Chromatography Forum

I am measuring VOCs in environmental samples and am trying to understand why a single compound (amoung 80 others) appears to elute at either of two very different times: 3.7 and 5.4 minutes. This is a much greater elution shift than seen for any other compound. The compound has been identified as hexamethyl-cyclotrisiloxane by matching the m/z spectrum with the NIST database, and for both peaks is an excellent (and nearly identical) match (match qualities > 90%, probability > 90%, InLib 396).

The method was GC-MS (Agilent 6890N GC, 5973NMSD) with a HP-5MS capillary column using helium as the carrier gas. The source of the compound is believed to be contamination from the sampling tubes, and abundances were high. However, abundances were also high with some other compounds. I also did not see 'split peaks' in the dataset. There are two different solutes in the cold trap and its been suggested to me that it might have something to do with the compound having a different affinity to each of these. Does this sound like a reasonable explanation (and why is only this compound affected)?

Im new to chromatography, so any ideas (especially in simple language ) would be greatly appreciated.

I think you have in fact 2 different compounds which are eluted from your column (siloxanes are present in stationary phase) so you got similar spectras.

Two different compounds with very similar spectra is the most likely answer. However you can get compound- specific retention shifts under some quite restricted circumstances. You say that you have two solutes in the cold trap - do you mean two solvents ? what are they and what role do they play in the analysis ?. How are you transferring samples to the GC - usually with VOCs it is adsorb thermal desorb, but then you would not have any solutes / solvents. What are the other compounds that do not shift around (very much) ? Do you ever see a peak at both of the retention times ?, or is it only ever at one or the other ?

Peter

Silanes can come from sampling tubes and other places, such as the septum on the instrument. (Crumbs of septa will fall into the inlet as multiple injections are made through a septum, partiularly if many injections are made through a single septum. These crumbs bleed silanes into the inlet.) If you are using an older HP-5 MS column, silanes can come from the column. And which particular silanes are seen in a given injection may depend on the history of the column, history of the inlet and chemistry of the current sample. History: how many hours since the last injection, as silanes can collect in a column between injections. And differences in temperature profiles from injection to injection.

Also be careful about residual air in the carrier gas. Use the high grade gas and an indicating trap at the back of the instrument. Any air present can result in degredation of the column - and generation of silanes.

You're probably looking at column bleed.

I vote for the column or trap ( if used) which I assume you use do to the cold trap statement. I set my cold trap to 20 durring purge and desorb and ramp it to 220 durring my bake cycle.
I see siloxanes every once in a while on my system.