8270 Calibration and Internal Standard Problems

[sw340496]

Hi,
I'm having some trouble getting linear response for 8270. Almost the whole back end of the run has be be calibrated using a quadratic curve fit which results in the CCV and LCS not to pass even though the areas count/calculated concentrations for the targets are hitting in the 90% recovery range. Also, my internal standard revoveries are fluctuating a lot in my CCV and LCS standards. For example the 1st 12hr check will be at 100% ITSD recovery and 12hrs later the ITSD recovery will be 300%. To rule out prep I re-preped the CCV and reshot and still got an increased response for the ITSD compounds. However, the response for the targets remains the same as before so it lis not an increased response across the whole method. I'm running a HP6990/7973 in pulsed splitless injection mode at constant pressure with the pressure pulse at 30psi and backing down to 9.5psi with 1.2 mL/min through the Rxi5-SilMS column with a Restek sky single gooseneck liner. The standards that I'm running for my calibration cure are 2.5,5,10,20,30,50,70,100ppm standards with ITSD at 40ppm.

Thanks for any insight.
Steve

[Steve Reimer]

So are the later PAHs fluctuating along with the internal standards? Or is it just the labeled ones in the internal standard mix?

[Jesse Robinson]

If the area counts keep climbing and you're already using a quadratic fit, it seems to me you are potentially over loading the detector. You could try running the GC in split mode. That could help with the linearity. I would start low at a 5:1 split, then if sensitivity isn't an issue move on to a 10:1 split. By doing this you will also protect the column from dirty samples. I also highly suggest using a single taper liner with a plug of glass wool in the center. This will allow for complete vaporization and mixing of the sample as well as preventing "trash" from getting to the column.

[James_Ball]

As Jessie said, try that 5:1 split injection with the glass wool. Also is your injection volume 1 or 2ul? If 2ul back it down to 1ul because it is easy to overload the injection port with 2ul of Methylene Chloride.

The higher weight PAH compounds are more difficult to maintain linearity with. Agilent sells an over sized drawout plate for the source that is supposed to help with linearity of heavier compounds. Having your Electron Multiplier voltage too high will also cause linearity problems, use the lowest voltage you can use that gives you enough sensitivity to see your low standard, drop the threshold down to 10 if necessary.

[Stunt]

Steve, I've had issues with linearity using the sky liners. Have you tried a non-coated single taper (gooseneck) or drilled uniliner or focus liner? Always use glass wool for PAH injections as Jesse and James have iterated. Also, you may wish to change your column to a DB-5. You're running a 1,4-bis(dimethylsiloxy)phenylene dimethyl polysiloxane phase, which should be ok, but an equivilent column, the DB-5 (5%-Phenyl)-methylpolysiloxane could work, and is better, in my opinion, than the DB-5 MS for Indeno and [GHI]. If you have to run splitless, you also might want to lengthen your runtime. If you're attempting to push a 100ppm std through the detector too fast, you could be overloading it, especially with the heavier molecular weight compounds, which is where you seem to be having your trouble.