Analytical Sciences

Glow Discharge Plasma Ionization Mass Spectrometry for Direct Detection of Oxygenated Organic Compounds in the Gas-phase

Yoko NUNOME,* Kenji KODAMA,** and Kazuaki WAGATSUMA***

*Graduate School of Integrated Sciences for Life, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8521, Japan
**X-ray Instrument Division, Rigaku Corporation, 14-8 Akaoji, Takatsuki, Osaka 569-1146, Japan
***Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba, Sendai, Miyagi 980-8577, Japan

This study describes a direct analysis of oxygenated organic compounds, such as ketones, esters and ethers, rapidly and easily using a soft plasma ionization (SPI) source combined with a Q-mass spectrometer. A related molecular ion, [2M+H]⁺, in which a sample molecule (M) can undergo protonation via water clusters, such as [(H₂O)_n+H]⁺ and [N₂(H₂O)_n+H]⁺, in an ambient air glow discharge plasma, can be dominantly detected as a base peak with little or no fragmentation at a pressure of several kPa. Oxygenated organic compounds with high proton affinity were found to generate their dimers through the hydrogen bonding interaction at higher pressures. A deuterated solvent was used to examine whether or not the adduct ion of analyte was derived from the solvent. The formation of [2M+H]⁺ strongly depended on the time. A two-dimensional spectrometer was used to obtain the distribution of several excited species and then to confirm the ionization reactions of the analyte in the SPI source. The sample molecule would be readily ionized through Penning-type collisions with excited N₂, which causes fragmentation for oxygenated compounds due to the lower pressures (approx. 1.0 kPa) while it is ionized by an attachment with protons from water clusters at higher pressures (several kPa). The SPI source can be a new and powerful tool for soft ionization in direct analysis of volatile organic compounds (VOCs).

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