Anal. Chem. 1986, 58, 2033-2036
2033
Hydrogen Isotope Ratio Determinations in Hydrocarbons Using the Pyrolysis Preparation Technique Zvi Sofer* a n d Craig F . Schiefelbein'
Cities Service Oil and Gas Corporation, P.O.Box 3908, Tulsa, Oklahoma 74102
Thls report describes the successful development of a new and eftlclent method for the determlnatlon of stable hydrogen isotope composttion of the C,,+ allphatlc fractlon of crude olls. Thls new technlque makes use of hlgh-temperature pyrolysis reactlons In which the allphatlc hydrocarbons In 011s are dlredly converted to hydrogen gas (plus methane and graphtte) followed by direct measurement for hydrogen kotoplc ratlos on the mass spectrometer. The conventlonal oxldatlonheductkn method for determlnlng hydrogen isotope ratios suffers from being very tkne-cmumlng; the new pyrolysis technlque allows for a 3-fold Increase of sample output wlth adequate reproduclblllty ( l a = &30/w) as determlned from extenslve replicate analyses of standard crude dl samples. TMS method wlll make It posslble to systematically test the usefulness of hydrogen Isotopes In petroleum exploratlon.
In an earlier report (I) the advantages of preparing hydrogen gas for the measurement of stable hydrogen isotopes in petroleum hydrocarbons by the pyrolysis method have been theoretically discussed. In this report the pyrolysis analytical procedures and the calculations of 6D values based on the mass spectrometric analyses are described. The isotopic composition of hydrogen gas generated by the pyrolysis method may vary because of not only real variations in the isotopic composition of samples but also pressure variations in the reaction vessel and differences in the temperature a t which the pyrolysis is carried out (I). It is therefore important to know before hand the effect of those two variables or, alternatively, to standardize the preparation procedure in order to eliminate their effects. Temperature variations can be easily eliminated by batch preparation; i.e., unknown samples and standards of known isotopic composition should be pyrolyzed at the same time and in the same furnace. When the temperature is uniform, pressure in the reaction vessel depends on (a) the sample size and (b) the size of the reaction vessel. The size of the reaction vessel can be standardized by using quartz tubes of equal lengths and diameter for the samples and standards. When pyrolysis temperature and reaction tube size are standardized, the effect of pressure (i.e., sample size) can be calibrated by pyrolyzing several different quantities of the same sample and plotting the resulting mass spectrometer readings (i.e., HD/H2 ratios as defined in ref 1) as a function of the total pressure (or the hydrogen partial pressure). Because the effect of pressure on the measured isotopic composition is theoretically the same for all samples, it can be accounted for by using the calibration plot. The total pressure in the reaction tube can be measured directly on the mass spectrometer inlet system by means of a pressure gauge (MKS Baratron type 22RB absolute pressure gauge with a measuring range of 0 to lo00 torr was used here). The reaction tube is opened (by means of a tube cracker) 'Present address: Conoco, Inc., P.O.Box 1267, Ponca City, OK 74603. 0003-2700/86/0358-203$01.50/0
under vacuum and the gases are allowed to expand into the modified inlet system. The measured pressure is directly proportional to the pressure in the reaction tube. This can be shown to be true by the following argument: P is the unknown pressure in the reaction tube, which has a volume V (V is constant for all samples). The volume of the modified inlet system is V , and it increases to V + V , after the tube is opened. The measured pressure at that time is Pz.From the gas laws
PV = P,(V
P = P2(
+ V,)
T) v + v,
(1)
