On‐Demand Ambient Ionization of Picoliter Samples Using Charge Pulses

In situ uncovering the catalytic cycle of electrochemical and chemical oxygen reduction mediated by an iron porphyrin

As one of the critical reactions in biotransformation and energy conversion processes, the oxygen reduction reaction (ORR) catalyzed by iron porphyrins has been widely explored by electrochemical, spectroscopic, and theoretical methods. However, experimental identification of all proposed intermediates of iron porphyrins in one catalytic cycle is rather challenging in the mechanistic studies of the ORR driven by electrochemical or chemical methods. Herein, we report the application of electrochemical mass spectrometry (EC-MS) and chemical reaction mass spectrometry (CR-MS) to in situ uncover the catalytic cycle of electrochemical and chemical ORRs mediated by an iron porphyrin molecular catalyst. Five crucial iron-oxygen intermediates detected by both EC-MS and CR-MS help to build the whole catalytic cycle and indicate the details of the 4e-/4H+ pathway to produce H2O in the electrochemical and chemical ORRs. By combining in situ MS methods with electrochemical and spectroscopic methods to characterize the intermediates and study the selectivities, this work provides a mechanistic comparison of the electrochemical and chemical ORRs catalyzed by one model iron porphyrin.

Recent Advances in On-Line Mass Spectrometry Toolbox for Mechanistic Studies of Organic Electrochemical Reactions

Electrochemical reactions are very complex and involve a variety of physicochemical processes. Accurate and systematic monitoring of intermediate process changes during the reaction is essential for understanding the mechanism of electrochemical reactions and is the basis for rational design of new electrochemical reactions. On-line electrochemical analysis based on mass spectrometry (MS) has become an important tool for studying electrochemical reactions. This technique is based on different ionization and sampling means and realizes on-line analysis of electrochemical reactions by establishing electrochemistry-MS (EC-MS) coupling devices. In particular, it provides key evidence for elucidating the reaction mechanism by capturing and identifying the reactive reaction intermediates. This review will categorize various EC-MS devices and the organic electrochemical reaction systems they study, highlighting the latest research progress in recent years. It will also analyze the properties of various devices and look forward to the future development of EC-MS.

Chain electrospray ionization mass spectrometry for ultra-low volume sample analysis

In this work, chain electrospray ionization (chain-ESI) was developed to efficiently ionize trace samples for mass spectrometry analysis. The primary ion source was found to have the ability to induce secondary electrospray ionization with an extraordinarily low sample consumption rate in the picoliters per minute (pLs/min). This allows low volume sample to generate substantial tandem mass spectrum (MS2) data for metabolite annotations. Notably, chain-ESI can effectively prevent the electro-redox reaction in the process of electrospray, so as to reflect the native state of the analytes. Furthermore, from a single Broussonetia papyrifera (B. papyrifera) trichome and a single A549 cancer cell, 1426 and 617 metabolites were detected respectively. All of those observations demonstrated that chain-ESI offers the advantages of direct, rapid analysis with extreme-low volumes and high coverage, enabling the measurement of bio-information in low volume samples.

Pulsed Direct Current Arc-Induced Nanoelectrospray Ionization Mass Spectrometry

This study explores the innovative field of pulsed direct current arc-induced nanoelectrospray ionization mass spectrometry (DCAI-nano-ESI-MS), which utilizes a low-temperature direct current (DC) arc to induce ESI during MS analyses. By employing a 15 kV output voltage, the DCAI-nano-ESI source effectively identifies various biological molecules, including angiotensin II, bradykinin, cytochrome C, and soybean lecithin, showcasing impressive analyte signals and facilitating multicharge MS in positive- and negative-ion modes. Notably, results show that the oxidation of fatty acids using a DC arc produces [M + O - H]- ions, which aid in identifying the location of C═C bonds in unsaturated fatty acids and distinguishing between isomers based on diagnostic ions observed during collision-induced dissociation tandem MS. This study presents an approach for identifying the sn-1 and sn-2 positions in phosphatidylcholine using phosphatidylcholine and nitrate adduct ions, accurately determining phosphatidylcholine molecular configurations via the Paternò-Büchi reaction. With all the advantages above, DCAI-nano-ESI holds significant promise for future analytical and bioanalytical applications.

