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Correction factors in the determination of natural abundance of stable S isotopes in soil
Appl. Radiat. ht. Vol. 45, NO. 8, pp. 889-893, 1994 Elsevier Science Ltd. Printed in Great Britain
0969-8043(94)EOO33-U
Correction Factors in the Determination of Natural Abundance of Stable S Isotopes in Soil J. ERIKSEN’
and H. SAABY JOHANSEN’
‘The Danish Institute of Plant and Soil Science, Research Centre Foulum, P.O. Box 23, DK-8830 Tjele, Denmark *Department of Mathematics and Physics, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark (Received 29 January 1993; in revised form 31 March 1994)
Sulphur cycling in soils has been investigated using natural abundances of stable S isotopes. A procedure for the determination of 6 “S in soil samples is presented here. After dry ashing, sulphate was precipitated as barium sulphate, which was decomposed at high temperatures to sulphur dioxide for mass spectrometric determination. Various mass spectrometer correction factors were examined and the total correction factor was determined by measurement of standard samples (reference material). The internal and external precision of S MSdetermination in soil samples was 0.03-0.55 and 0.20-0.58 delta value units, respectively.
Introduction Determination
of the natural abundance of stable S
isotopes has proved valuable in identifying sources of sulphur and in monitoring the fate of sulphur compounds in soil-plant systems. The isotopic composition of sulphur can serve as a label which delineates sources, as described by Krouse (1980). These techniques have been used to investigate sulphur cycling in forest soils (e.g. Krouse and Case, 1981; Krouse et al., 1984; Van Stempvoort et al., 1991) and in agricultural soils (e.g. Chae and Krouse, 1986; Schoenau and Bettany, 1989). For a review see Krouse et al. (1991). Differences in S isotopic composition are defined as:
ratios is poor. This fact is most likely explained by the variation in magnitude of the correction factor applied at different laboratories. The purpose of this paper is to describe a method for the determination of S “S in soil samples giving special attention to the correction factor. The total correction factor was determined by measurements on standard samples, but also contributions from single parameters were considered. Mass spectrometric corrections may be divided into: (a) general corrections (leak, valve mixing and background corrections); and (b) corrections specific to the measurement of S isotopes (inlet-line memory effects and O-isotope interference).
Experimental Analytical procedure
x 1000,
(1)
where the standard is sulphur from the troilite of the Canyon Diablo meteorite (CDT). Stable isotope analyses are performed using an isotope ratio mass spectrometer (IRMS), which simultaneously collects ion currents containing the two isotopes. The sample gas is usually SO1. It is increasingly common for isotopic analyses to be performed with commercially available mass spectrometers, but also for these instruments it is necessary to consider raw data corrections (Rees and Holt, 1991). According to Rees (1978) agreement between laboratories measuring sulphur isotope
A soil sample containing approx. 1 mg S was oxidized using the dry ashing procedure described by Steinbergs et al. (1962). 2 M HCl was added, until gas evolution stopped. The solution was filtered, supplied with methyl red indicator and acidified with 2 M HCl until the colour changed. Sulphate in the filtrate was precipitated using the procedure described by Piper (1944). After standing overnight the solution was centrifuged at 5000g for 10 min, the supernatant was discarded, and the precipitate was dried at 80°C. The dried barium sulphate precipitate was loaded into a tin capsule (Mikro Kemi AB, Sweden) and mixed with approx. 10 mg V,Os. The capsule was decomposed in the furnace of an elemental analyser (Carlo Erba 1108) at 1020°C in a stream of Oz. 889
J. ERIKSENand
890
Reduced copper wire was inserted into the combustion tube to reduce evolved SO, to SO2 (Kirsten and Nordenmark, 1987). In a stream of He, SO2 was passed through a water absorption tube filled with magnesium perchlorate and a 25 cm Teflon column filled with Porapak QS prior to condensation in a stainless-steel trap immersed in liquid nitrogen. The trap was connected to the inlet of the mass spectrometer [Tracermass Stable Isotope Analyzer (9002)], and noncondensible gas was pumped out. The liquid nitrogen was removed and SO2 was let into the dual inlet reservoir. The isotopic composition of the sample was determined by measuring the ion currents at mass 66 and 64, respectively, when sample and standard SOz were admitted alternately into the mass spectrometer. Mass Spectrometer Correction Factors Leak correction
As described by Deines (1970) for carbon and oxygen isotopes there is a possibility of fractionation differences in the viscous leaks of the dual inlet system. This is also true for sulphur. The leak correction can be determined by measuring the isotopic composition difference when the same sample is admitted through both the sample and the standard leak. The results of 5 runs of identical samples are listed in Table 1. According to these data a leak correction is not required. Valve mixing correction
