On cluster ions, ion transmission, and linear dynamic range limitations in electrospray (ionspray) mass spectrometry

Abstract

The ion transmission in Electrospray (Ionspray) Mass Spectrometry (ESMS) was studied in order to examine the instrumental factors potentially contributing to observed ESMS linear dynamic range (LDR) limitations. A variety of means used for the investigation of ion transmission demonstrated that a suspected loss in tetraalkylammonium ion signal in favour of formation of analyte ion-analyte ion-pair clusters is negligible. The ion/ion-pair cluster abundance continues to rise after the core analyte ion reaches the 10 −5M concentration LDR plateau. The relative cluster ion abundance changes observed with increasing concentration appear to reflect solution phase ion/molecule clustering reactions at the surface of charged droplets produced in the electrospray. The simultaneous measurement of the ES capillary spray current and ion currents at the first three stages of ion sampling revealed that the nozzle orifice which separates the atmospheric pressure ion source from the first vacuum stage receives a current that rises continuously with sample concentration. The skimmer current rises to a plateau then falls off at higher analyte concentrations, closely matching the final mass spectrometric response. Complete coverage of the charged droplet surface can explain the plateau, but a simple model that explains tetraalkylammonium ion signal suppression cannot be given. Dynamic range limitation was less pronounced with higher atmospheric pressure chemical ionization total ion currents, which showed linearity over greater total MS ion current ranges. MS signal suppression observed at high (> 0.2 mM) analyte concentrations may thus be attributed to unique ES ion formation/instrumental/space charge effects.