Ultrasensitive nanoelectrospray ionization-mass spectrometry using poly(dimethylsiloxane) microchips with monolithically integrated emitters

Liquid phase detection in the miniature scale. Microfluidic and capillary scale measurement and separation systems. A tutorial review

Microfluidic and capillary devices are increasingly being used in analytical applications while their overall size keeps decreasing. Detection sensitivity for these microdevices gains more importance as device sizes and consequently, sample volumes, decrease. This paper reviews optical, electrochemical, electrical, and mass spectrometric detection methods that are applicable to capillary scale and microfluidic devices, with brief introduction to the principles in each case. Much of this is considered in the context of separations. We do consider theoretical aspects of separations by open tubular liquid chromatography, arguably the most potentially fertile area of separations that has been left fallow largely because of lack of scale-appropriate detection methods. We also examine the theoretical basis of zone electrophoretic separations. Optical detection methods discussed include UV/Vis absorbance, fluorescence, chemiluminescence and refractometry. Amperometry is essentially the only electrochemical detection method used in microsystems. Suppressed conductance and especially contactless conductivity (admittance) detection are in wide use for the detection of ionic analytes. Microfluidic devices, integrated to various mass spectrometers, including ESI-MS, APCI-MS, and MALDI-MS are discussed. We consider the advantages and disadvantages of each detection method and compare the best reported limits of detection in as uniform a format as the available information allows. While this review pays more attention to recent developments, our primary focus has been on the novelty and ingenuity of the approach, regardless of when it was first proposed, as long as it can be potentially relevant to miniature platforms.

Proteomics analysis of circulating small extracellular vesicles: Focus on the contribution of EVs to tumor metabolism

The term small extracellular vesicle (sEV) is a comprehensive term that includes any type of cell-derived, membrane-delimited particle that has a diameter < 200 nm, and which includes exosomes and smaller microvesicles. sEVs transfer bioactive molecules between cells and are crucial for cellular homeostasis and particularly during tumor development, where sEVs provide important contributions to the formation of the premetastic niche and to their altered metabolism. sEVs are thus legitimate targets for intervention and have also gained increasing interest as an easily accessible source of biomarkers because they can be rapidly isolated from serum/plasma and their molecular cargo provides information on their cell-of origin. To target sEVs that are specific for a given cell/disease it is essential to identify EV surface proteins that are characteristic of that cell/disease. Mass-spectrometry based proteomics is widely used for the identification and quantification of sEV proteins. The methods used for isolating the sEVs, preparing the sEV sample for proteomics analysis, and mass spectrometry analysis, can have a strong influence on the results and requires careful consideration. This review provides an overview of the approaches used for sEV proteomics and discusses the inherent compromises regarding EV purity versus depth of coverage. Additionally, it discusses the practical applications of the methods to unravel the involvement of sEVs in regulating the metabolism of pancreatic ductal adenocarcinoma (PDAC). The metabolic reprogramming in PDAC includes enhanced glycolysis, elevated glutamine metabolism, alterations in lipid metabolism, mitochondrial dysfunction and hypoxia, all of which are crucial in promoting tumor cell growth. A thorough understanding of these metabolic adaptations is imperative for the development of targeted therapies to exploit PDAC's vulnerabilities.

Stable electrospray signal on a microfabricated glass chip with three-dimensional open edge and tiered depth geometries

