With regards to the following spectrum:
When asked for the M/Z for the molecular ion, the peak at 58 was taken, not 59.
What causes the peak at 59, and why isn't it taken as the molecular ion?
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Sign up to join this communityWith regards to the following spectrum:
When asked for the M/Z for the molecular ion, the peak at 58 was taken, not 59.
What causes the peak at 59, and why isn't it taken as the molecular ion?
Without knowing more details, it is hard to guess, but at this $m/z$, it seems likely that the peak is the result of one $^{12}\ce{C}$ being substituted by one $^{13}\ce{C}$. It is more useful to assume a uniform mass of 12 for carbon when analyzing such a spectrum.
Note that with more carbons in a larger molecule, it becomes more likely that at least one of them is a $^{13}\ce{C}$. Thus, the spectra become more complicated that way.
This whole analysis goes out the window if this is not an organic molecule, though similar patterns may arise for other molecules - it depends on the isotopes and their distribution.
The peak at $m/z = 59$ with lower intensity in respect to the one at $m / z =58$ (the molecular ion) is not overseen. Mass spectroscopy is capable to deliver information about the isotopic composition of your sample, too. Since -- accounting over all carbon atoms of your sample -- about 1.1% are of the non-radioactive isotope of $\ce{^{13}C}$, you may use this information to determine the carbon atoms in total present.
The story indeed contiues further, as to establish by this "isotope fingerprint" if you have one or two chlorines per molecule present ($\ce{^{35}Cl}$ and $\ce{^{37}Cl}$), bromines, presence / absence of nitrogen, etc.:
(source)