Quantitative

Determination of Tritium 

In Natural Waters

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1696-D
Quantitative

Determination of Tritium 

In Natural Waters

By C. M. HOFFMAN and G. L. STEW ART

RADIOCHEMICAL ANALYSIS OF WATER

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1696-D

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1966
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 15 cents (paper cover)
CONTENTS

Page

Abstract. _ _____________________________________________________ Dl

Introduction. ____________________________________________________ 1

Electrolytic enrichment,__________________________________________ 2

Factors influencing separation efficiency _________________________ 4

Multistage enrichments.______________________________________ 5

Counting procedures-_____________________________________________ 6

Gas phase technique._-_---______-_--_---___-____---__-___--_- 6

Liquid scintillation technique._________________________________ 10

Enrichment calculations.__________________________________________ 11

Theory and calculations.__-_---_-_-_____-__---__-____--___-_-_-___ 13

Analytical errors.________________________________________________ 14

Comparative sensitivities of counting techniques.--___-_-_-----_.- 15

Counting statistics.__--____________-__________-___-_____-_--_- 15

Summary_ _____________________________________________________ 17

References_ ____________________________________________________ 17

ILLUSTRATIONS

FIGURES 1-7. Diagrams of: Page

1. Nickel-steel electrode for electrolytic enrichment

process__ _________________________________ D3

2. Vacuum distillation apparatus for the quantitative

recovery of sample._________________________ 6

3. Gas production apparatus used to prepare H2 from

water. __ ________________-____-____________ 7

4. Internal sample Geiger-Mueller detector.________ 8

5. Counting array...._-_-_--____-____-__---__-_. 0

6. Block diagram of electronic circuitry.___________ 9

7. Falling drop apparatus for measuring of D2O in

water.___________________________________. ]2

8. Graph of falling time of sample droplet as a function of the

deuterium content-_________________________________ 12

9. Graph of percent error, sample activity and counting time

relationships for the liquid scintillation system. ------ 16

TABLES

Page 

TABLE 1. Effect of temperature and current on tritium recovery.------- D5

2. Analytical results of tritium concentration measurements. ____.. 15

m

RADIOCHEMICAL ANALYSIS OF WATER

QUANTITATIVE DETERMINATION OF TRITIUM IN 

NATURAL WATERS

By C. M. HOFFMAN and G. L. STEWART

ABSTRACT

This report describes the methods and procedures used by the U.S. Geological 

Survey Tritium Laboratory to analyze natural waters for tritium activity.

Samples having low activity are electrolytically enriched in cells with nickel 

and steel electrodes, and their tritium concentration is subsequently determined 

by liquid scintillation and gas phase counting techniques. Samples having high 

tritium concentrations can be counted directly, and no enrichment is required.

Theory, calculations, equipment, and procedures are described in some detail.

INTRODUCTION

In recent years, as the Nation's water supply problem has caused 

more concern, the hydrologist has turned to radioisotopes to aid him 

in his studies of water residence times and movement patterns. 

Tritium is superior to other water tracers in some kinds of hydrologic 

studies because it is incorporated directly into the water molecule.

Tritium, a naturally occurring radioactive isotope of hydrogen, 

has a mass number of 3. It is a soft beta emitter and has a maximum 

energy of 0.018 Mev (million electron volts) and a half life of 12.26 

years.

The discovery of tritium is attributed to Sir Ernest Rutherford 

and his coworkers at the Cavendish Laboratory in England; they 

produced tritium by deuteron-deuteron interaction:

.H*(d, p)JB*. (1)

In this reaction, He3 is also produced, and Rutherford was unable to 

ascertain which isobar was radioactive. Later, Alvarez and Cornog 

(1939) at Berkeley, Calif., determined tritium to be the radioactive 

component and He3 to be stable and naturally occurring.

Dl
D2 RADKXTHEtMHOAiL AmLYiSIS OF WATER

Rutherford also attempted to detect tritium in natural waters. 

He attempted to concentrate tritium by electrolyzing large quantities 

of natural water but was unable to detect tritium in the enriched 

water by mass spectrometry.

In 1951, Dr. Willard F. Libby, at the University of Chicago, 

measured the enriched water produced by Rutherford with a GeigerMueller detector instead of the less sensitive mass spectrometer and 

found the sample contained radioactive tritium.

