Buy Pure MDMA crystals is an organic amine which is an oily, unstable liquid in standard conditions. It is basic. During synthesis it is neutralized with hydrochloric acid to give a salt and water. The salt is MDMA.HCl and the water is removed.
This salt contains exactly 84% MDMA by mass and 16% HCl. When measured by a GC/MS, this is what it reads out. However MDMA.HCl is the only form produced anywhere so every dose, purity quote and mass you see is quoted for the HCl salt unless it is specifically stated otherwise.
The MDMA can only be up to 84% pure claims are based around either a poor understanding of the science or are a deliberate attempt to pass off inferior product as high quality.
But basically if anyone claims their MDMA is 84% pure then they have performed no analysis at all and should not be trusted. All doses you see for MDMA are referring to the HCl salt, which means that MDMA which is sold as “the purest you can get, 84%”, is treated as 100% pure.
The reason sellers do this is because they usually don’t have very pure MDMA, and they want people to make the mistake you have made and take more to account for crappy product. It is dishonest, confusing and misleading and if you have a choice you should avoid this vendor.