# Commission Regulation (EC) No 152/2009 of 27 January 2009 laying down the methods of sampling and analysis for the official control of feed (Text with EEA relevance), Division 3.17: Calibration standard solution of tryptophan and internal standard:

Extent: E+W+S+N.I.

Take 2,0 ml concentrated solution of tryptophan (3.15), and 2,0 ml of concentrated internal standard (α-methyl-tryptophan) solution (3.16). Dilute with water (3.1) and methanol (3.8) to approximately the same volume and to approximately the same concentration of methanol (10 %-30 %) as the finished hydrolysate.
This solution must be prepared freshly before use.
Protect from direct sunlight during preparation.

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Source: https://www.legislation.gov.uk/eur/2009/152/annex/III/division/G/division/3/division/3.17 (the official version)
Page: https://openlegislation.uk/eu-regulation-2009-152/annex-III/division-G/division-3/division-3.17
Contains public sector information licensed under the Open Government Licence v3.0. Source: legislation.gov.uk, The National Archives.