High-performance liquid chromatography on glass chips using precisely defined porous polymer monoliths as particle retaining elements

Comparison of experimental and simulated separation performance in capillary tube-in-manifold devices

A metal tube-in-manifold packed bed capillary column device, designed to overcome common limitations associated with capillary LC separations, is described. Experimental results of initial packing tests with sub-3 μm core-shell particles demonstrated efficiencies greater than 47,000 plates/m for a separation performed using the column device. Computational fluid dynamics (CFD) modeling of the multicomponent separation used for this work was validated against experimental LC results and the optimized model was able to effectively predict component peak retention times. However, the accuracy of predicted efficiencies requires further refinement. The tube-in-manifold design demonstrates that packed capillary columns with cylindrical cross-sectional channel geometry and ultrahigh pressure, low dead volume fluidic connections are achievable.

Modular Chip-Based nanoSFC-MS for Ultrafast Separations

This study presents the development of a miniaturized device for supercritical fluid chromatography coupled with mass spectrometry. The chip-based, modular nanoSFC approach utilizes a particle-packed nanobore column embedded between two monolithically structured glass chips. A microtee in the pre-column section ensures picoliter sample loads onto the column, while a microcross chip structure fluidically controls the column backpressure. The restrictive emitter and the minimal post-column volume of 16 nL prevent mobile phase decompression and analyte dilution, maintaining chromatographic integrity during transfer to the atmospheric pressure MS interface. This facilitates high-speed chiral separations in less than 80 s with high reproducibility.

Mass spectrometry coupling of chip-based supercritical fluid chromatography enabled by make-up flow-assisted backpressure regulation

This work introduces a novel microfluidic backpressure pressure control developed for chip-based supercritical fluid chromatography (chipSFC). The presented on-chip pressure control mechanism involves the post-column addition of a viscous make-up stream, which enables pressure regulation within the range of 73 to 130 bar range. In contrast to approaches using mechanical backpressure regulators, this chip-based make-up-assisted pressure regulation offers a wear-free alternative that functions entirely through fluidic means and contributes minimally to extra column volume. It prevents phase separation of the supercritical mobile phase and, therefore, expands the analytical scope of chipSFC to detection systems with an ambient pressure interface. This was demonstrated by a proof-of-principle experiment, where a model mixture was separated within 30 s and detected using atmospheric pressure ionisation mass spectrometry.

Rapid detection of glycosylated hemoglobin levels by a microchip liquid chromatography system in gradient elution mode

BACKGROUND

The determination of glycosylated hemoglobin (HbA1c) is crucial for diabetes diagnosis and can provide more substantial results than the simple measurement of glycemia. While there is a lack of simple methods for the determination of HbA1c using a point-of-care test (POCT) compared to glycemia measurement. In particular, high-performance liquid chromatography (HPLC) is considered the current gold standard for determining HbA1c levels. However, commercial HPLC systems usually have some sort of disadvantages such as bulky size, high-cost and need for qualified operators. Therefore, there is an urgent demand to develop a portable, and fast HbA1c detection system consuming fewer reagents.

RESULTS

We present a novel microchip that integrates a micromixer, passive injector, packed column and detection cell. The integrated microchip, in which all the microstructures were formed in the CNC machining center through micro-milling, is small in size (30 mm × 70 mm × 10 mm), and can withstand 1600 psi of liquid pressure. The integrated design is beneficial to reduce the band broadening caused by dead volume. Based on the microchip, a microchip liquid chromatography (LC) system was built and applied to the analysis of HbA1c. The separation conditions of HbA1c in blood calibrator samples were optimized using the microchip LC system. Samples containing four levels of HbA1c were completely separated within 2 min in optimal gradient conditions, with an inaccuracy (<3.2 %), a coefficient of variation (c.v. < 2.1 %) and a correlation coefficient (R2 = 0.993), indicating excellent separation efficiency and reproducibility.

SIGNIFICANCE

The POCT of HbA1c is critical for diabetes diagnosis. The microchip chromatography system was developed for HbA1c determination, which contains an integrated microchip and works under a gradient elution. It surpasses existing chip technology in terms of separation performance and detection speed, providing a competitive advantage for POCT of HbA1c. It is considered one important step for realizing efficient portable systems for timely and accurate diabetes diagnosis.

High-temperature ultrafast ChipHPLC-MS

Herein, we present a miniaturized chip-based HPLC approach coupled to electrospray ionization mass spectrometry utilizing temperature to achieve high-speed separations. The approach benefits from the low thermal mass of the microfluidic chip and can form an electrospray from the pre-heated mobile phase. With the help of this technology, isothermal and temperature-programmable operations up to 130°C were pursued to perform reversed-phase separations of pesticides in methanol and ethanol-containing eluents in less than 20 s.

