Sample loss during multiple injections

[Mike_D]

I have a problem, when I inject twice out of the same vial the 2nd injection gives a signal approx 1/20th of the first. I am running HPLC-RAD and uV and the problem appears with both.

Checks I have done: contents of vial has been checked and no separation of the analyte / mobile phase appears to be happening in the vial. The vial has also been radio checked and the concentration of what remains in the vial is normal.

Following the low 2nd injection run, I have run at 95% acetonitrile for half an hour and nothing comes off the column.

I have run a different hot compound and do not have this problem, multiple injections and no significant change in signal).

What could be happening, I have had the problem the other way round (low first then higher) but not seen this before.

[Uwe Neue]

If the sample has been freshly put into the vial, it could be due to sample adsorption in the vial.

[syx]

Could you give me a list about substances (maybe based on its functional groups) those have possibility to be bound by vial wall?

[HW Mueller]

Syx, anything not very soluble in the solvent, especially at concentration too low to precipitate. Also, remember that glass and silica are related. In radioisotope work the low concentrations often give rise to problems especially with cations (sometimes this is due to polymerization, gelling, of oxides rather than interaction with anions on glass).
Mike, with gammas it´s a cinch to find out where the stuff is, sometimes even with betas, for instance if Uwe is right the activity should stay mainly in the vial when you take the solution out. But be careful, it could be in a fine invisible precipitate and be removed with the liquid.