thermal effects

[moonchips]

Dear experts,

I saw the term of "thermal effects" ocassionaly. Can some one please explain in detail what "thermal effects" are?

My understanding is:
Thermal effects are effects generated on the column due to the mixing of organic/inorganic mobile phases. When the column temperature is relatively high, the injection size is fairly large, and the analytical sample diluent is a relatively strong solvent (say, pure IPA etc.), thermal effects take place by affecting the refrative index of the liquid getting into the detector. The symptom of the thermal effects are distorted peak shapes, and ghost peaks etc.

Please correct me if I am wrong.

Thank you very much.
Happy holidays.

[tom jupille]

You are "kind of" right, but conflating several different causes of peak shape problems.

One problem is that the phrase "thermal effects" is not well defined; it can mean different things to different people in different situations. For example:
- drift in retention time during the day as the lab warms up
- baseline noise resulting from ambient temperature fluctuations affecting the refractive index of the mobile phase
- peak shape issues caused by axial and radial temperature gradients within the column when the mobile phase in not adequately pre-heated (I think this is what you were referring to).

Peak shape problems can also be caused by the use of a too-strong diluent or too-large injection, as well as by a partially-plugged inlet frit, a head space at the top of the column, or a trapped air bubble (not to mention secondary interactions, e.g., with silanols).

Bottom line: while there is ovelap, not all "thermal effects" cause peak shape problems, and not all peak shape problems are caused by thermal effects.

[moonchips]

Tom,
Thank you very much for your reply. Could you please explain a bit more about secondary interactions?

Thank you again.

[tom jupille]

*That* is a very large "can of worms", and difficult to do without risking "talking down" to you on the one hand or "baffling" you on the other (depending on your experience level). The following is a one-paragraph, gross oversimiplification of a *very* complex topic:

Retention in reversed-phase chromatography is primarily driven by hydrophobicity; the water molecules in the solvent tend to "push" the analyte molecules into/onto the stationary phase. Once in/on the stationary phase, the analyte molecules can interact more strongly with active silanols on the underlying support (that's a "secondary" interaction). If only a small portion of the analyte molecules stick to the active silanols, that portion will be dragged out to longer retention, resulting in a tail on the usual distribution.

For more thorough (and more accurate) discussions, check out one or more of these books:

Snyder, Kirkland, & Dolan's Introduction to Modern Liquid Chromatography,

Dong's Modern HPLC for Practicing Scientists, or

Neue's HPLC Columns: Theory, Technology, and Practice