Oligonucleotides synthesized on a montmorillonite catalyst were analyzed directly. By mixing the catalyst with a matrix (2,4,6-trihydroxyacetophenone or 6-aza-2-thiothymine) and dibasic ammonium citrate, higher molecular weight products were detected compared with "classical" methods such as gel electrophoresis and HPLC with UV as a detector. The oligomers (30-mers and higher) were detected by mass spectrometry even though their concentration was less than 10(-4)% of the total content of the RNA. This method is different from the (MALDI) analysis of the eluates from montmorillonite, which otherwise requires desalting. Placing reaction mixtures with a high concentration of buffers on homoionic, preferably Li-containing, montmorillonite does not require desalting.