This is a follow-up question to Why are isotopes an issue in reading mass spectra?

A follow-up question: electrospray ionization (ESI) produces multiply charged species and charge can be >40 (for the purposes of m/z ratio), and assuming the difference in mass for an isotope is ~1 Da (with each additional neutron for an analyte at a charge of +1 in positive ionization mode) and assuming there is only one additional isotope, would the difference in weight for two isotopes each with a charge +10 be 0.1 Da?


2 Answers 2


More realistically if you have a molecule with Br, then the distance between isotopes:

  • Will be 2 $m/z$ units in case of 1 charge
  • And 1 $m/z$ units in case of 2 charges

You can use this information to determine if there are molecules ionized twice — their isotopes appear closer than expected.

Eventually since ESI/APCI adds $\ce{H+}$ for each charge, given original EMW was 100 Da, then:

  • $m/z$ in case of 1 charge: $(100+1)/1=101$ and $(102+1)/1=103$
  • $m/z$ in case of 2 charges: $(100+2)/2=51$ and $(102+2)/2=52$

Or in case of negative ionization you'll need to subtract 1 and 2 Da, respectively.


would the difference in weight for two isotopes each with a charge +10 be 0.1 Da?

Short answer: yes

Long answer: You said

assuming there is only one additional isotope

This is a weird assumption, as it is often not true. Molecules containing both $\ce{H}$ atoms and $\ce{C}$ atoms will have isotopic contributions from $\ce{H}$, $\ce{D}$, $\ce{^13C}$, and $\ce{^12C}$. Anything that has a sulfur atom will have contributions from $\ce{^32S}$, $\ce{^34S}$, $\ce{^33S}$, and $\ce{^36S}$. However none of those things really affect the answer to your question as I understand it. Isotopes are separated by almost 1 (or 2) Da increments.

The key is almost. Ions with a +10 charge would have isotopologues that were almost 0.1 Da apart, but not exactly. If the ion in question were only made of carbon, then a more exact value would be 0.10034 Da apart. Not all instruments can resolve a mass difference of 0.1 Da from 0.10034 Da, but some can.


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