Writing on the vials can decrease light transmission by several percent through optical quenching. Color quenching occurs when photons emitted by the fluors are attenuated as they pass through a colored sample. This can be especially significant if the sample is in the yellow to red spectrum. In chemical quenching, various chemical species absorb beta particle energy before it can be converted to photons. Newer scintillation counters can automatically correct for color and chemical quenching, but not for ink on the side of a vial.
Plastic vials are permeable to scintillation fluids.
They are economical and great for one-time counting, however over time the sample volume will decrease. Scintillation cocktail diffusing through plastic vials can cause increased chemical exposure to the worker. It also can cause the vial to swell, which potentially could cause it to become hung up in the transport mechanism. So, use plastic vials for one-time counts and use glass vials for counting and recounting over long periods of time. Make sure the caps are securely tightened.