Thermospray ionisation

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 

Solvents used successfully in TSP operation include water, methanol, acetonitrile, propan-2-ol, dichloro-methane and hexane. Any volatile buffer may be employed as an electrolyte, but involatile buffers and inorganic acids such as phosphate salts are to be avoided. This poses some limits on the analysis. 

Thermospray ionisation sources are usually outfitted with a quadrupole or magnetic sector mass spectrometer (including hybrids or tandem forms). Thermospray operation allows a reversed-phase solvent system, e.g. a 50 50 (v/v) water-methanol or acetonitrile mix containing 0.1 M ammonium acetate. This ensures compatibility with the universal HPLC procedures available in many industrial research laboratories. 

The TSP temperature needs to be high enough to prevent condensation of solvent on the source (typically 200-300 °C). Too high a temperature can lead to the thermal degradation of labile compounds. The vaporiser temperature, which is generally set at 

150-250 °C, is one of the most critical parameters in TSP operation, and should be optimised for different samples, wherever possible. This is considered to be a considerable drawback in routine operation of unknown polymer/additive extracts. Too low a vaporiser temperature results in the solute and solvent spraying into the ionisation source in their liquid form, without formation of gas-phase ions. Too high a vaporiser temperature causes premature evaporation of the solute and solvent before the outlet of the capillary is reached. This causes an unstable, pulsing ion beam. As ion formation in TSP operation depends very critically on the extent of desolvation and the energy of the nebulised droplets, it is clear that an inappropriate vaporiser temperature will cause loss of sensitivity. 

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Figure 6.10 Production of positive ions by thermospray ionisation. After Ashcroft [35]. From A.E. Ashcroft, Ionization Methods in Organic Mass Spectrometry, The Royal Society of Chemistry, Cambridge (1997). Reproduced by permission of The Royal Society of Chemistry...

Applications Various surfactant types (ABS, AES, secondary alkane sulfonates, and alkylphenol ethoxy-sulfates) have been analysed by means of a QQQ using a thermospray source [89]. Other applications of hyphenated thermospray ionisation mass-spectrometric techniques (LC-TSP-MS) are described elsewhere (Section 73.3.2). 

Many excellent reviews on the development, instrumentation and applications of LC-MS can be found in the literature [560-563]. Niessen [440] has recently reviewed interface technology and application of mass analysers in LC-MS. Column selection and operating conditions for LC-MS have been reviewed [564]. A guide to LC-MS has recently appeared [565]. Voress [535] has described electrospray instrumentation, Niessen [562] reviewed API, and others [566,567] have reviewed LC-PB-MS. For thermospray ionisation in MS, see refs [568,569]. Nielen and Buytenhuys [570] have discussed the potentials of LC-ESI-ToFMS and LC-MALDI-ToFMS. Miniaturisation (reduction of column i.d.) in LC-MS was recently critically evaluated [571]. LC-MS/MS was also reviewed [572]. Various books on LC-MS have appeared [164,433,434,573-575], some dealing specifically with selected ionisation modes, such as CF-FAB-MS [576] or API-MS [577],... 

LC-TSP-MS without tandem mass capabilities has only met with limited success for additive analysis in most laboratories. Thermospray ionisation was especially applied between 1987 and 1992 in combination with LC-MS for a wide variety of compound classes, e.g. dyes (Fig. 7.31). Thermospray, particle-beam and electrospray LC-MS were used for the analysis of 14 commercial azo and diazo dyes [594]. No significant problems were met in the LC-TSP-MS analysis of neutral and basic azo dyes [594,595], at variance with that of thermolabile sulfonated azo dyes [596,597], LC-TSP-MS has been used to elucidate the structure of Basic Red 14 [598]. The applications of LC-TSP-MS and LC-TSP-MS in dye analysis have been reviewed [599]. 

These problems have largely been solved by the development of a wide variety of powerful LC-MS interfaces (reviewed in Refs. [1-3]). In the following paragraphs, the two most widely used atmospheric pressure ionisation (API) systems, namely atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI), are briefly described, along with the older technique of thermospray ionisation... 

Several years later, the next step in the application of MS-MS for mixture analysis was developed by Hunt et al. [3-5] who described a master scheme for the direct analysis of organic compounds in environmental samples using soft chemical ionisation (Cl) to perform product, parent and neutral loss MS-MS experiments for identification [6,7]. The breakthrough in LC-MS was the development of soft ionisation techniques, e.g. desorption ionisation (continuous flow-fast atom bombardment (CF-FAB), secondary ion mass spectrometry (SIMS) or laser desorption (LD)), and nebulisation ionisation techniques such as thermospray ionisation (TSI), and atmospheric pressure ionisation (API) techniques such as atmospheric pressure chemical ionisation (APCI), and electrospray ionisation (ESI). 

