to diverse environmental conditions. Targeted controlled delivery of molecules to the vascular tissue can be a biomimetic way to induce long distance responses, providing a new tool for the fundamental studies and engineering of stress-tolerant plants. Here, a flexible organic electronic ion pump, an electrophoretic delivery device, for controlled delivery of phytohormones directly in plant vascular tissue is developed. The c-OEIP is based on polyimide-coated glass capillaries that significantly enhance the mechanical robustness of these microscale devices while being minimally disruptive for the plant. The polyelectrolyte channel is based on low-cost and commercially available precursors that can be photocured with blue light, establishing much cheaper and safer system than the state-of-the-art. To trigger OEIP-induced plant response, the phytohormone abscisic acid (ABA) in the petiole of intact Arabidopsis plants is delivered. ABA is one of the main phytohormones involved in plant stress responses and induces stomata closure under drought conditions to reduce water loss and prevent wilting. The OEIP-mediated ABA delivery triggered fast and long-lasting stomata closure far away from the delivery point demonstrating systemic vascular transport of the delivered ABA, verified delivering deuterium-labeled ABA.

C-Term Reduction in 3 μm Open-Tubular High-Aspect-Ratio Channels in AC-EOF Vortex Chromatography Operation

The performance of liquid chromatography operation in open-tubular channels, the ideal chromatographic column format, is limited by slow mass transport between the mobile and stationary phase. We recently introduced a lateral mixing methodology ("vortex chromatography") to reduce Taylor-Aris dispersion by employing (small) AC-EOF (alternating current electroosmotic flow) fields oriented perpendicular to the conventionally applied, axially oriented pressure gradient, resulting in the reduction of the C-term by a factor of 3, studied in 40 × 20 μm² (aspect ratio (AR) = 2) channels under unretained conditions. In the present contribution, a further increased performance gain for channel dimensions relevant for chromatographic applications is demonstrated. The impact of the applied voltage and salt concentration is studied for 3 × 20 and 5 × 20 μm² channels in ARs of up to 6.7, revealing a C-term reduction potential of a factor of up to 5 for large molecules (dextran) under unretained conditions. The decrease in κ_aris in a 5 μm channel (reduction of 80%) was larger than the decrease in a 3 μm channel (reduction of 44%).

Development of HPLC system that uses phase-separation multiphase flow as an eluent

We developed a new type of HPLC system that uses phase-separation multiphase flow as an eluent. A commercially available HPLC system with a packed separation column filled with octadecyl-modified silica (ODS) particles was used. First, as preliminary experiments, 25 kinds of mixed solutions of water/acetonitrile/ethyl acetate and water/acetonitrile were supplied to the system to act as eluents at 20 °C. 2,6-Naphthalenedisulfonic acid (NDS) and 1-naphthol (NA) mixture was used as a model and mixed analyte was injected into the system. Roughly speaking, they were not separated in organic solvent-rich eluents and well separated in water-rich eluents, in which NDS eluted faster than NA. This means that HPLC worked under a reverse-phase mode for separation at 20 °C. Next, the separation of the mixed analyte was examined on HPLC at 5 °C, and then after judging the results, four kinds of ternary mixed solutions were in detail as eluents on HPLC at 20 °C and 5 °C. Based on their volume ratio, the ternary mixed solutions acted as a two-phase separation mixed solution, leading to a phase-separation multiphase flow. Consequently, the solutions flowed homogeneously and heterogeneously in the column at 20 °C and 5 °C, respectively. For example, the ternary mixed solutions containing water/acetonitrile/ethyl acetate at volume ratios of 20:60:20 (organic solvent-rich) and 70:23:7 (water-rich) were delivered into the system as eluents at 20 °C and 5 °C. In the organic solvent-rich eluent, the mixture of NDS and NA was not separated at 20 °C but was separated at 5 °C, the elution of NA being faster than the one of NDS (phase-separation mode). In the water-rich eluent, the mixture of analytes was separated at both 20 °C and 5 °C, the elution of NDS being faster than the one of NA. The separation at 5 °C was more effective than at 20 °C (reverse-phase mode and phase-separation mode). This separation performance and elution order can be attributed to the phase-separation multiphase flow at 5 °C.

