Nebulization droplet formation

Nebulizers are used to introduce analyte solutions as an aerosol spray into a mass spectrometer. For use with plasma torches, it is necessary to produce a fine spray and to remove as much solvent as possible before the aerosol reaches the flame of the torch. Various designs of nebulizer are available, but most work on the principle of interacting gas and liquid streams or the use of ultrasonic devices to cause droplet formation. For nebulization applications in thermospray, APCI, and electrospray, see Chapters 8 and 11. 

Fig. 6 (A) Droplet formation. (B) Droplet delivery from an airblast nebulizer.

Droplet Formation. A general theory for the formation of droplets by nebulization has not been fully developed, although the understanding of the nebulization principle has been improved with mathematical... 

Regarding nucleation of solid particles from liquid droplets (antisolvent, nebulization, PGSS, microencapsulation processes), it is obvious that the droplets size is a basic parameter in the process. Some investigators reported results of experimental works supported by theoretical considerations on droplet formation by injection of a liquid into a pressurized fluid using a simple capillary nozzle (78) or a coaxial nozzle (79). Both articles, following previous works on liquid droplets formation into an insoluble liquid or into a gas, suggest the correlation of the experimental results with two dimensionless numbers... 

Whatever the technique used (thermal or ionotropic gelation), gel particles are generally formulated in a two-step procedure involving a droplet formation and hardening. The droplet formation step determines the mean size and the size distribution of the resulting gel particles. In the following, the main procedures used for droplet formation—droplet extrusion, nebulization (spray), and emulsification—are described. 

A DESI source is similar to an ESI source except that there is no secondary flow of heated nitrogen to dry the nebulized droplets (Figure 2.17), so a spray is produced. A mixture of methanol and water is subjected to a potential of 3-5 kV and then nebulized with nitrogen supplied at 5 x 10 Torr ( 7 bar). The spray mixture may be modified with acid (acetic or formic) or ammonia to enhance ion formation. The stream of charged droplets is then directed at the surface of solid samples, a few millimeters away, from which the analyte(s) is ejected. The desorbed compounds leave... 

Currently, ESI is the primary ionization technique for NBS of ACs and amino acids from dried blood specimens using LC-MS/MS. Ion sources are designed to enhance the formation of charged species in nebulized droplets from the flowing stream of mobile phase and sample. Positive ion generation is the desired result for ACs and amino acids. Details of electrospray and ionization can be found elsewhere. However, approaches to enhancing ionization of ACs and amino acids are described below. 

Let us now look at the other class of interference in ICP-MS—suppression of the signal by the matrix itself. There are basically three types of matrix-induced interferences. The first and simplest to overcome is often called a sample transport effect and is a physical suppression of the analyte signal, brought on by the level of dissolved solids or acid concentration in the sample. It is caused by the sample s impact on droplet formation in the nebulizer or droplet size selection in the spray chamber. 

With spray-coating the precursor solution is nebulized into small aerosol droplets which are deposited on the support surface. The droplet formation is influenced by the geometry of the spray chamber (capillary thickness, air jet velocity) and by the... 

Nebulizers can be divided into several main types. The pneumatic forms work on the principle of breaking up a stream of liquid into droplets by mechanical means the liquid stream is forced through a fine nozzle and breaks up into droplets. There may be a concentric stream of gas to aid the formation of small droplets. The liquid stream can be directed from a fine nozzle at a solid target so that, on impact, the narrow diameter stream of liquid is broken into many tiny droplets. There are variants on this approach, described in the chapter devoted to nebulizers (Chapter 19). 

Aerosols can be produced as a spray of droplets by various means. A good example of a nebulizer is the common household hair spray, which produces fine droplets of a solution of hair lacquer by using a gas to blow the lacquer solution through a fine nozzle so that it emerges as a spray of small droplets. In use, the droplets strike the hair and settle, and the solvent evaporates to leave behind the nonvolatile lacquer. For mass spectrometry, a spray of a solution of analyte can be produced similarly or by a wide variety of other methods, many of which are discussed here. Chapters 8 ( Electrospray Ionization ) and 11 ( Thermospray and Plasmaspray Interfaces ) also contain details of droplet evaporation and formation of ions that are relevant to the discussion in this chapter. Aerosols are also produced by laser ablation for more information on this topic, see Chapters 17 and 18. 

For a longitudinal disturbance of wavelength 12 pm, the droplets have a mean diameter of about 3-4 pm. These very fine droplets are ideal for ICP/MS and can be swept into the plasma flame by a flow of argon gas. Unlike pneumatic forms of nebulizer in which the relative velocities of the liquid and gas are most important in determining droplet size, the flow of gas in the ultrasonic nebulizer plays no part in the formation of the aerosol and serves merely as the droplet carrier. 

These solutions are not always practicable and HPLC flow rates of up to 2 mlmin may be accommodated directly by the use of electrospray in conjunction with pneumatically assisted nebulization (the combination is also known as lonspray ) and/or a heated source inlet. The former is accomplished experimentally by using a probe that provides a flow of gas concentrically to the mobile phase stream, as shown in Figure 4.8, which aids the formation of droplets from the bulk liquid, and will allow a flow rate of around 200 p. min to be used. 

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. 

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