Large-Volume Liquid Scintillation Counting of Carbon-14

Efforts have been undertaken to further improve the relatively simple technique of low-level liquid scintillation counting of 14C. Two different approaches have been made. By synthesizing more benzene for 14C measurement than usual (with up to 19.5g of carbon) an experimental detection limit of about 0.1 percent modern has been achieved (97.5% confidence level, 1000 min). Absorption of CO2 with up to 5.3g of carbon in 160ml of an absorbent-scintillation solution and counting in a special measuring chamber resulted in an experimental detection limit of about 1 percent modern, with the sample preparation taking only 1 hour. The detection limits achieved by the two techniques correspond to 14C ages of about 55,000 and 35,000 years BP, respectively.

U.S. Geological Survey, Denver, Colorado Radiocarbon Dates I

The U.S. Geological Survey, at the Denver Federal Center, Lakewood, Colorado assays C14 concentration of water samples using liquid scintillation techniques. The laboratory synthesizes benzene from precipitated barium carbonate using techniques described by Noakes, Kim, and Akers (1967). Three ml of the synthesized benzene is pipetted into a tared, low potassium-40 glass vial. The sample is weighed and 1ml of a scintillation solution is added to the vial. The scintillation solution used is a mixture of 10g PPO and 0.025g dimethyl-POPOP scintillators in 250ml toluene. Calculations of dates are made with the radiocarbon half-life of 5568 years; plus or minus numbers quoted herein are the standard error for the counting of radioactive disintegrations.

A Superior Counting Solution for Water-Soluble Tritiated Compounds

We describe a simple liquid-scintillation solution for use with water-soluble tritiated compounds that is superior to other such systems. Its advantages include: (1) a refrigerated-type liquid scintillation spectrometer is not needed because the solution has extremely good thermal stability from 22° to 75°C; (2) counting efficiencies are high from pH 1-12, therefore eliminating neutralization; (3) a sample is stable for at least one week, with a 7% variation in counting after three weeks (a comparison with counting in Bray's and Triton X-100 solutions is included); (4) there is no detectable phosphorescence or chemiluminescence; (5) toluene need not be analytical grade; (6) for more than 10,000 samples, efficiency has deviated less than 5%; (7) there is no odor (except for the toluene) as with naphthalene "cocktails" (Bray’s solution); (8) cost of the system compares favorably with that for other counting systems; (9) plastic vials can be used (40% less costly than glass) and are easily cleaned because of the detergent qualities of the cocktail. The only obvious disadvantage of the system is that high salt concentrations (>5 g of NaCl per 100 ml) cause a marked decrease in efficiency of aqueous radioactivity, and somewhat erratic counting for the toluene internal standard. However, additional solubilizer will sometimes correct this problem.