A method of detecting electrically charged particles of a particle stream extracted from a particle source by means of an extraction device , in particular an ion stream extracted from a plasma of a plasma source by means of an extraction device , wherein the electrically charged particles of the particle stream move, in a direction of propagation (z), towards an analysing device which can analyse ions of an analyte by mass spectroscopy, which analyte is supplied to the plasma source, wherein the particle stream passes a measuring probe which is constructed as an ion optical lens of the analysing device and / or of the particle source , as a sector electrode which has two or more sectors which form a central opening , as an iris electrode which has a central opening with an adjustable diameter, or as a corona electrode in which a plurality of sectors are formed from metal tips which are arranged radially around a central opening , wherein the method comprises the following steps: - detecting measurement signals (M) generated by the measuring probe , wherein the measuring probe is arranged in the particle stream extracted from the particle source and / or constructed in such a way that a portion of particles of the particle stream impinges on the measuring probe and the measurement signal (M) generated thereby is characterised by the number of impinging particles, wherein the measuring probe is moved to different measurement positions in an x-y plane perpendicular to the direction of propagation (z) of the particle stream ; - storing the measurement signal (M) generated and detected by the measuring probe in each of the measuring positions; - determining properties of the particle stream on the basis of the stored measurement signals (M), wherein the properties of the particle stream relate to a spatial distribution of the electrically charged particles in the particle stream along an x-axis and / or a y-axis extending perpendicularly to the direction of propagation (z) of the particle stream ; and - controlling the particle source , the extraction device and / or the analysing device on the basis of the properties of the particle stream that have been determined. - detecting measurement signals (M) generated by the measuring probe , wherein the measuring probe is arranged in the particle stream extracted from the particle source and / or constructed in such a way that a portion of particles of the particle stream impinges on the measuring probe and the measurement signal (M) generated thereby is characterised by the number of impinging particles, wherein the measuring probe is moved to different measurement positions in an x-y plane perpendicular to the direction of propagation (z) of the particle stream ; - storing the measurement signal (M) generated and detected by the measuring probe in each of the measuring positions; - determining properties of the particle stream on the basis of the stored measurement signals (M), wherein the properties of the particle stream relate to a spatial distribution of the electrically charged particles in the particle stream along an x-axis and / or a y-axis extending perpendicularly to the direction of propagation (z) of the particle stream ; and - controlling the particle source , the extraction device and / or the analysing device on the basis of the properties of the particle stream that have been determined. <EOS>