It was not until the introduction of Thermospray Ionisation in the 1980s that a system was available that enabled sufficiently reliable LC-MS instruments to be manufactured [6]. At this point the HPLC community began to realise this was a technique that might just answer their wish for a much needed universal detector. Thermospray ionisation never quite gave the degree of reliability, nor the sensitivity, required to fulfil this dream, but it did show that LC-MS was a very valuable technique to have available. The impetus for further development was therefore present. 

Cremin P, Guiry PJ, Wolfender J-L, Hostettmann K, Donnelly DMX (2000) A Liquid Chromatography-Thermospray Ionisation-Mass Spectrometry Guided Isolation of a New Sesquiterpene Aryl Ester from Armillaria novae-zelandiae. J Chem Soc Perkin Trans 1 2325... 

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. 

The use of LC-MS for the identification of minor components in BT derivatives was investigated by Niessen and co-workers [8] who evaluated the merits of GC-MS, moving belt LC-MS, particle-beam LC-MS and LC-MS-MS with thermospray ionisation (TSP). They found that LC-TSP-MS, LC-TSP-MS-MS and LC-PB-MS were the best techniques. Recent advances in LC-MS applied to food analysis [11] meant that LC-MS was the method of choice for the investigations reported here as it offered the potential to analyse with minimal sample clean up. A level of interest of 0.05 mg/kg food was set at the commencement of this work and this set the level of detection that should be achieved using LC-MS. 

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... 

The mass spectra of mixtures are often too complex to be interpreted unambiguously, thus favouring the separation of the components of mixtures before examination by mass spectrometry. Nevertheless, direct polymer/additive mixture analysis has been reported [22,23], which is greatly aided by tandem MS. Coupling of mass spectrometry and a flowing liquid stream involves vaporisation and solvent stripping before introduction of the solute into an ion source for gas-phase ionisation (Section 1.33.2). Widespread LC-MS interfaces are thermospray (TSP), continuous-flow fast atom bombardment (CF-FAB), electrospray (ESP), etc. Also, supercritical fluids have been linked to mass spectrometry (SFE-MS, SFC-MS). A mass spectrometer may have more than one inlet (total inlet systems). 

Thermospray (TSP) is another soft ionisation technique which produces predominantly MH+ or (M — H) ions, together with some fragmentation. TSP is best suited to the analysis of organic compounds of low molecular mass (<1000 Da) that exhibit some polarity. Polymer additive molecules fall in this wide category. 

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... 

LC-APCI-MS is a derivative of discharge-assisted thermospray, where the eluent is ionised at atmospheric pressure. In an atmospheric pressure chemical ionisation (APCI) interface, the column effluent is nebulised, e.g. by pneumatic or thermospray nebulisation, into a heated tube, which vaporises nearly all of the solvent. The solvent vapour acts as a reagent gas and enters the APCI source, where ions are generated with the help of electrons from a corona discharge source. The analytes are ionised by common gas-phase ion-molecule reactions, such as proton transfer. This is the second-most common LC-MS interface in use today (despite its recent introduction) and most manufacturers offer a combined ESI/APCI source. LC-APCI-MS interfaces are easy to operate, robust and do not require extensive optimisation of experimental parameters. They can be used with a wide variety of solvent compositions, including pure aqueous solvents, and with liquid flow-rates up to 2mLmin-1. 

Electrospray has been successful for numerous azo dyes that are not ionic salts. Several anthraquinone dyes have been analysed by LC-ESI-MS [552]. Electrospray achieves the best sensitivity for compounds that are precharged in solution (e.g. ionic species or compounds that can be (de)protonated by pH adjustment). Consequently, LC-ESI-MS has focused on ionic dyes such as sulfonated azo dyes which have eluded analysis by particle-beam or thermospray LC-MS [594,617,618]. Techniques like LC-PB-MS and GC-MS, based on gas-phase ionisation, are not suitable for nonvolatile components such as sulfonated azo dyes. LC-TSP-MS on... 

The thermospray, particle beam electrospray (ES) (Fig. 4.4) and atmospheric pressure chemical ionisation (APCI) have been used in HPLC-MS-MS. The ES and APCI are both atmospheric pressure ionisation systems. The column effluent is nebulised and ionised in the atmospheric pressure region and the ions are then... 

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. 

Barcelo [13] characterised selected pesticides by negative ion chemical ionisation thermospray high performance liquid chromatography-mass spectrometry. Ions observed... 

The most recent significant advance in plant hormone analysis has been the use of combined HPLC-MS for the analysis of GA conjugates, lAA conjugates and cytokinins. A number of interfaces have been developed for HPLC-MS, including thermospray, atmospheric pressure chemical ionisation, electrospray, particle beam, continuous flow fast atom bombardment (FAB) and frit-FAB (see reference [94]). GA standards have been analysed by HPLC-MS with a thermospray interface [95], an atmospheric pressure chemical ionisation interface has been used with GA conjugates [96] and cytokinins [97] while ion spray and plasma spray have been used to analyse ABA and its metabolites [98]. There are, however, many more reports on the use of frit-FAB HPLC-MS for the analysis of not only standards, but also endogenous hormones and their isotopically-labelled metabolites [18-23,99-101]. 

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