In an article I recently submitted, a reviewer asked that I provide a concentration in μg/kg instead of ppb (parts per billion), and mentions that the later is not correct. I am not a chemist, and I thought that 1 μg/kg = 1 ppb.
Is 1 ppb equal to 1 μg/kg ? What is a reason to consider ppb as incorrect?
You are correct suggesting that 1 μg/kg implies 1 ppb, however the reverse is not true. For instance, 1 ppb can also be 1 nmol/mol, and the reader will never have a chance to deduce which one is it unless you explicitly define the usage of the "parts per something" in the text. This clutters the manuscript with redundant notes and causes overall confusion.
IUPAC
IUPAC's “Green Book” lists ppb and related symbols (ppm, ppt, ppb etc.) as deprecated [1, p. 98]:
Although ppm, ppb, ppt and alike are widely used in various applications of analytical and environmental chemistry, it is suggested to abandon completely their use because of the ambiguities involved. These units are unnecessary and can be easily replaced by SI-compatible quantities such as pmol/mol (picomole per mole), which are unambiguous. The last column contains suggested replacements (similar replacements can be formulated as mg/g, μg/g, pg/g etc.).
$$ \begin{array}{lllll} \hline \text{Name} & \text{Symbol} & \text{Value} & \text{Examples} & \text{Replacement} \\ \hline \ldots & & & & \\ \text{part per billion} & \text{ppb} & 10^{-9} & \text{The air quality standard for ozone is a} & \pu{nmol/mol} \\ & & & \text{volume fraction of}~\varphi = 120~\text{ppb} & \\ \ldots & & & & \\ \hline \end{array} $$
ISO-80000-1
ISO-80000-1, section 6.5.5 The unit one [2, p.20] stresses out the necessity for the coherent usage of “per” units in general, and prohibits the usage of “parts-per” units:
Abbreviations such as ppm, pphm, ppb and ppt are language-dependent and ambiguous and shall not be used. Instead, the use of powers of 10 is recommended.
European Pharmacopoeia
According to European Pharmacopoeia, section 1.2. OTHER PROVISIONS APPLYING TO GENERAL CHAPTERS AND MONOGRAPHS [3, p. 4], in the absence of context ppm implies mass fraction:
The expression ‘parts per million’ (or ppm) refers to mass in mass, unless otherwise specified.
In 2020-02-26 there was a major accident with both engines stalling on the Airbus aircraft resulting in emergency landing due to the overdose of a biocide fuel additive. A base engineer was not familiar with the term “ppm”, and since the term was not defined in the maintenance manual, he incorrectly used an online calculator, which resulted in overshooting the correct biocide-to-fuel ratio 38 times. The fact that the instructions for two different biocides were not only extremely brief, but also referred to different ppm values by mass and by volume also greatly increased confusion:
Unit standardization
The dosage of biocide required was previously provided in ppm by weight for Biobor® JF and by ppm by volume for Kathon® FP 1.5. There is now industry-wide agreement to use mL/L unit for both Biobor® JF and Kathon® FP 1.5 biocides. Airbus maintenance procedures for all Airbus aircraft, the AMM for A300/A310/A320/A330/A340/A380, MP for A350 aircraft and AMP for A220 aircraft, were updated accordingly by removing the use of the ppm unit.
Luckily, the plane managed to land safely and there were no human casualties. This accident made the aircraft manufacturer to take action against using ppm notation in the future versions of technical documentation, and include transparent definitions for the “parts-per” notations still used in the current service manuals [4, p. 105].
