[khemauck]

https://i.imgur.com/L3bW6m2.jpg

This is it here. I have been reading and reading for hours and I still can't understand, or at least, I can't relate the examples given to my sample. If someone could help me with brief instructions for this (the first of four problems) I would be eternally grateful- I need to get some sleep tonight..

Thanks in advance,
Antony.

[MOTOBALL]

In general, use the following steps for MS problems.

1. Detect the molecular ion (given here as m/z 136)

2. Is the molecular weight (MW; here is 136 daltons) odd or even ?

3. The nitrogen (N) rule states that EVEN number of N (0, 2, 4 , 6, 8 etc) gives EVEN MW
                                                    ODD                     (1, 3, 5, 7, 9 etc) gives ODD MW
4. Inspect the spectrum in the region of the molecular ion, say +/- 10 from mol. ion; are there any relative intensity patterns that are characteristic of the presence of certain elements (due to isotopes) ? Common elements that have a significant % of isotopes include: Boron, Bromine, Chlorine, Copper, Iron, Sulfur, Tin. 

5. If so, examine the spectrum very carefully to see if any characteristic isotopic patterns are present at lower m/z values; what mass was lost from the mol. ion to give that lower mass ion ?

6.  Construct a table with several columns

1st col. is m/z value, starting with molecular ion at top, of all important ions (even low level intensity) down to low m/z values

2nd col. is mass difference between the molecular ion and the m/z value

3rd col. is elemental formula of the mass difference; a mass diff. of 15 would be -CH3, etc.

7. NOTE not all ions are formed by a sequential loss of relatively small masses going down a chain.
Some quite lower m/z values may also be formed directly from the molecular ion.

8. Combine any masses (e.g. -CH3 or CO etc) to generate the required MW.

I shall expect a report, in triplicate, on my desk(top) by 0900 tomorrow !!!

Good Luck.

[MOTOBALL]

I forgot one other step.

For the determination of APPROXIMATE count of carbon atom: select a suitable (high intensity) m/z value at say X.  What then is the relative intensity of the m/z (X + 1) ion to the m/z X ion ?

Either measure height of m/z X and m/z (X+1) signals or read from the printout.

Divide that relative intensity by 1.1 to give approx. C atom count.

Thus, a C9 ion (also containing N, O, H etc) at m/z 152 will have an isotopic peak (mainly 13C9) at m/z 153.  Ratio of intensities of m/z 153 to m/z 152 will be experimentally in range 9.5-10; theoretically should be 9.9.

Carry on !

[khemauck]

Wow, thank you so much for all this. It's been very helpful.

So between the M+ and the base peak there's a difference of 79. Does this mean a fragment with a mass of 79 has been lost or should I perhaps look at the much smaller peaks bewteen the base peak and the molecular ion?

Once again, thank you for all this.

[MOTOBALL]

So between the M+ and the base peak there's a difference of 79. Does this mean a fragment with a mass of 79 has been lost YES

should I perhaps look at the much smaller peaks between the base peak and the molecular ion? Absolutely YES

Please review items #4 and #5 from my list, then revisit these two questions.