Concentration dependence of nucleoside fast‐atom bombardment mass spectra

Abstract

The quantitative investigation of the four RNA nucleosides (adenosine, guanosine, cytidine and uridine) using fast-atom bombardment mass spectra is reported. Nucleoside concentrations are varied in the 0.8-4% range. The nucleoside hydrogen-bonded complexes are shown to form primarily on the liquid matrix surface.Fast-atom bombardment mass spectrometry (FAB-MS) is widely used in investigations of organic substances.' This technique is especially applicable to labile biological substances. FAB-MS also allows hydrogen-bonded complexes to be For example, Greco et a1.' measured the proton affinity of various nucleosides (Ns), utilizing FAB only as a source of specific ions, but the origin of complexes was not studied. Although accurate measurements of FAB mass spectra are quite their quantitative interpretation is still difficult due to complicated ion formation processes. The ion current can be influenced by a variety of substance properties, such as solubility, p~, , surface activity etc.6 Publications devoted to the elucidation of the ion formation mechanism are numerous, yet all the models described can be sorted into two groups: 'preformed ions' models and 'gas-phase reaction' models.In this paper we study the intensities of the main ion currents in the nucleoside FAB mass spectra as a function of the nucleoside concentration in the glycerol matrix and make an attempt to reveal some details of ion cluster formation. EXPERIMENTALAnalyses were performed with an MI-1201E mass spectrometer manufactured by SELMI (Sumy , Ukraine) and equipped with a FAB ion source. Samples were bombarded with a primary 5 keV argon atom beam. A copper strip served as a sample holder. Commercial nucleosides (Serva, Germany) were used in the experiments. Commercial synthetic glycerol (Gl) was dried in uacw before using it as a matrix. Dimethylsulfoxide (DMSO) was employed as solvent for all the nucleosides. Weights of all the components were measured with an accuracy of 0.05 mg. Approximately 10 mg of the nucleoside was dissolved in 0.06 mL of DMSO and then mixed with 0.1 mL of dried glycerol. The solution was then dried in uacuo to avoid concentration changes during ionization. The resulting solution was diluted to prepare five different concentrations ranging from 0.8 to 4%. Only the main peaks (not all) of the mass spectrum were recorded. Those peaks corresponded to: glycerol fragment ions (mlz = 45, 57, 75), protonated glycerol molecular ion (mlz = 93), protonated glycerol dimer ion (mlz = 185), ions of nucleoside bases, nucleoside molecular ions, glycerol-nucleoside complex ions and nucleoside dimer * Author for correspondence.ions. In processing the mass spectra, the intensity of each peak was multiplied by the square root of its mlz in order to correct for different sensitivity of the electron multiplier to different mass ions.' These corrected intensities were then normalized to the sum of the intensities of all the main peaks in the given spectrum. The normalized intensities were employed in our modelling. At least eight samples ...