INTENDED USE OF THE IAEA REFERENCE MATERIALS PART II: EXAMPLES ON REFERENCE MATERIALS CERTIFIED FOR STABLE ISOTOPE COMPOSITION

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1 INTENDED USE OF THE IAEA EFEENCE MATEIALS PAT II: EXAMPLES ON EFEENCE MATEIALS CETIFIED FO STABLE ISOTOPE COMPOSITION M. G r ö n i n g a, b, K. F r ö h l i c h a International Atomic Energy Agency Isotope Hydrology Section A V i e n n a, A u s t r i a P. De egge, P.. Danesi b International Atomic Energy Agency Agency s Laboratories A S e i b e r s d o r f, A u s t r i a 1 INTODUCTION In this paper IAEA reference materials distributed by the Isotope Hydrology Unit will be discussed. 1 These reference materials a r e i n t e n d e d f o r t h e d e t e r m i n a t i o n o f t h e r a t i o s o f s t a b l e i s o t o p e s o f l i g h t elements such as hydrogen, carbon, nitrogen, oxygen and sulfur in environmental materials. Some of the available materials are primary reference materials and define conventional scales for reporting of measurement results. An overview on other IAEA reference materials and the general scope of the Analytical Quality Control Services (AQCS) is given in an accompanying paper 2 a s w e l l a s i n t h e b i -a n n u a l l y p u b l i s h e d AQCS catalogu e. 3 The Isotope Hydrology Unit is also responsible for the distribution of 14 C quality assurance materials. These materials are intended for laboratories performing 14 C activity measurements on n a t u r a l m a t e r i a l s a n d a t e n v i r o n m e n t a l a c t i v i t y l e v e l s. A d e t a i l e d d e s c r i p t i o n o f t h e s e m a t e r i a l s a n d t h e i r c h a r a c t e r i s t i c s c a n b e f o u n d e l s e w h e r e Common Characteristics of the Stable Isotope eference Materials The reference materials distributed by the Isotope Hydrology Unit are chemically pure c o m p o u n d s such as carbonates, sulfates, sulfides, nitrates, graphite or polyethylene, or nearly pure natural m a t e r i a l s l i k e d i s t i l l e d w a t e r, c a r b o n a t e r o c k, s i l i c a t e s, r e f i n e d o i l, s u g a r, c e l l u l o s e a n d s i m i l a r c o m p o u n d s. 1 These materials are distinct from the other IAEA environmental matrix materials. M o s t of these materials have been prepared to serve as reference materials. Therefore much care w a s t a k e n i n t h e i n i t i a l p u r i f i c a t i o n a n d h o m o g e n i s a t i o n o f t h e r a w m a t e r i a l. The majority of all 30 stable isotope reference materials was produced by scientists under IAEA technical contracts or by cooperating institutions like the National Institute for Standards and Technology (NIST) in USA or the United States Geological Survey (USGS). Some of the materials were produced directly by the IAEA Isotope Hydrology Unit. In most cases the recommended values for stable isotope ratios in these materials were determined by interlaboratory comparison exercises. For those materials issued already twenty or more years ago, the number of participating laboratories was rather limited (in some cases l e s s t h a n t e n ).

2 1. 2 T e r m s u s e d f o r d i f f e r e n t c a t e g o r i e s o f r e f e r e n c e m a t e r i a l s i n t h i s p a p e r Several d e f i n i t i o n s f o r i n t e r n a t i o n a l d i s t r i b uted reference materials and for internal laboratory standards have to be clearly distinguished. Unfortunately there are no clear guidelines on the d e f i n i t i o n s t o b e u s e d i n t h i s f i e l d. S o m e o f t h e e x p r e s s i o n s a r e a m b i g u o u s, s o m e o t h e r u s e d i n different context by different authors. For this paper the d e f i n i t i o n s u s e d a r e e x p l a i n e d b e l o w. These d e f i n i t i o n s used for the various kind of materials should be clearly distinguished from each o t h e r : P r i m a r y r e f e r e n c e m a t e r i a l 1 ( o r i n t e r n a t i o n a l s t a n d a r d ) 6 : a natural, synthetic or virtual material versus which, by general agreement, the relative variations of stable isotope ratios in natural c o m p o u n d s a r e e x p r e s s e d. I t i s u s e d t o d e f i n e a c o n v e n t i o n a l s c a l e f o r r e p orting variations of s t a b l e i s o t o p e r a t i o s. C a l i b r a t i o n m a t e r i a l 1 ( o r p r i m a r y s t a n d a r d ) 6 : a n a t u r a l o r synthetic compound which has been carefully calibrated versus the primary reference material, and which calibration values have b e e n internationally agreed and adopted. It is used in case the primary reference material is not existing or available to calibrate measurements and instruments. Each primary reference material can be referred to as a calibration material as well. eference material (M): a natural or synthetic compound which has been carefully calibrated versus the primary reference material and which property values are sufficiently homogeneous a n d w e l l e s t a b l i s h e d a n d a r e a s s o c i a t e d w i t h w e l l d e t e r m i n e d u n c e r t a i n t i e s. I t i s u s e d t o c a l i b r a t e laboratory equipment and measurement methods for analysis of materials of a composition different from that of the primary reference material. The available reference materials cover a b r o a d s p e c t r u m o f c h e m i c a l c o m p o s i t i o n s a n d a w i d e r a n g e o f s t a b l e i s o t o p e