SFC Olefins problems

[madchemist83]

Hi. I'm new to this forum and i hope I'll find an answer to my question. I have SFC unit that i use to quantify olefins in gasoline. Lately I see a lot of peak tailing just for olefins. Tried changing temperature, flow, pressure .. nothing helps. Strange thing: it only does that on synthetic samples ( i.e. mix of 3-4 different hydrocarbons) and not on regular gasoline. Please help.

[carls]

the peak shape for alkanes is OK?

What columns are you using?

[madchemist83]

Alkanes and aromatics are fine. Also olefins peak only trailing in performance mix but not in actual gasoline sample. Silver column is from Selerity

[carls]

Have you prepared and/or purchased a fresh standard? What is the water content of the major components in your standard? This behavior points toward a difference in the matrices/solvent strength.

[madchemist83]

Performance mix is in-house made about a year ago. I'll try to make a new one. All components are hplc grade and shouldn't have much water in them

[madchemist83]

made another mix. same results.

[carls]

I may have barking up the wrong tree here. Its been many years since I ran this analysis and I was (mistakenly) thinking the silver column was before the silica column but it is the opposite.

However, even non-polar polar solvent like heptane can have as much as 100ppm water in them.

Is it possible the additive(s) in the gasoline samples are improving peak shapes? Perhaps its time to try another silver column.

Is the olefin mass load similar for both the standard and the samples (trailing peak(s) means tailing or fronting)?

[madchemist83]

Those chemicals do have water in levels 0.005% so that's 50 pppm. Will that affect chromatogram ? Standards olefin peak under same conditions will usually have about 0.5% less olefins then olefin peak of actual samples.

[carls]

Water is a strongly adsorbed on silica in normsl phase/ SFC so yes the water can affect the chromatography . I would expect the retention times of the alkanes/aromatics to be more affected. However the water could make it to the silver column and affect retention/chromatography there as well

[madchemist83]

Can you recommend any way of cleaning silver column?

[madchemist83]

Also how would I flush column with cleaning chemicals ? The only way I think is to inject it like a sample but it only injects small amounts

[carls]

I would contact selerity for their recommendation for flushing/cleaning-up the silver column. The conditions differ depending on the the type of silver loaded column used (silica or silica + SCX).