Development of a particle packed bed model for homogeneity evaluation of liquid chromatography column

Particle packed columns used for liquid chromatography (LC) can suppress a column internal band broadening (hereinafter referred to as band broadening) by packing monodisperse particles homogenously. However, a quantitative evaluation for the effects of particle shape and packed state on band broadening needs to be more investigated. In this study, we fabricated a model of particle packed bed using microfluid LC columns that have pillar array structure prepared by microfabrication technology, evaluating how structural factors inside of a column affect its band broadening. At first, microfluid LC columns was prepared using Si-quartz glass (Si-Q column) for the optimization of LC measurement system. Through the evaluation, it showed 11.6 times higher pressure tolerance compared to that of PDMS-soda lime glass (PDMS-g column). Then, an optimized LC measurement system was constructed using a microfluidic LC column made of Si-Q column, and it was confirmed that the measurement error was small enough and the LC measurement could be performed with high repeatability. Additionally, the effect of a distribution of structural size on band broadening was evaluated. It was confirmed that wide distribution of the structural size provided large band broadening in actual measurements. Comparing two columns having different structural log-normal distributions of 0 and 0.22 showed approximately 1.8 times difference in both real LC measurement. Lastly, the relationship between packed state and band broadening was evaluated. As packed state, we employed void arrangement and structural arrangement in the columns. Different location arrangements of 50 and 100 µm pillar sizes afforded different band broadening. Well-homogenized array showed approximately two times worse band broadening compared to that of delocalized array. Based on these results, the developed packed bed of particles model was able to evaluate the relation between structural factors and band broadening.

Integration of segmented microflow chemistry and online HPLC/MS analysis on a microfluidic chip system enabling enantioselective analyses at the nanoliter scale

In this work, we introduce an approach to merge droplet microfluidics with an HPLC/MS functionality on a single chip to analyze the contents of individual droplets. This is achieved by a mechanical rotor-stator interface that precisely positions a microstructured PEEK rotor on a microfluidic chip in a pressure-tight manner. The developed full-body fused silica chip, manufactured by selective laser-induced etching, contained a segmented microflow compartment followed by a packed HPLC channel, which were interconnected by the microfluidic PEEK rotor on the fused silica lid with hair-thin through-holes. This enabled the targeted and leakage-free transfer of 10 nL fractions of droplets as small as 25 nL from the segmented microflow channel into the HPLC compartment that operated at pressures of up to 60 bar. In a proof of concept study, this approach was successfully applied to monitor reactions at the nanoliter scale and to distinguish the formed enantiomers.

[Recent advances in microchip liquid chromatography]

Miniaturization is an important trend in modern analytical instrument development, including miniaturized gas chromatography and liquid chromatography, as well as micro bore columns and capillary-to-microfluidics-based platforms. Apart from the miniaturization of the separation column, which is the core part of a chromatographic system, other parts of the system, including the sampler, pumping system, gradient generation, and detection systems, have been miniaturized. Miniaturized liquid chromatography significantly reduces solvent and sample consumption while providing comparable or even better separation efficiency. When liquid chromatography is coupled with mass spectroscopy, a low flow rate can increase the ionization efficiency, leading to enhanced sensitivity of the mass spectrometer. In contrast, normal-scale liquid chromatography suffers from its relatively high volumetric flow rate, which challenges the scanning frequency of the mass spectrometer. On the other hand because of the small sample size, other detection strategies such as spectrometric methods cannot provide sufficient sensitivity and limits of detection. In this sense, mass spectrometry has become the detection method of choice for micro-scale liquid-phase chromatography. Miniaturized liquid chromatography can diminish sample dilution efficiently when extremely small amounts of samples are used. The main driving force for this miniaturization trend, especially in liquid-phase separations, is the desperate need for microscale analyses of biological and clinical samples, given these samples are precious and the sample size is usually very small. At present, microscale liquid-phase chromatography is the only method of choice for such small, precious, and highly informative samples. The miniaturization of liquid chromatography systems, especially chromatographic columns, would be advantageous to the modularization and integration of liquid chromatography instrumental systems. Chip liquid chromatography is an integration of chromatography columns, liquid control systems, and detection methods on a single microfluidic chip. Chip liquid chromatography is an excellent format for the miniaturization of liquid chromatography systems, and it has already attracted significant attention from academia and industry. However, this attempt is challenging, and great effort is required on fundamental techniques, such as the substrate material of the microfluidic chip, structure of the micro-chromatography column, fluid control method, and detection methods, in order to make the chips suitable for liquid chromatography. Currently, the major problem in chip liquid chromatography is that the properties of the chip substrate materials cannot meet the requirements for further miniaturization and integration of chip liquid chromatography. The strength of the existing chip substrate materials is generally below 60 MPa, and the material properties limit further advances in the miniaturization and integration of chromatographic chips. Therefore, new chip substrate materials and the standard of chip channel design such as channel size and channel structure should be the key for further development of chip liquid chromatography. Mainstream instrumentation companies as well as new start-up innovation companies are now undertaking efforts toward the development of microchip liquid chromatographic products. Agilent, the first instrumentation company that introduced commercial microchip liquid chromatographic columns to the market, has led this field. Apart from microchip-based columns, Agilent introduced trap columns for different kinds of biological molecules as well as gradient generation systems for microchip-based liquid phase chromatography. Recently, another start-up company introduced microchip columns based on the in situ microfabrication of the column bed rather than packing the column with a particulate material. Such developments in microfabrication may further propel the advancement of micro-scale liquid-phase chromatography to an unprecedented level, which is beyond the conventional components and materials employed in normal-scale liquid chromatography. This review introduces the recent research progress in microchip liquid chromatography technologies, and briefly discusses the current state of commercialization of microchips for liquid chromatography by major instrumentation companies.

