Use of High Resolution Mass Spectrometry in the St ud y of Petroleum Waxes, Microcrysta IIine Waxes, a nd Ozokerite W. K. REID BP Research Centre, The British Petroleum Co., Itd., Sunbury-on-Thames, Middlesex, England
b A preliminary investigation has been made on the application of the high resolution mass spectrometric technique to the examination of samples of petroleum waxes. Mass measurements, with an accuracy of better than 5 p.p.m., have been used to assign the most likely ionic formulas to some of the ions observed in the spectra of the samples studied. Aromatic ions, as well as ions containing sulfur and oxygen, have been observed and assignments, within the accuracy limits, are discussed. Spectra of two microwaxes have been obtained using a direct probe sample inlet system and some indication of the hydrocarbon ion distributions in their spectra is given.
F
many years, petroleum technologists have been aware of the complexity of petroleum waxes, which contain not only normal and isoparaffins but also cycloparaffins and aromatic hydrocarbons. The technique of wax analysis by low resolution mass spectrometry has been studied by O'Seal and Weir (8),Clerc, Hood, and O'Yeal ( S ) , and Levy, Galbraith, and Melpolder ( 5 ) . For some years, a modified AIS 2 single-focusing mass spectrometer (11) with a heated gallium inlet system has been used in our laboratories to obtain quantitative analyses of low molecular weight petroleum waxes. High resolution mass spectrometry, applied by Beynon ( I ) to organic chemistry, has been comparatively little used for the study of petroleum fractions. Carlson, Paulissen, Hunt, and O'Seal ( z ) , however, have made preliminary comments on this new technique, which has recently been used in the quantitative estimation of sulfur compounds in petroleum fractions ( 6 , 9 ) . -1 double-focusing AEI 11s 9 mass spectrometer (4) is available in our laboratories and a preliminary study of a range of petroleum waxes has been made: employing the higher resolution (approximately 1 in lO,OOO), higher sensitivity, and direct probe inlet of the 11s 9. OR
The accuracy of mass measurements that can be obtained using high resolution mass spectrometers has been mentioned by a number of authors. Tunnicliff et al. ( l a ) ,in a recent publication on the production of accurate mass tables by computer, note that the measured and assigned mass of a n ion must agree within 0.0002 amu to give confidence in the assignment. This would mean that, for example, a t m/e 200 the error in mass measurement must not exceed &1 p.p.m. No commercial high resolution instrument, including the AIS 9, a t present gives such average accuracy in mass measurement. Van Katwijk (IS) using the ?\IS9 quotes a more realistic experimental figure of 3 to 5 p.p.m. Other information-e.g., elemental analysis of the sample-must be used in order to restrict the number of possible molecular formulas. This paper demonstrates the ability of high resolution mass spectrometry to detect nonaliphatic hydrocarbons and heteroatom materials in waxes and comments on the assignment of a n ionic formula once the accurate mass has been determined. This preliminary study is intended to illustrate the potential of this new technique for the detection of minor components in waxes. The use of a direct probe Sample system to obtain spectra of the less volatile microwaxes, already studied by Carlson et al. (Z), is also discussed. EXPERIMENTAL
The following samples, readily available from commercial sources, were examined, no special precautions having been taken regarding storage or purification. Paraffin waxes from Middle East and S o r t h African crudes. A molecular distillation fraction of a paraffin wax from a Xiddle East crude. An ozokerite of Middle East origin. Two Middle East microcrystalline waxes. The mass spectra of the waxes were obtained with a n AEI LIS 9 mass spectrometer. The spectra were initially recorded with wide slits-Le., at a low resolution of approximately 1 in 600and the slit size was then reduced to
enable interesting parts of the spectra to be studied in detail-Le., at a resolution between 5 and 10,00O--with suitable scan rates and multiplier voltages. A typical set of instrument conditions is : Collector and source slits, inch 0.001 to Multiplier voltage, Kv. Temperature of inlet system, "C. Accelerating voltage, Kv. Electron voltage, e.v. Temperature of ion chamber, "C.
