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Sample preparation for hydrogen isotope analysis by mass spectrometry
Int. J. AppL Radiat. Isot. Vol. 36, No. 12, pp. 991-992, 1985 © Pergamon Press Ltd 1985. Printed in Great Britain. 0020-708X/85 S3.00 + 0.00
Sample Preparation for Hydrogen Isotope Analysis by Mass Spectrometry T. FLORKOWSKI* International Atomic Energy Agency, A-1400 Vienna, Wagramerstrasse 5, Austria (Received l April 1985; in revised form 17 May 1985)
Hydrogen isotope analysis of water by mass spectrometry is performed on the hydrogen gas which is obtained by quantitative reduction of water. In many laboratories the reduction is done in the oven, filled with the uranium turningsc~ or with granulated zinc. c:~ Procedures and the method of correcting results for isotope fractionation are described elsewhere.¢3~A new method of sample preparation by reduction of water with zinc in small containers described by Coleman et al. ¢4~can successfully replace older methods. Its advantages are simplicity and rapidness allowing for higher output of analyses per working day. There are, however, a few aspects not discussed by Coleman et al. which must be taken into account when introducing this method into the laboratory practice. The procedure of sample preparation is as follows: A small amount (e.g. 0.2 g) of previously prepared metal zinc shot is placed in the glass container equipped with a removable Teflon stopcock. Six of these containers are attached to the vacuum line and evacuated to 10 -3 torr during warming of the zinc for a few minutes by a hairdryer to about 100°C. Dry nitrogen or argon from the cylinder is introduced to the containers, in order to avoid contamination with air moisture when the container is opened for a few seconds for the addition of 8 ~ L of water sample with a microsyringe. After six water samples are introduced the bottoms of the containers are frozen with liquid nitrogen and quickly evacuated to about 10-3tort. Then all the containers are disconnected from the vacuum line and placed in a heating block, at a temperature of about 410°C. The reaction of water with zinc takes place in a few minutes, but the containers are kept in the heating block for 30 rain and then cooled down in open air. Samples are now ready for measurement in a mass spectrometer. Pretreatment of zinc consists of washing the sieved portion of zinc shot in a solution of concentrated nitric acid with distilled water (ratio 1:4) followed by rinsing with distilled water. Then the dried zinc shot is outgassed in a glass container under vacuum, at 300°C for I h. This batch of zinc is used for preparation of samples and is kept under vacuum after each use. Difficulties arise with the choice of a proper zinc shot. Firstly, zinc shot of a good granular size (below about 1.5 ram diameter) is made only by two or three producers. Secondly, many batches of zinc do not react properly with water samples. Pretreatment of zinc does not help in the case
of "bad" zinc shot. Finally there is a fractionation effect depending on the mass of zinc used for the reaction. The magnitude of this fractionation varies among different sorts of zinc from different producers, and even among different batches of zinc from the same producer. It is. therefore. necessary to perform certain tests in the laboratory before a routine use of the given lot of the zinc shot. Results of a series of experiments carded out in the Isotope Hydrology Laboratory of the IAEA in Vienna are summarized below. In the first experiment granular zinc (Riedel de Haen AG, Hannover, F.R.G.) was sieved to make three groups of granular size 0.2-0.5; 0.5-1.0 and above I mm. After pretreatment of each fraction, samples were prepared from standard water (SMOW) using various masses of zinc. For each mass and for each size fraction six samples were prepared, measured and mean ~sMow VS working standard was calculated. Figure I shows a plot of 6 vs mass of the zinc used. A steep curve is observed, although no differences were found for various granular fractions. This dependence on mass (and not on granular surface area) would suggest that fractionation of produced hydrogen is due to dissolving of hydrogen in the zinc. For a larger mass of zinc, higher fractionation is observed. It should be noted that in all cases the amount of zinc was in excess as the stoichiometric amount is below 0.04 g for 10 mg of H:O. In order to find the possible time effect several samples were measured with various time delay after preparation. No time effect was found. Second similar experiments were made using granular zinc from another producer (BDH Chemicals Lid, England, zinc metal shot 0.5-2.0 mm). In general, the fractional effect was found to be much smaller, although the slope of the curve depends on the granular size of zinc and also varies with different supply batches (Fig. 2). Zinc granules with a grey surface (as opposed to the shiny surface of other batches) indicated the lowest effect. Experiments with granular zinc from other producers (e.g. Merck) were negative, as no chemical reaction was observed under the same experimental conditions. For routine application, zinc from the BDH Chemicals Ltd has been chosen. The procedure begins with sieving zinc granules to obtain fractions 0.5-1.0 mm and above l mm. The larger fraction constitutes about 80% of the batch.
