The invention relates to a substance determining apparatus (19) for determining a substance within a fluid. A location event determination unit (34) determines location events indicating that a particle, which may have attached the substance, is located in a sensing region and a property of the location events from the sensing signal generated by sensing the sensing region. A location events selection unit (70) selects a part of the location events having a property in a predefined property range, and a substance determination unit (40) determines the substance within the fluid (3) based on the selected part of the location events. This allows tuning the determination of the substance within the fluid to a desired property range, which can relate to a desired kind of bound particles, thereby reducing the influence of unwanted effects on the determination of the substance and, thus, improving the determination accuracy.