Thermospray

Thermospray ionization is effected directly from a sample solution and may be readily interfaced with HPLC. The thermospray ion source, which was developed from direct liquid introduction interfaces, can accommodate a wide range of liquid flows (0.5-2.5 mL min"1) but is limited to the use of volatile buffers. It is a soft ionization technique and produces mass spectra dominated by ions yielding RMM information, i.e. (M + H) + /(M - H)_, but modifications to the source have allowed a certain amount of controlled fragmentation to be induced.

The sample solution is carried into the source via a capillary tube which terminates in a heated block. This results in the formation of a supersonic jet of vapour which contains charged droplets, the charging of the droplets being aided by the presence of a volatile electrolyte (e.g. ammonium acetate). By a combination of ion evaporation and ion-molecule reactions, sample ions are formed and exit the source via a small sampling orifice. The excess solvent vapours are removed by a backing rotary pump. In those situations where it is not possible or desirable to add a volatile electrolyte to the mobile phase, ionization may be effected by a mechanism akin to CI. This is achieved either by use of an electron beam (often termed filament on) or by creating a plasma within the vapour-rich source, usually by a high voltage discharge from a needle - a technique sometimes referred to as a plasmaspray. A simple schematic of a thermospray source is shown in Figure 3.

A wide variety of compounds are amenable to thermospray but its ability to cope with large, nonvolatile molecules is poor and the mass range appears to be limited for routine use to compounds below & 1500 Da. The operating temperatures of the source must satisfy the requirement of efficiently vaporizing the sample but without thermal degradation. Positive and negative ions are often formed with equal facility but, in general, basic compounds are best studied in the positive ion mode: negative ion operation is more sensitive for acidic molecules. Lack of knowledge with respect to proton affinities often means that the best ionization mode needs to be determined experimentally.

Figure 3 Simplified diagram of a thermospray ion source.

Lack of fragmentation is often observed in thermospray mass spectra but application of a higher voltage to a repeller electrode (located opposite the sampling cone) may be sufficient to induce the formation of fragment ions.

Criticisms of the technique have centred on the claims of poor reproducibility and compound dependence. The performance and optimization of the interface depend on the solution chemistry and solutions must be kept free of particulate matter that may lead to blockage of the capillary. Despite some disadvantages, many applications of thermospray have been described and it has been the mainstay for LC-MS development for a number of years. However, recent advances in alternative ionization methods may well see it superseded as the method of choice for LC-MS.

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