55 ILCS 5/5-36001

All contracts for supplies, material and work for the County of Cook shall be let as herein provided. All contracts for supplies, material or work for Cook County shall be approved by the board of commissioners and signed by the president of the board, the county purchasing agent and the comptroller. Supplies shall be issued only on the requisition of the responsible officers of the county institutions now or hereafter established by law, approved by the county purchasing agent.