Knowing the uncertainty of the net signal activity is essential in order to calculate the concentration uncertainty of an analyte in a sample or to decide whether the analyte activity is sufficient for detection. The net signal activity is usually calculated as a difference between the gross signal activity and the blank activity obtained from a material that is added to the sample during the analysis (e.g., acid to dilute the sample or gas to transport it as an aerosol). It is often assumed a priori that the analyte concentration in the blank is the same for any blank measurement, and the dispersion in the blank activities obtained from the individual measurements is related to the intrinsic imprecision of the measurement process only. This case can be called stationary; the relevant uncertainty calculation methods are widely known and used. However, in real applications where only some of the blank activity sources are well characterized, the blank stationarity needs to be demonstrated. The nonstationarity of the blank has serious consequences: (1) for a "well-known background", the net signal uncertainty increases, since it includes the variance of the nonstationary background component; (2) for paired measurements, the net signal uncertainty does not change but cannot be estimated from one single blank analysis, thus compromising the utility of the paired measurement approach. Presenting the lower and upper uncertainty limits is appropriate for the evaluation of the uncertainty of a net signal if the assumption of a stationary blank is questionable.
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