How many peaks of HCl are found in a mass spectrum?

Question

In a mass spectrometer some hydrogen chloride molecules will split into atoms. Chlorine has two isotopes. The hydrogen involved here is the isotope $\ce{^1_1H}$ only.

How many peaks does a mass spectrum will show?

My answer was two with $\ce{H^35Cl}$ and $\ce{H^37Cl}$ but I found the answer to be wrong. Where have I been deceived?

I also found the mass spectrum in CIE 9701/02 2003:

Why are there peaks for $m/z = 35$ and $37$? Even if I consider them to be chlorine's, why aren't there any for hydrogen?

Answer 1

Collision between an electron and the molecule during a MS-experiment usually results in two phenomena:

\begin{align} \begin{cases} \ce{AB + e &-> [AB]+ + 2e} &\text{electron ionization}\\ \ce{AB + e &-> [AB]-} &\text{resonance capture} \end{cases} \end{align}

Resonance capture is less frequent and usually doesn't add much to the spectra. For $\ce{HCl}$ considering electron ionization as the primary process the following process takes place:

$$\ce{HCl + e -> [HCl]+ + 2e}$$

Based on ionization method (e.g. applied energy) several fragmentation processes may accompany the formation of cation/cation-radical ($\ce{[HCl]+}$/$\ce{[HCl]^{+.}}$):

\begin{align} \ce{[HCl]+ &-> H+ + Cl^. }\\ \ce{[HCl]+ &-> H^. + Cl+ }\\ \ce{[HCl]+ + e &-> H+ + Cl-} \end{align}

Fragmentation is often followed by recombination acts of different degrees of probability, e.g.: \begin{align} \ce{H+ + Cl- &-> HCl}\\ \ce{H+ + HCl &-> [H2Cl]+}\\ \ce{H^. + [H2Cl]+ &-> [H3Cl]^.+} \end{align}

One can possibly observe peaks of $\ce{[H]+}$ and $\ce{[H2]+}$, but those are usually have negligible intensities due to recombination processes.

Regarding the spectra OP provided, it looks somewhat strange as the base peak @ $m/z = 36$ is not $100\%$ (approx. $90\%$ instead). I would recommend to check for the experimental data in more reliable sources, e.g. in NIST database:

Considering the fragmentation processes and the fact that $\ce{^{35}Cl} : \ce{^{37}Cl} = 3 : 1$, one can perform the following peak assignment:

\begin{array}{rrr} m/z & \text{r.i.}, \% & \text{fragment}\\ \hline 35 & 18 & \ce{[^{35}Cl]+}\\ 36 & 100 & \ce{[^1H^{35}Cl]+}\\ 37 & 6 & \ce{[^{37}Cl]+}\\ 38 & 33 & \ce{[^1H^{37}Cl]+}\\ \hline \end{array}

Alternatively, for $m/z = 37$ and $m/z = 38$ fragments $\ce{[^1H2^{35}Cl]+}$ and $\ce{[^1H3^{35}Cl]+}$, respectively, could be proposed, but it wouldn't be correct due to the following:

1. recombination acts leading to the formation of large molecules as detected end-products are less possible and usually result in much smaller peaks;
2. observed relative intensities are not in agreement with isotope distribution for chlorine ($\ce{^{35}Cl} : \ce{^{37}Cl} = 3 : 1$, which is in agreement with r.i. ratios for $\ce{[^{35}Cl]+} : \ce{[^{37}Cl]+} = 18 : 6$ and $\ce{[^1H^{35}Cl]+} : \ce{[^1H^{37}Cl]+} = 100 : 33$, but not with alternatively proposed fragments);
3. the problem itself states that

   hydrogen chloride molecules will split into atoms.