United States
Environmental Protection
Agency
Environmental Monitoring Systems -
Laboratory
Las Vegas NV 89114
Research and Development
EPA-600/S2-83-039 July 1983
&EPA Project Summary
Test Procedure for lodine-131 in
Drinking Water: Interlaboratory
Collaborative Study
W. H. Yanko, C. T. Bishop, and A A. Glosby
An interlaboratory collaborative study
was conducted on a test procedure for
measuring the iodine-131 concentra-
tion in drinking water. The purpose of
the study was to determine the esti-
mated precision and accuracy of test
results from participating laboratories
using this test procedure and analyzing
drinking water samples.
Drinking water samples containing
iodine-131 at concentrations on the
designated collection date of 7.8,25.9,
and 78.3 picocuries per liter were an-
alyzed in triplicate by 11 collaborators
(10 laboratories) using the test pro-
cedure.
A statistical analysis of the test re-
sults showed coefficients of variation
for repeatability (within-laboratory pre-
cision) of 8.7, 10.5, and 7.7 percent
respectively for the three samples for
an average repeatability precision of
9.0 percent The analysis also gave
coefficients of variation for reproduci-
bility (combined within-and between-
laboratory precision) of 17.4,15.5, and
15.7 percent for the respective samples
for an average reproducibility precision
of 16.2 percent
The average carrier iodine recoveries
were 76.4, 79.3, and 79.8 percent
respectively for the three samples, giving
an overall average of 78.5 percent. A
comparison of the grand average test
results for the three samples with the
known values for those samples shows
accuracy indexes of 97.4, 97.3, and
84.8 percent respectively, for an aver-
age accuracy for the test method of
93.2 percent.
This Project Summary was developed
by EPA's Environmental Monitoring
Systems Laboratory, Las Vegas, NV, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
This report is submitted as partial fulfill-
ment of an Interagency Agreement, EPA-
IAG-79-D-X0736, between the Environ-
mental Protection Agency and the Depart-
ment of Energy (DOE). Work was uone at
the Mound Facility of the Monaanto Re-
search Corporation under DOE Contract
Number DE-AC04-76-DP00053. The
work covered the period of September 1,
1979 to June 15, 1980.
The National Interim Primary Drinking
Water Regulations (NIPDWR) require the
use of approved test procedures for an-
alyzing public drinking water supplies for
contaminants. The Nl PDWR contains pro-
visions for the use of alternate test pro-
cedures with precision and accuracy
equivalent to or better than the approved
test procedure. This report describes a
multilaboratory test of a method selected
for the analysis of drinking water samples
for iodine-1 31 concentrations. The pur-
pose of the study was to determine what
precision and accuracy could be expected
in the test results from laboratories using
the method for analyzing drinking water
supplies for iodine-131 contamination.
The NIPDWR requires a sensitivity (lower
detection limit) of 1 picocurie per liter
(pCi/l) for measuring iodine-131 concen-
trations in drinking water supplies, lodine-
131 decays by emitting a beta particle and
a gamma photon. Gamma spectroscopy
has good specificity for gamma emitters,
but its sensitivity will meet the required
sensitivity (1 pCi /I) only if the iodine-131
-------
is first separated from multiliter size sam-
ples. The method tested, and described
here, requires separating the iodine-1 31,
using carrier iodine, from one or more
liters of sample and then counting the beta
activity produced by the separated iodine,
in a low beta background counting system.
Procedures
/. Analytical Test Procedure
The analytical procedure used in this
study consists of detailed steps in which
the iodine-1 31 with added iodate carrier is
separated from the sample as palladium
iodide. The iodine-131 (plus carrier) is
reduced to the iodide state with sodium
sulfite, precipitated as silver iodide, and
purified with zinc powder and by two
precipitations as palladium iodide. The
final palladium iodide is collected on a filter
and then counted for beta activity. The
recovery of the added carrier is determined
for each sample by weighing the palladium
iodide precipitate. The counting efficiency
is determined using the same amount of
iodate carrier as is used in the procedure
and standard iodme-131 activity, precipi-
tated as palladium iodide, and beta counted.
2. Collaborative Test
Procedure
A copy of the test procedure was sent to
potential participant laboratories; those
laboratories stating their willingness to
participate were used in this study.
Three reference sample solutions were
prepared using tap water, sodium thio-
sulfate and sodium carbonate preservatives,
known quantities of iodine-131 activity,
and lithium nitrate (for batch homogeneity
testing). The three reference solutions
contained iodine-131 concentrations of
7.8, 25.9, and 78.3 pCi/l respectively
(activity calculated for the collection date
to which analytical results were to be
normalized). The first, middle, and last
3.5-liter fractions of each sample batch
were used for homogeneity tests.