The valve that switches between the standard and sample gas reservoirs is not always perfectly sealed. Consequently, sample gas can enter the mass spectrometer when the standard gas is analyzed and vice versa. Deines (1970) determined the correction factor for valve mixing when measuring CO1 as 1 +fi +fi, wheref, is the fraction of the sample gas leaking into the mass spectrometer while the standard gas is being analyzed, and fr is the fraction of the standard gas leaking into the mass spectrometer while the sample gas is being analyzed. To obtain a similar correction for SO2 the following procedure was used: (a) the reservoirs for sample and standard gas were filled with SO1, and the pressure adjusted to normal operating conditions; (b) the mass 64 ion beam intensity was observed; (c) the gas in one of the reservoirs was pumped out; Table I. Determination of isotopic ratios in SO, from identical samples admitted through standard and sample leaks Ratio mass 66164 Sample No. 1 2 3 4 5
Standard
Sample
6 (66) (S)
0.049584 I2 0.04927523 0.04923852 0.04898068 0.05156726
0.04955833 0.04928922 0.04922278 0.04900586 0.05 156766
-0.52 0.28 -0.32 0.51 0.01
x*s/&
-0.01 +0.19
H. SAABYJOHANSEN Table 2. Measurement of the intensity drop, when pumping away SO, from the other reservoir in a dual inlet system,expressedas a fraction of total mass 64 ion beam intensity Reservoir Standard
Sample
8
7
0.011 0.003 0.020 0.002
0.020 0.009 0.031 0.004
No. of measurements Intensity drop (fraction of total) Mean Min Max SIJ;;
and (d) the drop in the mass 64 ion beam intensity of the gas from the other reservoir was determined. The intensity drop, expressed as a fraction of the total mass 64 ion beam intensity, gave the quantity fi . In a similar way, fz was determined by observing the intensity drop caused by pumping out SO2 from the other reservoir. The results of this procedure carried out on the instrument used in this laboratory are seen in Table 2. The relative standard deviation of fractions A and fi was high (~20%). Two main reasons were: (1) the apparent beam intensity fluctuated because of detector noise; and (2) the fraction determined was a small difference between two large numbers. This does not mean, necessarily, that the valve mixing correction varies between runs. Thus, the valve mixing correction factor is taken to be 1 + 0.011 + 0.020 = 1.031 (s = 0.004). Background correction
Before running a batch of samples the mass spectrometer is heated to release adsorbed SOr. The background mass 64 ion beam intensity is thus reduced to 10-14A. During a batch of samples the background slowly increases, and when the isotopic composition of the background and the sample differs, it is necessary to consider a background correction. The following quantities are defined: Z66.X : Unknown sample ion beam intensity at mass 66 Z64.X : Unknown sample ion beam intensity at mass 64 Z6b.s: Standard sample ion beam intensity at mass 66 Z64,s: Standard sample ion beam intensity at mass 64 Zf&b: Background ion beam intensity at mass 66 Z64,b : Background ion beam intensity at mass 64 The measured delta value [6 (66)] can be expressed as:
6 (66) =
&6.x+ zM.b z64,+ z64,_ z66.s + z,, z64,s + z64,b
1
1
x 1000
J
(2)
891
Natural abundance of S isotopes and the delta value (6 ?I) is defined as: . x looo.
The correction reduced to:
(3)
factor between (2) and (3) can be
against time, after closing the valve is shown. It appears that after 30 and 60 s, respectively, 0.09 and 0.04% of the original beam is persistent in the mass spectrometer. This effect varies depending on the type of instrument used. Rees (1978) made the inlet line as short as possible and maintained it at an elevated temperature (~60C). These precautions seem unnecessary for the mass spectrometer used in this laboratory, as a delay time between valve changeover and commencement of the measuring procedure of 30 s will reduce the correction factor to less than 1.0009. O-isotope interference
+ Z&+J z;,;:)].
(4)
The second correction term cancels out when ZMJ= Z,,s and the first depends on the difference between Z66,X and Zs6,s. It appears from (2) that the background increases the sample and the standard ion beam by the same absolute quantity, which implies that the background ion beam is only significant, when there are large differences in the isotopic composition between the standard and sample. Let the difference between the standard and the sample be 100 delta value units and Za4,s= ZM,,= 1 x 10e9 A, then Z66,s = 5 x lo-” A and I,, = 5.5 x lo-” A. If, in this situation, a background correction of less than 1% is desired, the background ion beam at mass 66 has to be kept below 6 x lo-” A. Since this is easily obtained, and since natural variations in S-isotope composition never approach lOO%o,a background correction is not required for this instrument.
