[shafaifer]

First organic mass spectrum to interpret (see attachment): Identify the molecule. I am just about to get into it, and I have done / thought this:

Molecular ion = 148 g/mol.

148 g/mol - 105 g/mol = 43 g/mol  Indicating loss of propyl.
43 g/mol - 27 g/mol = 16 g/mol  might indicate loss of oxygen.
120 g/mol - 105 g/mol = 15 g/mol  maybe loss of methyl.

43 g/mol + 16 g/mol = 59 g/mol - clearly there is more interpret. Is my interpretation so far correct? What advise could be given if my interpreration is correct?

Kindest regards/

Shafaifer

[MOTOBALL]

Let's start with some terminology regarding mass spectrometry----A mass spectrum is a plot of m/z values (X-axis), not g/mol vs. Relative Intensity (Y-axis).

Step 1

Perform a visual inspection of the mass spectrum.
The molecular ion, m/z 148, shows the absence of Br and Cl atoms (WHY )
The presumed molecular weight (MW 148 Da) is even, and therefore the molecule contains an even number of N atoms (0, 2, 4, 6, 8 etc) See the NITROGEN RULE !!!!!!
Check for absence/presence of "characteristic" m/z values; a "characteristic" ion is one that suggests the presence of a certain structural feature.
Is m/z 77 a characteristic ion  (WHY/WHY NOT ??)
 
Step 2

Set up a table with 4 columns.
Column 1: list m/z values from highest (m/z 148) to lowest (m/z 27). GENERALLY, as a first pass, ignore low intensity (< 5%) m/z values.
Column 2: mass difference to next highest m/z value (148 to 120 is 28); 120 to 105 is 15).
Column 3: possible elemental compositions for the mass difference (28 is C2H4 or CO)
Column 4: possible structural feature that corresponds to the elemental composition.

Step 3

On a piece of paper, draw the possible structural elements that you have derived from your analysis.  Try to connect them such that all of the major m/z values can be formed from the structure; be aware that sometimes, two (or more) different structures (isomers) can be constructed that will generate very similar mass spectra.
Check that the elemental composition (C, H, N, O, etc) of the assembled molecule gives MW 148.

Good Luck !

[shafaifer]

Thank you very much. I identified the molecule as an aromatic ketone. Why are you angry with me?

[MOTOBALL]

I'm not at all angry with you, just trying to get you started with the correct terminology for a topic that is obviously new to you.

[MOTOBALL]

"See the NITROGEN rule !!!!" is purely for emphasis, as is the WHY/WHY NOT and  the "GENERALLY." as these are all important things to consider in MS interpretation.

There was no reason for me to be angry, and I was not.  Had I been, I would not have spent so long typing out my original post.

[kriggy]

I would add that you should calculate the amount of carbon atoms in the molecule, if you know the intensity of molecular peak and M+1.