Analysis of trace oxygenates in propylene (or gasoline)

Hi dear community members!

I need your valuable thoughts and suggestions on below issue.

In the lab, an analyzer for analysis of trace oxygenates in propylene (or gasoline) is installed (SP1 7890-0178 or G3445 Option 618). It is a 7890B system, which consists of ALS, GSV, HP-5, GS-OxyPLOT columns, FID and a backflush system with PCM. The two columns are connected in series, first being the HP-5 column (see the attached picture). During the GC run, the heavy hydrocarbons are back flushed from the HP-5 column by the PCM (i.e the flow in the HP-5 column is set to a negative value) while the light hydrocarbons pass through both columns and elute first. Then ethers and alcohols elute from the GS-OxyPLOT column.

 

Method: Oven program: 50 C for 5 min then to 260 C with 10 C/min. Splitless injection. HP-5 column flow: initial 5.4 ml/min, hold time 3.5 min, then decreased to -5 ml/min at a rate of 90 ml/min per min. GS-OxyPLOT flow is constant at 6 ml/min.

 

We have the problem with the repeatability of retention times of some alcohols. What could be the reason? Is it because of the backflushing or the CFT connections?

 

Additionally the chromatograms of the same alcohols injected via ALS (liquid standards) and via GSV (alcohols in nitrogen) are completely different.

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  • Hello serdarche,

    Are you seeing the varying retention times on all components or just some?  If you run the same sample/standard several times, do you see varying responses for the same components?  If so, a leak could be present.

     

    If you do several runs as part of a sequence, do the retention times get steadier/more constant?  The OxyPLOT / LowOx columns seem to perform better when they are kept above 100C when the system is idle.

     

    When you say chromatograms look very different when comparing GSV to ALS injections, can you explain more about what you are seeing?  Are you able to share chromatograms showing this?

Reply
  • Hello serdarche,

    Are you seeing the varying retention times on all components or just some?  If you run the same sample/standard several times, do you see varying responses for the same components?  If so, a leak could be present.

     

    If you do several runs as part of a sequence, do the retention times get steadier/more constant?  The OxyPLOT / LowOx columns seem to perform better when they are kept above 100C when the system is idle.

     

    When you say chromatograms look very different when comparing GSV to ALS injections, can you explain more about what you are seeing?  Are you able to share chromatograms showing this?

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