Several combinations of oxygen and sulphur isotopes produce mass 66 ions. In the mass spectrometer the following ratio is measured: mass 66 mass 64 = 34s160'60
+
~33s170160
32~170170
+
“S’60’60 +
-+2;. '60'60
(5)
As the natural abundances of 33Sand I70 are 0.750 and 0.0374 mol%, respectively (Birkenfeld et al., 1969), they can be ignored in the calculation. Thus: “S -= 32s
mass --2;. 66 mass 64
(6)
Hulston and Shilton (1958) used this relationship to derive the following formula for the delta value:
6&WQx-(%-2Qs) R,-2Q,
x looo,
1 I 0
I 10
I 30
I 20 TIME
(1)
IN
I 40
I 50
I 60
, 70
SECONDS
Fig. 1. Variation of ion beam at mass 66 as a function of time after closing the valve.
(7)
where
and R, and Q, are the equivalent ratios for the standard. The above formula can be transformed to read as follows: 1 634S= 6 ‘80 , -6(66)-h (8) 11-3 2Qs * R, where 6 (66) is defined by equation (2), and 6 I80 is the delta value for the ratio ‘*O/‘6Oin the sample and the standard. Accordingly, the correction factor depends on which standard is used. When using CDT, with R, = l/22.1 and Q, = l/490 the correction is: 6 “S = 1.096 (66) - 0.1s ‘80.
0.01
~32sl6('~180
17(y70
Inlet-line memory effect
Desorption of SO2 from the walls of the tubing between the switching valves and the ion source is slow and can give rise to a memory effect. In Fig. 1 a plot of the decrease in intensity of the ion beam at mass 66
+
(9)
This correction is valid for systems, in which the O-isotope abundance in the sample Q, deviates from that in the standard Q,. When SO2 is extracted from sulphate by decomposition in a stream of oxygen (e.g. the furnace of an elemental analyzer), the amount of oxygen in the sample or the standard is much less than in the O2 stream introduced into the furnace..
J. ERIKSEN and H.
892
Table 3. Intercomparison samples with composition measured relative to CDT Name
Compound
6 “S
s/./;;
n
ZnS B&O, A@ Aa,S
+0.25 +20.32 -0.25 +21.70
0.14 0.11 -
13 10
NBS 122 NBS 127 NZ 1 NZ 2
If O2 (sample or standard (introduced), then QXx Q, and 6 “S = _-!L
h-28
of S isotopes
R, -?0041
SD of average
IAEA (1987) Stichler (1992) -
material)
~0,
6 (66),
‘@)
(11)
for this type of SO2 extraction. If CDT is used as the standard, the correction factor is 1.10. This is close to that found by Hulston and Shilton (1958) for a system without introduction of Oz. The reason being, that the ratio ‘*O/i60 in CDT is l/490, which by chance is very close to that in atmospheric oxygen. Determination
of the correction factor
In order to determine the “true” correction factor, measurements on standard samples were made. Since only a very limited amount of CDT, which defines the delta scale, exists, intercomparison samples from IAEA in Vienna were used for this purpose (Table 3). From measurements of isotopic differences between intercomparison samples, the correction factor can be determined (Table 4). On average the “true” total correction factor was 1.135 (s/J;; = 0.010). Since only small quantities of the intercomparison samples were available, it was necessary to use an internal laboratory standard. This standard (analytical grade
Table 4. Determination of the correction factor via comparison samples with known isotopic composition 6”s Sample names NBS 122 NBS122 NZ I
-----
Measured NBS 127
NZ2 NZ2
17.29 18.27 17.81 17.40 17.93 18.34 19.69
in %0 Litt. value 20.07 20.07 20.07 20.07 20.07 21.45 21.95 x+s/./;;:
Table 5. 5 “S in soil samoles Soil
Reference
where Q is the i80/i60 ratio in the gas introduced during decomposition of sample and standard. The origin of the introduced gas is atmospheric. Variations in O-isotope composition in the atmosphere Winkler (1984) are small. According to 6 ‘*O = 22-2% relative to the SMOW standard, which means that Q is 0.00205-0.00206. Assuming that fractionation of O-isotopes is negligible when preparing pure oxygen from atmospheric air, the corrected result is given by: S”S=
SAABY JOHANSEN
Calculated correction factor 1.161 1.099 1.127 1.153 1.119 1.170 1.115 1.135*0.010
of
2 3 4 5 6 7 8 9 10 11 12 13 14
No. of
6%
4
w
Internal
External
5.24 6.27 10.08 6.05 4.31 4.47 7.68 5.25 7.06 6.30 4.46 4.15 7.78 6.58
0.03 0.12 0.31 0.28 0.27 0.36 0.12 0.22 0.25 0.55 0.28 0.13 0.46 0.19
0.20 0.23 0.37 0.35 0.34 0.42 0.23 0.30 0.32 0.58 0.35 0.24 0.50 0.27
BaSO,) was determined relative to NBS 122 and NBS 127 (6 34S= 6.9; s/J;; = 0.2). Calculations From the ratios (R), between mass 66 ion beam and mass 64 ion beam measured in the mass spectrometer, 6 (66) was calculated as: 6(66)=(s-+
1000,
(12)
the 6 ‘% value was then calculated as follows: 6%=6(66)x1.1351+6.92+1O-3x6(66) x 1.135 x 6.g3 (13) ‘total correction factor, 2value of laboratory standard, 3correction term due to the transformation tween 6 -scales.
