Time‐of‐flight mass spectrometry of nucleic acids by laser ablation and ionization from a frozen aqueous matrix

Positive‐ion time‐of‐flight mass spectra of sodium salts of oligodeoxyribonucleic acids (DNA), and a protein, lysozyme, have been obtained using pulsed‐laser ablation and ionization of frozen aqueous solutions from a cooled copper substrate. The efficiency of positive‐ion formation was found to increase when the ablating laser was tuned to a frequency corresponding to resonant electronic transitions in sodium or copper atoms. Molecular ions of double‐stranded oligomeric nucleic acids, with little fragmentation, were observed at masses up to ∼18 500 Da. Single‐stranded nucleic acids could be fragmented by lowering the laser power‐density to reduce the the substrate heating rate. A dimer ion of lysozyme, at mass ∼29 000 Da, was detected at an impact energy on the electron multiplier of only 3.5 keV.