Electrospray Modifications for Advancing Mass Spectrometric Analysis

Abstract

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.

Keywords: atmospheric pressure; electrospray; ion source; ionization; mass spectrometry.

Fig. 1. Classification of electrospray-based ionization methods.

Fig. 2. Schematic illustrations of (A) paper spray ionization and (B) contactless tissue paper assisted spray ionization.

Fig. 3. Schematic representation of C-API MS, with sample delivery enabled by capillary action. A short tapered silica capillary [length, 1 cm; base o.d., 363 μm (or 323 μm without polyimide); tip o.d., 10 μm] was positioned vertically above an electrically isolated aluminum slide, with the outlet end placed orthogonal to the inlet of a metal capillary attached to the orifice of an ion trap mass spectrometer. The distance between the outlet of the silica capillary and the inlet of the metal capillary, attached to the MS orifice, was ∼1 mm. Before the measurements, the silica capillary was filled with a makeup solution [deionized water/acetonitrile (1 : 1, v∕v)] by means of capillary action. The inlet end of the silica capillary was then dipped into a droplet of a sample (10 μL) put onto the surface of the aluminum slide. The inset provides an illustration of the hypothetical mechanism of C-API.⁸⁴⁾ Reproduced with permission from ref. . Copyright (2011) American Chemical Society.

Fig. 4. Setup of the polarization induced ESI-MS. A cherry tomato was used as the sample loading substrate, where a sample droplet was deposited on the cherry tomato.⁸⁸⁾ Copyright (2015) John Wiley and Sons.

Fig. 5. Schematic representation of desorption electrospray ionization (DESI).

Fig. 6. Detailed illustration of the ELDI MS setup. (A) Sampling skimmer of a mass analyzer; (B) nitrogen laser beam; (C) electrospray capillary; (D) sample plate; (E) focusing lens; (F) reflecting lens; (G) syringe pump. Reprinted with permission from ref. . Copyright (2006) American Chemical Society.