r a t i o s. M o s t e x i s t i n g stable isotope ratio reference materials were first investigated in interlaboratory comparison exercises and distributed as intercomparison materials. Since they were used de facto as reference materials to calibrate equipment and measurements, some years ago their status was adjusted accordingly. I n t e r c o m p a r i s o n m a t e r i a l : a natural or synthetic compound with proven homogeneity which provide the means to check the overall quality of measurements performed i n c o m p a r i s o n w i t h t h a t of other laboratories. Its isotopic composition is computed by averaging the results of several l a b o r a t o r i e s o b t a i n e d i n i n t e r l a b o r a t o r y c o m p a r i s o n e x e r c i s e s a n d i n i n d i v i d u a l a s s a y s, a f t e r e l i m i n a t i o n o f o u t l i e r s u s i n g a 2σ i n t e r v a l c r i t e r i o n. 7 W o r k i n g s t a n d a r d ( o r t r a n s f e r s t a n d a r d ) 6 : This term in stable isotope mass spectrometry is somewhat misleading since it is not describing a standard, but an arbitrarily chosen gas used as a reference for analysis o f i s o t o p e r a t i o s o f s a m p l e s i n a d u a l -i n l e t m a s s s p e c t r o m e t e r ( s e e s e c t i o n 1.3). All measurements of prepared samples and reference materials are made versus this working s t a n d a r d a n d a r e l a t e r c o n v e r t e d i n t o a n i n t e r n a t i o n a l a c c e p t e d s c a l e. A b e t t e r t e r m would be laboratory reference gas I n t e r n a l s t a n d a r d ( o r r e f e r e n c e s t a n d a r d ) 6 : This term describes materials which are carefully s e l e c t e d a n d of similar composition a s t h e normal samples and which are used routinely as a s t a n d a rd to calibrate or check measurements and the measuring instruments. Page 2 o f 1 1

3 1. 3 C o n v e n t i o n a l S c a l e s f o r e p o r t i n g S t a b l e I s o t o p e a t i o s The analysis for stable isotope ratios is almost exclusively performed by mass spectrometry. The samples are firs t l y c o n v e r t e d i n t o a s u i t a b l e g a s c o n t a i n i n g i s o t o p e s o f t h e e l e m e n t u n d e r i n v e s t i g a t i o n a n d a r e t h e n a n a l y s e d i n d u a l -i n l e t m a s s s p e c t r o m e t e r, w h e r e t h e i s o t o p i c c o m p o s i t i o n o f t h e s a m p l e gas is directly compared with that of a gas working standard. The a n a l y t i c a l p r o c e d u r e i n c l u d e s a l s o the measurement of internal laboratory standards of similar chemical composition as the sample. The internal laboratory standards have to be carefully calibrated versus the available reference materials a n d e n s u r e t h e t r a ceability of results. The n a t u r a l v a r i a t i o n s o f t h e s t a b l e i s o t o p e r a t i o s o f l i g h t e l e m e n t s a r e r a t h e r s m a l l. F o r e x a m p l e, the total range of the variation of 18 O/ 16 O in water and ice on earth is in the order of only seven p e r c e n t a n d i n c r e a s e s t o a b out ten percent for all natural oxygen bearing materials. 8 For hydrogen isotopes with their large relative mass differences the variations of 2 H/ 1 H reach higher values of about 80 % for natural hydrogen bearing compounds. In virtually all application s o f s t a b l e i s o t o p e s i n e a r t h s c i e n c e s t h e r e l a t i v e d e v i a t i o n o f t h e i s o t o p i c r a t i o f r o m a s t a n d a r d i s o f i n t e r e s t r a t h e r t h a n t h e a b s o l u t e i s o t o p i c r a t i o o f t h e g i v e n s a m p l e. Therefore, a material is selected as a primary standard and its stable isotope r a t i o d e f i n e s t h e zero p o i n t o f a r e l a t i v e c o n v e n t i o n a l s c a l e. T h e e a s i e s t e x a m p l e i s n i t r o g e n, w h e r e t h e i s o t o p i c c o m p o s i t i o n o f n i t r o g e n i n a t m o s p h e r i c a i r i s t a k e n a s t h e p r i m a r y s t a n d a r d. F o r c o n v e n i e n c e t h e m e a s u r e m e n t s a r e n o t r e p o r t e d a s i s o t o p e ra t i o s, b u t g i v e n a s r e l a t i v e d e v i a t i o n f r o m t h e i s o t o p e r a t i o of the standard according to the following formula: δ = SA ST ST (1) w i t h δ (e.g. δ 2 H, δ 13 C, δ 15 N, δ 18 O, δ 34 S) being the normalised difference of the isotope concentration r a t i o s ( 2 H/ 1 H, 13 C/ 12 C, 15 N/ 14 N, 18 O/ 16 O, 34 S/ 32 S) of the sample and the standard (e.g. δ 15 N with air N 2 as standard). As the differences between sample and standard are normally very small, the δ-v a l u e s a r e usually expressed as per mille difference (parts per thousan d ). 