ESI with Modified Sprayers

The pure electrospray process of dispersing a liquid into an aerosol works best at flow rates of 1-20 pi min Conventional unassisted ESI has also limitations as a LC-MS interface due to the solvent properties in terms of volatility and polarity which can be electrosprayed without some type of assistance. Therefore, a number of sprayer modifications including a heated sprayer [55] have been developed to expand the range of ESI applications (Fig. 11.4). 

The design of a pneumatically assisted ESI interface differs from the pure electrospray interface in that it provides a pneumatic assistance for the spray process. This is achieved by admitting a concentric flow of an inert gas such as nitrogen around the electrospray plume. [56-58] Pneumatic assistance allows for higher flow rates and for a reduced influence of the surface tension of the solvent used. [59] Pneumatically assisted ESI can accommodate flow rates of 10-200 pi min  

In addition to high LC flow rates, solutions of high conductivity, and/or high surface tension are unsuitable for use with conventional ESI. An ultrasonic nebulizer can reduce such problems because it mechanically creates the spray. Unfortunately, the ultrasonically created droplets are comparatively large and this hin- 

For capillary zone electrophoresis (CZE) mass spectrometry coupling, another modification of an ESI interface has been developed. This interface uses a sheath flow of liquid to make the electrical contact at the CZE terminus, thus defining both the CZE and electrospray field gradients. This way, the composition of the electro sprayed liquid can be controlled independently of the CZE buffer, thereby providing operation with buffers that could not be used previously, e.g., aqueous and high ionic strength buffers. In addition, the interface operation becomes independent of the CZE flow rate. [62] 

Note Pneumatically assisted electrospray is also termed ion spray (ISP). However, the term ISP is not recommended instead of pneumatically assisted ESI because ISP i) represents a mere modification of the ESI setup and ii) is a company-specific term. [63] 

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