Question

Identify the compound from the following mass spectrum and information. Please tell me how you determined what the compound is.

4. Identify the compound from the following mass spectrum and information 29 100 50 28 90 10 50 60 70 80 100 110 a 30(M) 31 m/z 12 13 14 15 16 28 29 32 Rel 0.07 3.3 4.3 4.4 1.7 31 100 89 1.3 0.21 Abund CO

Answer 1

In order to determine the unknown compound using mass spectrometry follow the given sequence:

• First check for the molecular ion peak (it is the peak that represents the molecular ion) which corresponds to the peak with highest m/z value, i.e., the last peak in the spectrum. Here in this case the molecular ion peak corresponds to the m/z value = 30.
• Now check for the base peak (it is the highest intensity peak in the spectrum, i.e. the longest peak) which corresponds to the most stable fragment formed after disintegration. Here in this case the base peak is at m/z value= 29.
• From the molecular ion peak we get to know that the molecular mass of the compound is 30, this give rise to two possibilities: either the given compound is ethane or formaldehyde.
• Formaldehyde gives the most stable fragment peak at m/z=29 whereas that of ethane is obtained at 15.

I-U- с + си H I Ethane (most stable frag Mass= CH3 = 12+3 = 15 Molecular mass=C₂ HG =(12x2)+(6x) = 24+6 = 30 ► OCH (most stable fragment) Marss cho Arch formaldehyele Molecular mass= COH₂ =(1X12)+(1816)+ (2x1) = 12+16+2 = 30 = 16+1+12 - 29/

Thus the given mass spectrum suggests that the unknown compound is formaldehyde.