Chromatography Forum

We run a mix of 36 PAH and OPAH on an Agilent 5977e with HP5 MS UI
There are varying detection limits for each compound, but the 16 PAH have always been detectable to 2 ng/ml or lower.
The anthracene peak is typically detectable down to 2 ng/ml but the area counts for ANT are at least 10x lower than phenanthrene.

But recently the ANT is now almost undetectable, we can see it at around 2 ppm, which is not acceptable. With a new column, a cleaned source, and new liner/septum/syringe there has been no improvement.
I don't think ANT should be much more difficult to detect than PHE, so what would cause such a large difference between the two?
ANT is the very small peak at 26 minutes.

Normally the two peaks should be almost the same height when at the same concentration. I have seen low Anthracene peaks before and normally when I make a new standard they return to nearly the same height. For some reason Anthracene will break down or fall out of solution much faster than Phenanthrene.

Have you tried a new standard yet to see if that is the problem? It can fall out of the original stock so usually I have to purchase a new stock to correct the problem.

This standard was made using a new stock ampule.
Maybe there's a problem with it, its the 47930-U mix from Sigma.
I think I'll make a dilution using our solid anthracene to confirm.

The field samples I have run aren't showing any ANT peak either though.

Due to its shape, anthracene is more reactive than phenanthrene and less stable in your inlet.
Look up Clar's rule for details.

Steve Reimer wrote:
Due to its shape, anthracene is more reactive than phenanthrene and less stable in your inlet.
Look up Clar's rule for details.

And yet we have no problem detecting Anthraquinone, which should be even more reactive.

Anthraquinone is the oxidized version of anthracene. It was going to be my suggestion to look for it's presence as an indicator of oxidation. I've had bad ampules of PAHs where anthracene was 25% low. Anthraquinone was there after searching around for it, even though it wasn't supposed to be there.

We always run a confirmation from a second vendor on standards to make sure the primary used for calibration is accurate. It is extra cost but gives a good idea if everything is as it should be. For us it is required by the EPA methods but also a good practice even if it isn't. It helps to identify if any active compounds have been compromised during storage.