DEPENDENCE OF ION INTENSITY IN ELECTROSPRAY MASS-SPECTROMETRY ON THE CONCENTRATION OF THE ANALYTES IN THE ELECTROSPRAYED SOLUTION

The dependence of the mass-analyzed ion intensity I(A) and I(B) of analytes A+ and B+, whose concentration in the electrosprayed methanol solution was [A+] and [B+], was determined in a series of experiments and for a variety of concentrations. A proposed equation relating I(A) and I(B) to [A+] and [B+] (see eq 4) was found to provide useful predictions. Coefficients k for the ions Li+, Na+, K+, Cs+, and NH4+ and several protonated alkaloids, (ethyl)4N+, (n-propyl)4N+, (n-butyl)4N+, n-C7H15NH3+, and n-C11H23NH3+, were obtained. The values of the coefficients are compared with predictions of the Iribarne ion evaporation theory. Rates of solvent loss from the droplets and the fission of the droplets due to Rayleigh instability are also included in the analysis. It is concluded that the observed coefficients k depend on the surface activities of the analytes when [A+] and [B+] are in the range 10(-8)-5 x 10(-6) mol/L and on the ion evaporation rate constants and surface activities for [A+] and [B+] in the range 5 x 10(-5)-10(-2) mol/L. The ion evaporation rates predicted by the Iribarne equation are found just barely consistent with the experimental coefficients. Alternate methods for gas-phase ion production are also considered.