Surface charge-induced electrospray for high-throughput analysis of complex samples and electrochemical reaction intermediates using mass spectrometry

Electrospray-related ion sources are promising for direct mass spectrometric analysis of complex samples, but current protocols suffer from complicated components and low analytical sensitivity. Here, we propose a surface charge-induced electrospray ionization (SCIESI) inspired by flashover on an insulator surface under high voltage. This protocol not only effectively avoids contact between the sample solution and metal electrode, but also allows completion of the entire analytical process in less than 40 seconds and limits of detection in the pictogram per milliliter range. SCIESI coupled to mass spectrometry can also be used to monitor electro-chemical processes, and a number of oxidation and reduction reactions have been studied, demonstrating that it is a powerful tool for understanding electrochemical reaction mechanisms.

Capillary self-aspirating electrospray ionization (CSESI) for convenient and versatile mass spectrometry analysis

Electrospray ionization (ESI) is one of the most widely used ionization techniques, and its simplification can benefit many interested users. In this study, sample introduction by capillary action was studied and used to develop a simple ESI source called capillary self-aspirating electrospray ionization (CSESI). A conventional CSESI source requires only a common capillary of appropriate diameter in addition to the support of high voltage (HV). No pumps and sample loading is needed because the solution can spontaneously climb across the capillary by capillary action. With the proper modification of the glass capillary, the operation of CSESI can be further simplified and efficient. Specifically, cold plasma processing of the capillary creates a more hydrophilic surface that can facilitate sample introduction. Moreover, sputtering a thin platinum layer on the capillary tip makes the application of HV more convenient, and it also eliminates the influence of air bubbles in the capillary to ensure a sustained and stable electrospray. Overall, CSESI exhibits multiple desirable features such as simple structure, self-aspiration ability, low sample consumption, and inherent physical filtration capability. Apart from the routine ESI-MS analysis, it has also been applied in real-time monitoring of the oxidative dimerization of 8-methyl-1,2,3,4-tetrahydroquinoline, as well as direct analysis of muddy soil solutions without pretreatment.

Flexible Microtube Plasma for the Consecutive-Ionization of Cholesterol in Nano-Electrospray Mass Spectrometry

Electrospray ionization mass spectrometry (ESI-MS) is an established method for the identification of biomarkers. By nano-ESI (nESI), the polar molecular fraction of complex biological samples can be successfully ionized. In contrast, the less-polar free cholesterol, which serves as an important biomarker for several human diseases, is barely accessible by nESI. Although, complex scan functions of modern high-resolution MS devices are able to increase the signal-to-noise ratio, they are limited by the ionization efficiency of the nESI. One possible method to increase the ionization efficiency is the derivatization with acetyl chloride, however interferences with cholesteryl esters must be considered, so chromatographic separation or complex scan functions may be required. A novel approach to increase the yield of cholesterol ions of the nESI could be the application of a second consecutive-ionization process. This publication presents the flexible microtube plasma (FμTP) as a consecutive-ionization source, which allows the determination of cholesterol in nESI-MS analysis. Focusing on the analytical performance, the nESI-FμTP approach increases the cholesterol signal yield in a complex liver extract by a factor of 49. The repeatability and long-term stability could be successfully evaluated. A linear dynamic range of 1.7 orders of magnitude, a minimum detectability of 5.46 mg/L, and a high accuracy (deviation, -8.1%) demonstrates the nESI-FμTP-MS as an excellent approach for a derivatization-free determination of cholesterol.