This paper presents the microfabrication and performance of a three-dimensional electrospray ionization (ESI) emitter tip made from glass, which achieves stable current signals important for chemical analysis. Our novel microfabrication process and custom-built signal conditioning hardware provides the advantage of providing accurate features and steady signals. The fabrication process relies on standard microfabrication techniques (i.e., deposition, photolithography, and wet etching). This fabrication method involves the novel application of two layers of positive and negative photoresists in addition to Parafilm® wax tape. Open edge and tiered depth details were successfully created from a multilayer planar mask. This is a benefit for integrated miniaturized and microfluidic systems that often require micro features for their functionality but relatively large millimeter size features for their physical periphery. We demonstrate the fundamental performance of electrospray with this microfluidic chip. The emitter tip was fixed on a linear axis stage with high resolution (10 μm) to finely control the tip distance from a metal counter electrode plate. A custom printed circuit board system was built to safely control four voltages applied to the microchip ports from a single high voltage power supply. To readily form the electrospray, non-aqueous solvents were used for their low viscosity and a constant voltage of +2.7 kV was applied to the sheath electrospray microchannel. The liquid being sprayed was 80/20 (v/v) methanol/acetonitrile with 0.1% acetic acid in the sheath microchannel and with ammonium acetate (10-40 mM) in its remaining microchannels. The electrospray signal was measured in response to varying the distance (1.4 to 1.6 mm) between the electrospray emitter tip and a metal counter electrode plate in addition to the varying concentration of the background electrolyte, ammonium acetate. Stable and repeatable electrospray signal showed linear relationships with emitter tip distance and concentration (r2≥0.95).

Faster, better, and cheaper: harnessing microfluidics and mass spectrometry for biotechnology

High-throughput screening technologies are widely used for elucidating biological activities. These typically require trade-offs in assay specificity and sensitivity to achieve higher throughput. Microfluidic approaches enable rapid manipulation of small volumes and have found a wide range of applications in biotechnology providing improved control of reaction conditions, faster assays, and reduced reagent consumption. The integration of mass spectrometry with microfluidics has the potential to create high-throughput, sensitivity, and specificity assays. This review introduces the widely-used mass spectrometry ionization techniques that have been successfully integrated with microfluidics approaches such as continuous-flow system, microchip electrophoresis, droplet microfluidics, digital microfluidics, centrifugal microfluidics, and paper microfluidics. In addition, we discuss recent applications of microfluidics integrated with mass spectrometry in single-cell analysis, compound screening, and the study of microorganisms. Lastly, we provide future outlooks towards online coupling, improving the sensitivity and integration of multi-omics into a single platform.

Nanoparticle-Aided Nanoreactor for Nanoproteomics

Large-scale bottom-up proteomics of few even single cells is crucial for a better understanding of the roles played by cell-to-cell heterogeneity in disease and development. Novel proteomic methodologies with extremely high sensitivity are required for few even single-cell proteomics. Sample processing with high recovery and no contaminants is one key step. Here we developed a nanoparticle-aided nanoreactor for nanoproteomics (Nano3) technique for processing low-nanograms of mammalian cell proteins for proteome profiling. The Nano3 technique employed nanoparticles packed in a capillary channel to form a nanoreactor (≤30 nL) for concentrating, cleaning, and digesting proteins originally in a lysis buffer containing sodium dodecyl sulfate (SDS), followed by nanoRPLC-MS/MS analysis. The Nano3 method identified a 40-times higher number of proteins based on MS/MS from 2-ng mouse brain protein samples compared to the SP3 (single-pot solid-phase-enhanced sample preparation) method, which performed the sample processing using the nanoparticles in a 10 μL solution in an Eppendorf tube. The data indicates a drastically higher sample recovery of the Nano3 compared to the SP3 method for processing mass-limited proteome samples. In this pilot study, the Nano3 method was further applied in processing 10-1000 HeLa cells for bottom-up proteomics, producing 441 ± 263 (n = 4) (MS/MS) and 983 ± 292 (n = 4) [match between runs (MBR)+MS/MS] protein identifications from only 10 HeLa cells using a Q-Exactive HF mass spectrometer. The preliminary results render the Nano3 method a useful approach for processing few mammalian cells for proteome profiling.

