The following reference might be relevant.

Analytical techniques for biopharmaceutical development (pages 311-318) By Roberto Rodríguez-Díaz, Tim Wehr, Stephen F. Tuck Publisher CRC Press, 2005 ISBN 0824707060, 9780824707064

The following reference references might be relevant.

They write "...Detecting a low level-level (<10ug/ml) small molecule impurity in the presence of active protein and excipients at much higher concentrations is a daunting analytical problem. In many cases, NMR is an excellent method for solving such challenging analytical problems..."

The following references might be relevant.

1. Separation and Analysis of Trace Degradants in a Pharmaceutical Formulation Using On-Line Capillary Isotachophoresis-NMR. Stacie L. Eldridge, Valentino K. Almeida, Albert K. Korir, and Cynthia K. Larive. Anal. Chem., 2007, 79 (22), pp 8446–8453

2. NMR spectroscopy in pharmaceutical analysis. By Iwona Wawer, Ulrike Holzgrabe, Bernd Diehl ...discuss possibilities for use of NMR for detection of impurities in pharmaceuticals.

3. Analytical techniques for biopharmaceutical development (pages 311-318) By Roberto Rodríguez-Díaz, Tim Wehr, Stephen F. Tuck Publisher CRC Press, 2005 ISBN 0824707060, 9780824707064

They write "...Detecting a low level-level (<10ug/ml) small molecule impurity in the presence of active protein and excipients at much higher concentrations is a daunting analytical problem. In many cases, NMR is an excellent method for solving such challenging analytical problems..."