Monodisperse aerosol-generating devices

Figure 11 Magnified diagrams of monodisperse aerosol-generating devices. (A) Spinning-disk aerosol generator. (B) Vibrating orifice aerosol generator.

A particle-beam interface is a useful device that can provide library-searchable El spectra of LC-separated solutes [6,32-36]. This device uses a jet separator to free the solute molecules from volatile solvents and produces a high-velocity particle beam of gas-phase solute particles. The original device was termed MAGIC (monodisperse aerosol generating interface for chromatography) by its inventors [36]. 

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]. 

The first online DD-HPLC/FT-IR interface was reported by Lange et al. [42], who interfaced a concentric flow nebulizer to the Bio-Rad Tracer. Solutes were deposited on a continuously moving ZnSe substrate as a trace that was 150 pm in width. In an manner analogous to DD-GC/FT-IR measurements, the solute spots were transported into the beam of a rudimentary microscope, and transmission spectra were measured at intervals of 1 to 2 s. It was not possible to eliminate aqueous solvents at a rate of more than about 50pLmin with the concentric flow nebulizer, so the separations either had to be carried out on a 1-mm-i.d. column or to use a wider-bore column with an output stream splitter. Although Robertson et al. reported the elimination of aqueous mobile phases at flow rates up to 1 mL min using a monodisperse aerosol generator and achieved spot diameters of between 300 and 50 pm [35], it was not possible to direct the IR beam inside the vacuum chamber, and this device was never used online. 

Much effort and ingenuity have gone into the development of generators capable of producing monodisperse aerosols, and several are discussed at the end of this chapter. These are used for the testing of gas-cleaning equipment, the calibration of si .e measurement devices, and basic studies of aerosol behavior. 

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