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This research paper published in the journal of research of the national bureau of standards in 1956 investigates the mass spectrum of sulfur vapor and its isotope abundances. The authors, paul bradt, fred mohler, and vernon h. Dibeler, used a mass spectrometer to measure the appearance potentials of s8 and s2 ions, suggesting the presence of s2 molecules in the vapor. The document also includes information on the experimental procedure, results, and discussion.
Typology: Summaries
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Journal of Research of the Na tional Bureau of Standa rds Vol. 57, No.4, October 1956 Re s ear ch Pa per 2 71 3
Th e ma ss sp ect rum of s ulfur vapor ha s been measu red by evapo ra t in g sulfur from a heat ed t ube dir ect ly in to t he ionization ch ambe r of a ma ss sp e ct r omet er. I ons S;; w it h x ra nging fro m 1 to 8 are ob se rved with St mo st ab und a n t. I s otopc ab und anc es we re com- p u ted f r om t he S;- ions. Th e appearance p otent ials of S: and St are r es pectIv cl .v 8.9 ± 0. a nd 8.3 ± 0.2 el ect ron volt s. Thi s sugg est s t ha t t he vapor in t he ioni zat ion chamber is a mixture of mo lec ules co n ta ining S2 a nd S8 and po ss ib ly ot her molecules.
Th e ma ss spect rum of sulfur vapor ha s b een st udied in co nn ec t ion with a prog ram to establish reference
one objective of this s tud y was to check th e Isotop e ratios and the chem ical puri ty of the reference sa mpl e, the mass spect rum a nd the a pp ea ran ce potentials arc of research interest. Th e molec ular weigh t of sulfur va por indi cates that the va por in equilibri um wi th sulfur at 175 0 C is pr edominan tly S8 [2]. Thi s is a r el atively un sta ble co nfiguration, and in the presence of an c'Iectric dischluge an S2 band spect rum is observed [3]. Th e v ibration series and predissocia- tion spectra. give wi th so me uncertain ty a va lu e of 4.4 ev for the 8 2 bond energy, indi ca ting tlla t S2 is quit e sta bl e.
Th e s ulfur from the reference-sample stock is virgin sulfur from a dome in vVh ar to n County, T ex. Th e sulfur -va por pr eSS Ul' e is too small to meaSUl'e the ma ss spectr um at room te mp eratl~r e , a nd t.h e m eas urement s were m ade by evaporfl tmg the sulfur from a h eated tube in to the ioniza Lio n chamb er of a 60 0 ma ss spect rom eter. A few mi lli g ram s of coarse powder were held in a capi ll ary tube with a thermo- co uple in conta ct with t he tube. Th is in t urn wa s in a 6-mm tube, which ext ended about 2 cm to the entran ce port of the ionization chamb er. Th e ion- iza tion cham ber reached a tempernture of 186 0 C duri l1g operation and the sample attained a steady
tion. Thi s prov ed to b e a convenient temperature for recording the spectr um.. Sulfur dioxide was made by burning the sulfur 111 a,ir , an d the ma ss spect rum of S0 2 and the ail" oxygen was measured with a 180 0 gas-a nalysis ma ss spec- tr om eter. This is the co nventional method of meas- Ul'ing sulfur-isotope abundances.
1 Figur es ill bracket s intlicat e th e lit cra tlll'e references at th e end of this pa pC!'.
3 .1. Mass Sp e ctrum
Table 1 gives th e prin cipal ions observed in the ma ss spect rum of sulfur vapo r. Column 3 giv es the relativ e abu ndan ce of the ions S ~2, whereas co lumn 4 gives t h e monoi sotopic s pertrum. It is the s um. of th e isotope peaks in each Sx group r el ative to the S ions taken as 100. Th e rather compli cated isotope st ru cture iden tifies all these ions as predominan tly singly c har ged ion s, except for th e fo llowing.: l\1ass 16 from S ++ was 0.05 perce n t of t lte 64 peak m a spec- trum where 0 + from O 2 a nd CO 2 was neglig ible. ~la ss 32} ~ from S^328 3 3++ was 0.06 percent of the 64 peak , ancl is abou t 4 percen t of the 65 peak. Hen c e, doubly charged ions of mass 64 co ntribut e 4 percent to the 32 pea k. A peak at ma ss 80 from St+ is 0.06 percent of the 64 p eal:. Impuriti es th at ca n be ascribed to the sulfur ]' atl~ e r than the ma ss-spectrometer backg r ound arc vo latIle gase s, which decrease wit,h time. H 2 S+ ranged from. 1.2 to 0.1 5 p ercen t of the 64 pea k, an d Cs t WfiS 0. Lo 0.07 percent. A 48 peak rangi ng from 0.17 to 0.02 pOl' ce nt m a ~ T be 80 + from S0 2. ' l ~he molec:ule ion masked by St would b e a bou t LW lce the SO+ peak. l' ABLE 1. Jlrass sp ec/nl1 n oj su lfu r vapor
m le Ion
32 ______________ (^) S 32 64 ___ ___________ (^) S ~ 96 ____ __________ S (^323) 128 ___ __________ (^) S:' 160 _____________ (^) S ~'
192 _____________ (^) S ~' 224 _____________ (^) S ~' 256 ____________ _ (^) S ~
R ela ti ve int e ns it y
1:3. 5
Monoi so- topi c spec- trL1m
3.2. Isotope Ratios
8ulfur has four isotopes of abundance 8 32 , 95.0 ; 8 33 , 0. 76 ; 8 34 , 4.2 ; 8 36 , 0.014 (see table 2). The relative intensity of the isotope peaks in a molecule containing x atoms can be expressed formally by
S32 + a1 S 33 + az8 34 + a3 836 ,
where the a's are abundances relative to that of 8 32 as unity , but the 8 terms are chemical symbols, not algebraic terms. Thus the isotopes of 8 2 give the terms:
8 3Z^ S 32 + 2a1 8 32 S 33 + 2az83Z834 + ai8 33 8 33 + 2 a1 az S 338 35 +
2a3S3 2836 + a§ 8 34 8 34 + 2a1 a38 33 S 36 + 2aZa38HS3 6 +a~8 36 S 36.
