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The determination of zinc in standard reference materials by isotope dilution and atomic absorption analysis
Chemical Geology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
THE DETERMINATION OF ZINC IN STANDARD REFERENCE MATERIALS BY ISOTOPE DILUTION AND ATOMIC ABSORPTION ANALYSIS K.J.R. ROSMAN * and P.M. JEFFERY Physics Department, University of Western Australia, Nedlands, W.A. (Australia) (Received November 30, 1970) (Resubmitted May 18, 1971)
ABSTRACT Rosman, K.J.R. and Jeffery, P.M., 1971. The determination of zinc in standard reference materials by isotope dilution and atomic absorption analysis. Chem. Geol., 8:25-32. Zinc has been determined in a range of standard reference materials using stable isotope dilution and atomic absorption spectrophotometry. Provided that samples were first passed through an ion exchange column to reduce the level of contaminating species, the atomic-absorption method yielded results in general agreement with the isotope dilution values.
INTRODUCTION Zinc has been determined in eighteen standard reference materials from five different sources (Table I) by stable isotope dilution and atomic absorption spectrophotometry. The isotope dilution determinations are considered to be the more accurate, b u t the resuits from b o t h methods are in good agreement for most samples. Both techniques were developed as part of a program o f investigation into the elemental and isotopic abundance o f zinc in nature. The standard reference materials were analysed so that results from other laboratories could be meaningfully compared with analyses o f terrestrial and meteoritic matter (Rosman, in preparation). The results presented here may also help to resolve some of the reported discrepancies between determinations performed in different laboratories (Flanagan, 1969; Fleischer, 1969). No stable isotope dilution analysis of zinc in standard reference materials has so far been reported.
* Present address: Department of Physics, Western Australian Institute of Technology, Hayman Road, South Bentley, W.A. Chem. Geol., 8 (1971) 25-32
Description
diabase
granite
andesite granodiorite
basalt
dunite
peridodite
tonalite
dolomite
Code name
W-1
G-2
AGV-1 GSP-1
BCR-1
DTS-1
PCC-1
T-1
GFS 400
3
2
1
1
1
1 1
1
1
33.0±1.0 [37.0 ±4.01
37.0±0.6
104.5 ± 1.2 106.2 ± 1.2 103.8 ± 1.2
89.6±3.1 86.7±2.8 88.8±1.2 86.0±1.3 85.0±1.0
1 or2
88.4 ± 1.2 93.7 + 1.8 88.7 + 1.4
13.8 + 0.6
41.7±0.8 36.4±0.8 37.8±0.7 44.3±0.9 41.1±0.8 36.1±0.7 178.8±2.6
130.5±1.9
87.0±1.7 160.5±1.5]
80.9±1.6
177.0 + 1.6
32.6 + 1.2
128.3 + 3.5
86.3 + 3.0 105.9 + 2.3
13.8
177.9
38.6
37.1
129.4
105.1
83.9 86.7
89.3
171 179 191 13.9 15.8 16.7
37 37
80 89 85 80 82 86 99 100 112 131 124 40 37
15.5
180
37
39
128
104
81 86
85
average. 4
individual analyses
individual analyses average .4
atomic absorption
isotopic dilution *a
Source*1 Zn concentration.2
Results of isotope dilution and atomic absorption analysis
TABLE I
k
Z
Z
O
O~
b,J
7=
5
5
sulphide ore
syenite
syenite
SU-1
SY-2
SY-3
255.3 + 3.5
252.1 + 4 . 1
1.88 + .08 1.89 + .04 1.98 + .06
17.0 + 0.4 30.9 + 0.2
16.7 + 0.6
286.9 + 2.7
281.7 + 4 . 6
257.3 + 2.2
258.1 + 2.0
2.1 + 0 . 2
30.0 + 0.2
14.8 + 0.6
1.5 + 0 . 2
30.5 + 0.4
15.3 + 0.3
5.7 + 1.2 7.6 + 0.6
256
255
284
1.88
30.5
15.6 17.0
7.6
5.7 7.0 6.9 8.1 13.5 15.5 15.0 16.8 17.3 15.8 [35] 30.9 29.5 28.6 [4.3 1.6 2.5 2.0 247 264 292 250 258 261 257 +0.4] + 0.2 + 0.2 + 0.2
+ 0.8 + 0.7 +- 0.7 + 1.14 + 1.4 + 1.4 + 1.4 + 1.5
259
254
268
2.0
29.6
15.6 15.8
7.3
• 1 Code n u m b e r s for the source of sample analysed: 1 = United States Geological Survey, Washington; 2 = Geological Survey, Tanganyika; 3 = G. Frederick Smith Chemical C o m p a n y ; 4 = National Bureau o f Standards, Washington; 5 = Spectroscopy Society o f Canada. • 2 Concentration in p.p.m, by weight. • 3 1, 2, 3, 4 refer to the tracer used (see Table II). • 4 Values in square brackets have been excluded from the average.
