I'm measuring size distributions of aerosol Fe3O4 particles that first go through a water column, then through silica beads to dry the gas flow and then end up in an SMPS/Faraday cup. We get signals of very small particles (
To what size do you filter your water? How is the cleanliness of the system maintained and verified?
I use demiwater in the column, which is not further filtered. The system contains flexible carbon tubes and metal tubes that we don't extensively clean each time. But when there's no water in the column, there are no signals of small particles.
You need to filter better than you want to measure - in this case better than 10 nm. Plus there'll be particulate matter dislodged from surfaces. Contact the manufacturer for more advice.
I'd start at the beginning and ask how you generate or aerosolize the Fe3O4 particles. If it's from a solution then there'll be a surfactant to stabilize it. That surfactant unfortunately forms small nanoparticles when aerosolized.
It seems to me that the problem would most probably come from the water column used, e.g., contamination etc. We used to humidify filtered air by running it through water but did not see any signals in SMPS measurements as far as I remember.
Another comment just to add to Mr. Bischof´s suggestion: it´s a good point and you can test this assumption by reducing the amount of surfactant to be just enough to coat all the particles in the suspension (no free surfactant molecules) while maintaining the dispersion stability, and then see if you still get the peak. Or simply aerosolize surfactant solution without particles in it.
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