HPLC Products

Wash Solution a stronger eluent than the mobile phase

In a fixed volume autosampler, at least three times the loop volume of sample is injected, flushing the loop and making sure it contains exactly one loop volume of sample. Fixed volume autosamplers therefore use more sample than variable volume models and are less flexible, but they give much more reproducible injections and this problem does not apply to fixed volume autosamplers.

A variable volume autosampler uses a much larger loop, and meters in a small amount of sample with the syringe. The rest of the loop is filled with wash solution. So if the wash solution is a strong eluent, the same problem occurs as does if the sample solvent is a strong eluent, only much more so, because there is much more wash solution present than sample solvent! The problem is that the peaks become very broad, because the strong eluent disperses the sample into the column before it is itself washed away. The solution is that the wash solution must be no stronger than the eluent, and preferably the eluent itself only without any buffer. If running a gradient, the wash solution should be the eluent at the point of injection.

Other wash solution issues:

The wash solution must not be allowed to run out. Its obvious, but especially if there is a wash cycle after each injection, quite a large volume of wash solvent will be required.
The wash solvent must be degassed. Otherwise air can be sucked up, which may be in the syringe for a few injections before finally clearing itself. Hence the injection volume will be suspect.
Waste bottle for the wash solution must have enough capacity for all the wash solution used.