Recent trends in the biotechnology of functional non-digestible oligosaccharides with prebiotic potential

Prebiotics as a part of dietary nutrition can play a crucial role in structuring the composition and metabolic function of intestinal microbiota and can thus help in managing a clinical scenario by preventing diseases and/or improving health. Among the different prebiotics, non-digestible carbohydrates are molecules that selectively enrich a typical class of bacteria with probiotic potential. This review summarizes the current knowledge about the different aspects of prebiotics, such as its production, characterization and purification by various techniques, and its link to novel product development at an industrial scale for wide-scale use in diverse range of health management applications. Furthermore, the path to effective valorization of agricultural residues in prebiotic production has been elucidated. This review also discusses the recent developments in application of genomic tools in the area of prebiotics for providing new insights into the taxonomic characterization of gut microorganisms, and exploring their functional metabolic pathways for enzyme synthesis. However, the information regarding the cumulative effect of prebiotics with beneficial bacteria, their colonization and its direct influence through altered metabolic profile is still getting established. The future of this area lies in the designing of clinical condition specific functional foods taking into consideration the host genotypes, thus facilitating the creation of balanced and required metabolome and enabling to maintain the healthy status of the host.

Reversed-Phase Liquid Chromatography of Peptides for Bottom-Up Proteomics: A Tutorial

The performance of the current bottom-up liquid chromatography hyphenated with mass spectrometry (LC-MS) analyses has undoubtedly been fueled by spectacular progress in mass spectrometry. It is thus not surprising that the MS instrument attracts the most attention during LC-MS method development, whereas optimizing conditions for peptide separation using reversed-phase liquid chromatography (RPLC) remains somewhat in its shadow. Consequently, the wisdom of the fundaments of chromatography is slowly vanishing from some laboratories. However, the full potential of advanced MS instruments cannot be achieved without highly efficient RPLC. This is impossible to attain without understanding fundamental processes in the chromatographic system and the properties of peptides important for their chromatographic behavior. We wrote this tutorial intending to give practitioners an overview of critical aspects of peptide separation using RPLC to facilitate setting the LC parameters so that they can leverage the full capabilities of their MS instruments. After briefly introducing the gradient separation of peptides, we discuss their properties that affect the quality of LC-MS chromatograms the most. Next, we address the in-column and extra-column broadening. The last section is devoted to key parameters of LC-MS methods. We also extracted trends in practice from recent bottom-up proteomics studies and correlated them with the current knowledge on peptide RPLC separation.

Liquid Chromatography Column Design and Dimensional Analysis of the van Deemter Equation

The fundamental mechanisms of band broadening are usually introduced to students through the van Deemter equation. Dimensional analysis of this equation can give physical meaning to the equation coefficients and enhance our understanding relative to qualitative descriptions. This approach can also guide improvements to future liquid chromatography (LC) column designs.

Open tubular capillary ion chromatography with online dilution for small ions determination in drinks

This work aims to develop a more robust, easy-to-use, low-pressure, and cost-effective nonsuppressed open tubular capillary ion chromatography (NS-OTIC) approach with online dialysis for small ions determination in drinks. The fabricated device was applied for two selected columns, including poly(butadiene-maleic acid) on a 50-μm bore and AS18 Latex on 25-µm bore fused silica capillaries, for the separation of mixed cations (Na⁺, K⁺, Ca²⁺, Mg²⁺) and mixed anions (Cl^-, Br^-, NO₂^-, NO₃^-), respectively. High concentrations of ions (up to 100 mM) level can be directly introduced into the NS-OTIC system without an off-line (manual) dilution step. The linear relationship of the peak area and concentration of model ions can be obtained with a resolution > 1.1. The repeatability of the peak area for both OTIC columns was < 6% RSD. Juice and tea samples were successfully analyzed with % recoveries of 77-112 and 90-119 for cation and anion determinations, respectively.

