The more comprehensive approach to assessment of ion suppression is to constantly infuse an appropriate concentration into the mobile phase flow, downstream from the analytical column, using a syringe pump and a tee union. A typical sample should then be injected through the HPLC inlet as per the usual analytical parameters.
Monitoring of detector response during this experiment should yield a constant signal appropriate to the concentration of infused species. Once the sample has been injected, a drop in signal intensity (or a negative response) should be observed any time a species is ionised in the ion source. This should allow the retention time of any such species under the analytical parameters of the assay to be determined. Any species causing a negative response may be considered to be contributing to ion suppression, but only if such species co-elute with the analyte of interest.
It is also important to consider that species contributing to ion suppression may be retained by the column to a much greater extent than the analyte of interest. To this end, the detector response should be monitored for several times the usual chromatographic run time to ensure that ion suppression will not affect subsequent injections.