be-
Results and Discussion Of the examined correction factors only two are of significance: valve mixing and O-isotope interference correction. The former is specific to the instrument used in this laboratory, whereas the latter is applicable to any system using SO2 extraction with oxygen injection. The estimated total correction factor will be the product of these two: 1.100 x 1.031 = 1.134 (SD = 0.004). This is in good agreement with the factor: 1.135 observed correction actually (s/&r = O.OlO),which indicates that either there are no interactions between the different types of interferences or they cancel. In Table 5 results of the determination of 6 %i in 14 different soil samples, are given. The internal standard deviation originates from measurement uncertainties only, assuming that the correction factor and laboratory standard are truly constants. On average, the internal standard deviation was 0.26%0. For mutual comparison of the soils it is appropriate to use the internal deviation. But for comparison with the values measured in other laboratories. it is necess-
Natural abundance of S isotopes aty to consider deviations arising from the determination of the correction factor and the laboratory standard (the external standard deviation). This increases the standard deviation somewhat (Table 5).
Since the laboratory standard was determined relative to the intercomoarison samoles and these were determined relative *to CDT, deviations tended to accumulate. The correction little to the external standard
factor contributes very deviation for the instru-
ment used here. In conclusion, the data presented here demonstrate the importance of procuring information on the mass
spectrometric correction factors necessary for accurate determination
of 6 “S. In this study the precision of 6 34S in soil samples was limited by uncertainties on 6 34S in standard samples.
of the determination
Acknowledgements-The authors would like to express their gratitude to Dr Victor Middelboe for his comments on the manuscript.
References Birkenfeld H., Haase G. and Zahn H. (1969) Massen Spektrometrische Isotopenanalyse. VEB Deutscher Verlag der Wissenschaften, Berlin. Chae Y. M. and Krouse H. R. (1986) Alteration of sulfur-34 natural abundance in soil by application of feedlot manure. Soil Sci. Sot. Am. J. 50, 1425. Deines P. (1970) Mass spectrometer correction factors for the determination of small isotopic composition variations of carbon and oxygen. ht. J. Mass Spectrom. Ion Phys. 4, 283.
Hulston J. R. and Shilton B. W. (1958) Sulnhur isotonic variations in nature. Part Ameasurement of sulphur isotopic ratio by mass spectrometry. N.Z. J. Sci. 1, 91. IAEA (1987) Consultants’ Group Meeting on Stable Isotope Reference samples for Geochemical -and Hydrological Investigations. (G. Hut, Ed.). IAEA Vienna 16-18
September 1985.
893
Kirsten J. W. and Nordenmark B. S. (1987) Rapid, automatic, high-precision method for micro, ultramicro, and trace determinations of sulfur. Anal. Chim. Acta l%, 59
’ H. R. (1980) Sulphur isotopes in our environment. Krouse In P. Fritz and J. Ch. Fontes (Ed.) Handbook of Environmental Isotope Geochemistry, p. 435. Elsevier, Amsterdam. Krouse H. R. and Case J. W. (1981) Sulphur isotope ratios in water, air, soil and vegetation near Teepee Creek gas plant, Alberta. Water Air Soil Poffut. 15, 11. Krause H. R., Legge A. H. and Brown H. M. (1984) Sulphur gas emissions in the boreal forest: the West Whitecourt case study V. Stable sulphur isotopes. Water Air Soil Pollut. 22, 321.