Bipolar Electrospray from Electrodeless Emitters for ESI without Electrochemical Reactions in the Sprayer

A bipolar ESI source is developed to generate a simultaneous emission of charged liquid jets of opposite polarity from an electrodeless sprayer. The sprayer consists of two emitters, and the electrosprays are initiated by applying a high potential difference (HV) across the counter electrodes facing each emitter. The sprayer and the liquid delivery system are made of all insulators without metal components, thus enabling the total elimination of electrochemical reactions taking place at the liquid-electrode interface in the typical electrosprayer. The bipolar electrospray has been implemented using an online configuration that uses a syringe pump for flow rate regulation and an offline configuration that relies on HV for adjusting the flow rate. The voltage-current and flow rate-current relationships of bipolar electrospray were found to be similar to the standard electrospray. The application of bipolar ESI to the mass spectrometry of protein, peptide, and metallocene without electrochemically induced oxidation/reduction is demonstrated.

Mass Spectrometry Approach for Differentiation of Positional Isomers of Saccharides: Toward Direct Analysis of Rare Sugars

Rare sugars have gained popularity in recent years due to their use in antiaging treatments, their ability to sweeten with few calories, and their ability to heal infections. Rare sugars are found in small quantities in nature, and they exist typically as isomeric forms of traditional sugars, rendering some challenges in their isolation, synthesis, and characterization. In this work, we present the first direct mass spectrometric approach for differentiating structural isomers of sucrose that differ only by their glycosidic linkages. The method employed a noncontact nanoelectrospray (nESI) platform capable of analyzing minuscule volumes (5 μL) of saccharides via the formation of halide adducts ([M+X]-; X = Cl and Br). Tandem mass spectrometry analysis of the five structural isomers of sucrose afforded diagnostic fragment ions that can be used to distinguish each isomer. Detailed mechanisms showcasing the distinct fragmentation pattern for each isomer are discussed. The method was applied to characterize and confirm the presence of all five selected rare sugars in raw honey complex samples. Aside from the five natural α isomers of sucrose, the method was also suitable for differentiating some β isomers of the same glycosidic linkages, provided the monomeric sugar units are different. The halide adduct formation via the noncontact nESI source was also proven to be effective for oligosaccharides such as raffinose, β-cyclodextrin, and maltoheptaose. The results from this study encourage the future development of methods that function with simple operation to enable straightforward characterization of small quantities of rare sugars.

Pocket-Size Wireless Nanoelectrospray Ionization Mass Spectrometry for Metabolic Analysis of Salty Biofluids and Single Cells

Analysis of volume-limited biological samples such as single cells and biofluids not only benefits clinical purposes but also promotes fundamental research in life sciences. Detection of these samples, however, imposes strict requirements on measurement performance because of the minimal volume and concentrated salts of the samples. Herein, we developed a self-cleaning nanoelectrospray ionization device powered by a pocket-size "MasSpec Pointer" (MSP-nanoESI) for metabolic analysis of salty biological samples with limited volume. The self-cleaning effect induced by Maxwell-Wagner electric stress helps with keeping the borosilicate glass capillary tip free from clogging and thus increasing salt tolerance. This device possesses a high sample economy (about 0.1 μL per test) due to its pulsed high voltage supply, sampling method (dipping the nanoESI tip into analyte solution), and contact-free electrospray ionization (ESI) (the electrode does not touch the analyte solution during ESI). High repeatable results could be acquired by the device with a relative standard deviation (RSD) of 1.02% for voltage output and 12.94% for MS signals of caffeine standard. Single MCF-7 cells were metabolically analyzed directly from phosphate buffered saline, and two types of untreated cerebrospinal fluid from hydrocephalus patients were distinguished with 84% accuracy. MSP-nanoESI gets rid of the bulky apparatus and could be held in hand or put into one's pocket for transportation, and it could operate for more than 4 h without recharge. We believe this device will boost scientific research and clinical usage of volume-limited biological samples with high-concentration salts in a low-cost, convenient, and rapid manner.

