[TheBalrog]

I'm relatively new to the world of chemistry (am a biochemist) and need to use Thin Layer Chromatography. I've been using silica plates in a glass container with a Whatman 542 curtain and a mix of chloroform:methanol:acetic acid (100:25:2) that seems to give good separation of the chemical I'm interested in. Recently, however, the plates I run have shown dark streaks at the top towards the front when I visualize the plates using UV light, and even on dry runs without spotting any sample. When I visualize the plates with UV prior to running them in the solvent mix, i don't see any streak. Could it be a contaminant in the plates? I'm wondering also if the problem may have been caused by colleagues dipping plastic pipette tips into chloroform and consequent leaching of chemical contaminants from the plastic into the solvents.

Please help me identify the source of these dark smudges. Thanks...

[Edit] I've since tried the same solvent mix, but pipetting with clean glass pipettes from new containers and the dark smudges are still there. Is Whatman 542 an acceptable filter paper to use for a curtain or could it contain contaminants? Alternatively, the plates themselves may contain the contaminant? The plates were clean before, but I did leave them out on the bench for a couple of days, so maybe they picked up something from the air. Aarrghhhh!