I am currently studying the textbook Mass Spectrometry - A Textbook, third edition, by Jürgen Gross. In chapter 1.4.5 Statistical Nature of Mass Spectra, there's a following example:

It is important to be aware that a single molecule can only yield one ion of one $m/z$ value. This ion may either reflect the intact molecule or any of its fragment ions. Statistics on thousands of ion formations and dissociations are needed to generate a useful mass spectrum exhibiting signals at different $m/z$ where each of them can be assigned a relevant relative intensity. To understand this, simply imagine a single methane molecule that gets ionized and detected as a molecular ion: this would lead to a spectrum showing a single line at $m/z$ $16$, the intensity of which would have no meaning beyond ‘yes’ and ‘no’ (Fig. 1.7a). Alternatively, the molecular ion might fragment to yield a single $\ce{CH3+}$ ion at $m/z$ $15$; again, the spectrum would show only one line. In fact, such a spectrum could not tell us whether the peak is caused by a molecular or a fragment ion. Eight ions might lead to a spectrum where each ion corresponds to $33.3 \%$ relative intensity (as in Fig. 1.7c) even though other distributions would be possible. Eleven ions could scale the intensity in $20\%$ steps, while 23 ions could lead to a spectrum with $10\%$ steps. It is evident that a spectrum with intensity levels as accurate as $0.1\%$ has to be based on thousands of ions (Fig. 1.7f).

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Reading this, I do not understand how the author used this mass spectrum to infer that the molecule in question is $\ce{CH3+}$. The author seems to imply that, given this mass spectrum ($m/z$ values of $16$, $15$, etc.), we are able to infer that the molecule is $\ce{CH3+}$, but he doesn't really explain how this is done.

I would greatly appreciate it if people would please take the time to explain how the author identified $\ce{CH3+}$.

  • $\begingroup$ So what atom is the mathematical difference between 44 and 28? Then identify 28. $\endgroup$ – user55119 Jun 11 at 22:31
  • $\begingroup$ It seems you have trouble to fit our Q&A model. Usually 2 questions is one too many. In this case, if you don't get the spectrum of methane, there's IMO like no chance to solve this monster you mentioned earlier. $\endgroup$ – Mithoron Jun 11 at 22:32
  • $\begingroup$ @user55119 I've cut it out, for more focus. $\endgroup$ – Mithoron Jun 11 at 22:32
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    $\begingroup$ Focus on the 4 strongest peaks, 12, 16, 28 and 44. Peak 22 (m/2z) is a doubly charged 44 ( m/z). Look here: webbook.nist.gov/cgi/… $\endgroup$ – user55119 Jun 11 at 23:43
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    $\begingroup$ Yup! The 28 peak is CO. The peak 29 is C(13)O. Peak 45 is O=C(13)=O and peak 46 is O=C=O(18). Natural abundance of C13 = 1.1%; O18 = 0.2%. $\endgroup$ – user55119 Jun 12 at 1:37

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