9 The accuracy of measurements in experienced laboratories is in the order of ±0.1 for δ 18 O and ±1 for δ 2 H o r e v e n b e t t e r. T h i s i s p o s s i b l e d u e t o t h e s p e c i a l a n a l y s i s a r r a n g e m e n t w i t h d i r e c t c o m p a r i s o n o f s a m p l e i s o t o p e r a t i o s t o t h o s e o f t h e working standard during the measurement and due t o t h e d a i l y c a l i b r a t i o n o f t h e m a s s s p e c t r o m e t e r a n d t h e s a m p l e p r e p a r a t i o n l i n e s w i t h s u i t a b l e i n t e r n a l s t a n d a r d s ( i n t e r n a l l a b o r a t o r y s u b -s t a n d a r d s c a l i b r a t e d a g a i n s t i n t e r n a t i o n a l r e f e r e n c e materials). The s o c a l l e d δ-scales for stable isotope ratios of the elements hydrogen, carbon, nitrogen, oxygen a n d s u l f u r a r e g o o d e x a m p l e s f o r c o n v e n t i o n a l s c a l e s. T h e k n o w l e d g e o n t h e a b s o l u t e i s o t o p e r a t i o o f a given material is not necessary for their use, s ince these scales are defined completely arbitrarily versus a chosen primary reference material. For oxygen and hydrogen ocean water was chosen as the l a r g e s t a c c e s s i b l e r e s e r v o i r o n e a r t h a n d i n v i e w o f t h e s l i g h t v a r i a t i o n s i n i s o t o p i c c o m p o s i t i o n t h r o u ghout the oceans a hypothetically well mixed mean ocean water was selected as the reference (called SMOW - Standard Mean Ocean Water). This is a good example for a scale based on a virtual material not realised in nature. In case of carbon, marine carbonate of organic origin in a certain geological formation in the USA was selected (PeeDee Belemnite, PDB); for nitrogen, atmospheric nitrogen was the obvious choice; and for sulfur, material from a meteorite was selected and expected t o b e r e p r e s e n t a t i v e f o r t he mean cosmic abundance of sulfur isotopes (Cañon Diablo Troilite, CDT). However, t h e m a i n p e r s i s t i n g p r o b l e m w a s t h e p r o p e r c a l i b r a t i o n b e t w e e n d i f f e r e n t l a b o r a t o r i e s since the primary standard for oxygen and hydrogen did not exist at all, and both original materials Page 3 o f 1 1

4 defining the δ-scales for carbon and sulfur (PDB and CDT) were exhausted since some time. The IAEA therefore initiated the production of new reference materials which were calibrated with the b e s t a v a i l a b l e t e c h n i q u e s r e l a t i v e t o t h e o r iginal δ-scales of the respective elements. Beside many o t h e r m a t e r i a l s, t h e s e a c t i v i t i e s r e s u l t e d i n t h e p r o d u c t i o n o f t h e t h r e e m a t e r i a l s V S M O W ( w a t e r ), NBS19 (carbonate), and IAEA-S-1 (silver sulfide), which were adopted by international agreement as cal i b r a t i o n m a t e r i a l s a f t e r e x t e n s i v e t e s t s b y e x p e r i e n c e d l a b o r a t o r i e s. T h e s e c a l i b r a t i o n m a t e r i a l s were used to define the zero-p o i n t o f n e w δ-scales named VSMOW, VPDB and VCDT which should be as close as possible to the original SMOW, PDB and CDT δ-s c a l e s (the V standing for Vienna in a l l t h r e e c a s e s ). As an example the development of the scales in the case of hydrogen and oxygen isotope ratio s t a n d a r d s w i l l b e p r e s e n t e d D e f i n i t i o n o f t h e S c a l e s f o r e p o r t i n g H y d r o g e n a n d O x y g e n I s o t o p e a t i o s F i r s t t h e h i s t o r i c a l d e v e l o p m e n t i s p r e s e n t e d t o e s t a b l i s h t h e s c a l e s f o r r e p o r t i n g h y d r o g e n a n d oxygen isotope ratios, primarily intended for measurements on water samples and extended to other hydrogen and oxygen bearing materials. Already in 1953 average ocean water was suggested and used as reference point for measurements. 10 Since no average ocean water exists, this concept was refined in 1961 by defining the hypothetical Standard Mean Ocean Water (SMOW). 11 I t s i s o t o p i c c o m p o s i t i o n w a s d e f i n ed as a weighted average o f t h e a v a i l a b l e m e a s u r e m e n t s o f t h e i s o t o p i c c o m p o s i t i o n i n t h e m a i n o c e a n i c w a t e r m a s s e s. B u t s i n c e S M O W w a s j u s t a c o n c e p t a n d n e v e r e x i s t e d a s r e a l w a t e r s a m p l e, i t c o u l d n t b e u s e d f o r calibration of laboratory measurements. However, t h e i s o t o p i c r a t i o s o f S M O W w e r e d e f i n e d w i t h r e s p e c t t o t h e e x i s t i n g w a t e r s t a n d a r d NBS-1 12 of the US National Bureau of Standards, used earlier for an interlaboratory comparison. So for the first time a physically existing material was used t o c a l i b r a t e d i f f e r e n t l a b o r a t o r i e s t o t h e SMOW scale. NBS-1 was readily available for world-w i d e d i s t r i b u t i o n t o g e t h e r w i t h a n o t h e r w a t e r s t a n d a r d c a l l e d N B S -1 A o b t a i n e d f r o m m e l t e d s n o w w i t h a n l o w e r a b u n d a n c e o f t h e h e a v i e r i s o t o p e s. During an IAEA interlaboratory comparison in 1965 serious doubts were confirmed concerning the preservation of the NBS-1 w a t e r s t a n d a r d a n d p o s s i b l e c h a n g e s o f i t s i s o t o p i c c o m p o s i t i o n. At an IAEA Panel Meeting in 1966 it was therefore recommended to establish a pair of two new s t a n d a r d s, t h e f i r s t o n e b e i n g a s c l o s e a s p o s s i b l e t o t h e d e f i n e d S M O W a n d t h e o t h e r o n e w i t h a n a b u n d a n c e o f t h e h e a v i e r i s o t o p e s c l o s e t o t h e l o w e s t l i m i t s o b s e r v e d i n n a t u r a l w a t e r. T h e n e w s t a n d a r d w i t h a n i s o t o p i c c o m p o s i t i o n a s c l o s e as possible to SMOW was prepared by H. Craig. It was obtained by mixing distilled ocean water with small amounts of other waters in order to a d j u s t i t s i s o t o p i c c o m p o s i t i o n a s c l o s e a s p o s s i b l e t o t h a t o f t h e d e f i n e d S M O W. T h i s