The lead paint dust paper.

Single-Particle Detection of Lead-Based Paint Dust by Methylammonium Bromide Perovskite Fluorescence: A GFAAS-Calibrated, Single-Digit-Nanogram Per-Particle Detection Bound for Field Screening. Eric C. Ritter, Spirochaete Research Labs.

I ground up old white lead paint, picked out single particles under a microscope, and measured the lead in 54 of them one at a time on a graphite furnace AA. That gave me a way to estimate how much lead is in a particle from its size. Then I sprayed 58 more particles with methylammonium bromide in isopropyl alcohol and looked at them under 365 nm UV, down to the smallest ones I could move with an eyelash. All 58 glowed, including ones with about a nanogram of lead in them, and none of the 18 non-lead dust particles did.

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