l\’ith a distance of only 2 mm. betweii tlic main diffraction plane and the surface of the de Oude Delft lens, it is c,xtremely difficult to provide the space for a lieating or cooling device. This problem might b r solved by the use of fiber optics or a combination of fiber aiid lens optiw between fluorescent 5s(w(’nand image converter. The use of electronic image converters in the x-ray powder technique seems twtirely feasible \dienever a high time rcsolution is required and the angular wsolution of the diffraction pattern can sacrificed to a certain degree. 1 x 1
LITERATURE CITED
(1) Bertin, E. P., ANAL. CHEY. 2 5 , 708
(1953).
( 2 ) Boettcher, -4.,Thun, R. E., Opt& 11,
22 (1954). (3) G. L., L , “-4pplied “-4pplied X-Rays,” X-Rays,” (3) Clark, Clark, G. p. 38, hlcGrav--Hill, Sew York, 1940. Radiology 51, 359 ( 4 ) Coltman, J. IIT., T , RadTology (1948). (1948). (5) Hass, G., Thun, R. E., “Fifth Xa>-a(5j,Hass, tional Symposium on Vacuum Technology Transactions,“ Transactions,” p. 58, Pergamon Press, Sew York, 1959. (6) Hodges, P. C., Skaggs, L. S.,Am. J . Roentgenol. Radzrtni Radirtni Therapy 6 6 , 705 11951). ( 7 ) Lang, A . R.: Rev. Sci. Inst?. 2 5 , 1032 (1954).
(8) Morgan, R. H., Sturm, R. E., Radiology 57, 556 (1951). 19) Peiser, H. S., “X-Ray Diffraction bv PoIvcrvstaIIine Materials.” a. 70. Itracthyllcad as an antiknork agent, (4j. Although tlotcmiination of total nicwaptans is xatidac,tory for routine c,ontrol of c*onimcrc*iaic.stixctioii plant3 . additional information conrcrning mercaaptan composition i.; ncrdtd for the CIPvc.lopnicnt of ne\v 1xxml tion would separate and concentratc iiicwaptans for whwqucnt analytical rlctrrmination, I.mt the time reqiiirclm m t s and the cwnplications associated with azeot’rope formation make this approach unattractive. Cheinicxl concpnti,ationTfollov t ~ d1~)- gas chroniat,ograpliic I,.JIOTAL
determination of the mtrcaptans, can k~ used (e).Altwnatively, lon- voltage mass spectrometry, which has been applied to the determination of unsaturated hydrocarbons (,@), distinguishes easily between the parent peaks of the mercaptans of differcmt molecular weights and gives satkfactory analyses of merraptan concentrates. Ilercaptaiis n’crr rxtracted from light naphtha with acjueous potassiuni isobutyrntr solut’ion ( 8 ) , and further concentratrd bl- rstraction of the acidified solution viith iso-octnn(>. Finally, their concentration and molecular weight distribution in the iso-oct’ane concentratc \vert’ tirtcwiiincd using 1o~v voltagc mass spc>ctroinctry. Only alkyl mcwapt’ans ranging from on(’ through seven carbon atoms n-ere prcwnt. CONCENTRATION
The naphtha samples, as well as all glassware and reagents, are chilled to 20” F., and all rcxactions aiid phase separations are carried out as close to this temperature as possible to reduce evaporation losses and increase extraction efficiency. A measured volume of sample, typically 300 to 500 ml., is transferred to a 1-liter separator3 funnel that’ has been purged with nitrogen to minimize the possible oxidation of the mercaptans. To this sample is added 30 ml. of a mixture of equal volumes of aqueous 6 S potassium hydroxide and 3 5 potassium isobutyrate. The coiitents of the funnel are mixed by shaking vigorously for 5 minutes. Phase separation takes about 2 hours. The aqueous layer containing the mercaptides is drained into a 50-ml.
