Sample/Standard Carryover

I am running GC 6890 (FID detector) alongside G1888 headspace.

I had noticed 4 of my later eluting peaks got increasingly worse in terms of shape and reproducibility. After running blanks (no injection and also blank vials), it seemed like the cause of this turns out to be carryover. The more blank vials I run after my highest standard, the smaller the carryover peaks become. However, it is not time efficient for me to have to run 3 blanks in between standards in order to get decent peak shapes.

 

 

GC Paramaters

OVEN

Initial: 60C (hold 0.1min) ramp to 300C (12.5C/min), hold 6 min

Carrier gas: Helium 

 

I am using Rxi-624Sil MS column.

 

I have attached chromatograms to illustrate my problem. 

I am relatively new to GC so please do not hesitate to suggest something obvious.

Thanks!

 

Edit to add more instrument parameters:

 

Headspace

G1888 Headspace
Configuration
Headspace Device: Agilent G1888 Headspace Sampler(g1888)
Communications Mode: 10.1.1.102
Serial Number: IT00739016
Vial Size (mL): 20
Handshake Mode: Wait if GC Not Ready
Oven Stabilization Time (min): 1.00
Pressure Units:: psi
Carrier Connection: Front Inlet
Vial EPC: None
Loop Size (mL): 1
Method
GC Cycle Time (min): 40.00
Inject Time (min): 1.00
Loop Equilibration Time (min): 0.05
Loop Fill Time (min): 0.50
Loop Temperature (°C): 135
Oven Temperature (°C): 120
Shake: Off
Transfer Line Temperature (°C): 140
Vial Equilibration Time (min): 30.00
Vial Pressurization Time (min): 0.08
Vial Pressure (psi): 0.00
Carrier Gas Pressure (psi): 0.00

 

GC

OVEN

Initial: 60C (hold 0.1min) ramp to 300C (12.5C/min), hold 6 min

Carrier gas: Helium 

 

FID

Temp: 320C 

Hydrogen flow: 30mL/min

Air flow: 450mL/min

Helim: 45mL/min

attachments.zip
  • Hello, thank you for such a fast response!

     

    My split ratio is currently 20:1, so I will try increasing it. 

     

    I will also look into cleaning the inlet and injection port, as I don't believe that has been done in a long time.

     

    I am using headspace GC, so there is no solvent to wash with (that I am aware of), but please correct me if I'm wrong!

  • Hi nicoled,

     

    Two things contribute to carry over, one is sample preparation and handling, which I dont think is your problem. If you are running a split inlet on your GC, a high concentration standard can leave carry over if your split ratio is too low. It will cause excess vaporized injection to remain in cooler parts of the inlet causing carry over. Also, a pain staking cleaning of your inlet and injection port is probably in order. Another thing is to check how many solvent washes your auto injector performs per sample on the injector syringe. More washes might help. Also, if your solvent happens to be water, then a more intense cleaning regimen for your inlet may be needed, like as in daily.

  • Hi Nicoled:  I would try increasing the temperature of your loop and transfer line.  Seems like your highest boilers might be condensing in the loop/transfer line and then coming out in subsequent runs.  Try going up 10 C on each and see if that fixes it.   I'm not sure how high you can go on either of those.   

    I would also check your GC inlet just to make sure that that hasn't been set really low by accident.  Although the headspace is delivering vapor, the inlet temp still matters.  You wouldn't want your sample condensing (or partially condensing) there either.   

    Good luck!!

  • All of the above suggestions are excellent and should help. If, after trying those things, you find you still have some issue, another thing you might try is adding a post-run to your GC method. While these symptoms sound more like you are getting carryover somewhere prior to the column, adding a post-run to help ensure your column is clear could also help. A 1-2 minute post-run with increased column flow and 5-10 C above your final temp (if your column can support it) might help.

  • Thank you! 

    I think I will try this. Currently I have tried the above solutions and then also insert 2 blank vials between samples to ensure all carryover is eliminated. I think a post-run would be a more time-efficient way to do the same thing, so I will give it a shot!

  • Thank you! Increasing the loop and transfer line temperatures has definitely improved the amount of carryover I am experiencing! 

  • I hope it helps!

     

    I know nothing about headspace samplers, so this might be totally normal, but I just noticed the last two lines of your headspace parameters show "Vial Pressure (psi): 0.00" and "Carrier Gas Pressure (psi): 0.00". Is it normal not to have any pressure there? Like I said, I know nothing about headspace analysis, but whenever I have issues with GC, a flow/pressure that got turned off accidentally is a common cause.

  • Although it does say that those parameters have a setpoint of 0, they actually have pressures of 16 and 20 psi respectively! I am also not sure why they appear to be zero, but monitoring the system shows the 'actual' pressures at the appropriate setpoints! It's very curious, but rest assured there is pressure! This is a good observation, I will manually enter the setpoints (since the pressures are reaching that anyway, there should be no harm). 

     

    Thank you!  I am trying the post-run flow and temperature today so fingers crossed!

  • I will mark this as assumed answered due to its age. Feel free to update us.

    Regards

    James

  • This question has been marked as assumed answered.

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