5
4
steel
NBS 163
4 4
4
argillaceous steel
silica brick
NBS 198
4 4
NBS l b NBS 101f
K-feldspar bauxite
NBS 70a NBS 69a
bO
r-'
,..] r~
rn Z
Z
,.-]
O Z
Z
r~ ,.q
Z
N
28
K.J.R. ROSMAN AND P.M. JEFFERY
EXPERIMENTAL
Chemistry Dissolution All samples were available in the form of fine powders except for the steels which contained larger granules up to 0.5 mm in size. Dissolution of up to 3 g of silicate rock was effected with 20 ml HF, 1 ml HC104 and 1 ml H2 SO4. The addition of H2 SO4 was found necessary in order to obtain a high and reproducible extraction efficiency from the ion exchange columns. Huffman et at. (1963) also report a substantial improvement in column efficiency through the addition of H2 SO4. After evaporation of most of the H: SO4 the residue was dissolved in 20 ml of hot 6M HC1 and diluted to 1.2M in preparation for loading onto the ion exchange resin. The steel, dolomite and limestone samples were taken into solution in two stages. The first involved dissolution of most of the sample with 6 M HC1 with this being followed by a stage of filtration. The residue remaining after filtration was decomposed using the procedure already described above for silicate samples and the filtrate diluted to 1.2 M in preparation for the ion exchange resin.
Extraction Zinc was extracted from the sample solution using glass columns containing anion exchange resin (Dowex A g l - X 8 , 2 0 0 - 4 0 0 mesh, chloride form) supported on silica wool. A 4 cm × 0.75 cm 2 resin bed was used for the initial separation being followed by a smaller 3 cm × 0.2 cm 2 bed when further purification was necessary. Using atomic absorption spectrophotometry as a zinc monitor the ion exchange column procedure described by Huffman et al. (1963) was suitably modified. Each column was loaded with approximately 33-column volumes of the sample solution and washed with 13-column volumes of 0.6 M HC1. Zinc was eluted in 5 -column volumes of 0.01 M HC1. Over 14 estimates the mean extraction efficiency of the larger column was 94 % with 95 % confidence interval of-+ 4%.
Isotope dilution Three different zinc tracers were used during the period of the analyses and Table II shows their measured isotopic composition. Tracers 1 and 2 are solutions of zinc having the same isotopic composition but different concentrations. Tracers 3 and 4 have been synthesised by mixing singly enriched tracers. All singly enriched tracers were obtained from Oak Ridge National Laboratory. Each tracer solution was calibrated against the same zinc solution whose gravimetrically determined concentration was accurately known. The primary aim in preparing the double spikes was to reduce the effects of instrumental discrimination so that small differences in isotopic composition between samples could be detected. The elemental abundances that have been determined using a double spike are a byproduct of this investigation. (Rosman, in preparation). Other determinations were made using the well-established procedure of single spike
29
ZINC DETERMINATION IN STANDARD REFERENCE MATERIALS TABLE II The measured isotopic composition of the zinc tracers and normal zinc Isotopes
Abundance * (%) Tracer no. 1 and 2
64 66 67 68 70
1.847 4.506 89.43 4.168 0.048
3
4
normal zinc
6.137 6.009 53.36 6.288 28.20
82.46 0.969 15.74 0.814 0.02l
49.15 27.79 4.054 18.40 0.620
* These abundances are not corrected for instrumental discrimination. isotope dilution, including the optimisation methods discussed by Crouch and Webster (1963). The only ratio measured in the tracer-matrix mixture was the 67 Zn/66 Zn ratio since this gave short data collection times and minimised systematic errors that could be introduced by mass discrimination. Isotopic cross contamination was also less in the mixture than it would have been had mass 68 been chosen instead o f mass 66.