Each participant was sent an instruction
sheet, a data report sheet, and iodme-1 31
standard solution for calibrations, and a
3.5-hter portion of each of the three
sample solutions.
3. Data Processing Procedures
A statistical evaluation of the test results
was done by the procedures described in
E-691, E-177, and E-178 of the ASTM
Standard Part 41, 1 980, to determine the
repeatability precision (within-laboratory
variation); the reproducibility precision
(combined within- and between-laboratory
variation); and the accuracy of the test
procedure. The standard deviations and
equations for their calculations are listed
below.
Standard deviation of replicate test re-
sults within Lab i, for sample j, S,j
ra
l_h-i
•r
(1)
where: Xl)h = the result reported for
the h replicate of the j
sample material by Lab i
X,j = the mean of the individual
results of sample j for
Labi
n,j = the number of replicates
of sample j reported by
Labi
Repeatability (within-laboratory) standard
deviation for sample j, Sr
Since the number of replicates is the
same (3) for all participants for all three
samples, the equation can be given as
follows:
P
Sr = [ 1/P2S,2
'l I M
(2)
where: P = the number of participants
in the study.
Standard deviation of individual labora-
tory average from grand average for the j
sample material, S*
i= 1
_
where: X, = the average of the test re-
' suits for sample material j
by Lab i
Xj = the grand average for
sample material j
Standard deviation of between-labora-
tories for the j sample material, SL.
SL=
(4)
Reproducibility (combined within- and be-
tween-laboratory) standard deviation for
the ] sample material, SR
SR)= Srj2
The percent coefficient of variation for
repeatability (within- laboratory precision)
(also called repeatability index) for sample
J. Vr%
Vr%= 100Sr/X~
J '
(6)
The percent coefficient of variation for
between- laboratory precision for sample j,
V, %
VL%= 100 SL/X,
(7)
The percent coefficient of variation for
reproducibility (combined within- and be-
tween-laboratory precision) (also called
reproducibility index) for sample j, VR%
VR%=100SR)/X,
(8)
Accuracy index, a percent relationship of
the grand average to the known value for
the j sample material, Aj%
•i- = 100^-
O)
where: Y, = the known value for the
j sample material (pCi/l)
t-test to determine significant differences
or systematic error for sample j, t
t.=
(5)
, (P-1) degrees of freedom
Sxj/(P)1/2 (10)
where: P = number of participants
Y, = known value of the sam-
ple j iodine-131 concen-
tration
tc = 2.23, critical value for
11 participants, values
for t greater than 2.23
are significantly differ-
ent and show a system-
atic error.
Results and Discussion
A summary of the statistical evaluation
of the test results for the three samples is
given in Table 1, which lists the following
statistical parameters and values for the
three iodine-131 concentrations:
1. The known value, Y,, for each of the
iodine-131 concentrations^in pCi/I.
2. The grand average value, X,, for each
iodine-131 concentration (from 11
participants) in pCi/l.
-------
Table 1. Summary of Collaborative Study Results - Precision and Accuracy
lodine-131 (pCi/l)
Parameter e
Yj (pCi/l)
X, fpd/lj
A°/0
S, (pCi/l)
Sr(pCi/l)
Si (pCi/l)
c ' /*r;/n
&p \pL,l/l/
vr<%
v'%
Vfr%
1
1
7.8
7.6
97.4
1.19
0.66
1.14
1.32
8.7
15.0
17.4
2
25.9
25.2
97.3
3.25
2.64
2.87
3.90
10.5
11.4
15.5
3
78.3
66.4
84.8
9.54
5.14
9.07
10.43
7.7
13.7
15.7
Average
93.2
9.0
13.4
16.2
a Terms are defined in the text.
3. The accuracy index, A,% (from 11
participants) for each iodine-131
concentration, and the average ac-
curacy index over the concentration
range of 7.8 to 78.3 pCi/l.
4. The standard deviation of the grand
average values for the three iodine-
1 31 concentrations, S ^..
5. The repeatability (withirl-laboratory)
standard deviation, Sr, for each
iodine-131 concentration.
6. The between-laboratories standard
deviation, SL, for each iodine-131
concentrator!.
7. The reproducibility (combined with-
in- and between-laboratory) standard
deviation, SR, for each iodine-131
concentration.
8. The coefficients of variation for re-
peatability, Vr%; for between-labora-
tory precision, VL %; and for repro-
ducibility, VR%, fdreach iodine-1 31
concentrator1! and the averages of
each for the concentration range of
7.8 to 78.3 pCi/l.
