Please help me

on a mass spec graph it shows a single peak of 556 on the m/z axis for P
and then it asks what is the relative molecular mass of P
and you tick either
555
556
557
i ticked 556 and got it wrong, but the book says "for any compound, the m/z value of the molecular ion peak will be the same as the molecular mass of the compound" (assuming the ion has a 1+ charge)
can someone explain how it's not 556?
how will it be 555 because the molecular peak is clearly 556 even if its lost an electron

Reply 1

lordaxil

11

Difficult to say for sure without seeing the mass spectrum but, other than the answer given being incorrect, one other possibility is if there are radioisotopes present, e.g.

^{13} \rm{C}

which would give a small

M+1

peak in the spectrum. However, the main peak should still be at

M

.

Reply 2

username5390248

15

Original post by cherrybunny

on a mass spec graph it shows a single peak of 556 on the m/z axis for P
and then it asks what is the relative molecular mass of P
and you tick either
555
556
557
i ticked 556 and got it wrong, but the book says "for any compound, the m/z value of the molecular ion peak will be the same as the molecular mass of the compound" (assuming the ion has a 1+ charge)
can someone explain how it's not 556?
how will it be 555 because the molecular peak is clearly 556 even if its lost an electron

We've been told for our exam spec to minus 1 from the m/z value so maybe 555, but i would check with your teacher if you can :smile:

Reply 3

cherrybunny

OP

15

Original post by bacter1a

We've been told for our exam spec to minus 1 from the m/z value so maybe 555, but i would check with your teacher if you can :smile:

thank you

i thought so, but the book implied the m/z = the molecular mass

Reply 4

lordaxil

11

m/z is the molecular mass for +1 ions. If you are subtracting 1 it must be because there are heavier isotopes present, like

^{13} \rm{C}

or

^{2} \rm{H}

. Check your exam spec carefully.

(edited 3 years ago)

Reply 5

username5390248

15

Original post by cherrybunny

thank you

i thought so, but the book implied the m/z = the molecular mass

Yeah my teacher said it was because of the type of ionisation used but didn't explain further lol

Reply 6

jivinion

4

I think its because they used electronspray ionisation (not electron impact) during which all the atoms of the sample gain a proton (H+). So this increases the the mass by 1, but the relative molecular mass was originally 555 before it was ionised and gained a proton

Please help me

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