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    A sample of methane was analysed in a mass spectrometer and gave peaks which corresponded to relative masses of 16, 17, 18, 19, 20 and 21. If the natural abundance of 13C is 1% of 12C and that of 2H is 0.015% of 1H, suggest which ions are contributing to each peak. Which of these peaks would you expect to be the smallest?

    I don't get this question :confused: Can anyone explain it to me please?
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    Hmm, would the 21 peak consist of methane but with each H being a 2 and the carbon being a 13 C, and then the 20 peak would be the C12 with four H2s, the 19 peak the C12 with three H2 and one H1 (a normal hydrogen), and so on so forth for the rest of the peaks. I'm guessing the 21 peak would be smallest, as it contains all the rare isotopes in it? I am prolly not much help
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    (Original post by kohlstream)
    I'm guessing the 21 peak would be smallest, as it contains all the rare isotopes in it? I am prolly not much help
    That seems pretty sensible...I'd go with that :cool:
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    call (2)H deuterium (D)

    C(13)D4 = 21 amu
    C(12)D4, and C(13)HD3 = 20 amu
    C(13)D2H2, and C(12)HD3 = 19 amu
    C(13)DH3, and C(12)H2D2 = 18 amu
    C(13)H4, and C(12)H3D =17 amu
    C(12)H4 = 16 amu

    and, as stated, the most unlikely (smallest peak) will be the one containing the rare isotopes only i.e. C(13)D4
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    Thanks, I get it now
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    strictly speaking we've all been incorrect as the ionic charges have been missed out (only positive ions can be detected in the mass spectrometer) AND neither have the breakdown products been included.

    call (2)H deuterium (D)

    [C(13)D4]+ = 21 amu
    [C(12)D4]+, and [C(13)HD3]+ = 20 amu
    [C(13)D2H2]+, and [C(12)HD3]+, and [C(13)D3]+ = 19 amu
    [C(13)DH3]+, and [C(12)H2D2]+, and [C(12)D3]+, and [C(13)DH2]+, = 18 amu
    [C(13)H4]+, and [C(12)H3D]+, and [C(12)HD2]+ , and [C(13)D2]+ = 17 amu
    [C(12)H4]+, and [C(12)D2]+, and [C(13)H3]+, and [C(13)HD2]+, = 16 amu

    you would also expect peaks at 15, 14, 13 and 12
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    ****ing good mass spec, that's all I have to say.
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    (Original post by ChemistBoy)
    ****ing good mass spec, that's all I have to say.
    LOL - amen!
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    (Original post by ChemistBoy)
    ****ing good mass spec, that's all I have to say.
    LOL - amen!
 
 
 
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