NAA Application at the INCT

Halina Polkowska-Motrenko

Institute of Nuclear Chemistry and Technology, Warsaw, Poland

The Institute of Nuclear Chemistry and Technology

(INCT) was established in 1983 after partition of the

Institute of Nuclear Research in which it formerly

existed as a Chemistry Division.

The Institute has a leading role in Poland in

implementing nuclear technologies, methods and

instruments.

The INCT is appointed as the main institution

responsible for the development and implementation

of nuclear analytical techniques in Poland.

The Department of Analytical Chemistry of the INCT

has been involved in the elaboration of INAA and

RNAA methods for determination of selected elements

in many different materials.

INAA has been applied for multielemental analysis of rare and precious samples e.g. meteorites, concentrates and wastes from copper industry, forensic samples etc. and also for homogeneity testing of candidate CRMs.

Elemental standardization is used. Contents of up to 23 elements can be quantified on the basis of determination of medium- and long-lived radionuclides.

RNAA methods has been applied for determination of selected trace elements in different samples.

Many radiochemical separation schemes enabling selective and quantitative separation of selected elements have been elaborated. Mainly ion-exchange and extraction column chromatography but also liquid extraction and distillation have been applied.

The INCT has developed a concept of the high-accuracy (definitive) RNAA methods on selected elements in biological matrices and such methods have been elaborated since 1986. The high-accuracy RNAA methods are applied in the certification process of CRMs for inorganic trace analysis as well as for the validation of other analytical procedures.

High accuracy RNAA methods

for selected elements in biological matrices

• methods of guaranteed accuracy

• neutron activation

• very selective and quantitative post-irradiation separation of indicator radionuclide by column chromatography

• gamma-ray spectrometric measurements

based on combination of:

Devising of mineralization method and separation scheme involving column chromatography

Checking with the aid of radioactive tracers whether there is 100% recovery and high radiochemical purity of the analyte fraction

Basic steps in devising high-accuracy RNAA method

No Yes

NEUTRON IRRADIATIONsamples, standards,

CRMs, blanks, neutron flux monitors

Radiochemical separation - samples and one standard

gamma-ray spectrometric measurements of samples and all standards

Checking if all qualifying criteria have been met

No

result is not accepted

Result is accepted as obtained by the high-accuracy method

Yes

NoYes

The high-accuracy RNAA method is designed and elaborated according to rules assuring high accuracy:

•All conditions should be optimized enabling selective isolation of an element with 100% yield confirmed by tracer experiments,

•All potential sources of errors starting from sampling and sample dissolution up to gamma-spectrometric measurements should be identified at a stage of elaborating the method, and removed or appropriate corrective actions introduced into the procedure.

•Whenever possible, the color of the ion in question (or its complex) added as a carrier should be used for visual control to safeguard against unexpected failure of the separation procedure

•With each set of samples at least two standards should be irradiated, one of which is later on processed exactly as are the samples and the other is not. The specific activities of both standards should agree within predetermined limits;

•Residual blank resulting from the contact of the sample with sample container should be measured in each series of determinations;

In principle, the method should be a single element one while all parameters can be optimized with respect to this particular element.

The method is equipped with some classification criteria providing protection against making gross errors. The criteria are as follows:

•Visual control of the correctness of the separation procedure: in the retention stage on the chromatography columns the color band of analyte should not travel more than 1/3 of the bed length

•Residual blank (may originating form the contact of the sample with packing material) is small (below a few percent of the analyte contents in analyzed sample)

•Agreement of standards: the standard which has passed the whole analytical procedure should agree within predetermined limits (ca. few percent) with that measured directly

•The result for at least one CRM irradiated and analyzed together with the samples should be in agreement with the certified value

The result has to fulfil all of criteria to be accepted.

100 200 300 400 500 600 700 800 900 1000 1100 1200

0

1000

2000

3000

4000

115 C

d 5

27.7

keV

115 C

d 4

92.5

keV

115 C

d 2

61 k

eV11

5 Cd

- 115m

In 3

36.6

keV

115 C

d 2

31 k

eV

Cou

nts

per

chan

nel

Energy, keV

The limit of detection of the method has amounted to 0.5 ng g-1.

