Application of pulse height spectral analysis to double-label counting of 35S− 32P

Abstract

The characteristics of magnitude and shape of the liquid scintillation pulse height spectra of composite samples of 35S + 32P were studied as a function of their 32P: 35S activity ratios and fluor quenching properties. Pulse height spectra were characterized via the measurement of the spectral index of the samples (SIS). A linear relation exists between the value of the dual-label activity ratio plotted on a logarithmic scale against the SIS of the composite sample when the sample and fluor cocktail characteristics are maintained constant. This relationship was used to determine the activity ratios of composite radionuclide samples. The separate activities of 35SS and 32P of composite samples were calculated from the activity ratio and total activity. The method was tested against different mixtures of 35SS and 32P in plant matter. The mean % recovery of the calculated activities for 14 measurements of 32P and 32SS was 99.2%. The methods described are useful particularly with liquid scintillation counters not equipped with an external standard radiation source.