since V and V2 are constant for all samples, P is directly proportional to Pz and, therefore, the pressure calibration procedure can be carried out using Pz rather than the actual tube pressure P. The measurement of P2 should take place before the gas is allowed to expand into the variable sample reservoir and sample capillary in order to avoid any loss of gas into the waste pump or the analyzer pump. It should also be noted that this pressure calibration and correction are different from the usual H3+ correction, which is also pressure dependent and for which a separate calibration dnd correction are carried out electronically in the mass spectrometer. The actual development of the analytical procedure for the pyrolysis method involved two stages: first was the establishment of the optimal conditions for the achievement of the best isotopic results; this was done by varying the length of heating time. The second stage involved the actual determination of the isotopic composition of unknown samples prepared under optimal conditions by using two standards of known isotopic composition which were prepared in a similar manner. These two standards were also chosen to be used in establishing the optimal conditions. The two standards are the CIS+aliphatic fractions of Tara (NW Palawan) and Newport (North Dakota) crude oils and their hydrogen isotope composition is -85.5 f 0.7% and -157.5 f 1.3%, respectively, relative to SMOW. The isotopic composition of the two standards was independently determined by using the static oxidation method and reducing the resulting water (after cryogenically separating from COJ over hot uranium shavings. For comparison, the isotopic composition of some of the unknown samples was also independently determined by use of the oxidation/reduction method. EXPERIMENTAL SECTION Reagents and Materials. Reaction tubes made of standard wall quartz (20 cm X 9 mm 0.d.) sealed at one end and purged of organic contamination and absorbed water were used in pyrolysis. Purged, open-ended quartz capillary tubes were used to load samples into reaction tubes. Purging was done at 800-900 OC for 1 h followed by storage in an oven at 200 "C until used. Procedure. Approximately 4-10 mg of CI5+petroleum aliphatic fraction was weighed in a short quartz capillary tube (solid samples were heated to the melting point; all samples were loaded in the tube by capillary action); the capillary was then dropped into the reaction tube. All tubes were marked at a point 14.5 cm up from the closed end and were sealed off under vacuum at that 0 1986 American Chemical Society
2034
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986
Table I. Weight, Pressure, and HD/H2 Ratio Data for Tara and Newport Samples
wt, mg
Tara pressure, torr
HD/H2
ratio
wt, mg
Newport pressure, torr
HD/Hz
ratio
wt, mg
Tara pressure, torr
Temperature = 1140 "C for 0.5 h 7.6 7.5 5.6 6.8 5.2 5.5 4.5 8.0 6.1 7.1
366.4 350.1 318.8 317.8 254.8 279.2 232.8 378.5 303.5 355.9
96 304 95 280 91 713 92 089 90 215 89 279 86 870 97 925 92 463 96 580
6.1 4.6 5.9 7.3 5.2 6.0 5.8 6.1 4.9 5.5
303.3 236.1 306.3 361.1 263.7 292.7 284.9 292.8 243.1 268.2
339.6 378.9 301.9 271.6 249.0 254.6 267.5 310.5 323.3 407.1 448.4 404.5
7.0 8.0 5.7 6.1 6.8 7.0 8.4 7.1 8.1 7.4 5.4 7.0
341.3 377.1 274.0 299.4 317.8 338.3 393.6 330.8 335.5 368.8 249.1 321.6
73 926 76 232 73 060 71 120 70915 72 565 73 786 75 634 74 690 79 666 81 643 79 158
5.0 7.1 6.2 5.4 6.0 9.0 8.7 8.5 6.5 6.5 7.0
239.0 334.7 301.4 264.1 274.5 445.5 418.8 365.0 312.6 312.3 328.8
71 508 64 816 71 815 74 703 70 648 71 589 70 870 71 601 67 210 70 564
49 265 53 865 51 630 49 802 53 656 61 632 60 521 56 864 54 396 54 381 54 318
Temperature = 1140 "C for 1.5 h 70 487 69 865 66 297 67 560 69915 71 008 73 488 71 247 70 725 72 026 66 310 69 303
6.6 7.1 6.0 6.2 5.3 5.6 7.8 6.4 7.2 7.7 6.4
320.8 337.0 282.9 289.0 239.3 265.5 357.3 291.8 337.7 354.2 304.7
ratio
wt,
mg
HD/H,
ratio
Temperature = 1140 "C for 2.0 h
Temperature = 1135 "C for 1.0 h 6.6 7.6 6.0 5.6 5.0 5.0 5.1 6.2 6.4 8.2 9.0 8.0
HD/Hz
Newport pressure, torr
47 177 48 346 45 439 45 367 44 944 46 763 51 491 47 643 51 403 50 277 47 874
point with a torch. Sealed tubes were loaded on a heat-resistant rack (made of welded quartz tubing), modeled after a stainless steel rack described by Sofer (2),and placed in a furnace preheated to the maximum obtainable temperature (1140 "C). Mter periods varying from 30 min to 6.5 h the rack was pulled out of the furnace and cooled as quickly as possible (in order to quench any possible exchange reaction) by blowing cold air over the rack. Following pyrolysis, tubes were cracked open on the mass spectrometer inlet system by means of a tube cracker (3) and the pressure in the system was recorded. The gas was then allowed to expand into the variable reservoir and capillary,and the HD/H, ratio of the sample was determined by the mass spectrometer.
