Couple of questions about Mass Spectroscopy

[Hypercube]

I have a couple of questions regarding the field and terminology of mass spectroscopy. I think I understand the basic concept of how it works; molecules are ionized, bonds break, and the fragment ions are detected. But one thing I don't understand is that let's say the molecule has a molar mass of 458.37 g/mol; naturally there would be a m/z peak at ~458, that mass since it's the parent ion, but how is it possible for there also to be a m/z fragment at, say, 934.2003. That mass is greater than the mass of the molecule in its natural form.

 

And second; what does the term "MRM transition" refer to? I know that MRM stands for Multiple Reaction Monitoring, but what does it mean when someone telly you to look of the MRM transitions of a certain compound or molecule?

 

I really need to get this straight, and the sooner the better.

Edited July 17, 2011 by Hypercube

[John Cuthber]

It's not my field but here's my 3d worth.

The higher masses are due to pairs of molecules sticking together (and sometimes picking up another small ion or molecule like sodium) so you can get ions with masses like (2M +23).

 

MRM is a process as well as an abbreviation.

It refers to tandem mass spec systems typically with 3 quadrupole. The first selects some ion- say the molecular ion, but there might be lots of things with that mass. The second quad just focusses the ions so they travel in a tight beam, but it is placed in a chamber with a small amount of something else- for example nitrogen. The ions collide with it and get broken.

You then set the third quad to select one of the fragments.

The chances of the double coincidence are small so the selection

Mass, react, mass gives a very low background and so the stuff you are looking for is easier to detect.

[hypervalent_iodine]

As John Cuthber said, you can get dimerization, which will give you (usually) low intensity peaks of high mass.

 

As a side note, MS is not a spectroscopic method and as such is called mass spectrometry, not spectroscopy.

Edited July 18, 2011 by hypervalent_iodine

[John Cuthber]

There seems to be some debating room on that point.

 

http://www.amazon.co.uk/s/ref=nb_sb_noss?url=search-alias%3Dstripbooks&field-keywords=mass+spectroscopy&x=0&y=0#/ref=nb_sb_noss?url=search-alias%3Dstripbooks&field-keywords=%22mass+spectroscopy%22&rh=n%3A266239%2Ck%3A%22mass+spectroscopy%22

[hypervalent_iodine]

There seems to be some debating room on that point.

 

http://www.amazon.co...spectroscopy%22

 

Hm, well there you go. We were taught quite emphatically by our professors that it was certainly not a spectroscopic method and to never call it as such (it was in the same category as drawing pentavalent carbons on exam papers).

 

Edit to add: Upon further investigation I have found out that it used to be a form of spectroscopy by virtue of the way by which data was recorded (on a photographic plate), but is no longer considered to fall under that definition since MS no longer uses the same method of output. Spectroscopic techniques, so far as I am aware, are strictly those that measure interactions between matter and EMR. Since MS doesn't do this per se, it is not considered a spectroscopic technique.