0.003
-2 300 8 70
200
The spectra of the microwaxes were obtained using the direct probe manufactured by +LEIas an accessory of the 11s 9 instrument. The sample, either initially dissolved in a solvent or directly as a solid, was placed on the tip of the thin ceramic tube which is wired to a n external heater plug through a glass-metal seal on the vacuum flange. The tip of the probe was then placed in close proximity to the ion chamber, after being guided into position by a small cone structure in the side of the source block. The temperature of the ion chamber was slowly increased until the monitor current rose to a constant value sufficient to give adequate peak height throughout the spectrum. Mass measurements of peaks in the spectra obtained were made, using source and collector slits of 0.001 inch. To ascertain the accuracy of these measurements some peaks of Y pure hydrocarbon, n-tetracosane, were mass-measured using heptacosafluorotributylamine as standard. Typical results are given in Table 11. Detailed results from a range of peaks in a molecular distillation fraction of a Xiddle East paraffin wax are shou-n in Table 111, which is constructed as follows. The mass ratio of the unknown peak and the standard peak is obtained using the 11s 9 measuring system. .I correction factor is derived by meawring the mass ratio of two standard peaks, chosen in order to bracket the unknown peak, and the correction factor per amu is obtained and applied to correct the calculated mass of the unknovn peak. The corrected mass is subtracted from the mass of the equivalent hydrocarbon a t the same integral mass number to give a doublet difference, A X . Csing a graphical method previously described (IO) the doublet corresponding to this value of A31 is obtained and a formula for VOL. 38, NO. 3, MARCH 1966
445
Table I.
Hydrocarbon series
Doublet difference from hydrocarbon series
Table It.
Peak measured in
%-tetracosane 266 267 338 339
Series Overlapping with Saturated Hydrocarbon Ions
CnH2nt 2 Paraffin parents
C-Hi2 0.0939
CnHZn Small normal paraffin fragments, sizable isoparaffin fragments, monocyclo parents CnHZn- 14
Experimental mass 266 2970 267 3040 338 3898 339 3937
DISCUSSION AND RESULTS
Hydrocarbon Constituents. High resolution mass spectra can be used to separate ions with the same integral mass, provided that they have different molecular formulas and t h a t t h e mass difference is not prohibitively small for t h e resolution available. Usually the highest mas* peak in a doublet is the one containing t h e most hydrogen atoms and this fact can be used to indicate the compound types at each integral mass. Some possible isobaric series are shown in Table I to illustrate the ions that can produce multiple peaks a t one integral mass-e.g., naphthalenes (C,H?,-,?) occur a t the same integral mass as the paraffins (C,H2,+2) but with a mass difference of 0.0939 aniu. Similarly the heteroatom series in Table I are isobaric ITith the hydrocarbon ion series shown. HoTvever, since the major normal and isoparaffin fragment ions are identical in ionic formula-Le., C,,H2,+ l-high resolution obviously cannot distinguish between these ions and, as with low resolution mass spectrometry. an analysis of waxes in terms of normal and isoparaffins depends on the availability of calibration data. Xccurate mass measurement of the peaks occurring a t the CnHPnt2,C,H2,-12 series in a paraffin wax from a Middle East ANALYTICAL CHEMISTRY
CAn- 2 Monocvclo fragments, dicyclo parents
CnHzn-&
Mass Measurements in Spectrum of n-Tetracosane Using Heptacosafluorotributylamine (HFTB) as Standard
Standard peak of HFTB 264 264 338 352
CnHZn-1 Monocyclo fragments
CnH2n--20
0.0364 S~GHM 0.1810
the unknown ion is readily deduced. The correction factor varies slightly throughout the mass spectrum, but a typical figure is 1.000004 per amu.
