def library_dilution_calculator_fixed_volume(concentration, size, ul):
    """Calculate ng per sample (based on given ul_sample) needed for multiplexing."""
    nM_DNA = (float(concentration)*(10.0**6.0))/(660*float(size))
    ng_sample = (660*float(size)*nM_DNA*ul)/(10.0**6.0)
    return ng_sample