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I am looking at some data regarding a study around a channel-blocking agent called suxamethonium.

The histograms of the open times of the channels at varying concentrations of suxamethonium are given. However, I am having a difficult time figuring out the dosage. Here are examples of dose listing formats:

  • 200 μM-Sux
  • 500 μM-Sux

etc.

I understand that μ in this context is to mean micro and I believe the suffix "Sux" is supposed to stand for suxamethonium. But what is the capital "M"? I looked it up in a scientific notation table and it stands for mega. Am I supposed to infer that this is a liquid (i.e. ML)?

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    $\begingroup$ $\pu{M}$ is a unit, not an SI prefix, standing for molarity, a measure of the concentration of a chemical species in solution. $\endgroup$
    – a-cyclohexane-molecule
    Commented Jan 4, 2018 at 21:17
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    $\begingroup$ I think it's useful for future purposes to refer you to the IUPAC Green Book and IUPAP Red Book which contain a lot of similar (and much more) information. $\endgroup$
    – Linear Christmas
    Commented Jan 4, 2018 at 21:57

2 Answers 2

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You are right assuming that μ is a metric prefix, and that the number denotes the concentration of suxamethonium (suxamethonium chloride, I presume). However, this is supposed to be small caps "M" (as $\rm \small M$), not capital "M" (as $\rm M$). Also note that use of the term "molarity", as well as its notation $\rm \small M$, is discouraged [1, p.27]:

The term molarity and the symbol $\rm \small M$ should no longer be used because they, too, are obsolete. One should use instead amount-of-substance concentration of $\ce{B}$ and such units as $\rm mol/dm^3$, $\rm kmol/m^3$, or $\rm mol/L$.

And, of course, $1\,\rm {\small M} = 1\, mol/L$.

Also, check out the answer to a similar question "What does the unit M stand for?"

Reference

  1. Thompson, A.; Taylor, B. N. Guide for the use of the international system of units (SI). NIST Special Publication 811 2008.
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    $\begingroup$ I had never noticed that the unit is written as small caps instead of capital! :O (+1) $\endgroup$
    – Linear Christmas
    Commented Jan 4, 2018 at 22:17
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    $\begingroup$ @LinearChristmas No one really cares, I guess, and it's easier just to type capital M (I have this sin too). However, $\rm M$ is already reserved for Mega, so amount concentration units ideally needs another notation. I also caught up to this recently, probably from one of the great answers by Loong :) $\endgroup$
    – andselisk ♦
    Commented Jan 4, 2018 at 22:26
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    $\begingroup$ I’m sure you already know this, but technically, the prefix μ is not supposed to be italicised. But as far as I know it is not possible to have upright Greek characters in mhchem/MathJaX. $\endgroup$
    – orthocresol
    Commented Jan 6, 2018 at 0:55
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    $\begingroup$ @Alchimista If I understood you correctly, your web browser fails to render some MathJax code. Here is the screenshot:) I figured it might have to do with \pu{...} syntax, which is not properly rendered in mobile apps, so I replaced the code with the more compatible one. $\endgroup$
    – andselisk ♦
    Commented Jan 6, 2018 at 14:50
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    $\begingroup$ @orthocresol Although a bit annoying, upright 'mu' is possible (with Unicode), for example $\unicode[Times]{x3BC} \rm \small M$ gives $\unicode[Times]{x3BC} \rm \small M$; usage in mhchem is possible with additional $$ surrounding the Unicode character. It comes in handy for some formulae, e.g. this Loong's answer. $\endgroup$
    – Linear Christmas
    Commented Jan 6, 2018 at 15:09
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The capital M is simply an abbreviation or another way to denote mol/L, or moles per liter.

More specifically, the 'M' stands for the word 'molarity', which as a physical quantity is defined as: $$\ \frac{\text{Amount of Substance of Solute}}{\text{Volume of Solution}}$$

Therefore, $\pu{200 μM}$-Sux means $200$ micromoles of suxamethonium per liter, or $\pu{0.000200 mol/L}$ [because the micro (μ) symbol denotes $10^{-6}$].

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