Fluorobenzene best IS for TVH[GRO]?

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

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I have been using Fluorobenzene as the internal standard for GRO on my Agilent 5973/6890 GC-MS. I have an "H" in Compound Type and "FLAT" in the A2 parameter. Its been working very well and I am passing ERA performance tests.

However I recently got in a soil sample from a diesel spill. I started out weighing a minimal aliquot but quickly had to do a methanol extraction because DRO was so high. Ran M, MD, and MS that way and spike recovery was terrible.
So, on to the MeOH extraction... 10g in 10mL of P&T MeOH and running 50uL in the 5mL that is purged for a 100X dilution factor on the original sample.
All the BTEX and a few 8260 hits but about 10,000 ppb of GRO mostly due to peaks in the C9-C10 range. I ran M, MD, MD, M-dup, MS, MSD, and three GRO spikes. The very first M and GRO spike came in at ~1000 ppm for the M and spike recovery was great. But my MD and MS put on the next day (because reasons) were only ~600 ppm GRO as did another MD and M-dup. Recovery of GRO from MS and MSD was pretty good because MS gives about 2800 ppb of apparent GRO. But the GRO spikes had less than 70% recovery. All the chromatograms agreed out to about Ethylbenzene but beyond there quite variable. All the 8260 compounds were very close in every analysis. It was just the GRO peaks in between that varied.

So, it sppears that either Fluorobenzene is a poor IS for the heavy end of GRO; or that purge and trap split inlet gets variable at that end of the chromatogram. But like I said, the 8260 analytes and spikes were all solidly in agreement.

Any thoughts about how I can improve reproduceability at the heavy end of the GRO spectrum? Purging 5mL at 70C, valve, and transfer lines at 150C, inlet is at 200C, split ratio is 40:1.
Are you seeing any carryover of substituted naphthalenes in subsequent runs? I will see loss of heavier hydrocarbons and more variability in targets when I get contamination in the transfer line / concentrator plumbing.
I am indeed seeing substituted napthalene and higher straight chain (e.g. undecane) carryover beyond the range of GRO (so not detected as GRO) and napthalene itself still below my MDL (3 ppb).

So, yes and its persisting at a very low level. I guess this is a strong hint to me to clean my Archon to Tekmar 3000 purge transfer line.
LALman wrote:
I am indeed seeing substituted napthalene and higher straight chain (e.g. undecane) carryover beyond the range of GRO (so not detected as GRO) and napthalene itself still below my MDL (3 ppb).

So, yes and its persisting at a very low level. I guess this is a strong hint to me to clean my Archon to Tekmar 3000 purge transfer line.


You may also want to make a method for it that simply holds the final oven temperature for an extra 5 minutes to help clean out the high boilers so they don't carryover into the next sample. No real need to update method validations as it is the same as manually baking the oven between samples.
The past is there to guide us into the future, not to dwell in.
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