Practical implications of some recent studies in electrospray ionization fundamentals
✍ Scribed by Nadja B. Cech; Christie G. Enke
- Publisher
- John Wiley and Sons
- Year
- 2001
- Tongue
- English
- Weight
- 417 KB
- Volume
- 20
- Category
- Article
- ISSN
- 0277-7037
No coin nor oath required. For personal study only.
✦ Synopsis
Abstract
| I. | Introduction | 363 |
| II. | The Mechanics of ESI‐MS | 363 |
| III. | Analyte Characteristics and Selectivity | 365 |
| | A. Charging the Analyte | 366 |
| | 1. Ionization Through Charge Separation | 366 |
| | 2. Adduct Formation | 366 |
| | 3. Ionization Through Gas‐Phase Reactions | 366 |
| | 4. Ionization Through Electrochemical Oxidation or Reduction | 368 |
| | B. Analyte Surface Activity and Its Effect on ESI Response | 368 |
| | 1. Surface Activity and the Fissioning Process | 369 |
| | 2. Predicting ESI Response from Other Parameters | 369 |
| | C. The Role of Analyte p__K~a~__ and Solvent pH | 370 |
| | D. Improving ESI Response Through Derivatization | 371 |
| IV. | The Working Curve and Dynamic Range | 373 |
| | A. Detection Limits With ESI | 373 |
| | 1. Background Interferences | 373 |
| | 2. Random Noise | 374 |
| | 3. Ion Transmission and Sensitivity | 374 |
| | B. Sources of Signal Saturation at High Concentrations | 375 |
| | 1. Limited Amount of Excess Charge | 375 |
| | 2. Limited Space on Droplet Surfaces | 375 |
| | 3. Suppression and Competition at High Concentrations | 376 |
| | C. Improving the Detection Limit and Linear Dynamic Range | 376 |
| | 1. Extending to Higher Concentrations | 376 |
| | 2. Extending to Lower Concentrations | 376 |
| V. | Instrumental Parameters and Stability | 377 |
| | A. Current–Voltage Curves | 377 |
| | B. Effect of Instrumental Parameters on the Current–Voltage Curve | 378 |
| | C. Self‐Stabilizing Operation | 379 |
| | D. Non‐Conductive vs. Conductive Spray Capillaries | 379 |
| VI. | Solution Characteristics | 380 |
| | A. The Ideal ESI Solvent | 380 |
| | B. Solvent Choice for Analysis in the Positive Ion Mode | 381 |
| | C. Solvent Choice for Analysis in the Negative Ion Mode | 381 |
| | D. Compatibility Between ESI and Liquid Separation Techniques | 382 |
| VII. | Summary | 382 |
| VIII. | Acknowledgment | 383 |
| References | 383 |
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.10008