Krouse et al. (1991) Case studies and potential applications. In H. R. Krouse and V. A. Grinenko (Eds). Stab/e Isotopes: Natural and Anthropogenic Sulphur in the Environment, p. 307. Scope, 43. Wiley, New York. Piper C. S. (1944) Soil and Plant Analysis. Interscience, New York. Rees C. E. (1978) Sulphur isotope measurements using SO, and SF, . Geochim. Cosmochim. Acta 42. 383.
Rees C. E. and Halt B. D. (1991) The isotopic analysis of sulphur and oxygen. In H. R. Krause and V. A. Grinenko (Eds). Stable Isotopes: Natural and Anthropogenic Stdphur in the Environment, p. 43. Scope, 43. Wiley, New York. Schoenau J. J. and Bettany J. R. (1989) %S natural abundance variations in prairie and boreal forest soils. J. Soil Sci. 40, 397. Steinbergs A., Iismaa 0. and Freney J. R. (1962) Determination of total sulphur in soil and plant material. Anal. Chim. Acta. 27, 158.
Stichler W. (1992) Personal Communication. Isotope Hydrology Section, IAEA, Vienna. Van Stempvoort D. R., Wills J. J. and Fritz P. (1991) Above-ground vegetation effects on the deposition and cycling of atmosnheric sulfur: chemical and stable isotone evide&e. Water-Air Soil Pollut. 60, 55. Winkler F. J. (1984) Application of natural abundance stable isotope mass spectrometry in food control. In A. Frigerio and H. Milan (Eds) Chromatography and Mass Spectrometry in Nutrition Science and Food Safety, p. 173. Elsevier, Amsterdam.
0969-8043(94)EOO33-U
Correction Factors in the Determination of Natural Abundance of Stable S Isotopes in Soil J. ERIKSEN’
and H. SAABY JOHANSEN’
‘The Danish Institute of Plant and Soil Science, Research Centre Foulum, P.O. Box 23, DK-8830 Tjele, Denmark *Department of Mathematics and Physics, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark (Received 29 January 1993; in revised form 31 March 1994)
Sulphur cycling in soils has been investigated using natural abundances of stable S isotopes. A procedure for the determination of 6 “S in soil samples is presented here. After dry ashing, sulphate was precipitated as barium sulphate, which was decomposed at high temperatures to sulphur dioxide for mass spectrometric determination. Various mass spectrometer correction factors were examined and the total correction factor was determined by measurement of standard samples (reference material). The internal and external precision of S MSdetermination in soil samples was 0.03-0.55 and 0.20-0.58 delta value units, respectively.
Introduction Determination
of the natural abundance of stable S
isotopes has proved valuable in identifying sources of sulphur and in monitoring the fate of sulphur compounds in soil-plant systems. The isotopic composition of sulphur can serve as a label which delineates sources, as described by Krouse (1980). These techniques have been used to investigate sulphur cycling in forest soils (e.g. Krouse and Case, 1981; Krouse et al., 1984; Van Stempvoort et al., 1991) and in agricultural soils (e.g. Chae and Krouse, 1986; Schoenau and Bettany, 1989). For a review see Krouse et al. (1991). Differences in S isotopic composition are defined as:
ratios is poor. This fact is most likely explained by the variation in magnitude of the correction factor applied at different laboratories. The purpose of this paper is to describe a method for the determination of S “S in soil samples giving special attention to the correction factor. The total correction factor was determined by measurements on standard samples, but also contributions from single parameters were considered. Mass spectrometric corrections may be divided into: (a) general corrections (leak, valve mixing and background corrections); and (b) corrections specific to the measurement of S isotopes (inlet-line memory effects and O-isotope interference).
