Enhanced Nebulization Efficiency of Electrospray Mass Spectrometry: Improved Sensitivity and Detection Limit (original) (raw)
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Ionization and transmission efficiency in an electrospray ionization—mass spectrometry interface
Journal of the American Society for Mass Spectrometry, 2007
The ionization and transmission efficiencies of an electrospray ionization (ESI) interface were investigated to advance the understanding of how these factors affect mass spectrometry (MS) sensitivity. In addition, the effects of the ES emitter distance to the inlet, solution flow rate, and inlet temperature were characterized. Quantitative measurements of ES current loss throughout the ESI interface were accomplished by electrically isolating the front surface of the interface from the inner wall of the heated inlet capillary, enabling losses on the two surfaces to be distinguished. In addition, the ES current lost to the front surface of the ESI interface was spatially profiled with a linear array of 340-m-diameter electrodes placed adjacent to the inlet capillary entrance. Current transmitted as gas-phase ions was differentiated from charged droplets and solvent clusters by measuring sensitivity with a single quadrupole mass spectrometer. The study revealed a large sampling efficiency into the inlet capillary (Ͼ90% at an emitter distance of 1 mm), a global rather than a local gas dynamic effect on the shape of the ES plume resulting from the gas flow conductance limit of the inlet capillary, a large (Ͼ80%) loss of analyte ions after transmission through the inlet arising from incomplete desolvation at a solution flow rate of 1.0 L/min, and a decrease in analyte ions peak intensity at lower temperatures, despite a large increase in ES current transmission efficiency. (J Am Soc Mass Spectrom 2007, 18, 1582-1590 E lectrospray ionization (ESI) has become a prominent ionization technique for a broad range of chemical and biological applications of mass spectrometry (MS) [1, 2] because of its ability to create intact, multiply charged gas-phase ions (from, e.g., biomolecules in solution) and its facile coupling with on-line separation techniques [such as liquid chromatography (LC)] [3-6]. The sensitivity of ESI-MS is largely determined by the effectiveness of producing gas-phase ions from analyte molecules in solution (ionization efficiency) and the ability to transfer the charged species from atmospheric pressure to the low-pressure region of the mass analyzer (transmission efficiency) [7-10].
Biases in ion transmission through an electrospray ionization-mass spectrometry capillary inlet
Journal of the American Society for Mass Spectrometry, 2009
A heated capillary inlet for an electrospray ionization mass spectrometry (ESI-MS) interface was compared with shorter versions of the inlet to determine the effects on transmission and ionization efficiencies for low-flow (nano) electrosprays. Five different inlet lengths were studied, ranging from 6.4 to 1.3 cm. As expected, the electrospray current transmission efficiency increased with decreasing capillary length due to reduced losses to the inside walls of the capillary. This increase in transmission efficiency with shorter inlets was coupled with reduced desolvation of electrosprayed droplets. Surprisingly, as the inlet length was decreased, some analytes showed little or no increase in sensitivity, while others showed as much as a15 -fold gain. The variation was shown to beat least partially correlated with analyte mobilities, with the largest gains observed for higher mobility species, but also affected by solution conductivity, flow rate, and inlet temperature. Strategies for maximizing sensitivity while minimizing biases in ion transmission through the heated capillary interface are proposed.
ELECTROPHORESIS, 2004
A chemometrics approach has been used for evaluating the effect of four experimental parameters when coupling capillary electrophoresis (CE) to electrospray ionizationmass spectrometry (ESI-MS). Electrospray voltage, sheath-liquid flow rate, nebulizing gas flow rate, and spray needle position in respect to the MS orifice were varied according to a full factorial design. In addition to main effects, two interaction effects could be identified as significant when measuring the peak intensity of the analytes, from a sample mixture containing peptides and pharmaceuticals. The first interaction effects, between the nebulizing gas flow rate and the sheath-liquid flow rate, and the second interaction effect, between the nebulizing gas flow rate and the spray position, could further explain the impact that these variables have on the spray performance. The number of theoretical plates and the baseline noise were also measured. The sheath-liquid flow was found to significantly affect the separation efficiency, while the noise level mainly was controlled by the nebulizing gas flow. The same factorial design was also used for a CE capillary with lower internal diameter (ID) and the effects of the same variables were compared on those capillaries using equal injection volume for both capillaries. Similar trends were obtained in both capillaries but capillary ID was shown to be a significant variable when evaluating both capillaries in a single model. It was found that a capillary with 25 mm ID provided improved CE-MS performance over than corresponding 50 mm ID capillary. Enhanced sensitivity was obtained using the narrow-bore capillary, and at lower sheath-liquid flow rate the 25 mm ID capillary also gave rise to more efficient peaks.
