The detrimental effects of a too-strong injection solvent that exist in all forms of LC are magnified in case of polymers. This is due to two reasons.
- The effect of solvent composition on polymer retention is extremely large (“on-off mechanism”);
- In a strong eluent high-molecular-weight analytes show exclusion behaviour, i.e. the migrate through the column at a velocity higher than that of the mobile phase.
The effects are illustrated in the figure below, for the case of a large solvent peak (large injection volume). The dashed pink line indicates the solvent concentration above which exclusion occurs and below which retention is very high.
Almost all analyte molecules are seen to be focussed at the front of the solvent zone. If the run ahead, they are strongly retained; if they fall behind they are accelerated due to exclusion. The small number of analyte molecules that are in the tailing edge of the solvent peak upon injection will be stuck on the column, awaiting a gradient front to sweep them through the column. The chromatogram will show a very large and sharp breakthrough peak in comparison with a small peak at the correct elution composition of the analyte.
To avoid breakthrough, a weak solvent should be used. Even turbid solutions may be injected. “Sandwich” injections, in which the sample plug is preceded and followed by a plug of very weak eluent may alleviate the problem in case of relatively small injection volumes. A stronger remedy is to add a flow of weak solvent between the injector and the column. Good mixing of the stream of strong solvent through the injector and the weak make-up flow is then essential.
For more information see Section 4.3.2.
