sheath gas

https://doi.org/10.1351/goldbook.12571
Gas introduced through a tube that is coaxial with the electrospray emitter to pneumatically assist the formation of the sprayed droplets.
Source:
PAC, 2013, 85, 1515. (Definitions of terms relating to mass spectrometry (IUPAC Recommendations 2013)) on page 1590 [Terms] [Paper]