The concentration of Nuc-O3PO -in the solid sample can be estimated using the pKa of phosphoric acid or its diester. Assuming that the pKa of Nuc-O3POH is 2.0 and the concentration of [M+H] + in the sample with 3-HPA being the matrix is 0.27 M (see supporting information), the concentration of Nuc-O3PO -would be 1.6 × 10 -4 M when the amount of nucleic acid containing 10 pmol of phosphate diester group is dispersed in 25 nmol of 3-HPA, 93.6 mL/mole is taken as the molar volume of 3-HPA, and 1.14 is assumed as the pKa of 3-HPA. It is noted that the concentration of Nuc-O3PO -may be far away from 1.6 × 10 -4 M because the sample is a solid and it contains other materials besides the nucleic acid and the matrix.