3D-Printed Microfluidic Nanoelectrospray Ionization Source Based on Hydrodynamic Focusing

Nanoelectrospray ionization (nESI) mass spectrometry (MS) is an ideal detection method for microfluidic chips, and its performances depend on nESI emitters. However, the fabrication of monolithic nESI emitters in chips was difficult. Herein, we propose a three-dimensional (3D) printing method to develop a microfluidic nanoelectrospray ionization source (NIS), composed of a nESI emitter and other components. Firstly, the NIS was compatible with a 50 - 500 nL min^-1 nanoflows by imposing 3D hydrodynamic focusing to compensate for the total flow rate, achieving a 7.2% best relative standard deviation in the total ion current (TIC) profiles. Additionally, it was applied to probe thirteen organic chemicals, insulin, and lysozyme with adequate signal-to-noise ratios and an accuracy of m/z between 9.02 × 10^-1 and 1.48 × 10³ ppm. Finally, the NIS achieved comparable limits of detection compared with its commercial counterpart. Considering the standardized preparation of NIS, it would be a potential option to develop 3D-printed customized Chip-MS platforms.

Non-aqueous electrophoresis integrated with electrospray ionization mass spectrometry on a thiol-ene polymer-based microchip device

Non-aqueous capillary electrophoresis (NACE) on microfluidic chips is still a comparatively little explored area, despite the inherent advantages of this technique and its application potential for, in particular, lipophilic compounds. A main reason is probably the fact that implementation of NACE on microchips largely precluded the use of polymeric substrate materials. Here, we report non-aqueous electrophoresis on a thiol-ene-based microfluidic chip coupled to mass spectrometry via an on-chip ESI interface. Microchips with an integrated ESI emitter were fabricated using a double-molding approach. The durability of thiol-ene, when exposed to different organic solvents, was investigated with respect to swelling and decomposition of the polymer. Thiol-ene exhibited good stability against organic solvents such as methanol, ethanol, N-methylformamide, and formamide, which allows for a wide range of background electrolyte compositions. The integrated ESI emitter provided a stable spray with RSD% of the ESI signal ≤8%. Separation efficiency of the developed microchip electrophoresis system in different non-aqueous buffer solutions was tested with a mixture of several drugs of abuse. Ethanol- and methanol-based buffers provided comparable high theoretical plate numbers (≈ 6.6 × 10⁴-1.6 × 10⁵ m^-1) with ethanol exhibiting the best separation efficiency. Direct coupling of non-aqueous electrophoresis to mass spectrometry allowed for fast analysis of hydrophobic compounds in the range of 0.1-5 μg mL^-1 and 0.2-10 μg mL^-1 and very good sensitivities (LOD ≈ 0.06-0.28 μg mL^-1; LOQ ≈ 0.20-0.90 μg mL^-1). The novel combination of non-aqueous CE on a microfluidic thiol-ene device and ESI-MS provides a mass-producible and highly versatile system for the analysis of, in particular, lipophilic compounds in a wide range of organic solvents. This offers promising potential for future applications in forensic, clinical, and environmental analysis. Graphical abstract.

Recent technical progress in sample preparation and liquid-phase separation-mass spectrometry for proteomic analysis of mass-limited samples

Mass spectrometry (MS)-based proteomics has enabled the identification and quantification of thousands of proteins from complex proteomes in a single experiment. However, its performance for mass-limited proteome samples (e.g., single cells and tissue samples from laser capture microdissection) is still not satisfying. The development of novel proteomic methodologies with better overall sensitivity is vital. During the last several years, substantial technical progress has been achieved for the preparation and liquid-phase separation-MS characterization of mass-limited proteome samples. In this review, we summarize recent technological progress of sample preparation, liquid chromatography (LC)-MS, capillary zone electrophoresis (CZE)-MS and MS instrumentation for bottom-up proteomics of trace biological samples, highlight some exciting applications of the novel techniques for single-cell proteomics, and provide a very brief perspective about the field at the end.

