Effect of solvent on dynamic range and sensitivity in pneumaticcally-assisted electrospray (ion spray) mass spectrometry

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 (⩽5×10−6 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 2×10−6 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 2×10−6 M tetrabutylammonium bromide in different solvents is highest for dichloromethane. At high sample concentrations (⩾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