Multinozzle emitter arrays for nanoelectrospray mass spectrometry

Current state of hydrophilic interaction liquid chromatography of oligonucleotides

There has been a significant increase in the use of hydrophilic interaction liquid chromatography (HILIC) to separate oligonucleotides. This rise in the use of HILIC has correlated to the increasing success of oligonucleotides as therapeutic treatments and reagents in biomedical research. As more scientists need to routinely analyze oligonucleotides in addition to small molecules, peptides, and proteins using the same analytical instruments, it becomes difficult to use traditional types of analyses such as ion-pair reversed-phase chromatography. This increased use has led to new approaches that have improved the utility of HILIC to the point where it has become a legitimate alternative approach to ion-pair reversed-phase chromatography. This review highlights recent advances in HILIC separations of oligonucleotides with a focus on the underlying mechanisms of action. While HILIC has made significant gains in performance, there still remain challenges, which if properly addressed will continue to propel this approach forward.

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.

The Future of Proteomics is Up in the Air: Can Ion Mobility Replace Liquid Chromatography for High Throughput Proteomics?

The coevolution of liquid chromatography (LC) with mass spectrometry (MS) has shaped contemporary proteomics. LC hyphenated to MS now enables quantification of more than 10,000 proteins in a single injection, a number that likely represents most proteins in specific human cells or tissues. Separations by ion mobility spectrometry (IMS) have recently emerged to complement LC and further improve the depth of proteomics. Given the theoretical advantages in speed and robustness of IMS in comparison to LC, we envision that ongoing improvements to IMS paired with MS may eventually make LC obsolete, especially when combined with targeted or simplified analyses, such as rapid clinical proteomics analysis of defined biomarker panels. In this perspective, we describe the need for faster analysis that might drive this transition, the current state of direct infusion proteomics, and discuss some technical challenges that must be overcome to fully complete the transition to entirely gas phase proteomics.

A Microfluidic-Fabricated Rod Sprayer for Nanoelectrospray Mass Spectrometry

The nanoelectrosprayer is a key device in the hyphenation of nanoLC-ESI-MS, and its development plays a crucial role in pushing forward the mining depth of biological discovery and industrialization of omics science. In this work, a new type of nanoelectrospray emitter, a rod sprayer, was developed based on microfluidic manufacture. Due to its porous silica structure, the rod sprayer in effect worked as a multinozzle sprayer, which is composed of a bunch of micrometer sized spray channels. Without the need for sophisticated microfabrication equipment, a superclean environment, or a complicated assembling process, such sprayer rods can be facilely fabricated in a mass production style: 3,600 rods with excellent monodispersity have been fabricated in 1 h, and rod sprayers thus made have demonstrated excellent intraday, interday, and interbatch reproducibilities: RSD = 1.9, 4.9, and 6.1%, respectively. The rod sprayer can generate stable electrospray in a wide voltage range from 2.6 to 3.2 kV and flow rates from 50 to 1000 nL/min, covering typical flow rates of subnanoLC, nanoLC, to microLC, and work steadily even under complex matrix environments (e.g., Hank's balanced salt solution containing sodium, magnesium, and calcium ions) without clogging. Meanwhile, the rod sprayers exhibited 200-1800% ionization efficiency enhancement in comparison with commonly used tapered tip emitters, for small molecule drugs, peptides, and proteins, respectively, and provided a broadened linear dynamic range of 4 orders of magnitude. The excellent characteristics of the rod sprayer, together with its small size and mass production capacity, should provide a high quality, high durability, high consistency, and disposable use-supported nanoelectrospray solution for MS-based bioanalyses.

Expanding Native Mass Spectrometry to the Masses

At the 33rd ASMS Sanibel Meeting, on Membrane Proteins and Their Complexes, a morning roundtable discussion was held discussing the current challenges facing the field of native mass spectrometry and approaches to expanding the field to nonexperts. This Commentary summarizes the discussion and current initiatives to address these challenges.

