Applications of Delayed Extraction Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry to Oligonucleotide Analysis

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Abstract
A delayed ion extraction technique is shown to dramatically improve mass resolution and the overall quality of matrix-assisted laser desorption ionization (MALDI) mass spectra of oligonucleotides. Isotope limited mass resolution was obtained on samples up to 10-kDa molecular mass in linear mode, and as high as 7500 mass resolution (defined at half peak height) was observed in reflector mode. This performance is as good as that achieved to date for peptides and proteins. Applications included the detection of oxidized byproducts of phosphorothioate DNA and separation of components differing only by 15 Da at 9.5-kDa molecular mass. In addition to single components, complex mixtures could also be analyzed at greatly improved performance over conventional MALDI. An example is shown for sequence verification of an oligonucleotide of 31 bases in length by analyzing the failure products. Mass accuracy was adequate to verify sequences of oligodeoxyribonucleotides up to 9500-Da molecular mass. Fast fragmentation taking place between the ionizing pulse and the extraction pulse is demonstrated to be a sequencing tool for small oligonucleotides. By proper selection of matrix material, wavelength, and irradiance, fast fragmentation can be promoted efficiently. Fragment ions tend to form from cleavage of phosphodiester bonds, as previously observed in infrared MALDI.

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