The present invention includes a method and system of measuring alteration, alteration type, and alteration-causing species in process fluids and equipment and for controlling the process feeds and conditions to maximize the yields and equipment lifetime by minimizing the alteration. The invention includes an optical sensor comprising an optical fiber, a fiber grating written within the optical fiber, strain-tuned elements fixed to the optical fiber and an unaltered element, and an altered element fixed to the unaltered element.