281 RE SID U E ON IG N IT ION ... until constant weight is attained or until the percentage of residue complies with the limit in the individual monograph. Conduct the ignition in a wellventilated hood, but protected from air currents, and at as low a temperature as … Ignition Test, and USP <281> Residue on Ignition can be used as interchangeable in the ICH regions given the following: Unless otherwise specified in a monograph, an appropriate sample weight is chosen, typically 1-2 g, to result in a level of residue sufficient to be accurately measurable by weight (typically 1 …