# What is the difference in mosM and mOsm units, if there is any at all?

I'm reading Agre's paper on aquaporins [1] and he uses a unit called "mosM" throughout the paper, for example:

… ocytes were transferred from 200 mosM to 70 mosM modified Barth's buffer.

I just want to understand what this unit means, is it the same as mOsm? I know this might be an very stupid question but hey, I'm only a undergrad.

### Reference

1. Preston, G. M.; Carroll, T. P.; Guggino, W. B.; Agre, P. Appearance of Water Channels in Xenopus Oocytes Expressing Red Cell CHIP28 Protein. Science 1992, 256 (5055), 385–387. DOI: 10.1126/science.256.5055.385.
• Remember that "paper never refused ink". Having a wrong notation in Science magazine does not endorse its correctness. Dec 16 '20 at 15:26
• @M.Farooq It's not just Science, Nature uses this symbol too: doi.org/10.1038/46045. Dec 16 '20 at 17:06

I think the clinical chemistry notation has not been standardized but certainly it has been inspired by chemists.

The capital M in mosM emphasizes that it is milliosMolar. Chemists use capital M to denote molarity.

On the other hand, mosm indicates milliosmolal. Chemists use small m to symbolize molality.

This notation is not universal.

From this slightly dated article on Osmolality, which is behind pay-wall Osmolality, Harry F. Weisberg Laboratory Medicine, Volume 12, Issue 2, 1 February 1981, Pages 81–85.

### Terminology and Units

In 1934, Gamble, et al.,11 introduced the term ”osmillimole” which has been successively transformed into mosM (milliosmolar), mosm (milliosmolal), the noncommittal mOsm, and the present-day mosmol (milliosmole). Raoult’s law states that the lowering of the vapor pressure (one of the four ”colligative” properties of dilute solutions) is proportional to the mole-fraction (more easily calculated from molality) of solute present in the mixture.

Judging from the following table consisting of compiled notations presented in offline edition of The ACS Style Guide and AMA manual of style, section 13.12 Units of Measure:

$$\begin{array}{ll} \hline \text{Unit of measure} & \text{Symbol} & \text{Ref.} \\ \hline \text{osmolar} & \text{osm (also osM, Osm)} & \text{[1, p. 191]} \\ \text{osmole} & \text{osm} & \text{[2, p. 635]} \\ \text{microosmole} & \text{µOsm} & \text{[2, p. 634]} \\ \text{milliosmole} & \text{mOsm} & \text{[2, p. 634]} \\ \hline \end{array}$$

symbols $$\pu{Osm}$$ and $$\pu{osm}$$ refer to the amount-of-substance of an osmotically active entity, and symbol $$\pu{osM}$$ refers to osmolar concentration suggesting the following relation:

$$\pu{1 osm L^-1} = \pu{1 Osm L^-1} = \pu{1 osM}$$

The letter “o” in $$\pu{Osm}$$ appears to be capitalized for the sake of better readability when used in conjunction with metric prefixes, and the letter “m” in in $$\pu{osM}$$ is likely capitalized to resemble analogy with the unit symbol for molarity (small capital letter “m”: ᴍ).

### References

1. The ACS Style Guide: Effective Communication of Scientific Information, 3rd ed.; Coghill, A. M., Garson, L. R., Eds.; American Chemical Society; Oxford University Press: Washington, DC; Oxford; New York, 2006. ISBN 978-0-8412-3999-9.
2. AMA Manual of Style: A Guide for Authors and Editors, 11th ed.; American Medical Association, Ed.; Oxford University Press: New York, 2020. ISBN 978-0-19-024656-3.
• Does the ACS Guide say anything about the osmolality? IUPAC still has this term with a different notation. Dec 16 '20 at 19:16
• Thanks for the answer, please correct me if I'm wrong, it is the same as in 1mol/L = 1M, hence 1osmol/L=1osM? Dec 16 '20 at 19:18
• @M.Farooq No, the third edition of the ACS Style Guide doesn't mention osmolality. Good point regarding IUPAC though. Dec 16 '20 at 19:46
• @RafaelFranco Yes, this would be my interpretation. Minor correction: don't forget to put a space between the numerical value and the unit symbol. Dec 16 '20 at 19:46