Selected ion flow tube studies of several siloxanes

People are using increasing amounts of siloxanes that ultimately end up in landfills and then in landfill gas and biogas digesters. Their presence poses difficulties for industries seeking to utilize the energy content of landfill and biogas, as the combustion process oxidises silicon to silicon dioxide that in turn damages engine parts. Rapid, efficient and accurate methods are needed to quantify their presence.

Selected ion flow tube mass spectrometry (SIFTMS) is an emerging real-time technique that has found application for monitoring trace volatiles in air. Samples containing the trace volatiles are simply drawn into the flow tube and convected in a stream of helium. Chemical ionization reactions from mass-selected reagent ions with the volatiles ensue. To quantify the volatiles in the sample, the ion chemistry of the reagent ion with each volatile must be known.

Rate coefficients and product ion branching ratios were found for the compounds dodecamethylpentasiloxane, decamethylcyclopentasiloxane, decamethyltetrasiloxane, octamethylcyclotetrasiloxane, triethylsilanol, tetramethylsilane and hexamethyldisilazane.

The ion-molecule reactions of the seven silicon-containing compounds examined here were fast, occurring at or near the collision rate, thus allowing for detection at low levels. The very simple reaction chemistry found of proton transfer, electron transfer and methyl loss will enable easy quantitation of the siloxanes in landfill gas and biogas using the SIFTMS technique.