Emitter electrode

These thin wires are supported on a special carrier that can be inserted into the ion source of the mass spectrometer after first growing the whiskers in a separate apparatus. Although the wires are very fragile, they last for some time and are easily renewed. They are often referred to as emitter electrodes (ion emitters). 

For nonvolatile or thermally labile samples, a solution of the substance to be examined is applied to the emitter electrode by means of a microsyringe outside the ion source. After evaporation of the solvent, the emitter is put into the ion source and the ionizing voltage is applied. By this means, thermally labile substances, such as peptides, sugars, nucleosides, and so on, can be examined easily and provide excellent molecular mass information. Although still FI, this last ionization is referred to specifically as field desorption (FD). A comparison of FI and FD spectra of D-glucose is shown in Figure 5.6. 

Sometimes, in FD, the emitter electrode is heated gently either directly by an electrode current or indirectly by a radiant heat source to aid desorption of ions from its surface. 

Transistors, which control the current through a junction of semiconductor materials by a voltage signal from an emitter electrode. 

Debiasing of emitter-base pn junction can be minimized by using a double metal process. A simplified cross section of a 4H-SiC power BJT with double metal process is shown in Figure 6.11. In this structure, the emitter electrode covers most of the active area and is connected to emitter fingers through vias, whereas the base electrode is placed outside of the active area. Use of this structure eliminates most of the resistive voltage drop in the emitter fingers at an increased cost of the fabrication process. 

Field ionization occurs when gas-phase sample molecules are inteijected in a strong electrical field that is on the order of 10 Vcm The field distorts the electron cloud around the sample molecule and lowers the barrier for the removal of an electron. The quantum mechanical tunneling of this electron from the molecule to the conduction bands of the emitter produces M+ ions [10]. The heart of the FI ion source is an emitter electrode made fi om a sharp metal object such as a razor blade or thin wire. The emitter electrode is placed approximately 1 mm away from the cathode. The field is produced by applying a high potential (10 to 20 kV) to the tip of the emitter electrode. FI is a very soft ionization technique that produces primarily a molecular ion signal. It is applicable to volatile samples only. 

Bipolar transistors are realized using either an npn- or pnp-junction sequence. The different segments of the device are named as collector, base, and emitter electrode, respectively. In order to operate the transistor, one of the junctions is forward biased, while the other is biased in reverse. Using a small control current over the base electrode, a significant current between the collector and emitter electrodes is enabled. 

Comparison of Tubular, Planar Flow-By, and Porous Flow-Through Emitter Electrodes... 

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.)...

Direct evidence for the involvement of solution species in these redox reactions was reported at about this same time by our group. We found that under certain solution conditions, the molecular radical cations (M ) of some divalent metal porphyrins (e.g., Ni octaethylporphyrin (NiOEP), ZnOEP, and VOOEP) formed by this electrochemical process could be observed in positive-ion ES mass spectra.Certain other easy-to-oxidize species like polyaromatic hydrocarbons (PAHs), aromatic amines, and heteroaromatics were also oxidized at the emitter electrode and observed as cationic radicals. Molecular ions formed by loss of an electron had not been observed in ES mass spectra prior to those reports. Our work served to illustrate that analyte species, under the appropriate operational conditions, could be directly involved in the redox reactions in the metal spray capillary and that the products of their reactions could be observed in the gas phase. 

Figure 3,2. Diagrams of the electrical circuit for ES emitter systems of the (a) grounded and (b) floated emitter type. These geometries contain only the (downstream) ES circuit (with nominal resistance resulting in ES current /es) between the emitter and the mass spectrometer, (c) Electrical circuit of a floated emitter system with the typical commercial configuration incmporating an upstream grounded contact in the solution stream. In addition to the downstream ES circuit, this geometry includes an external upstream current loop (with resistance / ext resulting in current /ext) between the upstream grounding point and the emitter electrode. The total current at the emitter electrode (/-rcrr, equivalent to the faradaic current at the emitter electrode) is the sum of /es <1 /ext-...

As described by Konermann et al., the magnitude of /ext depends on the conductance, G = 1// ext, of the connection between the upstream ground point and the ES emitter electrode and the voltage applied to the ES capillary (i.e., /ext = I es/Rext)- Rext, in turn, depends on the solution conductivity, 2° Ce (2° is the limiting molar conductivity of electrolyte, Ce is the concentration of electrolyte), and the length, L, and cross-sectional area. A, of the tubing connection between the electrodes (i.e., /ext = Ves where... 

The extent of the one or more electrode reactions that occur at the ES emitter electrode, and, in part, the resulting solution compositional change is determined by the magnitude of the... 

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.

In general then, one would expect that the occurrence of redox reactions of the solvent or any other species sufficient to supply all the required current will effectively maintain or redox buffer " the interfacial potential of the emitter electrode at or near the eP values for those particular reactions. The interfacial potential in the emitter is expected to be only that magnitude necessary to supply the required current for a given availability of material for reaction (see Section 3.2.4). Redox buffering exploiting emitter electrode... 

Table 3.1. Standard Potentials for Some of the Reactions Anticipated to Occur at a Stainless Steel or Platinum Capillary ES Emitter Electrode when Using Typical ES-MS Solvent Systems and Solution Additives...

Emitter Electrode Reactions Emitter Electrode Reactions ... 

It is also important to consider that the emitter electrode material may participate in electrochemical reactions (e.g., anodic corrosion of iron in a stainless steel emitter). Table 3.2 lists the standard electrode potentials for some of the possible oxidation processes of metals used as ES emitters. The potential at which oxidation of a metal may take place strongly depends on the solution composition and follow-up reactions (e.g., precipitation, complexation, etc.) as well. As an example, the standard electrode potential for the reversible Ag/Ag couple is 0.80 V vs. SHE. However, at pH 14, Ag20 forms during the... 

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