Analytical Chemistry
We present a fully microfabricated and monolithically integrated capillary electrophoresis (CE)-electrospray ionization (ESI) chip for coupling with high-throughput mass spectrometric (MS) analysis. The chips are fabricated fully of a negative photoresist SU-8 by a standard lithographic process which enables straightforward batch fabrication of multiple chips with precisely controlled dimensions and, thus, reproducible analytical performance from chip to chip. As the coaxial sheath flow interface is patterned as an integral part of the SU-8 chip, the fluidic design is dead-volume-free. No significant peak broadening occurs so that very narrow peak widths (down to 2-3 s) are obtained. The sheath flow interface also enables comprehensive optimization of both the CE and the ESI conditions separately so that the same chip design is adaptable to diverse analytical conditions. Plate numbers of the order of 10 5 m-1 and good resolution are routinely reached for small molecules and peptides within a 2 cm separation length and a typical cycle time of only 30-90 s per sample. In addition, a limit of detection of 100 nM corresponding to a total amount of only 4.5 amol (per injection volume of 45 pL) and excellent quantitative linearity (R 2) 0.9999; 100 nM to 100 µM) were obtained in small-molecule analysis using verapamil as a test compound. The quantitative repeatability was proven good (8.5-21.4% relative standard deviation, peak area) also for the other drug substances and peptides tested. Electrospray ionization (ESI) 1-3 mass spectrometry (MS) provides a universal and highly sensitive detection tool, especially
Sheathless capillary electrophoresis-mass spectrometry using a pulsed electrospray ionization source
ELECTROPHORESIS, 2006
A sheathless interface has been developed for coupling CE with electrospray IT mass spectrometer. This interface utilized a pulsed ESI source. The use of a pulsed electrospray source allows the use of a sprayer with larger orifice, and thus alleviates the problem of column clogging during conductive coating and CE analysis. A pulsed ESI source operated at 20 Hz and 20% duty cycle was found to produce the optimal signals. For better signals, the maximum ion injection time in the IT mass spectrometer has to be set to a value close to the actual spraying time (10 ms). Using a sprayer with 50 mm od, more stable and enhanced signals were obtained in comparison with continuous CE-ESI-MS under the same flow rate (150 nL/min). The utility of this design is demonstrated with the analysis of synthetic drugs by CE-MS.
An LC/MS Method Providing Improved Sensitivity: Electrospray Ionization Inlet
Analytical chemistry, 2017
Electrospray ionization inlet (ESII) combines positive aspects of electrospray ionization (ESI) and solvent-assisted ionization (SAI). Similar to SAI, the analyte solution is directly introduced into a heated inlet tube linking atmospheric pressure and the initial vacuum stage of the mass spectrometer. However, unlike SAI, in ESII a voltage is applied to the solution through a metal union linking two sections of fused silica tubing through which solution flows into the inlet. Here, we demonstrate liquid chromatography (LC) ESII/MS on two different mass spectrometers using a mixture of drugs, a peptide standard mixture, and protein digests. This LC-ESII/MS approach has little dead volume and thus provides excellent chromatographic resolution at mobile phase flow rates from 1 to 55 μL min(-1). Significant improvement in ion abundance and less chemical background ions were observed relative to ESI for all drugs and peptides tested at flow rates from 15 to 55 μL min(-1). At a low inlet ...
Practical implications of some recent studies in electrospray ionization fundamentals
Mass Spectrometry Reviews, 2001
I.Introduction363II.The Mechanics of ESI-MS363III.Analyte Characteristics and Selectivity365 A. Charging the Analyte366 1. Ionization Through Charge Separation366 2. Adduct Formation366 3. Ionization Through Gas-Phase Reactions366 4. Ionization Through Electrochemical Oxidation or Reduction368 B. Analyte Surface Activity and Its Effect on ESI Response368 1. Surface Activity and the Fissioning Process369 2. Predicting ESI Response from Other Parameters369 C. The Role of Analyte pKa and Solvent pH370 D. Improving ESI Response Through Derivatization371IV.The Working Curve and Dynamic Range373 A. Detection Limits With ESI373 1. Background Interferences373 2. Random Noise374 3. Ion Transmission and Sensitivity374 B. Sources of Signal Saturation at High Concentrations375 1. Limited Amount of Excess Charge375 2. Limited Space on Droplet Surfaces375 3. Suppression and Competition at High Concentrations376 C. Improving the Detection Limit and Linear Dynamic Range376 1. Extending to Higher Concentrations376 2. Extending to Lower Concentrations376V.Instrumental Parameters and Stability377 A. Current–Voltage Curves377 B. Effect of Instrumental Parameters on the Current–Voltage Curve378 C. Self-Stabilizing Operation379 D. Non-Conductive vs. Conductive Spray Capillaries379VI.Solution Characteristics380 A. The Ideal ESI Solvent380 B. Solvent Choice for Analysis in the Positive Ion Mode381 C. Solvent Choice for Analysis in the Negative Ion Mode381 D. Compatibility Between ESI and Liquid Separation Techniques382VII.Summary382VIII.Acknowledgment383References383 In accomplishing successful electrospray ionization analyses, it