Spray desolvation application

For the chromatographic column, flow of solution from the narrow inlet tube into the ionization/desolvation region is measured in terms of only a few microliters per minute. Under these circumstances, spraying becomes very easy by application of a high electrical potential of about 3-4 kV to the end of the nanotube. Similarly, spraying from any narrow capillary is also possible. The ions formed as part of the spraying process follow Z-shaped trajectories, as discussed below. 

The first ESI design at the end of the 1980s proved to work properly as the HPLC interface with mobile phase flow rates between 1 and lOpL/min. Meanwhile, the development of the HPLC instrumentation and columns was oriented in the mL/min flow rate mode. In addition, the nebulization process based only on the application of an electrical field does not produce a stable spray from aqueous mobile phases. A modified ESI source, called ionspray, was then introduced [39], in which the nebulization of a liquid solution is pneumatically assisted by a coaxial flow of nitrogen (sheath gas) that allows the formation of a stable aerosol at mobile-phase flow rates between 10 and 500 pL/ min and the use of aqueous mobile phases. When working at higher flow rates (500-1000 pL/min), an additional nittogen flow rate can be used (auxiliary gas) to assist the desolvation of the droplets. This modified source is called turboionspray. 

Thermospray is a soft ionisation technique that relies on the production of a spray by direct heating of a metal capillary carrying the LC eluent. The presence of an electrolyte dissolved in the mobile phase causes this spray to consist of droplets carrying a statistical excess charge. Desolvation of these droplets results in the expulsion of ions, which are then extracted into the mass analyser. Thermospray is applicable to many polar materials but can give poor sensitivity for nonpolar species. Other... 

Until now, our discussion of electrospray was rather technical with an emphasis on interface design and occasional reference to applications. Next we shall consider the physicochemical aspects of the ESI process. This section presents answers to basic questions such as to why an electrospray does occur at all, how isolated gas phase ions are formed from droplets, and what rules are governing the charge state (distribution) of those incipient gas phase ions [88-90]. Ion formation in ESI can be considered to be composed of three steps i) creation of an electrically charged spray, ii) dramatic reduction of the droplets size, and finally iii) liberation of fully desolvated ions. 

Such is the added capability and widespread use of these nebulizers across all application areas that manufacturers are developing application-specific integrated systems that include the spray chamber and a choice of different desolvation techniques to reduce the amount of solvent aerosol entering the plasma. Depending on the types of samples being analyzed, some of these systems include a low-flow nebulizer, Peltier-cooled spray chambers, heated spray chambers, Peltier-cooled condensers, and membrane desolvation technology. Some of the commercially available equipment include the following ... 

Microflow nebulizer with heated spray chamber and Peltier-cooled condenser An example of this design is the Apex inlet system from ESI. This unit includes a microflow nebulizer, heated cyclonic spray chamber (up to 140°C), and a Peltier multipass condenser/cooler (down to -5°C). A number of different spray chamber and nebulizer options and materials are available, depending on the application requirements. Also, the system is available with Teflon or Nafion microporous membrane desolvation, depending on the types of samples being analyzed. Figure 17.12 shows a schematic of the Apex sample inlet system with the ctoss-Aow nebulizer. 

Alternative Sample Introduction Techniques Nonstandard sampling accessories like laser ablation systems, flow injection analyzers, electrothermal vaporizers, cooled spray chambers, desolvation equipment, direct injection nebulizers, and automated sample delivery systans and dilu-tors are considered critical to enhancing the practical capabilities of the technique. Their use has increased significantly over the past few years as ICP-MS is being asked to solve more and more diverse application problems. This chapter reflects the increased interest in sampling accessories, especially in the area of specialized sample introduction and desolvation devices to reduce the impact of conunon interferences. 

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