Cyclosiloxane peaks in blanks on GC/MS

[gmattz]

This may be a stupid question I just wanted a clear explaination of what occurs when a polydimethylsiloxane column (HP-1, HP-5, DB-1 etc.) begins to break down or bleed. Typically I see evenly spaced peaks of cyclosiloxanes such as D6, D7, D8, etc eluting stepwise with equal resolution to neighboring peaks. I have always been told that this is due to column bleeding, although I have always seen this as raising baseline. It appears as if sections of polydimethyl siloxane stationary phase of different lengths are being stripped from the stationary phase and appearing as peaks. Using an MS detector each peak appears as a cyclo instead of a linear siloxane with the typical M-15 loss caused by EI. I assume others see this?[/img]

[Ron]

Column bleed by definition is a continuous process that causes a baseline rise, not discrete peaks. There are other sources of siloxanes, such as injection port septa and vial septa that can produce peaks. These are generally smaller polymers than the column stationary phase, and have different characteristic ions that column stationary phases. 207 is a very common mass for column bleed, while for vial septa masses 73, 281, and 355 are common.

A common source of the siloxanes from septa is from autosampler wash vials. The siloxanes are washed from the needle after every injection and build up in the solvent in the wash vials.