Novel separation mode of HPLC based on phase-separation multiphase flow

A novel separation mode for high-performance liquid chromatography (HPLC) is proposed based on phase-separation multiphase flow. A commercially available HPLC system was used with a packed-separation column of octadecyl-silica (ODS)-modified particles. Water/acetonitrile/ethyl acetate ternary mixed solutions, (a) 1:8:1, (b) 1:3:1, and (c) 16:3:1 (v/v/v), were delivered into the system as an eluent at 20 and 5 °C. The ternary mixed solution acted as a two-phase separation solution leading to phase-separation multiphase flow. The solution flowed in the column homogeneously and heterogeneously at 20 and 5 °C, respectively. 1-Naphthol (NA) and 2,6-naphthalenedisulfonic acid (NDS) were injected into the system as model analytes. At 20 °C, the analyte mixture did not separate in solutions (a) and (b) while it separated in solution (c) with the elution order of NDS followed by NA. At 5 °C, it did not separate in solution (a), while it separated in solution (b) with elution order of NA followed by NDS and solution (c) with elution order of NDS followed by NA more effectively than 20 °C. The separation behavior and elution order are possibly caused by the phase-separation multiphase flow.

Porous layer open tubular nano liquid chromatography directly coupled to electron ionization mass spectrometry

A 25 µm i.d x 1.2 m length PS-DVB porous layer open tubular column (PLOT) was prepared and assessed in the configuration of a nano liquid chromatography coupled to an electron ionization mass spectrometry system (OT-nanoLC-EI-Ms), via the direct insertion of the column outlet into the ionization source. The developed system's operational parameters were comprehensively studied, and the setup performance was investigated employing both unidimensional and column switching configurations. As a result, the OT-nanoLC-EI-MS system demonstrated competitive applicability in separating non-amenable ESI compounds, such as polyaromatic hydrocarbons (PAHs) and non-amenable GC compounds such as thermolabile pesticides. Furthermore, with excellent chromatographic performance, the PLOT columns can work under more compatible EI-detection conditions - such as the elution with 100% organic solvent. For example, PAHs retention factors ranged between 1.5 and 2.2 for 100% MeCN mobile phase, and more than 33,000 plates per meter for naphthalene at 50 nL/min flow rate. In analyzing thermolabile pesticides, the column switching PLOT-nanoLC-EI-MS system provided LODs of 25 µg/L, demonstrating suitable intra e interday reproducibility (% RSD < 13%, n = 3), and possibilities the direct injection of raw samples with suitable robustness.

Chiral separation using cyclodextrins as mobile phase additives in open-tubular liquid chromatography with a pseudophase coating

The chiral separation of various analytes (dichlorprop, mecoprop, ibuprofen, and ketoprofen) was demonstrated with different cyclodextrins as mobile phase additives in open‐tubular liquid chromatography using a stationary pseudophase semipermanent coating. The stable coating was prepared by a successive multiple ionic layer approach using poly(diallyldimethylammonium chloride), polystyrene sulfonate, and didodecyldimethyl ammonium bromide. Increasing concentrations (0–0.2 mM) of various native and derivatized cyclodextrins in 25 mM sodium tetraborate (pH 9.2) were investigated. Chiral separation was achieved for the four test analytes using 0.05–0.1 mM β‐cyclodextrin (resolution between 1.11 and 1.34), γ‐cyclodextrin (resolution between 0.78 and 1.27), carboxymethyl‐β‐cyclodextrin (resolution between 1.64 and 2.59), and 2‐hydroxypropyl‐β‐cyclodextrin (resolution between 0.71 and 1.76) with the highest resolutions obtained with 0.1 mM carboxymethyl‐β‐cyclodextrin. %RSD values were <10%. This is the first demonstration of chiral open‐tubular liquid chromatography using achiral chromatographic coatings and cyclodextrins as mobile phase additives.

