Analytical Sciences

Multi-charged Oligonucleotide IonFormation in Sonic Spray Ionization

Min HUANG* and AtsumuHIRABAYASHI** 

*Life Science Group, Hitachi Ltd., 1-3-1,Minamidai, Kawagoe, Saitama 350-1165, Japan
**Central ResearchLaboratory, Hitachi Ltd., 1-280, Higashi-Koigakubo, Kokubunji, Tokyo185-8601, Japan

An oligonucleotide tends to release hydrogenatoms from a phosphoric acid group and to form negative ions that can bedetected by mass spectrometry. Usually, with a solution-spray basedionization technique, the negative ions are present in different chargestates. Ion formation for the nucleotide is quite complicated and iseasily influenced by matrix and other constituents in a sample solution, aswell as by the operating parameters for a mass spectrometer. In this work,we studied oligonucleotide ion formation by using an ion trap massspectrometer combined with a sonic spray ionization (SSI) source. Anoligonucleotide with 20 bases was measured. Effects from contaminants andparameters affecting the ion production, such as a high voltage applied tothe ionization source and sample solution-flow rate, were investigated.Our results showed that an ion with about one charge for every three baseswas most abundant. However, the signal intensity and the mass spectrumpattern were sensitive to the matrix and operating parameters. One of thereasons for such sensitivity is that there are various ion states for anoligonucleotide. Any change in the matrix or an operating parameter mayshift the balances between the ion states. Adding Tris, or(hydroxymethyl)aminomethane, enhanced the signal intensity of theoligonucleotide and promoted formation of the oligonucleotide ion withhigher charges, while adding acetic acid favored the ions with lowercharges, compared with that obtained in the medium without adding Tris andacetic acid. The effects on charged droplets and chemical enhancement wereinvestigated. The mechanism for oligonucleotide ion formation isdiscussed.

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