Trace-level results are often expressed using parts-per notation. The small number can look straightforward, but “parts” still needs a physical meaning. A mass fraction, an amount fraction and a mass concentration are different quantities, even when a report uses familiar-looking shorthand around them.
Read the scale and the quantity together
BIPM describes ppm as a relative value of 10⁻⁶ and notes the language-dependent ambiguity of abbreviations such as ppb. In an English document that defines a billion as 10⁹, one part per billion is a relative value of 10⁻⁹. Explicit ratio units avoid relying on that shorthand alone.BIPM — The International System of Units (opens in a new tab)
| Explicit expression | Equivalent expression |
|---|---|
| 1 mg/kg | 1 µg/g = 10⁻⁶ mass fraction |
| 1 µg/kg | 1 ng/g = 10⁻⁹ mass fraction |
| 0.20 mg/kg | 200 µg/kg |
These equalities concern mass divided by mass. They do not assert a liquid density or say whether the reported result refers to an element, a compound or a group of compounds. That analyte definition must come from the report.
Why ppm is not automatically mg/L
Milligrams per kilogram has a mass denominator; milligrams per litre has a volume denominator. To connect them, you need the mass of mixture per litre. A familiar approximation for some dilute aqueous systems should not be treated as a universal identity for every sample.
For a hypothetical mixture with density 1.25 kg/L and a constituent mass fraction of 2.0 mg/kg, the corresponding mass concentration is 2.0 mg/kg × 1.25 kg/L = 2.5 mg/L. If the density were 0.80 kg/L, the same mass fraction would correspond to 1.6 mg/L.
Identify what the reported mass belongs to
A trace result may refer to an element, a named compound or a defined sum. Keep that description with the number. Converting units does not convert one analyte definition into another, and a constituent's elemental mass is not automatically the mass of a compound containing it.
Similarly, distinguish a result stated for the original material from one stated for a prepared laboratory solution. The latter may require a documented preparation factor before it describes the original sample. Do not reconstruct that factor from a guess about the laboratory's routine practice.
A useful extraction record therefore includes analyte, value, unit, sample basis and any stated preparation factor. That is enough to show which conversions are justified and which remain unresolved.
Compare results and limits in compatible units
Suppose an illustrative maximum is 0.50 mg/kg and a result is 320 µg/kg. Converting the limit gives 500 µg/kg, so the numerical result is below that maximum on the same stated mass basis. The comparison still depends on the applicable decision rule and reporting context.
If the result is written “less than 320 µg/kg”, retain the inequality. It is not an exact value of 320, and it is not zero. A unit conversion must preserve what kind of result the laboratory supplied.
Check the prefix carefully when copying a maximum. A change from µg/kg to mg/kg introduces a thousand-fold difference. Matching the digits while losing the prefix can reverse the apparent interpretation of a trace result.
Prefer explicit ratio units in a summary
When the source's basis is clear, a summary such as “reported mass fraction, 320 µg/kg of the stated analyte in the stated material” is more informative than “320 ppb”. Keep the source's original wording available, especially if the abbreviation itself needs clarification.
Sources and further detail
- BIPM — The International System of Units (opens in a new tab)
Ninth edition, English version 4.01 (2026), tables 7 and 8 and section 5.4.7. Definitions and notation; examples in this article are independently constructed.
Sources checked 19 September 2026. Worked examples are illustrative unless a supplied report is explicitly identified. This article has not undergone independent scientific peer review.