You definitely can use an internal standard at specific concentration and use ratios of peak integrals to measure concentrations by NMR, but one thing to be mindful about is distortion of peak intensity by saturation - i.e. pulsing too frequently and thereby not allowing for sufficient relaxation of magnetization between scans.

A book "High-Resolution NMR Techniques in Organic Chemistry" (ISBN 978-0-08-042798-0) by Timothy Claridge has a very good explanation of this experimental detail in Chapter 4

So - yes it must be possible to set upper limit of concentration using NMR data.

I hope that somebody will correct me if I'm wrong.

You definitely can use an internal standard at specific concentration and use ratios of peak integrals to measure concentrations by NMR, but one thing to be mindful about is distortion of peak intensity by saturation - i.e. pulsing too frequently and thereby not allowing for sufficient relaxation of magnetization between scans.

A book "High-Resolution NMR Techniques in Organic Chemistry" (ISBN 978-0-08-042798-0) by Timothy Claridge has a very good explanation of this experimental detail in Chapter 4

So - yes it must be possible to set upper limit of concentration impurity content using NMR data.data, the lowest number that can be reported this way depends on sensitivity of the instrument and amount of sample in the tube or rotor (for solids).

I hope that somebody will correct me if I'm wrong.