It is important to select the appropriate substrate for PM collection based on the specific requirements of the study, the desired particle size range, the nature of the particulate matter, and the compatibility with subsequent analytical techniques. Substrate materials include cellulose, glass fibre, quartz fibre , polytetrafluoroethylene (PTFE) greased quartz reflector , mixed cellulose ester, greased aluminium foils, or Si wafer . The most widely used collecting substrate for EDXRF is a filtering membrane, offering a high capture efficiency, allowing for the collection of a wide range of particle sizes. While, in near real-time XRF instruments the substrate is a filtering tape . Reflective substrates are sometimes used when TXRF measurements are performed. The European Union's reference method for measuring heavy metals in PM consists in the analysis of air filtering membranes sampled at air monitoring stations . Filters have different pore sizes and filtration efficiencies, allowing for the collection of particles of various sizes and types. Filter membranes such as Teflon are compatible with a range of analytical techniques and are often used for gravimetric analysis, chemical analysis, and microscopic examination of PM. Compared to the other substrates, filters enable comparability of results across different monitoring sites and studies .