I'm slow at these sorts of things but have had to do these conversions a few times recently, and I see your difficulty. All the particles are different weights, it's not like a compound where you've got a particular weight per Mole, and the microplastics may each be a different dryness and have a bit of water or solution that you're counting them in, stored in their matrix as well...could you weigh two containers, each with the same liquid volume, but one containing the particles, and one without the particles, and get the weight of the particles that way? Then I guess it's just That Weight/That Volume and then convert to mg/mL...? There might be a quicker, smarter way...

To convert particles per cubic meter (particles/m³) to milligrams per milliliter (mg/mL), you need to estimate the mass of individual particles and use the relationship between volume units. First, determine the average mass per particle based on its volume and the material's density. For example, for spherical microplastic particles like PET (density ~1.38 g/cm³) or PP (density ~0.9 g/cm³) with a diameter of 200 µm (0.2 mm), calculate the volume of a single particle using the formula for the volume of a sphere, V = [4(pi)r^3] \3, where r = d/2. For a particle with a diameter of 0.2 mm, the volume is approximately 0.00419 mm³. Multiply this by the density to find the mass of one particle; for PET, this gives about 0.00579 mg per particle.

To convert the particle concentration, multiply the number of particles per cubic meter by the mass per particle, then divide by 10^6 to account for the conversion from cubic meters to milliliters ( 1 m³ = 10^6 mL ). For instance, if there are 10^6 particles/m³, the concentration in mg/mL would be (10^6 . 0.00579) / 10^6 = 0.00579 mg/mL). This approach assumes uniform spherical particles and may need adjustments for irregular shapes or size distributions.