I am used to identifying molecular structure in isolation using proton and carbon NMR, but have never worked with a mixture before. The mixture relates to the following spectrum:
The spectra for compounds A and E are as follows:
The spectra for the pure compounds were easy to delineate the structure, however it's the mixture that gave me the most trouble. I did some digging, and found a youtube video that gave me the following approach, in order to determine mole ratios:
mol(A)/mol(E) = integration(A)/integration(E)*N(E)/N(A)
The above relates to taking two signals, one from the pure spectrum of A, and one from the pure spectrum of E, and taking N, the number of hydrogens represented by those signals. Then looking at the NMR spectrum of the mixture, one can find the corresponding signal taken from A and E, and use the integration reported in the mixture NMR for the above equation.
Inspecting the pure spectra: there is a triplet in the spectra for unknown A at about 0.95 ppm, which represents three protons, and has a corresponding signal in the mixture at about the same chemical shift, with an integration of 3.056; and for the spectra of unknown E, there is doublet at about 1.32 ppm, which represents six protons, and has an integration in the mixed spectra of 9.272, with the signal shifted upfield to about 1.15 ppm. I suspect this could possibly be due to interactions between the components, perhaps van der Waal's forces.
Finally, returning these to the above equation:
mol(A)/mol(E) = (3.056/9.272)*(6/3) = 0.6592
Given the above equation can be related to the mass and molar mass of A and E as follows:
mol(A)/mol(E) = m(A)/m(E)*M(E)/M(A)
where M(A) = 148.08881 g/mol and M(E) = 134.10955 g/mol (taken from Mass Spectroscopy data)
and in the mixture, if the total mass m(T) = m(A)+m(E), then letting m(T)=100 g, one can solve for the percentage weight of each component, or %w/w as follows:
0.6592 = (100-m(E))/m(E)*(134.10955/148.08881)
m(E) = (100 g)/1.7279 = 57.87 g
m(A) = 100 g - 57.87 g = 42.13 g
This the %w/w are 57.87% and 42.13% respectively.
This is where I hit a snag however, because if I were to chose other signals from the pure spectra and plug them into the above equation, I don't get the same %w/w. I know this to be a limitation, given no internal standard was used, thus %w/w is not absolute but relative. But what are other limitations, besides NMR being inherently insensitive and requiring large concentrations? How could I fix these limitations?
Appreciate the help in advance.