+++2 SciDir, many thanks 2 Macro!!!

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Hodges RS, Merrifield RB. Monitoring of solid phase peptide synthesis by an automated spectrophotometric picrate
method. Anal Biochem 1975;65:241-272

DOI:

http://dx.doi.org/10.1016/0003-2697(75)90509-6

Sotiris E. Kakabakos, Philemon E. Tyllianakis, Gregory P. Evangelatos, Dionyssis S. Ithakissios. Colorimetric determination of reactive solid-supported primary and secondary amino groups. Biomaterials 1994 V.15(4), pp. 289-297

DOI:

http://dx.doi.org/10.1016/0142-9612(94)90054-X

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