Droplet charging thermospray

Katta V, Rockwood AL, Vestal ML. Field limit for ion evaporation from charged thermospray droplets. Int J Mass Spectrom Ion Proc. 1991 103 129-48. 

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. 

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... 

With the thermospray interface (Figure 4.38(a)), the mobile phase, usually containing an ammonium ethanoate buffer, is passed through a heated probe (350-400°C) into an evacuated source chamber where it forms a supersonically expanding mist of electrically charged droplets. The liquid evaporates to leave charged solid particles which then release molecular ions such as MH+ and, VI by an ammonia chemical ionization (Cl) process. The analyte ions are skimmed off into the mass spectrometer whilst the vaporized solvent is pumped away. An electron beam is also employed to enhance the production of ions by Cl. 

In the thermospray interface, aqueous mobile phases containing an electrolyte such as ammonium acetate are passed at flow rates of 1-2 ml/min through a heated capillary prior entering a heated ion source. The end of the capillary lies opposite a vacuum line. Nebulization takes place as a result of the disruption of the liquid by the expanding vapor formed at the capillary wall upon evaporation of part of the liquid in the capillary. This results in formation of a supersonic jet of vapor containing a mist of fine, electrically charged droplets. 

Thermospray. The thermospray interface was introduced and developed by Blakley and Vestal [14], In their approach, a liquid flow from HPLC was directed through a resistively heated capillary connecting to the MS ion source. The heat and vacuum would evaporate the solvent from a supersonic beam of mobile phase produced in the spray, creating charged small microdroplets. These small liquid droplets were further vaporized in the heated ion source. Ions present in the ion source were then transferred to the mass analyzer, and residual vapors were pumped away. 

Nebulization ionization is the process involved in the analyte ionization in thermospray [16] and electrospray [17] interfacing. No primary ionization, i.e., a filament or a discharge electrode, is applied. The ionization mechanism is not fully understood (Ch. 6.3). The general understanding can be snmmarized as follows Upon nebulization, charged droplets of a few pm ID are generated. The fate of these droplets is determined by a nnmber of competing processes, the relative importance of which may dependent on the natnre of the analyte ... 

The thermospray (TSP) interface is widely used for the determination of drug residues in foods. Thermospray is typically used with reversed-phase columns and volatile buffers. Aqueous mobile phases containing an electrolyte, such as ammonium acetate, are passed through a heated capillary prior to entering a heated ion source. As the end of the capillary lies opposite a vacuum line, nebulization takes place and a jet of vapor containing a mist of electrically charged droplets is formed. As the droplets move through the hot source area, they continue to vaporize, and ions present in the eluent are ejected from the droplet and... 

Thermospray and, more recently, electrospray ionization have found wide application as an interface technology between HPLC instruments and mass spectrometers. They represent powerful techniques for the analysis of complex lipids directly from solutions (Henion and Lee, 1990 Murphy, 1993). In most instances, the total HPLC eluant can be sent directly into the heated thermospray ion source. Here, the combination of heat and eluant velocity creates a plume of small-diameter particles suspended in a vapor (nebulization). A strong electric charge forms on the surface of the liquid particles and as the droplets evaporate the increase in charge ionizes analyte molecules that are discharged directly from the droplet into the gas phase. From here, they may enter the mass spectrometer directly. 

Electrospray ionization is similar in effect to the thermospray technique and is useful for similar applications. The difference resides in the use of a high electric field to nebulize the sample solution (or sample and eluant), creating droplets with excess electric charge. As the droplet solvent evaporates during traverse of a desolvation chamber, charge transfers to the analyte molecules and these are released as gaseous ions. A further refinement in this technique is the use of electronic lenses to direct ions more efficiently into the mass spectrometer. Because the analyte is not subject to heating, there is also less possibility for thermal decomposition of complex lipid components. 

The ion formation in thermospray ionization, introduced in the early 1980s by Vestal and co-work-ers, was explained in terms of ion evaporation. Preformed analyte ions are evaporated from small, fast-evaporating, charged droplets generated by the thermospray nebulization process. Although the importance of ion evaporation in thermospray ionization is questioned, the emphasis put at it at the time certainly stimulated further investigation into liquid-based ionization approaches for MS. 

Electrospray was described in 1968 by Dole et al. [104] and the ions were detected by ion mobility. Fenn and coworkers [105], [106], and, independently, Alexandrov etal. [107] readopted the idea as a means of creating ions for mass spectrometry and since then rapid development of this technique has occurred. The fundamental difference to thermospray is that the spray is formed by charging droplets to such an extent that they explode by coulomb repulsion into smaller and smaller droplets. The droplets are further decreased in size by collisions with gas and, finally, highly charged ions are liberated. The process in which ions are created from the droplets is still not clear and several partial models exist [108]. 

Thermospray is a soft ionisation technique that relies on the production of a spray by direct heating of a metal capillary carrying the LC eluent. The presence of an electrolyte dissolved in the mobile phase causes this spray to consist of droplets carrying a statistical excess charge. Desolvation of these droplets results in the expulsion of ions, which are then extracted into the mass analyser. Thermospray is applicable to many polar materials but can give poor sensitivity for nonpolar species. Other... 

In the technique known as thermospray [108, 117, 118] volatile electrolytes dissolved in the eluant are nebulized out of a heated vaporizer tube. As a result of heating, the liquid is nebulized and partially vaporized. The capillary is heated under controlled conditions to avoid complete evaporation of the liquid in the capillary. Unvaporized solvent and sample are carried into the ion source as micro droplets or particles in a supersonic jet of vapour. By applying efficient pumping directly at the ion source, up to 2 mL/min of aqueous solvents can be introduced into the MS vacuum system. The ionization of volatile analytes takes place by means of ion-molecule reactions in the gas phase non-volatile samples may be ionized by direct ion evaporation processes from highly charged droplets... 

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