Answer:  You need some additional information about the substances you are trying to convert.  In the future, we hope to provide a calculator for this that will make the calculation easy!

Explanation:  You need some additional information that is beyond the scope of our measurement conversion calculator.  First you need to know the molar volume of both of the substances (that would be their volume per mol in cubic meters per mol or compatible units), and you need to know the molar mass of the first substance (that would be its mass per mol in kilograms per mol or compatible units).  So, for example, if you are measuring parts per million of DDT in water, then you need the molar volume of DDT, the molar volume of water, and the molar mass of DDT.  Assume the molar volume of DDT is 1.5 liters per mol, and the molar volume of water is 1.2 liters per mol, and the molar mass of DDT is 1.8 kilograms per mol (these are not the actual values).  The goal is to find the number of micrograms for the DDT per milliliter of water given that there are 100 parts per million of DDT in the solution.  In order to have one mol of DDT, you would need 10,000 mol of water (because there are 100 ppm of DDT, or one part DDT for 10,000 parts water).  To have one mol of DDT, you would have 12,000 liters of water (10,000 mol of water at 1.2 liters per mol equals 12,000 liters), plus 1.5 liters of DDT (1 mol of DDT at 1.5 liters per mol equals 1.5 liters).  In total, you would have 12,001.5 liters of solution (12,000 liters of water plus 1.5 liters of DDT).  Since you have 12,001.5 liters of solution containing 1.8 kilograms of DDT (since there's one mole of DDT in that solution, and there are 1.8 kilograms per mol), said another way, that's 1.8 kilograms of DDT per 12,001.5 liters of solution. You can convert from that 1.8/12001.5 kilograms/liter to micrograms/ml to reach the result (in this example) of 149.981252343457 micrograms/ml.