LC-MS interfaces thermospray

He then joined the Central Research Establishment of the Home Office Forensic Science Service (as it then was) at Aldermaston where he developed thermogravimetry-MS, pyrolysis-MS, GC-MS and LC-MS methodologies for the identification of analytes associated with crime investigations. It was here that his interest in LC-MS began with the use of an early moving-belt interface. This interest continued during periods of employment with two manufacturers of LC-MS equipment, namely Kratos and subsequently Interion, the UK arm of the Vestec Corporation of Houston, Texas, the company set up by Marvin Vestal, the primary developer of the thermospray LC-MS interface. 

Figure 4.6 Schematic of a thermospray LC-MS interface. From applications literature published by Vestec (Applied Biosystems), Foster City, CA, and reproduced with permission.

Vestec Thermospray LC/MS Interface, Instruction/maintenance manual, Finnigan Edition, Vestec Corporation, Houston, TX. 

Figure 9.8. Schematic diagram of a thermospray LC-MS interface and insert indicating the mechanism of spray production.

The advantages and disadvantages of this type of interface, particnlarly in comparison to the moving-belt interface which was available at the same time, are listed below. This was one of the first LC-MS interfaces to be made commercially available and, although used in a number of laboratories, its development was halted premamrely by the introduction of the thermospray interface (as we shall see later). 

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. 

In this book, a number of different LC-MS interfaces have been described, where some of these have been included primarily from an historical standpoint. Currently, the most widely used interfaces are, undoubtedly, the electrospray and APCI interfaces and it is these that will be concentrated upon (a search of the Science Direct database [1] for 2001 nsing the term thermospray , previously the most widely used interface, yielded only one paper). 

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 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. 

Table 7.46 shows the LC-FTIR interface detection limits. Detection limits approaching those for GC-FHR light-pipe interfaces have been reported for flow-cell HPLC-FTIR when IR-transparent mobile phases are employed. For both the moving-belt and thermospray LC-MS couplings the detection limits are in the ng range. Selective evaporation consisting of fraction collection followed by DRIFT identification achieves a detection limit of 100 ng. 

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. 

After the development of larger and more efficient vacuum pumps, more user-friendly LC/MS interfaces of thermospray,4 5 and atmosphere pressure ionization,6 7 LC/MS earned its place in bio-analytical laboratories. The resulting device was a powerful instrument that required significantly more capital investment than HPLC/UV, GC, or GC/MS. 

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... 

Watson et al. (1985) performed analysis of conjugated steroids by coupling EG to MS by means of a thermospray (Th)-LC/MS interface. In their studies, negative ion Th-mass spectra were recorded for several steroid conjugates. This interface vaporizes the sample before it passes through a small orifice at the tip of a heated stainless steel tube in the ion source. Ionization occurs at the same time, usually promoted by a volatile buffer, such as ammonium acetate. 

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 use of liquid chromatography-mass spectrometry (LC-MS) is becoming more popular because of the increasing number of LC-MS interfaces commercially available thermospray (TSP), particle beam (PB), and atmospheric pressure ionization (API). Coupled with mass spectroscopy, HPLC provides the analyst with a powerful tool for residue determination. 

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). 

Over 30 years of liquid chromatography-mass spectrometry (LC-MS) research has resulted in a considerable number of different interfaces (Ch. 3.2). A variety of LC-MS interfaces have been proposed and built in the various research laboratories, and some of them have been adapted by instmment manufacturers and became commercially available. With the advent in the early 1990 s of interfaces based on atmospheric-pressure ionization (API), most of these interfaces have become obsolete. However, in order to appreciate LC-MS, one carmot simply ignore these earlier developments. This chapter is devoted to the older LC-MS interfaces, which is certainly important in understanding the histoiy and development of LC-MS. Attention is paid to principles, instrumentation, and application of the capillary inlet, pneumatic vacuum nebulizers, the moving-belt interface, direct liquid introduction, continuous-flow fast-atom bombardment interfaces, thermospray, and the particle-beam interface. More elaborate discussions on these interfaces can be found in previous editions of this book. 

For some years (1987-1992), thermospray LC-MS was the most widely apphed LC-MS interface. It has demonstrated its potential in qualitative as well as quantitative analysis in mat r application areas, such as drugs and metabolites, conjugates, nucleosides, peptides, natural products, pesticides. A few examples are given below. 

An interlaboratory comparison of the performance of thermospray and PBI LC-MS interfaces for the analysis of chlorinated phenoxyacid herbicides was reported by Jones et al. [94]. Except for Silvex, statistically significant differences were observed in the results from the two interfaces. PBI LC-MS exhibited a high positive bias, but a better %RSD at the highest concentration (500 pg/ml). A comparison of the official US-EPA method 515.1 for CPA analysis with on-line solid-phase extraction (SPE) in combination with GC with electron-capture detection (GC-ECD), LC-UV, and PBI LC-MS was reported by Bruner et al. [95]. In this method, liquid-liquid extraction (LLE), as prescribed in the US-EPA method, was replaced by SPE for sample preconcentration. In the LC methods, no derivatization was necessary. Detection limits were in the range of 0.07-0.8 ng/1 for GC-ECD, 0.7-7 ng/1 for PBI-LC-MS, and 6-80 ng/1 for LC-UV. The most accurate methods were LC-UV and GC-ECD, although PBI LC-MS is still more accurate than the US-EPA 515.1 method. 

Thermospray (TSP). While the LC/MS interfaces described above provided partial solutions to on-line capability, the two major problems confronting such devices still remained unsolved. These difficulties were the capability to handle flow rates of 1-2 mL/min of high polarity solvent systems and the need to analyze compounds of low volatility or poor thermal stability. The development of thermospray (TSP) mass spectrometry by Marvin Vestal (9) provided an immediate solution to both these problems. 

Nevertheless, Figure 5 demonstrates that the enantiomeric separation using a phosphate buffer in the mobile phase can be coupled via the PSS approach on-line to an LC/MS moving belt interface. Other examples of the PSS approach or similar procedures with other compounds and other LC/MS interfaces have been described (2, 9-14). Besides the actual phase-system switching, which enables the choice of the most favorable solvent for a particular interface, the PSS approach offers some other features as well. The desorption flow-rate used can be adjusted to the capabilities of the LC/MS interface applied. While in the present example with the moving belt a flow-rate of 0.4 ml/min of methanol was used, desorption has also been demonstrated with 1.2 ml/min for a thermospray interfaced),... 

The two examples discussed show the bioanalytical applicability of LC/MS, both in qualitative analysis and in target compound analysis. Although the moving belt interface owing to its El capabilities is ideally suited for the identification of unknown compounds and is used often for that purpose, identification problems can sometimes also be solved with thermospray LC/MS, for instance by applying repeller-induced fragmentation, and LC/MS/MS. The PSS approach is an example that indicates that LC/MS still can be improved. A more elaborate discussion on the so-called multidimensional approaches in LC/MS is given elsewhere (14). 

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