Identifying Metal-Oxo/Peroxo Intermediates in Catalytic Water Oxidation by In Situ Electrochemical Mass Spectrometry

Molecular catalysis of water oxidation has been intensively investigated, but its mechanism is still not yet fully understood. This study aims at capturing and identifying key short-lived intermediates directly during the water oxidation catalyzed by a cobalt-tetraamido macrocyclic ligand complex using a newly developed an in situ electrochemical mass spectrometry (EC-MS) method. Two key ligand-centered-oxidation intermediates, [(L^2-)Co^IIIOH] and [(L^2-)Co^IIIOOH], were directly observed for the first time, and further confirmed by ¹⁸O-labeling and collision-induced dissociation studies. These experimental results further confirmed the rationality of the water nucleophilic attack mechanism for the single-site water oxidation catalysis. This work also demonstrated that such an in situ EC-MS method is a promising analytical tool for redox catalytic processes, not only limited to water oxidation.

High-Throughput Screening Using a Synchronized Pulsed Self-aspiration Vacuum Electrospray Ionization Miniature Mass Spectrometer

With the advantages of rapid analysis, high sensitivity, and multicomponent identification, mass spectrometry (MS) is recognized as an appealing choice for high-throughput screening (HTS) analysis. Aiming at the small size, simple operation, and adequate performance, the development of miniature mass spectrometers has made great progress over the last 2 decades. Besides the essential analytical performance, simple operation and HTS capability are two other crucial features desired in miniature MS instruments. In this paper, an induced self-aspiration vacuum electrospray ionization source (ISA-VESI) was developed and coupled to a miniature ion trap mass spectrometer. A special timing sequence was designed to synchronize all the operation steps in each measurement, including dual-pulse sample injection, multipulse gas injection, MS analysis, and the movement of the homemade HTS platform used as the sampler. Then, the automatic high-throughput analysis of multiple samples can be accomplished with close coordination among the sample delivery, the sample introduction and ionization, and the ion trap operation. The measurement time of each ISA-VESI-MS analysis was about 7 s, with a sample consumption of less than 100 nL.

Conducting and characterizing femto flow electrospray ionization

Femto flow electrospray ionization (ESI) with flow rates ranging from 240 fL min^-1 to the low pico level (<10 pL min^-1) was conducted and measured using a submicron emitter tip and relay ESI configuration. Signature analyte ion current intensities and profiles were observed. The obtained flow rate and ionization current enabled size calculation for initial charged nanodroplets.

Revealing the Sulfur Redox Paths in a Li-S Battery by an In Situ Hyphenated Technique of Electrochemistry and Mass Spectrometry

The lithium-sulfur (Li-S) battery is one of the most promising next generation energy storage systems due to its high theoretical specific energy. However, the shuttle effect of soluble lithium polysulfides formed during cell operation is a crucial reason for the low cyclability suffered by current Li-S batteries. As a result, an in-depth mechanistic understanding of the sulfur cathode redox reactions is urgently required for further advancement of Li-S batteries. Herein, the direct observation of polysulfides in a Li-S battery is reported by an in situ hyphenated technique of electrochemistry and mass spectrometry. Several short-lived lithium polysulfide intermediates during sulfur redox have been identified. Furthermore, this method is applied to a mechanistic study of an electrocatalyst that has been observed to promote the polysulfides conversion in a Li-S cell. Through the abundance distributions of various polysulfides before and after adding the electrocatalyst, compelling experimental evidences of catalytic selectivity of cobalt phthalocyanine to those long-chain polysulfide intermediates are obtained. This work can provide guidance for the design of novel cathode to overcome the shuttle effect and facilitate the sulfur redox kinetics.