Experimental Analytical procedure
x 1000,
(1)
where the standard is sulphur from the troilite of the Canyon Diablo meteorite (CDT). Stable isotope analyses are performed using an isotope ratio mass spectrometer (IRMS), which simultaneously collects ion currents containing the two isotopes. The sample gas is usually SO1. It is increasingly common for isotopic analyses to be performed with commercially available mass spectrometers, but also for these instruments it is necessary to consider raw data corrections (Rees and Holt, 1991). According to Rees (1978) agreement between laboratories measuring sulphur isotope
A soil sample containing approx. 1 mg S was oxidized using the dry ashing procedure described by Steinbergs et al. (1962). 2 M HCl was added, until gas evolution stopped. The solution was filtered, supplied with methyl red indicator and acidified with 2 M HCl until the colour changed. Sulphate in the filtrate was precipitated using the procedure described by Piper (1944). After standing overnight the solution was centrifuged at 5000g for 10 min, the supernatant was discarded, and the precipitate was dried at 80°C. The dried barium sulphate precipitate was loaded into a tin capsule (Mikro Kemi AB, Sweden) and mixed with approx. 10 mg V,Os. The capsule was decomposed in the furnace of an elemental analyser (Carlo Erba 1108) at 1020°C in a stream of Oz. 889
J. ERIKSENand
890
Reduced copper wire was inserted into the combustion tube to reduce evolved SO, to SO2 (Kirsten and Nordenmark, 1987). In a stream of He, SO2 was passed through a water absorption tube filled with magnesium perchlorate and a 25 cm Teflon column filled with Porapak QS prior to condensation in a stainless-steel trap immersed in liquid nitrogen. The trap was connected to the inlet of the mass spectrometer [Tracermass Stable Isotope Analyzer (9002)], and noncondensible gas was pumped out. The liquid nitrogen was removed and SO2 was let into the dual inlet reservoir. The isotopic composition of the sample was determined by measuring the ion currents at mass 66 and 64, respectively, when sample and standard SOz were admitted alternately into the mass spectrometer. Mass Spectrometer Correction Factors Leak correction
As described by Deines (1970) for carbon and oxygen isotopes there is a possibility of fractionation differences in the viscous leaks of the dual inlet system. This is also true for sulphur. The leak correction can be determined by measuring the isotopic composition difference when the same sample is admitted through both the sample and the standard leak. The results of 5 runs of identical samples are listed in Table 1. According to these data a leak correction is not required. Valve mixing correction
The valve that switches between the standard and sample gas reservoirs is not always perfectly sealed. Consequently, sample gas can enter the mass spectrometer when the standard gas is analyzed and vice versa. Deines (1970) determined the correction factor for valve mixing when measuring CO1 as 1 +fi +fi, wheref, is the fraction of the sample gas leaking into the mass spectrometer while the standard gas is being analyzed, and fr is the fraction of the standard gas leaking into the mass spectrometer while the sample gas is being analyzed. To obtain a similar correction for SO2 the following procedure was used: (a) the reservoirs for sample and standard gas were filled with SO1, and the pressure adjusted to normal operating conditions; (b) the mass 64 ion beam intensity was observed; (c) the gas in one of the reservoirs was pumped out; Table I. Determination of isotopic ratios in SO, from identical samples admitted through standard and sample leaks Ratio mass 66164 Sample No. 1 2 3 4 5
Standard
Sample
6 (66) (S)
0.049584 I2 0.04927523 0.04923852 0.04898068 0.05156726
0.04955833 0.04928922 0.04922278 0.04900586 0.05 156766
-0.52 0.28 -0.32 0.51 0.01
x*s/&
-0.01 +0.19
H. SAABYJOHANSEN Table 2. Measurement of the intensity drop, when pumping away SO, from the other reservoir in a dual inlet system,expressedas a fraction of total mass 64 ion beam intensity Reservoir Standard
Sample
8
7
0.011 0.003 0.020 0.002
0.020 0.009 0.031 0.004
No. of measurements Intensity drop (fraction of total) Mean Min Max SIJ;;
and (d) the drop in the mass 64 ion beam intensity of the gas from the other reservoir was determined. The intensity drop, expressed as a fraction of the total mass 64 ion beam intensity, gave the quantity fi . In a similar way, fz was determined by observing the intensity drop caused by pumping out SO2 from the other reservoir. The results of this procedure carried out on the instrument used in this laboratory are seen in Table 2. The relative standard deviation of fractions A and fi was high (~20%). Two main reasons were: (1) the apparent beam intensity fluctuated because of detector noise; and (2) the fraction determined was a small difference between two large numbers. This does not mean, necessarily, that the valve mixing correction varies between runs. Thus, the valve mixing correction factor is taken to be 1 + 0.011 + 0.020 = 1.031 (s = 0.004). Background correction
Before running a batch of samples the mass spectrometer is heated to release adsorbed SOr. The background mass 64 ion beam intensity is thus reduced to 10-14A. During a batch of samples the background slowly increases, and when the isotopic composition of the background and the sample differs, it is necessary to consider a background correction. The following quantities are defined: Z66.X : Unknown sample ion beam intensity at mass 66 Z64.X : Unknown sample ion beam intensity at mass 64 Z6b.s: Standard sample ion beam intensity at mass 66 Z64,s: Standard sample ion beam intensity at mass 64 Zf&b: Background ion beam intensity at mass 66 Z64,b : Background ion beam intensity at mass 64 The measured delta value [6 (66)] can be expressed as:
6 (66) =
&6.x+ zM.b z64,+ z64,_ z66.s + z,, z64,s + z64,b
1
1
x 1000
J
(2)
891
Natural abundance of S isotopes and the delta value (6 ?I) is defined as: . x looo.