Consecutive Sample Injection Analysis in Tube Radial Distribution Chromatography

Tube radial distribution chromatography based on the tube radial distribution flow, or annular flow, in an open-tubular capillary has been reported, where the annular flow is created through phase-separation multiphase flow. We have proposed the first-ever procedure for consecutive sample injection analysis using chromatography. In basic terms, a commercially available HPLC system could be used with a sample injector (0.2 μL volume) and a fused-silica capillary tube (250 cm long) as a separation column instead of a normal packed one, while the built-in detection cell was replaced by improved on-capillary detection. A ternary mixed solution of water/acetonitrile/ethyl acetate (3:8:4 volume ratio) was delivered into the capillary tube as an eluent at a flow rate of 2.0 μL min^-1. Model sample solutions of 1-naphthol and 2,6-naphthalenedisulfonic acid were consecutively analyzed by the present chromatography with a processing rate of 6 samples per hour. Simple and rapid consecutive analysis could be performed because washing and initialization of the separation tube was no longer necessary. The obtained results provide clues to developing new methodologies which combine features of both chromatography (separation) and the flow injection method (consecutive analysis).

How does DNA 'meet' capillary-based microsystems?

DNA possesses various chemical and physical properties which make it important in biological analysis. The opportunity for DNA to 'meet' capillary-based microsystems is rapidly increasing owing to the expanding development of miniaturization. Novel capillary-based methods can provide favourable platforms for DNA-ligand interaction assay, DNA translocation study, DNA separation, DNA aptamer selection, DNA amplification assay, and DNA digestion. Meanwhile, DNA exhibits great potential in the fabrication of new capillary-based biosensors and enzymatic bioreactors. Moreover, DNA has received significant research interest in improving capillary electrophoresis (CE) performance. We focus on highlighting the advantages of combining DNA and capillary-based microsystems. The general trend presented in this review suggests that the 'meeting' has offered a stepping stone for the application of DNA and capillary-based microsystems in the field of analytical chemistry.

Diffraction-based label-free photothermal detector for separation analyses in a nanocapillary

Miniaturization of column diameter in liquid chromatography is one of the major trends in separation sciences toward single-cell proteomics and metabolomics. Micro/nanoscale open tubular (OT) capillaries are promising tools for efficient separation analyses of the ultra-small volume of samples. However, highly sensitive and label-free on-column detection is still challenging for such ultra-small capillaries. In this study, we developed a photothermal detector using optical diffraction phenomena by a single nanocapillary. Our detection method realized concentration determination of unlabeled sample solutions in a nanocapillary with 460 nm inner diameter. The calculated limit of detection was 0.12 µM, which corresponds to 16 molecules in a detection volume of 0.23 fL. Furthermore, normal-phase chromatography was performed on a 12 cm long nanocapillary, and femtoliter sample injection, efficient separation, and label-free detection of dye molecules were demonstrated. Our photothermal detector will be widely used as a universal tool for chemical/biological analyses using capillaries with micro/nanoscale diameters.

Mixed-mode open tubular column for peptide separations by capillary electrochromatography

Mixed-mode chromatography open tubular column has been developed for peptide separation in electrochromatography. A column with 92 cm effective length and 50 μm internal diameter is fabricated internally with a copolymer sheet of restricted thickness. Catalyst facilitated binding of the coupling agent 3,5-bis (trifluoromethyl) phenyl isocyanate has been carried out at the interior surface of the column. The initiator sodium diethyldithiocarbamate was bound to the coupling agent. A small amount of N-[2-(acryloylamino) phenyl] acrylamide was used along with methacrylic acid and styrene in the monomer mixture to induce a little polar character in the stationary phase fabricated inside the column. Twenty-three peptides have been separated from a chemically digested protein mixture present in cytochrome C in capillary electrochromatography, in addition to the separation of six commercial peptides. We achieved an average plate count of over 1.5 million/m with the column of current study both for the digested protein components and commercial peptides using 70/30% v/v (acetonitrile/20 mM ammonium formate) at pH 6.5. In addition, the column resulted in baseline separation of all the peptides with very good resolution, enhanced peak capacity, and better retention time span.

