TM00-01S Analysis and interpretation of reporting environmental data with a non-proper calibration curve for Kjeldahl nitrogen and phosphorus methods; Supplementary Infromation

For the time period between April 20, and October 4, 1999, samples were analyzed by a modified calibration model. An additional low concentration standard was added that improperly eliminated the blank standard from the calibration model. It was initially thought that the blank was critical in the calibration model, by introducing potentially a 0.01 and 0.05 mg/L high bias in results for phosphorus and nitrogen, respectively. Additional data analysis described here, shows that the nonstandard calibration model was still a good model to determine the phosphorus and nitrogen concentrations in environmental samples. The correct 8-point calibration model with the blank standard was developed and all original sample data were processed through the calculation, giving paired results. The variances observed in the data set would be caused entirely by the original nonstandard calibration model and the corrected 8-point calibration model, since the original instrument signal for the samples was used. The larger issue is "should the NWIS and LIMS databases be updated"? In conclusion, the differences between the calibration models is well within the routinely observed method variability with a median bias of 0 mg/L for phosphorus and 0.002 mg/L for nitrogen. A majority of differences calculated in the original result and the 8-point calibration result is within the LT-MDL and LRL for each method. Results in this range are already in a low confidence for quantitation region and data are reported with the estimated "E" attached to each result, reflecting the higher relative variability for measurement.