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## AP®︎/College Chemistry

### Course: AP®︎/College Chemistry>Unit 1

Lesson 2: Mass spectrometry of elements

# Mass spectrometry

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.

## Want to join the conversation?

• 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.
• At , Sal says that you have to make the appropriate adjustments based on the charge of the ions. I don't understand how the charge of the ions will affect the mass of the different isotopes in any way. Aren't the masses of electrons so low that they don't affect the mass of the ions? • In the mass spectrum, the x-axis is not simply the atomic mass of the ions, rather the mass-to-charge ratio of the ions, or M/Z. If the charge of the ions is +1, then Z = 1 and the ratio is M/1, or simply just the mass of the ions. But if the charge of the ions is +2, then Z = 2, and the ratio becomes M/2, which gives us half of the ion’s mass. So, the adjustment we would have to make is multiply the M/2 by 2 to yield M/1 or simply just M.

Hope that helps.
• Is there chemistry in middle school level ? • 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.
• 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? • Spectroscopy and Spectrometry are not basically the same thing. Spectroscopy measures light wavelengths to determine chemical composition. Spectrometry has to do with mass and isotopes. • Sal just said that sometimes people refer to mass spectrometry as mass spectroscopy, and use the two as synonyms. Even though it's not correct, mass spectroscopy is used in place of mass spectrometry sometimes.

So you are correct that spectroscopy and spectrometry are different, but the difference is the the use of light in spectrometry and not in spectrometry.
• can cations and anions form or only cations, and how do the ions accelerate what is their relationship between the electronic plates, also does the sample contain isotopes from the beginning so that the process can separate them and organize?
Thank you • 1. Can Cations and Anions form? Or is it only cations?
In mass spectrometry, both anions and cations can form when the unknown substance is vaporized (depending on the type of mass spectrometry). The charge is important for the mass/charge ratio.
2. How do the ions accelerate and what is their relationship to the electronic plates?
How the ions are accelerated depends on the type of mass spectrometer (different mechanisms for anions and cations). There are six primary types of mass spectrometers.
A. Quadropole Mass Analyser: accelerates particles with a DC bias (which acts like a magnetic field).
B. TOF (Time of Flight) mass analyser: accelerates ions by time (without electric or magnetic fields). It is like chromatography but with velocity and kinetic energy of ions.
C. Magnetic Sector Mass Analyser: ions are separated with a magnetic feild.
D. Electrostatic Sector Mass Analyser: ions are separated with an electric field.
E. Quadropole Trap Ion Mass Analyser: ions are separated with magnetic field. Different from Quadropole Ion Mass Analyser because it uses a magnetic field as opposed to DC bias; the mechanics are also different).
F. Ion Cyclotron Resonance: uses a magnetic field that traps electrons in an orbit within the device.
3. Does the sample contain isotopes in the beginning so that the process can separate and organize them?
Yes, the sample does contain various isotopes which are separated by their mass difference. The isotopes with heavier mass won't travel as far as the lighter isotopes, creating a pattern in the detector. Scientists record how many of each isotope are detected, which gives us the percentage of each isotope, and therefore the abundance of the isotope in nature. The isotope percentages in a sample can say a lot about a sample's age (ex: radio carbon dating).