A tom& absorption spectrophotometry Atomic absorption spectrophotometry was initially introduced to establish ion exchange column procedures and to standardize sample sizes for the mass spectrometer. A single column treatment of the sample solution was found to be sufficient to reduce interferences to a level that permitted reproducible results to be obtained for samples o f widely varying elemental composition. When a correction for the column efficiency was applied the results showed good agreement with isotope dilution measurements. The accuracy and precision o f this m e t h o d depend mainly upon uncertainties in column efficiency, blank correction and estimation o f solution concentration, The question of column efficiency has already been discussed. The blank correction depends markedly on the composition o f the containers used for the sample decomposition as well as on the quantities o f reagents used. The silicate dissolution procedure yielded a blank of 1.7 -+ 0.6 #g for up to 3 g o f rock sample when a platinum crucible was used or 1.2 #g for teflon crucibles. The concentration o f zinc in column treated solutions was estimated from a calibration curve constructed prior to each batch o f sample analyses, by using standard solutions whose concentrations were accurately known. Calibration curves proved to be linear for concentrations o f less than 1.5 #g/g when the 2139 A wavelength line was used. The error arising from the measurement o f solution concentrations was estimated to be less than 0.015 #g o f zinc per gram o f solution.
DISCUSSION OF RESULTS Full details o f the results from b o t h methods are displayed in Table I.
Che~ Geol., 8 (1971) 25-32
30
K.J.R. ROSMAN AND P.M JEFFERY
TABLE Ill Comparison of isotope dilution results with other published values Sample
W-1 G-2 AGV-1 GSP-1 BCR-I DTS-1 PCC-I Tq GFS 400 NBS 70a NBS 69a NBS 198 NBS lb NBS 101~ NBS 163 SU-1 SY-2 SY-3
Zn concentration (p.p.m.) this work
other published values
89.3 83.9 86.7 105.1 129.4 37.1 38.6 177.9 13.8 5.7 7.6 15.6 17 30.5 1.88 284 255 256
826 , 887, 84.98 74.94, 887, 80.68 1124 , 1187 , 81.18 1434, 1067, 96.78 1324, 1307, 127.48 614 534 , 607 1903, 1647 291 7.52, 7.32
131
294 s
References 1 Thompson et al. (1970): emission spectroscopy. 2 Ball and Filby (1965): neutron activation analysis and X-ray fluorescence. 3 Msusule Tonalite Supplement (1963): a compilation of results. 4 Flanagan (1969): a compilation of results. s Sine et al. (1969): a compilation of results. 6 Fleischer (1969): a compilation of results. 7 Medlin et al. (1969): atomic absorption spectrophotometry. 8 Johansen and Steinnes (1970): neutron activation analysis.
Isotope dilution results T h e e r r o r limits d i s p l a y e d in T a b l e I are c o n s i d e r e d to be 95 % c o n f i d e n c e limits. T h e y were c o m p u t e d b y a s s u m i n g t h e c o n c e n t r a t i o n to be a f u n c t i o n o f i n d e p e n d e n t variates w h o s e u n c e r t a i n t i e s were k n o w n . U n c e r t a i n t i e s in weighing, t r a c e r c a l i b r a t i o n , b l a n k corr e c t i o n a n d mass s p e c t r o m e t r i c m e a s u r e m e n t s have b e e n t a k e n i n t o a c c o u n t . N o error h a s b e e n allowed for possible loss o f s a m p l e zinc w i t h r e s p e c t t o tracer d u r i n g the s a m p l e d i s s o l u t i o n since cross c h e c k s lead us to believe t h a t losses at this stage are trivial.
