Mass spec..

[Zebra]

Are all the fragments +1??
How do you calculate m/e?

:C arghargh

[iNerd]

No you get +2's.

[Zebra]

So, how do you know when the fragments is 2+ ?

[Aurelian]

So, how do you know when the fragments is 2+ ?

Generally you will only deal with 1+; most MS's have the ability to focus on just one charge group.

Otherwise, the peaks of the 2+ group will be in the exact same relative positions as the 1+ group just further along the spectrum.

[luken93]

So, how do you know when the fragments is 2+ ?

Otherwise, the peaks of the 2+ group will be in the exact same relative positions as the 1+ group just further along the spectrum.

Are you sure? I thought that because it is a mass/charge ratio, if the charge is 2+ then it will have a peak at the mass of the fragment divided by 2?

[Zebra]

how do you even work out mass charge ratio?
for example, take ethanoic acid (CH3COOH) whats the m/e of this??

[luken93]

Draw out the structure:
       H       O
       |       //
 H - C - C
       |      \
       H     OH

The furthest peak along the spectrum will be the full molecular ion, which in this case is 60 (As the Mr = 60)
As well is this peak, when the molecule is inserted into the mass spectrometer, it is bombarded with electrons that will not only ionise the molecule, but may create fragments. Basically, a fragment can form at ay breakage of a bond, such as the bond between the CH3 and COOH could break, forming a CH3+ ion and/or a COOH+ ion.

As Aurelion said, you will probably only ever use a 1+ charge, hence the m/z of each fragment is simply the Mr of that fragment (CH3COOH = 60, CH3 =15, etc)

[Mao]

No you get +2's.

For all intents and purposes of VCE Chemistry, you will only get fragments with +1 charge for organic molecules.

Cases when you will get more than just +1:
-Elemental MS, single atoms (such as Mg2+)
-MS of an already positively charged species (e.g. MS of CH3NH3+)

[schnappy]

how do you even work out mass charge ratio?
for example, take ethanoic acid (CH3COOH) whats the m/e of this??

You need to learn what m/e is (Can someone explain why in chem it's either m/e or m/z? Intuitively it should be m/Q, or is that because Q is conventionally in coulomb which is obviously inconvenient - so e or z differ from Q?)

The m/e ratio is exactly what it says, the mass of the ion divided by its charge. Your ethanoic acid as you wrote it out has no net charge on it, so lets substitue:
m/e=60/0=WORMHOLE
So if you get the reference, an uncharged particle has no m/e ratio. Hence why you charge them in a MS. If you do physics, check out the magnetism bit in Unit 4 - you'll learn that currents (moving charges - not moving particles) are deflected by a mag field.

The heavier something is (higher m) the less it is deflected (Newton's second law). Since in VCE we only deal with +1 charges, the m/e ratio is always just m - the molecular mass of the ion.

It's an incomplete explanation, but hope it helps.

[iNerd]

No you get +2's.

For all intents and purposes of VCE Chemistry, you will only get fragments with +1 charge for organic molecules.

Cases when you will get more than just +1:
-Elemental MS, single atoms (such as Mg2+)
-MS of an already positively charged species (e.g. MS of CH3NH3+)

Lol you neg me because I say the right thing.

I got two +2 questions wrong in just a Unit 1 test simply because I hadn't prepared for them due to people saying "+1 will only come up"...

Be prepared for anything OP.

[iNerd]

So, how do you know when the fragments is 2+ ?

Otherwise, the peaks of the 2+ group will be in the exact same relative positions as the 1+ group just further along the spectrum.

Are you sure? I thought that because it is a mass/charge ratio, if the charge is 2+ then it will have a peak at the mass of the fragment divided by 2?

You're right luken, the +2 peaks are smaller and exactly half of the +1.

[luken93]

So, how do you know when the fragments is 2+ ?

Otherwise, the peaks of the 2+ group will be in the exact same relative positions as the 1+ group just further along the spectrum.

Are you sure? I thought that because it is a mass/charge ratio, if the charge is 2+ then it will have a peak at the mass of the fragment divided by 2?

You're right luken, the +2 peaks are smaller and exactly half of the +1.

Yeah, I didn't know they existed until there was one single question in our Chem Textbook, then I looked it up and found that it could be...

[Aurelian]

Are you sure? I thought that because it is a mass/charge ratio, if the charge is 2+ then it will have a peak at the mass of the fragment divided by 2?

Yes, that's correct; sorry, my phrasing "further along" was probably not the best to use.

[Mao]

No you get +2's.

For all intents and purposes of VCE Chemistry, you will only get fragments with +1 charge for organic molecules.

Cases when you will get more than just +1:
-Elemental MS, single atoms (such as Mg2+)
-MS of an already positively charged species (e.g. MS of CH3NH3+)

Lol you neg me because I say the right thing.

I got two +2 questions wrong in just a Unit 1 test simply because I hadn't prepared for them due to people saying "+1 will only come up"...

Be prepared for anything OP.

No mate. I voted you down because your post doesn't give any explanation. That kind of answers don't help anyone.

[iNerd]

No you get +2's.

For all intents and purposes of VCE Chemistry, you will only get fragments with +1 charge for organic molecules.

Cases when you will get more than just +1:
-Elemental MS, single atoms (such as Mg2+)
-MS of an already positively charged species (e.g. MS of CH3NH3+)

Lol you neg me because I say the right thing.

I got two +2 questions wrong in just a Unit 1 test simply because I hadn't prepared for them due to people saying "+1 will only come up"...

Be prepared for anything OP.

No mate. I voted you down because your post doesn't give any explanation. That kind of answers don't help anyone.

No mate the question was "Are all fragments +1"

I answered the question. He then asked a follow-up.

...but I didn't answer it 'cause I wasn't available.