Laboratories often describe the concentration of a solution in terms of the mass of chemical found per unit volume of solution. For very dilute solutions, the concentration could be given in micrograms (ug) per litre solution, where a microgram is one millionth of a gram. Parts per million (ppm) is another common way to describe concentration. It stands for the number of "parts" of chemical (such as grams) per parts (also grams) of total solution. You can convert between these two types of measurement by using the density of the solution.
Divide your value of micrograms per litre (ug/L) by 1000. Because a litre (L) is equal to 1000 millilitres (ml), this calculation will give you the concentration value in units of micrograms per millilitre. For example, if your solution is 250 ug/L, you would divide 250 by 1000 to get 0.250 ug/ml.
Divide the value you just obtained by the density of your solution in grams per millilitre. This will change the units again, this time to micrograms per gram (ug/g) of solution. In the case of the example, if the solution density was 1.08 grams per millilitre, you would perform the following calculation; 0.250/1.08 = 0.231 ug/g.
Rewrite the value you just obtained, keeping the same numerical value but using units of milligrams per kilogram (mg/kg). Because a milligram is equal to 1000 micrograms and a kilogram is equal to 1000 grams, this change in units does not change the actual value of the number because the ratio of the units has not changed. You now have your solution concentration in units of milligrams per kilograms, which is the same as parts per million. In the example, you would rewrite 0.231 ug/g as 0.231 mg/kg, which equals 0.231 ppm.
In dilute water solutions, where the density of the solution is close to 1 gram per millilitre, the concentration in micrograms per litre is almost exactly equal to the concentration in parts per billion (ppb).