High-Throughput Glycan Profiling of Human Serum IgG Subclasses Using Parallel Reaction Monitoring Peptide Bond Fragmentation of Glycopeptides and Microflow LC-MS

LC-MS-based N-glycosylation profiling in four human serum IgG subclasses (IgG1, IgG2, IgG3, and IgG4) often requires additional affinity-based enrichment of specific IgG subclasses, owing to the high amino acid sequence similarity of Fc glycopeptides among subclasses. Notably, for IgG4 and the major allotype of IgG3, the glycopeptide precursors share identical retention time and mass and therefore cannot be distinguished based on precursor or glycan fragmentation. Here, we developed a parallel reaction monitoring (PRM)-based method for quantifying Fc glycopeptides through combined transitions generated from both glycosidic and peptide bond fragmentation. The latter enables the subpopulation of IgG3 and IgG4 to be directly distinguished according to mass differences without requiring further enrichment of specific IgG subclasses. In addition, a multinozzle electrospray emitter coupled to a capillary flow liquid chromatograph was used to increase the robustness and detection sensitivity of the method for low-yield peptide backbone fragment ions. The gradient was optimized to decrease the overall run time and make the method compatible with high-throughput analysis. We demonstrated that this method can be used to effectively monitor the relative levels of 13 representative glycoforms, with a good limit of detection for individual IgG subclasses.

Microflow Liquid Chromatography – Multi-Emitter Nanoelectrospray Mass Spectrometry of Oligonucleotides

While the most sensitive LC-MS methods for oligonucleotide analysis contain ion-pairs in the mobile phase, these modifiers have been associated with instrument contamination and ion suppression. Typically, entire LC-MS systems are reserved for oligonucleotide LC-MS when using ion-pairing buffers. To overcome these limitations, numerous HILIC methods, liberated from ion-pairs, have been recently developed. Since ion-pairs play a role in analyte desorption from ESI droplets, their removal from mobile phases tend to impact method sensitivity. An effective way to recover MS sensitivity is to reduce the LC flow rate and therefore reduce ESI droplet size. With a focus on MS sensitivity, this study investigates the applicability of a microflow LC- nanoelectrospray MS platform in oligonucleotide ion-pair RP and HILIC LC-MS methods. The platform is effective and substantially increased the MS sensitivity of HILIC methods. Furthermore, LC method development for both types of separations provide insight into microflow chromatography of oligonucleotides, an under investigated chromatographic scale.

Advances in ionisation techniques for mass spectrometry-based omics research

Omics analysis by mass spectrometry (MS) is a vast field, with proteomics, metabolomics and lipidomics dominating recent research by exploiting biological MS ionisation techniques. Traditional MS ionisation techniques such as electrospray ionisation have limitations in analyte-specific sensitivity, modes of sampling and throughput, leading to many researchers investigating new ionisation methods for omics research. In this review, we examine the current landscape of these new ionisation techniques, divided into the three groups of (electro)spray-based, laser-based and other miscellaneous ionisation techniques. Due to the wide range of new developments, this review can only provide a starting point for further reading on each ionisation technique, as each have unique benefits, often for specialised applications, which promise beneficial results for different areas in the omics world.

On the potential of micro-flow LC-MS/MS in proteomics

INTRODUCTION

Due to its excellent sensitivity, nano-flow liquid chromatography tandem mass spectrometry (LC-MS/MS) is the mainstay in proteome research; however, this comes at the expense of limited throughput and robustness. In contrast, micro-flow LC-MS/MS enables high-throughput, robustness, quantitative reproducibility, and precision while retaining a moderate degree of sensitivity. Such features make it an attractive technology for a wide range of proteomic applications. In particular, large-scale projects involving the analysis of hundreds to thousands of samples.

AREAS COVERED

This review summarizes the history of chromatographic separation in discovery proteomics with a focus on micro-flow LC-MS/MS, discusses the current state-of-the-art, highlights advances in column development and instrumentation, and provides guidance on which LC flow best supports different types of proteomic applications.

EXPERT OPINION

Micro-flow LC-MS/MS will replace nano-flow LC-MS/MS in many proteomic applications, particularly when sample quantities are not limited and sample cohorts are large. Examples include clinical analyses of body fluids, tissues, drug discovery and chemical biology investigations, plus systems biology projects across all kingdoms of life. When combined with rapid and sensitive MS, intelligent data acquisition, and informatics approaches, it will soon become possible to analyze large cohorts of more than 10,000 samples in a comprehensive and fully quantitative fashion.

The ever expanding scope of electrospray mass spectrometry-a 30 year journey

John Fenn’s electrospray mass spectrometry (ESMS) was awarded the chemistry Nobel Prize in 2002 and is now the basis of the entire field of MS-based proteomics. Technological progress continues unabated, enabling single cell sensitivity and clinical applications.

