Electrospray diagnostics by Fourier analysis of current oscillations and fast imaging

The different spraying modes in electrospray ionization sources exhibit large variations in their ion yield and stability. To achieve consistently optimal ion production, active control of the spray parameters is desirable. To diagnose the changes in spraying mode, the spray current and its Fourier spectrum were monitored under a wide range of conditions, that is, as a function of the spray voltage, liquid flow rate, and composition. Most Fourier spectra indicated a strong dc component, a low-frequency branch at low flow rates and applied voltages, and a high-frequency branch and their harmonics. Changing of these parameters resulted in several spraying mode changes that were reflected in the Fourier spectra of the spray current. Significant mode changes and the malformation of the Taylor cone were detected as peak shifts or sudden changes in the spectrum quality. This was confirmed by fast imaging that showed a reduction in the size of the Taylor cone under hydrophobic tip conditions and rapid periodic ejection of filaments and droplets for high conductivity solutions. Comparing the oscillation frequencies of Taylor cones of different sizes, good correlation was found with the frequencies of capillary waves on comparablly sized liquid spheres. Spray stability was also linked with the positional stability of the contact line between the liquid meniscus and the capillary tip.