The correction reduced to:
(3)
factor between (2) and (3) can be
against time, after closing the valve is shown. It appears that after 30 and 60 s, respectively, 0.09 and 0.04% of the original beam is persistent in the mass spectrometer. This effect varies depending on the type of instrument used. Rees (1978) made the inlet line as short as possible and maintained it at an elevated temperature (~60C). These precautions seem unnecessary for the mass spectrometer used in this laboratory, as a delay time between valve changeover and commencement of the measuring procedure of 30 s will reduce the correction factor to less than 1.0009. O-isotope interference
+ Z&+J z;,;:)].
(4)
The second correction term cancels out when ZMJ= Z,,s and the first depends on the difference between Z66,X and Zs6,s. It appears from (2) that the background increases the sample and the standard ion beam by the same absolute quantity, which implies that the background ion beam is only significant, when there are large differences in the isotopic composition between the standard and sample. Let the difference between the standard and the sample be 100 delta value units and Za4,s= ZM,,= 1 x 10e9 A, then Z66,s = 5 x lo-” A and I,, = 5.5 x lo-” A. If, in this situation, a background correction of less than 1% is desired, the background ion beam at mass 66 has to be kept below 6 x lo-” A. Since this is easily obtained, and since natural variations in S-isotope composition never approach lOO%o,a background correction is not required for this instrument.
Several combinations of oxygen and sulphur isotopes produce mass 66 ions. In the mass spectrometer the following ratio is measured: mass 66 mass 64 = 34s160'60
+
~33s170160
32~170170
+
“S’60’60 +
-+2;. '60'60
(5)
As the natural abundances of 33Sand I70 are 0.750 and 0.0374 mol%, respectively (Birkenfeld et al., 1969), they can be ignored in the calculation. Thus: “S -= 32s
mass --2;. 66 mass 64
(6)
Hulston and Shilton (1958) used this relationship to derive the following formula for the delta value:
6&WQx-(%-2Qs) R,-2Q,
x looo,
1 I 0
I 10
I 30
I 20 TIME
(1)
IN
I 40
I 50
I 60
, 70
SECONDS
Fig. 1. Variation of ion beam at mass 66 as a function of time after closing the valve.
(7)
where
and R, and Q, are the equivalent ratios for the standard. The above formula can be transformed to read as follows: 1 634S= 6 ‘80 , -6(66)-h (8) 11-3 2Qs * R, where 6 (66) is defined by equation (2), and 6 I80 is the delta value for the ratio ‘*O/‘6Oin the sample and the standard. Accordingly, the correction factor depends on which standard is used. When using CDT, with R, = l/22.1 and Q, = l/490 the correction is: 6 “S = 1.096 (66) - 0.1s ‘80.
0.01
~32sl6('~180
17(y70
Inlet-line memory effect
Desorption of SO2 from the walls of the tubing between the switching valves and the ion source is slow and can give rise to a memory effect. In Fig. 1 a plot of the decrease in intensity of the ion beam at mass 66
+
(9)
This correction is valid for systems, in which the O-isotope abundance in the sample Q, deviates from that in the standard Q,. When SO2 is extracted from sulphate by decomposition in a stream of oxygen (e.g. the furnace of an elemental analyzer), the amount of oxygen in the sample or the standard is much less than in the O2 stream introduced into the furnace..