Rational Strategy for Space-Confined Seeded Growth of ZnO Nanowires in Meter-Long Microtubes

Seeded crystal growths of nanostructures within confined spaces offer an interesting approach to design chemical reaction spaces with tailored inner surface properties. However, such crystal growth within confined spaces tends to be inherently difficult as the length increases as a result of confinement effects. Here, we demonstrate a space-confined seeded growth of ZnO nanowires within meter-long microtubes of 100 μm inner diameter with the aspect ratio of up to 10 000, which had been unattainable to previous methods of seeded crystal growths. ZnO nanowires could be grown via seeded hydrothermal crystal growth for relatively short microtubes below the length of 40 mm, while any ZnO nanostructures were not observable at all for longer microtubes above 60 mm with the aspect ratio of 600. Microstructural and mass spectrometric analysis revealed that a conventional seed layer formation using zinc acetate is unfeasible within the confined space of long microtubes as a result of the formation of detrimental residual Zn complex compounds. To overcome this space-confined issue, a flow-assisted seed layer formation is proposed. This flow-assisted method enables growth of spatially uniform ZnO nanowires via removing residual compounds even for 1 m long microtubes with the aspect ratio of up to 10 000. Finally, the applicably of ZnO-nanowire-decorated long microtubes for liquid-phase separations was demonstrated.

An overview of open tubular liquid chromatography with a focus on the coupling with mass spectrometry for the analysis of small molecules

Open tubular liquid chromatography (OT-LC) can provide superior chromatographic performance and more favorable mass spectrometry (MS) detection conditions. These features could provide enhanced sensitivity when coupled with electrospray ionization sources (ESI-) and lead to unprecedented detection capabilities if interfaced with a highly structural informative electron ionization (EI) source. In the past, the exploitation of OT columns in liquid chromatography evolved slowly. However, the recent instrumental developments in capillary/nanoLC-MS created new opportunities in developing and applying OT-LC-MS. Currently, the analytical advantages of OT-LC-MS are mainly exploited in the fields of proteomics and biosciences analysis. Nevertheless, under the right conditions, OT-LC-MS can also offer superior chromatographic performance and enhanced sensitivity in analyzing small molecules. This review will provide an overview of the latest developments in OT-LC-MS, focusing on the wide variety of employed separation mechanisms, innovative stationary phases, emerging column fabrication technologies, and new OT formats. In the same way, the OT-LC's opportunities and shortcomings coupled to both ESI and EI will be discussed, highlighting the complementary character of those two ionization modes to expand the LC's detection boundaries in the performance of targeted and untargeted studies.

Open tubular ion chromatography: A state-of-the-Art review

This review summarizes the progress in open tubular ion chromatography (OTIC) over the period from 1981 to 2020. Although OTIC columns provide superior column efficiency, require very little sample volumes, and consume a minimum level of eluents compared to regular packed columns, not many reports can be found from the literature mainly due to the difficulties in the preparation of OTIC columns and the harsh system requirements, such as pL-nL injections and extremely small detection volumes. However, technical advances, e.g., capacitively coupled contactless conductivity detectors (C⁴Ds), hydroxide eluent compatible polymer-based OTIC columns, electrodialytic capillary suppressors, and nanovolume gas-free hydroxide eluent generators (EGs), have removed the obstacles to OTIC. As such, in this review, the author focused on the development of the key components in an OTIC system from the perspective of instrument development. A brief revisit of open tubular (OT) column theory is first presented, followed by a discussion of the system configuration and component development. Attention is given to the advances in the development of the suppressed open tubular ion chromatography (SOTIC) system.

A comparison of 2 micron inner diameter open tubular column liquid chromatography with pressure-driven isocratic, slip-flow, and electrochromatographic modes of operation: a theoretical study

Modifications to the flow profile used in open tube capillary liquid chromatography (OT-CLC) include using slip-flow walls and using electroosmosis as a fluid pump as practiced in electrochromatography. These modifications are implemented experimentally by changing the capillary surface and solvent conditions which results in the change of boundary conditions at the capillary wall. In this paper we employ a theory-based study and compare the zone broadening of simple solutes using parabolic flow from a liquid pump, slip-flow from a highly hydrophobic inner surface with water eluent, and electroosmosis for the conditions of pure water and dilute salt utilizing 2 µm inner diameter OT capillaries. In general, the two types of flow other than parabolic exhibit thin zones in the early part of the chromatogram, consistent with previous studies of slip-flow and electroosmotic flow used in electrochromatography. Electrochromatography is shown to yield higher efficiency and less zone broadening than parabolic and slip-flow conditions used in this study. Nonetheless, it is found that the zone standard deviations are shown to be similar for these flow profiles as is the number of plates for these different flow profiles under the conditions utilized here. It is revealed that these modifications do not warrant the effort to maintain the special solvent conditions when compared to gradient elution OT-CLC, which gives a nearly constant peak width throughout the chromatogram, is easiest to implement, and is the method of choice for complex analysis.

