A comparison of instrument performance for six liquid scintillation counters

The counting characteristics of six liquid scintillation counters are compared. Each instrument was tested to determine the figure of merit ($\text{E}{}^2\text{B}$), the change in percentage counting efficiency and external standard ratio over sample volumes from 1 to 20 ml, the reproducibility of chemical quench curves over sample volumes from 5 to 20 ml, and the effects of increasing sample activity on the actual and calculated counting efficiency. Tests were performed using both ³H and ¹⁴C. The results show that differences in counting efficiency are more important than maximum sensitivity ($\text{E}{}^2\text{B}$). All instruments demonstrated the same response to changes in sample volume. The external standard ratio varied with sample volume in all instruments except one. Quench curves were essentially volume independent, with two exceptions. This study demonstrates that periodic performance tests must be conducted to assure that these instruments are operating in an efficient and reproducible manner.