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Name: Gisselle Y.
Status: Student
Age: 16
Location: N/A
Country: N/A
Date: May 2003


Question:
Why is methanol a stronger solvent in chromatography than water?



Replies:
Methanol is somewhat less polar than water so many organic compounds have a significantly higher solubility in methanol than in water. Thus the methanol elutes them more quickly than does water.

Vince Calder


It depends on what the stationary phase is. Chromatography works by the partitioning of substances between a mobile phase and a stationary phase. The more time something spends in the stationary phase instead of the mobile phase, the slower it will move in the chromatography.

When you speak of a "stronger solvent", you probably mean a mobile phase that tends to make things move faster. The speed of an analyte in a chromatographic system depends on the chemical nature of the analyte itself, the mobile phase, and the stationary phase.

On a highly hydrophobic ("reverse phase") stationary phase such as C-18, most organic compounds will move faster with a methanol mobile phase than with a water mobile phase. This is because the analytes will more strongly partition into the stationary phase from water than from methanol, which is not as polar as water. More hydrophilic analytes on a more polar stationary phase (like silica gel), however, may move faster in water than in methanol.

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois



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