In our lab, we use the liquid linear alkylbenzene (LAB) cocktail mixed with 2g/L 2,5 diphe-nyloxazole (PPO) as the organic scintillator for radiation detection. The new LAB cocktails are stored in the transparent glass bottles provided by the company. Although they are exposed to the daylight lamp in the lab, they are always very clear(see picture 1, the sample was put in a small glass vial). However, when we put some liquids into a transfer system, they turned to be very yellow after about two weeks (see picture 2). In the system, the LAB liquid was first stored in a large Acrylic vessel (acrylic is proved to be compatible with the LAB) and then went through the stainless steel tubes and valves (from Swagelok company), finally went into a small acrylic container or a glass jar for preventing spilling. There is no heating during the experiment, just some pressure changes created by a pump to make the LAB transfer from the large vessel to the small container. The LAB liquid may also contact with the rubber from O-ring for vacuum sealing, the Teflon tape, the plastic(PTFE) gas tube, copper lid, etc. So in our case, what may cause the yellowing for the LAB?

clear LAB

yellowing LAB

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    $\begingroup$ The role of LAB in your cocktail is partly just a solvent. Usually, the cocktail also contains at least one primary scintillator (e.g. PPO = 2,5-diphenyloxazole) and maybe a secondary scintillator. The yellowing may be caused by degradation of these scintillators. Could you add the name or composition of your cocktail? In addition to heat and UV, cocktails can be sensitive to various chemicals, including atmospheric oxygen. In my experience, all cocktails turn yellow sooner or later, except when stored in flame-sealed glass vials in the dark. $\endgroup$ – Loong Oct 16 '15 at 0:43
  • $\begingroup$ Thanks Loong ! We have 2g/L PPO in the LAB. I may try to let one sample unsealed and expose to the air. $\endgroup$ – jwhoo Oct 16 '15 at 22:29
  • $\begingroup$ After sample testing, we know that it is the Buna O-ring causing the yellowing. The mechanism may be the triple bond Nitrile affects the cocktail. $\endgroup$ – jwhoo Oct 19 '15 at 20:58

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