Dear All,
I am currently working with NMR spectra and could not understand fully if a compound amount strongly influence the peak intensity in Proton NMR. Also, has an influence the solvent amount used in for NMR analysis on the peak intensity of a studying compound?
Thanks,
Iryna.
In 1H spectra, if you follow certain precautions in the data acquisition (in particular use sufficiently long recycling delay in accmulations, then RELATIVE intensities (integrals!) of individual multiplets (for single peaks the discussion is more complicated) respect very nicely the relative molar ratios of the protons that are responsible for those multiplets. Total compound amount is a very different thing (just consider the spectrum off a molecule that has no protons).
For decoupled 13C spectra the intensities follow the molar ratios very approximately because of differences in NOE enhancements, particularly between protonated (strong) and nonprotoneated (weak) carbons. Non-decoupled 13C spectra would behave as 1H from this point of view, but they take longer to measure and interpret, so nobody is using them.
For ABSOLUTE molar concentrations, the situation is much different since a calibration is needed and there exists a number of approaches. Unless you do some special calibration, the absolute integral of a multiplet is to be considered as having "arbitrary units", nly the ratios have any chance to be rigorous.
This whole branch of "NMR spectroiscopy science" is often referred to as qNMR (q for quantitative). You can use the term to google it.
As for solvent influence, in first approximation it has no direct impact. What matters is the molarity of the nuclei you observe. In first approximation, of course; upon close inspection, solvent may have a few indirect effects.
Hi,
This you should be able to verify by preparing several samples with varied amount of solute (the compound) keeping everything else same (e.g. solvent type and volume, temp. etc.). In principle, intensity should be affected by the concentration as more the concentration -> more the spins -> expect a higher magnetization. Now how strongly it will affect is a matter of investigation.
Best
Amandeep
In 1H spectra, if you follow certain precautions in the data acquisition (in particular use sufficiently long recycling delay in accmulations, then RELATIVE intensities (integrals!) of individual multiplets (for single peaks the discussion is more complicated) respect very nicely the relative molar ratios of the protons that are responsible for those multiplets. Total compound amount is a very different thing (just consider the spectrum off a molecule that has no protons).
For decoupled 13C spectra the intensities follow the molar ratios very approximately because of differences in NOE enhancements, particularly between protonated (strong) and nonprotoneated (weak) carbons. Non-decoupled 13C spectra would behave as 1H from this point of view, but they take longer to measure and interpret, so nobody is using them.
For ABSOLUTE molar concentrations, the situation is much different since a calibration is needed and there exists a number of approaches. Unless you do some special calibration, the absolute integral of a multiplet is to be considered as having "arbitrary units", nly the ratios have any chance to be rigorous.
This whole branch of "NMR spectroiscopy science" is often referred to as qNMR (q for quantitative). You can use the term to google it.
As for solvent influence, in first approximation it has no direct impact. What matters is the molarity of the nuclei you observe. In first approximation, of course; upon close inspection, solvent may have a few indirect effects.
It's not exactly tells about compound amount. In case of a crude product, the intensity (area) tells the percentage of conversion. Thus yield can b assigned from peak intensity in a mixture of components
As Stan already said, NMR can be used both in a qualitative and in a quantitative way. The key aspect for quantification is to use a sufficiently long relaxation delay (in the order of 15-20 seconds will do in most cases). For absolute quantification, a qNMR standard is needed with a well known purity. The solvent can have an effect on the relaxation delay, but if it is sufficiently long, you will likely not notice it.
- Badges
- Science topic