Microfluidic Probe for In-Situ Extraction of Adherent Cancer Cells to Detect Heterogeneity Difference by Electrospray Ionization Mass Spectrometry

The pathological studies of cancer tissues and cell molecules could provide an early diagnosis for the treatment of cancer. In this work, we have designed a microfluidic surface extractor (MSE). The MSE has been coupled with electrospray mass spectrometry (extraction reagent, methanol; optimum flow rate, 0.5 mL/h) to analyze the phospholipid content of different tumor cells. Three types of cancer cell lines, including A549 cells, HepG2 cells, and U87 cells, were investigated, and the principle component analysis (PCA: linear discriminant analysis (LDA), PC1 97.2%; PC2, 2.8%) was carried out to analyze the difference in the lipid contents. The LDA revealed heterogeneity among the cancer cells. The designed MSE could have potential applications in the clinical analysis of cancer tissues.

Benchtop-compatible sample processing workflow for proteome profiling of < 100 mammalian cells

Extending proteomics to smaller samples can enable the mapping of protein expression across tissues with high spatial resolution and can reveal sub-group heterogeneity. However, despite the continually improving sensitivity of LC-MS instrumentation, in-depth profiling of samples containing low-nanogram amounts of protein has remained challenging due to analyte losses incurred during preparation and analysis. To address this, we recently developed nanodroplet processing in one pot for trace samples (nanoPOTS), a robotic/microfluidic platform that generates ready-to-analyze peptides from cellular material in ~200 nL droplets with greatly reduced sample losses. In combination with ultrasensitive LC-MS, nanoPOTS has enabled >3000 proteins to be confidently identified from as few as 10 cultured human cells and ~700 proteins from single cells. However, the nanoPOTS platform requires a highly skilled operator and a costly in-house-built robotic nanopipetting instrument. In this work, we sought to evaluate the extent to which the benefits of nanodroplet processing could be preserved when upscaling reagent dispensing volumes by a factor of 10 to those addressable by commercial micropipette. We characterized the resulting platform, termed microdroplet processing in one pot for trace samples (μPOTS), for the analysis of as few as ~25 cultured HeLa cells (4 ng total protein) or 50 μm square mouse liver tissue thin sections and found that ~1800 and ~1200 unique proteins were respectively identified with high reproducibility. The reduced equipment requirements should facilitate broad dissemination of nanoproteomics workflows by obviating the need for a capital-intensive custom liquid handling system.

Nanodroplet processing platform for deep and quantitative proteome profiling of 10-100 mammalian cells

Nanoscale or single-cell technologies are critical for biomedical applications. However, current mass spectrometry (MS)-based proteomic approaches require samples comprising a minimum of thousands of cells to provide in-depth profiling. Here, we report the development of a nanoPOTS (nanodroplet processing in one pot for trace samples) platform for small cell population proteomics analysis. NanoPOTS enhances the efficiency and recovery of sample processing by downscaling processing volumes to <200 nL to minimize surface losses. When combined with ultrasensitive liquid chromatography-MS, nanoPOTS allows identification of ~1500 to ~3000 proteins from ~10 to ~140 cells, respectively. By incorporating the Match Between Runs algorithm of MaxQuant, >3000 proteins are consistently identified from as few as 10 cells. Furthermore, we demonstrate quantification of ~2400 proteins from single human pancreatic islet thin sections from type 1 diabetic and control donors, illustrating the application of nanoPOTS for spatially resolved proteome measurements from clinical tissues.

There is a great need of developing highly sensitive mass spectrometry-based proteomics analysis for small cell populations. Here, the authors establish a robotically controlled chip-based nanodroplet processing platform and demonstrate its ability to profile the proteome from 10–100 mammalian cells.

