Thermospray interface operation

Most of the direct and indirect (transport) interfaces described here use chemical ionization (c.i.) ion-sources, which are not well suited to such polar, non-volatile compounds as tri- and higher oligosaccharides. The thermospray interface, which can operate on an ion-evaporative mode, is capable of producing intact molecular ions from such nonvolatile, polar molecules and should be useful in oligosaccharide analysis. Molecules of this type, however, can also be easily analyzed by fast-atom-bombardment ionization, and use of this technique, coupled to direct liquid introduction and moving-belt interfaces, has been reported. The latter system has been applied to complex oligosaccharide analysis. ... 

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 thermospray interface (Vestec Inc., Houston, TX) was operated at a vaporizer temperature of 180-195°C and a source temperature of 250 C. The vaporizer temperature was adjusted to optimize the solvent signal which correlated closely to optimal conditions for the analyte [14]. Most of the compounds were analyzed using ion evaporation ionization, although a few proved more sensitive under discharge ionization conditions (discharge needle at 1000 V). 

The thermospray interface for LC-MS is generally considered as a difficult interface. This is due to the fact that for a proper operation the careful... 

The thermospray interface was the first LC-MS system that allowed reliable quantitative bioanalysis for a wide variety of compounds. Numerous examples were published in the literature. An excellent example is the automated analysis of bambuterol. The automated system, described by Lindberg and coworkers, contained a series of feedback steps in order to assure the various components of the system were operating properly during overnight, unattended analysis and to avoid the loss of valuable sample material. The same approach was applied to the quantitative bioanalysis of cortisol and related steroid compounds. In order to enhance the response of cortisol in thermospray ionization, the compound was derivatized to the 21-acetate using acetic... 

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. 

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. 

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],... 

Thermospray ionization 8,000 Can be interfaced with standard-bore HPLC can be used with NP- or RP-HPLC simple to operate suitable for large, polar, and involatile molecules as well as ions Decomposition of thermolabile compounds only volatile solvents and volatile buffers can be used limited structural information... 

The instrumentation and interfaces that had been used up to 1998 in CWC-related LC/MS analysis were summarized previously (4). At that time, sources that operate at atmospheric pressure, using electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI), were displacing their predecessors that used thermospray ionization or continuous flow fast atom bombardment. Atmospheric pressure ionization (API), either ESI or APCI, is now the method of choice in CWC-related analysis and will be the focus of this current review. A small number of recent applications involving alternative types of ionization are also included. For earlier applications of LC/MS to chemical weapons (CW) analysis, using thermospray and other ionization methods, the reader is referred to our previous review (4). The other major trend has been the increasing availability and ease of use of less-expensive bench-top quadrupole and... 

The main components of an LC-MS are the HPLC apparatus, an optional UV or photodiode array detector, the interface, the mass spectrometer and a computer system for data management and evaluation. The interface is the key component of the LC-MS system. All other components must be adapted to the particular interface that is used. Most commercially available systems work with thermospray, electrospray, or particle beam interfaces. Each interface has a distinct mode of action and its own operational parameters. 

Tandem MS. The thermospray HPLC/MS/MS was performed on a Finnigan MAT TSQ-46C triple quadrupole mass spectrometer interfaced to an INCOS Data System (Finnigan MAT, San Jose, CA). The triple quadrupole was operated with the first and second quadrupoles in the RF mode during HPLC/MS operation. For HPLC/MS/MS analysis, the first quadrupole selected the [M+H] ion of the compound, while the third quadrupole was scanned over the mass range of 12-300 daltons. The second quadrupole serves as a collision chamber. Argon collision gas was added to the enclosed chamber of this quadrupole to give a pressure of 2 mtorr for collisional activation of the sample ions. 

The new Thermospray "Universal Interface" was been developed to allow HPLC to be properly coupled to conventional El and Cl mass spectrometry. A block diagram of the new interface is shown in Figure 1. The LC effluent is directly coupled to a Thermospray vaporizer in which most, but not all, of the solvent is vaporized and the remaining unvaporized material is carried along as an aerosol in the high velocity vapor jet which is produced. The operation and control of the thermospray device has been described in detail elsewhere. (1)... 

The two stage momentum separator used in this interface is shown schematically in Figure 3 coupled to the combination Thermospray/EI source. This device is conceptually similar to those used in other MAGIC (2) or particle beam (3,4) interfaces. However, since most of the solvent vapor is removed in the gas diffusion cell, this separator is required primarily to remove sufficient helium to allow the standard MS pumping system to achieve the good vacuum required for El operation. The performance of this device can be optimized much more readily for separating helium from macroscopic particles than when copious quantities of condensible vapors are present as in the more conventional particle beam systems. 

The PB interface accommodates common reverse and normal phase chromatographic mobile phases at flow rates up to 0.5 ml/min, and is mechanically simple, rugged, and easy to operate. Unlike the TSP interface, buffer ions are not required to effect ionization, which is instead accomplished by El or Cl in a conventional MS source (3,8). Thermospray LC-MS allows higher flow rates, but requires a special source, and generally requires a volatile buffer to ionize neutral molecules. Thermospray LC-MS provides mostly molecular weight information, whereas PB-LC-MS provides El as well as Cl spectra. 

Currently, the main breakthrough in environmental analysis is observed in the application of LC-MS and LC-MS/MS techniques. One of the obstacles to routine analytical applications of LC-MS had been the unavailability of rugged and reliable LC-MS interfaces. The development of atmospheric pressure ionization (API) overcame such limitations as poor structural information or sensitivity seen with thermospray (TSP) or particle-beam (PB). API is used as a generic term for soft ionization obtained by different interface/ionization types, such as APCI and electrospray (ESI) that operate under atmospheric pressure conditions. Today, LC-MS has become a routine analytical tool, allowing the detection of polar and nonvolatile compounds not amenable to GC analysis. 

For the HPLC-MS systems, many different ionization techniques have been described in the past. Various interface, ionization methods, and operating techniques applicable to LC-MS are discussed in [117J for instance Thermospray, particle beam, electrospray (ES), field desorption (FD), fast atom bombardment (FAB), time of flight (TOF), etc. The electrospray technique produces a soft ionization for thermally labile compounds, while FAB has the advantage that higher molecular mass samples can be introduced into the mass spectrometer. Table 8 offers a rough guide to the applicability of various LC-MS interfaces. For more detailed information on LC-MS, see [118]. 

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