- 10
o X
0 . 2 - 0.5 mm 0 . 5 - 1 . 0 mm > 1.0 m m
A g o -20
#0 o
-30
-40 0
I 0.1
I 0.2
I 0.3
I Q4
I Q5
I 0.6
I 0.7
1 0.8
Zn(g)
* Present address: Institute of Physics and Nuclear Techniques, University of Mining and Metallurgy, Al. Mickiewicza 30, 30-059 Krakow, Poland. 991
Fig. I. Effect of zinc amount on 6 value. 6 means 6SMOW VS working standard. Zinc from Riedel de Haen AG, Hannover.
992
Technical Note o 05-1.0
mm
& > 1.0 mm V 0 5 - 1 0 mm } X • 1.0 mm eltfferent O 0 . 5 - 1 . 0 m m t different + > 1.0mm J
lot lot ( g r t y )
- 10
:~a
,
a
standard water (SMOW). Portions of 0.25 g of zinc are weighed with an accuracy of 0.01 g. The time necessary for one person to prepare a batch of six samples is about I h. The reproducibility (precision) of analysis is determined on the basis of 6 values of standard water. In practice, standard deviation (I a) calculated for a certain period of time (several weeks) for standard water samples is below I%~.
Acknowledgements--Technical help of Mr Piotr Obrocki and Mr Ahmed Tanweer in carrying out laboratory experiments is gratefully acknowledged. Thanks are due to Dr R. Gonfiantini and Dr J. E. Rouse for helpful discussions.
°30
0
I
I
I
I
I
0,1
O.Z
0.3
0.4
0.5
Zn (g)
Fig. 2. Effect of zinc amount and granular size on 5 value (tsMow vs working standard). Zinc from BDH Chemicals Ltd, England.
References I. Bigeleisen J., Perlman M. L. and Prosser H. C. Anal. Chem. 24, 1356 (1952). 2. Friedman I. Geochim. Cosmochim. Acta 4, 89 (1953). 3. Gonfiantini R. In Stable Isotope Hydrology--Deuterium and Oxygen-18 in the Water Cycle (IAEA, Vienna,
1981). Both fractions are used but not mixed at the same time. A batch of six samples is prepared in which one sample is a
4. Coleman M. L., Shephard T. J., Rouse J. E. and Moore O. M. Anal. Chem. 53, 993 (1982).
Sample Preparation for Hydrogen Isotope Analysis by Mass Spectrometry T. FLORKOWSKI* International Atomic Energy Agency, A-1400 Vienna, Wagramerstrasse 5, Austria (Received l April 1985; in revised form 17 May 1985)
Hydrogen isotope analysis of water by mass spectrometry is performed on the hydrogen gas which is obtained by quantitative reduction of water. In many laboratories the reduction is done in the oven, filled with the uranium turningsc~ or with granulated zinc. c:~ Procedures and the method of correcting results for isotope fractionation are described elsewhere.¢3~A new method of sample preparation by reduction of water with zinc in small containers described by Coleman et al. ¢4~can successfully replace older methods. Its advantages are simplicity and rapidness allowing for higher output of analyses per working day. There are, however, a few aspects not discussed by Coleman et al. which must be taken into account when introducing this method into the laboratory practice. The procedure of sample preparation is as follows: A small amount (e.g. 0.2 g) of previously prepared metal zinc shot is placed in the glass container equipped with a removable Teflon stopcock. Six of these containers are attached to the vacuum line and evacuated to 10 -3 torr during warming of the zinc for a few minutes by a hairdryer to about 100°C. Dry nitrogen or argon from the cylinder is introduced to the containers, in order to avoid contamination with air moisture when the container is opened for a few seconds for the addition of 8 ~ L of water sample with a microsyringe. After six water samples are introduced the bottoms of the containers are frozen with liquid nitrogen and quickly evacuated to about 10-3tort. Then all the containers are disconnected from the vacuum line and placed in a heating block, at a temperature of about 410°C. The reaction of water with zinc takes place in a few minutes, but the containers are kept in the heating block for 30 rain and then cooled down in open air. Samples are now ready for measurement in a mass spectrometer. Pretreatment of zinc consists of washing the sieved portion of zinc shot in a solution of concentrated nitric acid with distilled water (ratio 1:4) followed by rinsing with distilled water. Then the dried zinc shot is outgassed in a glass container under vacuum, at 300°C for I h. This batch of zinc is used for preparation of samples and is kept under vacuum after each use. Difficulties arise with the choice of a proper zinc shot. Firstly, zinc shot of a good granular size (below about 1.5 ram diameter) is made only by two or three producers. Secondly, many batches of zinc do not react properly with water samples. Pretreatment of zinc does not help in the case
