Particle-beam interface nebulizer

Detector UV 369 MS Hewlett-Packard model 59SSA, particle beam interface nebulizer 60°, helium 35 psi, m/z 299... 

The nebulization and evaporation processes used for the particle-beam interface have closely similar parallels with atmospheric-pressure ionization (API), thermospray (TS), plasmaspray (PS), and electrospray (ES) combined inlet/ionization systems (see Chapters 8, 9, and 11). In all of these systems, a stream of liquid, usually but not necessarily from an HPLC column, is first nebulized... 

The particle beam interface [55] borrowed and built upon some of the key elements and concepts of its predecessors. Eluent from the HPLC was nebulized into a spray of small droplets by a flow of helium. The spray of droplets entered a heated chamber where evaporative processes further reduced the droplet size creating an aerosol. The next step of the process involved the spraying of the aerosol (i.e., the... 

The particle-beam interface is an analyte-enrichment interface in which the column effluent is pneumatically nebulized into a near atmospheric-pressure desolvation chamber connected to a momentum separator, where the high-mass analytes are preferentially directed to the MS ion source while the low-mass solvent molecules are efficiently pumped away (71, 72). With this interface, mobile phase flow rates within the range O.l-l.O ml/min can be applied (73). Since the mobile phase solvent is removed prior to introduction of the analyte molecules into the ion source, both EI and CI techniques can be used with this interface. 

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. 

In a particle-beam interface (PBI), the column effluent is nebulized, either pneumatically or by TSP nebulization, into a near atmospheric-pressure desolvation chamber, which is connected to a momentum separator, where the high molecular-mass analytes are preferentially transferred to the MS ion source, while the low molecular-mass solvent molecules are efficiently pumped away. The analyte molecules are transferred in small particles to a conventional EI/CI ion source, where they disintegrate in evaporative collisions by hitting a heated target, e.g., the ion source wall. The released molecules are ionized by El or conventional CL... 

In subsequent years (1988), the MAGIC system was commerciahzed, first by Hewlett-Packard (nowadays Agilent Technologies), and subsequently by other instrument manufacturers. Four commercial versions of the system have been available (1) the particle-beam interface, featuring an adjustable concentric pneumatic nebulizer, (2) the thermabeam interface with a combined pneumatic-TSP nebulizer, (3) the universal interface, in which TSP nebulization and an additional gas diffusion membrane is applied, and (4) the capillary-EI interface, which resulted from systematic modifications to existing PBI systems by Cappiello [83]. The first system was most widely used, and is discussed in more detail below. For some years, PBI was widely used for environmental analysis, especially in the US. 

A comparison of various LC-MS systems for the analysis of complex mixtures of PAHs showed that (1) the moving belt interface was mechanically awkward and is compatible only with a limited range of mobile phases, (2) particle-beam interface had low sensitivity, and the response was nonlinear, (3) a heated nebulizer interface that uses atmospheric pressure chemical ionization (APCI) was the preferred procedure (Anacleto et al. 1995). 

A schematic of a particle beam interface is shown in Figure 21.13. The eluent from the HPLC column is nebulized using helium gas to form an aerosol in a reduced pressure chamber heated at 70°C. A cone with a small orifice is at the end of the chamber, which leads into a lower pressure area. The difference in pressure causes a supersonic expansion of the aerosol. The hehum and the solvent molecules are lighter than the analyte molecules and tend to diffuse out of the stream and are pumped away. The remaining stream passes through a second cone into a yet lower pressure area, and then the analyte vapor passes into the ion source. The particle beam interface produces electron ionization (El) spectra similar to those of GC-MS, so the vast knowledge of El spectra can be used for analyte identification. 

As shown in the schematic of the particle-beam interface (Figure 5.7), three steps, nebulization, desolvation, and momentum separation, are involved in the conversion of analytes from LC effluents into gas-phase species. First, the LC effluent is nebulized pneumatically by forcing the liquid and helium carrier gas through a small orifice. Desolvation of the droplets in the mist occurs when they travel at very high velocity through the desolvation chamber. The temperature of this chamber is maintained a few degrees above ambient temperature to provide thermal energy just sufficient to compensate for the latent heat of vaporization of the solvent. The third step is momentum separation of the solute particles fi om... 