Quantitation of Intact Proteins in Human Plasma Using Top-Down Parallel Reaction Monitoring-MS

Direct quantitation of proteins in complex biological matrices by mass spectrometry remains a challenge. Here, we describe a novel top-down parallel reaction monitoring-mass spectrometry (PRM-MS) assay, enabled by microflow LC-nanospray MS using a silicon microfluidic LC-MS chip. We demonstrated direct analysis of intact proteins such as somatropin in human plasma, achieving sensitivity (0.1-1.0 fmole) and speed (1-5 min) on par with enzyme-linked immunosorbent assay (ELISA).

Direct Analysis of Biofluids by Mass Spectrometry with Microfluidic Voltage-Assisted Liquid Desorption Electrospray Ionization

Signal suppression by sample matrix in direct electrospray ionization-mass spectrometric (ESI-MS) analysis hampers its clinical and biomedical applications. We report herein the development of a microfluidic voltage-assisted liquid desorption electrospray ionization (VAL-DESI) source to overcome this limitation. Liquid DESI is achieved for the first time in a microfluidic format. Direct analysis of urine, serum, and cell lysate samples by using the proposed microfluidic VAL-DESI-MS/MS method to detect chemical compounds of biomedical interest, including nucleosides, monoamines, amino acids, and peptides is demonstrated. Analyzing a set of urine samples spiked with dihydroxyphenylalanine (DOPA) showed that the assay had a linear calibration curve with r² value of 0.997 and a limit of detection of 0.055 μM DOPA. The method was applied to simultaneous quantification of nucleosides, that is, cytidine, adenosine, uridine, thymidine, and guanosine in cell lysates using 8-bromoadenosine as internal standard. Adenosine was found most abundant at 26.5 ± 0.57 nmol/10⁶ cells, while thymidine was least at 3.1 ± 0.31 nmol/10⁶ cells. Interestingly, the ratio of adenosine to deoxyadenosine varied significantly from human red blood cells (1.07 ± 0.06) to cancerous cells, including lymphoblast TK6 (0.52 ± 0.02), skin melanoma C32 (0.82 ± 0.04), and promyelocytic leukemia NB4 cells (0.38 ± 0.06). These results suggest that the VAL-DESI-MS/MS technique has a good potential in direct analysis of biofluids. Further, because of the simplicity in its design and operation, the proposed microfluidic liquid DESI source can be fabricated as a disposable device for point-of-care measurements.

Fourier transform ion mobility spectrometry with multinozzle emitter array electrospray ionization

Fourier transform ion mobility spectrometry (FT-IMS) is a useful multiplexing method for improving the duty cycle (DC) of IMS from 1 to 25% when using an entrance and exit ion gate to modulate the ion current with a synchronized square wave chirp.

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.

Development of cell metabolite analysis on microfluidic platform

Cell metabolite analysis is of great interest to analytical chemists and physiologists, with some metabolites having been identified as important indicators of major diseases such as cancer. A high-throughput and sensitive method for drug metabolite analysis will largely promote the drug discovery industry. The basic barrier of metabolite analysis comes from the interference of complex components in cell biological system and low abundance of target substances. As a powerful tool in biosample analysis, microfluidic chip enhances the sensitivity and throughput by integrating multiple functional units into one chip. In this review, we discussed three critical steps of establishing functional microfluidic platform for cellular metabolism study. Cell in vitro culture model, on chip sample pretreatment, and microchip combined detectors were described in details and demonstrated by works in five years. And a brief summary was given to discuss the advantages as well as challenges of applying microchip method in cell metabolite and biosample analysis.

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.

