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window ofB, it is possible to separately count the two isotopes. A pulse height

analyser set with a threshold and window for a particular isotope is known as a

channel. Modern counters operate with a so-calledmultichannel analyser that

records the entire energy spectrum simultaneously. This greatly facilitates multi-

isotope counting and in particular allows the effect of quenching on dual-label

counting to be assessed adequately.

Dual-label counting has proved to be useful in many aspects of molecular biology

(e.g. nucleic acid hybridisation and transcription), metabolism (e.g. steroid synthesis)

and drug development.

Determination of counting efficiency

When we detect radioactivity we usually need to know the actual rate of decay (the

d.p.m.) (see Section 14.2.6). This is because we want to record and publish data that are

independent of the types of equipment used to do the detection. To calculate d.p.m. we

need to know the efficiency of counting. What’s more, in liquid scintillation counting

we have to contend with quenching. Samples may vary in nature so the levels of

quenching may vary from one to the next. Therefore the efficiency of counting needs

to be determined for every sample.

One way to do this is to use an internal standard (called aspike). The sample is

counted (and gives a reading of, say,Ac.p.m.), removed from the counter and a small

amount of standard material of known disintegrations per minute (Bd.p.m.) is added.

The sample is then recounted (Cc.p.m.) and the counting efficiency of the sample

calculated:

counting efficiency¼½ 100 ðCAÞ=BŠ% ð 14 : 4 Þ

Carefully carried out, it is the most accurate way of correcting for quenching. On the

other hand, it is tedious since the process has to be done for every sample. Therefore

automated methods have been devised; however, these all use the internal standard as

the basis for establishing the parameters.

Isotope S

Isotope T

Channel 1

Proportion of pulses at a given energyEnergy of pulses

XYAChannel 2 B

Fig. 14.7Diagram to illustrate the principle of counting dual-labelled samples.

567 14.3 Detection and measurement of radioactivity