Tritium is produced naturally in the atmosphere by cosmic radiation, and the following mechanism is believed to be mainly responsible for its production:

6C12. (2)

It is also produced naturally in geologic formations by the following 

reaction:

3Li6 M iH3. (3)

In addition, tritium is produced synthetically in thermonuclear explosions, and large concentrations are trapped in rain and snow.

The usefulness of tritium in dating or tracer studies depends on the 

accuracy of its analysis. Tritium levels for some natural waters are 

extremely low, and the analysis may require special enrichment techniques 

and other critical steps, all of which must be performed with great care 

to produce accurate and meaningful results. The following discussion presents methods employed by the U.S. Geological Survey Tritium Laboratory to determine quantitatively the tritium concentrations in natural waters.

Acknowledgments. The authors are grateful to Mr . T. A. Wyerman 

and Mr. R. K. Reaves for the assistance they rendered in the Laboratory while the authors were working out the analytical procedures reported here.

ELECTROLYTIC ENRICHMENT

Pre-hydrogen bomb tritium concentrations in water are low, and it 

was calculated by Faltings and Harteck (1950) that there is only one 

mole of natural tritium in the entire atmosphere, or one tritium atom 

for every 10 cc (cubic centimeters) of air. As a convenience, tritium 

concentrations are normally expressed in tritium units, 1 TU being defined as one tritium atom for every 1018 hydrogen atoms. This ratio 

is approximately equal to 3.2 picocuries per liter.

Low tritium concentrations in natural waters are not directly measurable and therefore the water samples must be enriched. Four methods of enrichment are currently employed: electrolysis, thermal 

diffusion, distillation, and gas chromatography. Fractional electrol-
DETEEiMIN'ATiaN" OF TETTIUM D3

ysis is the method most commonly used and is the method used in the 

tritium laboratory.

The electrolysis is performed in cells of the type shown in figure 1. 

At present, cells of 1-liter and 250-, 40-, and 10-milliliter capacity are 

used, the size of the cell being determined by the degree of enrichment 

required. As a general proposition, samples are enriched according to 

their estimated original tritium concentrations as follows:

Volume reduction 

Concentration (tritium units) (mittillters)

> 1,000 __ _ __--____-____-___-____ None measurable directly.

100-1,000.__. _____________________ 40-4

10-100. ---_----_-_--___----__-___ 250-4

-_-___---_________-_____.__ l,OpO-4

These ranges are flexible and may be adjusted according to the 

amount of time available for counting a sample. For example, a 

sample containing 400 TU may be measured directly but this requires 

a long counting time and monopolizes the counting equipment 

unnecessarily.

The electrodes are constructed from uniform steel and nickel tubing, 

and the two segments are separated by a Teflon spacer of uniform

/Steel

Nickel

las vent

Spacer

FIGURE 1. Nickel-steel electrode for electrolytic enrichment process.
D4 KADKJCHEIMIGAL ANlALYiSIS OF WATER

thickness. The cells are fitted with stoppers to eliminate cross contamination and a gas outlet to allow the H2 and O2 produced during 

electrolysis to escape. 

The reactions that occur at the electrodes are

Nickel anode: 2H2O-*4H++O2 +4e-, (4) 

Steel cathode: 2H++2e--*H2 , (5)

where H may be protium (H1 ), deuterium (H2), or tritium (H3).

Nickel and steel are used as electrode materials because of the good 

separation efficiency obtained (that is, the reduction of protium in 

reaction (5) is more rapid than the reduction of tritium), their 

corrosion resistant character, and their ready availability.

FACTORS INFLUENCING SEPARATION EFFICIENCY

Factors that adversely affect the separation efficiency during the 

electrolysis are:

1. Variation in interelectrode spacing and associated current density 

differences.

2. High temperatures in the electrolysis cell which produces water 

vapor losses.

3. Irregularities on electrode surface.

4. High salt concentrations which react with the electrodes.

During electrolysis, protium is preferentially reduced and discharged 

at the cathode, and the solution surrounding the cathode increases in 

tritium concentration relative to the rest of the solution. This condition is undesirable since a larger proportion of tritium is available 

for reduction at the cathode, and the separation efficiency of the 

process is reduced. This concentration gradient is eliminated to some 

extent by diffusion, convection caused by thermal differentials, and 

mixing through gas evolution (Urey, 1933). If irregularities are 

present on the electrode surface, electrolysis takes place hi the electrode cavities since gas bubbles tend to form on the projections. 