J. ERIKSEN and H.
892
Table 3. Intercomparison samples with composition measured relative to CDT Name
Compound
6 “S
s/./;;
n
ZnS B&O, A@ Aa,S
+0.25 +20.32 -0.25 +21.70
0.14 0.11 -
13 10
NBS 122 NBS 127 NZ 1 NZ 2
If O2 (sample or standard (introduced), then QXx Q, and 6 “S = _-!L
h-28
of S isotopes
R, -?0041
SD of average
IAEA (1987) Stichler (1992) -
material)
~0,
6 (66),
‘@)
(11)
for this type of SO2 extraction. If CDT is used as the standard, the correction factor is 1.10. This is close to that found by Hulston and Shilton (1958) for a system without introduction of Oz. The reason being, that the ratio ‘*O/i60 in CDT is l/490, which by chance is very close to that in atmospheric oxygen. Determination
of the correction factor
In order to determine the “true” correction factor, measurements on standard samples were made. Since only a very limited amount of CDT, which defines the delta scale, exists, intercomparison samples from IAEA in Vienna were used for this purpose (Table 3). From measurements of isotopic differences between intercomparison samples, the correction factor can be determined (Table 4). On average the “true” total correction factor was 1.135 (s/J;; = 0.010). Since only small quantities of the intercomparison samples were available, it was necessary to use an internal laboratory standard. This standard (analytical grade
Table 4. Determination of the correction factor via comparison samples with known isotopic composition 6”s Sample names NBS 122 NBS122 NZ I
-----
Measured NBS 127
NZ2 NZ2
17.29 18.27 17.81 17.40 17.93 18.34 19.69
in %0 Litt. value 20.07 20.07 20.07 20.07 20.07 21.45 21.95 x+s/./;;:
Table 5. 5 “S in soil samoles Soil
Reference
where Q is the i80/i60 ratio in the gas introduced during decomposition of sample and standard. The origin of the introduced gas is atmospheric. Variations in O-isotope composition in the atmosphere Winkler (1984) are small. According to 6 ‘*O = 22-2% relative to the SMOW standard, which means that Q is 0.00205-0.00206. Assuming that fractionation of O-isotopes is negligible when preparing pure oxygen from atmospheric air, the corrected result is given by: S”S=
SAABY JOHANSEN
Calculated correction factor 1.161 1.099 1.127 1.153 1.119 1.170 1.115 1.135*0.010
of
2 3 4 5 6 7 8 9 10 11 12 13 14
No. of
6%
4
w
Internal
External
5.24 6.27 10.08 6.05 4.31 4.47 7.68 5.25 7.06 6.30 4.46 4.15 7.78 6.58
0.03 0.12 0.31 0.28 0.27 0.36 0.12 0.22 0.25 0.55 0.28 0.13 0.46 0.19
0.20 0.23 0.37 0.35 0.34 0.42 0.23 0.30 0.32 0.58 0.35 0.24 0.50 0.27
BaSO,) was determined relative to NBS 122 and NBS 127 (6 34S= 6.9; s/J;; = 0.2). Calculations From the ratios (R), between mass 66 ion beam and mass 64 ion beam measured in the mass spectrometer, 6 (66) was calculated as: 6(66)=(s-+
1000,
(12)
the 6 ‘% value was then calculated as follows: 6%=6(66)x1.1351+6.92+1O-3x6(66) x 1.135 x 6.g3 (13) ‘total correction factor, 2value of laboratory standard, 3correction term due to the transformation tween 6 -scales.
be-
Results and Discussion Of the examined correction factors only two are of significance: valve mixing and O-isotope interference correction. The former is specific to the instrument used in this laboratory, whereas the latter is applicable to any system using SO2 extraction with oxygen injection. The estimated total correction factor will be the product of these two: 1.100 x 1.031 = 1.134 (SD = 0.004). This is in good agreement with the factor: 1.135 observed correction actually (s/&r = O.OlO),which indicates that either there are no interactions between the different types of interferences or they cancel. In Table 5 results of the determination of 6 %i in 14 different soil samples, are given. The internal standard deviation originates from measurement uncertainties only, assuming that the correction factor and laboratory standard are truly constants. On average, the internal standard deviation was 0.26%0. For mutual comparison of the soils it is appropriate to use the internal deviation. But for comparison with the values measured in other laboratories. it is necess-
Natural abundance of S isotopes aty to consider deviations arising from the determination of the correction factor and the laboratory standard (the external standard deviation). This increases the standard deviation somewhat (Table 5).
Since the laboratory standard was determined relative to the intercomoarison samoles and these were determined relative *to CDT, deviations tended to accumulate. The correction little to the external standard
factor contributes very deviation for the instru-
ment used here. In conclusion, the data presented here demonstrate the importance of procuring information on the mass
spectrometric correction factors necessary for accurate determination
of 6 “S. In this study the precision of 6 34S in soil samples was limited by uncertainties on 6 34S in standard samples.
of the determination
Acknowledgements-The authors would like to express their gratitude to Dr Victor Middelboe for his comments on the manuscript.
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