Thermospray techniques

This technique is complementary to the thermospray technique. Relative advances of the particles beam technique over thermospray include library searchable electron impact spectra, improved reproducibility, easier use and increased predictability over a broad range of compounds. But since a particle beam requires same sample volatility, very large and polar compounds such as proteins may not provide satisfactory results using particle beam liquid chromatography-mass spectrometry. Additionally, certain classes of compounds such as preformed ions, azo dyes and complex sugars may not yield satisfactory electron impact spectra, but can be run on thermospray. In other words, both liquid chromatography-mass spectrometry techniques complement each other s limitations and the analyst may want to add both to address a broader range of samples. 

The next major advance in LC-MS interfacing was developed by Blakely and Vestal (55, 56). To circumvent the solvent elimination problem, Blakely et al. (55) developed the thermospray interface that operates with aqueous-organic mobile phase at typical 4.6-mm i.d. column flow rates, 1-2 mL/min. The thermospray technique works well with aqueous buffers. This feature is an advantage when the versatility of the reversed-phase mode is considered. In fact, with aqueous buffers, ions are produced when the filament is off. A recent improvement in the thermospray technique is the development of an electrically heated vaporizer that permits precise control of the vaporization (56). This... 

The detection of samples in the gas phase through katharometry, electron capture, etc. is well developed and reliable. The linking of a gas chromatograph with a mass spectrometer is increasingly found to be useful in research studies, to identify the components of mixtures whose quantitation has been accomplished by other means. Thermospray techniques that have entered into routine use in mass spectrometry (Section 4.11), have made the CZE MS and various HPLC MS combinations particularly useful. 

Thallium compounds, as oxidants 1278, 1281, 1286, 1288, 1311-1325 Thebaine, enzymatic formation of 1216 Thermal analysis 1002 Thermal desorption 938 Thermospray techniques 319 Thiacahx[4]anilines, synthesis of 1416 ThiacaUxarenes,... 

Particle beam LC-MS is a rapidly developing complimentary interface to thermospray techniques and provides a method of linking conventional HPLC systems with eluant flow-rates of 0.3-1.0mlmin , to an El ion source to obtain the classical El spectra which can be compared to conventional reference spectra (Figure 7.12). A capillary GC column may be connected to the same interface [10]. LC eluant enters the interface together with a stream of helium to form an aerosol of droplets which move through the desolvation chamber maintained at room temperature and pressure. The... 

Electrospray ionization is similar in effect to the thermospray technique and is useful for similar applications. The difference resides in the use of a high electric field to nebulize the sample solution (or sample and eluant), creating droplets with excess electric charge. As the droplet solvent evaporates during traverse of a desolvation chamber, charge transfers to the analyte molecules and these are released as gaseous ions. A further refinement in this technique is the use of electronic lenses to direct ions more efficiently into the mass spectrometer. Because the analyte is not subject to heating, there is also less possibility for thermal decomposition of complex lipid components. 

The thermospray technique [84—87] uses a heated vaporizer from which the HPLC eluent containing the dissolved electrolyte is sprayed as a jet into a heated chamber. A sampling orifice is positioned normal to the axis of the vaporizer probe. The ions and molecules are pumped through the sampling orifice into the mass spectrometer. Electron impact or collision-activated ionization, although optional, provides structural information. 

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. 

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. 

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. 

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. 

The mass spectrometry of diazo compounds was reviewed by Zeller (1983) and by Lebedev (1991). It is difficult to record mass spectra of diazonium salts using conventional techniques. With the water thermospray method, however, Schmelzeisen-Redeker et al. (1985) observed the diazonium ion and various fragments such as [Ar+ - N2 + 2H]+ and [Ar + N2 + H20]+. Ambroz et al. (1988) applied the fast atom bombardment (FAB) technique using a 3-nitrobenzylalcohol matrix. Peaks for ArNJ, Ar+, and [M + ArN2]+ and further peaks due to solvated ions were found. 

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. 

In summary, it can be said that prior to the development of the thermospray interface there were an increasing nnmber of reports of the analytical application of LC-MS [3] bnt in this present anthor s opinion, based on a nnmber of years of using a moving-belt interface, the technique could not be considered to be routine . The thermospray interface changed this and with the commercial intro-dnction of the combined APCI/electrospray systems in the 1990s the technique, for it now may be considered as a true hybrid technique, has reached maturity (although this should not be taken as a suggestion that there will be no further developments). 

Examples of such compounds include anionic surfactants whose analysis had previously been limited to desorption techniques such as FAB and thermospray but which yielded interpretable El spectra when using a parUcle-beam interface... 

The introduction of the thermospray interface provided an easy-to-use LC-MS interface and was the first step in the acceptance of LC-MS as a routine analytical technique. It soon became the most widely used LC-MS interface of those available in the mid to late 1980s. 

Electrospray ionization occurs by the same four steps as listed above for thermospray (see Section 4.6). In contrast to thermospray, and most other ionization methods nsed in mass spectrometry, it shonld be noted that electrospray ionization nnnsnally takes place at atmospheric pressure. A similar process carried out under vacuum is known as electrohydrodynamic ionization and gives rise to qnite different analytical results. This technique has not been developed into a commercial LC-MS interface and will not be considered further. 

Atmospheric-pressure chemical ionization (APCI) is another of the techniques in which the stream of liquid emerging from an HPLC column is dispersed into small droplets, in this case by the combination of heat and a nebulizing gas, as shown in Figure 4.21. As such, APCI shares many common features with ESI and thermospray which have been discussed previously. The differences between the techniques are the methods used for droplet generation and the mechanism of subsequent ion formation. These differences affect the analytical capabilities, in particular the range of polarity of analyte which may be ionized and the liquid flow rates that may be accommodated. 

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-... 

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. 

Thermospray was quite popular before the advent of electrospray, but has now given way to the more robust API techniques, although TSP sources continue to operate. Developed as an LC-MS interface, this technique calls for a continuous flow of sample in solution. 

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). 

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