t a s k w a s c o m p l i c a t e d d u e t o t h e r e q u i r e d a d j u s t m e n t o f b o t h t h e i s o t o p i c c o m p o s i t i o n o f h y d r o g e n a n d o f oxygen. This standard was ready in 1968 and was called Vienna Standard Mean Ocean Water (VSMOW). According to the control analyses performed by Craig VSMOW has the same 18 O/ 16 O r a t i o as the defined SMOW, but a slightly lower 2 H/ 1 H ratio or respectively a slightly negative δ 2 H v a l u e (-0.2 ). However, this slight difference is about a factor of four to five lower than the analytical u n c e r t a i n t y o f m o s t l a b o r a t o r i e s. A b s o l u t e i s o t o p e r a tios on VSMOW were determined for 18 O/ 16 O, 13 for 17 O/ 16 O 14 as well as for 2 H/ 1 15, 16, 17 H. The second standard was obtained by E. Picciotto from melting a firn sample at Plateau Station, Antarctica. This material was named Standard Light Antarctic Precipitation (SLAP). The absolute isotope ratios of SLAP were determined only for 2 H/ 1 H, 15, 16, 17 a b s o l u t e 18 O/ 16 O ratios for SLAP were computed from those of VSMOW by using its assigned value of δ 18 O= versus VSMOW (see below). 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5 All four materials VSMOW, SLAP, NBS-1, NBS-1A (the latter both transferred to IAEA from the National Bureau of Standards) were then distributed by the IAEA Isotope Hydrology Laboratory. In a n I A E A C o n s u l t a n t s M e e t i n g w a s c o n v e n e d i n o r d e r t o d i s c u s s t h e i s o t o p e r e s u l t s o n t h e s e s t a n d a r d s o b t a i n e d s o f a r f r o m l a b o r a t o r i e s a n d t o a d v i s e o n f u t u r e a c t i o n o n s t a n d a r d i z a t i o n o f s t a b l e i s o t o p e m e a s u r e m e n t s. 18 T h e r e c o m m e n d a t i o n o f t h e e x p e r t s co n c e r n i n g w a t e r s t a n d a r d s w a s t o e x p r e s s a l l f u t u r e r e s u l t s a s δ-v a l u e s r e l a t i v e t o V S M O W i n o r d e r t o r e s o l v e c o n f u s i o n o n r e s u l t s e x p r e s s e d i n n o n -c o r r e s p o n d i n g scales. It was stated that the coherence between δ-values reported by different laboratories c o u l d b e improved by adopting fixed δ-values for a second water reference standard. The experts recommended to adopt SLAP for this purpose and to normalize the 18 O and 2 H δ-s c a l e s r e l a t i v e t o t h i s s t a n d a r d. I n T a b l e 1 t h e δ-v a l u e s a r e l i s t e d w i t h t h o s e o f VSMOW being by definition at zero and those of SLAP established by the weighted mean of results from different laboratories. The established δ 2 H-value for SLAP is in remarkable good agreement with the available absolute hydrogen isotope ratio d e t e r m i n a t i o n o f t h e t w o m a t e r i a l s u s i n g i s o t o p e d i l u t i o n m e t h o d s ( d e v i a t i o n e x p r e s s e d a s δ 2 H smaller t h a n 0. 5 ). Table 1 O x y g e n a n d h y d r o g e n δ-v a l u e s v e r s u s V S M O W a s s i g n e d t o t h e e x i s t i n g w a t e r r e f e r e n c e materials (NBS -1 and NBS -1A δ-v a l u e s a r e r e p o r t e d v e r s u s S M O W) Name δ 18 O [ ] δ 2 H [ ] VSMOW 0 0 SLAP GISP ± ± 1. 0 NBS NBS-1 A The definition of VSMOW as zero of the oxygen and hydrogen δ-s c a l e s a n d t h e a d o p t i o n o f f i x e d δ-values for SLAP is therefore a slight modification of the original definition of a δ-scale in formula ( 1 ) : δ = S A ST ST δ SLAP SLAP VSMOW VSMOW, ( 2) w i t h t h e a d d i t i o n a l t e r m i n t h e b r a c k e t b e i n g t h e n o r m a l i z a t i o n o f t h e r e s p e c t i v e s c a l e i n t e r m s o f pre-defined hydrogen and oxygen isotope ratios o f t h e t w o s t a n d a r d s ( t a b l e 1 ). The two scales defined in equation (1) for SMOW and in (2) for VSMOW coincide only if S M O W V S M O W a n d i f t h e a d o p t e d δ-values for SLAP in table 1 correspond to the true ones as defined by e q u a t i o n ( 1 ) f o r b o t h h ydrogen and oxygen. From the reported measurements of NBS1 and VSMOW a slight offset of the zero-p o i n t o f t h e t w o s c a l e s c o u l d b e c o n c l u d e d ( o f f s e t s o f a n d 0. 5 f o r δ 18 O and δ 2 H, respectively), but which was well within the limits of measurement uncertainty of most l a b o r a t o r i e s. 19 Both offsets were a bit larger t h a n evaluated before by Craig. However, due to the s c a t t e r o f t h e i n d i v i d u a l r e s u l t s t h e s t a t e d m e a n o f f s e t s h o u l d n o t b e a p p l i e d f o r c o n v e r s i o n f r o m o n e s c a l e t o t h e o t h e r. A t h i r d w a t e r s tandard was proposed during the same meeting in w i t h a n i s o t o p i c composition intermediate between VSMOW and SLAP. This material was obtained from Greenland firn in 1978 and was called GISP (Greenland Ice Sheet Precipitation). G I S P i s i n t e n d e d t o p r o v e t h e successful calibration as performed with VSMOW and SLAP and the linearity of the measuring system. esults of two interlaboratory comparisons investigating GISP are published in IAEA Page 5 o f 1 1

