Our understanding of the N cycle is affected by how accurately we can measure NH4 in natural waters. Measuring NH4 concentrations requires accounting for matrix effects (ME) that are caused by substances in the sample that attenuate or intensify the signal of the samples relative to the standards. We show that the ME calculation in the recently published fluorometric NH4 method is mathematically incorrect, producing results that consistently underestimate NH4 concentration as a nonlinear function of the ME. We provide the correct equation and offer an alternative approach that accounts for ME by using sample water rather than deionized water to make the standards, thereby producing a standard curve that contains the same background chemical properties as the samples. In addition, we show that the previous method for measuring a sample's background fluorescence does not include the background signal of the reagent or its interaction with the matrix constituents of the sample. We provide a new method for measuring a sample's background fluorescence that includes the background fluorescence of the sample, reagent, and their interaction. The simple changes we suggest produce more accurate and precise NH4 measurements.
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