Carry over in 6890

Discussions about GC and other "gas phase" separation techniques.

6 posts Page 1 of 1

I am seeing very persistent carry over from my terpenes. No matter how many blanks I run or how many components we clean and replace, the carry over returns and stays. I have seen the carry over both using the Tekmar Headspace autosampler as well as just the GC. Here is my configuration and all of the maintenance performed to attempt to remove source of carry over:

Tekmar HT3 Headspace Autosampler
1 mL loop
Agilent 6890 GC with FID

h2 flow: 3.0 mL/min
split injection 25:1
GC oven hold at the end of ramp 5 min @ 310C

Our team performed thorough cleaning and maintenance this week to try to prevent the system from consistently experiencing buildup of the terpenes. I figured there must be a cold spot on the headspace side so I took my IR thermometer and looked for cold spots. I insulated the transfer line and ran at 300C. Upon confirmation using the thermometer the transfer line was staying sufficiently hot, as were all other components.

I flushed the transfer line with methanol, flushed the 8-port valve system on the Tekmar with hexanes, acetone, and methanol. I replaced the septum, gold seal, inlet liner, and column. I removed the split vent carbon filter and bypassed it ( we didn't have a new one in house). I cleaned the inlet thoroughly using methanol on a cotton swab.

I performed a manual injection directly into the injection port of the GC of a terpene standard and then ran a blank and saw no carry over. Then I ran a 25 ppm terpene standard on the Tekmar and the peaks were tailing and broad. I ran a second 25 ppm terpene standard on the Tekmar and the chromatogram appeared the same. I ran an instrument blank (no vial) on the Tekmar and saw carry over of the entire chromatogram. I also ran two blanks on the GC only and saw the same carry over.

I replaced the column and baked at 300C for one hour. I ran a blank on the GC only and saw the carry over on the brand new column. I ran a second blank on the GC only and see remnants of carry over. It is to note that the carry over is at the same retention time as the standards and if I change the method for a faster ramp or higher flow rate the carry over peaks do elute sooner.

Does anyone know of any solutions to attempt? I am not sure what else can be done at this point.


* If substances lighter than terpenes (or other class substances) also cause carry-over?
This maybe caused by your HS tubing, which became sorbent of terpenes.
* If rising the temperature of valve instead of transfer line changes the game?
This maybe caused by sample loop inside coating which also became sorbent (e.g. rust)
Wow! 25 ppm? I use SPME for my terpene/terpenoid analysis and 25 ppm b-myrcene would be way too much for me to get linear chromatography. It would be terrible. SPME is a concentrating technique so perhaps straight HS analysis can deal with that much material.

Could it be that you're just feeding too much sample to your system for a headspace analysis? Has your system run well under these conditions in the past?

Are you doing something like what's described here: ... gement(110).pdf
Any chance someone installed a smaller volume liner and you're getting some injection volume blowing back into the split vent line or elsewhere and slowly migrating back into the inlet on each injection.

The only other thing I can suggest is the split vent trap but you said you bypassed that.
We had a terpene carryover issue - actually it was more of a contamination issue that I initially thought was carryover due to similarity with some of my samples. Did not effect direct injections significantly but was heavily present in headspace or SPME type analysis. Turned everything inside out trying to find it. Turns out the night janitor was mopping the floor with Pine sol once a week.
There is a lot of info needed to provide an answer.
IS the Headspace directly plumbed to the weldment of the GC, or is the the transfer line connected to the inlet vial a needle or tubing through the septa?

Also need all the parameters from the HS.

Some HS method parameters are counter intuitive, and lead to excessive tailing and/or carryover if not set correctly.

As for system related carryover on "GC only"... is the HS unit disconnected from the GC module for this test?
Was the septa on the inly replaced in between these tests?

If you share all flows, temps, times, I can evaluate the method parameters.
Scott Allison
HawkEye Analytical, LLC
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