PPO, 2,5-Diphenyloxazole

The emission from the radioisotopes is often insufficient to penetrate the window of a Geiger-Muller counter. Therefore, the compound whose activity is to be measured is often mixed in solution with a scintillator, called a fluor, which transforms / rays into luminescence proportional to the number of /3 particles emitted. The sample is dissolved in a solvent (toluene, xylene or dioxane, the latter being used for water-soluble compounds) that acts as a relay to transfer the energy to the scintillator. The scintillation mixture contains PPO (2,5-diphenyloxazole), which emits in the UV and POPOP, which emits in the visible and is well adapted to detection with photomultiplier tubes (Fig. 17.2). The quantum yield of emission will depend on the energy of the emitted particles. 

Most liquid scintillation counters that count fi particles utilize PPO (2,5-diphenyloxazole) as a fluor. Excited PPO emits a light flash in the blue region of the visible spectrum. Recognizing this fact, would 1 nCi of yellow-colored DNP-14C-alanine count more or less effi-... 

Dissolve 4.0 g PPO (2,5-diphenyloxazole) and either 0.050 g POPOP [p-bis(5-phenyloxazol-2-yl)benzene] or 0.080 g bis-MSB [p-bis(o-methylstyryl)benzene] in 1 liter of a solution composed of 667 ml toluene and 333 ml Triton X-100. Make certain that the contents are completely dissolved before the solution is used. This solution should be stored in a brown bottle in a cool dark space. 

The samples were absorbed in 9 ml of Carbo-Sorb and collected in low potassium glass scintillation vials. To this were added 13 ml of homogeneous blend toluene based scintillator (Permafluor V ). Permafluor V contains 91% PPO (2,5 diphenyloxazole) as the primary scintillator and 9% POPOP (l,4-bis-2(5-phenyloxazolyl)-benzene) as the secondary scintillator. ... 

The scintillator solution is prepared by dissolving naphthalene (200 grams) PPO (2,5-diphenyloxazole) (14.0 grams) and POPOD (1,4 bis-2-(5-phenyloxazolyl)benzene) (0.06 gram) in scintillation grade dioxane, diluting to 2 litres and adding 200 ml of methanol. 

Radioactivity Analysis. Samples of urine, feces, and tissues were combusted to COo and analyzed for radioactivity (5). By using this method the recovery of radioactivity from samples spiked with C was 95 dt 5%. To determine the radioactivity expired as CO2, 5-ml aliquots of the solution used to trap the CO2 were added to 15 ml of a scintillation counting solution containing 4 grams 2,5-diphenyloxazole (PPO) and 0.1 grams l,4-bis-2(5-phenyloxazolyl)-benzene (POPOP) per liter of 1 1 toluene 2-methoxyethanol. Samples were counted for radioactivity in a Nuclear Chicago Mark II liquid scintillation counter. Counting eflSciency was corrected by the internal standard technique. 

The resulting solution consisted of 5 ml. of water and 20 ml. of dioxane containing 7 grams 2,5-diphenyloxazole (PPO), 1.5 grams bis-(o-methylstyryl) benzene (bis-MSB), and 120 grams naphthalene/liter dioxane. The Y-value of this solution was approximately 1 nCi/liter. 

The light, hv2, from F2 is of longer wavelength than hvx from F and is more efficiently detected by a PMT. Two widely used primary and secondary fluors are 2,5-diphenyloxazole (PPO) with an emission maximum of 380 nm and l,4-bis-2-(5-phenyloxazolyl)benzene (POPOP) with an emission maximum of 420 nm. 

The plates were impregnated with the scintillating medium by dipping them into 20% 2,5,Diphenyloxazole (PPO) in toluene, dried and exposed to X-ray film (Kodak, X-Omat R XR2) for several days at -70°. Fluorgraphs were then developed as described (8). 

Excited solvent molecules transfer their energy to other solvent molecules, and eventually to a primary fluor molecule (Fx), which then emits a photon (hiq) and decays to its ground state (Equations 3-7 and 3-8). A widely used fluor is 2,5-diphenyloxazole (PPO). 

Several approaches have been used to circumvent these problems. First, scintillation cocktails have been developed that will accept more water and associated hydrophilic compounds. Bray s solution (4 g PPO, 0.2 g POPOP, 60 g naphthalene, 20 ml ethylene glycol, 100 ml methanol, and dioxane up to 1 liter), Kinard solution (xylene, p-dioxane, ethanol (5 5 3) containing 0.5% PPO, 0.005% a-NPO, and 6% naphthalene), and ethanol systems (e.g., 3 parts ethanol, 4 parts 0.8% PPO, 0.01% POPOP in toluene) are notable examples. In these examples, PPO denotes 2,5-diphenyloxazole, POPOP denotes l,4-bis-2-(5-phenyloxazolyl)-benzene, and a-NPO denotes 2-(l-naphthyl)-5-phenyloxazole. (POPOP and a-NPO are secondary fluors and can usually be omitted with modern scintillation counters.)... 

Figure 10.12. Typical primary and secondary scintillators, (a) Primary 2,5-diphenyloxazole (PPO). (b) Secondary 1,4-bis-2-(5-phenyloxazolyl)-benrene...

Liquid scintillation counting is usually carried out using organic solvents such as toluene which are readily excited by ft particles. The number of solvent molecules, and hence fluorophores exdted depends on the energy of the ft particles. It is in principle possible to determine the phosphorescence of the excited solvent molecules directly however, this phosphorescence is at short wavelength (Imix < 300 nm) and is therefore not readily accessible technically. The fluorophore most often used is 2,5-diphenyloxazole (PPO) with an emission maximum of 380 nm, either alone or in combination with a secondary fluorophore such as l,4-bis-(5-phenyloxazole)-benzene (POPOP) whose fluorescence maximum... 

Radioisotope Procedures. Radioactive volatile acids separated by Wiseman-Irvin chromatography were collected in 1 ml of 0.5N KOH. The aqueous fraction containing the acid was transferred to a scintillation vial and evaporated to dryness in a vacuum oven at 20 psi and 50 °C. The residue was redissolved in 0.1 ml distilled H2O before adding 4 ml of absolute ethanol and 15 ml of scintillation fluid 2,5-diphenyloxazole (PPO) and 0.01% 1,4-bis[2-(5-phenyloxazolyl)]benzene (POPOP) in toluene. Samples were counted in a liquid scintillation counter (Nuclear Chicago Corp.). 

The fixed wet gel is impregnated with 2,5-diphenyloxazole (PPO). The P-particles excite these molecules to emit photons in the blue region, so blue-sensitive x-ray film is used. Development at low temperatures (-70 to -80 C) increases detection by lO-to-12 fold for H and by 9-fold for C and S. 

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