Mass Spectrometry HELP!

[kimk2kr]

alright... ive just finished learning about MS at school and i dont understand few of the concepts and how the instrument works.

so after the sample is vapourised and ionised (bombarded with electron) and the positive ions start to accelerated.

Why is it that it is necessary for the positive ions to be deflected? Wouldnt it provide better result if the positive ions were directed directly towards the detector?

Help please.... thanks!

[illuminati]

Without any deflection comes no means of identifying which ions are which.
So say you have a range of fragmentated ions. Without an external magnetic field, all the ions would end up in the same spot, and what you get is one big peak on your mass spectrum.
This is why we apply an external magnetic field
Different ions deflect more based on mass/charge ratio. The higher the mass charge ratio, the lower the deflection. By placing detectors at the places where the ions are deflected to, you can actually gauge how much of a certain fragment ion there is, as well as identify the mass/charge ratio of the ion. This provides much more information about the substance in question

[kimk2kr]

Without any deflection comes no means of identifying which ions are which.
So say you have a range of fragmentated ions. Without an external magnetic field, all the ions would end up in the same spot, and what you get is one big peak on your mass spectrum.
This is why we apply an external magnetic field
Different ions deflect more based on mass/charge ratio. The higher the mass charge ratio, the lower the deflection. By placing detectors at the places where the ions are deflected to, you can actually gauge how much of a certain fragment ion there is, as well as identify the mass/charge ratio of the ion. This provides much more information about the substance in question

ahhhhh i think i kinda get what you mean... haha

but if the fragmentated ions were to deflect, isnt there a possibility or some possibility some ions wont even reach the detector? hence you cant accurately identify the substance?

:/ cheers anyways man!

[Mao]

but if the fragmentated ions were to deflect, isnt there a possibility or some possibility some ions wont even reach the detector? hence you cant accurately identify the substance?

That's right. The mass spectrometer can only look at a narrow mass range at any given time.

But to put it into perspective, you would typically work on samples of a few milligrams. That may seem very little, but also remember that 1 mole = 602 000 000 000 000 000 000 000 molecules, so even a small fraction of a mole still has a lot of molecules. The mass spectrometer doesn't need much, and it can work very fast (a few milliseconds), so we can afford to throw most of the sample away.

Imagine this (overly-)simplified scenario.
Suppose the mass spectrometer can only look at a m/z range of 10. We'll look at m/z = 0 -> 10. So we ionize 1 billion molecules and let them fragment/etc. Only a million fragments end up in the range of 0 -> 10, but that's still plenty for the MS detector. So we take this small spectrum, and move on to the next range m/z 10 -> 20. Rinse and repeat, after 10 cycles, we've scanned everything up to m/z=100. Over this time, we've only used 10 billion molecules (~0.00000000000001 moles). Since the MS can work very fast, and it can change the scanning range automatically, we can get a full spectrum in less than a second from a very small amount of sample.

Note that the numbers are mostly made up to convey a point, reality may be a few million times different, but when we're working on the real life scale of moles, millions are insignificantly small in comparison. In reality, we get an efficiency of 1/10,000,000 to 1/100,000,000,000 depending on various circumstances.

[kimk2kr]

but if the fragmentated ions were to deflect, isnt there a possibility or some possibility some ions wont even reach the detector? hence you cant accurately identify the substance?

That's right. The mass spectrometer can only look at a narrow mass range at any given time.

But to put it into perspective, you would typically work on samples of a few milligrams. That may seem very little, but also remember that 1 mole = 602 000 000 000 000 000 000 000 molecules, so even a small fraction of a mole still has a lot of molecules. The mass spectrometer doesn't need much, and it can work very fast (a few milliseconds), so we can afford to throw most of the sample away.

Imagine this (overly-)simplified scenario.
Suppose the mass spectrometer can only look at a m/z range of 10. We'll look at m/z = 0 -> 10. So we ionize 1 billion molecules and let them fragment/etc. Only a million fragments end up in the range of 0 -> 10, but that's still plenty for the MS detector. So we take this small spectrum, and move on to the next range m/z 10 -> 20. Rinse and repeat, after 10 cycles, we've scanned everything up to m/z=100. Over this time, we've only used 10 billion molecules (~0.00000000000001 moles). Since the MS can work very fast, and it can change the scanning range automatically, we can get a full spectrum in less than a second from a very small amount of sample.

Note that the numbers are mostly made up to convey a point, reality may be a few million times different, but when we're working on the real life scale of moles, millions are insignificantly small in comparison. In reality, we get an efficiency of 1/10,000,000 to 1/100,000,000,000 depending on various circumstances.

ahhh cheers man! i get it now and by m/z do you mean mass charge ratio? cause i learnt mass charge ratio as m/e... :/

and also is it possible for you to tell me some advantages and disadvantages of analytical techniques (spectroscopy mostly) and if they are used for quantitative or qualitative analysis ?


thanks!

[ggxoxo]

Hey can someone please help me with a mass spec relate qn; I've put it in another forum but no one has answered it yet

Do base peaks have any significance in mass spectrometry?

Also: The base peak on the mass spectrum of pentan-2-one occurs at a mass/charge ratio of 43, whereas on the
mass spectrum of pentan-3-one the base peak occurs at a mass/charge ratio of 57. Account for these two
peaks.

[Mao]

@kimk2kr,
- m/z is the same thing as m/e
- re: adv/disadv of spectroscopic techniques, this question is a bit too broad. All the info should be in your textbook.

@ggxoxo,
- base peaks are quite important, but for reasons that aren't taught in VCE.
- By 'accounting' for peaks, you simply have to work out what fragment they correspond to. You don't need to explain why or how they are different depending on the analyte.

[kimk2kr]

@kimk2kr,
- m/z is the same thing as m/e
- re: adv/disadv of spectroscopic techniques, this question is a bit too broad. All the info should be in your textbook.

@ggxoxo,
- base peaks are quite important, but for reasons that aren't taught in VCE.
- By 'accounting' for peaks, you simply have to work out what fragment they correspond to. You don't need to explain why or how they are different depending on the analyte.

by adv/disadv i mean like in comparasion to each other, which one is preferred and stuff.