Mass spec

Can someone please explain mass spec to me? I don't understand it at all and I have an open question for it, and have literally no idea what to write, because I don't know what it means by a parent ion, or anything.

Have you tried looking it up?

It's a pretty standard instrumental technique with one or two web sites that explain it all ...

Yep, but I'm just not getting it. I've looked on scholar as well, and none of it helps.

What exactly do you not understand from, say, the chem guide explanation ...?

Okay, so when they say that the peak at *number* must be *fragment*, how do they know that? And how do they know what ratio the peaks are in? And if you're given the molecule that the spec is for, and you're told to assign fragments to the peaks, how do you do that? And what the hell does it mean by the parent ion?

Okay, so when they say that the peak at *number* must be *fragment*, how do they know that? And how do they know what ratio the peaks are in? And if you're given the molecule that the spec is for, and you're told to assign fragments to the peaks, how do you do that? And what the hell does it mean by the parent ion?

Okay, so when they say that the peak at *number* must be *fragment*, how do they know that? And how do they know what ratio the peaks are in? And if you're given the molecule that the spec is for, and you're told to assign fragments to the peaks, how do you do that? And what the hell does it mean by the parent ion?

Can someone please explain mass spec to me? I don't understand it at all and I have an open question for it, and have literally no idea what to write, because I don't know what it means by a parent ion, or anything.

Each peak has a mass value, you can work out the fragment by calculating the fragments Mr and comparing it to the structural formula.

You can tell something is a fragment fairly easily from how the machine works. The sample is ionized by a metal coil emitting high speed electrons, some of which will essentially knock electrons off the sample forming positive ions. Say for example you had a sample of some compound where the molecular mass was 56. The x axis of a mass spec is the mass/charge ratio (i.e. the mass of molecule/fragment divided by it's positive charge). It is only possible to lose an integer number of electrons (1,2,3etc) so if you had a peak at any mass/charge ratio that isn't a factor of 56 it cannot be the original compound and must be a fragment. I assume a parent ion is just an ion that can also form fragments.

By parent ion, do you mean molecular ion, M+ ? Because this is the peak with the largest molecular mass and is the original molecular with a +1 charge, due to the loss of an electron.

There are four main components to a mass spectrometer: ionisation, acceleration, deflection and detection.

In the first part, ionisation, a sample is injected into the mass spectrometer and high energy electrons are fired at the sample using an electron gun. These high energy electrons usually 'knock' out one electron from each atom/molecule in the sample, although they can remove two or more electrons, to form cations (if they have a high enough energy).

Now that the atoms/molecules are ionised they can be accelerated through a high potential difference using negatively charged plates (which attract the cations).

The accelerated cations are then deflected around an arc. It does this by using a very large electromagnet which produces a magnetic field. Only those atoms/molecules that have the right mass:charge (m/z) ratio will have the correct trajectory to be deflected onto the detector. Those that have a m/z value that is too high for that particular magnetic field strength are not deflected enough. Those that have a m/z value that is too low for that particular magnetic field strength are deflected too much.In either case, the atom/molecule does not reach the detector and hence is not detected.

The magnetic field strength is altered until the detector detects a cation, the magnetic field strength tells you what the m/z value is. For ions that have a higher m/z value, the magnetic field strength will have to be stronger in order to deflect the ions enough to be detected, and the reverse argument is true for ions that have a lower m/z value.

Hopefully this gives you a basic understanding of the mass spectrometer.

What do you mean by "Mr"?


Aw okay, that makes a bit more sense. Thank you.


It'd make sense if it was the molecular ion. So is it just the molecule you're sampling minus an electron?

What do you mean by "Mr"?
Aw okay, that makes a bit more sense. Thank you.
It'd make sense if it was the molecular ion. So is it just the molecule you're sampling minus an electron?

Relative molecular mass, sorry.