Electrospray electrochemical process

J. Fernandes de la Mora, G.J. Van Berkel, C.G. Enke, R.B. Cole, M. Martinez-Sanchez and J.B. Fenn, Electrochemical processes in electrospray ionization mass spectrometry, J. Mass Spectrom., 35 (2000) 939-952. 

Figure 3.3. Theoretical solid line plots of the concentration of the electrochemical product, [EP], added to or removed from the solution sprayed via the electrochemical processes in the electrospray emitter as a function of flow rate through the emitter. Plots were calculated using Eq. (3.1), assuming that only one electrochemical reaction,7, occurred in which 1. Actual experimental currents measured at flow rates from l.OpL/min to lOOOmL/min using a pneumatically assisted floated ES source with and without an upstream ground are also plotted black circles represent es, white circles represent /ext> triangles represent /es + Iext- Solution composition was acetonitrile/water (1/1 v/v), 5 mM ammonium acetate, 0.75% by volume acetic acid (pH 4). Voltage drop between the ES emitter electrode and counter electrode of mass spectrometer is 4 kV.

Van Berkel, G. J. Asano, K. G. Schnier, P. D. Electrochemical processes in a wire-in-a-capillary bulk-loaded, nano-electrospray emitter. J. Am. Soc. Mass Spectrom. 2001, 12, 853-862. 

Part I of the book is dedicated to explaining fundamental aspects of the electrospray process. First, the detailed fundamentals of electrospray are considered from a mechanistic viewpoint (Chapter 1). Special attention is then given to the root causes of the observed selectivity of ionization in electrospray (Chapter 2). Inherent to electrospray ionization sources are electrochemical processes that are explained in detail in Chapter 3. The ES fundamentals section is completed with a comparative inventory of source hardware (Chapter 4). 

The ion formation may occur in the bulk solution before the electrospray process takes place or in the gas phase by protonation or salt adduct formation, or by an electrochemical redox reaction. Polar compounds already exist in solution as ions therefore, the task of the electrospray is to separate them from their counterions. This is the case of many inorganic and organic species and all those compounds that show acidic or basic properties. Proteins, peptides, nucleotides, and many other bio- and pharmaceutical analytes are typical examples of substances that can be detected as proto-nated or deprotonated species. 

S. Liu, W. J. Griffiths, and J. Sjovall, On-Column Electrochemical Reactions Accompanying the Electrospray Process, AnaL Chem. 2003, 75, 1022. 

Liu, S., Griffiths, W. J., and Sjovall, J. (2003). On-column electrochemical reactions accompanying the electrospray process. Anal. Chem. 75 1022-1030. 

Xie et al. [20] reported the fabrication chip for pumps and an electrospray nozzle. The process used to fabricate the electrochemical pump chips with electrospray nozzle is shown in Fig. 2.11. A 1.5 xm layer of Si02 was grown on the surface of a 4 inch silicon wafer by thermal oxidation. The front side oxide layer was patterned and removed with buffered FIF. XeF2 gaseous etching was used to roughen the silicon surface in order to promote the adhesion between subsequent layers and the substrate. The first 4.5 p,m parylene layer was deposited. 

Figure 3.1. Schematic representation of the processes that occur in electrospray in positive ion mode. The imposed electric field between the emitter electrode and counter electrode leads to a partial separation of positive from negative ions present in solution at the meniscus of the solution at the metal capillary tip. This net charge is pulled downfield, expanding the meniscus into a cone that emits a fine mist of positively charged droplets. The droplets carry off an excess of positive ions. Solvent evaporation reduces the volume of the droplets at constant charge, leading to fission of the droplets. Continued production of charged droplets requires an electrochemical oxidation at the emitter electrode-solution interface—that is, a conversion of ions to electrons. Electrochemical reduction is required to be the dominate process in negative ion mode. (Adapted from the original figure in Ref. 26.)...

Considering the requirements for charge balance in such a continuous electric current device and the fact that only electrons can flow through the metal wire supplying the electric potential to the electrodes, one comes to the conclusion that the electrophoretic charge separation mechanism [of droplet charging and formation] requires that the [positive-ion] electrospray process should involve an electrochemical conversion of ions to electrons [within the metal ES capillary[. 

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