Ionization thermospray

The thermospray ionization interface was the first device to provide a viable means of coupling HPLC to MS for quantitative analysis, particularly for relatively involatile analytes. In its first form (Blakley 1978), HPLC eluant 

This extended series of developments culminated in the thermospray design (Blakley 1983) that was subsequently commercialized as an LC/MS interface. A solution of the analyte (e.g., HPLC eluant) and a volatile buffer (typically O.IM ammonium acetate added post-column) was evaporated from a heated capillary at a flow rate of up to LSml/min into a heated chamber (whence the name thermospray ), forming a mist of droplets containing relatively involatile analytes and solvent vapor as the solvent evaporated the analyte formed adducts with ions from the added salt. It is believed that the formation of free gaseous ions from the microdroplets then proceeds in a manner similar to that discussed for ESI in Section 5.3.6. Most of the neutrals are removed by a vacuum pump and the ions are extracted orthogonally by some electrostatic lenses and a repeUer through a pinhole (restricted to 25 (j,m to protect the vacuum in the mJz analyzer, typically a quadrupole because of its better tolerance to poor vacuum). Such an arrangement is found to be efficient 

Improved Vaporization of Involatile Analytes By Rapid Heating of Microparticles 

The advantage of rapid heating for volatilization of thermally labile compounds (Beuhler 1974, Daves 1979) can be understood by assuming that both proceed with unimolecular rate constants Iq. At lower temperatures (high 

kj for decomposition is larger than for vaporization and, on heating, mainly decomposition products are formed in the vapor. 

A new technique, known as material-enhanced laser desorption/ionization (MELDI), has become available for profiling proteins in complex mixtures [67], It uses derivatized carrier materials, such as cellulose, silica, poly(glycidyl methylacrylate/divinylbenzene), or diamond powder, to bind proteins from the sample. The carrier particles are derivatized with iminodiacetic acid (IDA) and loaded with ions to form [carrier-IDA-Cu +] complex, which is then added to the protein sample. The protein-bound carrier materials are spotted on the MALDI target, mixed with a MALDI matrix and analyzed by MALDI-TOF-MS. 

CONDENSED-PHASE IONIZATION TECHNIQUES IONIZATION OF LIQUID-STATE SAMPLES 

The direct sampling of solutions is often necessary in a variety of situations such as biological fluids and eluants from liquid chromatography and capillary electrophoresis separation devices. Liquid solutions are difficult to handle by the mass spectrometry vacuum system and require some novel introduction and ionization systems. The last two decades have witnessed the development of some unique ionization methods that are suitable for direct analysis of sample solutions the important ones are discussed below. 

Thermospray, a predecessor of currently popular and more versatile ESI, has proven to be a convenient mode of ionization for liquid-phase samples, especially for HPLC effluents [68,69]. The liquid stream is passed through a heated capillary at a flow rate of 0.5 to 2 mLmin to produce a spray of fine liquid droplets. Ionization occurs through three distinct processes  

The first process is direct desorption of the preformed sample ions into the gas phase from the liquid droplets after solvent evaporation. 

With the development of TSl, Vestal et al. [39,44-46] also introduced a new ionization technique. They demonstrated that collision of the vapor-droplet beam from 

Under these conditions, mainly protonated molecules [M+H]+ are observed in positive-ion mode. In ion-evaporation and thermospray buffer ionization modes. Alkali -cationization may be observed as well, but generally not as frequently as in ESI operation. In general, little attention was paid to such effects in TSI-MS. 

Another instrumental development is based on the fact that the generation of smaller droplets is more favorable in terms of droplet evaporation during ESI, of sensitivity and the abihty to preserve non-covalent molecular associates. Thus, nanoelectrospray ionization (nESI) has been developed [68], where the analyte is sprayed from a gold-coated fused-silica capillary with a tip diameter of 1-5 pm rather than from capillaries with a 100-150-pm tips that are used in conventional (pneumatically assisted) ESI. In nESI, flow-rates as low as 20 nl/min can be nebulized. Thus, gentler operating conditions (temperature, gas flows, needle voltage) can be achieved. In order to more readily implement nESI in LC-MS operation, integrated chip-based nano-LC-nESI devices have also been developed [69]. 

ESI-MS is very widely applied, for instance in all application areas discussed in Sect. 7.5. This is partly due to the fact that until today, ESI-MS is the most convenient and most widely applied ionization technique for LC-MS. ESI-MS helped to open new application areas for MS. 

In ESI, as with most other ionization techniques discussed here, the addition of alkali-metal ions (other types of metal ions) can be used to extend the applicability 

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One of the reasons for lack offlterature was probably because environmental analysis depends heavily on gas chromatography/mass spectrometry, which is not suitable for most dyes because of their lack of volatility (254). However, significant progress is being made in analyzing nonvolatile dyes by newer mass spectral methods such as fast atom bombardment (EAB), desorption chemical ionization, thermospray ionization, etc. 