The results obtained by this method are in excellent agreement with certified values for the CRMs spanning the Cd concentration range of more than 5 orders of magnitude.

10-3 10-2 10-1 100 101 102 103

10-3

10-2

10-1

100

101

102

103

Determination of Cd by the definitive methodin the certified reference materials

8

7

5

43

2

1

y = 0.98805x + 0.02852

R2 = 0.99992

6

5. INCT-MPH-2 Mixed Polish Herbs6. CTA-OTL-1 Oriental Tobacco Leaves7. IAEA MA-M-2 Mussel Tissue8. IAEA H-8 Horse Kidney

1. Versieck's Human Serum2. IAEA H-4 Animal Muscle3. INCT-TL-1 Tea Leaves4. IAEA H-9 Human Diet

Ex

pe

rim

en

tal v

alu

es

g

g-1

Certified values g g-1

1E-10 1E-9 1E-8 1E-7 1E-6 1E-5 1E-4 1E-30

20

40

60

80

100

120

140

(Horwitz curve) x 1/2

Horwitz curve

Thompson and Lowthian curve

RS

D (

%)

Mass fraction, (g g -1)

The results are also precise as can be seen when the results are shown on the background of Horwitz and Thompson and Lowthian curves representing variation of reproducibility precision

This method can be also apply for Ni determination when the samples are irradiated with the high flux of fast neutrons by reaction

58Ni(n,p)58Co

The limit of detection for Co amounted to 4x10-2 ng g-1 and 20 ng g-1 for Ni.

High accuracy RNAA method for the determination of Mo in biological materials

Nuclear reaction used for Mo determination

98Mo(n,γ)99Mo 99mTc

may be subjected to interference originating from the nuclear fission reaction

235U(n,f)99Mo 99mTc

The elaborated analytical scheme allows for the simultaneous determination of trace amount of Mo and U in biological materials.

Uranium is determined by the reaction:

238U(n,γ)239U 239Np

The detection limits amounted 0.15 ng g-1 for U and 2.5 ng g-1 for Mo.

ng g-1 10.4 ng g-1

Uncertainty budget for Co determination in INCT-Tl-1 Tea Leaves

source of uncertainty unit valueRel. std.

uncertainty, %Mass of sampleMass of standardMass of neutron flux monitorResidual blankNeutron flux gradientNeutron self-shielding/scatteringSample counting statisticsStandard counting statisticsCounting positioning of sampleCounting positioning of standardPulse pile-up effect - samplePulse pile-up effect – standardPeak calculation – samplePeak calculation – standardRadiochemical separation

mgmgmgng

2502050

0.011.00±0.025

3000030000

1.00 ± 0.06

0.10.10.10.10.10.20.60.80.30.40.10.10.20.20.4

Mass fraction of Co: 399 ng g-1 10.4 ng g-1 (k=2) (2.6%)

Certified value 387 ng g-1 42 ng g-1

Uncertainty budget for Mo determination in NIST 1547 Peach Leaves

source of uncertainty unit valueRel. std.

uncertainty, %Mass of sampleMass of standardResidual blankNeutron flux gradientInterfering reactionNeutron self-shielding/scatteringSample counting statisticsStandard counting statisticsCounting positioning of sampleCounting positioning of standardPulse pile-up effect - samplePulse pile-up effect – standardPeak calculation – samplePeak calculation – standardRadiochemical separation

mgmgmg

ng

Bq /countsBq /counts

150200

1.00± 0.0252

3000150000

1.00 ± 0.01

0.10.10.10.12.10.21.80.40.30.30.10.10.20.20.6

Mass fraction of Mo: 58 ng g-1 3.4 ng g-1 (k=2) (5.8%)

Certified value 60 ng g-1 8 ng g-1

In 1998 CCQM adopted the following definition of primary method of measurement (PMM).