6 r e p o r t s. 20, 7 It was noted that the accordance of results from different laboratories improved by a factor of more than two when the data were normalized using SLAP as second standard in addition to VSMOW. 2 INTENDED USE OF EFEENCE MATEIALS 2.1 ecent situation for IAEA Ms The r e f e r e n c e m a t e r i a l s d e d i c a t e d f o r t h e d e t e r m i n a t i o n o f s t a b l e i s o t o p e r a t i o s a t e n v i r o n m e n t a l level are distributed by the IAEA and on its behalf by NIST. A considerable number of units of these reference materials are distributed per year by the IAEA (figure 1) in spite of the established rigid rule on the distribution of these reference materials allowing only the order of one unit of each material per laboratory in a given three years period (see paragraph below) C-14 S-34 N-15 C-13 H-2,O-18 Figure 1 N u m b e r o f r e f e r e n c e m a t e r i a l s d i s t r i b u t e d b y t h e I A E A I s o t o p e H y d r o l o g y U n i t p e r y e a r. T h e s e f i g u r e s i n c l u d e a l l t y p e s o f M s a n d a l s o t h o s e M s w h i c h a r e d i s t r i b u t e d f r e e o f charge (e.g. 14 C ), f o r c a l i b r a t i o n s t u d i e s o f M s a n d f o r i n t e r l a b o r a t o r y c o m p a r i s o n s. Most of these IAEA reference materials are described by eports of Investigation produced by the National Institute of Standards and Technology in USA. The IAEA Isotope Hydrology Laboratory i s i n t h e p r o c e s s o f u p d a t i n g t h e a v a i l a b l e i n f o r m a t i o n a c c o r d i n g t o t h e r e q u i r e m e n t s a s s t a t e d i n ISO-G u i d e 3 1 a n d i t s r e c e n t r e v i s i o n. 21 In the available NIST reports, the intended use of these materials is stated as in the following example for VSMOW, SLAP, GISP: These efe rence Materials are intended to provide samples of known isotopic composition with 2 H/ 1 H and 18 O/ 16 O isotope ratios stated in parts per thousand difference ( ) from the VSMOW isotope-ratio standard. These Ms are not certified, but their use allows comparability of stable hydrogen and oxygen isotope-ratio data obtained by investigators in different laboratories. VSMOW and SLAP are intended for stable hydrogen and oxygen isotope-ratio calibration of water samples and of mass spectrometers for isotope-ratio analysis of all other oxygen- or hydrogen-bearing substances. 22 A similar statement is provided for the other reference materials in other issued NIST eports of Investigation. In the following sections, these IAEA reference materials will be discussed in respect to the ISO G u i d e recommendations. Page 6 o f 1 1

7 S t o r a g e a n d T r a n s f e r o f a p r o p e r t y v a l u e. 23 Individual units of the water and gas reference materials are stored in sealed 20 ml glass ampoules ready for distribution and any remaining bulk m a t e r i a l i s s t o r e d i n s e a l e d t e n l i t e r g l a s s c o n t a i n e r s. W h e n o p e n i n g a s e a l e d g l a s s c o n t a i n e r t o f i l l n e w s a m p l e s f o r d i s t r i b u t i o n, t h e i s o t o p i c c o m p o s i t i o n o f t h e w a t e r r e f e r e n c e m a t e r i a l i n t h e f i l l e d ampoules is always compare d against ampoules filled earlier from other containers. This cross-c h e c k verifies the proper storage of the materials and excludes the possibility of any significant changes of t h e i s o t o p i c c o m p o s i t i o n d u r i n g s t o r a g e. In case of the other solid reference materials they consist of materials which can be stored under a m b i e n t l a b o r a t o r y c o n d i t i o n s i n a i r -tight cap-s e a l e d b o t t l e s a n d c o n t a i n e r s. I n s o m e i n s t a n c e s t h e head space air is replaced by dry nitrogen or argon. Two carbonate materials are stored in des i c c a t o r s under vacuum to prevent any reaction and isotopic exchange between the carbonate material, air m o i s t u r e a n d a i r c a r b o n d i o x i d e E s t a b l i s h i n g t r a c e a b i l i t y o f t h e m e a s u r e m e n t r e s u l t. 23 In case of the primary reference material VSMOW or the calibration materials NBS19 and IAEA-S-1 t h e t r a c e a b i l i t y t o S I u n i t s, i n t h i s c a s e t h e m o l e, i s a c h i e v e d t h r o u g h t h e d e t e r m i n a t i o n o f t h e a b s o l u t e i s o t o p e r a t i o s o f t h e s e m a t e r i a l s using a definitive analysis method (isotope d i l u t i o n m e t h o d ). I n s p i t e o f t h i s, u n t i l n o w t h e s e m a t e r i a l s w e r e n o t c o n s i d e r e d a s c e r t i f i e d M s. O n e r e a s o n i s t h e l a c k o f i n f o r m a t i o n o n s o m e d e t a i l s o f t h e formerly performed measurements (up to 28 years ago) required nowadays for a certification. In any c a s e a n a b s o l u t e i s o t o p e r a t i o d e t e r m i n a t i o n i s q u i t e u s e f u l, b e i n g a n a d d i t i o n a l m a t e r i a l -i n d e p e n d e n t information which can facilitate in future the production of successor materials. However, the main p u r p o s e o f t h e s e c a l i b r a t i o n m a t e r i a l s i s t o g i v e the means to the laboratory to trace their measurements back to an international accepted uniform δ-s c a l e. Virtually all other reference materials were established through interlaboratory comparison e x e r c i s e s. F r o m a p r a c t i c a l p o i n t o f v i e w, t h e s e Ms are in most cases the only available reference material for a specific type of substances and are therefore needed to calibrate any sample measurements versus a common scale. Some of these Ms should be considered as being traceable back only to the originating