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.

Parents
  • There are two possible causes of RT shift.
    The first is column status.
    GS-OxyPLOT columns have the properties of a trap
    If the carrier gas purity is poor, it is more affected.
    It is recommended to maintain the standby oven temperature at 200 degrees after the end of the analysis.
    The second is that the GS-OxyPLOT column is a kind of PLOT.
    A sudden change in flow rate in the column will change density of the packing material, which causes a change in RT.
    In fact, if you backflush the PLOT columns, the symptoms get worse.

    CP-Lowox column is recommended over GS-OxyPLOT.

Reply
  • There are two possible causes of RT shift.
    The first is column status.
    GS-OxyPLOT columns have the properties of a trap
    If the carrier gas purity is poor, it is more affected.
    It is recommended to maintain the standby oven temperature at 200 degrees after the end of the analysis.
    The second is that the GS-OxyPLOT column is a kind of PLOT.
    A sudden change in flow rate in the column will change density of the packing material, which causes a change in RT.
    In fact, if you backflush the PLOT columns, the symptoms get worse.

    CP-Lowox column is recommended over GS-OxyPLOT.

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