Super-atmospheric pressure ionization mass spectrometry and its application to ultrafast online protein digestion analysis

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.

Generation of Ions from Aqueous Taylor Cones near the Minimum Flow Rate: "True Nanoelectrospray" without Narrow Capillary

Generating ultrafine charged droplets using electrospray is crucial for attaining high ionization efficiency for mass spectrometry. The size of the precursor charged droplets depends on the spray flow rate, and conventional wisdom holds that an electrospray of a nL/min flow rate (nanoelectrospray) is only possible using narrow capillaries with an inner diameter of ∼1 μm or smaller. Here, the electrospray of aqueous solutions with high electric conductivities generated from a large off-line capillary of 0.4 mm i.d. has been performed using a high-pressure ion source. The electric discharge is avoided by operating the ion source at 2.5 bar gauge pressure. The highly stable Taylor cone can be tuned to a near-hydrostatic state that exhibits the "true nanoelectrospray" properties, i.e., high salt tolerance and minimal ion suppression. The Q^1/2 scaling law describing the electrospray current I and flow rate Q is found to be valid down to the nanoflow regime under a condition that is free of electric discharge. For a given solution, the flow rate and the size of the initial droplets and ionization species can be controlled with the spray current as the indicator for the instantaneous flow rate without changing the emitter capillary of different sizes. In regard to the application, the nanoelectrospray with a large micropipette tip is easy to use, free of clogging when dealing with viscous and high-salt buffer solutions, and with reduced surface interaction with the emitter inner surface. An acquisition of very clean mass spectra of proteins from concentrated solutions of nonvolatile salts such as phosphate-buffered saline is demonstrated.

Temperature-Controlled Electrospray Ionization: Recent Progress and Applications

Native electrospray ionization (ESI) and nanoelectrospray ionization (nESI) allow researchers to analyze intact biomolecules and their complexes by mass spectrometry (MS). The data acquired using these soft ionization techniques provide a snapshot of a given biomolecules structure in solution. Over the last thirty years, several nESI and ESI sources capable of controlling spray solution temperature have been developed. These sources can be used to elucidate the thermodynamics of a given analyte, as well as provide structural information that cannot be readily obtained by other, more commonly used techniques. This review highlights how the field of temperature-controlled mass spectrometry has developed.

Electrospray Ionization Inside the Ion Inlet Tube: Multijet Mode Operation

We investigated the electrospray ionization inside the narrow channel of the ion inlet tube. An insulating emitter capillary made of fused silica with a 0.2 mm outer diameter was inserted into the ion inlet tubes with a 0.5 and 0.6 mm inner diameter to aspirate all the charged droplets. A custom-made ion inlet tube with two side holes near its entrance is used to observe the spraying condition. The spray current is measured and monitored during the MS acquisition using isolation amplifiers. Because the emitter is cylindrically surrounded in close proximity by the metallic inner wall, it is difficult to obtain a stable and symmetric Taylor cone with its apex at the center of the emitter. Instead, a stable operation under a flow rate of 1-4 μL/min is found to be in the form of a multicone-jet mode with two or more Taylor cones anchoring around the rim of the emitter. The emitted charged droplet jets are dragged from hitting the wall by the fast-flowing air inside the inlet tube. Comparison with the typical cone-jet and multijet mode operated several millimeters outside the inlet capillary shows signal enhancements for protein standards.

High-pressure ESI-MS made easy using a plug-and-play ion source and its application to highly conductive aqueous solutions

The performance of a compact high-pressure electrospray ionization (HP-ESI) source that can be readily used for commercial atmospheric pressure ionization (API) mass spectrometers is reported. The ion source employs a converging-diverging outlet nozzle, and ions/droplets generated inside the high-pressure compartment are carried by the high-velocity air jet toward the mass spectrometry (MS) ion inlet placed under the atmospheric pressure. With the use of a shielding electrode, the HP-ESI can also be operated with its emitter held at ground potential. This feature prevents the flow of current from the emitter to other electrically grounded components and facilitates the connection of ion source to liquid chromatography (LC) columns or capillary electrophoresis. Sensitive detection of proteins from highly conductive aqueous solutions such as 0.1% trifluoroacetic acid (TFA) solution and the prevention of electrochemical artifacts by the grounded emitter operation are demonstrated.

A Plug-and-Play High-Pressure ESI Source with an Emitter at Ground Potential and Its Application to High-Temperature Capillary LC-MS

A new high-pressure ESI source that can be readily used for commercial API mass spectrometers in a plug-and-play manner without any modification on the ion sampling interface is introduced. The emitter can be operated at ground potential, and the positive mode electrospray is generated by applying a negative high potential to the counter electrode. A shielding electrode effectively shields the opposing electric field and improves the ion transmission. This feature facilitates the direct connection of the ESI emitter to the electrically grounded components. The application of the present ion source to the high-temperature (>100 °C) capillary liquid chromatography for high-speed separation of peptide and proteins is demonstrated using a monolithic polymeric column.