Babcock bottle having an enlarged neck, and 1.0 ml. of iso-octane is added to the mixture. The bottle is then closed with a rubber stopper in which a 60-ml. separatory funnel and a pressure-release tube are inserted. T n e n t y millilitws of concentrated hydrochloric acid is added in small increments from the 60-ml. separatory funnel to liberate the mercaptans and iqobutyric acid. K h e n the bottle is sinirled, they collect in the iso-octane layer. The volume of this layer (about 5 ml.) is read, and a few microliters are charged, with x capillary dipper, into the inlet s j stem of the mass sppctrometer. SPECTROMETRY
Analyses are performed with a Consolidatcd Electrodynaniics Corp. Ilodel 21-103 mass spectrometer, modifid to operate a t low ionizing voltages (‘7). In srlecting t’Le ionizing voltage for -pes of conipounds, such as nitr, it is necessary to compromiw b e t w e n two alternatives. High voltage causes undesirable fragmentation but j-ieltls good sensitivity ; lou- voltagc eliniinatcs fragmrlntation but decreases sensitivity. For dc+mnining mcrcaptans, the ionizing 1-oltagc, repellcr voltagc., and ionizing elect’ron current are niaiiitained a t 9 volts, 3 volts, and 20 pa. lliese ronditioiis cause only minor fragmentation and, a t the niolecular niass, yield a satisfactory sensitivity of approsiniately 7 chart divisions per micron pressure. Scanning is st’arted a t an accelerating voltage of 1500 volts. The mass spectrometer is d i b r a t e d with American Petroleuni Institute r .
VOL. 32, NO. 8 , JULY 1960
941
standard mercaptans and portions of the isobutyric acid and iso-octane used in the concentration procedure. The sensitivities of the mercaptans of interest range from 6.55 to 8.13 divisions per micron pressure (Table I), but differences among isomers are small enough so that average sensitivities introduce little error. However, variations in sensitivity among mercaptans having different carbon numbers are significant. Sensitivities of isobutyric acid and isooctane are 0.4 and 0.8 division per micron. The spectrum of the mercaptan concentrate is obtained under conditions identical to those used in the calibration. Day-to-day variations in sensitivity are much greater with low voltage than with high voltage spectrometry because small uncontrollable electron energy
Table 1.
Molecule-Ion Sensitivities
(Ionizing voltage:
9.0 volts) Divisions Per Mercaptan Micron Methyl 7.73 Ethyl 8.13 7.94 n-PiopyI 7.99 Isopropyl 6.71 n-Butyl 7.06 sec-Bu tyl 6.61 Isobutyl n-Amyl 7.28 n-Hexyl 6.55 n-Heptyl 7.76
Mle 48 62 76 76 90 90 90 104 118 132
TEST OF METHOD
Table II. Mercaptan Distribution in Three Synthetic Mixtures
(Weight %) Ethyl Propyl Butyl Known 10.3 29.4 40.5 Found 10.1 29.0 41.1 Found 10.0 28.4 41.4
Amyl 19.8 19.8 20.2
Known Found Found
15.9 15.5 15.3
23.3 22.8 22.8
37.6 38.1
23.2 23.6 23.6
Known Found Found
8.6 8.6 8.6
33.6 32.6 32.7
49.4 49.8 50.1
8.4 9.0
38.3
changes at low voltage levels cause much larger changes in peak height than equivalent changes a t high levels (1, 6). Sensitivity variations as great as 3370 over short periods and 70% after installing a new filament have been reported (6). T o correct calibration factors for these variations in instrument sensitivity, a spectrum of n-propyl mercaptan is obtained a t the time the concentrate is analyzed. This compound was chosen because propyl mercaptans were often the major mercaptans in the light naphthas analyzed. Sensitivity variations as great as 60% have been satisfactorily corrected by this means. Complete recalibration is rarely needed. Computation of the Components in mole per cent is based on standard procedures. Except for iso-octane, only parent peak heights are used. Isooctane must be determined from a fragment peak because compounds containing quaternary carbon atoms show very weak parent peaks a t low ionizing voltage. The peak a t m/e 99 is free of interference and was chosen for computation. Solution of simultaneous equations is unnecessary.