On-Demand Mass Spectrometry Analysis by Miniature Mass Spectrometer

Electrospray ionization (ESI) has become a powerful tool for the analysis of biomolecules by mass spectrometry (MS). The process of ESI is difficult to control, and side reactions such as electrochemical reactions can occur during the ESI process because of the high voltages applied. Herein, a novel on-demand MS analysis method was developed based on discontinuous ion injection-induced ESI on a miniature MS system. Highly efficient ionization was enabled under low voltages (<300 V) using a discontinuous atmospheric pressure interface. On-demand ionization showed comparable sensitivity with regular nanoESI for the analyses of a series of compounds. It was found to be softer than regular ESI or nanoESI methods for ionization of proteins such as myoglobin and cytochrome C. As the ionization finished as soon as the interface was closed, the sample consumption was observed to reduce significantly for MS analysis, allowing single-cell analysis with multiple MS and MS/MS measurements.

Microdroplet Impact-Induced Spray Ionization Mass Spectrometry (MISI MS) for Online Reaction Monitoring and Bacteria Discrimination

Microdroplet impact-induced spray ionization (MISI) is demonstrated involving the impact of microdroplets produced from a paper and their impact on another, leading to the ionization of analytes deposited on the latter. This cascaded process is more advantageous in comparison to standard spray ionization as it performs reactions and ionization simultaneously in the absence of high voltage directly applied on the sample. In MISI, we apply direct current (DC) potential only to the terminal paper, used as the primary ion source. Charge transfer due to microdroplet/ion deposition on the flowing analyte solution on the second surface generates secondary charged microdroplets from it carrying the analytes, which ionize and get detected by a mass spectrometer. In this way, up to three cascaded spray sources could be assembled in series. We show the detection of small molecules and proteins in such ionization events. MISI provides a method to understand chemical reactions by droplet impact. The C-C bond formation reactions catalyzed by palladium and alkali metal ion encapsulation using crown ether were studied as our model reactions. To demonstrate the application of our ion source in a bioanalytical context, we studied the noninvasive in situ discrimination of bacteria samples under ambient conditions.

Nested-channel for on-demand alternation between electrospray ionization regimes

On-demand electrospray ionization from different liquid channels in the same emitter was realized using filamented capillary and gas phase charge supply. The solution sub-channel was formed when back-filling solution to the emitter tip by capillary action along the filament. Gas phase charge carriers were used to trigger electrospray ionization from the solution meniscus at the tip. The meniscus at the tip opening may be fully filled or partially empty to generate electrospray ionization in main-channel regime and sub-channel regime, respectively. For emitters with 4 μm tip opening, the two nested electrospray (nested-ESI) channels accommodated ESI flow rates ranging from 50 pL min-1 to 150 nL min-1. The platform enabled on-demand regime alternations within one sample run, in which the sub-channel regime generated smaller charged droplets. Ionization efficiencies for saccharides, glycopeptide, and proteins were enhanced in the sub-channel regime. Non-specific salt adducts were reduced and identified by regime alternation. Surprisingly, the sub-channel regime produced more uniform responses for a peptide mixture whose relative ionization efficiencies were insensitive to ESI conditions in previous picoelectrospray study. The nested channels also allowed effective washing of emitter tip for multiple sampling and analysis operations.

Induced Self-aspiration Electrospray Ionization Mass Spectrometry for Flexible Sampling and Analysis

Electrospray ionization (ESI) operating in pulse mode can enhance the utilization efficiency of the electrospray ions by a mass spectrometer. Herein, a novel ionization technique called induced self-aspiration-electrospray ionization (ISA-ESI) was developed based on self-aspiration sampling and capacitive induction. The sample solution polarized in a strong electric field was pulsed drawn into a capillary that was connected to a subambient chamber. The sample solution with polarized ions forms a charged liquid column, which can initiate an electrospray when reaching the capillary outlet. In addition to the self-aspiration ability, the use of a constant high voltage supply and no electrical contact with the solution can also simplify the sampling and ionization operation, enabling a convenient ESI mass spectrometry analysis. The developed ISA-ESI source has been used for multidimensional monitoring of chemical reactions as well as liquid extraction surface analysis of plant tissues. It was expected that this special ionization method could be extended to automated high-throughput ESI-MS analysis.

