http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
Mass spectrometric analysis with cluster projectiles and coincidence counting
Methods for maximizing the amount of secondary ion information, per primary projectile, are described. The method is based on time-of-flight mass spectrometry and event-by-event coincidence counting. Each projectile in a secondary ion mass spectrometry experiment is essentially a nano-probe of the surface. Coincidence counting takes advantage of this fact by analyzing the results of each event individually. This type of event-by-event analysis is the only way to insure that the secondary ion chemical and spatial information is preserved. The information obtained from coincidence counting time-of-flight mass spectrometry includes: a) surface com position, b) relative concentrations, and c) degree of intermolecular mixing. The technique was applied to the study of an important new class of polymers: polymer blends. Secondary ion mass spectrometry, when applied to the analysis of synthetic polymers, induces backbone fragmentation which is characteristic of the homopolymer. The characteristic "fingerprint" peaks from polystyrene and poly(vinyl methyl ether) were used to identify the presence of these two polymers in a polymer blend. The percent coincidence between the characteristic secondary ions from each component of the blend were used to determine both the relative concentration and the degree of molecular mixing. Results indicate molecular segregation of the two polymers on the film surface. The largest degree of segregation was determined for the phase separated blends. The performance of this technique depends on the desorption efficiency of the primary projectiles. In practice then, one seeks primary ions which are surface sensitive, have controllable parameters such as size, velocity, and charge state, and generate high secondary ion yields. Focus was placed on the use of keV organic cluster projectiles to meet these criteria. Of particular interest to this study were C[18] (chrysene), C[24] (coronene), and C[60] (buckminster-fullerene). Results indicate enhanced secondary ion yields for C[60]. For example, when CsI is bombarded with 30keV C[60], the yields for I- and Csl2- are in excess of 20%. This yield is comparable to values obtained for MeV/amu primary ions. The high secondary ion yields for C60 are attributed to the unique structure of this molecule.