8.6
Precision and accuracy of the mass spectrometry were tested by analyzing synthetic concentrates of mercaptans in iso-octane. Accuracy of the method, including concentration, was tested by analyzing naphthas whose total mercaptan contents had been determined by potentiometric titration with silver nitrate. Three synthetic concentrates, containing 10 to 15 weight mercaptans in iso-octane, were prepared at 30” F. by measuring appropriate volumes of the different components with graduated precision pipets and mixing in glass-stoppered containers. Except for the absence of isobutyric acid, these samples simulated the isooctane concentrates obtained from naphthas. The samples were analyzed twice within a %day period. Normalized results are given in Table I1
~~
Table 111.
Mercaptan Sulfur in Eight Naphthas
(Milligrams per 100 ml. of naphtha) Sample
Methyl
...
Ethyl ..
4
18
..
1
942
Mass Spectrometric Propyl Butyl Amyl Hexyl 2 1
7 8
ANALYTICAL CHEMISTRY
13
14 17
Potentiometric Heptyl Total Tota‘i ... 6 5
3
..
...
,..
19
10
5
‘4‘
12
6
3
38 42 64
38 41 68
Standard deviations are excellent. They show the repeatability and accuracy of the lorn voltage mass spectrometric part of the niethod t o be 0.2 and 0.6 weight %. Eight straight-run and catalytic naphthas were analyzed for total mercaptan content by mass spectrometry and also by potentiometric titration (Table 111). Standard deviation is only 1.4 mg. per 100 nil. and the results obtained by the two methods are not significantly different. Contributing t o the low result obtained on sample H by the mass spectronietric method is the decreasing extraction efficiency of potassium isobutyrate with increasing carbon number of the mereaptans. Even with the relatively large concentrations of hexyl and heptyl mercaptans, however, t.he discrepancy between methods is still not significant. CONCLUSION
The method was developed for the determination of members of the alkyl mercaptan series in light naphthas. Aromatic mercaptans, which might be present in heavier naphthas, could alqo be extracted by potassium isobutyrate solution. For alkyl mercaptans heavier than hexyl, and for mercaptans of other molecular configuration, a different solvent mould be needed. The use of methanol-potassium hydroxide (S) or sodium aniinoethoxide in anhydrous ethylenediamine ( 6 ) , for quantitative extraction from gasoline, appears to be feasible. For the deterniination of mercaptans having lower vapor pressures than heptyl, a mass spectrometer n-ith a heated inlet system would be required. LITERATURE CITED
(1) Field, F. H., Franklin, J. L., “Electron
Impact Phenomena and the Properties of Gaseous Ions,” pp. 12-52, Academic Press, Kew York, 1957. (2) Field, F. H., Hastings, 9. H., ANAL. CHEM. 28,1248 (1956). (3) Field, H. W., Oil Gas J . 40 [20], 40 (Sept. 25, 1941). (4) Greek, B. F., Duval, C. A., Kalichevsky, V. A., Ind. Eng. Chem. 49, 1938 (1957): (5) Honig, R. E., J . Chem. Phys. 16, 105 (1948). (6) Liberti, A,, Cartoni, G. P., Chim. e ind. (Mdan) 10,821 (19571. (7) . , LumDkin. H. E., -4s.4~.CHEM.30. 321 (1958): (8) Thompson, C. J., Coleman, H. J., Rall, H. T., Smith, H. IT.,Ibid., 27, 175 (1955). (9) Yahroff, D. L., Border, L. E., Proc. A?n. Petrol. Znst. 20M(III), 95 (1939).
RECEIVED for review FebruaLy 15, 1960. Accepted April 27, 1960. i t h Annual Meeting, ASTM Committee E-14 on Mass Spectrometry, Los Xngeles, Calif., May 18, 1959.
