8260 high B.P. non linearity and R.T. locking question.

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

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I did a source cleaning and installed new filaments and put on a new 8260 cal. The 8260 calibration had <~7% RSD for every compound except 1,2,4- and 1,3,5- trichlorobenzene, and napthalene. These are distinctly non linear and 40 ppb reads out at about 20 ppb in RSD mode. I tried changing the trap; no effect. I changed the liner, gold seal, and trimmed column; no effect.

I am thinking I have active sites somewhere in the system. So, I am left with possibly the P&T inlet transfer line or Tekmar P&T valve being dirty? I suppose its possibly the Archon transfer line but seems less likely.

Locked R.T.....
I have enabled the RT locking now for a couple of calibrations and use toluene-d8 for the locking compound. I still get some variability in RT especially in the early eluting peaks. Is this normal or should I be looking at the switching solenoids in the Tekmar 3000?
Today I set the Tekmar 3000 temps to 50C. Then put the 3000 in desorb and cut power. Then flushed the transfer line from the GC inlet to the top of the Tekmar 3000 trap with 40 mL P&T MeOH then 40mL DIW, then 10 minutes of dry nitrogen. Buttoned it all up and am trying a run.
I just assume that the late eluters are going to be quadratic to some extent and trying to make them fit a linear calibration isn't realistic.
Of course, your sample sets may not include the creosote and PERC that I regularly see. Sometimes literally.
Does the Tekmar 3000 use an empty tube as the moisture trap like the Encon does?

I found that if you bake the moisture traps at 250C like the analytical traps this problem becomes worse. I bake our Encon moisture traps at 180C and see it happen less. Also changing the traps often seemed to reduce the effect for a time, but the lower temperature helped the most. It is almost like the higher temperature is creating active sites and the higher the total analyte loading the better those last analytes respond, eg, greater adsorption at lower loading.

But those analytes are very rare to get average response. I do it when calibrating from 5 to 200 ppb but any wider range it is impossible.
The past is there to guide us into the future, not to dwell in.
Huh, I have been fortunate then because I've had very good fits for those compounds in the past. Which is why I posted this because it was anomalous behavior. I have had problems with Bromoform for example due to active sites.

Its true I do not see creosote but I do see PERC and TCE. Usually the heavy compounds I see are old gasoline residue and old or fresh DRO. But I run the DRO samples at a higher dilution to avoid killing my K Vocarb3000 Trap.

Yes the Tekmar 3000 uses a coiled empty tube. I have been baking it at 310C with the valve body, transfer line, and trap bottom all at 150C. I desorb at 250C and bake at 260C. Purge line is 110C and Purge temperature is 70C.
Purge time 11 minutes; dry purge 1 minute; desorb 1 minute; bake 8 minutes. On Desorb the moisture trap temperature is 40C. Inlet is at 150C and split at 40:1 and He carrier gas is 1mL/min. System is a fastscan 5973inert / 6890N GC-MS. I run the 8260 VOC in RT Locked SIM/Scan. Then I break out subsets of analytes with other methods that are calibrated off of the full 8260 files.
LALman wrote:
Today I set the Tekmar 3000 temps to 50C. Then put the 3000 in desorb and cut power. Then flushed the transfer line from the GC inlet to the top of the Tekmar 3000 trap with 40 mL P&T MeOH then 40mL DIW, then 10 minutes of dry nitrogen. Buttoned it all up and am trying a run.


This seemed to increase my low end recoveries of napthalene. So I think my next calibration will come out better.
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