Liquid Scintillation Counting of ¹⁴CO₂ in Aqueous Carbonate Solutions Modification of Harlan's Method

Abstract

Harlan's method for liquid scintillation counting of ¹⁴CO₂ was modified to achieve rapid sample preparation and optimum counting results for ¹⁴CO₂ in aqueous Na₂¹⁴CO_3-NaOH sample solutions, in which total concentration of Na₂CO₃ and NaOH ranged from 0.2N to 1.0N and carbonate fraction in normality varied from 10% to 50%.
The original procedure for sample preparation was not simple enough to prepare many samples. It also had a possibility to increase error owing to use of eye-measured gel powder, Cab-O-Sil. An attempt was made to find an improved procedure.
Suitability of the original scintillator solution for counting of the above sample solutions was open to question, because relation between samples and counting results had not been fully described in Harlan's report. Various scintillator solutions were examined to select the optimum one.
Scintillation mixtures subjected to examination were counted in Packard Tri-Garb liquid scintillation spectrometer Model 3002, the sample chamber being kept at 5°C.
As a result, the following modified procedure was found to be advantageous for rapid sample preparation and accurate counting of our samples. Four handred milliliters of absolute ethanol and 25g of the gel powder are mixed and the resultant slurry is partially diluted with toluene. The mixture, 4g of PPO, and 15mg of POPOP are placed in a 1-liter volumetric flask and again diluted to the mark with toluene. A counting sample is made up by mixing 15ml of the scintillator solution and 0.5ml of a Na₂¹⁴CO_3-NaOH sample solution in the standard glass counting vial. A piece of aluminum foil is attached on top of the vial prior to capping, if protection of the vial cap against contamination is desired. The scintillation mixtures are counted in the counter at a gain setting of 21 with window openings 50-1000 to obtain maximum counting efficiency.
The aluminum foil attached on top of a vial increased counting efficiency by 2%. This effect being included, the mean counting efficiency over scintillation mixtures, which were made up in four combinations of total alkalinity and carbonate fraction levels, was 73.5%. Counting efficiency increased within the range of ±0.6% from the mean as total alkalinity and carbonate fraction increased. Background was about 65cpm.