When bnffer is not present, i.e. when normal-phase chromatography is being nsed, thermospray ionization is not possible and a filament or discharge electrode is nsed to generate a plasma in which Cl-type processes can occnr. In addition to allowing ionization nnder these conditions, it is found that the ionization of compounds may be enhanced under conditions in which true thermospray can operate. 

The Cl reagent-gas plasma is either generated by thermospray ionization if buffer is present or, as described above, by the filament or discharge electrode. 

In many cases, a protonated molecular ion (M - - H)+ is the only ion observed in a thermospray spectrum but if ammonium acetate buffer is used, depending upon the relative proton affinities of the species present, an ammonium adduct (M - - NH4)+ may be the predominant ion. In addition, clusters may be formed with components of the mobile phase. Although the thermospray ionization process involves less energy than conventional Cl, and very little intense fragmentation is usually observed, the presence of ions due to the elimination of small molecules, e.g. water, methanol and ketene, is not unknown. These latter ions are usually of relatively low intensity when compared to the protonated or... 

If a buffer is present in the HPLC mobile phase, and this is essential for true thermospray ionization, it should ideally be volatile and this may necessitate modifying existing HPLC methodology. 

Repeller An electrode nsed in thermospray ionization to effect fragmentation of molecnlar species. 

Thermospray ionization The formation of ions from droplets produced by the thermospray process. 

The most recent progress in MS analysis of chlorophylls has been obtained with the development of atmospheric ionization methods such as atmospheric pressure chemical ionization (APCl) and electrospray ionization (ESI). These techniques have demonstrated much more sensitivity than thermospray ionization, detecting chloro-... 

The feasibility of online SPE LC/MS/MS has been tested since the introduction of thermospray ionization. In an early research paper by Lant and Oxford (1987), a prototype online SPE LC/MS system was set up and successfully applied for the measurement of labetalol, a hypertension drug and a- and /J-adrenergic receptor, in plasma. This system was set up by coupling an advanced automated sample processor (AASP, Varian, Walton-on-Thames, UK) with a reversed-phase column, a ten-port switching valve, and an MS equipped with thermospray interface (Vestec, Houston, Texas) (Blakley et al. 1980, Blakley and Vestal 1983). 

After overcoming the instability of the thermospray ionization source, a sensitivity of 2 ng/mL was achieved with a calibration range of 10 to 103 ng/mL for human plasma samples. In recent years, the online SPE LC/MS/MS technology has matured and is now easy to build and use. It is used widely in the pharmaceutical industry (Jemal et al. 2000 Hsieh 2004 Hennion 1999). 

Thermospray ionization TSI Spray Nonvolatile molecular ions LC-MS... 

M ammonium acetate, which yields [NH4] +, clustered [NH4] + and [CH3COO] ions protonation to [M + H] + or ammonium addition to [M + NH4] + is observed and the spectra are similar to ammonia Cl or field desorption spectra327,374 possible reactions between gas-phase ions and analyte molecules have been investigated374. The ionization mechanism is still not entirely understood, nor is the influence of a number of experimental parameters such as the nature and concentration of the electrolyte, the flow rate and the thermospray temperature375. Thermospray ionization exhibits better sensitivity than conventional Cl and is widely used in pesticide residue analyses327,336,337,340. 

Yinon et al. (228) used an HPLC interfaced with a triple-quadrupole mass spectrometer by means of a particle beam for the identification of several azo dyes. Characterization of the dyes was achieved by observing typical fragment ions formed by cleavage of the N-C and C-N bond on either side of the azo linkage and/or cleavage of the N=N double bond with the transfer of two hydrogen atoms to form an amine. Sensitivity was observed to be two to three orders of magnitude worse than with thermospray ionization. 

LD Betouski, JM Ballard. Identification of dyes by thermospray ionization and mass spectrome-try/mass spectromelogy. Anal Chem 56 2604 -2609, 1984. 

A microchip-atmospheric pressure thermospray ionization technique was developed to investigate ofloxacin in the presence of other compounds (09RCM3313). Ofloxacin was detected by a voltammetric method in human urine (08ELA144). The trace of ofloxacin was detected with poly (crystal violet) film modified electrodes (07MI48). 

Fischer, J. and J. Michael (1995). Thermospray ionization liquid chromatography-mass spectrometry and chemical ionization gas chromatography-mass spectrometry of hexazione metabolites in soil and vegetation extracts. J. Chromatogr. A, 704(1) 131-139. 

In the late 1980s, thermospray ionization (TSI) techniques offered what would be the precursor to a universal and reliable LC/MS interface for compounds of pharmaceutical interest. Conventional HPLC flow rates (l-2mL/min) were accommodated by this interface, using volatile buffers that contain ammonium acetate. New applications were realized, and higher standards of analytical performance were established for pharmaceutical analysis (Voyksner et al., 1985 Beattie and Blake, 1989 Oxford and Lant, 1989 Malcolm et al., 1990 Bowers et al., 1991). 

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