Just to give you a bit of background info, I essentially draw air through a device that separates out particulates based on their inertia and therefore size. There are 8 stages, each separates out the particles in the size range given in my initial post. Now, we know the mass of air that passes through the device (let's say 10 grams), and we know the mass of particles collected on each stage, so what I then do is calculated the amount of particles in a given air of mass (mgparticles/gair), for example: Stage 1 mass collected = 1mg. 10 grams air drawn through. Therefore, 0.1mgparticles/1gramair. (Particles between 0.1-0.2 um) Stage 2 mass collected = 0.1mg. 10 grams air drawn through. Therefore, 0.01mgparticles/1gramair. (Particles between 0.2-0.5 um) etc etc. Now, typically the 8 stages are summed and we get total particles (mg) per gram of air. This doesn't require normalisation. However, if we want to to see which size range of particles are predominant, then we need to take the size range of each stage into account. That's where we do the normalisation outlined previously. I understand the principle of the normalisation. Indeed, how can you compare the mass collected in each stage when some stages collect a bigger range of particles than others? What I can't get my head around is how the two are directly comparable. If you look at my original post the total mass collected is 9.1 mg, or 0.91mgparticles/gair if we assume 10g of air again. dM/dlogDp has the same units as mg or mgparticles/gair (you can do normalisation for either), but the numbers are far higher (72.5mg in fact). So, ultimately, which one is correct? Are they directly comparable? Does the normalised have any physical meaning. Both each individual stage and the total mass of particles is much higher. If someone asked me "What mass of particles are in 10g of air?", would I tell them 9.1mg or 72.5mg?