A “Retention Map” is a graph of the changes in retention time for an analyte with the Y axis plotted against changes % B solvent composition of the mobile phase against the X Axis “Time”.

In the graphics shown here, the “B solvent” is Acetonitrile and the “A solvent” of the mobile phase is buffered Water (pH 2.18, 6.80 or 8.00).

The acid pH is 0.05% v/v H₃PO₄ in Distilled Water (standard 85% conc. H3PO4) the others mobile phases are 20mM NH₄COOH buffered with NH₄OH in Distilled Water to give the appropriate desired pH.

The column dimension and flow rate is shown, so simple change of axis scale will accommodate different flows and column size, i.e. if 3ml / min, divide time by 3 for 1 ml / min, if 75 x 4.6 id and want 150 x 4.6 times time by 2, or combine both as required.

Why not show conventional Log K plots?

With currently standard reverse phase HPLC columns there will be an exponential increase of retention as the % B reduces or % aqueous buffer increases. Conversion to a log plot yields a simple straight line, which is easy to interpret...

The plots for Type-C™ phases can be, V or U shaped (RP,NP) thus interpretation as log plots is less simple and can be confusing.

Therefore presentation of the fundamental data is not shown in log plots, for ease and simplicity of interpretation.

Several publications have shown that there is a casual relationship of Log P and retention time at given % organic in the mobile phase for irregular as well as for type A & B reverse phase columns. That is, as Log P increases, the compound is becoming less polar and retains longer on a conventional RP columns for a given % of organic solvent.

For neutral compounds this is usually independent of eluent pH.

For acidic compounds maximum retention occurs when the compound’s ionization is suppressed, that is, typically two pH units below the compounds pKa value.

For basic compounds maximum retention occurs when the compound’s ionization is suppressed, that is, typically two pH units above the compounds pKb value. If you use target compounds of neutral, acid & base character of different Log P values, then plot retention maps at different pH’s of eluents on different phases, then if you know from the nature of your compound its Log P, you can extrapolate your compounds retention behavior...?
YES !!!

So let us see if theory holds up in practice?

Click here to compare Retention Maps with UDC-Cholesterol™ Columns

What is the significance of Octanol-Water Partition Coefficients (LogP) to HPLC method development? A free web site, below, will calculate the Log P ( which is regarded as a definition of the polarity ie relative solvency in octanol and water ) of most known chemicals from their structure / molecular makeup. http://syrres.com/esc/datalog.htm