interlaboratory comparison exercise, especially those determined in the Seventies with a lack of available information on the formerly used methods, the laboratory c a l i b r a t i o n p r o c e d u r e a n d t h e u n c e r t a i n t y. T h i s l i m i t a t i o n s h o u l d c e r t a i n l y n o t a p ply to these Ms which were tested only by few selected laboratories. In this case the traceability chain could be followed back directly to the primary reference material provided that these laboratories have given a detailed uncertainty budget for their measurements and sufficient information on the analysis p r o c e d u r e s a n d p r o v i d e d a l l l a b o r a t o r y m e a n v a l u e s a n d t h e i r u n c e r t a i n t i e s w e r e t a k e n p r o p e r l y i n t o a c c o u n t f o r t h e f i n a l e v a l u a t i o n w i t h o u t a p u r e s t a t i s t i c a l l y d r i v e n o u t l i e r r e j e c t i o n. D u r i n g t h e o ngoing evaluation of the available reference materials their traceability will be re-a s s e s s e d D e t e r m i n i n g t h e u n c e r t a i n t y o f t h e m e a s u r e m e n t r e s u l t s. 23, 24 The reporting of uncertainty of measurement results is requested from participants in IAEA interlaboratory comparisons. However, in many cases only some of the sources of uncertainty are stated like the standard deviation of individual determinations or no uncertainty statement is provided at all. Only a few laboratories comply by stating a total uncertainty budget for their measurement process. For future assessments of the suitability of reference materials only such laboratories should be considered which provide sufficient data on their measurement uncertainty. This would certainly eliminate some of the major constraints hindering the full use of interlaboratory comparison exercises for the characterization of new reference materials in terms of traceability. As long as the principle of an uncertainty budget is not implemented i n a l l l a b o r a t o r i e s, a p r o v i s i o n a l a p p r o x i m a t i o n o f t h e c o m b i n e d s t a n d a r d u n c e r t a i n t y f o r m e a s u r e m e n t s c o u l d b e t h e a s s e s s m e n t o f i n t e r n a l s t a n d a r d s a n d t h e i r l o n g t e r m b e h a v i o u r. T h e s t a n d a r d d e v i a t i o n c a l c u l a t e d f r o m s u c h a t i m e s e r i e s s h o u l d b e q u i t e u s e f u l f o r e s t i m a t i o n o f a laboratory uncertainty as expanded standard uncertainty. Page 7 o f 1 1

8 C a l i b r a t i o n o f a n a p p a r a t u s a n d a s s e s s m e n t o f a m e a s u r e m e n t m e t h o d. A s p o i n t e d o u t, o n e of the main applications of the existing reference materia l s f o r s t a b l e i s o t o p e r a t i o a s s a y i s t h e calibration of equipment and measurement procedures. Due to the large spectrum of investigated substances in isotope hydrology and isotope geochemistry a variety of reference materials was p r o d u c e d t o e n a b l e a c a l i b r a t i o n w i t h a m a t e r i a l a s s i m i l a r a s p o s s i b l e U s e o f r e f e r e n c e m a t e r i a l s f o r q u a l i t y c o n t r o l p u r p o s e s. The available reference materials are i n t e n d e d t o c a l i b r a t e i n t e r n a l s t a n d a r d s p r e p a r e d b y t h e i n d i v i d u a l l a b o r a t o r i e s. T h e M s a r e N O T i n t e n d e d t o b e u s e d t h e m s e l v e s f o r q u a l i t y c o n t r o l p u r p o s e s. F o r t h e d i s t r i b u t i o n o f a l l s t a b l e i s o t o p e ratio reference materials a rather strict rule applies: Each laboratory is entitled to order one unit of any reference material only once in a three years p e r i o d. T h i s l i m i t a t i o n w a s s e t t o p r e s e r v e t h e a v a i l a b i l i t y of the valuable reference materials for the maximal possible time and therefore to ensure the comparability of results from laboratories over long times. 2.2 Future Plans Holding a key role in producing and distributing reference materials for the determination of ratios of stable isotopes in light elements, the International Atomic Energy Agency follows as much as p o s s i b l e t h e l a t e s t r e q u i r e m e n t s f o r t h e p r o d u c t i o n o f r e f e r e n c e m a t e r i a l s a n d d e v e l o p m e n t s i n t h e metrological part of isotope geochemistry. In regular intervals of two to three years Advisory Group a n d C o n s u l t a n t s M e e t i n g a r e c o n v e n e d t o d i s c u s s t h e l a t e s t a p p r o a c h e s a n d n e w d e v e l o p m e n t s i n stable isotope reference materials an d t o f o c u s t h e I A E A a c t i v i t i e s o n t h e m a i n p r i o r i t i e s. ecently d e v e l o p e d a n a l y t i c a l t e c h n i q u e s e n a b l e l a b o r a t o r i e s t o a n a l y z e s a m p l e a m o u n t s t e n t o hundred times smaller than some years ago on their isotopic composition (for example by gas chromatography isotope ratio mass spectrometry GC -IMS, or the use of Elementar Analyzers for o n -line combustion of samples). At the same time these methods challenge the established system of reference materials in view of the required homogeneity of the existing Ms for such small amounts, which was not taken in consideration at the time of their production. The Isotope Hydrology Unit t h e r e f o r e h a s s t a r t e d t h e