On-line pre-treatment, separation, and nanoelectrospray mass spectrometric determinations for pesticide metabolites and peptides based on a modular microfluidic platform

In order to address time-consuming sample pre-treatment and separation prior to mass spectrometry (MS) identifications, highly integrated chips were developed, but damage to any functional unit in these chips would result in complete replacement. Herein, we propose a modular microfluidic platform comprising pre-treatment, liquid chromatography (LC) separation and nanoelectrospray ionization (nESI) chips for on-line enrichment, separation and nESI MS detection of pesticide metabolites and peptides. The pre-treatment chip is applicable in enriching pyridalyl and its metabolites, and it achieves optimal desalination efficiency, 98.5%, for polymerase chain reaction products. Additionally, the LC separation chip was fully characterised, and it demonstrated satisfactory separation efficiency, quantification ability and pressure durability. Finally, the modular microfluidic platform was used to identify the peptides in trypsin-digested casein. Four additional peptides were identified, indicating an improvement in detection ability compared with using off-line zip tips coupled with MS investigations. Because the proposed modular platform can significantly reduce manual work, it would be a potential tool to achieve high throughput and automatic MS identifications with low sample consumptions.

A microfluidic platform, composed of enrichment, separation and nanoelectrospray ionization modulations was developed to on-line-investigate pesticide metabolites and peptides.

A piezo-ring-on-chip microfluidic device for simple and low-cost mass spectrometry interfacing

Mass spectrometry (MS) interfacing technology provides the means for incorporating microfluidic processing with post MS analysis.

Multi-channel microfluidic chip coupling with mass spectrometry for simultaneous electro-sprays and extraction

Considering the advantages and research status of microfluidic chip coupling with mass spectrometry (MS), a microfluidic chip-based multi-channel ionization (MCMCI) for the extraction of untreated compounds in complex matrices without sample pretreatments was developed. Quantitative analysis of human urine spiked with various rhodamine B concentrations was also performed, and good linearity was obtained. Comparing to the macro ionization device, MCMCI significantly improved the integration of ionization source, simplified the operation of such a device, and greatly increased the signal intensity with much lower gas pressure. Comparison of our MCMCI with two and three gas channels indicated that the liquid–liquid extraction process before spraying and after spraying produced similar MS results. Moreover, this MCMCI with three gas channels also implemented simultaneous dual sprays with high DC voltages, the interference of two samples was minor and ion suppression effect was drastically alleviated. Such advantages may easily enable internal calibration for accurate mass measurement. Furthermore, dual extraction can be implemented by integrating such multi-spray configuration, which can improve the extracted signal intensity and sensitivity. These technologies open up new avenues for the application of microfluidic chip coupling with MS.

Precision-Trimming 2D Inverse-Opal Lattice on Elastomer to Ordered Nanostructures with Variable Size and Morphology

A low-cost and scalable method is developed for producing large-area elastomer surfaces having ordered nanostructures with a variety of lattice features controllable to nanometer precision. The method adopts the known technique of molding a PDMS precursor film with a close-packed monolayer of monodisperse submicron polystyrene beads on water to form an inverse-opal dimple lattice with the dimple size controlled by the bead selection and the dimple depth by the molding condition. The subsequent novel precision engineering of the inverse-opal lattice comprises trimming the PDMS precursor by a combination of polymer curing temperature/time and polymer dissolution parameters. The resultant ordered surface nanostructures, fabricated with an increasing degree of trimming, include (a) submicron hemispherical dimples with nanothin interdimple rims and walls; (b) nanocones with variable degrees of tip-sharpness by trimming off the top part of the nanothin interdimple walls; and (c) soup-plate-like submicron shallow dimples with interdimple rims and walls by anisotropically trimming off the nanocones and forming close-packed shallow dimples. As exemplars of industrial relevance of these lattice features, tunable Young's modulus and wettability are demonstrated.

