Mystery Peak

[Foobs]

I have a DB-5 column on an FID GC. I'm analyzing a mixture of hydrocarbons of various lengths and types (syncrude).

I've found a peak whose identity I've been a bit puzzled by (and thus far GC-MS hasn't helped). The 1-olefin and linear paraffin peak are very predictable (olefin then paraffin, about .3 minutes apart) for each carbon number. The mystery peak starts out (at C7) between the olefin and paraffin, then merges with the paraffin, then comes out the late side of the paraffin peak (C15 or so).

Any thoughts on what this mystery peak might be? It isn't an alcohol (we've run enough standards of those to rule that out). It isn't an internal olefin (the reaction behavior isn't right). The best guess I've come to is an aldehyde. Does the description I'm giving of its retention time fit that?

I realize I'm asking a question that requires very specific knowledge so I may have to figure this thing out myself. Still, if anyone can offer me any insight I would very much appreciate it.

Thank You

[AICMM]

I would suggest a visit to the NIST webbook. They have retention time index for a significant number of compounds and columns. Type in some suspected compounds and see if this fits the bill for what you are proposing.

Best regards.

[freemab]

The way you state this, "The mystery peak starts out (at C7) between the olefin and paraffin, then merges with the paraffin, then comes out the late side of the paraffin peak (C15 or so)." makes it sound like you have a broad peak overlapping several narrow ones. Is that what you meant?

If so, are you working isothermally or with a programmed temperature? In either case, do you consistently use the same cycle time (and program)?

What I have in mind is a carry-over peak from some previous injection. Is this feasible?