Answer:
There are several ways to go about this.
If you want to determine ppm of a solute in water for example, you can determine the number of grams of water in 1 L by its density (1.0 g/mL=1000.0 g/L). Therefore, 1L of water has a mass of 1000.0 grams. If you add 1 gram of a solute to this, you end up with 1 gram, solute per 1000.0 grams solvent (1 part per thousand). Converting from ppt to ppm is done by multiplying by 1000; or, in this example, 1 ppt=1000ppm.