The Chromatography Forum

of Delaware Valley

From the Solvophobic Theory to Modelling HPLC

December 10, 2025: 12 pm ET/6 pm CET/9 am PT

In the last 50 years, HPLC method development changed a great deal from a trial&error-based process to a well-planned systematic procedure using software tools. Method development times are drastically shorter and time to market is well decreased.

This development started in 1975 with a surprise: As long ion exchange chromatography was unable to separate vanillylmandelic acid (VMA) and homovanillic acid (HVA), which differ only in a single Oxygen atom, the separation was unexpectedly successful on a Reversed Phase C18-column with a huge difference of the retention btw. both molecules. This could only be explained with differences in the solvatation of the sample molecules by the aqueous eluent. Other compounds with similar differences in their structures have shown a similar behaviour as well, as we will describe it in this paper. This was the start of the of the Solvophobic Theory [1-3], which is the Theory of Reversed-Phase-HPLC, developed by Csaba Horváth with Wayne Melander and Imre Molnár at Yale. The question was: Which energetic contribution is dominating retention in RPC? The research exhibited surprising results, showing, that the aqueous eluent plays a strong role in the retention process of RP-gradient elution. Later the cooperation with Lloyd R. Snyder, and many interested scientists led to the birth of several modeling theories [4], many of which are used today worldwide in pharmaceutical analysis. An important progress was the elaboration of the fundamentals of gradient elution [5] in RPC, which changed the landscape of analytical routines to the performance we apply today, going  alongside with modeling retention in multifactorial separation processes, leading to a large number of new, important and safe drugs.
 

Modelling HPLC separations requires a Design of Experiments with a number of input runs. Out of those a model is calculated with a potential to evaluate the Design Space (DS) in which a decent separation is possible. As the Design Space includes typically a “baseline separation” of all present components, the robustness of working inside the DS is depending on the tolerance limits of the input parameters, and they are responsible for method robustness and smooth routine operation. The results are HPLC-Methods with a high degree of flexibility, which includes possible changes for parameters settings, if necessary.

Conclusion: The main advantage of modelling is: The capability to change conditions, and if we remain inside of the DS, there is no need for a revalidation.  This allows the fast production of new drugs with a great degree of flexibility for the desperate millions of sick patients with a limited time to live.

REFERENCES

[1] Solvophobic Interactions in Liquid Chromatography with Nonpolar Stationary Phases – Solvophobic Interactions Part I, Csaba Horváth, Wayne Melander, Imre Molnár, J. of Chromatogr., 125, 129-156 (1976).

[2] Liquid Chromatography of Ionogenic Substances with Nonpolar Stationary Phases – Solvophobic Interactions Part II, Csaba Horváth, Wayne Melander, Imre Molnár, Anal.Chem., 49, 1, 142-154 (1977).

[3] Enhancement of Retention by Ion-Pair Formation in Liquid Chromatography with Nonpolar Stationary Phases – Solvophobic Interactions Part III, Csaba Horváth, Wayne Melander, Imre Molnár, Petra Molnár, Anal. Chem., 49, 14, 2295-2305 (1977).

[4] “Computer Assisted Method Development for High-Performance Liquid Chromatography”, edited by J.L.Glajch and L.R.Snyder, J.Chromatogr. Vol. 485, Elsevier, 1990.

[5] High-Performance Gradient Elution, The practical application of the linear solvent strength model, John W. Dolan and Lloyd R. Snyder, John Wiley and Sons, 2007, Hoboken, New Jersey, USA.