The particle beam interface (Fig. 6) was created under the acronym MAGIC (monodisperse aerosol generator interface for chromatography) [28]. Now, the aerosol is produced by a variety of means (with auxiliary gas, thermospray, or ultrasonic nebulizers) at atmospheric pressure and a uniform distribution of the droplets results in particles of a narrow size distribution, which can be handled more efficiently by the separator. The droplets are dried to particles in a heated expansion chamber, and a momentum separator isolates the particles from the gas. In the source, the particles are destroyed by impact and the sample is released and ionized by using El, Cl, or even FAB. The appearance of the El spectra is almost identical to conventional El spectra obtained by direct probe or GC/MS. Therefore, library searches are possible, which is the major advantage of this interface. 

Figure 6. Particle beam interface a) Nebulizer b) Desolvation chamber c) Momentum separator d) Ion source...

A particle beam interface (PBI) better serves the purpose of LC-EI-MS [55-57]. The PBI removes the solvent by nebulization into an evacuated desolvation chamber from where the evolving microscopic sample particles are transferred into an El ion source via a jet separator (Fig. 5.14). Designs different from PBI have also been developed [57]. The PBI is comparatively robust and attained popularity in particular for low- to medium-polarity analytes, but has some drawbacks such as poor sensitivity especially with water-rich mobile phases, moderate linearity with polar compounds, and low tolerance for heat-sensitive compounds [58,59]. The most recent addition to LC-EI interfaces, also the simplest and most elegant solution, makes use of the very low liquid flow rates of nano-LC equipment [60,61]. The flow from a 30 pm i.d. fused silica capillary column is passed... 

At present, the most powerful and promising interfaces for drug residue analysis are die particle-beam (PB) interface that provides online EI mass spectra, the thermospray (TSP) interface diat works well with substances of medium polarity, and more recently the atmospheric pressure ionization (API) interfaces that have opened up important application areas of LC to LC-MS for ionizable compounds. Among die API interfaces, ESP and ISP appear to be the most versatile since diey are suitable for substances ranging from polar to ionic and from low to high molecular mass. ISP, in particular, is compatible with the flow rates used with conventional LC columns (70). In addition, both ESP and ISP appear to be valuable in terms of analyte detectability. These interfaces can further be supplemented by preanalyzer collision-induced dissociation (CID) or tandem MS as realized with the use of triple quadrupole systems. Complementary to ESP and ISP interfaces with respect to the analyte polarity is APCI with a heated nebulizer interface. This is a powerful interface for both structural confirmation and quantitative analysis. 

The APCI interface uses a heated nebulizer to form a fine spray of the HPLC eluate, which is much finer than the particle beam system but similar to that formed during thermospray. A cross-flow of heated nitrogen gas is used to facilitate the evaporation of solvent from the droplets. The resulting gas-phase sample molecules are ionized by collisions with solvent ions, which are formed by a corona discharge in the atmospheric pressure chamber. Molecular ions, M+ or M , and/or protonated or de-protonated molecules can be formed. The relative abundance of each type of ion depends upon the sample itself, the HPLC solvent, and the ion source parameters. Next, ions are drawn into the mass spectrometer analyzer for measurement through a narrow opening or skimmer, which helps the vacuum pumps to maintain very low pressure inside the analyzer while the APCI source remains at atmospheric pressure. 

The APCI interface uses a heated nebulizer to form a fine spray of the HPLC eluate, which is much finer than the particle beam system... 

More recent efforts became geared toward the development of a device that is analogous to the DD-GC/FT-IR interface (i.e., one in which the mobile phase is eliminated while depositing the analytes in as small an area as possible), so that ideally at least, the spectrum of each eluate can be measured in real time. Many different techniques for solvent elimination have been applied to the DD-HPLC/ FT-IR interface, including thermospray [33], concentric flow nebulizer [34], particle beam (sometimes called a monodisperse aerosol generator) [35], ultrasonic nebulizer [36], and pneumatic nebulizer [37,38]. A comparison of many of these techniques has been made by Somsen et al. [39], but at the time of this writing, the book is still out as to the identity of the optimum approach. An excellent summary of HPLC/FT-IR interfaces is to be found in a review article by Kalasinsky and Kalasinsky [40]. 

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