a) Name two technical approaches to adapt ESI to high liquid flow rates.
Pneumatically-assisted ESI provides pneumatic assistance for the spray process by admitting a concentric flow of an inert gas such as nitrogen around the electrospray plume.
Solutions of high conductivity, and/or high surface tension are also not suitable for use with conventional ESI. An ultrasonic nebulizer can reduce such problems because it mechanically creates the spray. With both pneumatically-assisted ESI and ultrasonic nebulizer ESI the role of the high voltage almost reduces to the mere supply of electric charging of the droplets.
b) What is the purpose of the so-called sheath flow in CZE-ESI applications?
For capillary zone electrophoresis (CZE) mass spectrometry coupling, another modification of an ESI interface has been developed. A sheath flow of liquid is used to make the electrical contact at the CZE terminus, thus defining both the CZE and electrospray field gradients.
c) Can you tell the difference between pneumatically-assisted ESI and ion spray?
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.
d) Compare the liquid flow rates in nanoESI to those of conventional ESI and pneumatically assisted ESI.
Pneumatically assisted ESI can accommodate flow rates of 10-200 microliters per minute, conventional ESI works best at 1-20 microliters per minute, and nanoESI gives access to 20-50 nanoliters per minute.