Nucleic acid purification

A process for purifying plasmid DNA from a nucleic acid containing sample comprising plasmid DNA and contaminants, which process comprises a step of contaminant removal, comprising: (a) treating the sample to form a nucleic acid solution having a concentration of monovalent cations; (b) contacting the nucleic acid solution with an ultrafiltration membrane having a molecular weight exclusion limit of at least 30kDa under conditions in which substantially no gel-layer forms and in which the concentration of monovalent cations is sufficiently high for a time sufficient to remove substantially all RNA and form a retentate containing plasmid DNA; and (c) collecting the retentate.