As known now that 21st and 22nd standard amino acid, both are coded by two different stop codons (UAG and UGA). Both the amino acid are incorporated into ribosome by class-II tRNA for protein synthesis.There three stop codons( UAG, UGA and UAA). Stop codon UAA may also code for a standard amino acid, which may be 23rd amino acid.
It seems that the 23rd AA doesn't exists. No tRNA was found to code for UAA. Maybe you found something else?
Mining prokaryotic genomes for unknown amino acids: a stop-codon-based approach: http://www.biomedcentral.com/1471-2105/8/225
Also thought fMet to be a historical oversight in the succession of amino acids following the big 20. It is after all a proteinogenic amino acid. Maybe it could be an honorary 23rd?
The full list of the 23 amino acids is:
1. Alanine 2. Arginine 3. Asparagine 4. Aspartic acid 5. Cysteine 6. Cystine 7. Glutamine 8. Glutamic acid 9. Glycine 10. Histidine 11. Hydroxyproline
12. Isoleucine - Essential 13. Leucine - Essential 14. Lysine - Essential 15. Methionine - Essential 16. Phenylalanine - Essential 17. Proline 18. Serine 19. Threonine - Essential 20. Tryptophan - Essential 21. Tyrosine 22. Valine - Essential
23. Ornithine
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