Use of Polymer Surfaces for Molecular Ion Adsorption and Desorption

Abstract

The desorption of molecules and molecular ions from surfaces as a consequence of excitation by an incident fast or slow particle is a classic example of a good news-bad news scenario. The good news is that organic mass spectrometry has experienced a revolution in development and application. The bad news is that the understanding of the processes involved is still in a state of chaos and there is no evidence that the atmosphere has improved to a level where an orderly evolution of concepts will emerge. Part of the problem is emotional, linked to the personalities and ambitions of those, like ourselves, who are actively promoting a particular variant and model, but another part is related to the complexity of the problem. Unlike studies of atomic and molecular ion emission using highly disciplined surfaces where theory and experiment are sharpened to elegant simplicity, the fragile and reactive nature of the organic molecule makes the study of the emission of their molecular ions a dirty business. One aspect of the high vacuum technology that has made possible a study of adsorbate-substrate interaction is the use of Langmuir adsorption at a solid-gas interface. By careful control of pressure and exposure time, it is possible to lay down adsorbate layers with varying degrees of surface coverage and to study the influence of surface concentration on the desorption-ionization process [1]. It is also possible to perform Langmuir adsorption experiments at a solid-liquid interface. In these studies, solution concentration plays the role of partial pressure in controlling the surface concentration of adsorbate. Rapid removal of the liquid phase after equilibrium has been reached leaves a surface containing adsorbate in a particular state dictated by the conditions of the thermodynamics of adsorption for that particular system.