Figure shows the mass spectrum of the same compound from an electron- impact ion

Question

Figure shows the mass spectrum of the same compound from an electron- impact ionization source and an ionization source. Figure: Electron-impact spectrum (a) and chemical ionization spectrum (b) of the same biologically important compound. (From H. M. Fales, H. A. Lloyd, and G. A. W. Milne, J. Amer. Chem. Soc., 1970, 92, 1590-1597. American Chemical Society.) (a)Which mass spectrum would be best for determining the molecular mass of the compound? Why? (b)Which mass spectrum would be best for determining the chemical structure? Why? (c)Which spectrum is which? How do you know?

Explanation / Answer

Ans – Here, in this problem (ref. J. Am. Chem. Soc., 1970, 92, 1590-1597), the molecule of interest is ephedrine (A) which is actually a substituted benzyl alcohol.

Ephedrine Ephedrine - H2O    Ephedrine - NHCH3

A  C10H15NO B C10H13N C C9H11O+
Molecular Mass: 165.12 Molecular Mass: 147.1 Molecular Mass: 135.08

b) The Electron Impact (EI) mass spectra can be a complete proof for both molecular ion as well as chemical structure determination. As the ion peaks at m/e 148 and m/e 135 reveals the presence of the two functional groups, -OH and -NHCH3, and the ions at m/e 58 (CH3CH=NH+CH3) and 107 (C6H5CH=OH+).

c) The first spectrum is Electron Impact (EI) mass spectra and the second one is Chemical Ionisation (CI) mass spectrum. This is because the chances of molecular ion ((M+1)+ ion) is consistently more relatively abundant in the chemical ionization mode in the electron impact mode.

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