GC DRO Surrogate problems...still.

[crosemeyer]

Hi all. I've asked a bit about this before, but now think I may have an answer, and am looking for some advice. I run Diesel Range Organics on an HP6890 GC/FID, and constantly have surrogate recovery issues. A lot of "ND" samples have very high surrogates. Well, I am pretty convinces that this is because our reporting limit is artificially high, so the samples actually are giving a baseline contribution, especially those with some heavier hydrocarbons. Here are a method blank, and a sample below the RL that had a ~200% surrogate recovery:





We calibrate the instrument down to 10ppm, but our reporting limit is 250ppm. So what I was thinking of doing was extracting from a smaller amount of sample. Thoughts?

edit: I swear both of those images are 400x300

[8jw8]

what is your surrogate? and what is your sample prep?

for our hydrocarbons we used a surrogate that eluted before everything else

[crosemeyer]

what is your surrogate? and what is your sample prep?

for our hydrocarbons we used a surrogate that eluted before everything else

Oops, forgot to mention.. The surrogate is Octocasone. CA defines DRO as C10-C28, so it's a good mark for integration. Issue is, anything that has heavier stuff than #2 Diesel in it interferes.

Extract from 1L water with 2x40mL MeCl2, concentrate to ~2mL.

[Bigbear]

The thing that jumps out at me is, your integration parameters could use tweaking. If your surrogate is the big peak on the left, there is quite a lot of integrated area below the baseline.

[crosemeyer]

The thing that jumps out at me is, your integration parameters could use tweaking. If your surrogate is the big peak on the left, there is quite a lot of integrated area below the baseline.

Actually, the surrogate is the large peak near the right, around the 8min mark, where I have manually integrated.

[Yama001]

We often get artifact peaks from prep that can coelute with octacosane, though if that was a routine issue you should have occasional problems with the blank.

We use ortho-terphenyl - elutes close to C19, I would have guessed it would be a problem eluting with the DRO range, but it seems to work well on a DB-5 type phase. It looks like we have an elution pattern similar to yours.

I would be careful reducing the sample size - it is routine to keep the reporting limit high to avoid false positives to to baseline rise from a deteriorating column, prep artifacts, etc. Some labs like to shave the baseline or increase the reporting limit when they need to - I was never comfortable with that approach.