A primary method of measurement is a method having the highest metrological qualities, whose operation can be completely described and understood, for which a complete uncertainty statement can be written down in terms of SI units.

A primary direct method measures the value of an unknown without reference to a standard of the same quantity.

A primary ratio method measures the value of a ratio of an unknown to a standard of the same quantity, its operation must be completely described by a measurement equation.

As can be seen for the high-accuracy RNAA methods, the expanded uncertainty (k=2) varies from 2.6% for more favorable case of Co determination to 5.8% for the less favorable case of Mo determination. Such values are characteristic for the methods of the highest metrological qualities.

The high-accuracy RNAA methods have the detection limits of the order of ng g-1 and provide the results with very low value of uncertainty, traceable to SI units. The methods can be fully described by measurement equations.

The above statement means that the high-accuracy RNAA method fulfils the CCQM definition of a primary ratio method (PMM) and can be complementary to isotope-dilution mass spectrometry (IDMS).

IDMS is the only one trace analysis method indicated by the CCQM so far as PMM.

From the point of view of QA/QC of chemical measurements it is very important that at least two trace analysis PMMs based on different chemical principles can be available. Despite of the advantages of IDMS, this method has also some limitations like the all other analytical techniques.

The high-accuracy RNAA method can serve for checking accuracy of the other methods used in analytical chemistry and can be applied in the certification of CRMs.

Preparation and certification of CRMs

•In the INCT the certification of the candidate reference materials is performed on the basis of worldwide ILC.

•The applied methods of data evaluation and classification criteria have been the same with only minor modifications.

•The adopted approach of data evaluation is based on outlier’s rejection procedure which uses concurrently four statistical criteria (Dixon, Grubbs, skewness and kurtosis) at the significance level of 0.05, followed by calculation of overall means of laboratory means remaining after outlier rejection, standard deviations, confidence intervals etc.

Whenever possible, the process of assigning of reference values is confirm by comparison of assigned values with results obtained by the high-accuracy RNAA method.

Assigning of certified value for Cd in CTA-OTL-1

0 0,5 1 1,5 2 2,5 3 3,5 4 4,5

High-accuracy RNAA result

Certified value

Range after outlier rejection

Original data range

The same material CTA-OTL-1 was analyzed in the course of ILCs organized by the INCT

•in the year 1991 (as a candidate reference material CTA-OTL-1)

•and the later years 2000 and 2003 (as an unknown to participants reference material accompanying new candidate RMs).

In the following figures the comparison of certified values with the values obtained by the high-accuracy RNAA method can be seen.

Cd

199019921994199619982000200220042006

0,5 0,7 0,9 1,1 1,3 1,5

mg/kg

high-accuracy RNAA method

Co

199019921994199619982000200220042006

0,4 0,6 0,8 1 1,2 1,4

mg/kg

high-acuracy RNAA method

Ni

199019921994199619982000200220042006

3 4 5 6 7 8 9

mg/kg

high-accuracy RNAA method

Zn

1990

1995

2000

2005

40 45 50 55 60 65

mg/kg

In all figures, the good agreement of originally certified values with those that could be assigned on the basis of results from 2000 and 2003 ILCs has been obtained. The mean values with their

confidence limits always overlap.

The assigned values agree well with the results obtained by the high-accuracy RNAA methods.

Good agreement of the mean values obtained in the period of more than 10 years, as well as with the results obtained by the high-accuracy RNAA methods shows a correctness of the adopted procedure of assigning certified values.

Concluding:

INAA procedures applied at the INCT enable the determination of up to 23 elements in one sample

INAA procedure has been elaborated and used for testing of homogeneity of candidate RMs

Many RNAA procedures have been elaborated enabling radiochemical separation of selected elements from various irradiated materials and their determination at very low level

The high-accuracy RNAA methods meeting requirements of PMM have been elaborated for Cd, Co, Cu, Ni, Mo and U determination in biological materials.

Thank you for your attention.