446
CnH2nt 1 Small normal paraffin fragments, large isoparaffin fragments
Formula Ci9Ha C19H39 C~&O CzsC'3HSo
Calculated mass 266 2974 267 3052 338 3913 339 3946
Error, p.p.m. -1 2 -4 3 -4 2 +3 4
crude has demonstrated the presence of very small amounts of naphthalenes (or less likely seven-ring condensed naphthenes, fragmentation peaks from cyclic hydrocarbons, etc.) and the distribution of these compounds can be easily ascertained. I n this case, the C,Hin-12 distribution, corrected for background fragmentation, covered the range between C12 and C18 and since this is below the distribution of the normal paraffins to GO), these CnHln--lJ peaks probably correspond to trace contaminants in the was. The certainty of assignment of molecular formulas to the peaks in a high resolution mass spectrum is dependent on the accuracy with which mass measurements can be made. The 11s 9 is capable of making ma:\s nieasurements accurate to within a few parts per million (4, 23). Under the conditions used to obtain the spectra and mass measurements reported in this paper the agreement between theoretical masses and measured masses was better than 5 p.p.m. This is demonstrated in Table 11, where some typical results of mass measurements of n-tetracosane and heptacosafluorotributylamine are given. The figures quoted are single determinations of the mass of the peak being measured. tT7ith the knowledge that mass measurements may be expected to be w-ithin at least 5 p.p.m. of the true values, the formulas of some unknown peaks in a molecular distillation wax fraction were obtained. The standard peaks employed were those on the CnH?,+2 series
due to the normal paraffins in the wax. The results obtained are shown in Table 111. The formulas assigned to the peaks are those hydrocarbons whose calculated masses are nearest to the corrected masses. The final column of Table 111 is the difference in parts per million between the calculated and experimental figures. When the difference is greater than about 5 p.p.ni. the possibility of the presence of a heteroatom in the ion must be considered and thus the ion at m/e 332 could contain a sulfur atom. The formulas assigned to the other ions in this paper were those whose calculated masses were closest to the values measured, though in some cases chemical information vias also taken into consideration t o decide between two formulas that nere very close in mass to the measured maw. The isotope peaks with ionic forinulas containing t n o C13 atoms were observed in the spectra of the waxeb studied, two mass units higher than the C,H2,+2 peaks. One such peak was observed in a triplet a t mass 312 ab C&i13H46 with the other peaks as C23H36and C24H34. The corresponding mass measurements are shown in Table 111. Because of the high relative intensities of some of these isotopic peaks, care must be taken in their assignment, since without consideration of accurate maSs measurements and/or isotope ratios, such peaks occurring in a multiplet a t one integral mass may possibly be incorrectly assigned a t first inspection. Nonhydrocarbon Constituents. As the presence of minute amounts of nonhydrocarbon material in petroleum fractions can have major effects on their properties, the detection of sulfur, oxygen, and nitrogen compounds ( 2 , 6 , 9. 10) is of major importance. .%Ithough waxes contain mainly saturated hydrocarbons, high resolution spectra will frequently indicate the presence of heteroatom compounds. I n general, only a .mall portion of the spectrum can be studied in detail, because of the time required to mass-measure the
II
PORTION OF OZOKERITE SPECTRUM
Figure 1. resolution
Portion of
l
an ozokerite spectrum at high
peaks, but the information obtained is sufficient to pinpoint the major compound types, in many cases using the paraffin peaks as standards for mass measurement. d North African wax was found to contain oxygen compounds occurring a t the same integral mass as the C,H2,+z and C " H P ~hydrocarbon series but differing from these series by the O-CH4 doublet (value 0.0364 amu). The formulas of these oxygen compounds are C,H2,0 and C,H2,-20, as shown in Table I, and the distribution of the compounds can be easily obtained. The values observed were n = 16 to 25 in C,H2,0 and n = 17 to 24 in C,H2,-20. These oxygen compounds could, for example, be aliphatic aldehydes or ketones (C,H2,0) and cyclic oxygen compounds (C,H2,-20). A41though it is not known if they arise by oxidation of the wax. by the refining process, or by the solvent extraction of the wax, high resolution mass spectrometry provides strong evidence of the presence of such oxygenated material. A Middle East ozokerite was studied using the conventional ills 9 heated inlet system a t 300' C. and a part of the complex spectrum obtained is shown in Figure 1. The region for examination was a t as high a mass as possible consistent with both adequate peak intensity and keeping the number of possible ionic formulas reasonably small. The triplets a t m/e 225 and 226 were studied in detail and the mass measurements are shown in Table IV. The two peaks of the highest mass in each of these triplets were assigned to hydrocarbon ions on the basis of mass meas-
9
6
5
333333133333333
w 0
VOL 38, NO. 3, MARCH 1966
447
CARBON NUMBER
Figure 2. Distribution of hydrocarbon ion series in a Middle East (Agha Jari) microwax A
E
7
300
-
280
-
260
-
240-
kzj 220 IO Eo
g5200-
Y x
2 180$8 ‘ 6 0 -
Ym
‘: 8u
140-
b$ ,20:E 100 -
$s
80-
40 60
20
-
2 CARBON NUMBER
Figure 3. wax B
2o
Distribution of hydrocarbon ion series in a Middle East (Kirkuk) micro-
22
24
2~
2a
x)
3z
34
36
3a
4o
42
44
CARBON .NUMBER
Figure 4.
