Hello all!

I would like to start a nice, civilized discourse on the material presented in several papers by Dr. D. V. McCalley.

Dr. McCalley has published many excellent papers which have focused on the problem of peak tailing of basic compounds in RP-HPLC analysis. He has examined factors such as the nature of the organic modifier and mobile phase pH and their effects on peak shape. Overall, I have found the papers to be very informative and well-done.

However, upon closer examination, I have noticed that, in many cases, the k values for many compounds which he evaluates are less than 1.0 (J. Chrom. A 738 p.169-179, J. Chrom. A 844, p. 23-28). To some extent, this fact troubles me. I have the following questions:

1. Is it valid to evaluate column performance for peaks with k values of less than 1?

2. Do the results he presents have practical value, since few chromatographers would develop methods where the target analyte had such a low k value?

Please post your opinions on the subject. If any readers know Dr. McCalley, I would be very grateful if he would respond to this query.

Thank you.

Thanks very much for your interest in my papers. I'm glad somebody is reading them. If you are interested in the analysis of a single compound, it is not a good idea to report performance data for peaks with k < 1.0. This is because extra column effects can contribute significantly to peak shape. However, in my work, I am using a system with very low dead volume- a 1ul flow cell detector and 2 ul injections in conjunction with a relatively large diameter column (4.6mm). This ensures that any contributions of extra column effects are small.The aim of my work is to evaluate columns for a wide range of different bases, and to draw general conclusions on the relative performance of columns-for instance at different pH values and with different modifiers. I am not really intending people interested in the analysis of say codeine alone to look up in the tables and decide which column yields the highest efficiency at a k value of 0.5. There are other problems to consider if you increase k for early eluting peaks. The first is that it will not be possible to elute more hydrophobic compounds in a reasonable time with the same mobile phase. Changing the modifier concentration can change the state of the ODS ligands or even the pH of the mobile phase (even if that of the aqueous part is maintained constant). Furthermore, lowering the concentration of the organic modifier to increase k increases the danger of producing ligand collapse-or at least anomalous effects. Finally, there is greater danger of overloading the column at higher k values, producing other complicating effects. It is really a matter for debate which way you go. Certainly, if you are only interested in a narrow group of compounds eg amphetamines, I would choose conditions very similar to those appropriate for their analysis. I would certainly not recommend that anyone without specialist instrumentation (particularly with regard to the detector) whould report column performance data for peaks with k<1.0

David McCalley PhD