- 1). Ensure that the parts per million value you are converting is a mass per mass value. This is the typical usage of parts per million. If you are making a conversion to mg/kg (which are both mass units), the ppm value you are converting is most likely a mass per mass value.
- 2). Write out the numerical value of your parts per million measurement. For example, if your measurement value was 328 parts per million, you would write out 328.
- 3). Write the units mg/kg after your numerical value. In the case of the example, you would write out 328 mg/kg. This is all that is required because parts per million on a mass per mass basis are identical to milligrams per kilogram. You can prove this to yourself by considering that a milligram is one one-thousandth of a gram, or 0.001 gram, and a kilogram is 1,000 grams. So the ratio of milligrams to kilograms is 0.001/1000 which is 0.000001, which is 1/1,000,000.
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