.....
Figure 4. Electron
... . , ,
. ., -.~,
impact mass spectrum of Yb(thd)s
By contrast, Figure 4 shows the E1 mass spectrum of Yh(thd)s. The spectrum displays a large nnmher of fragment peaks. Similarly for reasons previously described these spectra show only that the CI mass spectrum is free from fragment peaks as compared to the E1 mass spectrum. Figure 5 shows the CI mass spectrum of a mixture of Er(thd)j, Tm(thd)a, Yh(thd)a, and Lu(thd)j. As can he seen by looking at the last four columns of Tahle I, a spectrum due to a mixture of these four elements could he analyzed readily since the 716 and 717 peaks are indicative of Er, the 719 peak is largely indicative of Tm, the 722, 723, and 724 peaks are indicative of Yh, and 725 peak is largely indicative of Lu. This spectrum could he used only for qualitative or semiqnantitative analysis but with computer time averaging, this method could he readily used for quantitative analysis as there are no fragment peaks. If the same mixture were run on an E1 mass spectrometer, fragmentation of the [Lu(thd)a]+ hy loss of CH3 would cause overlap with the parent peak of erbium, thus making E
Figure 5. lsobutane chemical ionization mass spectrum of Er(thd)l. Tm(thd)S. Yb(thd)s, and Lu(thd)S in a mixture
Yh(thd)j. Ytterbium has seven naturally abundant isotopes: YhI6' = 0.13570, Yh"0 = 3.0370, Yhl7' = 14.3170, Yb172 = 21.82%, Yb173 = 16.13%, Yb1T4 = 31.86%, YhlTe = 12.73%. Tahle I shows that one expects to find six larger peaks, and Figure 3 shows this to he the case. There are no fragment peaks okserved in the data.
search Laboratories, Wright-Patterson Air Force Base, Ohio, who supplied the chelates used in this study. Also, we thank David Rosenthal for the use of the AEI MS-902 facilities of the Center for Mass Suectrometrv. .. Research Triangle Institute, Durham, N.C. Received for review Auril9.1973. Accented Julv 23. 1973
Preparation of Sulfur Dioxide f'or kass 3pec;iru11ieier Analyses by Combustion of SIJlfides with Copper Oxide Peter Fritz, R. J. Drimmie, and V. K. Nowicki Department at Earth Sciences, University of Waterloo, Ontario, Canada
The mass-spectrometric determination of 34S/32S ratios in natural materials is most commonly done with sulfur dioxide. The standard technique for the preparation of sulfur dioxide from sulfides or native sulfur consists of their combustion in a stream of oxygen at temperatures between 900 and 1350 "C (1-3). This technique allows the almost total recovery of the sulfur in the form of sulfur dioxide and, therefore, has been accepted by most lahoratories. However, this conversion is not carried out under vacuum, and contamination of the sulfur dioxide by atmospheric gases of impure oxygen is possible. To overcome this disadvantage and to have another independent method, a technique for the conversion of sulfides into hexafluorides has been developed (4, 5). This technique permits the handling of extremely small samples, but its disadvantage lies in the use of extremely reactive fluorinating agents. (1) H G Thode, J. Macnarnara. and C. B Collins, Can. J. Res., 27, 361
,,"_",.
ilCll0,
(2)T. A Rafler, N. 2. J. Sci. Tech.. Sect. B , 38,849 (1957a). (3) T. A. Rafter. N. 2.J. Sci. Tech., Sect. B , 38, 969 (1957b) 14) J. R. Hulstanand H. G. Thode. J. Geoohvs. Res.. 70. 3475 11965) , ~~~. (5) H. Pucheit, B. R. Sabels, &T. C. hiering. Geochim. Cosmochim. Acta, 35, 625 (1971).
164
It is mererore aesiraDie to nave a simple preparation technique in which the preparation line can he evacuated hut which does not involve dangerous chemicals or complicated procedures. This is possible if an oxygen donor is intimately mixed with the sulfides and the combustion is carried out a t elevated temperatures. Several laboratories have begun to experiment with such a preparation technique using reagent grade cupric oxide (CuO) or cuprous oxide (CnzO) as oxygen donors. Our findings using this technique are described.
EX1 Details of the preparation line used for these experiments ai'e shown in Figure 1. The essential parts are a combustion chamhiII and a vacuum line for the purification and collection of the sulfilr dioxide. " .^ ^ ^^^ .. . .. .*.. m e cnarge ot IUU-mu mg or inrimareiy mixen oxme ana sumde is packed between quartz wool plugs in an open-ended quartz tube. It is pushed into the combustion chamber nnee the system is evacuated, and the furnace is heated to the desired temperature. The evolving SO2 is trapped with liquid nitrogen in the first cooling trap. The combustion i s complete after about 3 minutes and any excess oxygen can be pumped off. The SO3 is then distilled into the second trap hy replacing the liquid nitrogen with a cooling mixture held at about -40 "C.A total yield measurement
_. .
ANALYTICAL CHEMISTRY. VOL. 46, NO. 1, JANUAR'I' 1974
1
.
