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Help please chemistry mass spectroscopy

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Hi guys, I don't understand what they are trying to say in the second and third bullet points and how it relates to the question. Can someone please help?
Reply 1
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High-res/accurate mass MS gives more detailed ion data - so the example formula you've got is 102.104467 au (C=12, H=1.007825, O=15.994917). That would be different given a different elemental composition because of the exact molecular mass of each element. The last two points are focused on that, that as elemental masses are not exact whole numbers, they will affect accurate molecular mass.
Reply 3
Original post by Stiff Little Fingers
High-res/accurate mass MS gives more detailed ion data - so the example formula you've got is 102.104467 au (C=12, H=1.007825, O=15.994917). That would be different given a different elemental composition because of the exact molecular mass of each element. The last two points are focused on that, that as elemental masses are not exact whole numbers, they will affect accurate molecular mass.

But aren't these elemental masses/Ar values averages so it wouldn't really affect other compounds. For example, H=1.007825 in all compounds.
Original post by sienna2266
But aren't these elemental masses/Ar values averages so it wouldn't really affect other compounds. For example, H=1.007825 in all compounds.


Hydrogen will always have that mass, but something that's mass 102 but with a different molecular formula won't be 102.104467, it'd be close but not the same. Hence why accurate mass is used to determine molecular formula of fragments.
Reply 5
Original post by Stiff Little Fingers
Hydrogen will always have that mass, but something that's mass 102 but with a different molecular formula won't be 102.104467, it'd be close but not the same. Hence why accurate mass is used to determine molecular formula of fragments.

So am i correct in saying high resolution mass spec gives the exact mass of the element in the compound and not the Ar value of the element in the compound? So for example Oxygen will have a mass of x.25636 in compound A but it will have a mass of x.64773 in compound B sort of thing? So the two different compounds have the same Mr but the Mr is different in the decimal places like 102.211 and 102.209

Also, since the mass is the same but different formula, then other atoms could be making up the compound. This is completely random but for explanation purposes CH3CNCH2 and FHCH2CH3 both have a mass of 102 but different atoms making it up. Is this what the qquestion is talking about or the above I mentioned or neither?:colondollar:
(edited 5 years ago)
Original post by sienna2266
So am i correct in saying high resolution mass spec gives the exact mass of the element in the compound and not the Ar value of the element in the compound? So for example Oxygen will have a mass of x.25636 in compound A but it will have a mass of x.64773 in compound B sort of thing?


Technically yes (when i run accurate mass i do it via profile scanning and get a distribution across the mass covering the slightly different weights) rather than distinct peaks but for this it's not something you need to concern yourself with.


Also, since the mass is the same but different formula, then other atoms could be making up the compound. This is completely random but for explanation purposes CH3CNCH2 and FHCH2CH3 both have a mass of 102 but different atoms making it up. Is this what the qquestion is talking about or the above I mentioned or neither?:colondollar:

Sort of, its an example of application. Your hypothetical compounds don't have the same mass but as an example of same mass different elemental formula: 2,3,5-trimethylpyrazine (C7H10N2) and benzoic acid (C7H6O2) - both mass 122. C=12, H=1.007825, O=15.994915, N=14.003074. Accurate mass of 2,3,5-TMP = 122.084398. Accurate mass of Benzoic Acid = 122.036780. The two have distinct masses at the second decimal place because elemental masses are not actually integers and so can be distinguished in high-resolution mass spectrometry.
Reply 7
Original post by Stiff Little Fingers
Technically yes (when i run accurate mass i do it via profile scanning and get a distribution across the mass covering the slightly different weights) rather than distinct peaks but for this it's not something you need to concern yourself with.



Sort of, its an example of application. Your hypothetical compounds don't have the same mass but as an example of same mass different elemental formula: 2,3,5-trimethylpyrazine (C7H10N2) and benzoic acid (C7H6O2) - both mass 122. C=12, H=1.007825, O=15.994915, N=14.003074. Accurate mass of 2,3,5-TMP = 122.084398. Accurate mass of Benzoic Acid = 122.036780. The two have distinct masses at the second decimal place because elemental masses are not actually integers and so can be distinguished in high-resolution mass spectrometry.

Thanks so much:biggrin: Also, is it just me or the mark scheme is worded quite poorly. They shouldn't be using Mr and Ar as Ar for example represents the mass of oxygen averaged over many different compounds. High resolution mass spec gives you the exact mass of the element and not an average. But then that got me thinking what do you use to replace these terms?

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