Abstract: Direct detection of poisoning of chemically amplified and other lithographic resists by ambient gases is achieved in substantially real time and with extremely high sensitivity by making test exposures of the resist at different doses such as by a graded density feature. Sensitivity may be made unconditionally adequate to ensure that uniform and predictable by separating test exposures by a time interval in excess of the amount of time necessary for lithographic exposure of chip areas on a wafer. Sensitivity increase may also obtained, particularly in combination with time separation of test exposures, from the use of sub-resolution features for grading density of a test mask pattern feature which quantizes exposure sensitivity at high resolution. Measurement of test exposure patterns may be accomplished in an overlay optical metrology tools in the same process in which lithographic resist patterning is measured to reduce time and process steps.