Mass determination of phosphoramidites

Nucleoside phosphoramidites are the most widely used building blocks in contemporary solid-phase synthesis of oligonucleotides. The accurate molecular weight measurements of such molecules, which are acid-labile compounds, may be easily determined by mass spectrometry using a matrix system, triethanolamine/NaCl, on a liquid secondary ion mass spectrometer (LSIMS) or fast-atom bombardment (FAB) MS equipped with a double-focusing mass spectrometer. The present method rapidly and easily measures the accurate molecular weights of various phosphoramidites as adduct ions [M+Na]+ with an average mass error smaller than 0.4 ppm, allowing determination of the formulas of the phosphoramidites in place of elemental analysis. Further, it was found that intensities of molecular-related ions could be enhanced to the highest degree by adjustment of the molar ratio of phosphoramidite and NaCl, fixing the amount of triethanolamine on LSIMS, making the present method a powerful tool for identification of phosphoramidites by mass spectrometry.