We have synthesized a small 9-mer with 1-trp,3-arg,1-tyr, 1-thr, 1-ala. 1-ile, and an N-term ser. We did Fmoc on for the N-term serind and removed it later after cleavage from the resin. The calculated mw is 1208.39. Our mass spec (ion spray) shows a molecular ion at 1220 with a nice 610 peak too. What in the world could have a mw difference of 12???