Thermospray ionization technique

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). 

This method is still in use but is not described in this book because it has been superseded by more recent developments, such as particle beam and electrospray.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. 

Thermospray interface. Provides liquid chromatographic effluent continuously through a heated capillary vaporizer tube to the mass spectrometer. Solvent molecules evaporate away from the partially vaporized liquid, and analyte ions are transmitted to the mass spectrometer s ion optics. The ionization technique must be specified, e.g., preexisting ions, salt buffer, filament, or electrical discharge. 

Cl is not the only ionization technique where this aspect of interpretation must be considered carefully fast-atom bombardment, thermospray, electrospray and atmospheric-pressure chemical ionization, described below in Sections 3.2.3, 4.6, 4.7 and 4.8, respectively, all produce adducts in the molecular ion region of their spectra. 

For many years, electron ionization, then more usually known as electron impact, was the only ionization method used in analytical mass spectrometry and the spectra encountered showed exclusively the positively charged species produced during this process. Electron ionization also produces negatively charged ions although these are not usually of interest as they have almost no structural significance. Other ionization techniques, such as Cl, FAB, thermospray, electrospray and APCI, however, can be made to yield negative ions which are of analytical utility. 

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-... 

Much data on the structure of flavonoids in crude or semipurified plant extracts have been obtained by HPLC coupled with MS, in order to obtain information on sugar and acyl moieties not revealed by ultraviolet spectrum, without the need to isolate and hydrolyze the compounds. In the last decade, soft ionization MS techniques have been used in this respect, e.g., thermospray (TSP) and atmospheric pressure ionization (API). However, the most used methods for the determination of phenols in crude plant extracts were the coupling of liquid chromatography (LC) and MS with API techniques such as electrospray ionization (ESI) MS and atmospheric pressure chemical ionization (APCI) MS. ESI and APCI are soft ionization techniques that generate mainly protonated molecules for relatively small metabolites such as flavonoids. 

In the early 1980s, ionization techniques such as FAB, PD, and thermospray (TSP) made it possible to use MS in analysis of high-mass macromolecules since the production of gas phase ions from charged and polar compounds can be done without prior chemical derivatization. FAB is a soft ionization technique that performs well... 

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). 

MS has been used for a number of years as a powerful tool for the study of drug biodeposition and metabolism [83-87]. These types of studies have traditionally been carried out by gas chromatography-MS techniques. However, the use of LC-MS has been growing, especially since the introduction of the thermospray interface. With the advent of liquid-phase ionization techniques, such as electrospray, it has become possible to use LC-MS for the structural characterization of highly polar molecules at very low concentrations. 

The emergence of soft ionization techniques such as FAB, thermospray (TSP), APCI and ESI, combined with MS/MS, will considerably simplify these analysis. On a much smaller sample quantity, bile acids can be analysed without prior separation or derivati-zation. Soft ionization techniques are well suited for such polar, non-volatile thermolabile compounds. 

An ideal interface should not cause extra-column peak broadening. Historical interfaces include the moving belt and the thermospray. Common interfaces are electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCl). Several special interfaces include the particle beam—a pioneering technique that is still used because it is the only one that can provide electron ionization mass spectra. Others are continuous fiow fast atom bombardment (CF-FAB), atmospheric pressure photon ionization (APPI), and matrix-assisted laser desorption ionization (M ALDl). The two most common interfaces, ESI and APCI, were discovered in the late 1980s and involve an atmospheric pressure ionization (API) step. Both are soft ionization techniques that cause little or no fragmentation hence a fingerprint for qualitative identification is usually not apparent. 

The breakthrough of TSP was partly due to the introduction of a new ionization technique [58], based on the ion-evaporation mechanism (Ch. 6.2-3). Ammonium acetate or another volatile buffer is assumed to assist in the process. However, in the vast majority of the applications, TSP is best considered as a solvent-mediated Cl method. Four modes of ionization in TSP LC-MS can be distinguished, i.e., two liquid-based ionization modes (applied in 60% of the applications), ion-evaporation, and thermospray buffer ionization, and two electron-initiated ionization modes (applied in 40% of the applications) filament-on ionization, and discharge-on ionization. 

Besides thermospray and FAB, other LC/MS ionization techniques are known such as electrospray, or atmospheric pressure ionization (APCI). As LC/MS becomes more common, more studies on pyrolysates using these techniques are likely to be done. 

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