THE FUNDAMENTALS OF APPLYING ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY TO LOW-MASS POLY(METHYL METHACRYLATE) POLYMERS

Electrospray ionization (ESI) is capable of ionizing many soluble poly mers. The ESI spectra are complex because of overlap of the multiply c harged ions of the oligomer distribution, causing current computer tra nsform programs to fail. However, it is possible to determine the orig in of the multiply charged ions, making it feasible to write a program designed to transform ESI polymer spectra. To assess the value of suc h a program for polymer analysis, isolated monodisperse methyl methacr ylate (MMA) oligomers (25 and 50 repeat units) were used to determine molar signal response and propensity for fragmentation. The sum of the peak areas for the multiply charged MMA 50-mer was found to be only a bout 66% of the summed peak areas for the 25-mer for the same molar co ncentration. However, conversion of the multiply charged peak areas to the singly charged representations, with peak area compression taken into account, gave equal signal responses for the 25-and 50-mers. Sign al response variations due to the tacticity of the MMA oligomers were not observed. Fragmentation of the MMA oligomers also was shown not to occur under normal ESI conditions. Therefore, transformation of the p olymer spectra to the singly charged molecular ion distribution should allow accurate calculation of average molecular weights, polydispersi ty, end group mass, and repeat unit mass.