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.

  • 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?

  • 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.

  • Hi serdarche,

    When the GC is idle, the system should be kept in backflush mode with the HP-5 pre-column at a negative flow and the OxyPlot/Lowox at the normal analytical flow.  Also, as gil-ho suggests, the oven should be hot, 200 deg C is a good suggestion.  This will keep any moisture or other trace contaminants in the carrier gas from accumulating on the polar column.  You can create "Stand-by" method with these condition and load this method whenever the GC is not being used.  This method should also be used to re-condition the system after instrument maintenance.  You should also consider placing moisture traps on your carrier gas supply.  Small amounts of water will trap on the PLOT, column and have an adverse effect on retention time and peak shape.

     

    I would not worry about a "sudden change in flow" in the polar PLOT column.  The flow to this column is supplied by the PCM and should never change.  Also, the PLOT column is never backflushed so the flow direction is always from the CFT to the detector.  The only column that is backflushed if the HP-5.

     

    It is not surprising that the alcohols have different retention times depending on the sample matrix.  When injecting a gasoline sample, the heavier hydrocarbon matrix will condense on the HP-5 column and act as a "solvent".  This will temporarily act as a thicker stationary phase resulting in longer retention times.  This effect is not seen with the propylene gas injection because the sample matrix (propylene) cannot condense on the HP-5 column.  If this is the cause, the alcohols should have a longer retention time is gasoline and a shorter retention time in propylene.

     

    lancenelson offers some good advice regarding leaks.  You should check for leaks, especially where the columns and 1/8" stainless steel line connect to the Capillary Flow Technology (CFT) Splitter.  Also check for leaks at the inlet column nut, GSV, and carrier gas supplies in the back of the 7890.  Only use an electronic leak detector.  Do not use a soap solution such as Snoop.

  • Dear All, thank you very much for your valuable recommendations. Below you can see the chromatogram of gas std and comparison of two consecutive analysis. As you some peaks have bad repeatability for retention time and others for area. Actually I can not figure out three things: 

     

    1) According to the certificate the gas std is composed of 8 components (ethers: MTBE- 9.93 mkmol/mol, DIPE- 9.87 mkmol/mol, DME- 10.1 mkmol/mol, TAME- 9.73 mkmol/mol and alcohols: methanol-4.87 mkmol/mol, n-propanol- 5 mkmol/mol, isopropanol- 4.95 mkmol/mol, n-butanol- 4.91 mkmol/mol in Nitrogen). However in the chromatogram there are more peaks than 8 and I can not figure out which is which.

     

    2) Why there are no repeatability in retention times and peak areas.

     

    3) How can I check if there is a leak in the CFT connections if I do not have an electronic leak detector?

  • Hello serdarche,

    1) I think the extra peaks could be due to impurities in the standard.  Trace oxygenates is not an easy standard to make, as they can be a little bit reactive.  (One reason why having inert tubing for this analysis is recommended.)  My guess is that DME is the peak around 10 min, the ethers are 12-14 min, methanol around 15 min, and propanol and butanol are the last two large peaks at 18 and 20 min.  The two propanols track closely on this column and sometimes co-elute.  Do you have any other standards, perhaps with different amounts of the components of interest?  That may help with identification.  Our application note for LoxOx column may help with this as well: https://www.agilent.com/cs/library/applications/A01613.pdf 

     

    2) For repeatability testing, it's best to have more than two runs.  I recommend 5-7 minimum.  If the instrument has been sitting for a while, often the first run or two can look poor.  This is especially the case with the LowOx column if it is not kept at an elevated temperature while idle (as mentioned above).

     

    3) Some will leak check with a water/isopropanol mixture and watch for bubbles; I haven't used this mixture myself.  Avoid a soap solution, as it can leave residue behind and can get into the system if a leak is present.  An electronic leak detector for your lab would be a good investment.  Agilent's G3388B works well and lasts for several years.  (As I look on Agilent's website, the leak detector costs less than a replacement LowOx column.)

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