Bubbles held in this manner decrease the stirring effect, and the 

tritium concentration increases near the cathode surface, thus unfavorably increasing the concentration gradient and decreasing the 

separation efficiency.

To have the electrolysis proceed at a reasonable rate, an electrolyte 

is added to the water sample. The solution is made alkaline by the 

addition of Na2O2 :

(6)
QUALITATIVE' DErTEiRMESTATIOOSr OF TRITIUM D5

An alkaline, rather than acidic, electrolyte is used because of the 

competing hydrogen gas the metal-acid combination would produce. 

The Na202, unlike the acid, contains no hydrogen and reduces the 

possibility of contamination. Electrolytes such as NaCl are undesirable because they attack the electrodes, making them unsuitable 

for extended use. For this reason, the conductance of the sample is 

measured prior to analysis, and if the salt concentration is too high, 

the sample is quantitatively distilled.

During electrolysis, a fractionation decrease, and hence a separation 

efficiency decrease, occurs as cell temperature increases because the 

ratio PH^O/PTZO (where P is the vapor pressure) decreases with increasing temperature. It is desirable to maximize this ratio by cooling the electrolysis cell to improve the separation efficiency. The 

cooling bath is a converted deepfreeze unit fitted with a thermoswitchrelay temperature control, a centrifugal circulating pump, and an 

exhaust duct. Power for the electrolysis cells is furnished by lowripple battery eliminators.

Carlston and Thatcher (1962) in a study of the parameters involved 

in the electrolysis process found that low-temperature and low-current 

density conditions are desirable for good recovery. Table 1 shows 

the effect of temperature and current on tritium recovery.

TABLE 1. Effect of temperature and current on tritium recovery 

[After Carlston and Thatcher, 1962]

Temperature (°C)

8_ -_ __ ..

25

8

25

Applied volts

 »

/

Average current 

(amperes)

j[

f3 -.- __

Percent recovery

f 83

58

48

MULTISTAGE ENRICHMENTS

When the NaOH concentration is between 1 and 20 percent during 

the electrolysis (Ostlund and Werner, 1962), the most rapid electrolysis rate is achieved and the recovery is reproducible. If a high 

degree of enrichment is to be achieved, a sample must be electrolyzed 

in a number of stages to maintain this optimum range. When transferring a sample from stage to stage, the partially enriched water 

sample is first neutralized by slowly bubbling C02 gas through the 

solution. Neutralization liberates any hydrogen bound in the OH~ 

by forming sodium carbonate and water, and speeds up the distillation 

process:

C02+2NaOH-*Na2C03+H20 (7)
D6 RADIOCHEMICJAiL ANALYSIS! OF WATER 

Heating tape

FIGURE 2. Vacuum distillation apparatus for the quantitative recovery of sample.

Neutralized samples are vacuum distilled using the apparatus shown 

in figure 2. Heat is supplied with a Variac controlled heating tape 

(only partly shown), and a glass wool trap helps prevent salt particles 

from passing through the system to the receiver bulb. After quantitative distillation, the sample is again made alkaline by the addition 

of Na2O2 and the enrichment continued in the next electrolysis stage.

COUNTING PROCEDURES 

GAS PHASE TECHNIQUE

The tritium content of an enriched sample is determined by gas 

phase counting. Although the method offers high sensitivity and 

efficiency in measuring the beta decay, it has the disadvantage that 

a skilled technician is needed to prepare the sample for counting. 

The sample is counted as hydrogen gas, and the tritium recovery is 

quantitative, thus eliminating fractionation effects.

The gas production apparatus is shown in figure 3. An aliquot 

of the enriched water sample, approximately 0.6 ml, is placed in 

bulb E where it is frozen with liquid nitrogen, and the system evacuated to remove all the gaseous impurities. This bulb is then warmed 

and the water vapor allowed to pass through the hot zinc column. 

The water vapor reacts with the zinc to form hydrogen gas and zinc 

oxide:

The bulb to trap any unreacted water vapor is cooled with liquid 

nitrogen during this reaction. This water vapor may be subse-
QUAiNOTTAiTIVE DETERMINATION OF TRITIUM D7

quently recycled through the hot zinc to effect a complete reaction. 

As the hydrogen is formed by reaction (8), it is forced into the detector tube by a Toepler pump and there mixes with a measured 

quantity of previously added toluene vapor. The amount of hydrogen may be calculated from the pressure observed on the mercury 

manometer because the volume of the detector is known.

