Isotopic Peaks:

 The peak at m /z = 33 in the case discussed above is an isotopic peak and is called as [M+1] peak. The origin of this peak can be understood if we take into account the natural abundance of the isotopes of constituent atoms of a molecule. You know that, most of the elements exist in nature predominantly as a single entity i.e., as a collection of identical atoms. However, some elements have isotopes i.e., they exist as a mixture of atoms having same atomic number but different mass numbers. For example, carbon exists in nature as a mixture of ¹²₆C as well as ¹³₆C atoms. The natural abundance of ¹³₆C is 1.1% as compared to ¹²₆C.  This means that if we have a thousand atoms of carbon (having ¹²₆C isotopes) then there would be 11 atoms that would be of the ¹³₆C isotope. In case of the above example it amounts to saying that for every 1000 molecules of methanol containing a ¹²₆C isotope, there would be 11 methanol molecules with a ¹³₆C in them. These molecules would have a molar mass of 33 gmol^-1 as against the 'normal' value of 32 and hence the peak at m/z = 33. Further, since oxygen ( ¹⁶₈O and ¹⁸₈O ) and hydrogen ( ¹₁H , and ²₁H ) can also exist as isotopes, would also influence the spectrum and we may expect a very small signal at m /z = 34 i.e., at [M + 2]  in addition to the M  and [M + 1]  peaks.