add gc-ms protocol and reference in isa assay

assays/CMML_18-0008_GC-MS/protocols/GC-MS.md

+## Metabolite profiling
+
+Approximately 50 mg of ground leaf material was used to obtain extracts for metabolite profiling by gas chromatography-mass spectrometry (GC-MS) as described by Fiehn et al. (2000). Data analysis was performed using MassHunter Software, version B.07.00 (Agilent). For relative quantification, all metabolite peak areas were normalized to the peak area of the internal standard ribitol added prior to extraction and the amount of leaf material. From each sample, two extracts were prepared and analysed.
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