Plasmaspray

The advent of atmospheric-pressure ionization (API) provided a method of ionizing labile and nonvolatile substances so that they could be examined by mass spectrometry. API has become strongly linked to HPLC as a basis for ionizing the eluant on its way into the mass spectrometer, although it is also used as a stand-alone inlet for introduction of samples. API is important in thermospray, plasmaspray, and electrospray ionization (see Chapters 8 and 11). 

For a more detailed description of the ionization process inherent in thermospray and plasmaspray please consult Chapter 9, Atmospheric Pressure Ionization. The reader should also compare thermospray with electrospray (see Chapter 8). 

In many applications in mass spectrometry (MS), the sample to be analyzed is present as a solution in a solvent, such as methanol or acetonitrile, or an aqueous one, as with body fluids. The solution may be an effluent from a liquid chromatography (LC) column. In any case, a solution flows into the front end of a mass spectrometer, but before it can provide a mass spectrum, the bulk of the solvent must be removed without losing the sample (solute). If the solvent is not removed, then its vaporization as it enters the ion source would produce a large increase in pressure and stop the spectrometer from working. At the same time that the solvent is removed, the dissolved sample must be retained so that its mass spectrum can be measured. There are several means of effecting this differentiation between carrier solvent and the solute of interest, and thermospray is just one of them. Plasmaspray is a variant of thermospray in which the basic method of solvent removal is the same, but the number of ions obtained is enhanced (see below). 

Schematic diagram of a thermospray ion. source. This source, of current design, also incorporates (a) a discharge electrode so that the source can be operated in plasmaspray mode and (h) a repeller electrode to induce fragmentation. The vaporizer itself can be used as a discharge electrode.

Most of the ions produced by either thermospray or plasmaspray (with or without the repeller electrode) tend to be very similar to those formed by straightforward chemical ionization with lots of protonated or cationated positive ions or negative ions lacking a hydrogen (see Chapter l).This is because, in the first part of the inlet, the ions continually collide with neutral molecules in the early part of their transit. During these collisions, the ions lose excess internal energy. 

In the latter, small quantities of emerging mixture components dissolved in elution solvent would be laborious to deal with if each component were to be first isolated by evaporation of solvent before its introduction into the mass spectrometer. In such circumstances, the direct introduction, removal of solvent, and ionization provided by plasmaspray or electrospray is a boon and puts LC/MS on a par with GC/MS for mixture analysis. Furthermore, GC is normally concerned with volatile, low-molecular-weight compounds and is of little or no use for the many polar, water-soluble, high-molecular-mass substances such as the peptides, proteins, carbohydrates, nucleotides,... 

By rapidly vaporizing a solution by heat, a spray is produced from which the solvent can be removed, leaving sample ions that pass straight into the analyzer region of a mass spectrometer. Plasmaspray is very similar, but ion yield is vastly improved through use of a corona discharge. 

The nebulization and evaporation processes used for the particle-beam interface have closely similar parallels with atmospheric-pressure ionization (API), thermospray (TS), plasmaspray (PS), and electrospray (ES) combined inlet/ionization systems (see Chapters 8, 9, and 11). In all of these systems, a stream of liquid, usually but not necessarily from an HPLC column, is first nebulized... 

Aerosols can be produced as a spray of droplets by various means. A good example of a nebulizer is the common household hair spray, which produces fine droplets of a solution of hair lacquer by using a gas to blow the lacquer solution through a fine nozzle so that it emerges as a spray of small droplets. In use, the droplets strike the hair and settle, and the solvent evaporates to leave behind the nonvolatile lacquer. For mass spectrometry, a spray of a solution of analyte can be produced similarly or by a wide variety of other methods, many of which are discussed here. Chapters 8 ( Electrospray Ionization ) and 11 ( Thermospray and Plasmaspray Interfaces ) also contain details of droplet evaporation and formation of ions that are relevant to the discussion in this chapter. Aerosols are also produced by laser ablation for more information on this topic, see Chapters 17 and 18. 

This method is still in use but is not described in this book because it has been superseded by more recent developments, such as particle beam and electrospray. These newer techniques have no moving parts, are quite robust, and can handle a wide variety of compound types. Chapters 8 through 13 describe these newer ionization techniques, including electrospray, atmospheric pressure ionization, plasmaspray, thermospray, dynamic fast-atom bombardment (FAB), and particle beam. 

Finally, note that the ions produced by the combined inlet and ion sources, such as electrospray, plasmaspray, and dynamic FAB, are normally molecular or quasi-molecular ions, and there is little or none of the fragmentation that is so useful for structural work and for identifying compounds through a library search. While production of only a single type of molecular ion may be useful for obtaining the relative molecular mass of a substance or for revealing the complexity of a mixture, it is often not useful when identification needs to be done, as with most general analyses. Therefore,... 