Collecting te rms of equal-mass numbers gives the relative intensities of the 8 z ions as listed in col umn 2 of table 2. Because of intensity and resolution, the S2 ions are best adapted to deriving isotope ratios, and mass peaks 64, 65, 66, and 68 were used to deter- mine ai, a z, and a 3' Five successive slow scans of the 82 peaks under steady conditions gave the rela- tive intensities listed in column 3, where uncertain- ties listed are the maximum spread of the data. In these measmem ents t he 64 peak was about 5, scale divisions on the most sensitive scale, and a small drift in the 64-peak height was corrected by a linear int er polation to the positions of the mea s- .med peaks on the record. The fourth column gives the derived relativ e abundances.
TABLE 2. I sotope abundances and relative intensities of 8 2 i ons
m le R.elative Observ ed Deriv ed ral at ive intensities inten sities abundanc es
64 ____ ___ (^1) ------------ 65 _______ (^) 2a, O. 01596 ± 7 al= O. 00798 ± 4 66 _______ (^) 2az+ a; 08897 ± 20 a ,= O. 04445 ± 20 67 _______ (^) 2a,az (^) --------- (^) ----------- - 68 _______ (^) 2a 3+ a ~ 00228 ± 2 a3= 0. 00015 69 _______ (^) 2a,a3 (^) --------- ----- ---- -- - 70 ___ ____ (^) 2aZ a3 (^) - - ------- 72 _______ ------^ -^ ----- a ~ (^) -- - ------ - --- ------- -
Table 3 gives percentage abundances from da ta of table 2 and values derived from the mass spectrum of S02 made from this sulfur. The correction for O 2 isotopes wa" based on measurements of the ail' oxygen used in making SO;;. The table also includes published values from S02 spectra. The S2 ions are not favorable for the computation of a3, for the contribution of 2a3 to the 68 peak is only 15 percent of the a ~ term. A somce of un- certainty in evaluat.ing a) from the 65 peak arises from the possibility that a trace of S~2S34 ++ may be present. The comparative values of tabl e 2 give no evidence of this.
TABLE 3. Sulfur-isotope abundances
l!lass num bel'
32 33 34 36
S t(s ulfur vapor ) ___ _____ (^) 95.0 O. 76 ±0. 004 4. 22±0. OJ 0. SO, (same s ulfur ) ______ 95. 0. 77 ,±. 01 4. 23±. Niel' [51 ---------------- 95.1^.^74 ±.^02 4.20±^. 1^.^ 0l6±^.^0016 Thode (^) [61 sulfur from same regioll __________ (^) --- - - --- ---- 4.
3.3. Appearance Potentials
Some measurements of appearance potentials were made to see whether or not Si was a fragment ion from S8 ionization. The experimental conditions were not well adapted to accurate measur ements. The ion-repeller voltage was kept rather high to maintain sensitivity, and there were irregularities in the current -voltage curves that may come from surface charges on adsorbed sulfur. }.If ercury vapor wa s introduced with the sulfur vapor , and the ap- pearance potentials of sulfur ions were measured relative to tha t of Hg + (spectroscopic value 10. ev) [4]. Measurements ar e based on semilog plots, with current plott ed on a scale to make the ion cur- rent at 50 v unity. Values of the appearance po- tentials at an ordinate of 0.003 of the c urr ent at 50 v are: S it 8.9 ± 0.2 ev and 8i 8.3 ± 0.2 ev. Varia- tions in the slopes of the current-vol tage curves give some added uncertaint y. 80me measurement s on S+ indicate an appearance pot ential roughly 2 v higher than Si and Sit. A search for n egat ive ions gave negative results, but there was no basis to appraise the sensitivity for negative-ion d etection.
Th e fa ct that the appearanc e potential of 8; is somewhat less than that of S it suggests that S molecules are pre se nt and 8.3 ev is the ionization potential of S2. Ionization resulting in a pair of positive and n ega tive ions could give fragment ions at an appearance pot ential less than the ionization pot ent ial of the S8 mol ec ule, but there is no evidence that this occurs. As vapor in the ionization chamber is a t a pressure less than 10- 4 mm and at a tempera- ture of 186 0 C, dissociation of S8 into the r elatively stable Sz molecule is not unexpected. Th e mass spectrum of sulfur vapor given in table 1 is probably to b e interpreted as the spectrum of a mixture of molecules. The appearance potentials are unexpectedly low. Smyth and Blewett [7] reported an appearance po- tential of 10.7 ± 0.3 ev for Si from thermally dis- sociated CS2, as compared with 8.3 ev found in this work. An ionization potential of 8 2 lower than that observed by 8myth and Blewett is suggested by analogy with O2 • The ionization potential of 0 is 13.61 ev [4] and that of O 2 is 12.2 ev [8]. As the ionization potential of 8 is 10.36 ev [4], the ionization potential of S2 is expected to be considerably lower than this, not slightly higher as reported by 8myth and Blewett.