Extraction of ions from solutions under atmospheric pressure as a method for mass spectrometric analysis of bioorganic compounds

Abstract

The basic features of mass spectrometry, such as high sensitivity, molecular mass determination capacity, and acquisition of structural information, are not used practically to analyze non-volatile, polar, and thermally unstable compounds, in particular the majority of bioorganic substances. This is because it is impossible to evaporate these compounds without decomposition under ordinary heating. It is necessary to use non-destructive methods of evaporation and ionization of the substances in the ion source of the mass spectrometer or, alternatively, chemical protection of polar and labile groups. Existing methods of ’soft’ ionization require obligatory sample preparation procedures. This fact limits the application of these methods. Recently, certain progress has been achieved in this field of mass spectrometry applying the generation of the ions directly from solutions under atmospheric pressure. In this paper we report a method called the extraction of dissolved ions under atmospheric pressure (EDIAP). This method allows the mass spectra of non-volatile and thermolabile bioorganic compounds to be recorded with controllable fragmentation or clustering of the quasimolecular ion. Control of the mass spectrum allows unique information about the molecular mass of a substance, its structural features, and the nature of interaction between the ions and the neutral solvent molecules to be obtained. Moreover, the kinetics and mechanisms of ion-molecule reactions can be also investigated. These numerous tasks can be solved using only the ion source invented at the Institute of Analytical Instrumentation of USSR Academy of Sciences. The developed ion source is schematically shown in Fig. 1. The ion source has been installed in a MX 1320 double focusing mass spectrometer with accelerating potential of 2.5 kV. Commercially available pumps have been used in the differential pumping system. The pump speed was 5 L/s in the fore-vacuum chamber and 700 L/s in the high-vacuum chamber.