Toluene acts as a quench gas and eliminates multiple discharges 

and cascading. Several gasses have been investigated for suitability 

as quenching agents, and toluene and ethylamine were found to have 

exceptionally good counting characteristics. Plateaus of 800 volts 

and <C3 percent slope are obtained by using toluene that has been 

frozen with liquid nitrogen and then purified by pumping off the 

gaseous impurities prior to adding the pure quench to the sample 

detector. A toluene to hydrogen pressure ratio is approximately 

1:40 at 40 cm (centimeters) of mercury pressure, but this ratio is 

not critical. Although a satisfactory quench gas, ethylamine rapidly 

attacks the detector Plexiglas endpieces and makes the unit unsuitable 

for further use after a short time.

Toepler pump

FIGURE 3. Gas production apparatus used to prepare Hj from water. C bulb for trapping any unreacted 

water vapor; enriched water sample is added to bulb E.
D8 RADIOiCHElMIOAiL ANiALYlSIS OF WATER

A cross section of the Geiger-Mueller detector is shown in figure 4. 

A gas-tight seal is obtained between the body wall and the endpieces 

with epoxy cement. The inner surface of the tube must be clean and 

smooth, and great care must be taken to center the stainless steel 

anode wire properly.

Since the count rate may be low for even enriched samples, it is 

necessary to reduce the high natural background to a statistically 

usable level. The counting array shown in figure 5 serves this purpose. The array consists of a mercury shield, an anticoincidence 

ring, and the mercury shroud inside of which the detector is placed. 

The mercury shield is a large stainless steel tank filled with 5,000 

pounds of purified mercury metal which forms a 4-inch-thick mercury 

wall surrounding the detector. This dense metal absorbs a large 

part of the cosmic radiation naturally present. The anticoincidence 

ring is a multiple anode Geiger detector which reduces the background 

by eliminating pulses that are simultaneously triggered in the ring 

and sample detector. The mercury shroud is similar in construction 

to the large shield and serves to reduce the background count still 

further by absorbing radiation which passes through the anticoincidence ring without triggering a pulse and by absorbing radiation 

produced by impurities in the metal of the anticoincidence ring. 

This counting array reduces the unshielded background count of 350 

cpm (counts per minute) to a background of 2.5 cpm. The background value is determined by filling the detector with a tritium-free 

electrolytic hydrogen and quench mixture and counting it in the 

array.

The tritium soft beta emission is measured by a Sharp's Laboratory 

Low Beta System, which consists of the anticoincidence ring and detector preamps, high voltage supply, main electronics console and 

dual printout registers. The console houses the logic circuits, main 

amplifiers and sealers. A block diagram of the system is shown in 

figure 6.

Plexiglas 

ST1

3/32" walk

Groun

No. 30 chromel wire

Gas inlet

FIGURE 4. Internal sample Geiger-Mueller detector.
QUANTITATIVE DETERMINATION OF TRITIUM D9

Scope

Preamps

High 

voltage

Sealers

Printers

o

ooo a

OOO CD 

OOO CD

FIGURE 5. Counting array: A, mercury shield; B, anticoincidence ring; C, mercury shroud; D, cavity

in which detector is placed.

FIGURE 6. Block diagram of electronic circuitry.
D10 RADHOCHEOVHOAiL ANiALYISIS OF WATEK

LIQUID SCINTILLATION TECHNIQUE

If tritium is present in sufficient amounts, the tritium concentration 

may be determined directly by use of a liquid scintillation spectrometer. An aliquot of water sample is mixed with a scintillator solution in a polyethylene vial and placed between two photomultiplier 

tubes in the spectrometer. As the tritium decays, the emitted beta 

particles excite the scintillator solution generating light pulses that are 

detected and amplified by the photomultiplier tubes and electronically 

sorted according to pulse energy into the appropriate sealer channel 

(Bell and Hayes, 1958). The window on the analyzer channel is 

adjusted to accept the major part of the tritium spectrum while :xcluding excessive background noise and the higher part of the eneigy 

spectrum.

The spectrophotometer used was fitted with a 200-position sample 

changer, automatic printout, and a time base modified to extend the 

counting tune to 1,000 minutes. The dioxane cocktail used is 

basically that described by Werbin, Chaikoff, and Imada (1959). 