How electrospray potentials can disrupt droplet microfluidics and how to prevent this

A pressure-resistant microfluidic glass chip that integrates a packed-bed HPLC column, a droplet generator and a monolithic electrospray emitter is presented. This approach enables a seamless coupling of chip-HPLC and droplet microfluidics with ESI-MS detection. For the electrical contacting of the emitter, an electrode was integrated into the channel, which reaches up to the emitter tip. The incidental finding that under certain circumstances, the electrospray potential can strongly disturb the droplet microfluidics by electrowetting, was investigated in detail. Strategies to avoid this are evaluated and include electrical shielding and/or chip layouts, where the droplet generator is positioned at a long distance from the emitter.

On-Line Coupling of Chip-Electrochromatography and Ion Mobility Spectrometry

We report the first hyphenation of chip-electrochromatography (ChEC) with ion mobility spectrometry (IMS). This approach combines the separation power of two electrokinetically driven separation techniques, the first in liquid phase and the second in gas phase, with a label-free detection of the analytes. For achieving this, a microfluidic glass chip incorporating a monolithic separation column, a nanofluidic liquid junction for providing post-column electrical contact, and a monolithically integrated electrospray emitter was developed. This device was successfully coupled to a custom-built high-resolution drift tube IMS with shifted potentials. After proof-of-concept studies in which a mixture of five model compounds was analyzed in less than 80 s, this first ChEC-IMS system was applied to a more complex sample, the analysis of herbicides spiked in the wine matrix. The use of ChEC before IMS detection not only facilitated the peak allocation and increased the peak capacity but also enabled analyte quantification. As both, ChEC and IMS work at ambient conditions and are driven by high voltages, no bulky pumping systems are needed, neither for the hydrodynamic pumping of the mobile phase as in high-performance liquid chromatography nor for generating a vacuum system as in mass spectrometry. Accordingly, the approach has great potential as a portable analytical system for field analysis of complex mixtures.

Single-particle-frit-based packed columns for microchip chromatographic analysis of neurotransmitters

The fabrication of effective microchip liquid chromatography (LC) systems tends to be limited by the availability of suitable chromatographic columns. Herein, we developed a glass microchip LC system in which porous single-particle silica was adopted as frits and a freeze-thaw valve was utilized to achieve sample injection without interfering with sampling. The fabrication of single-particle-frit-based packed columns did not require an additional packing channel, thus avoiding dead volumes within the channel interface that can influence chromatographic separation. Further, the length of the packed column could be adjusted using the location of single-particle frits within the column channel. The fabricated frits exhibited high mechanical strength, good permeability, and tolerance for high pressures during chromatographic separation. In particular, the developed microchip LC system was able to withstand high separation pressures of more than 5000 psi. The microchip LC system was applied to the separation of neurotransmitters. Three different monoamine neurotransmitters were completely separated within 5 min with theoretical plate numbers on the order of 100,000 plates m^-1. The microchip LC system has a potential for application in a variety of fields including environmental analysis, food safety, drug analysis, and biomedicine.

Confinement of Monolithic Stationary Phases in Targeted Regions of 3D-Printed Titanium Devices Using Thermal Polymerization

In this study, we have prepared thermally initiated polymeric monolithic stationary phases within discrete regions of 3D-printed titanium devices. The devices were created with controllable hot and cold regions. The monolithic stationary phases were first locally created in capillaries inserted into the channels of the titanium devices. The homogeneity of the monolith structure and the interface length were studied by scanning a capacitively coupled conductivity contactless detector (C4D) along the length of the capillary. Homogeneous monolithic structures could be obtained within a titanium device equipped with a hot and cold jacket connected to two water baths. The confinement method was optimized in capillaries. The sharpest interfaces (between monolith and empty channel) were obtained with the hot region maintained at 70 °C and the cold region at 4 or 10 °C, with the latter temperature yielding better repeatability. The optimized conditions were used to create monoliths bound directly to the walls of the titanium channels. The fabricated monoliths were successfully used to separate a mixture of four intact proteins using reversed-phase liquid chromatography. Further chromatographic characterization showed a permeability (Kf) of ∼4 × 10-15 m2 and a total porosity of 60%.

