Electrospray process

Two mechanisms for electrospray process have been proposed  

Charged residue mechanism. This was proposed by Dole and suggests that after multiple coloumbic explosions droplets are formed, which contain only one ion [10]. This mechanism is thought to be important for the ionisation of macromolecules. 

Iribame and Thompson proposed the ion-evaporation mechanism. This suggests desorption of the ions from the droplets when they are less than 10 nm in size [14]. This is thought be the dominant mechanism for small molecules. 

What is certain about the process is that from the time the initial (neutral) solution is passed through the capillary to the detection of the ions in the mass spectrometer a complex series of reactions has taken place. 

The early ESI interfaces were all optimised for flow rates between 1 and 10 fil/min. In trying to achieve direct compatibility with analytical HPLC, much development work has been done to accommodate higher flow rates and increase the efficiency of the nebulisation process. The present pneumatically assisted ESI interface is optimised around flow rates of 50-300 [lEmin. It is not the intention here to describe all the different manufacturers interfaces and source designs for API but the technology has been well documented [15]. 

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The lack of a definitive explanation, however, does not affect our ability to appreciate the analytical capabilities of the technique, the HPLC characteristics that will affect the production of ions by the electrospray process and the mass spectra that may be obtained. 

The flow rate of liquid in the HPLC-electrospray system is paramount in determining performance both from chromatographic and mass spectrometric perspectives. The flow rate affects both the size and size distribution of the droplets formed during the electrospray process (not all droplets are the same size) and, consequently, the number of charges on each droplet. This, as we will see later, has an effect on the appearance of the mass spectrum which is generated. It should also be noted that the smaller the diameter of the spraying capillary, then... 

There are a number of properties of the solvent, such as its viscosity, conductivity, surface tension and polarity, that have an effect on the electrospray process. 

The electrospray process is susceptible to competition/suppression effects. All polar/ionic species in the solution being sprayed, whether derived from the analyte or not, e.g. buffer, additives, etc., are potentially capable of being ionized. The best analytical sensitivity will therefore be obtained from a solution containing a single analyte, when competition is not possible, at the lowest flow rate (see Section 4.7.1 above) and with the narrowest diameter electrospray capillary. 

Figure 11.2 A schematic of the electrospray process, showing the release of charged droplets from the Taylor cone and the Z-spray arrangement with respect to the sample inlet, sample cone, and the subsequent path of the ions into the analyzer.

Hydrophilic interaction liquid chromatography (HILIC) can handle the matrix effects nonvolatiles, which may reduce the evaporation of volatile ions during the electrospray process. Ion-exchange methods that require the use of salts could otherwise interfere with MS. 

Met-0-Me /(R,R,R,R)-1 in the FAB spectra reduces to ca. 1.5 1, if ESI is used to generate the ionic complexes. This observation appears to be general for all host-guest pairs studied and it was attributed to the electrospray process, although no real... 

The pure electrospray process of dispersing a liquid into an aerosol works best at flow rates of 1-20 pi min" Conventional unassisted ESI has also limitations as a LC-MS interface due to the solvent properties in terms of volatility and polarity which can be electrosprayed without some type of assistance. Therefore, a number of sprayer modifications including a heated sprayer [55] have been developed to expand the range of ESI applications (Fig. 11.4). 

Fig. 1.9 Pneumatically assisted electrospray. The coaxial nitrogen gas assists the electrospray process allowing to operate at flow rates of several hundred microliters.

Non-volatile buffers such as phosphates, borates, perchlorates and phosphoric acid should be avoided at all costs because of high background ion current, source contamination and blockages, and in the case of perchlorates, explosions. Figure 6.4 shows the mass spectrum of typical background when using phosphoric acid in the eluent. If the solvent system for a particular analysis does not assist the electrospray process, it is possible to enhance ionisation by postcolumn addition of a suitable volatile buffer. 

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. 

The electrospray process consists of feeding a liquid through a metal capillary which is maintained at a high electrical potential with respect to some nearby surface. As the liquid reaches the capillary tip, the liquid is dispersed into fine electrified droplets by the action of the electric field at the capillary tip. If the liquid is volatile, as the liquid evaporates the droplets shrink in size, become electrically unstable, and break down into smaller size droplets. This process has been experimentally demonstrated by Doyle, Moffett, and Vonnegut (10) and by Abbas and Latham (11). If the liquid contains macromolecules, after the solvent has evaporated completely the macromolecules are left as electrically charged particles in the gas phase, that is, as gaseous macroions. 

Although numerous attempts to provide theoretical explanations of the electrospray process have been made (see, for example, ref. 12-17), a good quantitative theory of the phenomenon would require simultaneous solutions of the hydrodynamic and electrostatic differential equations, and, to our knowledge, no such theory has yet been proffered. However, experimental observations have provided some insight into the process. 

All of the work with macroions reported to date has been done using mixtures of either ethanol and water or acetone and benzene. Further investigations into the electrospray process should permit optimization of the system and production of a wide variety of solvent mixtures suitable for use in the electrospray process. 

After the macroions are produced by the electrospray process, they are injected into a vacuum by use of a nozzle-beam system of the type first suggested by Kantrowitz and Grey (20) and later modified by Becker and Bier (21). Articles describing in detail the principles of operation of such a system have been published, among them being articles by Anderson, Andres, and Fenn (22, 23). 

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

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