They found that optimum counting results were obtained with 100 

grams naphthalene, 10 grams 2,5-diphenyloxazole, and 0.25 grams 

l,4-bis-2(5-phenyloxazolyl)-benzene per liter of spectroquality dioxane. These investigators cited as optimum a dioxane-naphthalenewater ternary of 20 percent water having an efficiency of 8.6 percent. 

However, this laboratory found optimum conditions to be a 10- 

percent-water ternary in a 20-ml plastic vial yielding a counting 

efficiency of 24 percent and a background of 16 cpm. Although the 

efficiency of the system increases with decreasing water contrnt 

(because of an internal pulse-height-attenuation process called 

quenching), this increase in efficiency from 8.6 to 24 percent is due 

mainly to refinements in electronics.

The liquid scintillation method has some disadvantages not found 

in the gas phase technique. The high background of the liquid 

scintillation spectrometer (16 cpm for liquid scintillation versus 2 

cpm for gas phase) is a distinct disadvantage. This background is 

due to thermal noise within the photomultiplier tubes and to small 

conccentrations of K-40 or other radioactive impurities in the glass 

sheath of the phototubes. The photomultiplier tube faces are made 

of quartz to reduce the K-40 background.

Chemical quenching and quenching due to matter or coloration 

in the water sample present a problem since they hinder the transfer 

of light and decrease the counting efficiency. High-activity samples 

are corrected for quenching by using the channel-ratio method 

(Bush, p. 1024), in which the relative spectrum shift for the analyzer 

and monitor channels is correlated to a quench correction factor,
DETEiRfMUSTATOlON OF TRITIUM Dll

Q. A curve is prepared by quenching a standard with chloroform and 

plotting Q versus the monitor to analyzer ratio.

Quenching due to the formation of peroxides also occurs as the 

dioxane cocktail ages and it is inadvisable to prepare more scintillator 

solution than can be used within a few days after preparation.

It is necessary to minimize exposure of the sample cocktail to 

light since the phosphors become excited, causing an initial high count 

rate. The samples are allowed to cool in the darkened freezer unit 

at least 30 minutes prior to counting.

ENRICHMENT CALCULATIONS

An enrichment efficiency factor, E, based on NBS (National 

Bureau of Standards) tritium standards is used to calculate the amount 

of enrichment:

TV E-^-0 (9)

where

F°=initial volume, 

F=final volume, 

T°=initial TU, 

T=final TU.

Most recent precipitation samples only require a single stage of enrichment to obtain measurable levels, and the use of E to calculate 

enrichment is convenient.

When high enrichment is necessary, the deuterium enrichment can 

be measured and the tritium enrichment computed by the method 

described in the section on "Theory and Calculations."

Normally, the final deuterium concentrations are measured with the 

falling drop apparatus shown in figure 7. The constant temperature 

bath is filled with a large volume of water (about 10 gal) for thermal 

stability, and the temperature is maintained by a thermoswitch and 

heating system. The falling tube, filled with isobutyl benzoate (a 

water insoluble substance having a density close to that of water), is 

thus kept at a constant temperature in the mechanically stirred bath. 

Heat reflectors keep the infrared radiation of the heating lamps from 

directly striking the falling tube.

A small amount of sample is drawn into a micropipet assembly, and 

the time required for a drop of measured volume to pass between two 

marks on the falling tube is measured by a tinier. The first mark is 

positioned low enough on the tube so that the drop reaches its terminal 

velocity before timing is begun. Since Stoke's Law is obeyed and the 

density of the drop is proportional to the deuterium concentration, the
D12 RADIQCHEiMIOAiL ANALYSIS OF WATER

OO

oo

FIGURE 7. Falling drop apparatus for measuring deuterium in water. A, Micropipet assembly; B, motordriven stirrer; C, heating system; D, heat reflectors; E, thermoswitch; F, falling tube; G, constant temperature bath; H, timer.

5.0

5 4.0

§ 3.0

Q.

O 

UJ

cc

-. 2.0

1.0

0.0

10 

D2 0, IN PERCENT

15 20

FIGURE 8. Falling time of sample droplet as a function of the DjO content.
QUANTITATIVE DETERMINATION OF TRITTUM D13

deuterium content may be measured by comparing a sample's falling 

time to those of a series of standards. A standard curve is shown in 

figure 8, in which the reciprocal of the falling time in minutes is 

plotted versus percent D2O.