Atomic absorption results T h e p r o c e d u r e u s e d for c o m p u t i n g error l i m i t s was the same as t h a t d e s c r i b e d for isot o p e d i l u t i o n . U n c e r t a i n t i e s in weighing, c o l u m n efficiency, b l a n k c o r r e c t i o n a n d use o f
ZINC DETERMINATION IN STANDARD REFERENCE MATERIALS
31
the calibration curve have all been included. Again the final error limit was considered to be a 95 % confidence limit. Where error limits are not displayed in Table I for atomic absorption measurements the 95 % confidence limits are less th,an -+ 8% of the final concentration. Due to the uncertainty in the blank correction this value is exceeded at lower concentrations. In these cases the computed error limits are displayed. The accuracy of this method relies critically on the column extraction efficiency whwh will be drastically reduced for silicate samples if tta SO4 is not added General
In cJculating average concentrations certain values, indicated by square brackets in Table I, have been excluded. No explanation can be offered for the single low analysis ot GSP-I The values of 35 p.p.m, and 4.3 p.p.m, for NBS 101 f and NBS 163 respectively were rejected on the grounds that succeeding replicate analyses o f the same material were in good agreement. Although 37 p.p.m, for PCC-1 is consistent with other determinations excessive ion beam scattering in the mass spectrometer during this analysis led to a poorly formed mass spectrum with the result that the measured 6~ Zn/66 Zn ratio was in doubt and has been rejected. Even when these values are excluded there are stall materials that exhibit variations in concentration which cannot readily be accounted foi by experimental errorso It appears, therefore, that some of the standard materials may be slightly i~d~,,mogeneous with respect to their zinc content. Table III is a comparison of this work with other published values. Unfortunateb, relatively few determinations of zinc have been made using GFS, NBS and CAAS standards. Where comparison is possible there is approximate agreement between the results of this work and some of the published data. There appears however, to be a consistent discrepancy in the case of DTS-1 and PCC-1.
CONCI USIONS The values recommended for the standard materials from this work are those obtained by the isotope dilution method of analysis. It is interesting that when all interferences are removed and appropriate corrections are made, atomic absorption analysis gives vahles which are in good agreement with the more accurate method.
ACKNGWI EDGEMENTS The authors wish to thank the many donors who supplied the samples fi)r this work and the Institute of Agriculture of the University of Western Australia for the use of t,ne of its atomic absorption spectrophotometers. This project was financed by the Research Grants Committee of the University of Western Australia. One of the authors, K.J.R Rosman, was the recipient of a University Research Studentship for the major period of this research. Chem. Geol.. 8 {1971) 25-32
32
K.J.R. ROSMAN AND P.M. JEFFERY
REFERENCES Ball, T.K. and Filby, R.H., 1965. The zinc content of some geochemical standards by neutron activation and X-ray fluorescence analysis. Geochim. Cosmochim. Acta, 2 9 : 7 3 7 - 7 4 0 . Crouch, E.A.C. and Webster, R.K., 1963. Choice of optimum quantity and constitution of the tracer used for isotope dilution analysis. J. Chem. Soc., 1963: 118-131. Flanagan, F.J., 1969. U.S. Geological Survey Standards - II. First compilation of data for the new U.S.G.S. rocks. Geochim. CosmochirrL Acta, 33: 81-120. Fleischer, M., 1969. U.S. Geological Survey Standards - I. Additional data on rocks G-1 and W-I, 1965-1967. GeochirrL Cosmochim. Acta, 33: 65-79. Huffman, Jr., C.H., Lipp, H.H. and Rader, L.F., 1963. Spectrophotometric determinations of micro quantities of zinc in rocks. Geochim. Cosmochim. Acta, 27: 209-215. Johansen, O. and Steinnes, E., 1970. Determination of Co, Cu, Fe, Ga, W and Zn in rocks by neutron activation and anion-exchange separation. Talanta, 17: 407-414. Medlin, J.H., Suhr, N.H. and Bodkin, J.B., 1969. Atomic absorption analysis of silicates employing LiBO 2 fusion.AtomicAbsorption Newsletter, 8: 25-29. Msusule Tonalite Supplement No. 1, 1963. Tanganyika. Rosman, K.J.R. and Jeffery, P.M., 1971. An improved mass spectrometer ion source for use with solid samples of high ionisation potential elements. J. Phys., E: J. ScL Instr., 4 : 1 3 4 - 1 3 6 . Sine, N.M., Taylor, W.O., Webber, G.R. and Lewis, C.L., 1969. Third report of analytical data for CAAS sulphide ore and syenite rock standards. Geochim. Cosmochim. Acta, 3 3 : 1 2 1 - 1 3 1 . Thompson, G., Bankston, D.C. and Pasley, S.M., 1970. Trace element data for reference carbonate rocks. Chem. GeoL, 6: 165-170.