High-Throughput Bioassays using "Dip-and-Go" Multiplexed Electrospray Mass Spectrometry

A multiplexed system based on inductive nanoelectrospray mass spectrometry (nESI-MS) has been developed for high-throughput screening (HTS) bioassays. This system combines inductive nESI and field amplification micro-electrophoresis to achieve a "dip-and-go" sample loading and purification strategy that enables nESI-MS based HTS assays in 96-well microtiter plates. The combination of inductive nESI and micro-electrophoresis makes it possible to perform efficient in situ separations and clean-up of biological samples. The sensitivity of the system is such that quantitative analysis of peptides from 1-10 000 nm can be performed in a biological matrix. A prototype of the automation system has been developed to handle 12 samples (one row of a microtiter plate) at a time. The sample loading and electrophoretic clean-up of biosamples can be done in parallel within 20 s followed by MS analysis at a rate of 1.3 to 3.5 s per sample. The system was used successfully for the quantitative analysis of BACE1-catalyzed peptide hydrolysis, a prototypical HTS assay of relevance to drug discovery. IC₅₀ values for this system were in agreement with LC-MS but recorded in times more than an order of magnitude shorter.

Mechanistic Study of Oxygen Reduction at Liquid/Liquid Interfaces by Hybrid Ultramicroelectrodes and Mass Spectrometry

Proton-coupled electron transfer (PCET) reactions at various interfaces (liquid/membrane, solid/electrolyte, liquid/liquid) lie at the heart of many processes in biology and chemistry. Mechanistic study can provide profound understanding of PCET and rational design of new systems. However, most mechanisms of PCET reactions at a liquid/liquid interface have been proposed based on electrochemical and spectroscopic data, which lack direct evidence for possible intermediates. Moreover, a liquid/liquid interface as one type of soft interface is dynamic, making the investigation of interfacial reactions very challenging. Herein a novel electrochemistry method coupled to mass spectrometry (EC-MS) was introduced for in situ study of the oxygen reduction reaction (ORR) by ferrocene (Fc) under catalysis from cobalt tetraphenylporphine (CoTPP) at liquid/liquid interfaces. The key units are two types of gel hybrid ultramicroelectrodes (agar-gel/organic hybrid ultramicroelectrodes and water/PVC-gel hybrid ultramicroelectrodes), which were made based on dual micro- or nanopipettes. A solidified liquid/liquid interface can be formed at the tip of these pipettes, and it serves as both an electrochemical cell and a nanospray emitter for mass spectrometry. We demonstrated that the solidified L/L interfaces were very similar to typical L/L interfaces. Key CoTPP intermediates of the ORR at the liquid/liquid interfaces were identified for the first time, and the four-electron oxygen reduction pathway predominated, which provides valuable insights into the mechanism of the ORR. Theoretical simulation has further supported the possibility of formation of intermediates. This type of platform is promising for in situ tracking and identifying intermediates to study complicated reactions at liquid/liquid interfaces or other soft interfaces.

Electric modeling and characterization of pulsed high-voltage nanoelectrospray ionization sources by a miniature ion trap mass spectrometer

A better understanding of nanoelectrospray ionization (nano-ESI) would be beneficial in further improving the performances of nano-ESI. In this work, the pulsed high-voltage (HV) nano-ESI has been electrically modeled and then systematically characterized by both voltage-current and mass spectrometry measurements. First, the equivalent resistance of a nano-ESI source changes with respect to both emitter tip diameter and the HV applied. Increased voltage could improve both spray current and ionization efficiency of the pulsed HV nano-ESI. Compared with conventional DC HV method, a pulsed HV has less heating effect on the capillary tip and thus allowing the application of a much higher voltage onto a nano-ESI source. As a result, a pulsed HV nano-ESI could further boost the ionization efficiency of nano-ESI by employing even higher voltages than conventional DC nano-ESI sources.

