As wafer diameters increase, monitor wafers become more costly for process characterization and real-time sensors become more attractive. Process-state sensors are generally easier to implement than direct wafer-state sensors, but process-state sensors are usually not thought to provide wafer-state analysis. Utilizing a process-state sensor, such as a mass spectrometer, for wafer-state information will result in new approaches for sensing, optimizing, and controlling integrated circuit fabrication processes. We used a two stage differentially pumped mass spectrometer system sampling directly from an electron cyclotron resonance (ECR) chamber to sense the process-state and monitor end-point during silicon dioxide etch. The end-point uniformity is characterized using mass spectroscopy and material balance modeling. Specifically, using CF4 and D2 etch gases, the partial pressure of CO-containing etch products decays near the endpoint, and the average initial slope is directly correlated with the uniformity determined from optical interferometry thickness measurements. Changing the ECR magnet geometry defines the etch uniformity, and the resulting CO+ end-point signal is sensed with the mass spectrometer. By analyzing the initial slope of the CO+ signal near the end-point, a correlation exists between uniformity and the mass spectrometry signal. A COF2 etch product material balance provides a concentration versus time end-point model which substantiates the correlation between the mass spectrometer signal and the etch uniformity. This model is valid for any sensor that detects a change in the COF2 concentration at the end-point.