Mass Spectrometry Watch

Fleuryette
Badges: 5
Rep:
?
#1
Report Thread starter 3 years ago
#1
Hello,

Basically, I suck at mass spectrometry, I don't understand about m/z values and m+ ions etc. and the textbook doesn't help at all!
Please can some smart person explain it to me?

Thank you x
0
reply
Pigster
Badges: 17
Rep:
?
#2
Report 3 years ago
#2
That counts me out, then.
0
reply
Pigster
Badges: 17
Rep:
?
#3
Report 3 years ago
#3
Which exam board are you?
0
reply
Zahid~
Badges: 18
Rep:
?
#4
Report 3 years ago
#4
Could you be more specific?
0
reply
Fleuryette
Badges: 5
Rep:
?
#5
Report Thread starter 3 years ago
#5
(Original post by Pigster)
That counts me out, then.
OCR A, it's a complete bugger, for example how to calculate the molecular mass of a compound from a peak, WHICH PEAK?! AHHH
0
reply
gapyear2018
Badges: 18
Rep:
?
#6
Report 3 years ago
#6
(Original post by Fleuryette)
OCR A, it's a complete bugger, for example how to calculate the molecular mass of a compound from a peak, WHICH PEAK?! AHHH
The molecular ion peak Is the last peak on the right of a mass spectrum


Posted from TSR Mobile
0
reply
Pigster
Badges: 17
Rep:
?
#7
Report 3 years ago
#7
The molecular mass is the same as the molecular ion peak (the one on the right).

OCR A doesn't worry about M+ peaks.

Often, they ask about the base peak (or a named peak). You have to be able to work out an ion that has the Mr the same as the base peak. Drawing the structure of said ion, with the charge in the correct place, is not something I can do over the Interwebs.
0
reply
gapyear2018
Badges: 18
Rep:
?
#8
Report 3 years ago
#8
The peak with the largest mass corresponds to an ion with just one electron removed.

So if the molecular ion peak was 30 the substance would have a relative mass of 30


Posted from TSR Mobile
0
reply
joshohill
Badges: 3
Rep:
?
#9
Report 3 years ago
#9
Each peak corresponds to a molecular ion...

One the furthest to the right gives Mr or the molecule you are looking it

Other peaks give parts of that molecule

Always right them with a '+' as they are IONS. There is always a mark for just writing the mr with a + sign next to it.

If you see a tiny little peak on the very right hand side, ignore it. It's irrelevant. ( usually has a height of 1)

You have to be patient and play around to try and find the structure of your molecule but if you subtract the Ar of each atom as you build it onto your molecule from your peak value you will see if you're on the right track or not... eg an m/z peak at 30... put two carbons down on your page and then draw on your 6 hydrogens ( subtract 24 then subtract 6 and you will soon see youve got the right molecule)

Quite often you will be given a infrared spectroscopy bit with the question too...use this it's essential.

Google 'common fragment ions a level' and you might be able to remember a few of the structures but it's not essential as you can work it out with a bit of thought and time in the exam.

check this site out....http://www.chemguide.co.uk/analysis/masspecmenu.html
0
reply
joshohill
Badges: 3
Rep:
?
#10
Report 3 years ago
#10
eg question 6a jan 11 f322 ..... look at the peak furthest to the right.. It has a value of 58. W are given in the question that we have 3 carbons and 6 hydrogens... so subtract the masses of those and you will be left with 16.... This is the value of the mr of the other atom in the molecule. In the question it has told us that it is oxygen we are looking for . So divide 16 by the mass of 1 oxygen which is 16 and you get 1.


aooo then says suggestion for formula of m/z peak at 29

I started off with 2 carbons and 6 hydrogens (ethane) and realised the masses of those gave me 30 not 29. So just take a hydrogen off and hey presto 29... (Dont forget to put a plus sign after as once again it is a fragment ION)


Hope all of this helps

Just treat it all as a puzzle, dont over complicate it... Possibly some of the easier stuff in the paper as it gives you all the info you need to know and the maths is just addition and subtraction
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

University open days

  • Cardiff University
    Undergraduate Open Day Undergraduate
    Wed, 27 Mar '19
  • University of Portsmouth
    Postgraduate and Part-Time Open Evenings Postgraduate
    Wed, 27 Mar '19
  • Middlesex University London
    Postgraduate Open Evening Postgraduate
    Wed, 27 Mar '19

How old were you when you first saw porn?

I've never seen it (78)
22.03%
Before I was 12 (124)
35.03%
13 (53)
14.97%
14 (46)
12.99%
15 (25)
7.06%
16 (9)
2.54%
17 (2)
0.56%
18 (3)
0.85%
Between the ages of 19 - 24 (4)
1.13%
Over 25 (0)
0%
12 (10)
2.82%

Watched Threads

View All