A Unique Model to Study the Attrition Effects of Biofuels on Polymers using Hybrid Liquid Chromatography and Plasma Mass Spectrometry
We have developed a unique method to evaluate the rates of adsorption and desorption by utilizing natural trace chemical species Cr3+/Cr6+ and As3+/As5+ present in biodiesel to serve as tracking agents. Immersing a polypropylene sample in biodiesel and computing the difference in levels of these trace chemical species over time (relative to the control) would indirectly reflect the degree of adsorption and desorption. These particular trace species were selected because they tend to be absent in most polymers so the study could be free of undesirable matrix effects. Obviously, species of major elements such as sodium or magnesium were avoided because they tend to be present as impurities in most materials and could affect the results of the study. The chromium and arsenic species can be well separated by a High Performance Liquid Chromatograph (HPLC). In fact, among the analytical techniques currently available, HPLC is the only technique that can be used to separate these chemical species. However, while attempting to employ HPLC for this purpose there was a barrier to overcome. Some of the chromium and arsenic peaks had very close retention times, which created a serious overlap problem. This difficulty could only be overcome by linking the HPLC to a mass spectrometer. Hence, measurement of the species of interest was undertaken by a hyphenated facility combining high performance liquid chromatography with inductively coupled plasma mass spectrometry (HPLC-ICP-MS). The built in dynamic reaction cell (DRC) within the mass spectrometer added to the analytical capability of the hyphenated equipment by forming useful polyatomic combinations when desired and by eliminating unwanted spectral interferences. The adduct formation of AsO+ (when oxygen was used as the cell gas) helped in measuring arsenic at very low concentrations. This combination of equipment possessed the special ability of isolating all four species [Cr3+/Cr6+ and As3+/As5+] simultaneously in a single run followed by high resolution mass detection.
Keywords - Biofuels, HPLC, ICP-MS, mass spectrometry, metal leaching.