Unfortunately, the DoE did not give clear results for all the vari-ables because the statistical analysis revealed an unusually highstandard deviation (>10). This means that only factors affectingvery heavily the recoveries of the analytes could be identified.We discovered that both dilution and acidification of the sam-ple after deproteinization and before SPE were necessary, whereasacidification of the sample before extraction did not improve recov-eries significantly. Recoveries also increased as the concentrationof the displacer HCOONH4increased from 5 to 10 mmol L−1and