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Chemistry library

Unit 4: Lesson 1

Mass spectrometry

Mass spectrometry

AP.Chem:
SPQ‑1 (EU)
,
SPQ‑1.B (LO)
,
SPQ‑1.B.1 (EK)
In the analytical technique of mass spectrometry, atoms or molecules are ionized using a high-energy electron beam and then separated based on their mass-to-charge ratios (m/z). The results are presented as a mass spectrum, which shows the relative abundances of the ions on the y-axis and their m/z ratios on the x-axis. This data can be used to calculate the exact masses of the atoms or molecules in the sample. Created by Sal Khan.

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• Sal says mass spectrometry can be used to measure the abundance of isotopes of a certain element in nature. But when he says in nature does he mean in all of nature or just 'the nature of the sample'? If the former, then how can we make inferences of the abundance of isotopes in nature from a single sample? If the latter, is there any technique that chemists use to infer the abundance of isotopes in all of nature?
• In statistics, we can go around, take samples, and make an inference from it.

Whenever we take a random sample in nature & use mass spectrometry, we are finding the percentage that different isotopes occur in a sample of that element.

If we just take one sample & use the percentages from that one sample, then I would think that isn't the best inference for all of nature, though we can still make a claim for it.

Though I obviously did not find the relative abundances of the elements myself, here is what I have done.

Take multiple samples, use mass spectrometry to to find the relative abundances of each isotope, and then write that down.
Continue this process for n=K samples, & then I can calculate the mean relative abundances for each isotope.

If K (our number of samples) is small then we wouldn't be as accurate as if we take more samples.

If anything I said here is incorrect, please someone let me know. I don't have experience using mass spectrometry so I don't actually know the process they used, but from my experience in AP Statistics that is the type of process we would use.

Hope this helps,
- Convenient Colleague
• This is probably way too advanced of a question
But how do we guarantee that the electrons we shoot will knock off the electrons of the atoms? Electrons are very tiny, aren't we just gonna miss? Also, what if some of the electrons we shoot end up joining the atom to form negative ions?
• When we ionize a sample in MS, we bombard it with quite a lot of electrons. So even if most of the electrons we fire from the electron source miss the target, enough are making contact for us to be able to measure it in MS.

The electrons that we fire are of a high enough energy (~70 eV) that they ionize a sample by knocking existing electrons from the sample rather than simply add to the neutral species to create anions. In either case though whether we produce positive cations or negative anions, we can still detect them in MS since they are charged. Hope that helps.
• when you says in nature does he mean in all of nature or just 'the nature of the sample'? If the former, then how can we make inferences of the abundance of isotopes in nature from a single sample?
• In statistics, we can go around, take samples, and make an inference from it.

Whenever we take a random sample in nature & use mass spectrometry, we are finding the percentage that different isotopes occur in a sample of that element.

If we just take one sample & use the percentages from that one sample, then I would think that isn't the best inference for all of nature, though we can still make a claim for it.

Though I obviously did not find the relative abundances of the elements myself, here is what I have done.

Take multiple samples, use mass spectrometry to to find the relative abundances of each isotope, and then write that down.
Continue this process for n=K samples, & then I can calculate the mean relative abundances for each isotope.

If K (our number of samples) is small then we wouldn't be as accurate as if we take more samples.

If anything I said here is incorrect, please someone let me know. I don't have experience using mass spectrometry so I don't actually know the process they used, but from my experience in AP Statistics that is the type of process we would use.

Hope this helps,
- Convenient Colleague
• How do we know that most of the elements will have the same charge after they get bombarded by electrons? Does the beam knock off every single electron in the atoms, so that they have no electrons? Otherwise, why would the charge be predictable?

If some atoms end up having a different charge after bombardment by electrons, then the particular isotope that is represented by the mass to charge ratio would be hard to identify on the x axis. Suppose for example the mass is 4 and the charge is 2+, that wouldn't be distinguishable from a mass of 2 and a charge of 1+, right?
• Good question. It requires energy to remove electrons from atoms or molecules in MS. Since it takes more energy to remove more than one electron from your sample particles, most cations produced in the ionization chamber carry a +1 charge. There will be a small number of atoms or molecules with a +2 charge but they will be too small to make much an impact on the results.

And yes you are correct since the MS only records a mass to charge ratio, m/z, those two ions would be indistinguishable on the spectrum.

Hope that helps.
• i don't get it, doesn't this just changes the electrons in the sample? where does the neutrons come from? do they attach to each other depending on their charge?
• Because isotopes exist. Zirconium has 5 natural isotopes, each of these atoms have different numbers of neutrons so has a different mass.
(1 vote)
• At , Sal says that the atoms are ionized so that they can pass through an electric plate. He says that the purpose of the electric plates is to accelerate their speed. Why does the speed of the ions need to be accelerated? Another question — why are heavier atoms deflected less, as Sal says at ?
(1 vote)
• The whole point of mass spectrometry is to separate ions based on their mass. Ions need to be accelerated or else they wouldn't be able to reach the detector at the end of the spectrometer. For mass spectrometry to work the ions need to enter the magnetic field with velocity.

When they are accelerated and pass through the magnetic field they are deflected based on their masses. We need this or else we wouldn't be able to sort out ions with the same charge but different masses. Ions with higher masses have higher inertia and therefore require more force to change their direction of motion, and vice versa for ions with lower masses. So if the magnetic field is held constant then ions with a lot of mass and inertia will therefore be slightly deflected, while ions with less mass and inertia will be deflected to a higher degree.

Hope that helps.
• Is there a name for this particular set-up? I've seen other types of mass spectrometer in textbooks and online, and this one has a slightly different process to what I've read about. It doesn't look like TOF either.
• If the Mass Spectrometry machine thing ionizes the atoms, how will the number of neutrons be detected? If you're trying to find the atomic mass of the atom, that's the protons+neutrons, but I don't get why shooting off electrons will have anything to do with it.
(1 vote)
• In MS we ionize a sample by essentially firing electrons at the sample which has the effect of knocking other electrons bound to the sample particles off. The loss of these electrons creates cations with a 1+ charge most often. When these cations are finally detected at the end of MS, they are detected as a mass-to-charge ratio, or m/z. Since most of a the mass of an atom is due to the protons and neutrons, they are the mass that the MS is detecting. And since the charge is usually 1+ the denominator of the m/z ratio is usually 1, so the m/z ratio is often just the atomic mass, m.

Hope that helps.
• How do you determine the molecular formula from MS? Which peak do you look at?
(1 vote)
• Depends on which type of MS you're doing.

If you're just sending a sample with a single element in it like Sal's example then you use that to find the isotopes and their abundances.

But if you're doing something in organic chemistry where we send organic compounds with multiple carbons of various chain lengths then we usually look at the M+, M+1, and M+2 peaks. The M+ peak is the largest atomic weight peak referred to as the molecular ion peak. It's essentially the entire molecule, minus a single electron, turning it into what we call a radical cation. This is more or less your molecular weight of your molecule. The M+1 peak would just be the peak with an atomic mass 1 greater than the M+ peak. From the M+1 peak we can usually determine the number of carbons, oxygens, and nitrogens in the molecule using their known isotope abundances. The M+2 peak is similar to the M+1 in that it is 2 atomic mass units greater than the M+ peak. If there's any sulfurs or halogens in your molecule, they'll show up in the M+2 if there is one.

The rest of the MS data will be the fragments of your molecule cut off and turned into smaller radial cations. These are only really helpful if your organic molecule is a straight carbon chain, otherwise it's not much help.

Hope that helps.