Biomonitoring of perfluorinated compounds in a drop of blood

Biomonitoring of pollutants and their metabolites and derivatives using biofluids provides new opportunities for spatiotemporal assessment of human risks to environmental exposures. Perfluorinated compounds (PFCs) have been used widely in industry and pose significant environmental concerns due to their stability and bioaccumulation in humans and animals. However, current methods for extraction and measurement of PFCs require relatively large volumes (over one hundred microliters) of blood samples, and therefore, are not suitable for frequent blood sampling and longitudinal biomonitoring of PFCs. We have developed a new microassay, enabled by our silicon microfluidic chip platform, for analyzing PFCs in small volumes (less than five microliters) of blood. Our assay integrates on-chip solid-phase extraction (SPE) with online nanoflow liquid chromatography-electrospray ionization-mass spectrometry (nanoLC-ESI-MS) detection. We demonstrated high sample recovery, excellent interday and intraday accuracy and precision, and a limit of detection down to 50 femtogram of PFCs, in one microliter of human plasma. We validated our assay performance using pooled human plasma and NIST SRM 1950 samples. Our microfluidic chip-based assay may enable frequent longitudinal biomonitoring of PFCs and other environmental toxins using a finger prick of blood, thereby providing new insights into their bioaccumulation, bioavailability, and toxicity.

A pulsed pinhole atmospheric pressure interface for simplified mass spectrometry instrumentation with enhanced sensitivity

RATIONALE

A proof-of-concept pulsed pinhole atmospheric pressure interface, namely PP-API, was developed and characterized for mass spectrometry instrumentation.

METHODS

The PP-API was analyzed and optimized theoretically with respect to gas flow rate, opening force and dimensions. A PP-API interfaced mass spectrometry system was then constructed and tested.

RESULTS

As a discontinuous pressure interface, the PP-API allows efficient ion transfer from atmosphere pressure ionization sources to ion traps directly. Both nano-ESI and APCI ionization sources have been successfully integrated with the PP-API interface for the detection of volatile, organic, peptide and polymer samples. The use of multiple ionization sources has also been demonstrated to enhance signal intensity, as well as avoid charge competitions in ionization sources.

CONCLUSIONS

With simplified geometry and high ion transfer efficiency, this PP-API would enable miniaturized mass spectrometry systems with high sensitivity.

Fabrication of a polymer nozzle array in a microstructured fibre as a nanoelectrospray emitter for mass spectrometry

We report a modified silica microstructured fibre (MSF) as a multiple electrospray (MES) emitter, with dimensional compatibility with conventional liquid chromatography and mass spectrometry equipment, to generate stable electrospray from a wide range of applied potentials and flow rates. An array of polymer nozzles is fabricated in the MSF by a procedure involving templated polymerization of microtubes and wet chemical etching of the silica at the tip. The structure of the emitting end of the MSF was optimized with respect to the etching process, and the morphology of the polymer nozzles was optimized with respect to polymerization conditions. The mechanisms of the etching and of the templated polymerization of the microtubes were explored. Optimization experiments were performed using commercially available MSF having 126 tubular air channels arranged in a hexagonal pattern with channel diameter of ∼5.6 μm. However, the flexibility and versatility in the pattern, shape, and size of channels in MSFs allowed a custom-designed MSF to be fabricated and tested for MES. In the new design, six channels were evenly spaced in a radial pattern, and when polymer nozzles were made, six stable electrosprays were observed over a wide range of electrospray conditions. Using these MES emitters, the spray current is enhanced by a factor related to the number of nozzles.

On the ionization and ion transmission efficiencies of different ESI-MS interfaces

The achievable sensitivity of electrospray ionization mass spectrometry (ESI-MS) is largely determined by the ionization efficiency in the ESI source and ion transmission efficiency through the ESI-MS interface. These performance characteristics are difficult to evaluate and compare across multiple platforms as it is difficult to correlate electrical current measurements to actual analyte ions reaching the detector of a mass spectrometer. We present an effective method to evaluate the overall ion utilization efficiency of an ESI-MS interface by measuring the total gas-phase ion current transmitted through the interface and correlating it to the observed ion abundance measured in the corresponding mass spectrum. Using this method, we systematically studied the ion transmission and ionization efficiencies of different ESI-MS interface configurations, including a single emitter/single inlet capillary, single emitter/multi-inlet capillary, and a subambient pressure ionization with nanoelectrospray (SPIN) MS interface with a single emitter and an emitter array, respectively. Our experimental results indicate that the overall ion utilization efficiency of SPIN-MS interface configurations exceeds that of the inlet capillary-based ESI-MS interface configurations.