is imperative to have an understanding of the effects of variables such as analyte structure, instrumental parameters, and solution composition. Here, we review some fundamental studies of the ESI process that are relevant to these issues. We discuss how analyte chargeability and surface activity are related to ESI response, and how accessible parameters such as nonpolar surface area and reversed phase HPLC retention time can be used to predict relative ESI response. Also presented is a description of how derivitizing agents can be used to maximize or enable ESI response by improving the chargeability or hydrophobicity of ESI analytes. Limiting factors in the ESI calibration curve are discussed. At high concentrations, these factors include droplet surface area and excess charge concentration, whereas at low concentrations ion transmission becomes an issue, and chemical interference can also be limiting. Stable and reproducible non-pneumatic ESI operation depends on the ability to balance a number of parameters, including applied voltage and solution surface tension, flow rate, and conductivity. We discuss how changing these parameters can shift the mode of ESI operation from stable to unstable, and how current–voltage curves can be used to characterize the mode of ESI operation. Finally, the characteristics of the ideal ESI solvent, including surface tension and conductivity requirements, are discussed. Analysis in the positive ion mode can be accomplished with acidified methanol/water solutions, but negative ion mode analysis necessitates special constituents that suppress corona discharge and facilitate the production of stable negative ions. © 2002 Wiley Periodicals, Inc., Mass Spec Rev 20: 362–387, 2001; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mas.10008I.Introduction363II.The Mechanics of ESI-MS363III.Analyte Characteristics and Selectivity365 A. Charging the Analyte366 1. Ionization Through Charge Separation366 2. Adduct Formation366 3. Ionization Through Gas-Phase Reactions366 4. Ionization Through Electrochemical Oxidation or Reduction368 B. Analyte Surface Activity and Its Effect on ESI Response368 1. Surface Activity and the Fissioning Process369 2. Predicting ESI Response from Other Parameters369 C. The Role of Analyte pKa and Solvent pH370 D. Improving ESI Response Through Derivatization371IV.The Working Curve and Dynamic Range373 A. Detection Limits With ESI373 1. Background Interferences373 2. Random Noise374 3. Ion Transmission and Sensitivity374 B. Sources of Signal Saturation at High Concentrations375 1. Limited Amount of Excess Charge375 2. Limited Space on Droplet Surfaces375 3. Suppression and Competition at High Concentrations376 C. Improving the Detection Limit and Linear Dynamic Range376 1. Extending to Higher Concentrations376 2. Extending to Lower Concentrations376V.Instrumental Parameters and Stability377 A. Current–Voltage Curves377 B. Effect of Instrumental Parameters on the Current–Voltage Curve378 C. Self-Stabilizing Operation379 D. Non-Conductive vs. Conductive Spray Capillaries379VI.Solution Characteristics380 A. The Ideal ESI Solvent380 B. Solvent Choice for Analysis in the Positive Ion Mode381 C. Solvent Choice for Analysis in the Negative Ion Mode381 D. Compatibility Between ESI and Liquid Separation Techniques382VII.Summary382VIII.Acknowledgment383References383
CHIMIA International Journal for Chemistry, 2009
The present work shows that the electrochemical properties of electrospray ionization (ESI) can be used to add functions to the process. As example, we show how the choice of the electrode material can be used to study interactions between metal ions and biomolecules in mass spectrometry (MS). In positive ionization MS, an electrospray device acts as anode, which implies oxidation reactions. Sacrificial electrodes (made of copper or zinc) are used to supply the electrospray current and to produce cations that are able to react on-line with compounds of interest. Thus, the interactions between copper ions and ligands or peptides were investigated by using a copper electrode. Another example is the in situ electrogeneration of a dinuclear zinc(ii) complex for the mass tagging of phosphopeptides when working with a zinc electrode. In order to perform these reactions on the same microchip, a dual-channel microsprayer was used, where one channel was dedicated to the tag electrogeneration and the other to the infusion of a phosphopeptides solution. Finally, this dual-channel microsprayer was used to study complexation at liquid-liquid interfaces in biphasic ESI-MS, such as thioether crowns and lead ions or peptides and phospholipids complexes. These examples illustrate the use of electrochemistry and on-chip reactions in ESI-MS analysis.
Electrophoresis, 1996
A miniaturized, integrated capillary electrophoresis-ultraviolet detection-electrospray ionization-mass spectrometry (CE-UV-ESI-MS) interface has been constructed and evaluated. The device incorporates a fiber optic detection cell close to the electrospray tip to allow UV monitoring of separated zones just prior to their admittance into the mass spectrometer. This configuration provides precise information about the time when UV-active zones enter the electrospray and allows easy location of analyte mass information in the ion current profile. The miniaturized dimensions of the interface allow the use of short capillaries for fast separations.