Chromatography Forum

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

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

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

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.

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.

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.

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.