448
Distributions occurring in initial fractionation of microwax A
ANALYTICAL CHEMISTRY
4e
urements within the known accuracy limits. The average of the measured A M values for the third peak in these triplets is seen in Table IV to be 0.1797 amu. This led to considering three main doublets in assigning this peakthe Sz-CaH16 doublet (0.1810 amu) , the (0.1792 amu), and N O Z - C ~ Hdoublet ~~ the S - C H ~ Odoublet (0.1844 amu). Chemically, the S02-C2H22doublet is unlikely and since the wax is known to contain sulfur (0.23 %) a sulfur doublet was preferred, the nitrogen content being less than 0.05 %. The shape of the peak (as observed on the oscilloscope on the instrument) indicated that it was a single peak and not due to two or more ionic species. This, taken with the fact that the difference between the measured A M value of 0.1797 amu and that of 0.1844 amu for the S-CH2o doublet was greater than the expected error in mass measurement, favored the S2-C4H16 doublet. It was thus concluded that the ion a t m/e 226 is ClzHlsSz (as shown in Table IV) and belongs to the C,Hz,-6S2 series. Microcrystalline Waxes. The spect r a of two microcrystalline waxes were obtained using the direct inlet probe and distributions of hydrocarbon ion series obtained are shown in Figures 2 and 3. These results are similar t o those obtained by Carlson et al. (2) using a heated inlet system a t 500’ C. and to those predicted bv ;\finchin ( 7 ) , who suggested that C34to Cm was the range of paraffins in microwax. From results of other analyses-e.g. , lubricating oils and “bright stocks”- it has been found that the use of the direct probe inlet system tends to produce the maxima in the distributions a t higher carbon numbers than those obtained by a conventional heated inlet system, presumably because of more representative sampling and minimal thermal decomposition. However, comparison with earlier work ( 2 ) indicates that the range of carbon numbers obtained is probably the same using both inlet systems. Use of the direct probe also tends to produce some initial fractionation as the ion chamber warms up and the distributions shown in Figure 4 can be attribut’ed to crude distillation of one of the microwaxes. As this wax was shown by XMR to contain about 30% “oil,” it is possible that this initial fractionation contains a concentration of the “oily” component. The mass spectra of the microwaxes were, as expected, complex a t high resolution and part of one of the mass spectra obtained a t a fast scan speed is shown in Figure 5 . Accurate mass measurement was difficult because of the low sensitivity of some of the more interesting peaks and the relatively short time available for a detailed mass study. The spectrum changed after
LITERATURE CITED
Figure 5. Portion of a microwax spectrum at fast scan showing presence of CIHZn-l* peaks at m / e 170 and 184
Table IV.
Peak 225 226
Measured mass 225.2582 225,1637 225.0782 226,2604 226.1732 226.0867
Mass Measurements on Ozokerite
Eauivalent hydrocarbon Formula Mass CiBHaa 225.2582 Ci&a 225.2582 C1&33 225.2582 Cl6Ha.r 226.2661 CiaHa4 226.2661 CiaH34 226.2661
about half a n hour and peaks had to be retuned a t stages of a quarter of an hour to new ion repeller and beamcentering positions. However, the presence of C,H2,-2 ions is confirmed
AM
Assignment
0.0945 0.1800 0.0057 0.0929 0.1794
C-Hii S&Hi6 C"-CH C-Hi2 St-CaHis
Peak C16H33 Ci7Hzi CizHi7Sz CisC'aHaa CuHn CizHi8Sz
by mass measurement of the doublets at m/e 170 and 184 (Figure 5). Future work is planned to provide a detailed study of the complex high resolution spectra of microwaxes.