r e -e v a l u a t i o n o f t h e e x i s t i n g m a t e r i a l s i n o r d e r t o d e r i v e t h e b e s t p o s s i b l e i n f o r m a t i o n o n t h e f o r m e r pr o d u c t i o n s t e p s a n d o n t h e h o m o g e n e i t y a n d u n c e r t a i n t y o f t h e e s t a b l i s h e d recommended values. This implies significant efforts for this Unit in order to carefully assess the e s t a b l i s h e d m a t e r i a l s. T h i s t a s k s h o u l d n o t b e d e l e g a t e d t o o t h e r l a b o r a t o r i e s i n view of the problems of the lack of traceability and rigorous uncertainty assessment in many laboratories and in recognition o f t h e l i m i t a t i o n s f o r e x p e r i e n c e d l a b o r a t o r i e s t o d e d i c a t e t h e m s e l v e s t o t i m e c o n s u m i n g a n d e x p e n s i v e t a s k s a t a n o n p r i n c i p l e c o s t -free basis. As part of the development of an quality assurance system for the IAEA laboratories the Isotope Hydrology Unit aims to update the existing protocols to comply to the requirements of the quality assurance system as well as to the necessary prec o n d i t i o n s for the characterization and certification of reference materials E x i s t i n g M s f o r t h e a s s e s s m e n t o f s t a b l e i s o t o p e r a t i o s i n w a t e r s a m p l e s. The existing k n o w-how in the Isotope Hydrology Unit for high precision analysis of stable i s o t o p e r a t i o s i n w a t e r samples has already resulted in a project to produce a successor material for VSMOW. VSMOW is expected to be exhausted within the next seven to ten years. Since all measurements of hydrogen and o x y g e n i s o t o p e r a t i o s b a s e i n p r i n c i p l e o n t h i s m a t e r i a l, t h e p r o d u c t i o n o f a s u i t a b l e r e p l a c e m e n t h a s high priority. It was decided in accordance with the recommendation of an IAEA consultants meeting t o i n i t i a t e t h e p r o d u c t i o n o f a w a t e r i n d i s t i n g u i s h a b l e i n i t s h y d r o g e n a n d o x y g e n i s o t o p i c c o m p o s i t i o n from the existing VSMOW. In order to be able to also reproduce the ratios of 17 O and 18 O, isotopically e n r i c h e d w a t e r c o u l d n o t b e u s e d t o a d j u s t i t s i s o t o p i c c o m p o s i t i o n. T h e r e f o r e t h r e e n a t u r a l r a w w a t e r w e r e i d e n t i f i e d w i t h t h e i r i s o t o p i c co m p o s i t i o n a s c l o s e a s p o s s i b l e t o V S M O W b u t w i t h s l i g h t differences therein to allow a triangular mixing approach to produce the final water (figure 2). Page 8 o f 1 1

9 delta2h vs. VSMOW [per mille] 7 IAEA 6 Lab 2 5 Lab Lab 4 2 Lab 5, 18O only 1 0 Mix (IAEA) delta18o vs. VSMOW [per mille] Figure 2 C a l i b r a t i o n o f t h r e e r a w w a t e r f o r m i x i n g a r e p l a c e m e n t f o r VS M O W b y f i v e l a b o r a t o r i e s ( V S M O W b e i n g a t t h e o r i g i n o f t h e c o o r d i n a t e c r o s s ) a n d t h e p r e l i m i n a r y r e s u l t o f a f i r s t t r i a l m i x i n g o f t h e t h r e e c o m p o n e n t s ( b i g s q u a r e c l o s e t o t h e c o o r d i n a t e o r i g i n o f t h e plot). The three raw water were sampled, transfe rred to the Isotope Hydrology Unit and calibrated. This c a l i b r a t i o n w a s v e r i f i e d w i t h t h e r e s u l t s o f f o u r o t h e r h i g h p r e c i s i o n s t a b l e i s o t o p e l a b o r a t o r i e s w h i c h were in accordance by better than ± 0.02 for δ 18 O and by better than ±0.3 for δ 2 H (see figure 2 ). In the next step the three raw water will be mixed gravimetrically to produce the new standard which will then be tested carefully for any isotopic deviation from VSMOW. M e a n w h i l e t h e f i r s t a t t e m p t s a r e i n i t i a t e d t o o b t a i n i c e f r o m t h e i n n e r p a r t s o f A n t a r c t i c a t o p r o d u c e a replacement for SLAP within the next years E x i s t i n g M s f o r c a r b o n, s u l f u r a n d n i t r o g e n i s o t o p e s. A n i n t e r c a l i b r a t i o n f o r t h e a v a i l a b l e s u l f u r s t a b l e i s o t o p e r e f e r e n c e m a t e r i a l s w a s s t a r t e d i n i n o r d e r t o p r o d u c e a c o n s i s t e n t s e t o f r e c o m m e n d e d v a l u e s o f a l l t h e s e M s b a s e d o n t h e V C D T s c a l e. T h e e v a l u a t i o n o f t h e r e s u l t s i s ongoing. A similar re-a s s e s s m e n t i s p l a n n e d f o r s o m e i n o r g a n i c c a r b o n M s f o r i n c o o p e r a t i o n w i t h NIST and invited laboratories. The Isotope Hydrology Unit has already upgraded its equipment to be a b l e t o t e s t a l l e x i s t i n g i n o r g a n i c c a r b o n M s a n d t o p r o d u c e n e w o n e s. No s i m i l a r a c t i v i t y i s p l a n n e d s o f a r f o r n i t r o g e n i s o t o p e M s. T h e s i t u a t i o n t h e r e s e e m s t o b e s a t i s f a c t o r y. In view of the increasing importance of the determination of stable isotope ratios in organic materials (food authentication, medicine, biology, hydrogeology and isotope geochemistry), steps are foreseen to implement an appropriate analytical capability in the Isotope Hydrology Unit within the next two years for a high -p recision determination of carbon isotopes in organic matter, mainly to be used for the certification of new reference