The present invention is directed to a method for modifying the retention time of RNA on a chromatographic column. The present invention also concerns a method for purifying RNA from a mixture of at least two RNA species. Furthermore, the present invention relates to a method for co-purifying at least two RNA species from a mixture of at least two RNA species. In particular, the present invention provides a method for harmonizing the numbers of A and/or U nucleotides in at least two RNA species. The present invention is also directed to RNA obtainable by said methods, a composition comprising said RNA or a vaccine comprising said RNA and methods for producing such RNA and compositions. Further, the invention concerns a kit, particularly a kit of parts, comprising the RNA, composition or vaccine. The invention is further directed to a method of treating or preventing a disorder or a disease, first and second medical uses of the RNA, composition and vaccine. Moreover, the present invention concerns a method for providing an adapted RNA sequence or an adapted RNA mixture.