High ohmic resistor hyphenated gel loading tip nano-electrospray ionization source for mini mass spectrometer

The deployment of mini mass spectrometers on the field strongly demands efficient ionization sources that are easy-to-operate. Nano-electrospray (nESI) ion source has been widely used in the field of chemistry, biology, medicine, pharmaceutical industry, clinical assessment and forensic science. In this study, a high ohmic resistor hyphenated gel loading tip nESI source was coupled with our home developed mini mass spectrometer. This ionization source has the advantages of simple-in-design, disposable and low-in-cost, therefore it could be frequently used for analysis of aqueous samples without leading to cross contamination. Performances of the gel loading tip nESI emitter were similar to pulled glass capillary, and highly compatible for the analysis of biomolecule in aqueous solution. Different peptide and small molecules have been confirmed with a continuous atmospheric pressure-interfaced (CAPI) mini mass spectrometer. The corona discharge, which was usually observed at nESI emitter tip under high aqueous solvent conditions, resulting in low ion intensity, has been successfully quenched using a 10 GΩ resistor in both a pulled glass capillary and a gel loading tip as nESI emitter in this study. Compared with conventional ESI, the metal wire assisted gel loading tip facilitated loading and direct analysis of biological samples without sample pretreatment.

High-Temperature Liquid Chromatography and the Hyphenation with Mass Spectrometry Using High-Pressure Electrospray Ionization

Increasing the operating temperature of the liquid chromatography (LC) column has the same effect as reducing the diameter of the packing particles on minimizing the contribution of C-term in the van Deemter equation, flattening the curve of plate height vs. linear velocity in the high-speed region, thus allowing a fast LC analysis without the loss of plate count. While the use of smaller particles requires a higher pumping pressure, operating the column at higher temperature reduces the pressure due to lower liquid viscosity. At present, the adoption of high-temperature LC lags behind the ultra-high-pressure LC. Nevertheless, the availability of thermally stable columns has steadily improved and new innovations in this area have continued to emerge. This paper gives a brief review and updates on the recent advances in high-temperature liquid chromatography (HTLC). Recent efforts of hyphenating the capillary HTLC with mass spectrometry via a super-atmospheric pressure electrospray ionization is also reported.

Hyphenation of high-temperature liquid chromatography with high-pressure electrospray ionization for subcritical water LC-ESI-MS

High-pressure electrospray ionization (HP-ESI) performed under super-atmospheric pressure allows a stable and efficient electrospray of pure aqueous and/or superheated solutions even under a μL min-1 flow rate regime. In this paper, we report the direct coupling of the HP-ESI source to high-temperature liquid chromatography (HT-LC) operated at ≤30 μL min-1 flow rates. In addition to ESI, the ion source functions as a back-pressure regulator to keep the mobile phase in the liquid phase when the column is heated to >100 °C. Under an ion source pressure of 7 bar, the LC column can be operated up to 160 °C. LC is performed under isocratic elution, and besides the isothermal mode, the temperature of the column can also be programmed to increase the selectivity while keeping the ion source at a constant temperature. For a given solution flow rate, the analytical time can be shortened by increasing the column temperature. HT-LC-ESI-MS using pure water as the mobile phase with a capillary column is also demonstrated.

Electrospray ionization source with a rear extractor

A new electrospray source design is introduced by having an extractor electrode placed at 1 to 2 mm behind the emitter tip. The extractor was integrated into the sprayer body as a single device. An insulating tube was used to isolate the emitter from the extractor and to deliver the sheath gas for the electrospray. The electric field strength at the emitter was primarily determined by the relative position and the potential between the needle and the extractor; therefore, the spraying condition was insusceptible to the change of sprayer position or orientation with respect to the ion sampling inlet. Such design allowed the use of much lower operating voltage and facilitated the optimization of sprayer position by keeping the electric field parameter constant. Using an emitter capillary of 150 and 310 μm in inner and outer diameters, strong ion signal could still be acquired with 2-kV emitter potential even if the distance between the emitter and ion inlet was extended to >70 mm. Charge reduction of protein ions using 2 extractor-based electrosprays of opposite emitter polarities was also demonstrated.

Towards Practical Endoscopic Mass Spectrometry

In this paper, we briefly review the remote mass spectrometric techniques that are viable to perform "endoscopic mass spectrometry," i.e., in-situ and in-vivo MS analysis inside the cavity of human or animal body. We also report our experience with a moving string sampling probe for the remote sample collection and the transportation of adhered sample to an ion source near the mass spectrometer. With a miniaturization of the probe, the method described here has the potential to be fit directly into a medical endoscope.

Electrospray Modifications for Advancing Mass Spectrometric Analysis

Generation of analyte ions in gas phase is a primary requirement for mass spectrometric analysis. One of the ionization techniques that can be used to generate gas phase ions is electrospray ionization (ESI). ESI is a soft ionization method that can be used to analyze analytes ranging from small organics to large biomolecules. Numerous ionization techniques derived from ESI have been reported in the past two decades. These ion sources are aimed to achieve simplicity and ease of operation. Many of these ionization methods allow the flexibility for elimination or minimization of sample preparation steps prior to mass spectrometric analysis. Such ion sources have opened up new possibilities for taking scientific challenges, which might be limited by the conventional ESI technique. Thus, the number of ESI variants continues to increase. This review provides an overview of ionization techniques based on the use of electrospray reported in recent years. Also, a brief discussion on the instrumentation, underlying processes, and selected applications is also presented.

In vivo endoscopic mass spectrometry using a moving string sampling probe

A novel moving string sampling probe and sample transportation system for performing in situ and in vivo endoscopic MS.