Effect of Solvent on Dynamic Range and Sensitivity in Pneumatically-assisted Electrospray (Ion Spray) Mass Spectrometry

Abstract

Mass-analyzed detector signal and spray current have been measured in pneumatically-assisted electrospray mass spectrometry. The sample was tetrabutylammonium bromide dissolved in water, methanol, acetonitrile, chloroform, dichloromethane or toluene. At low sample concentrations (less than or equal to 5 x 10(-6) M) the ion signal rose with increasing sample concentration. Above 10(-5) M the ion signal was fixed and independent of sample concentration, Comparison of signals with spray currents for tetrabutylammonium bromide at 2 x 10(-6) M in different solvents revealed a strong correlation between ion signal and spray current. Apparently, the abundance of the tetrabutylammonium ion at m/z 242 is fully controlled by the amount of charge on droplets, while other solvent properties such as volatility, surface tension and polarity do not play a role at low tetrabutylammonium bromide concentrations. Thus, water is a poor solvent for electrospray because it does not allow efficient droplet charging, not because it is less volatile and more difficult to spray than organic solvents. The ion signal at 2 x 10(-6) M tetrabutylammonium bromide in different solvents is highest for dichloromethane. At high sample concentrations (greater than or equal to 10(-5) M) the dependence of the ion signal on spray current is lost. It appears impossible to convert a high charge on droplets into sample ions. Creation of droplets having a surface fully covered with sample is assumed to be the cause of ion signal saturation. Increasing the sample concentration will only increase the number of ions inside a droplet. The number of sample ions at the surface escaping into the gas phase is fixed and independent of sample concentration above 10(-5) M.