A Question About Running PAHs by Reversed Phase

[adam]

Hey

I wanted to ask about running Polynuclear Aromatic Hydrocarbons (or for that matter, highly non-polar analytes in general) by Reversed Phase HPLC. I understand that these analytes are separated quite well in the reversed phase mode. My question actually has to do with the issue of transferring the analytes from the injection system to the head of the column. Specifically, I'm wondering if it is a problem that these analytes are so poorly water soluble, but the initial mobile phase is usually mostly water. And, as a result of this, do the analytes sometime "crash out" in the connecting tubing.

For anyone that has commonly done this kind of analysis, I'm wondering if you've ever found it necessary to minimize the length of tubing connecting the column and the injection system, to avoid the issue mentioned above.

Thanks for any feedback.

[dblux_]

... Specifically, I'm wondering if it is a problem that these analytes are so poorly water soluble, but the initial mobile phase is usually mostly water. And, as a result of this, do the analytes sometime "crash out" in the connecting tubing.

For anyone that has commonly done this kind of analysis, I'm wondering if you've ever found it necessary to minimize the length of tubing connecting the column and the injection system, to avoid the issue mentioned above.

Thanks for any feedback.

If the initial concentration of ACN is in the range 40 to 60% then it is not mostly water. It's a good idea to have samples dissolved in mobile phase of initial concentration and this completely eliminate the possibility of PAH percipitation in the system.
BTW - shortening connection tubing is not a solution at all, if a stuff has to percipitate it will, no matter in tubing or in column (if tubing is to short).

[adam]

Hi All

I thought this was a pretty interesting question (if I don't say so myself).

So I thought I would give it another try. Anyone have any experience or comments to offer on the question below.

Thanks
Adam

[James_Ball]

Our analyses start with 50/50 water/acetonitrile as the mobile phase at injection and the samples and standards are in 100% Acetonitrile and we don't have any problems with the PAH analytes.

You may have problems if you have much heavier hydrocarbons in the sample matrix as those may not be as soluble with the water content, but usually those will flush out once you reach the high end of your gradient. The only problem we have ever really had is if we fail to remove all of the Methylene Chloride extraction solvent when switching over to Acetonitrile, that causes very bad peak shapes and peak splitting.

Keeping the dead volume between the injector and column, and actually any part of the system after the sample is introduced will always help improve the results of the chromatography.

[DJ]

There are thousands of publications on PAH analysis using HPLC dating back to the Nixon administration. This isn't real cutting edge stuff here.

[adam]

Not cutting edge enough for you? Well, I didn't ask if it's possible to analyze PAHs by reversed phase. I asked a very specific question, about a fine point that is not often discussed.

If you were not interested to offer a comment, that's fine.

But, honestly, why would you jump on the thread just to make an obnoxious remark? Merry Christmass to you too.

[DJ]

Not cutting edge enough for you? Well, I didn't ask if it's possible to analyze PAHs by reversed phase. I asked a very specific question, about a fine point that is not often discussed.

If you were not interested to offer a comment, that's fine.

But, honestly, why would you jump on the thread just to make an obnoxious remark? Merry Christmass to you too.

I wanted to ask about running Polynuclear Aromatic Hydrocarbons (or for that matter, highly non-polar analytes in general) by Reversed Phase HPLC. I understand that these analytes are separated quite well in the reversed phase mode. My question actually has to do with the issue of transferring the analytes from the injection system to the head of the column. Specifically, I'm wondering if it is a problem that these analytes are so poorly water soluble, but the initial mobile phase is usually mostly water. And, as a result of this, do the analytes sometime "crash out" in the connecting tubing.

For anyone that has commonly done this kind of analysis, I'm wondering if you've ever found it necessary to minimize the length of tubing connecting the column and the injection system, to avoid the issue mentioned above.

It is always desirable to minimize extra column volume. This includes all connections from the injection port down to the detector.


PAHs are probably dissolved in 100% methanol or MeCN (check http://www.epa.gov/). Injecting an analyte in a stronger solvent (than what is at the head of the column) can be tricky. Are you injecting into a column equilibrated with 100% aqueous mobile phase>?