Adaptation to Different Flow Rates

The actual process of electrospraying disperses a liquid into an aerosol, which works best at flow rates of 1-20 pi min . This sets certain limits to its use as an LC-MS interface in respect to solvent properties such as volatility and polarity. Consequently, a number of sprayer design modifications have led to an expansion of the range of ESI applications (Fig. 12.5). 

The design of a pneumatically assisted ESI interface differs from the sinple electrospray interface in that it provides a pneumatic assistance for the spray process. This is achieved by supplying a concentric flow of an inert gas such as nitrogen around the electrospray plume [58-60]. Assistance by a nebulizer gas stream of about 1-21 min allows for higher liquid flow and for a reduced influence of the surface tension of the solvent [61], Pneumatically assisted ESI can accommodate flows of 10-200 pi min . In fact, all modem ESI interfaces equipped with a nebulizer gas or sheath gas line enclosing the spray capillary. Thus, most routine ESI measurements are done using pneumatically assisted ESI. 

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 for establishing the electrical contact at the CZE terminus, thus defining both the CZE and electrospray field gradients. This way, the composition of the electrosprayed liquid can be controlled independently of the CZE buffer, thereby enabling 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 [64]. 

Note Pneumatically assisted electrospray is also termed ion spray (ISP). However, the term ISP is not recommended in place of pneumatically assisted ESI because ISP represents a mere modification of the ESI setup and is a company-specific term [65]. 

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There may be several reasons for this set-up. One reason is that the intake capacity can be adapted to different flow rates from the recipient. It also increases the operational reliability. If one vacuum pump fails, it does not cause the whole production line to stop. Additional vacuum pumps can also be used to cover peak loads. This parallel set-up is used for vacuum control stations or where the pump sizes needed for the required intake capacity are not available. 

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