Improved Sensitivity of Ultralow Flow LC-MS-Based Proteomic Profiling of Limited Samples Using Monolithic Capillary Columns and FAIMS Technology

In this work, we pioneered a combination of ultralow flow (ULF) high-efficiency ultranarrow bore monolithic LC columns coupled to MS via a high-field asymmetric waveform ion mobility spectrometry (FAIMS) interface to evaluate the potential applicability for high sensitivity, robust, and reproducible proteomic profiling of low nanogram-level complex biological samples. As a result, ULF LC-FAIMS-MS brought unprecedented sensitivity levels and high reproducibility in bottom-up proteomic profiling. In addition, FAIMS improved the dynamic range, signal-to-noise ratios, and detection limits in ULF LC-MS-based measurements by significantly reducing chemical noise in comparison to the conventional nanoESI interface used with the same ULF LC-MS setup. Two, three, or four compensation voltages separated by at least 15 V were tested within a single LC-MS run using the FAIMS interface. The optimized ULF LC-ESI-FAIMS-MS/MS conditions resulted in identification of 2,348 ± 42 protein groups, 10,062 ± 285 peptide groups, and 15,734 ± 350 peptide-spectrum matches for 1 ng of a HeLa digest, using a 1 h gradient at the flow rate of 12 nL/min, which represents an increase by 38%, 91%, and 131% in respective identifications, as compared to the control experiment (without FAIMS). To evaluate the practical utility of the ULF LC-ESI-FAIMS-MS platform in proteomic profiling of limited samples, approximately 100, 1,000, and 10,000 U937 myeloid leukemia cells were processed, and a one-tenth of each sample was analyzed. Using the optimized conditions, we were able to reliably identify 251 ± 54, 1,135 ± 80, and 2,234 ± 25 protein groups from injected aliquots corresponding to ∼10, 100, and 1,000 processed cells.

Open-tubular admicellar electrochromatography of charged analytes

Fundamental studies on the separation of cationic and anionic analytes in open-tubular admicellar electrochromatography (OT-AMEC) using cetyltrimethylammonium bromide (CTAB) and fused silica capillaries are presented. OT-AMEC was compared with open-tubular admicellar liquid chromatography (OT-AMLC) by running the two methods using the same mobile phases. The mobile phases were buffered at pH ≥ 6 and contained a low concentration (above the critical surface aggregation concentration and below the critical micelle concentration) of CTAB. The stationary pseudophase of CTAB admicelles were formed at the solid surface and liquid interface inside the capillary by simply conditioning the capillary with the mobile phase. Separations were performed in a 30 cm (21.5 cm to UV detector) long and 50 μm inner diameter capillary, using low pressure (50 mbar) in OT-AMLC and high voltage (15 kV at negative polarity) in OT-AMEC. The appropriate equations for the experimental estimation of retention factor (k) values of analytes were discussed. For anionic analytes, k in OT-AMEC were carefully determined by considering the observed interaction between CTAB monomers and tested analytes. The calculated k for each analyte was found similar in OT-AMLC and OT-AMEC, although the mechanism of retention was not entirely different due to the contribution of electrophoresis in OT-AMEC. Studies on the addition of a typical (i.e., acetonitrile) and atypical modifier (i.e., nonyl-β-glucoside) into the mobile phase, and sample focusing with >10x improvement in peak height under isocratic conditions were also conducted.

Nano liquid chromatography columns

Nano liquid chromatography (nanoLC), with columns having an inner diameter (ID) of ≤100 μm, can provide enhanced sensitivity and enable analysis of limited samples.