Improved mass spectrometry characterization of organic compounds using analyte preconcentration and radio-frequency ionization.
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Olaitan, Abayomi D.
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Mass spectrometry (MS) is a powerful and sensitive analytical technique that can be used for analysis of organic compounds in diverse areas of research including biomedical, environmental, and forensic sciences. Detection and identification of volatile organic compounds (VOCs) in real world samples, which are often present at low concentrations and within complex mixtures, require the use of efficient sampling techniques and versatile ionization sources. Presented research projects focus on the use of analyte preconcentration and radio-frequency ionization to improve sensitive characterization of organic compounds with MS. A home-built post-column cryogenic trap (PCCT) coupled to a commercial gas chromatography (GC)/quadrupole MS system was utilized for VOC analysis. Analytical figures of merit, including limit of detection (LOD) and signal-to-noise ratio, for GC/PCCT/MS were evaluated and compared to “conventional” GC/MS (i.e., without PCCT) using direct headspace and solid-phase microextraction techniques. An approximately two orders of magnitude improvement in LOD for the analysis of VOCs using GC/PCCT/MS over the use of conventional GC/MS was observed. Radio-frequency ionization (RFI), a new and highly efficient ionization method discovered in our lab, coupled with Fourier transform-ion cyclotron resonance (FT-ICR) MS is an ideal approach for analysis of volatile and semi-volatile organic compounds. RFI FT-ICR MS was employed for sample fingerprinting and chemical characterization of organic compounds present in bio-oil and commercial gasoline samples. Experimental results, designed to study the effects of electric field amplitude and frequency on ion generation yield, indicate that signal reproducibility is enhanced at higher frequencies (for RF voltage amplitude between 175 Vp-p and 400 Vp-p). Moreover, experimental results indicate that the ionization threshold RF voltage amplitude in RFI is independent of either the analyte ionization energies or polarizabilities, suggesting that RFI is a universal ionization technique. The possibility of electron emission in RFI, using both direct charged particle current measurements and indirect electron detection, was also studied. We show that RF-generated electrons can be captured by high electron affinity molecules (e.g., hexafluorobenzene) in a 9.4 tesla FT-ICR mass spectrometer. Results from the electron attachment rate constant for hexafluorobenzene, using a post-RF electron trapping FT-ICR experiment, indicate that RFI process involves RF-generated electrons.