Say NO to PPM

Say NO to PPM

The problem with parts per million (ppm) is that its meaning is ambiguous when used as a mass fraction concentration unit. Its use is to be avoided in place of more specific terminology.

Examples of good and bad practice

Bad practice

The concentration of this analyte in SRM 610 is 1234 ppm …

Good practice

The mass fraction of this analyte in NIST SRM 610 was 1234 µg g-1 …


A WIKI search of the acronym ‘ppm’ reveals a bewildering range of definitions in computing, finance, medicine and the names of organisations (my favourite being the Scouts Association of Malaya – in Malayan, of course). So, if we limit ourselves to the measurement sciences, what is wrong with ‘ppm’ as a mass fraction concentration unit?

The problem is that ‘ppm’ is not sufficiently specific and can lead to ambiguities. This ambiguity is best illustrated by the following examples:

Replace ‘ppm’ as a solution mass concentration by µg ml-1.

Replace ‘ppm’ as a mass fraction concentration in the solid by µg g-1 (or mg kg-1).

Retain ‘ppm’ if its meaning is designed to represent a fraction (in a similar way to ‘%’).

Index to terms

Concept Metrological terms considered Metrological terms covered
Say no to ppm The ambiguities associated with the use of ‘ppm’
Sigma is out, standard deviation is the way to go! What symbols are used to represent the properties of population- and sample-distributions
Standard ≠ reference material Avoiding the use of the term standards when referring to (certified) reference materials or calibrators Reference material, certified reference material, standard reference material, calibrator, calibration, validation, measurement standard (étalon), verification
Please, no more errors from your laboratory Explaining the difference between uncertainty and error Measurement uncertainty, measurement error, systematic measurement error, measurement bias, random measurement error, confidence level
Precision Distinguishing between repeatability, intermediate precision and reproducibility and discouraging the use of ‘internal precision’ Measurement precision, repeatability condition of measurement, intermediate precision of measurement, reproducibility condition of measurement, repeatability, intermediate precision, reproducibility
Small bias yes, high accuracy no Explaining the difference between accuracy, bias and trueness Measurement accuracy, measurement trueness, measurement bias

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