THE DETERMINATION OF ZINC IN STANDARD REFERENCE MATERIALS BY ISOTOPE DILUTION AND ATOMIC ABSORPTION ANALYSIS K.J.R. ROSMAN * and P.M. JEFFERY Physics Department, University of Western Australia, Nedlands, W.A. (Australia) (Received November 30, 1970) (Resubmitted May 18, 1971)
ABSTRACT Rosman, K.J.R. and Jeffery, P.M., 1971. The determination of zinc in standard reference materials by isotope dilution and atomic absorption analysis. Chem. Geol., 8:25-32. Zinc has been determined in a range of standard reference materials using stable isotope dilution and atomic absorption spectrophotometry. Provided that samples were first passed through an ion exchange column to reduce the level of contaminating species, the atomic-absorption method yielded results in general agreement with the isotope dilution values.
INTRODUCTION Zinc has been determined in eighteen standard reference materials from five different sources (Table I) by stable isotope dilution and atomic absorption spectrophotometry. The isotope dilution determinations are considered to be the more accurate, b u t the resuits from b o t h methods are in good agreement for most samples. Both techniques were developed as part of a program o f investigation into the elemental and isotopic abundance o f zinc in nature. The standard reference materials were analysed so that results from other laboratories could be meaningfully compared with analyses o f terrestrial and meteoritic matter (Rosman, in preparation). The results presented here may also help to resolve some of the reported discrepancies between determinations performed in different laboratories (Flanagan, 1969; Fleischer, 1969). No stable isotope dilution analysis of zinc in standard reference materials has so far been reported.
* Present address: Department of Physics, Western Australian Institute of Technology, Hayman Road, South Bentley, W.A. Chem. Geol., 8 (1971) 25-32
Description
diabase
granite
andesite granodiorite
basalt
dunite
peridodite
tonalite
dolomite
Code name
W-1
G-2
AGV-1 GSP-1
BCR-1
DTS-1
PCC-1
T-1
GFS 400
3
2
1
1
1
1 1
1
1
33.0±1.0 [37.0 ±4.01
37.0±0.6
104.5 ± 1.2 106.2 ± 1.2 103.8 ± 1.2
89.6±3.1 86.7±2.8 88.8±1.2 86.0±1.3 85.0±1.0
1 or2
88.4 ± 1.2 93.7 + 1.8 88.7 + 1.4
13.8 + 0.6
41.7±0.8 36.4±0.8 37.8±0.7 44.3±0.9 41.1±0.8 36.1±0.7 178.8±2.6
130.5±1.9
87.0±1.7 160.5±1.5]
80.9±1.6
177.0 + 1.6
32.6 + 1.2
128.3 + 3.5
86.3 + 3.0 105.9 + 2.3
13.8
177.9
38.6
37.1
129.4
105.1
83.9 86.7
89.3
171 179 191 13.9 15.8 16.7
37 37
80 89 85 80 82 86 99 100 112 131 124 40 37
15.5
180
37
39
128
104
81 86
85
average. 4
individual analyses
individual analyses average .4
atomic absorption
isotopic dilution *a
Source*1 Zn concentration.2
Results of isotope dilution and atomic absorption analysis
TABLE I
k
Z
Z
O
O~
b,J
7=
5
5
sulphide ore
syenite
syenite
SU-1
SY-2
SY-3
255.3 + 3.5
252.1 + 4 . 1
1.88 + .08 1.89 + .04 1.98 + .06
17.0 + 0.4 30.9 + 0.2
16.7 + 0.6
286.9 + 2.7
281.7 + 4 . 6
257.3 + 2.2
258.1 + 2.0
2.1 + 0 . 2
30.0 + 0.2
14.8 + 0.6
1.5 + 0 . 2
30.5 + 0.4
15.3 + 0.3
5.7 + 1.2 7.6 + 0.6
256
255
284
1.88
30.5
15.6 17.0
7.6
5.7 7.0 6.9 8.1 13.5 15.5 15.0 16.8 17.3 15.8 [35] 30.9 29.5 28.6 [4.3 1.6 2.5 2.0 247 264 292 250 258 261 257 +0.4] + 0.2 + 0.2 + 0.2
+ 0.8 + 0.7 +- 0.7 + 1.14 + 1.4 + 1.4 + 1.4 + 1.5
259
254
268
2.0
29.6
15.6 15.8
7.3
• 1 Code n u m b e r s for the source of sample analysed: 1 = United States Geological Survey, Washington; 2 = Geological Survey, Tanganyika; 3 = G. Frederick Smith Chemical C o m p a n y ; 4 = National Bureau o f Standards, Washington; 5 = Spectroscopy Society o f Canada. • 2 Concentration in p.p.m, by weight. • 3 1, 2, 3, 4 refer to the tracer used (see Table II). • 4 Values in square brackets have been excluded from the average.