Improving the sensitivity of mass spectrometry by using a new sheath flow electrospray emitter array at subambient pressures

Arrays of chemically etched emitters with individualized sheath gas capillaries were developed to enhance electrospray ionization (ESI) efficiency at subambient pressures. By incorporating the new emitter array in a subambient pressure ionization with nanoelectrospray (SPIN) source, both ionization efficiency and ion transmission efficiency were significantly increased, providing enhanced sensitivity in mass spectrometric analyses. The SPIN source eliminates the major ion losses of conventional ESI-mass spectrometry (MS) interfaces by placing the emitter in the first reduced pressure region of the instrument. The new ESI emitter array design developed in this study allows individualized sheath gas around each emitter in the array making it possible to generate an array of uniform and stable electrosprays in the subambient pressure (10 to 30 Torr) environment for the first time. The utility of the new emitter arrays was demonstrated by coupling the emitter array/SPIN source with a time of flight (TOF) mass spectrometer. The instrument sensitivity was compared under different ESI source and interface configurations including a standard atmospheric pressure single ESI emitter/heated capillary, single emitter/SPIN and multi-emitter/SPIN configurations using an equimolar solution of nine peptides. The highest instrument sensitivity was observed using the multi-emitter/SPIN configuration in which the sensitivity increased with the number of emitters in the array. Over an order of magnitude MS sensitivity improvement was achieved using multi-emitter/SPIN compared with using the standard atmospheric pressure single ESI emitter/heated capillary interface.

Top-down proteomics of a drop of blood for diabetes monitoring

The most common markers for monitoring patients with diabetes are glucose and HbA1c, but additional markers such as glycated human serum albumin (HSA) have been identified that could address the glycation gap and bridge the time scales of glycemia between transient and 2-3 months. However, there is currently no technical platform that could measure these markers concurrently in a cost-effective manner. We have developed a new assay that is able to measure glucose, HbA1c, glycated HSA, and glycated apolipoprotein A-I (apoA-I) for monitoring of individual blood glycemia, as well as cysteinylated HSA, S-nitrosylated HbA, and methionine-oxidized apoA-I for gauging oxidative stress and cardiovascular risks, all in 5 μL of blood. The assay utilizes our proprietary multinozzle emitter array chip technology to enable the analysis of small volumes of blood, without complex sample preparation prior to the online and on-chip liquid chromatography-nanoelectrospray ionization mass spectrometry. Importantly, the assay employs top-down proteomics for more accurate quantitation of protein levels and for identification of post-translational modifications. Further, the assay provides multimarker, multitime-scale, and multicompartment monitoring of blood glycemia. Our assay readily segregates healthy controls from Type 2 diabetes patients and may have the potential to enable better long-term monitoring and disease management of diabetes.

Introducing samples directly into electrospray ionization mass spectrometers by direct infusion using a nanoelectrospray interface

Procedures are described for constructing and using a microscale electrospray interface for direct infusion of samples into mass spectrometers. The sensitivity of the nanospray interface is a result of greatly reducing the flow of sample solution while preserving the analyte signal intensity. The described methodology provides a simple and robust way to analyze individual purified peptide and protein samples, i.e., samples that do not require liquid chromatography separation.

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.

Multinozzle emitter array chips for small-volume proteomics

High-throughput multiplexed proteomics of small-volume biospecimens will generate new opportunities in theranostics. Achieving parallel top-down and bottom-up mass spectrometry analyses of target proteins using a unified apparatus will improve proteome characterization. We have developed a novel silicon-based microfluidic device, multinozzle emitter array chip (MEA chip), as a new platform for small-volume proteomics using liquid chromatography-nanoelectrospray ionization mass spectrometry (LC-nanoESI-MS). We demonstrate parallel, on-chip, and online LC-MS analysis of hemoglobin and its tryptic digests directly from microliters of blood, achieving a detection limit of less than 5 red blood cells. Our MEA chip will enable clinical proteomics of small-volume samples.

Microfluidics for single cell analysis

Substantial evidence shows that the heterogeneity of individual cells within a genetically identical population can be critical to their chance of survival. Methods that use average responses from a population often mask the difference from individual cells. To fully understand cell-to-cell variability, a complete analysis of an individual cell, from its live state to cell lysates, is essential. Highly sensitive detection of multiple components and high throughput analysis of a large number of individual cells remain the key challenges to realise this aim. In this context, microfluidics and lab-on-a-chip technology have emerged as the most promising avenue to address these challenges. In this review, we will focus on the recent development in microfluidics that are aimed at total single cell analysis on chip, that is, from an individual live cell to its gene and proteins. We also discuss the opportunities that microfluidic based single cell analysis can bring into the drug discovery process.