(1) Beynon, J. H., "Mass Spectrometry
and Its Applications to Organic Chemistry," Elsevier, Amsterdam, 1960. (2) Carlson, E. H., Paulissen, G. T., Hunt, R. H., O'Neal, M. J., Jr., ANAL. CHEM.32, 1489 (1960). (3) Clerc, R. J., Hood, A., O'Neal, M. J., Jr., Zbid., 27, 868 (1955). (4) Elliott, R. M., Craig, R. D., Errock, G. A., Proceedings of Fifth International Instruments and Measurements Conference, Vol. I, p. 271, Academic Press, New York, 1961. (5) Levy, E. J., Galbraith, F. J., Melpolder, F.,W., "Advances in Mass S ectrometry, R. M. Elliott, ed., Vof 2, p. 395, Pergamon, London, 1963. (6) Lumpkin, H. E., ANAL. CHEM. 36, 2399 (1964). (7) Minchin, J., J. Znst. Petrol. 34, 542 (1948). (8) O'Neal, M. J., Jr., Weir, T. P., Jr., ANAL.CHEM.23, 830 (1951). (9) Reid, W. K., Xead, W. L., Bowen, K . hl., Mass Spectrometry Conference, Paris, IP/ASTM/GAMS, September 1964. (10) Reid, W. K., Mead, W. L., West, A. R., ANAL. CHEM.36, 1140 (1964). (11) Thornton, E., West, A. R., 2.Anal. Chem. 170, 348 (1959). (12) Tunnicliff, D. D., Wadsworth, P. A., Schissler, D. O., ANAL. CHEM.37, 543 (1965). (13) Van Katwijk, J., Appl. Spectros. 18, 102 (1964). RECEIVEDfor review August 16, 1965. Accepted November 29, 1965.
Estimation of Solubility of Bismuth Compounds in Liquid Ammonia ANNIE G. SMELLEY,' FRANCIS E. BRANTLEY,2 and ARTHUR F. FINDEW U. S. Bureau of Mines, University, Ala.
b The solubilities of six bismuth compounds in liquid ammonia are reported. The compounds were dissolved in liquid ammonia and equilibrated for 2 hours at - 3 3 " C. An aliquot of the solution was then analyzed for bismuth. The bismuth was determined polarographically using a dropping mercury electrode with 1M HzS04 as a supporting electrolyte, Triton X-100 as a maximum suppressor, and a mercury pool reference electrode. Numerical results are given for the solubility of the triiodide, trichloride, tribromide, nitrate, sulfide, and lactate of bismuth.
P
to a n investigation of electrodeposition of bismuth from liquid ammonia, a literature search was made to obtain solubilities of bismuth salts which were being considered for use in the study. I n spite of the RELIMIKARY
large amount of work reported in the literature concerning solubilities in liquid ammonia, numerical values for solubilities are cited for only a small number of compounds. Solubilities for bismuth compounds are listed in descriptive phrases such as soluble, very soluble, slightly soluble, and insoluble (3, 4, 6). Much information is reported concerning the reactions of bismuth halides with ammonia, but only general statements concerning solubilities have been made. For these reasons this work was started to obtain quantitative solubility data. Several methods are described in the literature for determining solubilities in liquid ammonia. These are either elaborate and time consuming for precision measurement (IO), or rather simple techniques yielding qualitative statements of solubilities (5). The method and apparatus used for studying the solubility of the bismuth salt was a
modification of those described in the A conventional polaroliterature. graphic method of analysis was used to determine the amount of bismuth dissolved in the liquid ammonia. It was decided that for the work reported here, any additional accuracy that might be gained by the use of special procedures in drying ammonia, purifying chemicals used, and establishing equilibrium conditions would not be justified. Errors are introduced when these factors are disregarded; however, the objective was to establish a n order of magnitude rather than expend the time necessary for obtaining more accurate results. Chemist, Bureau of Mines, U. S. Department of the Interior, Tuscaloosa, Ala. Present address, Division of Minerals, Bureau of Mines, U. S. Department of the Interior, Washington, D. C. a School of Chemistry, University of Alabama, University, Ala. VOL. 38, NO. 3, MARCH 1966
449