5
4
steel
NBS 163
4 4
4
argillaceous steel
silica brick
NBS 198
4 4
NBS l b NBS 101f
K-feldspar bauxite
NBS 70a NBS 69a
bO
r-'
,..] r~
rn Z
Z
,.-]
O Z
Z
r~ ,.q
Z
N
28
K.J.R. ROSMAN AND P.M. JEFFERY
EXPERIMENTAL
Chemistry Dissolution All samples were available in the form of fine powders except for the steels which contained larger granules up to 0.5 mm in size. Dissolution of up to 3 g of silicate rock was effected with 20 ml HF, 1 ml HC104 and 1 ml H2 SO4. The addition of H2 SO4 was found necessary in order to obtain a high and reproducible extraction efficiency from the ion exchange columns. Huffman et at. (1963) also report a substantial improvement in column efficiency through the addition of H2 SO4. After evaporation of most of the H: SO4 the residue was dissolved in 20 ml of hot 6M HC1 and diluted to 1.2M in preparation for loading onto the ion exchange resin. The steel, dolomite and limestone samples were taken into solution in two stages. The first involved dissolution of most of the sample with 6 M HC1 with this being followed by a stage of filtration. The residue remaining after filtration was decomposed using the procedure already described above for silicate samples and the filtrate diluted to 1.2 M in preparation for the ion exchange resin.
Extraction Zinc was extracted from the sample solution using glass columns containing anion exchange resin (Dowex A g l - X 8 , 2 0 0 - 4 0 0 mesh, chloride form) supported on silica wool. A 4 cm × 0.75 cm 2 resin bed was used for the initial separation being followed by a smaller 3 cm × 0.2 cm 2 bed when further purification was necessary. Using atomic absorption spectrophotometry as a zinc monitor the ion exchange column procedure described by Huffman et al. (1963) was suitably modified. Each column was loaded with approximately 33-column volumes of the sample solution and washed with 13-column volumes of 0.6 M HC1. Zinc was eluted in 5 -column volumes of 0.01 M HC1. Over 14 estimates the mean extraction efficiency of the larger column was 94 % with 95 % confidence interval of-+ 4%.
Isotope dilution Three different zinc tracers were used during the period of the analyses and Table II shows their measured isotopic composition. Tracers 1 and 2 are solutions of zinc having the same isotopic composition but different concentrations. Tracers 3 and 4 have been synthesised by mixing singly enriched tracers. All singly enriched tracers were obtained from Oak Ridge National Laboratory. Each tracer solution was calibrated against the same zinc solution whose gravimetrically determined concentration was accurately known. The primary aim in preparing the double spikes was to reduce the effects of instrumental discrimination so that small differences in isotopic composition between samples could be detected. The elemental abundances that have been determined using a double spike are a byproduct of this investigation. (Rosman, in preparation). Other determinations were made using the well-established procedure of single spike
29
ZINC DETERMINATION IN STANDARD REFERENCE MATERIALS TABLE II The measured isotopic composition of the zinc tracers and normal zinc Isotopes
Abundance * (%) Tracer no. 1 and 2
64 66 67 68 70
1.847 4.506 89.43 4.168 0.048
3
4
normal zinc
6.137 6.009 53.36 6.288 28.20
82.46 0.969 15.74 0.814 0.02l
49.15 27.79 4.054 18.40 0.620
* These abundances are not corrected for instrumental discrimination. isotope dilution, including the optimisation methods discussed by Crouch and Webster (1963). The only ratio measured in the tracer-matrix mixture was the 67 Zn/66 Zn ratio since this gave short data collection times and minimised systematic errors that could be introduced by mass discrimination. Isotopic cross contamination was also less in the mixture than it would have been had mass 68 been chosen instead o f mass 66.
