The definition:

The abbreviation m/z is used to denote the dimensionless quantity formed by dividing the mass number of an ion by its charge number. It has long been called the mass-to-charge ratio although m is not the ionic mass nor is z a multiple or the elementary (electronic) charge, e. The abbreviation m/e is, therefore, not recommended. Thus, for example, for the ion $\ce{C7H7^2+}$, m/z equals 45.5.

Could someone please explain the definition? I found it a bit difficult to comprehend.

  • What is "ionic mass"?
  • What do they mean by "z a multiple or elementary charge"?
  • $\ce{C7H7^2+}$, m/z equals 45.5 -> what did they do here?

4 Answers 4


$m$ is the atomic mass of the ion (in units ($u$); $1u=1.665402\times10^{-27} \mathrm{kg}$), $z$ is the number of its positive/negative charges. $z$ should not be used in interchange with $e$ because it is dimensionless, while $e$ has the unit $\mathrm{C}$ (Coulomb). For $\ce{[C7H7]^2+}$, $m/z$ is given by (the unit of $m$ is omitted):

$$m(\ce{C7H7})/2=(7 \cdot 12+7 \cdot 1)/2=91/2=45.5$$


There are no differences simply the second notation is not recommended by IUPAC. The explanation is below:

What is ionic mass? The ionic mass is the real mass of the ion that takes in account the weight of the electrons in the overall weight. However electrons are much more lighter (0.548 mDa) then nuclei, so according to IUPAC definition m is intend to be A:

Total number of heavy particles (protons and neutrons jointly called > nucleons) in the atomic nucleus. Also called nucleon number. Symbol m in mass spectrometry.

And not the ionic mass (that in fact would more accurate).

What do they mean by "z a multiple or elementary charge"? This is another approximation they suppose that we could use an elementary charge +1, +2, +3, -1, -2, -3 etc. etc. that is not a real quantification in Coulumb of the electrical charge of the ion as stated Jannis Andreska.
$\ce{C7H7^2+}$, m/z equals 45.5 -> what did they do here? They simply divide m (see definition above) by z in this case (2+=2).

Addendum: This definition silently says that the m/z ratio is an approximation m is not the real weight of the ion (in fact is the weight only of the heavier particles and z is not the real charge of the ion) so it says: "don't use 'e' because this is an approximation we are not using the real charge nor the real weight, and the m/z ratio is is a dimensionless quantity!". I believe that in the future when even the lighter electrons, due the increasing accuracy of the instrument, will matter we will use $m_{i}/q_e$ where $m_i$ is the ionic mass and $q_e$ is the real charge... Only matter of time (and matter!)!

Reference here!

  • $\begingroup$ Thank you, but I have one last question - Normally in my exam papers they use m/z and m/e interchangeable. When the paper uses m/e is the calculation going to change? Because I normally simply divide the mass number by the elementary charge. $\endgroup$
    – confused
    Commented Apr 16, 2014 at 4:13
  • $\begingroup$ @confused yw no, is the same thing however the second notation is discourage because of the facts mentioned above. $\endgroup$
    – G M
    Commented Apr 16, 2014 at 8:11
  • $\begingroup$ I do not completely agree with your definition: in a mass spectrum, m should not refer to the total number of nucleons but really to the actual experimental mass (expressed in a.m.u.) of the molecule. Unfortunaltely, there are multiple definitions of the masses used in mass spec (see en.wikipedia.org/wiki/Mass_(mass_spectrometry)), and the notation m/z can apply to most of them. One should in addition to the m/z notation also indicate which mass definition is being used. $\endgroup$
    – PLD
    Commented Apr 16, 2014 at 11:21
  • $\begingroup$ @PDL thanks a lot for your comment, I've rearranged the question to remove the misconception, please add your answer so the question could be more complete to the others users. More answers make this site better! $\endgroup$
    – G M
    Commented Apr 16, 2014 at 12:29

One should note that the IUPAC definition for $m/z$ has changed in the latest recommendation (Definitions of terms relating to mass spectrometry (IUPAC Recommendations 2013)) which helps clarifying the question asked above.

The question of what the labeling of the x-axis in a mass spectrum is supposed to represent is listed as the most prominent Controversial term in the introductory section of this definition document. Thus the question remains somewhat open. The definition of $m/z$ has slightly changed compared to the 1991 one cited in the above question:

Abbreviation representing the dimensionless quantity formed by dividing the ratio of the mass of an ion to the unified atomic mass unit, by its charge number (regardless of sign). The abbreviation is written in italicized lowercase letters with no spaces.

The major change from the 1991 definition (quoted in your question) is the replacement of the mass number by the actual ratio of mass to the unified atomic mass unit. This follows the deveolopment of high resolution instruments and of ESI, which means that the $m/z$ scale has become abundantly used for multiply charged ions and that one cannot approximate anymore the ion mass divided by $1 u$ as the total mass number of the atoms present in the ion.

  • $\begingroup$ +1 so the gold book definition seems obsolete, it depends from which definition the chemist choose... $\endgroup$
    – G M
    Commented Apr 18, 2014 at 15:04

m/z is chemical jargon that no longer complies with the modern rules of metrology. Therefore: just foreget it. Instead follow this:

  1. Mass spectrometers measure the quantity mass/charge.
  2. The recomended symbol of mass is m
  3. The recomended symbol of charge is Q
  4. therefore, the best symbol of the derived quantity mass/charge is m/Q
  5. the unit of mass that is usually used in mass spectrometry is the dalton Da
  6. the unit of charge that is used in mass spectrometry is the atomic charge unit e
  7. therefore the unit of m/Q is Da/e which is also referred to as a thomson Th

To go back to your questions:

  • e is not a symbol of charge, it is a unit of charge
  • ionic mass is again chemistry jargon. It is like saying "my humic mass is 80 kg". Nobody would do this. It should say "m is the mass of the ion ..."
  • the mass m(C7H7^2+) = 91 Da
  • the charge Q(C7H7^2+) = 2e
  • therefore m/Q(C7H7^2+) = (91 Da)/2e = 45.5 Da/e = 45.5 Th

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