Compounds and methods for purifying oligonucleotides such as RNA and DNA. A target oligonucleotide is reacted with an orthoester linker comprising an affinity tag to form an orthoester oligonucleotide-orthoester linker conjugate which is subjected to a purification technique to separate the target oligonucleotide from impurities such as truncated oligonucleotides. The orthoester linker can be then removed under mild conditions to generate the target oligonucleotide in high purity.