Low recovery rate in Kjeldahl Procedure

[Mox-TT]

Hi, everyone
We have a product that can peform determinations of nitrogen/protein according to the Kjeldahl method
However we found recovery rate was a little low recently. See below steps:
1. H2SO4 + K2SO4 + catalyst → NH4+(AcidicSolution) -------Digestion in 420℃
2. NH4 + NaOH + steam distillation = NH3 -------Distillation
We tested tubing system like shortening the whole lenght but no more findings
Can anyone provide suggestions that the reason why recovery rate is low or some factors can influence recovery rate?
Thanks in advance!

 

[25362]

I wonder whether published information as in the following link could be of help:

https://fr.wikipedia.org/wiki/Méthode_de_Kjeldahl#Mode_op.C3.A9ratoire