2.4.3 - Dual Labeling (Exclusion Method)

Introduction

The liquid scintillation counting technique is universal as it detects all effects, which lead to the emission of light (fig. 13). However, the analysis of β-spectra among each other causes problems because of their continuous energy distribution.

The spectra of dual labeled β-emitters (e.g. ³H and ¹⁴C) in one sample overlap in the low energy region (fig. 14).

A discrimination measurement may be attained by setting a high energy channel 2 which measures only pulses from the higher energetic isotope. In a second low energy channel 1, pulses from both nuclides are registered of which the interferences from the high energetic isotope may be calculated (screening or exclusion method).

According to figure 14 only ¹⁴C-pulses are measured in channel 2, while in channel 1 the high energetic pulses of ¹⁴C are discriminated partly in order to obtain a pulse ratio in favor to ³H.

For the calculation of the activities, the derived efficiency values for ³H and ¹⁴C in both counting channels have to be taken into consideration.

In the procedure described below, ³H stands as example for any low energy β-emitter and ¹⁴C for any higher one.

Materials and Equipment

- Standard solution of ¹⁴C and ³H

Procedure

1. The measuring channels are selected according to figure 14.

2. The counting efficiencies of ¹⁴C in channel 1 and 2 and ³H in channel 1 are determined by measuring the corresponding standard samples.

3. The unknown sample is measured in both channels. While the measurement of the unknown sample in channel 2 registers only ¹⁴C pulses as R_X(2), channel 1 contains both the ³H and the ¹⁴C counts as R_X(1).

Evaluation

The unknown activities are calculated according to:

where R_3H (1) = R_X (1) – R_14C (1) = R_X (1) - A_{14C *} ε_14C (1)

The method can be used for all β-emitters, unless their energies differ sufficiently (factor 2). If three radionuclides are present, the method can be applied accordingly with 3 different channel settings and evaluation starting with the highest energetic β-emitter. If quench correction has to be applied, correction curves for each measuring efficiency (3 for 2 radionuclides, 9 for 3!) have to be recorded. The procedure gets comprehensively more difficult if more radionuclides are present, and a computerized approach gets inevitable (see 2.3.15.). The application of TDCR may help considerably.

The procedure applied for the determination of dual labeled β-emitters may be derived as well for the differentiation of α- and β-emitters in one sample.