Quantitative Capability of Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry for a Complex Mixture

Abstract

Electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been evaluated for the quantitation of complex mixtures, by using a crude oil (Taching vacuum residue) and polyethylene glycol (PEG600) as model analytes. The crude oil is an extremely complex mixture consisting of hundreds of organic compounds, which are mostly non-polar hydrocarbons. The crude oil ESI FT-ICR mass spectrum contained approximately five hundred distinguishable peaks accompanied by plenty of isobars. These ionic species were singly charged but not protonated. Although the crude oil response line was saturated at high concentration, the linearly responding range was extended by shortening the duration of the external accumulation which was carried out in the hexapole ion guide. No improvement in the linear response was observed for the exclusively trapped crude oil ions in a narrow m/z range which was trimmed by a correlated sweep ejection event. The crude oil mass distribution pattern shifted toward higher m/z as the analyte concentration became higher. A linear relationship was observed between the analyte concentrations and the average molecular weights, in both the crude oil and PEG600 experiments. The average molecular weights increased on extending the external accumulation time. This phenomenon was attributable to a novel space charge effect taking place for externally accumulated ions. Optimizing the external accumulation period was vital to get a linear response up to a reasonably high concentration region. Basically, complexity and plurality of mixture samples need not significantly affect quantitation by means of ESI FT-ICR MS.