Why do I get Ampliseq for Illumina libraries of low concentration?

Question

[stefaniab]

I have prepared 4 libraries using a custom Ampliseq for Illumina panel. Despite that the libraries have the expected size based on Agilent 2200 tapestation run, their quantities are really low ranging from 0.123 ng/ul -0.266 ng/ul (Qubit HS DNA assay) / 247pM-1nM (NEBNext® Library Quant Kit for Illumina). Do you have any idea on how to increase the libraries' concentrations?