[Aznhmonglor]

Okay so we're doing a lab on Mass Spectroscopy and one of our prelab question asks

When acetone is injected into the Mass Spectrometer, what is the m/z value of its molecular ion? If this molecular ion, fragments by loss of a methyl radical, what is the m/z value of the resulting ion?

For that question I have no clue where to start. How do I know the m/z for any of the elements? Is it suppose to be memorized? Shown on a graph?

Next question asks for the molecular weight of pyridine, cocaine, and trimethylamine and asks what the three molecular weights have in commmon?

I seriously just see 3 numbers? Suggestions on what they want me to focus on?

and Lastly, this question asks for teh molecular weight of Nicotine and asks if this weight has the same property as the last three above?

Again no clue.

Suggestions?

[Arkcon]

When acetone is injected into the Mass Spectrometer, what is the m/z value of its molecular ion? If this molecular ion, fragments by loss of a methyl radical, what is the m/z value of the resulting ion?

For that question I have no clue where to start. How do I know the m/z for any of the elements? Is it suppose to be memorized? Shown on a graph?

Actually, it's pretty easy to calculate.  Do you know what a mass spec does, and how it works?  You can memorize some element masses, if you want to, but you might be able to find them, in a table, of some sort ... periodically.

Next question asks for the molecular weight of pyridine, cocaine, and trimethylamine and asks what the three molecular weights have in commmon?

I seriously just see 3 numbers? Suggestions on what they want me to focus on?

Well, just for the fun of it, what are they.  Let's see if a pattern emerges.  This probably has something to do with how a mass spec works, I'm guessing.

and Lastly, this question asks for teh molecular weight of Nicotine and asks if this weight has the same property as the last three above?

Again no clue.

Suggestions?

Well, lets see that as well.  As fr suggestions, you seem very sure that you have no chance of solving this problem.  I suppose it would have stymied me in school too.  But I've actually used a mass spec this way, in training courses I elected to take, and in industry, so I don't agree that your situation is as hopless as you imply.

[Aznhmonglor]

Wow it just struck me that a Mass Spec is something that measures the molecular weight of the molecules without a periodic table. But yes as for the seconds question all three of the molecules are amines but I can't compare them in any way, I know their MW's and thats about it. What is this pattern you talk about? Do you mean they all might contain the same molecular ion?

Molecular Weight of:
Pyridine = 79.1
Trimethylamine = 59.1
Cocaine = 303.4

[Arkcon]

Wow it just struck me that a Mass Spec is something that measures the molecular weight of the molecules without a periodic table.

Well, yeah, but if you didn't use the periodic table, on your possible unknowns, how would you know your mass spec was correct?  So, what molecular ion do you expect to see with acetone?

But yes as for the seconds question all three of the molecules are amines but I can't compare them in any way, I know their MW's and thats about it. What is this pattern you talk about? Do you mean they all might contain the same molecular ion?

Molecular Weight of:
Pyridine = 79.1
Trimethylamine = 59.1
Cocaine = 303.4

No, not same molecular ion.  But they may fragment into the same fragments.  I'd play around with the structures, and see what you can figure out.  You'll have to know how your particular mass spec will fragment, and ionize your analyte.

[Aznhmonglor]

You would know your mass spec is correct by looking at the fragments. The molecular ion I expect to see would be something like a base peak at 58 or 43 maybe somewhere there Im not sure how I would know though.

Hmm what you said about the last part there sounds complicated. These questions are from my prelab and haven't really learned about Mass Spec just going over these few questions (They should have been simple) but lol yeah seems hard to me.

[Borek]

I would suggest looking only at the integer part of molar masses.

[Arkcon]

You would know your mass spec is correct by looking at the fragments. The molecular ion I expect to see would be something like a base peak at 58 or 43 maybe somewhere there Im not sure how I would know though.

OK, acetone has a formula mass of 58, that's true.  But, a mass spec can only analyze ions.  So what will need to happen to the molecule so it can be analyzed?  How will that change the mass?  There are at least two different ways the mass spec can ionize a sample, and two modes of operation, for each method.  Your lab write-up, or text book should explain this to you.