Thermospray interface droplet charging

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. 

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... 

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. 

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