ON CLUSTER IONS, ION TRANSMISSION, AND LINEAR DYNAMIC-RANGE LIMITATIONS IN ELECTROSPRAY (IONSPRAY) MASS-SPECTROMETRY

The ion transmission in Electrospray (Ionspray) Mass Spectrometry (ESM S) was studied in order to examine the instrumental factors potentiall y contributing to observed ESMS linear dynamic range (LDR) limitations . A variety of means used for the investigation of ion transmission de monstrated that a suspected loss in tetraalkylammonium ion signal in f avour of formation of analyte ion-analyte ion-pair clusters is negligi ble. The ion/ion-pair cluster abundance continues to rise after the co re analyte ion reaches the 10(-5) M concentration LDR plateau. The rel ative cluster ion abundance changes observed with increasing concentra tion appear to reflect solution phase ion/molecule clustering reaction s at the surface of charged droplets produced in the electrospray. The simultaneous measurement of the ES capillary spray current and ion cu rrents at the first three stages of ion sampling revealed that the noz zle orifice which separates the atmospheric pressure ion source from t he first vacuum stage receives a current that rises continuously with sample concentration. The skimmer current rises to a plateau then fall s off at higher analyte concentrations, closely matching the final mas s spectrometric response. Complete coverage of the charged droplet sur face can explain the plateau, but a simple model that explains tetraal kylammonium ion signal suppression cannot be given. Dynamic range limi tation was less pronounced with higher atmospheric pressure chemical i onization total ion currents, which showed linearity over greater tota l MS ion current ranges, MS signal suppression observed at high (> 0.2 mM) analyte concentrations may thus be attributed to unique ES ion fo rmation/instrumental/space charge effects. (C) 1997 Elsevier Science B .V.