text: The first paper on site-specific enrichment used the ninhydrin reaction to cleave the carboxyl site off alpha-amino acids in photosynthetic organisms. The authors demonstrated an enriched carboxyl site relative to the bulk δC of the molecules, which they attribute to uptake of heavier CO through the Calvin cycle.  A recent study applied similar theory to understand enrichments in methionine, which they suggested would be powerful in origin and synthesis studies.