TABLE 1—Effect of Paper Density 
Variation on Self-absorption 
Variation of self- 
absorption correc- 
tion factor corre- 
sponding to +10% 
variation in paper 
Proportion of 
beta particles 
unabsorbed by 
paper of mean 
density 8.785 
Isotope mg/cm? density 
Cc 0.366 +7.4% 
$3 0.431 +6.5% 
Br? 0.787 +2.4% 
[131 0.828 E19 
C136 0.843 +1.7% 
ps2 0.957 +0.5% 
upon the density or weight per unit 
area of the paper strip. For practical 
purposes, the density can be considered 
constant. 
On this basis, the total weight w 
of the labeled component is propor- 
tional to the sum total of the net rates 
of count (corrected for background, 
radioactive decay, deadtime losses, 
etc.) of the relevant sections, 1.e. 
w= Ka (1) 
where a is the corrected net rate of 
count of each relevant section. The 
constant A depends upon the specific 
radioactivity of the original com- 
ponent and upon the over-all counting 
efficiency. 
In practice it has been found con- 
venient to determine K by scanning a 
chromatogram run with a known 
weight of compound of known specific 
activity. Its position also serves to 
identify the peaks of the original 
chromatogram. 
When the radiochromatogram is 
plotted as net rate of count against 
distance along the strip, w is propor- 
tional to the area enclosed by the 
relevant part of the curve (see below). 
With the scanner described, it has been 
found convenient to measure the areas 
plotted on the recorder by means of a 
planimeter. In comparing different 
radiochromatograms quantitatively, it 
is important that they be plotted on 
this basis since the total radioactivity 
is the product of the mean rate of 
count (over the relevant sections) and 
the distance in an absolute sense over 
which the radioactivity is spread. For 
the purposes of identification, however, 
the R; value is the important property, 
and the radiochromatogram should 
then be plotted as net rate of count 
against Ry, value. Other factors of 
180 
significance in quantitative work are 
discussed below. 
Decay corrections. Usually the 
time taken to scan a unidimensional 
paper chromatogram is small com- 
pared to the half-life of the isotope 
being assayed. Thus corrections for 
decay may be applied to the measured 
areas as a whole and based on the 
mean time of scanning. When the 
time of scanning is not relatively small, 
the time scale corresponding to each 
part of the strip must be recorded and 
different corrections applied according 
to position on the strip. 
Geometry effects. Ideally, the geo- 
metrical distribution of radioactivity 
would be the same for every exposed 
section of the paper chromatogram. 
In practice, variations occur, especially 
in strips cut from a two-dimensional 
sheet when the radioactive zone would 
not be expected to be in the middle 
region of the section, except by chance. 
The beta-particle sensitivity of an 
end-window-type Geiger tube varies 
according to the position of the source 
below the window; the more off center 
the source, the less the sensitivity. 
For this reason, Boursnell (7) has used 
a large-area window tube for scan- 
ning. In unidimensional chromato- 
grams, however, the radioactive zones 
appear to be sufficiently similar to 
permit assays based on adequate con- 
trols with known amounts of radioac- 
tive material. 
For two-dimensional chromato- 
grams, a tube of larger window area 
than the one illustrated on page 55 
might be desirable. A simpler alterna- 
tive, however, is to cut the two-dimen- 
sional sheet into the equivalent number 
of strips sufficiently narrow to eliminate 
significant geometrical effects. A 
width of 1 cm has been found to be 
effective; strips of this width are run 
over the middle of the drum. This can 
be done by simply winding the narrow 
strips in the middle of drum H. 
Deadtime corrections. Corrections 
for counter deadtime or quench time 
losses for every section of a radio- 
chromatogram would be tedious to 
make. For this reason, it is recom- 
mended that quench and ratemeter 
paralysis times be kept to a minimum. 
For example, if a Geiger counter is 
effectively quenched for 100 usec fol- 
lowing each pulse, then this correction 
for a maximum rate as high as 100 
counts per second will be only 1%. 
Since the rate of count necessarily 
varies from zero to zero through the 
maximum for a resolved component, 
the deadtime error in its estimation 
will be less than 1%. 
Self-absorption. Self-absorption of 
a constant source will vary according 
to the variations in paper density. 
Corrections for this factor in quantita- 
tive comparisons along a unidimen- 
sional strip are tedious, but the follow- 
ing calculations indicate the effect to 
be small in any case. For the purpose 
of the calculation, it was assumed that 
a resolved component would be spread 
over about 5 cm? of paper. The coeffi- 
cient of variation in mean density of 
5-cm? sections cut from a random sheet 
TABLE 2—Radiometric Assay of Br8?-labeled Derivatives Separated on Paper 
Chromatograms 
Weight applied 
to chromatogram by scanning 
Compound on 
Recovery 
mixture (micrograms) (%) Remarks 
(p-BrCsH,)2CH.CCl; 0.2 109 
de 1.0 104 
a 1.0 111 Labeled compound applied 
Me 2.0 105 singly 
a 5.0 85 
i 5.0 92 
(p-BrCsH,4)2CH.CCls 1 0 111 
+ (p-BrCsH,)2C:CCls 0.5 97 
(p-BrC.H,).CH.CCls 50 107 Applied as a labeled mixture 
+(p-BrC.H,)2C:CClz 50 118 
+ (p-BrC.H,),.CH.COOH 50 100 
(p-BrCeH,4)20:CCl2 20 126 Applied as a mixture; acti- 
+(p-BrCsH4)2CO 20 62 vated after chromatogra- 
Average recovery 
101.2 
LUE E ESE 
