Measurement of electrostatic charge of an aerosol using a grid-probe

This paper describes an electrostatic grid-probe which can be used to study the charge of an aerosol emitted from a dry powder inhaler (DPI). Electrostatic charge plays a significant role in the function of an inhaler, separation of the drug and additive particles, and drug deposition in the lungs. The measurement system consists of three conducting grids through which the charged aerosol passes. The outer ones are connected to earth and the one in the middle is connected to an electrometer and a PC. Charge inside the probe induces a potential to the middle grid which is recorded as a function of time. When the distance between the grids is relatively small, the probe gives information about the charge distribution as a function of time. Therefore, the probe can be used to study whether the drug and additive particles are emitted separately from the DPI and the charge levels of both compounds. Measurements have been performed using several example powders and mixtures. Results show that the probe can separate signals from the drug and additive particles. Care has to be taken in interpreting the signals due to particles colliding with the grid and exchanging charge with it. Results were also verified by separating the aerosol physically using a high electric field separator. Separated particles were identified optically and also by using a differential scanning calorimeter.