of "bad" zinc shot. Finally there is a fractionation effect depending on the mass of zinc used for the reaction. The magnitude of this fractionation varies among different sorts of zinc from different producers, and even among different batches of zinc from the same producer. It is. therefore. necessary to perform certain tests in the laboratory before a routine use of the given lot of the zinc shot. Results of a series of experiments carded out in the Isotope Hydrology Laboratory of the IAEA in Vienna are summarized below. In the first experiment granular zinc (Riedel de Haen AG, Hannover, F.R.G.) was sieved to make three groups of granular size 0.2-0.5; 0.5-1.0 and above I mm. After pretreatment of each fraction, samples were prepared from standard water (SMOW) using various masses of zinc. For each mass and for each size fraction six samples were prepared, measured and mean ~sMow VS working standard was calculated. Figure I shows a plot of 6 vs mass of the zinc used. A steep curve is observed, although no differences were found for various granular fractions. This dependence on mass (and not on granular surface area) would suggest that fractionation of produced hydrogen is due to dissolving of hydrogen in the zinc. For a larger mass of zinc, higher fractionation is observed. It should be noted that in all cases the amount of zinc was in excess as the stoichiometric amount is below 0.04 g for 10 mg of H:O. In order to find the possible time effect several samples were measured with various time delay after preparation. No time effect was found. Second similar experiments were made using granular zinc from another producer (BDH Chemicals Lid, England, zinc metal shot 0.5-2.0 mm). In general, the fractional effect was found to be much smaller, although the slope of the curve depends on the granular size of zinc and also varies with different supply batches (Fig. 2). Zinc granules with a grey surface (as opposed to the shiny surface of other batches) indicated the lowest effect. Experiments with granular zinc from other producers (e.g. Merck) were negative, as no chemical reaction was observed under the same experimental conditions. For routine application, zinc from the BDH Chemicals Ltd has been chosen. The procedure begins with sieving zinc granules to obtain fractions 0.5-1.0 mm and above l mm. The larger fraction constitutes about 80% of the batch.
- 10
o X
0 . 2 - 0.5 mm 0 . 5 - 1 . 0 mm > 1.0 m m
A g o -20
#0 o
-30
-40 0
I 0.1
I 0.2
I 0.3
I Q4
I Q5
I 0.6
I 0.7
1 0.8
Zn(g)
* Present address: Institute of Physics and Nuclear Techniques, University of Mining and Metallurgy, Al. Mickiewicza 30, 30-059 Krakow, Poland. 991
Fig. I. Effect of zinc amount on 6 value. 6 means 6SMOW VS working standard. Zinc from Riedel de Haen AG, Hannover.
992
Technical Note o 05-1.0
mm
& > 1.0 mm V 0 5 - 1 0 mm } X • 1.0 mm eltfferent O 0 . 5 - 1 . 0 m m t different + > 1.0mm J
lot lot ( g r t y )
- 10
:~a
,
a
standard water (SMOW). Portions of 0.25 g of zinc are weighed with an accuracy of 0.01 g. The time necessary for one person to prepare a batch of six samples is about I h. The reproducibility (precision) of analysis is determined on the basis of 6 values of standard water. In practice, standard deviation (I a) calculated for a certain period of time (several weeks) for standard water samples is below I%~.
Acknowledgements--Technical help of Mr Piotr Obrocki and Mr Ahmed Tanweer in carrying out laboratory experiments is gratefully acknowledged. Thanks are due to Dr R. Gonfiantini and Dr J. E. Rouse for helpful discussions.
°30
0
I
I
I
I
I
0,1
O.Z
0.3
0.4
0.5
Zn (g)
Fig. 2. Effect of zinc amount and granular size on 5 value (tsMow vs working standard). Zinc from BDH Chemicals Ltd, England.
References I. Bigeleisen J., Perlman M. L. and Prosser H. C. Anal. Chem. 24, 1356 (1952). 2. Friedman I. Geochim. Cosmochim. Acta 4, 89 (1953). 3. Gonfiantini R. In Stable Isotope Hydrology--Deuterium and Oxygen-18 in the Water Cycle (IAEA, Vienna,
1981). Both fractions are used but not mixed at the same time. A batch of six samples is prepared in which one sample is a
4. Coleman M. L., Shephard T. J., Rouse J. E. and Moore O. M. Anal. Chem. 53, 993 (1982).