Hey there! Sign in to join this conversationNew here? Join for free
Turn on thread page Beta
    • Thread Starter
    Offline

    4
    ReputationRep:
    I really don't understand mass spectrometry and I have a chemistry mock next week and no lessons before then, so can't ask my teacher, I would really appreciate if someone could help me!

    The specification says that we need to know how to:
    (u) use data from a mass spectrometer to: (i) calculate relative atomic mass and the relative abundance of isotopes, (ii) work out the relative molecular mass of molecules and understand that other peaks are caused by fragments of the molecule (no detail required at this stage).
    • Peer Support Volunteers
    • Study Helper
    Offline

    19
    Peer Support Volunteers
    Study Helper
    (Original post by lizz-ie)
    I really don't understand mass spectrometry and I have a chemistry mock next week and no lessons before then, so can't ask my teacher, I would really appreciate if someone could help me!

    The specification says that we need to know how to:
    (u) use data from a mass spectrometer to: (i) calculate relative atomic mass and the relative abundance of isotopes, (ii) work out the relative molecular mass of molecules and understand that other peaks are caused by fragments of the molecule (no detail required at this stage).
    It may be easier to explain how to calculate RAM if you give us an example of a question.

    To calculate percentage relative abundance of isotopes:

    \dfrac{\mathrm{relative \ abundance}}{\mathrm{total \ relative \ abundance}} \times 100

    The line furthest to the right of the spectrum (ignoring isotopes) is called the Molecular Ion (M+). The mass of the Molecular ion gives you the Mr of the molecule.

    When the molecule is bombarded with electrons, some of the molecules break up into fragments of the molecule, showing a fragmentation pattern on the spectrum. These fragmentation patterns can be used to identify molecules.

    I think this is all you need to know?
    Offline

    4
    The mass spectrometer is an instrument used for measuring the masses of atoms and molecules. It can also be used to measure the relative abundance of different isotopes and to predict the structure of more complex molecules.



    The four stages of Spectroscopy are:
    Ionisation
    A vaporised sample is placed in the spectrometer. The atom/molecules diffuse into the path of high energy electrons fired from an electron gun and are ionised.
    e.g.
    M(g) → M+(g) + e-

    Some doubly charged ions may also be produced, but in smaller amounts are more energy is required to ionise the sample twice.

    Acceleration
    The positive ions accelerated by an electric field and focused into a beam by passing them through a series of negative plates.

    Deflection
    The beam of fast moving, positive ions is deflected by a strong magnetic field. The magnitude of the deflection depends on the m∕z of the ion. When the ratio is small, the deflection is large. The magnetic field can be controlled for different m∕z.

    Detection
    The ions in the beam are detected electrically. When an ion hits the detector, it accepts an electron. This creates a small electric current which is amplified and produces a signal on the computer. By looking at the relative intensity of the peaks, we can produce a spectrum to compare the different m∕z peaks.


    After detection, the question might give you a mass spectra too look at, such as:

    You have to multiply the m/z ratio by its abundance, and then divide by the total abundance. So here, it would be:
    (64 x 12) + (66 x 8 ) + (67 x 1) + (68 x 6 )
    12+8+1+6

    = 65.6 (answer)

    Hope that helped.
    • Community Assistant
    • Study Helper
    Offline

    17
    ReputationRep:
    Community Assistant
    Study Helper
    (Original post by lizz-ie)
    I really don't understand mass spectrometry and I have a chemistry mock next week and no lessons before then, so can't ask my teacher, I would really appreciate if someone could help me!

    The specification says that we need to know how to:
    (u) use data from a mass spectrometer to: (i) calculate relative atomic mass and the relative abundance of isotopes, (ii) work out the relative molecular mass of molecules and understand that other peaks are caused by fragments of the molecule (no detail required at this stage).
    Check out this page:

    http://www.ibchem.com/IB/ibnotes/full/ato_htm/2.2.htm
    • TSR Support Team
    Offline

    21
    ReputationRep:
    TSR Support Team
    Right, do you understand the concept - ionisation (and fragmentation) of gaseous molecules, then acceleration and detection to produce the mass spec graph?

    As for (i): To calculate the relative abundance of isotopes, you look for peaks where the difference is due to the different isotopes. For example, if I had CH3Cl, I'd look for the peaks at m/z 50 & m/z 52, as the difference between those is due to the Cl35 and Cl37 isotopes. Then you look at the intensity of the peaks - the difference in the sizes of the peaks shows the relative abundancy - so in my example, the peak at m/z 50 will be 3 times the size of the peak at m/z 52, telling me the relative abundancy of the isotopes is 75% Cl35 to 25% Cl37
    Then, calculating the relative atomic mass of an isotope - you're looking for the average - so if element X existed as X57, X59 & X61, in the ratio of 2: 2 :1 then I know:
    40% is X57
    40% is X59
    20% is X61

    and, the RAM would be \frac {(2 * 57) + (2 * 59) + 61} {5} which equals 58.6, making that the RAM of X.

    For (ii) What this is is about spotting the parent molecule, which will be the one at the highest m/z, as it's the molecular ion. After that it's working out what the other peaks show the loss of - for instance, if my parent molecule was CH3CH2CH2CH3 then the parent ion peak would be m/z 58, and I could see that the peak at m/z 43 corresponded to a loss of 15 mass units - so it'd be the loss of CH3
    It essentially tests your ability to do the maths with atomic masses and knowledge of the structure to find what each fragment lost would be.
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: January 1, 2013
Poll
“Yanny” or “Laurel”

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.