Thermospray interface discharge electrode

A group of techniques employing differential selection of solute ions relies on nebulisation and ionisation of the eluent, with some discrimination of ion selection in favour of the solute. Main representatives are APCI [544] and thermospray [545]. In a thermospray interface a supersonic jet of vapour and small droplets is generated out of a heated vaporiser tube. Controlled, partial vaporisation of the HPLC solvent occurs before it enters the ion source. Ionisation of nonvolatile analytes takes place by means of solvent-mediated Cl reactions and ion evaporation processes. Most thermospray sources are fitted with a discharge electrode. When this is used, the technique is called plasmaspray (PSP) or discharge-assisted thermospray. In practice, many... 

In thermospray interfaces, the column effluent is rapidly heated in a narrow bore capillary to allow partial evaporation of the solvent. Ionisation occurs by ion-evaporation or solvent-mediated chemical ionisation initiated by electrons from a heated filament or discharge electrode. In the particle beam interface the column effluent is pneumatically nebulised in an atmospheric pressure desolvation chamber this is connected to a momentum separator where the analyte is transferred to the MS ion source and solvent molecules are pumped away. Magi and Ianni (1998) used LC-MS with a particle beam interface for the determination of tributyl tin in the marine environment. Florencio et al. (1997) compared a wide range of mass spectrometry techniques including ICP-MS for the identification of arsenic species in estuarine waters. Applications of HPLC-MS for speciation studies are given in Table 4.3. 

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 option of high flow rates, combined with TSP ionisation, helped to improve the sensitivity because of the quantitative transfer of analytes in the column effluents into the mass spectrometer. High flow rates under reversed-phase conditions (RP) as well as normal-phase separations (NP) were amenable to this interface type. Thermospray ionisation takes place by means of a solvent-mediated chemical ionisation (Cl) process, where a filament or discharge electrode is employed, or by an ionisation process which is enabled, and supported by a volatile buffer such as, for example, ammonium acetate, that is added to the eluent to improve positive ionisation. 

In a thermospray interface (Figure 3B), the column effluent is rapidly heated in a narrow bore capillary such that partial (ca. 90%) evaporation of the solvent is achieved inside the capillary. As a result, a mist of vapour and small droplets is formed in which the heated droplets further evaporate and ions are generated, either by the thermospray ionization process based on ion evaporation or by solvent-mediated chemical ionization initiated by electrons from a heated filament or a discharge electrode. The excess vapour is pumped away directly from the ion source. 

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