Aqueous and non-aqueous microchip electrophoresis with on-chip electrospray ionization mass spectrometry on replica-molded thiol-ene microfluidic devices

This work describes aqueous and non-aqueous capillary electrophoresis on thiol-ene-based microfluidic separation devices that feature fully integrated and sharp electrospray ionization (ESI) emitters. The chip fabrication is based on simple and low-cost replica-molding of thiol-ene polymers under standard laboratory conditions. The mechanical rigidity and the stability of the materials against organic solvents, acids and bases could be tuned by adjusting the respective stoichiometric ratio of the thiol and allyl ("ene") monomers, which allowed us to carry out electrophoresis separation in both aqueous and non-aqueous (methanol- and ethanol-based) background electrolytes. The stability of the ESI signal was generally ≤10% RSD for all emitters. The respective migration time repeatabilities in aqueous and non-aqueous background electrolytes were below 3 and 14% RSD (n=4-6, with internal standard). The analytical performance of the developed thiol-ene microdevices was shown in mass spectrometry (MS) based analysis of peptides, proteins, and small molecules. The theoretical plate numbers were the highest (1.2-2.4×10⁴m^-1) in ethanol-based background electrolytes. The ionization efficiency also increased under non-aqueous conditions compared to aqueous background electrolytes. The results show that replica-molding of thiol-enes is a feasible approach for producing ESI microdevices that perform in a stable manner in both aqueous and non-aqueous electrophoresis.

Integrated Electrodes and Electrospray Emitter for Polymer Microfluidic Nanospray-MS Interface

Interfacing of microfluidic devices to mass spectrometry has challenges including dilution from sheath liquid junctions, fragile electrodes, and excessive dead volumes which prevent optimum performance and common use. The goal of this work is to develop a stable nanospray chip-MS interface that contains easily integrated electrodes and an embedded capillary emitter to mitigate current chip-MS problems. This system uses a hybrid polystyrene-poly(dimethylsiloxane) (PS-PDMS) microfluidic platform with an embedded electrode and integrated capillary emitter used as the nanospray interface. Two chip designs were used to evaluate the performance, illustrate on-chip reaction capabilities. By direct infusion, this system showed good performance with LODs of GSH and caffeine of 9 nM and 1 nM, R² of 0.996 and 0.992 and sensitivity of 12 counts/nM and 332 counts/nM over a linear dynamic range of 40 nM to 50 μM and 1 to 50 μM respectively. A reaction was performed on the chip with syringe pumps showing the oxidation of glutathione (GSH) to oxidized glutathione (GSSG) using H₂O₂. The on-chip reaction of GSH oxidation to GSSG, with online-MS detection, successfully demonstrate the stability and robustness of the nanospray interface.

Mass spectrometry-based monitoring of millisecond protein-ligand binding dynamics using an automated microfluidic platform

Characterizing protein-ligand binding dynamics is crucial for understanding protein function and for developing new therapeutic agents. We present a novel microfluidic platform that features rapid mixing of protein and ligand solutions, variable incubation times, and an integrated electrospray ionization source for mass spectrometry-based monitoring of protein-ligand binding dynamics. This platform offers many advantages, including solution-based binding, label-free detection, automated operation, rapid mixing, and low sample consumption.

Three-dimensional hydrodynamic focusing microfluidic emitter: a strategy to inhibit sample ion expansion in nanoelectrospray ionization

A proposed 3D HFNE was used to generate a wrapped electrospray to achieve the inhibition of sample ion expansion, thus restraining injection loss in electrospray ionization.

Microfluidic self-aspiration sonic-spray ionization chip with single and dual ionization channels for mass spectrometry

In consideration of the miniaturization, integration, and universal disadvantages of microfluidic chip-based ionization coupled with mass spectrometry, this study proposed a novel microfluidic self-aspiration sonic-spray ionization chip.

Advances in coupling microfluidic chips to mass spectrometry

Microfluidic technology has shown advantages of low sample consumption, reduced analysis time, high throughput, and potential for integration and automation. Coupling microfluidic chips to mass spectrometry (Chip-MS) can greatly improve the overall analytical performance of MS-based approaches and expand their potential applications. In this article, we review the advances of Chip-MS in the past decade, covering innovations in microchip fabrication, microchips coupled to electrospray ionization (ESI)-MS and matrix-assisted laser desorption/ionization (MALDI)-MS. Development of integrated microfluidic systems for automated MS analysis will be further documented, as well as recent applications of Chip-MS in proteomics, metabolomics, cell analysis, and clinical diagnosis.

