Capture of magnetic particulate matter directly from air on silicon substrates

Abstract

A major fraction of airborne particulate matter is magnetic. The origin of these magnetic particles is anthropogenic (industry, traffic), and often accompanied by traces of very toxic heavy metal pollutants. The magnetic fraction of airborne particulate matter is therefore a good indicator for the amount of air pollution, and can serve as a first indicator for toxicity.We collected airborne magnetic particulate matter directly onto silicon substrates by means of a stack of permanent NdFeB magnets (Figure 1). Collection was performed in open air along a campus road for a period of 2-4 weeks. For comparison, a control substrate was mounted in a similar fashion, nearby but not on a magnet. The silicon substrates can be readily used in a variety of surface analysis techniques. The morphology of the captured particles was analyzed by optical and electron microscopy, as well as by AFM. The substrate that was on the magnet stack showed an increased amount of particles. The particle size varied from 50 micrometer down to diameters below 100 nm. Many of the particles are perfectly round spheres with a diameter of a few up to tens of micrometers. SEM-EDX analysis showed that particles that are captured magnetically mainly consist of iron-oxide. VSM analysis showed that the substrate that was on the magnet has a magnetic moment that is six times higher than the substrate that was not in the magnetic field gradient (Figure 2). The average collection of magnetic particulate matter on the 5mm×5mm area is approximately 4 nAm2/day, which translates to 46 ng/day if the particles are made entirely of iron-oxide.These experiments show that one can collect magnetic particulate matter directly from the air, without using filters and complex post-treatment procedures that change the particle composition and cause clustering. Magnetic particulate matter sampling is therefore a promising complementary technique that can be applied in for instance air quality monitoring stations. **