I think the 13th Amendment that safetalker is referring to, is an amendment which was passed by congress but never ratified by a sufficient majority of states to become an official amendment. Interestingly, it is an outstanding amendment, which means if 3/4ths of the states were to ratify it today, it would become law. Its wording is as follows:

"If any citizen of the United States shall accept, claim, or receive or retain any title of nobility or honour, or shall, without the consent of Congress, accept and retain any present, pension, office or emolument of any kind whatever, from any emperor, king, prince or foreign power, such person shall cease to be a citizen of the United States, and shall be incapable of holding any office of trust or profit under them, or either of them."

Those who argue that this amendment was ratified are generally of the ilk who incorrectly believe that it would strip lawyers of their citizenship (and are working toward that end) due to their use of the "Esq." suffix.