Abstract: For determining the content and the chemical composition of particulate compounds in a liquid flow (15), the particulate compounds are collected on a filter (20) in a container (19) of a material which is permeable to microwaves. When carrying out the determination, samples of a fixed size of the liquid in the flow are supplied batchwise to the container on one side of the filter via an openable and a closable connection (16, 17) between the liquid flow and the container. At the same time, liquid from each liquid sample supplied batchwise, which has passed through the filter, is discharged from the container via an openable and a closable outlet (30, 31) on the other side of the filter.