To achieve sufficient vapor pressure for El and Cl, a nonvolatile liquid will have to be heated strongly, but this heating may lead to its thermal degradation. If thermal instability is a problem, then inlet/ionization systems need to be considered, since these do not require prevolatilization of the sample before mass spectrometric analysis. This problem has led to the development of inlet/ionization systems that can operate at atmospheric pressure and ambient temperatures. Successive developments have led to the introduction of techniques such as fast-atom bombardment (FAB), fast-ion bombardment (FIB), dynamic FAB, thermospray, plasmaspray, electrospray, and APCI. Only the last two techniques are in common use. Further aspects of liquids in their role as solvents for samples are considered below. 

Plasmaspray, or discharge-assisted thermospray, is a modification of thermospray in which the degree of ionization has been enhanced. 

As the name implies, thermospray uses heat to produce a spray of fine droplets. Plasmaspray does not produce the spray by using a plasma but, rather, the droplets are produced in a thermospray source and a plasma or corona is used afterward to increase the number of ions produced. 

To increase the number of ions, a plasma or corona discharge is produced in the mist issuing from the capillary. The electrical discharge induces more ionization in the neutrals accompanying the few thermospray ions. This enhancement increases the ionization of sample molecules and makes the technique much more sensitive to distinguish it from simple thermospray, it is called plasmaspray. 

Ions formed by thermo- or plasmaspray are extracted through a small hole into the mass spectrometer analyzer, where a mass spectrum of the original dissolved sample is obtained. 

Thermospray and plasmaspray can be used with both sector and quadrupole instruments. They have been used extensively to couple liquid chromatographs to mass spectrometers. 

Dynamic FAB is an interface between a liquid chromatograph and a mass spectrometer and is, at the same time, an ion source. As an inlet/ion source, this technique fulfils a similar function to plasmaspray and electrospray, both of which are combined inlet/ion sources. 

LC can be combined with all kinds of mass spectrometers, but for practical reasons only quadrapolar, magnetic/electric-sector, and TOP instruments are in wide use. A variety of interfaces are used, including thermospray, plasmaspray, electrospray, dynamic fast-atom bombardment (FAB), particle beam, and moving belt. 

AIR. (atmospheric) air, a standard for nitrogen and chlorine isotopes APCL atmospheric-pressure chemical ionization, also called plasmaspray API. atmospheric-pressure ionization... 

Fast atom bombardment (FAB) Plasma desorption (PD) Liquid secondary-ion mass spectrometry (LSIMS) Thermospray (TSP)/plasmaspray (PSP) Electrohydrodynamic ionisation (EHI) Multiphoton ionisation (MPI) Atmospheric pressure chemical ionisation (APCI) Electrospray ionisation (ESI) Ion spray (ISP) Matrix-assisted laser desorption/ionisation (MALDI) Atmospheric pressure photoionisation (APPI) Triple quadrupole (QQQ) Four sector (EBEB) Hybrid (EBQQ) Hybrid (EB-ToF, Q-ToF) Tandem ToF-ToF Photomultiplier... 

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... 

Application of HPLC-MS to the analysis of a black tea liquor was studied in a paper by Bailey 39 a great deal of useful information could be obtained without sample pretreatment. A tea liquor was applied to a wide-pore HPLC column connected to a mass spectrometer by a VG Plasmaspray interface. Pseudo-molecular ions were obtained from the flavanols, flavanol gallates, chlorogenic acids, 4-coumarylquinic acids, and caffeine, but the flavanol glycosides were extensively fragmented by the interface. Fragments were obtained from unresolved polymer that supported its previous designation as a flavanol polymer. 

Compound 87 in acid-hydrolyzed urine serves as a tracer for occupational exposure to the corresponding diisocyanate. It was derivatized with pentafluoropropionic anhydride and determined by LC using TSP-MS and plasmaspray (PSP) MS (discharge-assisted TSP-MS). The [M — 2](—) ion was measured instrumental LOD 0.1 pg/pL LOD about 0.2 -ig/L urine, RSD 10% for 0.5 pg/L230. Another determination of 87 in urine is with isobutyl chloroformate231. 

A modification/enhancement of the TSP is the plasmaspray interface for discharge ionization which has also been commercialized. A TSP system can be operated in three different modes (a) filament-off mode, i.e., TSP ionization mode, (b) the filament-on mode, and (c) the discharge ionization mode. 

A Valeur, P Michelsen, G Odham. On-line straight phase liquid chromatography/plasmaspray tandem mass spectrometry of glycerolipids. Lipids 28 255-259, 1993. 

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