Fixed solids" is the term applied to the residue of total, suspended, or dissolved solids after heating to dryness for a specified time at a specified temperature. the weight loss on ignition is called "Determinations of fixed and volatile solids do not distinguish precisely between inorganic and organic matter because the loss on ignition is not confined to organic matter. it includes losses due to decomposition or vol-atilization of some mineral salts. better characterization of or-ganic matter can be made by such tests as total organic carbon (Section 5310), BOD (section 5210), and COD (section 5220).