Mass Spectrometer Problem

Physics Ninja looks at a mass spectrometer problem and break down three of its components - accelerator, the velocity selector, and the spectrometer. We calculate the radius of curvature of charge particles and calculate the separation between isotopes

[thaopham]

Incredible explanation. I appreciate your attention to details, it helps me understand the matter much better. Thank you so much

[ZaksLab]

I've read a lot of comments on the fact that ions of different mass will emerge at different velocities from the accelerating potential.

These are really good questions about the accelerating potential issue: absolutely the heavier particles will emerge with a lower velocity. If the speed selector was fixed at one value and the accelerating potential was fixed at one value, you would miss the vast majority of atoms/'molecules emerging from the ionization chamber. I ran into these comments while preparing to produce a video for a physics course, which inspired me to do more research on the issue, and the answer is very simple: the experimenter can turn the knobs to change these parameters in order to scan through ions with different charge to mass ratios! Mass spectrometers simply modify the parameters to rapidly scan through m/z values to produce a relative abundance vs. m/z plot. In the physics textbooks I've taught from, they always present it as "one velocity, many different landing positions", but clearly this can't be true unless the particles are very close in mass.


In the article, we see that Dempster's magnetic sector analyzer (1918) looks very much like the schematic we're used to seeing in introductory physics, except that *it has only a single detector* instead of a spread-out photographic plate. This made me realize that with experimental control over the accelerating potential and the magnetic field in the analyzer region, we can detect different particles at the same position by sweeping through the parameters of the device. It is interesting to note that Dempster's design did NOT have a speed selector, but he acknowledged the loss of resolution due to the distribution of ion velocities entering the analyzer and proposed adding the Wien filter (velocity selector) in order to improve the resolution of the device. I think the addition of the Wien filter is what distinguishes this canonical example as the "Bainbridge Mass Spectrometer".



I ended up making my video with a spectrometer schematic that has a single fixed detector, assuming we have the experimental control to sweep through parameters and detect different ions by modifying the accelerating potential, speed selector and/or analyzer magnetic field.

[teycheneya]

Best video on the topic so far. And I've seen many.

[matastunkevicius]

This saved me in physics 1, thank you so much!

[multivision]

The separation is not the difference of the radii, but the diameters, so it is not 1, but 2cm

[jh]

omg thank you thank you thank you!!!! I was looking for this!!! I love your videos please make more!!! I only have 14 more days till ap physics 2, and 11 days till ap physics 1self studying them. these hellps super super much

[mayraa]

thank you so much!!! this was incredibly helpful and so easy to understand!!! God bless!!!

[tychophotiou]

I am a physics teacher. I would like to correct you on the hand rules…. To find the the direction of a force on a moving charge you ALWAYS use Fleming’s left hand rule. The right hand rule is only used to find the direction of an induced current in a magnetic field.

[anitag.]

I tried the exercise and everything is okay for the first radius, in the second part though I cannot understand how did you calculated the mass. Because if I try the result is 9.95x10^-26kg and if i put this in the formula the second radius results 5.2 instead of 6.. What did I do wrong?

[tychophotiou]

If the ratio of masses is 60:58 then the ratio of the radii is 60:58, this gives a 2nd radius of 5.17 cm (where did 6cm come from?)

[orangutan]

How are the isotopes moving with the same velocity after the velocity selector? Wouldn't the orange isotope move slower than the red one before entering the selector, so it will get deflected?

[seniorshimhanda]

Electrostatic propulsion for spacecraft adopts the same principle to achieve high exhaust velocities and specific impulse.

[rezaaliharsini]

thank you so much, such a great energy and video.

[freeelectron]

Really good Mass Spec physics lecture! May the F=ma be with you Ninja! :)

[jh]

Also, I had a similar mass spectrum question on my textbook and in order to find velocity, they used conservation of energy equations... why could they not use the equation for circular motion and find velocity?

[gregorydevenport]

According to my calculations I get 100, 000 m/s for the velocity.

[jh]

Also I got... 9.989 times tenth to the -22. How did you get 10 000 m/s

[adosar]

The energy that the ions gain during accelaration is same if we consider that all ions have same charge. So The heavier molecules will have smaller velocity and the ligther will have greater velocity. But velocity selector let pass only the ions with certain velocity. So how the spectrometer detects the other ions? It doesn't make sense.

[smyThegmc]

I don understand how isotopes with different masses reach the same speed under the same accelerating conditions I try to do my takehome assignment but I am desperate now