A method of ionising a sample is disclosed comprising performing an initial experiment comprising: (i) adding one or more reagents to an analyte sample; (ii) varying the composition and/or concentration of the one or more reagents; (iii) ionising the analyte sample including the one or more reagents; (iv) determining the composition and/or concentration of the one or more reagents which results in a desired, improved or optimised ionisation or other condition or parameter for one or more analytes of interest; and (v) determining one or more first retention times or one or more first retention time windows for the one or more analytes of interest. The method then further comprises separating an analyte sample using a first separation device and during the course of a single experimental run or acquisition varying the composition and/or concentration of one or more reagents which are added to an eluent which emerges from the first separation device. The composition and/or concentration of the one or more reagents which are added to the eluent is varied at the one or more the first retention times or during the one or more the first retention time windows so that an ionisation or other condition or parameter for the one or more analytes of interest is as desired or is improved or optimised.