Concentration-dependent MSD response factor-heavy compounds

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

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Hello,
We are using a HAPSITE ER GCMS (dimethyl polysiloxane column) system to analyze TO-14 analytes (chloroethene through hexachlorobutadiene). The internal standard is Bromopentafluorobenzene, but we calibrate using a commercial cylinder containing the TO-14 compounds (just got a brand new cylinder and new IS canister). During the last few rounds of biannual calibrations, we've begun to notice that the MSD response factor (RF) is concentration-dependent for the higher boiling point (B.P.) analytes only (i.e., the RF is concentration-independent--as it should be, for the lower B.P. compounds). For the high B.P. compounds, the RF is higher at high concentrations and lower at low concentrations of the standard injections.

We're trying to determine potential reasons for this problem with the higher B.P. compounds. Could the quadrupole not be conducting the heavier mass fragments to the detector efficiently? Could the electron ionizer be failing to properly ionize the heavier compounds? Could the column be spent (and would this be a symptom of a column that is past its prime?). The column (and the whole system in general) is pretty old and we do run the oven temp all the way up to the highest column operating temperature (200 deg C) for 90 seconds at the end of each run which isn't how I'd do things, but I just started here 3 months ago.

If anyone has ideas about how we might diagnose this issue and identify the failing component in this process, I'd be very grateful! About our injector: the HAPSITE units have a heated inlet with a triple-adsorbent bed concentrator (which we just tried changing out & which didn't solve the issue). We inject our standards using gas-tight glass syringes.

Thanks so much for your input!
Woof, you're eluting HCBD at 200? That must be a long runtime. The heavy bois usually look like trash in my experience in an air matrix (To-15). Are you straight spiking into a vacc'ed cannister or using a dynamic diluter to hit your concentrations? I've seen the 1,2,4 TCB, nap, and HCBD recoveries fluctuate as much as 500% depending on the method you use.
The HCBD is eluting around 155 degC-- I don't know why we kick it all the way up to 200. The standard injections are made by injecting different volumes of standard (taken directly from a sampling port/volume attached to the concentrated cylinder via gastight syringe) into the GCMS inlet under N2 flow, but I'm not sure this should influence the behavior of the heavier cmpds since all are pre-concentrated on a multi-stage sorbent bed (within the GCMS) and subsequently desorbed.

I calculated the % standard deviation in RFs for each compound (across all concentrations used in the cal. curve) as a function of elution order (a proxy for boiling point), and this shows a pretty good correlation:

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If there has been some high boiling contamination in the system it could be adsorbing some of the heavier analytes, and this can appear as higher responses at higher concentrations. If your curve is say going from 10ng of a target to 400ng of a target and the contaminate saturates at around 50ng, then the lowest standards will be reduced quite a bit, but the higher standards will be their mass minus up to 50ng, so the farther you go from 50ng, the lower the percentage loss which will result in a quadratic shaped calibration curve.

An old column can cause this if over time it has picked up some contaminate that is sticking around on the column, or if something is hanging around in the inlet. If you are using an Agilent system, check the split vent line leaving the inlet, which should be a 1/8" copper line, those can become contaminated and cause a problem like this. Also a good idea to keep the split vent carbon trap changed at least annually. If not an Agilent, it could still have a similar setup, just be sure to keep it all clean.
The past is there to guide us into the future, not to dwell in.
Thanks James, 10-100's of ng is the amount of mass we're injecting. We have no way to control the inlet (it's a HAPSITE GCMS), but your hypothesis about the column makes sense, so we're going to look into baking it out at a higher temp. and see if that solves our problem. Cheers,--Tiffany
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