materials N e w r e f e r e n c e m a t e r i a l s t o b e p r o d u c e d i n f u t u r e. The analytical capabilities of the Isotope Hydrology Unit should be upgraded to enable it to determine the isotopic composition of all existing r e f e r e n c e m a t e r i a l s. S o f a r, n i t r o g e n a n d s u l f u r i s o t o p e s c a n n o t b e m e a s u r e d a t a l l a n d t h e c a p a b i l i t y for carbon analysis in organic matter is just being developed. Due to the increasing constraints for other laboratories to perform measurements on IAEA reference materials for homogeneity tests on a c o s t -f r e e b a s i s a n d i n o r d e r t o k e e p c o n t r o l o v e r t h e d a ta evaluation, this effort seems to be necessary. The establishment of recommended values for future reference materials will not rely anymore on interlaboratory comparison exercises, but will be based on extensive measurements in the IAEA Page 9 o f 1 1

10 Isotope Hydrology Unit and the comparison of t h o s e r e s u l t s w i t h a few selected laboratories of proven competence I n t e r l a b o r a t o r y c o m p a r i s o n e x e r c i s e s a n d p r o f i c i e n c y t e s t s. The main focus of such e x e r c i s e s i n t h e future will be t o s u p p o r t quality assurance i n s t a b l e i s o t o p e l a b o r a t o r i e s. P r o f i c i e n c y tests will be performed with selected laboratories where appropriate and desired by using well calibrated internal standards stored and used in the Isotope Hydrology Laboratory D i s t r i b u t i o n o f r e f e r e n c e m a t e r i a l s. In general it would be desirable to change t h e distribution policy for future reference materials a n d a l l o w l a b o r a t o r i e s t o o r d e r t h e s e M s m o r e frequently. This implies that much larger batches of future Ms h a v e t o b e p r e p a r e d a n d h a n d l ed. In the case of the successor material for VSMOW, about 200 l i t r e w i l l b e p r o d u c e d, w h i c h i s a b o u t t h r e e times more than previously for VSMOW. eferences 1 IAEA-TECDOC-825, eference and intercomparison materi a l s f o r s t a b l e i s o t o p e s o f l i g h t e l e m e n t s, P r o c e e d i n g s o f a C o n s u l t a n t s M e e t i n g h e l d i n V i e n n a, 1-3 D e c , S e p A. Fajgelj, Z. adecki, K. Burns, J. Moreno Bermudez, P. De egge, J. La ossa, P.. Danesi, this i s s u e. 3 International Atomic Energy Agency, IAEA AQCS Catalogue for eference Materials and I n t e r c o m p a r i s o n E x e r c i s e s / , V i e n n a, K. o z a n s k i, e p o r t o n t h e C o n s u l t a n t s G r o u p M e e t i n g o n C -14 eference Materials for a d i o c a r b o n L a b o r a t o r i e s, h e l d o n F e b , V i e n n a, K. o z a n s k i, W. S t i c h l e r,. G o n f i a n t i n i, E. M. S c o t t,. P. B e u k e n s, B. K r o m e r, J. v a n d e r P l i c h t, The IAEA 14C Intercomparison Exercise 1990, adiocarbon, 1992, 3 4, N o. 3, International Vocabulary of Basic and General Terms in Metrology, In ternational Organization for S t a n d a r d i z a t i o n, 2 nd e d., G e n e v a, S w i t z e r l a n d, L. Araguás Araguás, K. ozanski, eport on Interlaboratory Comparison for Deuterium and Oxygen-18 Analysis of Precipitation Samples, International Atomic Energy Agency, V i e n n a, M. Magaritz and J.. Gat, in: Stable Isotope Hydrology, deuterium and oxygen-1 8 i n t h e w a t e r cycle (J.. Gat and. Gonfiantini, Ed.), Technical eport Series No. 210, International Atomic Energy Agency, Vienna, 1981, Chapter 5, p Gonfiantini, in: Stable Isotope Hydrology, deuterium and oxygen-18 in the water cycle (J.. Gat and. Gonfiantini, Ed.), Technical eport Series No. 210, International Atomic Energy Agency, V i e n n a, , C h a p t e r 4, S. Epstein and T. Mayeda, Geochim. et Cosmochim Acta 1953, 4, H. Craig, Science, 1961, 1 3 3, N o , F.L. Mohler, Natl. Bur. Standards (U.S.), Tech. Note No. 51, 1960, P. Baertschi, Earth and Planetary Science Letters, 1976, 3 1, W. Li, B. Ni, D. Jin, and T.L. Chang, Kexue Tongbao, Chinese Science Bulletin, 1988, 3 3, Hagemann, G. Nief, and E. oth, Tellus, 1970, 2 2, J.C. De Wit, C.M. van der Straaten, W.G. Mook, Geostandards Newsletter, 4, n o. 1, S. Tse, S.C. Wong, C.P. Yuen, Analytical Chemistry, 1980, 5 2, G o n f i a n t i n i, S t a n d a r d s f o r S t a b l e I s o t o p e M e a s u r e m e n t s i n N a t u r a l C o m p o u n d s, N a t u r e , 2 7 1, N o , Gonfiantini, eport on the IAEA Consultants Meeting on Stable Isotope Standards and Intercalibration in Hydrology and in Geochemistry at 8-10 Sep 1976, IAEA, Vienna, March Page 1 0 o f 1 1

11 20. Gonfiantini, eport on the IAEA Consultants Meeting on Stable Isotope Standards and Intercalibration in Hydrology and in Geochemistry at 8-10 Sep 1976, IAEA, Vienna, March ISO/EMCO No.488, eference Materials - Contents of Certificates and Labels, Apr.1998, evision of ISO Guide 31:1981, International Organisation for Standardisation. 22 eport of Investigation, eference Materials VSMOW-GISP-SLAP, Na t i o n a l I n s t i t u t e of Standards & Technology, Gaithersburg, USA, 15 Oct ISO Guide 33, Uses of Certified eference Materials, evision of ISO Guide 33: 1989, I n t e r n a t i o n a l O r g a n i s a t i o n f o r S t a n d a r d i s a t i o n, G e n e v a, S w i t z e r l a n d, , t o b e p u b l i s h e d. 24 Quantifying Uncertainty in Analytical Measurements, EUACHEM, First edition, Page 1 1 o f 1 1