THEORY AND CALCULATIONS

During the electrolysis process, the enrichment of the isotopes 

relative to each other are related by the equation:

&*_0*, (10) 

pat

where a and & are enrichment factors and p, d and t are the number 

of moles of protium, deuterium and tritium, respectively. The 

enrichment factors may be defined as the ratio of concentrations of 

the isotopes in the gas phase to those in the liquid phase:

~(p/d) liquid

(pit} gas ( . 

^-(p/t) liquid' (12)

(d/Qgas (l . 

y~(d/t) liquid {6)

As the value of 7 apparently decreases with increasing temperature 

(Roy, 1962), the ratio of log /3/log a appears to be quite constant 

at 1.40 ±0.01 (Kaufman and Libby, 1954; Bigeleisen, 1960). An 

expression for the original tritium concentration of the enriched 

water may be derived from the relationship found in equation (10):

C'dp Cd dd C l O dt

 = a -r= |8 JP° p Jd° d Jt° t

where the superscript ° indicates initial conditions. 

Assuming that t<^<^d<^<^p, then

V /d\« /* v-°=(d-°) =(r)

or, letting X signify the mole fraction,

v

The original tritium concentration, X°t , is calculated using equation 

(15).
D14 R'ADIQCEIEMEOAk ANALY1SIS OF WATER

The final tritium concentration, Xt , is calculated from sample 

activity, detector efficiency, effdet, and observed hydrogen pressure, 

fobs, to give disintegrations per minute per milliliter

where P is a water equivalency factor that relates hydrogen pressure 

in the detector to milliliters of water, and

The enrichment factor ratio a/ ft is determined by preparing standard s 

of known initial deuterium and tritium concentrations, measuring 

these concentrations after enrichment, and substituting these values 

into equation (14).

ANALYTICAL, ERRORS

The measurement of such physical quantities as pressure, weight, 

and volume are negligible sources of error. A pressure reading error 

of 1 mm for a total mercury pressure of 60 cm would introduce an 

error of ±0.17 percent, whereas the volumetric error of a 40-ml 

pipet is on the order of ±0.05 percent. Losses in transfer operations 

are small and have been shown to be about ±0.1 percent.

Cross contamination and detector memory effect may present a 

problem after the analysis of an active sample, but if the distillation equipment is properly dried and the detector evacuated to a 

low pressure, then cross contamination may be regarded as an insignificant error factor. This intersample contamination is not 

important for liquid scintillation counting since new vials are used 

for each sample and pipets are cleaned and completely dried in an 

oven prior to use.

There is a possibility of erratic performance of the counting equipment due to such factors as excessive line noise, electronic oscillations 

or component breakdown. This source of error is indeterminate 

and is best guarded against by frequent equipment checks using 

blanks, standards and a printer set for short count times.

Table 2 shows the analytical results for a variety of samples. 

These data indicate the effect of enrichment on the assay and gives 

a comparison between counting systems for a range of tritium 

concentrations.
DEITEiRMINATION OF TBTTOUM D15

TABLE 2. Analytical results of tritium concentration measurements

Sample

Type

Surface water _ 

Precipitation.. _.

Location

Sacramento River, Sacramento, 

Calif.

Mississippi River, Anoka, 

Minn. 

Mississippi River, Anoka, 

Minn. 

Missouri River, Nebraska City, 

Nebr.

IAEA* (Vienna) » .. .........

. -do.»-... - . - 

.... .do.».-. . . ..

 . .do.«-- ~ . . 

.... .do------.... ... ...

Date 

collected

July 27, 1964 . .

Julyl963. .

Ttllv 1QA3

June 1963.. 

May 5-18, 1963. . 

August 1962..... 

October 1962... .

Enrichment 

factor

5.65

5.20 

0

5.57 

0

0 

6.10 

0 

39.79 

11.43 

0

Tritium units

Gas 

counting

240 

110

950 

2,440

12, 950 

119

45±4 

558±30 

4, 050±200

Liquid 

scintillation 

counting

212

96 

920

2,240 

12,550

1,430 

43±6 

495±31 

4, 130±200

1112 TU.

*1,448TU.

s <100 TU.

«200-800 TU.

"International Atomic Energy Agency.

COMPARATIVE SENSITIVITIES OF COUNTING TECHNIQUES

A comparison between the detection limits of the gas phase system 

and the liquid scintillation technique shows the greater sensitivity 

of the gas phase method.

