Mo. Rev. Stat. § 106.160

At the time and place appointed for trial, and before proceeding thereon, some person authorized by law so to do shall administer to the members of the special commission, in cases of impeachment of the governor or a judge of the supreme court, or to the judges of the supreme court, in cases of impeachments of other officers, an oath or affirmation impartially to try and determine the charges and to do justice according to the law and the evidence; and no member shall sit or give his vote until he shall have taken such oath or affirmation.