Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: adamdolan123 on November 23, 2014, 06:09:10 AM
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I need help with analysing this Mass spectrum im absolutely clueless on how to do it :/
http://s1295.photobucket.com/user/adamdolan/media/ScreenShot2014-11-23at110112_zpsdbd37ed6.png.html?o=0 (http://s1295.photobucket.com/user/adamdolan/media/ScreenShot2014-11-23at110112_zpsdbd37ed6.png.html?o=0)
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OK, that's a healthy stack of data. But to know what it means, we have to know something about the sample. No one gets useful answers from an array of peaks without background information.
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all i know is that it comes for a gel air freshener and the solvent was hexane
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OK. But I need more about your application. You told me what you have, but how did you treat it, what do you want to know, and what do you know about similar types of samples. These are things that an analytical chemist must know to make useful conclusions from laboratory results.
I can take a bad guess, if you want it, that you've fed an unknown sample into one instrument, you have a stack of peaks, and you want to know everything about it. Well, that I can help you with -- that would be a pointless exercise, you will never figure out what all these peaks mean.
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i just added some of the gel to hexane, shook it for a bit, place the hexane solution in a vial and put it in a gc/ms machine. The reason i find it so hard is because thats all the information we were given.
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but if u could take a whack at it that would be great
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OK. What does hexane by itself look like under these particular gs-ms conditions? You can find out what those peaks are, and ignore them. Warning: depending on the possibility of fragment formation, the hexane may give different peaks when it reacts in the fragments of other products, so my simple blank subtraction above may not be so simple.
What's the ingredient list on this product? Can you exclude some components that aren't hexane soluble, and then test others, as pure substances, under these particular gc-ms conditions?
What are the conditions, of the GC and the MS? We would need to know more about the sample to use these analysis conditions, but you know, baby steps.
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well the hexane spectrum doesn't coincide with my spectrum that much. There are no ingredients on the air freshener but i think the aim of the exercise is to try and guess a product obv it may not be the correct one but as long as you show some reasoning. I know the molecular is 162 but i cant work out a product that this would be. Most air fresheners have linalool in them but it has a mass of 154.
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so ye struggling
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Well, you're assuming that the largest (one of the largest) fragment is the parent ion. That's often true, but ou have many fragments. Either your ionization parameters are "hard" or you molecule is very fragmentation prone. In that case, it may be much larger.
OK, you know the MW of linalool is 154. What sort of ion would have to have been created to increase the MW +7? Although, something that big, would likely not survive harsh ionization.
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Try this procedure to organize the data and your thoughts.
Capital letters are only for EMPHASIS.
Step 1
Perform a visual inspection of the mass spectrum.
The PRESUMED molecular ion, m/z 163, shows the presence or absence (WHICH ??) of Br and Cl atoms (WHY ???)
The presumed molecular weight (MW 163 Da) is ODD, and therefore the molecule contains an ODD number of N atoms (1,3,5,7 etc) See the NITROGEN RULE.
Check for absence/presence of "characteristic" m/z values; a "characteristic" ion is one that suggests the presence of a certain structural feature.
For instance, m/z 77 is a characteristic ion (C7H7)+; indicates alkyl-substituted phenyl ring. Is this in your spectrum ?
Step 2
Set up a table with 4 columns.
Column 1: list m/z values from highest (m/z 163) to lowest. GENERALLY, as a first pass, ignore low intensity (< 5%) m/z values.
Column 2: mass difference to next highest m/z value OR next-but-one group of ions.
Column 3: possible elemental compositions for the mass difference (e.g. 28 is C2H4 or CO)
Column 4: possible structural feature that corresponds to the elemental composition.
Step 3
On a piece of paper, draw the possible structural elements that you have derived from your analysis. Try to connect them such that all of the major m/z values can be formed from the structure; be aware that sometimes, two (or more) different structures (isomers) can be constructed that will generate very similar mass spectra.
Check that the elemental composition (C, H, N, O, etc) of the assembled molecule gives MW 163.
Good Luck !