EFFECT OF MULTIPLE SPRAYERS ON DYNAMIC-RANGE AND FLOW-RATE LIMITATIONS IN ELECTROSPRAY AND IONSPRAY MASS-SPECTROMETRY

The sample-ion signal in electrospray mass spectrometry is an approximately linear function of sample concentration up to 10(-5) mol/L. Dynamic range for the tetrabutylammonium ion signal and spray current leaving the nebulizer are determined with one and two sprayers, at flow rates of 5 and 10 muL/min, and at different positions of the sprayer. The dynamic range does not appear to be limited by the total amount (mass) of sample, but by the concentration of the sample solution introduced into the ion source, as is demonstrated by the use of two parallel sprayers. Limitation of dynamic range as a result of increasing repulsion between charged droplets at higher sample concentration appears to be unlikely. The efficiency of droplet charging gradually decreases with increasing sample concentration, while the detector signal abruptly reaches a plateau. Limitation just by insufficient charge on droplets is ruled out. Limitation of sample-ion signals can be attributed to an upper limit to the number of free quaternary ammonium ions at the surface of electrically charged droplets. Multiple sprayers can be used to increase the range of flow rates accepted by the electrospray interface. In the case of ionspray (pneumatically assisted electrospray) the stability of the ion signal at higher flow rates is improved by the use of multiple sprayers.