SHORT TIME-SCALE OBSERVATION OF AN ELECTROSPRAY CURRENT

The design and use of an experimental device for investigating the time-dependent structure of an electrospray current is described. Various electrolytes, including aqueous solutions, were submitted to electrospray and the instantaneous current intensities were monitored by means of a fast current measurer. The intensity proved discontinuous and roughly periodic in almost every case. By recording the intensity pattern on a fast oscilloscope, an estimate of the 'period' was obtained. A typical structure (obtained from a 10 mm NaBr methanolic solution, 'reduction mode') consists of series of groups of peaks. The charge per peak is approximately 10(-10) C and does not change much with the experimental parameters (electrolyte concentration, nature of the solvent, value of the high voltage). The current variation seems to be mainly related to the number of peaks rather than to the heights of the peaks. These results are consistent with commonly observed averaged spray intensities (0.1 to 1 muA) and suggest a pulsed droplet emission frequency of the order of 300 kHz.