Herein, we introduce three multimodal SPE methods using two to (n) purification columns to separate full length 5'-DMT-on oligonucleotides with size ranging from 40 to 180-mers from short length 5'-DMT-on oligonucleotides. Two of the said methods require using some columns sequentially with the collection and reprocessing of an intermediate fraction and are used for oligonucleotides with length ranging from 70 to 180-mers. A third method is carried out with columns stacked and used in series and is best used to purify oligonucleotides with length ranging from 40 to 80-mers. Preferentially, a series of stacked columns contains from top to bottom hydrophobic porous sorbents with increasing pore sizes. Short length DMT-on oligonucleotides arise from depurination or branching during phosphoramidite based synthesis. Reversed phase partitioning and binding of short length DMT-on oligonucleotides take place simultaneously with the size exclusion of the full length DMT-on oligonucleotides. In the presence of a high ionic strength buffer, the short length DMT-on oligonucleotides bind to the top stacked columns while the less hydrophobic contaminant or DMT-off failures do not bind and/or are being washed off. In a stacked configuration, the full length DMT-on oligonucleotides are retained by the bottom column while in a sequential configuration, full length DMT-on oligonucleotides are collected and reprocessed. After detritylation of the full length oligonucleotides from the bottom column or last column in a sequence, full length nucleic acids are eluted with purity typically ranging from 90 to 95% for oligonucleotides about 80-mers in size and purity around 80 to 90% for oligonucleotides about 150-mers in size. This invention yields purified long oligonucleotides at a fraction of traditional purification costs which could spur their wider use in biological applications.