The 66.4 pCi /I grand average compared
to the 78.3 pCi/l known value showed a
significant difference or systematic error.
A t-test gave a t( value of4.13 compared to
the critical value (tc) of 2.23 for 11 partici-
pants Therefore, 66.4 pCi/l is significantly
different than 78.3 pCi/l and shows a low
bias for the method for that level of iodine-
1 31 concentration.
Conclusions
The repeatability precision (within-la-
boratory precision), reproducibility preci-
sion (the combined within- and between-
laboratory precision), and accuracy have
been determined by a multilaboratory test
of the method. Criteria by which to evaluate
an alternate test procedure for equivalency
have been established.
Accuracy indexes of 97.4, 97.3, and
84.8 percent for the 7.8, 25.9, and 78.3
pCi/l iodine-131 concentration samples
respectively, and an average accuracy index
of 93.2 percent were obtained with this
test procedure. The study did not reveal an
explanation for the lower 84.8 percent
accuracy for the sample with the highest
iodine-131 concentration.
The estimated repeatability (within-lab-
oratory), single-operator, single-machine,
same-day, relative precision of the test
procedure for the determination of iodine-
131 concentrations in drinking water at
the 7.8 pCi /I level (averaged over the 11
participants) is ±1.3 pCi/l(17.4 percent
2Spercent); atthe25.9 pCi/l level(aver-
aged over the 11 participants) is ±5.3
pCi/l (20.9 percent, 2S percent); and at
the 78.3 pCi/I level (averaged over the 11
participants) is±10.3 pCi/l(1 5.5 percent,
2S percent), for an average 2S percent of
18.0 percent for the range of iodine-131
concentrations of 7.8 to 78.3 pCi/l.
The estimated reproducibility (combined
within- and between-laboratory), multiop-
erator (multilaboratory), single-machine,
same-day, relative precision of the test
procedure in the determination of iodine-
131 concentrations at the 7.8 pCi/llevel
(averaged over the 11 participants) is
±2.7 pCi/l (35.8 percent, 2S percent); at
the 2 5.9 pCi/I level (averaged over the 11
participants) is ±8.2 pCi/l(32.6 percent,
2S percent); and at the 78.3 pCi/l level
(averaged over the 11 participants) is
±21.5 pCi/l (32.3 percent 2S percent),
for an average 2S percent of 33.6 percent
for the range of iodine-131 concentra-
tions of 7.8 to 78.3 pCi/l.
The variations observed in the iodine
carrier recoveries by the 11 participants
for the three samples showed a depen-
dence on analyst technique for the method.
Also, the similar average iodine carrier
recoveries for the three samples (76.4,
79.3, and 79.8 percent respectively) does
not offer any possible explanation for the
low bias observed for the sample with the
78.3 pCi/l iodine-131 concentration.
The sensitivity of the test procedure is
not limited as much by the chemistry of
the procedure (iodine recovery) as it is by
the background and the counting efficiency
of the counting system used. With a beta
background of <5 cpm and a counting
efficiency of >20 percent, the sensitivity
of measurement (lower detection limit)
required in the NIPDWR (1 pCi/l) can
easily be met All participants whose test
results were used in the precision and
accuracy analysis of this study used count-
ing instruments with beta backgrounds
<5 cpm and counting efficiencies >20
percent.
Recommendations
It is recommended that the method
tested in this study be investigated for
procedure steps that result in analyst tech-
nique dependence (as demonstrated by
the variations in the iodine carrier re-
coveries ranging from 48 to 100 percent).
One possible weakness in the procedure is
in the instructions for the reaction of solid
zinc powder with solid silver iodide in an
aqueous system.
The procedure should be modified to
include a more detailed procedure for the
preparation of a palladium iodide-131
counting standard and for the determina-
tion of the counting efficiency. A revision
of section 6.5, the standardization of po-
tassium iodate, is recommended.
The method should be re-tested to de-
termine if the low bias is real for the 78.3
pCi/l iodine-131 concentration and higher
levels.
-------
W. H. Yanko, C. T. Bishop, and A. A. Glosby are with Monsanto Research Corp.,
Miamisburg, OH 45342.
Earl L. Whittaker is the EPA Project Officer (see below).
The complete report, entitled "Test Procedure for Iodine-131 in Drinking Water:
Interlaboratory Collaborative Study,"(Order No. PB 83-207589; Cost: $10.00,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
P.O. Box 15027
Las Vegas, NV 89114
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
PS «>0?912PROTECTION
saaRfcs-
------- |