Calibration methods for determination of ammonium and excess acid in Kjeldahl digests by flow injection analysis

Several modes of calibration for the determination of ammonium (protein) by gas diffusion flow injection analysis (FIA) in Kjeldahl sample solutions have been evaluated with the goal of reducing analytical variability caused by deviations in acid content. Univariate calibration based on peak area as a response function allows accurate and precise determination of the protein content only if small injection volumes, i.e., less than 80 μl, are used. No possibility to estimate the concentration of excess acid then exists. However, by using the entire FIA response curve (1098 data points) and partial least squares (PLS) calibration both protein and excess acid can be determined in one single run. The calibration set must be comprised of solutions covering both the expected protein and the excess acid concentration ranges. The method has been tested on feed samples. Typical values for the root mean square errors of prediction (RMSEP) were 0.8% for protein and 0.025 mol/l for sulphuric acid.