Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: d9717 on April 10, 2014, 06:43:41 AM
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When a sample of isotopes is injected into the mass spectrometer to find the relative atomic mass (RAM), doesn't the elements in the sample need to be in a uncombined state?
For example oxygen is found occurring as O2 rather than just O. So to find the RAM of oxygen, then doesn't these O2 molecules need to be separated into its individual elements? Because if it isn't separated then doesn't this mean that a molecular ion would form instead!?
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Hmmm ... you seem to be missing something. Briefly, a mass spec only measures ions. So by definition, all molecules are broken into ions, or they are missed by the detector. Now, molecular ions may form, but they're not formed from molecules, that's just what we call a molecule, that has a charge, that the system detects. You probably having trouble, because you're conflating atomic mass spec, of the sort used to understand the elements and their isotopes, and compound mass spec, which we use to understand (usually) large molecules.
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@Arkon, for the purpose of atomic mass spec, I was watching a revision video and the teacher said the sample, for example, injected was pure chlorine gas, Cl2. So I just got confused that if you inject chlorine gas which comes in cl-cl (diatomic molecules) then how are you supposed to find the mass of an individual chlorine isotope? I hope this make sense, Thanks for your help by the way.
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1. Assuming just Cl2 - what possible masses would the molecular ion have?
2. What happens during ionization stage?
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@Borek At the ionisation stage, the electron gun knocks off an electron from the atom/molecule thus it contains a +ve charge. But this doesn't really separate the molecules into their atoms, does it?
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@Borek At the ionisation stage, the electron gun knocks off an electron from the atom/molecule thus it contains a +ve charge. But this doesn't really separate the molecules into their atoms, does it?
It doesn't separate molecules into atoms, but it starts decomposition of the molecules into smaller chunks. Identifying them is what the MS analysis is about.
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@Arkon, for the purpose of atomic mass spec, I was watching a revision video and the teacher said the sample, for example, injected was pure chlorine gas, Cl2. So I just got confused that if you inject chlorine gas which comes in cl-cl (diatomic molecules) then how are you supposed to find the mass of an individual chlorine isotope? I hope this make sense, Thanks for your help by the way.
The Cl-Cl bond gets broken by bombardment by electrons, so you get Cl· free radicals. These register with the masses of individual chlorine atoms.
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Thank you!