A tom& absorption spectrophotometry Atomic absorption spectrophotometry was initially introduced to establish ion exchange column procedures and to standardize sample sizes for the mass spectrometer. A single column treatment of the sample solution was found to be sufficient to reduce interferences to a level that permitted reproducible results to be obtained for samples o f widely varying elemental composition. When a correction for the column efficiency was applied the results showed good agreement with isotope dilution measurements. The accuracy and precision o f this m e t h o d depend mainly upon uncertainties in column efficiency, blank correction and estimation o f solution concentration, The question of column efficiency has already been discussed. The blank correction depends markedly on the composition o f the containers used for the sample decomposition as well as on the quantities o f reagents used. The silicate dissolution procedure yielded a blank of 1.7 -+ 0.6 #g for up to 3 g o f rock sample when a platinum crucible was used or 1.2 #g for teflon crucibles. The concentration o f zinc in column treated solutions was estimated from a calibration curve constructed prior to each batch o f sample analyses, by using standard solutions whose concentrations were accurately known. Calibration curves proved to be linear for concentrations o f less than 1.5 #g/g when the 2139 A wavelength line was used. The error arising from the measurement o f solution concentrations was estimated to be less than 0.015 #g o f zinc per gram o f solution.
DISCUSSION OF RESULTS Full details o f the results from b o t h methods are displayed in Table I.
Che~ Geol., 8 (1971) 25-32
30
K.J.R. ROSMAN AND P.M JEFFERY
TABLE Ill Comparison of isotope dilution results with other published values Sample
W-1 G-2 AGV-1 GSP-1 BCR-I DTS-1 PCC-I Tq GFS 400 NBS 70a NBS 69a NBS 198 NBS lb NBS 101~ NBS 163 SU-1 SY-2 SY-3
Zn concentration (p.p.m.) this work
other published values
89.3 83.9 86.7 105.1 129.4 37.1 38.6 177.9 13.8 5.7 7.6 15.6 17 30.5 1.88 284 255 256
826 , 887, 84.98 74.94, 887, 80.68 1124 , 1187 , 81.18 1434, 1067, 96.78 1324, 1307, 127.48 614 534 , 607 1903, 1647 291 7.52, 7.32
131
294 s
References 1 Thompson et al. (1970): emission spectroscopy. 2 Ball and Filby (1965): neutron activation analysis and X-ray fluorescence. 3 Msusule Tonalite Supplement (1963): a compilation of results. 4 Flanagan (1969): a compilation of results. s Sine et al. (1969): a compilation of results. 6 Fleischer (1969): a compilation of results. 7 Medlin et al. (1969): atomic absorption spectrophotometry. 8 Johansen and Steinnes (1970): neutron activation analysis.
Isotope dilution results T h e e r r o r limits d i s p l a y e d in T a b l e I are c o n s i d e r e d to be 95 % c o n f i d e n c e limits. T h e y were c o m p u t e d b y a s s u m i n g t h e c o n c e n t r a t i o n to be a f u n c t i o n o f i n d e p e n d e n t variates w h o s e u n c e r t a i n t i e s were k n o w n . U n c e r t a i n t i e s in weighing, t r a c e r c a l i b r a t i o n , b l a n k corr e c t i o n a n d mass s p e c t r o m e t r i c m e a s u r e m e n t s have b e e n t a k e n i n t o a c c o u n t . N o error h a s b e e n allowed for possible loss o f s a m p l e zinc w i t h r e s p e c t t o tracer d u r i n g the s a m p l e d i s s o l u t i o n since cross c h e c k s lead us to believe t h a t losses at this stage are trivial.
