We created a general setup for quantitation by NMR, using a neat solvent as an external reference. The method, described in the flowchart, is simple to use. An external reference can be used for any sample of analyte, because signal overlap between analyte and reference or reactions between the two are not a concern. Quantitation with an external reference must take into account the different conditions in which the two spectra are acquired. Differences in sample volumes can be made &lt;0.3% by using tubes of the same manufacturer/model and filling them with ≥ 50 mm. A larger source of errors, up to 25%, comes from the difference in RF energy absorption by the two samples. This is accounted for using the reciprocity principle which states that the area of the signal is proportional to the reciprocal of the 90° pulse. The parameter set we use for quantitation includes a macro which measures the 90° pulse in the background and uses the value for the excitation pulse. The processing software, Eretic2 in Topspin, will automatically read the values for the 90° pulses in the two spectra and use them to calculate the concentration of the analyte using the PULCON relationship: [analyte] = (Aanalyte/NSanalyte) / (Areference/NSreference) × (pw90analyte/pw90reference) × [reference] [ ] - molar concentration, A – area of the signal, NS – number of spins responsible for the area, pw90 – the 90° pulse. The molar concentration of neat liquids is calculated from molar weight and density. Using them as external references eliminates the inconvenience and intrinsic errors associated with preparing a standard by gravimetric and volumetric measurements and affords an accuracy &lt;1%. We measured the concentrations of 9 organometal reagents using a sample of water as reference and compared the results with an internal standard NMR method and with 2 titration methods.

Precise NMR method for titering solutions of organometal reagents using neat solvents as external references

ACS Fall 2021

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