The present invention provides antibodies and methods for preparing antibodies to metabolites in the serotonin, tryptophan, kynurenine pathways. The prepared antibodies have low cross-reactivity to related metabolites, and are useful reagents for specific and sensitive immunoassays The present invention also provides stable derivatives of various metabolites and short chain fatty acids. The derivative can be conjugated to a biomolecule such as a carrier protein and combined with an adjuvant to stimulate an immune response. The derivatives can also be conjugated to other biomolecules.