Atomic absorption results T h e p r o c e d u r e u s e d for c o m p u t i n g error l i m i t s was the same as t h a t d e s c r i b e d for isot o p e d i l u t i o n . U n c e r t a i n t i e s in weighing, c o l u m n efficiency, b l a n k c o r r e c t i o n a n d use o f
ZINC DETERMINATION IN STANDARD REFERENCE MATERIALS
31
the calibration curve have all been included. Again the final error limit was considered to be a 95 % confidence limit. Where error limits are not displayed in Table I for atomic absorption measurements the 95 % confidence limits are less th,an -+ 8% of the final concentration. Due to the uncertainty in the blank correction this value is exceeded at lower concentrations. In these cases the computed error limits are displayed. The accuracy of this method relies critically on the column extraction efficiency whwh will be drastically reduced for silicate samples if tta SO4 is not added General
In cJculating average concentrations certain values, indicated by square brackets in Table I, have been excluded. No explanation can be offered for the single low analysis ot GSP-I The values of 35 p.p.m, and 4.3 p.p.m, for NBS 101 f and NBS 163 respectively were rejected on the grounds that succeeding replicate analyses o f the same material were in good agreement. Although 37 p.p.m, for PCC-1 is consistent with other determinations excessive ion beam scattering in the mass spectrometer during this analysis led to a poorly formed mass spectrum with the result that the measured 6~ Zn/66 Zn ratio was in doubt and has been rejected. Even when these values are excluded there are stall materials that exhibit variations in concentration which cannot readily be accounted foi by experimental errorso It appears, therefore, that some of the standard materials may be slightly i~d~,,mogeneous with respect to their zinc content. Table III is a comparison of this work with other published values. Unfortunateb, relatively few determinations of zinc have been made using GFS, NBS and CAAS standards. Where comparison is possible there is approximate agreement between the results of this work and some of the published data. There appears however, to be a consistent discrepancy in the case of DTS-1 and PCC-1.
CONCI USIONS The values recommended for the standard materials from this work are those obtained by the isotope dilution method of analysis. It is interesting that when all interferences are removed and appropriate corrections are made, atomic absorption analysis gives vahles which are in good agreement with the more accurate method.
ACKNGWI EDGEMENTS The authors wish to thank the many donors who supplied the samples fi)r this work and the Institute of Agriculture of the University of Western Australia for the use of t,ne of its atomic absorption spectrophotometers. This project was financed by the Research Grants Committee of the University of Western Australia. One of the authors, K.J.R Rosman, was the recipient of a University Research Studentship for the major period of this research. Chem. Geol.. 8 {1971) 25-32
32
K.J.R. ROSMAN AND P.M. JEFFERY
REFERENCES Ball, T.K. and Filby, R.H., 1965. The zinc content of some geochemical standards by neutron activation and X-ray fluorescence analysis. Geochim. Cosmochim. Acta, 2 9 : 7 3 7 - 7 4 0 . Crouch, E.A.C. and Webster, R.K., 1963. Choice of optimum quantity and constitution of the tracer used for isotope dilution analysis. J. Chem. Soc., 1963: 118-131. Flanagan, F.J., 1969. U.S. Geological Survey Standards - II. First compilation of data for the new U.S.G.S. rocks. Geochim. CosmochirrL Acta, 33: 81-120. Fleischer, M., 1969. U.S. Geological Survey Standards - I. Additional data on rocks G-1 and W-I, 1965-1967. GeochirrL Cosmochim. Acta, 33: 65-79. Huffman, Jr., C.H., Lipp, H.H. and Rader, L.F., 1963. Spectrophotometric determinations of micro quantities of zinc in rocks. Geochim. Cosmochim. Acta, 27: 209-215. Johansen, O. and Steinnes, E., 1970. Determination of Co, Cu, Fe, Ga, W and Zn in rocks by neutron activation and anion-exchange separation. Talanta, 17: 407-414. Medlin, J.H., Suhr, N.H. and Bodkin, J.B., 1969. Atomic absorption analysis of silicates employing LiBO 2 fusion.AtomicAbsorption Newsletter, 8: 25-29. Msusule Tonalite Supplement No. 1, 1963. Tanganyika. Rosman, K.J.R. and Jeffery, P.M., 1971. An improved mass spectrometer ion source for use with solid samples of high ionisation potential elements. J. Phys., E: J. ScL Instr., 4 : 1 3 4 - 1 3 6 . Sine, N.M., Taylor, W.O., Webber, G.R. and Lewis, C.L., 1969. Third report of analytical data for CAAS sulphide ore and syenite rock standards. Geochim. Cosmochim. Acta, 3 3 : 1 2 1 - 1 3 1 . Thompson, G., Bankston, D.C. and Pasley, S.M., 1970. Trace element data for reference carbonate rocks. Chem. GeoL, 6: 165-170.