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n I
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Figure 1. Combustion line for the conversion of sulfides to sulfur dioxide
stopcocks are made by Ace Glass: a) No. 8194-03 or 8195-03, b) No. 8196-05, and c ) No. 8194-05. The only grease joint used connects the storage vessel with the preparation line All
r
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-7
-6
50
60
70
80
90
100
%YIELD
Figure 2. Yields and sulfur isotope data obtained by combustion of CuS (reagent grade) with different amounts of CuO or Cu20
(SO2 + COz) is made and the SO2 is frozen back by a mixture of ethanol and liquid nitrogen held a t approximately - 120 "C. Any C02 present is pumped off and a SO2 yield measurement can be made. It is important to note that the SO2 solidifies a t about -74 "C and thus condenses above the level of the cooling mixture. Therefore, before any pumping is done, the level of the freezing mixture should be raised above the ring of frozen S02. If this is not done, significant amounts of SO2 are lost and the reproducibility of the data decreases sharply. After this purification, the SO2 is vaporized by a mixture also kept at about -40 "C and transferred into a storage vessel a t liquid nitrogen temperature. Most commercially available quartz wool is highly contaminated, probably by organic compounds, and it was therefore purified by heating to at least 1200 "C in a stream of oxygen.
RESULTS AND DISCUSSION The method was tested by using CuO and Cu20 as oxygen donors. However, reagent grade Cu20 can contain up to 0.3% oil as a preservative, which results in an extremely high COz production. Because heating of the CuzO up to 500 "C under vacuum does not totally remove this preservative, it is recommended that CuO be used as oxygen donor. However, if Cu2O is used and the SO2 is carefully purified, reproducible data can be obtained, but all samples combusted with Cu20 gave slightly lower P 4 S values than those mixed with CuO (Figures 2 and 3). This is most probably due to different I S 0 contents in the two oxides for which the data presented here have not been corrected.
Purification of the CuO (Fisher reagent grade) was not necessary, since only very minor amounts of COS could be detected. This is demonstrated by the data shown in Table I. For the "purified Son,'' the CuO was preheated to 250 "C and any traces of C02 were pumped off. For the "non-purified S02," neither procedure was applied. More important than the choice of the oxygen donor are the combustion temperatures and the oxygen/sulfur ratio. Both control the yield of sulfur dioxide and, depending on the sulfide, the isotopic composition of the sulfur dioxide. Figure 2 shows the P 4 S values and yields for different combustion temperatures and ratios for mixtures of CuO and CuS. The reproducibility of the 634Sdecreases sharply if stoichiometric mixtures (oxygen/sulfur = 2/1) are burned below 900 "C. However, if excess oxygen is available, reproducible data can be obtained even at lower burning temperatures with relative low yields (Figure 2b). This is not the case for ZnS (Figure 3). The yields and the reproducibility of the 634S values decrease sharply at combustion temperatures below 1000 "C even if excess oxygen is available. At higher burning temperatures, the data are highly reproducible if the oxygen/sulfur ratio is above three. Table I1 lists the yields and isotope data obtained from silver sulfide, galena, and chalcopyrite. Silver sulfide (AgZS) is probably the most commonly burned sulfide, since it is the final product in the reduction of sulfates for sulfur isotope analyses. It has excellent
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1, JANUARY 1974
165
'1
1150 7
v
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-
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-
1000
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900
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850
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9
10
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40
30
%.
50
70 %YIELD
80
60
90
100
Yields and sulfur isotope data obtained by combustion of ZnS (reagent grade) with different amounts of CuO or CuzO
Table I. Comparison of 634SValues Obtained with Purified and Unpurified SO2 Sample
cus cus cus cus ZnS ZnS ZnS
Oxygen/ sulfur ratio
Combustion temperature, " C
634S,'/oo
purified
purified
2: 1 2:l 2:l 4:l 4: 1 5: 1 5: 1
1100 1000 900 800 900
-7.4 -7.6 -7.7 -7.5 f11.1 f11.3 f11.0
-7.5 -7.6 -7.8 -7.5 +10.9 4-11.3 +11.0
1100 1000
non-
Table II. Yields and 634SValues of Sulfides Combusted at Different Temperatures. Atomic Ratio Oxygen (Cu0):Sulfur = 4 : l
6 3 4 s . 0,'oo
burning characteristics, and, if mixed with an excess of CuO, yields above 90% and highly reproducible 6 3 4 s values are obtained even at low combustion temperatures. Similarly good are the yields and reproducibility of isotope data obtained from chalcopyrite. Galena, however, gave low yields and poor isotope data at low burning temperatures. At combustion temperatures above 1000 "C, the yields are normally above 90% and the 634Svalues are reproducible within fO.lO/oo. We did not investigate in which form sulfur was lost from runs with low yields, but most probably so3 or s04'- is formed ( 3 ) .
166
+8-
(4
PbS (Galena)
42s
Teomp, Yield, C %
800 900 1000 1050 1100
94 101 104 ,
..
100
634S.
Oleo
+10.35 f10.45 4-10.45 ... +10.38
Chalcopyrite
Yield,
634S,
Yield,
%
Oioo
%
64
? 94 90 97
f1.71 f3.66 f3.84 f3.86 f3.94
95 97 100 ... 100
634S, 0
loo
-9.70 -9.91 -9.69
... -9.62
The overall reproducibility of 634S values measured on SO2 obtained from the combustion of mixtures with excess oxygen at temperatures above 1000 "C is better than &0.2"/00; the analytical reproducibility on our mass spectrometer (Varian, GD-150) is better than ~ O . l " / & . The ZnS samples used for these experiments have been analyzed by J. Monster (McMaster University) and a 6 3 4 s ZnS us. CuS of f 2 0 . 6 was found for combustion in an oxygen stream. Our measurements (corrected values) give a 634SZnS us. CuS of +20.4. Received for review March 28, 1973. Accepted July 20, 1973.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1, JANUARY 1974