A reliable and simple method for fabricating a poly(dimethylsiloxane) electrospray ionization chip with a corner-integrated emitter

Monolithically integrated emitters have been increasingly applied to microfluidic devices that are coupled to mass spectrometers (MS) as electrospray ionization sources (ESI). A new method was developed to fabricate a duplicable structure which integrated the emitter into a poly(dimethylsiloxane) chip corner. Two photoresist layers containing a raised base which guaranteed the precise integration of the electrospray tip emitter and ensured that the cutting out of the tip exerted no influence even during repeated prototyping were used to ease the operation of the process. Highly stable ESI-MS performance was obtained and the results were compared with those of a commercial fused-silica capillary source. Furthermore, chip-to-chip and run-to-run results indicated both reliability and reproducibility during repeated fabrication. These results reveal that the proposed chip can provide an ideal ion source for MS across many applications, especially with the perspective to be widely used in portable MS during on-site analysis.

Microfluidics-to-mass spectrometry: a review of coupling methods and applications

Microfluidic devices offer great advantages in integrating sample processes, minimizing sample and reagent volumes, and increasing analysis speed, while mass spectrometry detection provides high information content, is sensitive, and can be used in quantitative analyses. The coupling of microfluidic devices to mass spectrometers is becoming more common with the strengths of both systems being combined to analyze precious and complex samples. This review summarizes select achievements published between 2010 and July 2014 in novel coupling between microfluidic devices and mass spectrometers. The review is subdivided by the types of ionization sources employed, and the different microfluidic systems used.

Small-volume analysis of cell-cell signaling molecules in the brain

Modern science is characterized by integration and synergy between research fields. Accordingly, as technological advances allow new and more ambitious quests in scientific inquiry, numerous analytical and engineering techniques have become useful tools in biological research. The focus of this review is on cutting edge technologies that aid direct measurement of bioactive compounds in the nervous system to facilitate fundamental research, diagnostics, and drug discovery. We discuss challenges associated with measurement of cell-to-cell signaling molecules in the nervous system, and advocate for a decrease of sample volumes to the nanoliter volume regimen for improved analysis outcomes. We highlight effective approaches for the collection, separation, and detection of such small-volume samples, present strategies for targeted and discovery-oriented research, and describe the required technology advances that will empower future translational science.

Magnetite-doped polydimethylsiloxane (PDMS) for phosphopeptide enrichment

A moldable, reusable magnetite-doped polydimethylsiloxane (PDMS) substrate for phosphopeptide enrichment.

Recent advances in microfluidics combined with mass spectrometry: technologies and applications

Instrument miniaturization is one of the critical issues to improve sensitivity, speed, throughput, and to reduce the cost of analysis. Microfluidics possesses the ability to handle small sample amounts, with minimal concerns related to sample loss and cross-contamination, problems typical for standard fluidic manipulations. Moreover, the native properties of microfluidics provide the potential for high-density, parallel sample processing, and high-throughput analysis. Recently, the coupling of microfluidic devices to mass spectrometry, especially electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI), has attracted an increasing interest and produced tremendous achievements. The interfaces between microfluidics and mass spectrometry are one of the primary focused problems. In this review, we summarize the latest achievements since 2008 in the field of the technologies and applications in the combining of microfluidics with ESI-MS and MALDI-MS. The integration of several analytical functions on a microfluidic device such as sample pretreatment and separations before sample introduction into the mass spectrometer is also discussed.