Based on 2<r statistics, a 500-minute count time, a background of 

2.20 cpm, and a water equivalent of 0.61 ml, the lower limit of detectability for the gas phase system would be 46 TU.

A similar calculation for a 2-ml water sample, a counting efficiency 

of 23.75 percent, and a background of 16.20 indicates that the sensitivity for the liquid scintillation spectrometer would be 1.49 TU.

These concentrations are the minimum tritium that must be 

present to be detectable above the statistical background of the 

system under the described conditions. It would not be possible to 

measure these minimum concentrations with any accuracy, however.

COUNTING STATISTICS

The counting error is statistical since radioactive decay is a random 

quantity, and the frequency of the measured events follows a bellshaped normal distribution curve. A common measure of the 

analytical precision is the standard deviation, <r, which is expressed as

<r=

N 1/2

(18)
D16 RADlOCHEaVCECJAI, ANALYISIS OF WATER

where X is the measured value in a set of N measurements and X is 

the arithmetic mean of the set of N measurements. For a normal 

distribution curve, a probability of 68.3 percent exists that a single 

measured value X will have a maximum deviation of lor. For a 

2<r deviation, the probability is 95.5 percent.

When a single measurement is made that produces a statistically 

significant number of counts, the standard deviation for a large 

number of counts is equal to the square root of the total counts, 

NT- To convert these results into counts per minute, both the total 

counts, NT, and the square root of the total counts, NT%, are divided 

by the time in minutes, t. Thus, for a Iff deviation and a 68.3-percent chance that the measurement is correct, the activity of the 

sample may be expressed as

~r NT ±. ~~r~' (NTY<2

To make the most efficient use of the counting equipment and maximize the number of samples analyzed, the total counting time needed 

to make a measurement within the desired probable error limits is 

predetermined from the approximate sample activity.

The series of curves shown in figure 9 were calculated for the 

liquid scintillation spectrometer and were based on a background of

50

40

CC

§30

cc

LJ

20

10

2 4 6 8 10 15 20 25 

COUNTS PER MINUTE ABOVE BACKGROUND

30

FIGURE 9. Percent error, sample activity, and counting time relationships for the liquid scintillation

system.
DETERMINATION OF TRITIUM D17

16 cpm and 2<r statistics. For example, if the sample had an activity of 10 cpm above background and was counted for a total of 

500 minutes, the probable error for the measurement would be about 

5 percent. Since 1 cpm above background equals 292 TU for the 

liquid scintillation system, there would be a 95.5 percent chance that 

the tritium concentration of the sample would be

SUMMARY

The tritium content of water samples is determined by either of 

two methods: gas phase counting or liquid scintillation techniques. 

In the gas phase method, hydrogen from the water molecule is reduced 

to Ha gas; this gas is placed in a detector tube, and the activity of the 

sample is" measured under anticoincidence counting. The liquid 

scintillation technique involves mixing an aliquot of enriched sample 

with a dioxane scintillator solution and counting the resulting cocktail 

in a dual channel liquid scintillation spectrometer.

Prehydrogen bomb tritium concentrations in precipitations were 

low and not directly measurable; therefore fractional electrolysis was 

used to increase the tritium concentration to easily measurable levels. 

Samples are placed in a cell fitted with nickel and stainless steel 

electrodes and a potential placed across the cells results in H2 and O2 

evolution at the electrode surfaces. Since protium is preferentially 

reduced, tritium tends to be enriched in the solution. The increase 

of tritium in solution thus becomes a function of volume reduction 

during electrolysis, and the overall tritium increase is expressed in 

an enrichment factor. Knowledge of the enriched sample's activity 

and the enrichment factor permits calculation of the original tritium 

concentration of the water sample.

The natural production of tritium in the atmosphere and its subsequent incorporation into the water molecule gives the hydrologist 

a useful tool for ground-water dating and associated studies. The low 

concentrations of this natural tritium requires a reproducible enrichment process and a sensitive means of detecting the low energy beta 

emission. The process described fulfills these requirements and is 

readily adaptable to low cost analysis and large-scale production.

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Bigeleisen, Jacob, 1960, Correlation of tritium and deuterium isotope effects:

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D18 RAMOCHEMKSAk ANALYiSIS OF WATER

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in physical and biological sciences, Vienna 1961, Proc.: Internat. Atomic 

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Roy, L. P., 1962, Influence of temperature on the electrolytic separation factor 

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