Considerations for uranium isotope ratio analysis by atmospheric pressure ionization mass spectrometry

The accurate measurement of uranium isotope ratios from trace samples lies at the foundation of achieving nuclear nonproliferation. These challenging measurements necessitate both the continued characterization and evaluation of evolving mass spectrometric technologies as well as the propagation of sound measurement approaches. For the first time in this work, we present the analysis of uranium isotope ratio measurements from discrete liquid injections with an ultra-high-resolution hybrid quadrupole time-of-flight mass spectrometer. Also presented are important measurement considerations for evaluating the performance of this type and other atmospheric pressure and ambient ionization mass spectrometers for uranium isotope analysis. Specifically, as the goal of achieving isotope ratios from as little as a single picogram of solid material is approached, factors such as mass spectral sampling rate, collision induced dissociation (CID) potentials, and mass resolution can dramatically alter the measured isotope ratio as a function of mass loading. We present the ability to accurately measure 235UO2+/238UO2+ down to 10s of picograms of solubilized uranium oxide through a proper consideration of mass spectral parameters while identifying limitations and opportunities for pushing this limit further.

Analytical methods for cholesterol quantification

Cholesterol is an important lipid molecule in cell membranes and lipoproteins. Cholesterol is also a precursors of steroid hormones, bile acids, and vitamin D. Abnormal levels of cholesterol or its precursors have been observed in various human diseases, such as heart diseases, stroke, type II diabetes, brain diseases and many others. Therefore, accurate quantification of cholesterol is important for individuals who are at increased risk for these diseases. Multiple analytical methods have been developed for analysis of cholesterol, including classical chemical methods, enzymatic assays, gas chromatography (GC), liquid chromatography (LC), and mass spectrometry (MS). Strategy known as ambient ionization mass spectrometry (AIMS), operating at atmospheric pressure, with only minimal sample pretreatments for real time, in situ, and rapid interrogation of the sample has also been employed for quantification of cholesterol. In this review, we summarize the most prevalent methods for cholesterol quantification in biological samples and foods. Nevertheless, we highlight several new technologies, such as AIMS, used as alternative methods to measure cholesterol that are potentially next-generation platforms. Representative examples of molecular imaging of cholesterol in tissue sections are also included in this review article.

Fabrication and Use of Nanopipettes in Chemical Analysis

This review summarizes progress in the fabrication, modification, characterization, and applications of nanopipettes since 2010. A brief history of nanopipettes is introduced, and the details of fabrication, modification, and characterization of nanopipettes are provided. Applications of nanopipettes in chemical analysis are the focus in several cases, including recent progress in imaging; in the study of single molecules, single nanoparticles, and single cells; in fundamental investigations of charge transfer (ion and electron) reactions at liquid/liquid interfaces; and as hyphenated techniques combined with other methods to study the mechanisms of complicated electrochemical reactions and to conduct bioanalysis.

A cautionary note on the effects of laboratory air contaminants on ambient ionization mass spectrometry measurements

RATIONALE

Ambient ionization mass spectrometry methods are convenient, sensitive and require little sample preparation. However, they are susceptible to species present in air surrounding the mass spectrometer. This study identifies some challenges associated with the potential impacts of indoor air contaminants on ionization and analysis involving open-air methods.

METHODS

Unexpected effects of volatile organic compounds (VOCs) from floor maintenance activities on ambient ionization mass spectrometry were studied using three different ambient ionization techniques. Extractive electrospray ionization (EESI), direct analysis in real time (DART) and ionization by piezoelectric direct discharge (PDD) plasma were demonstrated in this study to be affected by indoor air contaminants. Identification of contaminant vapors was verified by comparison with standards using EESI-MS/MS product ion scans.

RESULTS

Emissions of diethylene glycol monoethyl ether and ethylene glycol monobutyl ether are identified from floor stripping and waxing solutions using three ambient ionization mass spectrometry techniques. These unexpected indoor air contaminants are capable of more than 75% ion suppression of target analytes due to their high volatility, proton affinity and solubility compared with the target analytes. The contaminant vapors are also shown to form adducts with one of the target analytes.