Miniaturized high-performance liquid chromatography instrumentation

Miniaturized high performance liquid chromatography (HPLC) has attracted increasing attention for its potential in high-throughput analyses and point-of-care applications. In this review we highlight the recent advancements in HPLC system miniaturization. We focus on the major components that constitute these instruments along with their respective advantages and drawbacks as well as present a few representative miniaturized HPLC systems. We discuss briefly some of the applications and also anticipate the future development trends of these instrumental platforms.

Solid supports for extraction and preconcentration of proteins and peptides in microfluidic devices: A review

Determination of proteins and peptides is among the main challenges of today's bioanalytical chemistry. The application of microchip technology in this field is an exhaustively developed concept that aims to create integrated and fully automated analytical devices able to quantify or detect one or several proteins from a complex matrix. Selective extraction and preconcentration of targeted proteins and peptides especially from biological fluids is of the highest importance for a successful realization of these microsystems. Incorporation of solid structures or supports is a convenient solution employed to face these demands. This review presents a critical view on the latest achievements in sample processing techniques for protein determination using solid supports in microfluidics. The study covers the period from 2006 to 2015 and focuses mainly on the strategies based on microbeads, monolithic materials and membranes. Less common approaches are also briefly discussed. The reviewed literature suggests future trends which are discussed in the concluding remarks.

On-chip integration of organic synthesis and HPLC/MS analysis for monitoring stereoselective transformations at the micro-scale

We present a microfluidic system, seamlessly integrating microflow and microbatch synthesis with a HPLC/nano-ESI-MS functionality on a single glass chip. The microfluidic approach allows to efficiently steer and dispense sample streams down to the nanoliter-range for studying reactions in quasi real-time. In a proof-of-concept study, the system was applied to explore amino-catalyzed reactions, including asymmetric iminium-catalyzed Friedel-Crafts alkylations in microflow and micro confined reaction vessels.

Enantioselective reaction monitoring utilizing two-dimensional heart-cut liquid chromatography on an integrated microfluidic chip

Chip-integrated, two-dimensional high performance liquid chromatography is introduced to monitor enantioselective continuous micro-flow synthesis. The herein described development of the first two-dimensional HPLC-chip was realized by the integration of two different columns packed with reversed-phase and chiral stationary phase material on a microfluidic glass chip, coupled to mass spectrometry. Directed steering of the micro-flows at the joining transfer cross enabled a heart-cut operation mode to transfer the chiral compound of interest from the first to the second chromatographic dimension. This allows for an interference-free determination of the enantiomeric excess by seamless hyphenation to electrospray mass spectrometry. The application for rapid reaction optimization at micro-flow conditions is exemplarily shown for the asymmetric organocatalytic continuous micro-flow synthesis of warfarin.

Integrated on-chip mass spectrometry reaction monitoring in microfluidic devices containing porous polymer monolithic columns

Chip-based microfluidics enable the seamless integration of different functions into single devices. Here, we present microfluidic chips containing porous polymer monolithic columns as a means to facilitate chemical transformations as well as both downstream chromatographic separation and mass spectrometric analysis. Rapid liquid phase lithography prototyping creates the multifunctional device economically.

Advances in and prospects of microchip liquid chromatography

This review focuses on recent advances in the field of microfluidic liquid chromatography from January 2013 through April 2015. Articles are organized by the type of stationary phase support focusing on device fabrication, column preparation, and use for specific applications. Additionally, a comprehensive table comparing chromatographic figures of merit for the work described is included as Appendix A as a reference for readers.

Methacrylate Polymer Monoliths for Separation Applications

This review summarizes the development of methacrylate-based polymer monoliths for separation science applications. An introduction to monoliths is presented, followed by the preparation methods and characteristics specific to methacrylate monoliths. Both traditional chemical based syntheses and emerging additive manufacturing methods are presented along with an analysis of the different types of functional groups, which have been utilized with methacrylate monoliths. The role of methacrylate based porous materials in separation science in industrially important chemical and biological separations are discussed, with particular attention given to the most recent developments and challenges associated with these materials. While these monoliths have been shown to be useful for a wide variety of applications, there is still scope for exerting better control over the porous architectures and chemistries obtained from the different fabrication routes. Conclusions regarding this previous work are drawn and an outlook towards future challenges and potential developments in this vibrant research area are presented. Discussed in particular are the potential of additive manufacturing for the preparation of monolithic structures with pre-defined multi-scale porous morphologies and for the optimization of surface reactive chemistries.