(A) Different shapes of stainless steel electrospray emitters investigated for optimal ESI performance: symmetrically tapered electrospray needle (1), blunt tapered tip (2), sharp tapered needle (3), 30° bevel tip made from 2 (4), 45° bevel tip made from 2 (5), 35° bevel tip with a smaller surface area (6). (B) Average signal-to-noise ratio of arginine as a function of flow rate and electrospray potential. Emitter geometry for each plot is indicated in the top right corner. Sample: 20 µM each of proline, threonine, isoleucine, and arginine in 0.2% formic acid and 50% methanol. Reprinted with permission from Ref [29]. Copyright American Chemical Society 2010.