The Clean Air Act Amendments of 1990 and resultant rulemaking outlined in the Code of Federal Regulations (CFR) require powerplants to strictly control and monitor stack emissions (figure). One of the many challenges imposed by these regulations is certification of the continuous emissions monitoring system (CEMS), including the performance of a relative accuracy test audit (RATA). The RATA is particularly challenging for powerplants that emit very low levels of NOx-such as gas-fired combined cycles that are permitted in the single digits (less than 10 ppm). Sources of possible errors are the calibration-gas concentrations, monitor biases, monitor linearity over the span range, monitor drift over time, and absorption, quenching, or interferences by the sample train.