Controlled dispensing and mixing of pico- to nanoliter volumes using on-demand droplet-based microfluidics

We present an integrated droplet-on-demand microfluidic platform for dispensing, mixing, incubating, extracting and analyzing by mass spectrometry pico- to nanoliter-sized droplets. All of the functional components are successfully integrated for the first time into a monolithic microdevice. Droplet generation is accomplished using computer-controlled pneumatic valves. Controlled actuation of valves for different aqueous streams enables accurate dosing and rapid mixing of reagents within droplets in either the droplet generation area or in a region of widening channel cross-section. Following incubation, which takes place as droplets travel in the oil stream, the droplet contents are extracted to an aqueous channel for subsequent ionization at an integrated nanoelectrospray emitter. Using the integrated platform, rapid enzymatic digestions of a model protein were carried out in droplets and detected on-line by nanoelectrospray ionization mass spectrometry.

Controlled generation of double emulsions in air

This Letter describes the controlled generation of double emulsions in the gas phase, which was carried out using an integrated emitter in a poly(dimethylsiloxane) (PDMS) microfluidic chip. The integrated emitter was formed using a molding approach, in which metal wires with desirable diameters were used as emitter molds. The generation of double emulsions in air was achieved with electrohydrodynamics actuation, which offers controllable force exerting on the double emulsions. We developed this capability for future integration of droplet microfluidics with mass spectrometry (MS), where each aqueous droplet in the microchannel is introduced into the gas phase as a double emulsion for subsequent ionization and MS analysis.

Mass spectrometry-based proteomics: existing capabilities and future directions

Mass spectrometry (MS)-based proteomics is emerging as a broadly effective means for identification, characterization, and quantification of proteins that are integral components of the processes essential for life. Characterization of proteins at the proteome and sub-proteome (e.g., the phosphoproteome, proteoglycome, or degradome/peptidome) levels provides a foundation for understanding fundamental aspects of biology. Emerging technologies such as ion mobility separations coupled with MS and microchip-based-proteome measurements combined with MS instrumentation and chromatographic separation techniques, such as nanoscale reversed phase liquid chromatography and capillary electrophoresis, show great promise for both broad undirected and targeted highly sensitive measurements. MS-based proteomics increasingly contribute to our understanding of the dynamics, interactions, and roles that proteins and peptides play, advancing our understanding of biology on a systems wide level for a wide range of applications including investigations of microbial communities, bioremediation, and human health.

Mass spectrometry-based proteomics: existing capabilities and future directions

Mass spectrometry (MS)-based proteomics is emerging as a broadly effective means for identification, characterization, and quantification of proteins that are integral components of the processes essential for life. Characterization of proteins at the proteome and sub-proteome (e.g., the phosphoproteome, proteoglycome, or degradome/peptidome) levels provides a foundation for understanding fundamental aspects of biology. Emerging technologies such as ion mobility separations coupled with MS and microchip-based-proteome measurements combined with MS instrumentation and chromatographic separation techniques, such as nanoscale reversed phase liquid chromatography and capillary electrophoresis, show great promise for both broad undirected and targeted highly sensitive measurements. MS-based proteomics increasingly contribute to our understanding of the dynamics, interactions, and roles that proteins and peptides play, advancing our understanding of biology on a systems wide level for a wide range of applications including investigations of microbial communities, bioremediation, and human health.

Membrane-based emitter for coupling microfluidics with ultrasensitive nanoelectrospray ionization-mass spectrometry

An integrated poly(dimethylsiloxane) (PDMS) membrane-based microfluidic emitter for high-performance nanoelectrospray ionization mass spectrometry has been fabricated and evaluated. The ∼100-μm-thick emitter was created by cutting a PDMS membrane that protrudes beyond the bulk substrate. The reduced surface area at the emitter enhances the electric field and reduces wetting of the surface by the electrospray solvent. As such, the emitter enables highly stable electrosprays at flow rates as low as 10 nL/min and is compatible with electrospray solvents containing a large organic component (e.g., 90% methanol). This approach enables facile emitter construction and provides excellent stability, reproducibility, and sensitivity as well as compatibility with multilayer soft lithography.