INVESTIGATION OF THE CLUSTERING PROCESSES OCCURRING IN LIQUID SECONDARY-ION MASS-SPECTROMETRY FOR ALKYL QUATERNARY AMMONIUM BROMIDES

Upon investigation of the clustering behavior of a series of alkyl qua ternary ammonium bromides (MBr) in liquid secondary ion mass spectrome try (LSIMS), only the dimer ion, M(2)Br(+), was of significant abundan ce in the resultant mass spectra. The ion abundance of the dimer grows relative to that of the molecular ion, M(+), with increasing analyte concentration in LSIMS until a concentration is reached where the ion abundance ratio M(2)Br(+)/M(+) remains constant upon subsequent analyt e addition, The relationship between the M(2)Br(+)/M(+) ion abundance ratio and analyte concentration can be explained in terms of the solut ion equilibrium between these species, where the plateau region of the plot corresponds to the presence of an analyte monolayer at the liqui d surface. The M(2)Br(+)/M(+) values obtained in this region provide r elative dimer bond strengths for the alkyl quaternary ammonium salts, so the effect of different substituents on dimer stability can be asse ssed. Using this method, the importance of the electrostatic interacti on on the dimer strength and the destabilizing effect of the hydrophob ic interaction between long alkyl chains is revealed.