CONCLUSIONS

The common practice in MS analysis of subtracting a background air spectrum may not be appropriate if the presence of ionizable air contaminants alters the spectrum in unexpected ways. For example, VOCs released into air from floor stripping and waxing are capable of causing ion suppression of target analytes.

Triboelectric nanogenerators for sensitive nano-coulomb molecular mass spectrometry

Ion sources for molecular mass spectrometry are usually driven by direct current power supplies with no user control over the total charges generated. Here, we show that the output of triboelectric nanogenerators (TENGs) can quantitatively control the total ionization charges in mass spectrometry. The high output voltage of TENGs can generate single- or alternating-polarity ion pulses, and is ideal for inducing nanoelectrospray ionization (nanoESI) and plasma discharge ionization. For a given nanoESI emitter, accurately controlled ion pulses ranging from 1.0 to 5.5 nC were delivered with an onset charge of 1.0 nC. Spray pulses can be generated at a high frequency of 17 Hz (60 ms in period) and the pulse duration is adjustable on-demand between 60 ms and 5.5 s. Highly sensitive (∼0.6 zeptomole) mass spectrometry analysis using minimal sample (18 pl per pulse) was achieved with a 10 pg ml^-1 cocaine sample. We also show that native protein conformation is conserved in TENG-ESI, and that patterned ion deposition on conductive and insulating surfaces is possible.

Recent advances in real-time and in situ analysis of an electrode–electrolyte interface by mass spectrometry

Novelty: Recent advances in real-time and in situ monitoring of an electrode–electrolyte interface by mass spectrometry are reviewed.

Mass spectrometric snapshots for electrochemical reactions

A hybrid ultramicroelectrode containing one micro-carbon electrode and one empty micro-channel was employed to be a micro-electrochemical cell and a mass spectrometric nanospray emitter. This setup can combine MS with an electrode directly and provide in situ information about an electrochemical reaction. The mechanisms proposed by Bard et al. for a Ru(bpy)32+ (bpy = 2,2'-bipyridine) electrochemiluminescence (ECL) system were confirmed by the MS detection of key intermediates. The short-lived diimine intermediate of electrochemical oxidation of uric acid was also detected, which affirms that the novel technique is able to catch fleeting intermediates. These experimental results demonstrate that this new method is simple, easy to implement and can be coupled with many commercial mass spectrometric instruments to provide very useful information about electrochemical reactions.

In Situ Ion-Transmission Mass Spectrometry for Paper-Based Analytical Devices

Current detection methods for paper-based analytical devices (PADs) rely on spectroscopic and electrochemical properties, which place special requirements on the analyte or need analyte labeling. Here, ion-transmission mass spectrometry (MS) was proposed for coupling with PADs to enable rapid in situ MS analysis of the sample on paper. The sample was analyzed directly on paper via analyte ionization by ions transmitted through the paper, generated by a low-temperature plasma probe. Prior to MS analysis, the sample can be separated by paper electrophoresis or by paper chromatography, among a variety of other features offered by PADs. The versatility of this technique was demonstrated by MS analysis of a paper microarray, a mixture of amino acids, and whole blood doped with drugs on PADs.

Structure of faustovirus, a large dsDNA virus

Many viruses protect their genome with a combination of a protein shell with or without a membrane layer. Here we describe the structure of faustovirus, the first DNA virus (to our knowledge) that has been found to use two protein shells to encapsidate and protect its genome. The crystal structure of the major capsid protein, in combination with cryo-electron microscopy structures of two different maturation stages of the virus, shows that the outer virus shell is composed of a double jelly-roll protein that can be found in many double-stranded DNA viruses. The structure of the repeating hexameric unit of the inner shell is different from all other known capsid proteins. In addition to the unique architecture, the region of the genome that encodes the major capsid protein stretches over 17,000 bp and contains a large number of introns and exons. This complexity might help the virus to rapidly adapt to new environments or hosts.