Charged droplets

Evaporation from a spray of charged droplets produced from a stream of liquid yields ions that can be analyzed in a mass spectrometer. Thermally labile and normally nonvolatile substances such as sugars, peptides, and proteins can be examined successfully. 

A solution of an analyte in a solvent can be sprayed (nebulized) from an electrically charged narrow tube to give small electrically charged droplets that desorb solvent molecules to leave ions of the analyte. This atmospheric-pressure ionization is known in various forms, the one most relevant to this section being called electrospray. For additional detail, see Chapters 8, 9, and 11. 

The electric field and gas flow work together to provide a finely dispersed spray of charged droplets. An electric potential of about 3-5 kV is used for capillaries of about 50-100-pm diameter. 

Because the electrically charged droplets retain their charge but get smaller, their electric field increases. At some point, mutual repulsion between like charges causes charged particles (ions) to leave the surface of the droplet (ion evaporation). These ions can be detected by the mass spectrometer. 

Evaporation of solvent from a spray of electrically charged droplets at atmospheric pressure eventually yields ions that can collide with neutral solvent molecules. The assemblage of ions formed by evaporation and collision is injected into the mass spectrometer for mass analysis. 

An AC corona discharge in the throat leads to a cloud of charged droplets whose large momentum allows very long travel distances. Multiple devices for eliminating static discharges in powder silos have been tested [41] but the tests did not address typical flow rates for large capacity, dense phase pneumatic transfer operations. 

Electrospray ionization mass spectrometry (ESI-MS) is an analytical method for mass determination of ionized molecules. It is a commonly used method for soft ionization of peptides and proteins in quadmpole, ion-trap, or time-of-flight mass spectrometers. The ionization is performed by application of a high voltage to a stream of liquid emitted from a capillaty. The highly charged droplets are shrunk and the resulting peptide or protein ions are sampled and separated by the mass spectrometer. 

In trne thermospray, charging of the droplets is dne to the presence of a bnffer in the mobile phase. Both positively and negatively charged droplets are formed dne to the statistical flnctnation in anion and cation density occnrring when the liqnid stream is disrnpted. As with the interfaces previonsly described, involatile bnffers are not recommended as blocking of the capillary is more likely to occnr if temperatnre control is not carefnlly monitored and for this reason ammoninm acetate is often nsed. 

Schematic view of Miiiikan s oii drop experiment. An atomizer generated a fine mist of oil droplets (yellow circles). Bombarding the dropiets with X rays gave some of them extra negative charge orange circle). In the presence of sufficient eiectricai force, these negativeiy charged droplets could be suspended in space. ...

Principles and Characteristics Thermospray ionisation (TSP) involves introduction of a relatively high flow (0.2-2mLmin ) of solvent into the ion source of a mass spectrometer, and is therefore suitable as an interface for HPLC-MS, using standard bore columns. A vaporiser probe (essentially a resistively heated capillary tube of about 100 xm i.d.) acts as a transfer line for taking solvent and solute into the source. The source is heated to prevent condensation of the solvent, and the temperature of the capillary is chosen so as to ensure vaporisation of the solvent. In this way, a vapour jet is generated, which contains small, electrically charged droplets if the solvent is at least partially aqueous and... 

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.

Irbarne, J. V. Thomson, B. A. Evaporation of small ions from charge droplets. J. Chem. Phys. 1976, 64, 2287-2294. 

C. Dependence of Charged Droplet Current /, Droplet Radius R,... 

D. Evolution of Initial Charged Droplets to Very Small... 

Electrospray (ES) existed long before its application to mass spectrometry (MS). It is a method of considerable importance for the electrostatic dispersion of liquids and creation of aerosols. The interesting history and notable research advances in that field are very well described in Bailey s book Electrostatic Spraying of Liquids. 37 Much of the theory concerning the mechanism of the charged droplet formation was developed by researchers in this area. The latest works can be found in a special issue38 of the Journal of Aerosol Science devoted to ES. 

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