For an example of qNMR, I want to talk about an analytical method – standard addition. If you’re anything like me, you HATED standard addition in undergraduate analytical chemistry. I’m not sure why I had trouble wrapping my brain around this as an undergrad, but believe you me, it did not come easy at all – which, if you really think about, makes no sense at all.
This is a great technique, with relatively easy sample preparation that provides quantitative information without having to add anything else to your sample. Brilliant. Right?!?! Moreover, this technique can be very useful if you have matrix effects that prevent you from extracting the information you’re really interested in.
Standard addition is a simple procedure where you prepare a series of samples that have the same total volume and the same amount of unknown but vary in the amount of known concentration stock solution that each flask is doped with. The concentration range of these standard addition standards should reflect a reasonable approximation of the unknown concentration. Following a biologically relevant procedure, we attempted this for an unknown solution of sodium acetate. Six samples were prepared ranging from 0 mM stock solution up to 50 mM. A sample preparation table and example cartoon is depicted below:
Three samples were prepared at each concentration and the 1H NMR was acquired on the NMReady-60e. Each spectrum was phased, baseline corrected, referenced and integrated with MNova to generate a stacked plot.
If all the scans are superimposed, and we zoom in on the sodium acetate region (δ = 1.5-2.5 ppm) we can see that the concentration create ‘steps’ where the absolute integration changes depending on the concentration.
If we tabulate and plot this data, we get generate a linear calibration curve. An example is shown here:
This graph can be used to find the x-intercept, by either algebra or extrapolation, this is where the analyte signal is 0. Here is where we can extract the concentration of our unknown.
It’s here that standard addition can be tricky – be careful to use the correct solutions. You’ve diluted along the way so 1) make sure you’re using the right concentrations around the way; and 2) be sure that you account for that when you are preforming the calculations!
If you’re interested in learning more about this experiment, or other ways to incorporate a benchtop NMR spectrometer into your undergraduate teaching curriculum please see our sample experiments page or contact us directly!
[1] Bharti, S. K.; Roy, R. Trends. Anal. Chem. 2012, 35, 5
[2] Harris, D. C. ‘Quantitative Chemical Analysis’, 5th Ed.; W.H. Freeman and Company: New York, 1999
[3] Rajabzadeh, M.; J. Chem. Educ. 2012, 89, 1454
[4] Bruce, G. R.; Gill, P. S. J. Chem. Educ. 1999, 76, 805