2.4.2 - Quench Correction Curves (Channel Ratio Method)

Introduction

In order to obtain the quench correction curve through the channel ratio method, the standard sample (e.g. ³H) is measured in two different energy channels according to figure 3. From the ratio of the count rates of the high energetic and the whole energy channel, the channel ratio (R_N(1) / R_N(2) or vice versa) is calculated and is used as quenching parameter.

Always the same cocktail, same type and size of vial and total sample volume for calibration and samples to be analyzed are always to be used for correct quench correction. For further theory see chapter 1.2.

Materials and Equipment

• CHCl₃ or Nitromethane

• Standard of ³H (¹⁴C)

Procedure

1. The standard sample is measured in the selected channels.

2. The measurements are repeated after addition of increasing amounts of a quenching agent (e.g. Chloroform CHCl₃, Nitromethane CH₃NO₂) (50 μL, 50 μL, 100 μL, 100 μL, 200 μL).

3. The unknown sample is measured with the same channel setting and its channel ratio is calculated.

Measuring times should be chosen in order to achieve a statistical deviation of 2σ < 0.5 % (about 160,000 counts)

Evaluation

From the count rates in the whole energy channel 2 and the known activity of the standard, the counting efficiency is calculated and plotted against the channel ratio (fig. 12).

The corresponding efficiency of the unknown sample is interpolated from the resulting correction curve.

Analogueously, the instrument specific quench parameter QP (e.g. H#, tSIE) may be recorded and plotted simultaneously versus counting efficiency.