Sample/Standard Carryover

Question

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 (&deg;C): 135 Oven Temperature (&deg;C): 120 Shake: Off Transfer Line Temperature (&deg;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

megaflorch · Answer

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.

crystalnipp · Answer

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!!

mkpbphd · Answer

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.

mkpbphd · Answer

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.

Sample/Standard Carryover

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