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Offline mir

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NMR vs MS
« on: February 26, 2007, 07:02:21 AM »
Is it just because NMR are more easy to intepret, than MS - since everybody chooses NMR?

There is a lot(!) of information an an mass spectra. Are you using all of it to determine structure?
No single thing abides, but all things flow.
Fragment to fragment clings, and thus they grow
Until we know and name them.
Then by degrees they change and are no more
The things we know.
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Offline movies

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Re: NMR vs MS
« Reply #1 on: February 26, 2007, 12:16:30 PM »
Mass spec will only tell you about fragment masses, not their structure.  Take for example the amino acids leucine and isoleucine.  They tend to look just about the same by mass spec, but are easily discerned by NMR.  Fragmentations aren't always so reliable.  You can find fragments consistent with how you expect the molecule to break down, but it's not conclusive.  Also, the absense of a fragment doesn't necessarily mean that you don't have the target molecule, it might just not fragment.  In NMR, if you don't have a resonance in the aromatic reagion, you probably don't have an aromatic proton!

You are right, however, in stating that MS is underused for structure determination.

Offline english

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Re: NMR vs MS
« Reply #2 on: February 26, 2007, 02:07:28 PM »
H NMR tells you everything you need to know!

I think MS is just a good way to validate things.  I don't hold any one way more important than the other.  I think they all are important at validating structures.

Offline Dan

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Re: NMR vs MS
« Reply #3 on: February 26, 2007, 02:18:39 PM »
H NMR tells you everything you need to know!

You need other nulei too, and more advanced techniques (DEPT, HMQC, HMBC, nOe etc.) but NMR is much better for structure elucidation than MS in my opinion.
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Offline Yggdrasil

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Re: NMR vs MS
« Reply #4 on: February 26, 2007, 02:51:21 PM »
However, it is often much easier to analyze your NMR spectra if you have some prior information from other techniques like MS.

Offline Dan

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Re: NMR vs MS
« Reply #5 on: February 26, 2007, 03:27:58 PM »
However, it is often much easier to analyze your NMR spectra if you have some prior information from other techniques like MS.

Agreed. You should always get a mass spectrum etc to strenghten your case.
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Offline Custos

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Re: NMR vs MS
« Reply #6 on: February 27, 2007, 08:58:28 PM »
Since NMR and MS tell you different information it's pretty pointless trying to decide which one you need for structure elucidation. As others have said you need both, and either can be more useful depending on the structure you are trying to work out. Mass Spect is much more useful than NMR for peptide analysis (for example) or for some heterocyclic systems that have not many carbons and hydrogens. In addition, even for simple organic molecules it is nearly impossible to work out the structure from carbon and proton NMR alone. You need a molecular formula as well and that most easily comes from high resolution MS.

Offline chiralic

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Re: NMR vs MS
« Reply #7 on: February 27, 2007, 11:49:52 PM »
MS is a chemical process whereas NMR (as well as is IR) is a physical process. The fragmentation process in MS is a chemical process. IMHO, It's easier to predict and to describe a physical process that a chemical process.

Even though that I work with equipments as GC-MS and ICP-MS in the Forensic Chemistry area, I must recognize that to do a structure elucidation by the evaluation of a mass spectra (either by identification or interpretation based on the chemical structure) to extract all the information contained in itself is difficult.

The interpretation of a mass spectrum is based on the use of characteristic peaks (key ions) and/or the key differences of masses between the molecular Ion and abundant ion fragments or between abundant ion fragments with functional groups, getting complicated relationships between MS data and Chemical Structures

The occurrence of rearrangement reactions means that structure identification from mass spectra is often difficult o impossible; furthermore functional groups do not always produce the same peaks.

IMHO, I think that it is the reason WHY NMR (mainly 13NMR than 1HNMR) is being used in organic chemistry more than MS (or IR or UV).
« Last Edit: February 27, 2007, 11:57:42 PM by chiralic »

Offline mike

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Re: NMR vs MS
« Reply #8 on: February 28, 2007, 12:30:27 AM »
NMR is very expensive compared to MS. I know this is not entirely related to your query but it is worth thinking about the various costs associated with intrumentation when comparing.
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Offline mir

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Re: NMR vs MS
« Reply #9 on: February 28, 2007, 05:19:45 AM »
People oftens only look at the losses, and use very little of the exact information the intensities gives them. There must be some kind of way to evaluate this information against possible ions? Isnt there a handbook or something for this? Or a modeling software of some sort?

Intensities is the second dimension of MS. The third dimension must be the fragmentation mechanisms.

This is my recipe for determining structures from MS:
- Rule out silica-peaks if your column is bleeding
- Look up in a MS-database, if you get a good or only bad hits you should doublecheck anyway
- Find the molecular ion peak (check this against the losses), and determine the number of carbons at that peak (if possible, estimate the number of A+2 elements also)
- Look up in a molecular weight table, and find the possible combination of atoms
- Work out the DBE-value (Double bond equivalents)
- Look at your substrate, use it as an backbone
- Now, draw all possible combinations of your molecular formula and DBE
- Look for low-ion series. A series of 39 and 65 is a strong evidence that 91 is the tropylium ion (and your your substance is aromatic).
- Then look at the nearest ion-peak. Determine the losses, and carboncontent if possible.
- Look for strong odd-ions, determine the losses
- Now evaluate your selection of molecules

At this step, you should isolate the product and do an NMR. If you cant isolate, remove the solvent, and take your crude mixture and do an analysis of that, and your pure substrate.

« Last Edit: February 28, 2007, 05:28:18 AM by mir »
No single thing abides, but all things flow.
Fragment to fragment clings, and thus they grow
Until we know and name them.
Then by degrees they change and are no more
The things we know.
- Titus Lucretius Carus

http://www.ife.no

Offline AhmedEzatAlzawalaty

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Re: NMR vs MS
« Reply #10 on: February 28, 2007, 08:58:36 AM »
in order to etermine structure by spectroscpy first keep in mind that the absence of a signal is more informative than its presence.
first look at NMR then IR then Mass and UV you wont probably make use of it.
THX
Ahmed Ezzat Alzawalaty
Pharmacy
Zagazig
Egypt

Offline mir

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Re: NMR vs MS
« Reply #11 on: February 28, 2007, 02:30:35 PM »
Well, that is correct.
But GC/MS is still fast and accurate.
Something to keep in mind.
No single thing abides, but all things flow.
Fragment to fragment clings, and thus they grow
Until we know and name them.
Then by degrees they change and are no more
The things we know.
- Titus Lucretius Carus

http://www.ife.no

Offline Custos

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Re: NMR vs MS
« Reply #12 on: February 28, 2007, 11:49:22 PM »
Also, in the MS look for isotope patterns (1:1 for bromine, 3:1 for chlorine) to help figure out if you have those halogens present. If the molecular weight is an odd number it means you have an odd number of nitrogen atoms in the molecule. Every bit of data is helpful in working out a structure.

Offline english

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Re: NMR vs MS
« Reply #13 on: March 01, 2007, 02:55:34 AM »
Generally I think MS is good for validation and determination of a possible molecular formula.

Then IR for significant functional groups.  Then H NMR and 13C NMR for determination of the actual structure.

After that I go back and use MS to see if the fragments and peaks make sense.  Sometimes NMR can be a hassle.  Splitting patterns aren't always the prettiest thing.

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