RESULTS AND DISCUSSION Table I summarizes the results of the various heating periods for the Tara and Newport samples. Shown are sample weights (mg), pressure (torr) in the inlet system, and the isotopic ratios (HD/H, ratios) expressed in machine units. Data in Table I are plotted in Figure 1 (sample weight vs. pressure) and Figure 2 (HD/H, ratios vs. pressure). In both figures, solid and open symbols represent the Tara and Newport samples, respectively. Figure 1 shows that for a given sample weight, after 6.5 h of heating, gas pressure in the pyrolysis tube ;3 much lower than after shorter heating periods. This indicates that under these experimental conditions, gas (most likely hydrogen) is escaping out of the pyrolysis tube, probably by diffusion through the quartz walls ( 4 ) . Comparison of the 2.5-h to the
7.4 6.3 7.4 5.4
331.0 282.2 346.6 188.7 219.2 342.5 179.0 205.2 350.4 217.0 256.4 265.4
7.6 4.3 5.0 7.7 4.7 6.1 5.7
65 175 62 732 65 916 60 038 60 723 65 859 61 244 60 325 66 446 60 941 62 395 62 695
9.8 8.0 8.0 6.0 5.0 4.8 4.3 6.6 7.0 5.4 8.3 6.1
431.7 354.6 378.9 268.2 225.7 218.0 195.1 281.8 300.3 228.1 364.7 278.4
47 257 44 674 45 710 40 875 40 054 40 246 39 490 42 415 42 862 40 643 45 675 41 846
Temperature = 1140 "C for 2.5 h 6.2 6.9 5.4
280.6 311.2 240.9 219.2 342.5 179.0 205.2 350.4 217.0 256.4 265.4
7.6 4.3 5.0 7.7 4.7 6.1 5.7
62 641 64 055 61 091 60 723 65 859 61 244 60 325 66 446 60941 62 395 62 695
6.7 9.4 7.6 5.0 4.8 4.3 6.6 7.0 5.4 8.3 6.1
288.3 382.0 320.5 225.7 218.0 195.1 281.8 300.3 228.1 364.7 278.4
42 243 45 480 43 545 40 054 40 246 39 490 42 415 42 862 40 643 45 675 41 846
Temperature = 1140 "C for 6.5 h 7.0 7.4 5.8 6.3 6.5 9.7 7.2 4.7 6.9 4.8 5.7 7.0
193.9 195.1 161.3 174.0 182.9 274.6 200.9 119.4 215.3 137.0 165.1 175.5 NEWPORT
70 520 72 256 70 423 70 114 70 257 72 633 69 744 69 090 69724 68 826 69468 70 777 TARA
5.1 6.5 7.7 9.8 7.1 4.9 6.1 5.9 6.3 6.0 5.7 7.0
132.6 186.6 217.1 267.7 201.9 147.8 160.5 177.2 183.0 178.3 175.5 196.4
47 955 48 338 49 877 51 587 48 163 45 687 47 700 47 612 47 157 46 003 47 391 48 574
TIME Ihrl
1c 05 Q
E9
150
Figure 1. 1140 'C.
200
250
PRESSUI%
150
YO0
us0
TORR
Weight-pressure relationship for different heating times at
0.5-h experiment also shows a slightly lower pressure after longer heating periods for a given sample weight, again suggesting some loss. However, the difference is very small and, therefore, the loss appears insignificant. Figure 2 shows the predicted ( I ) linear relationship between sample pressure and
ANALYTICAL CHEMISTRY, VOL. 58, NO.9, AUGUST 1986
2035
Table 11. Linear Regression Data slope
heating time, h
Tara
Newport
0.5 1.0 1.5 2.0 2.5 6.5
70.76 48.55 47.74 38.63 35.06 21.41
68.62 63.20 53.30, 38.53 34.25 36.66
intercept Tara Newport 70 529 59 192 54 147 52 730 53 443 66 403
HD/H2 ratio difference at 200 torr
corr coeff Tara Newport 0.9652 0.9447 0.8985 0.9890 0.9474 0.7339
50 962 33 672 31 509 31 560 32 629 41 210
0.9083 0.9840 0.8654 0.9650 0.9914 0.8048
19 996 22 590 21 536 21 190 20 976 22 143
Table 111. Weight, Pressure, and HD/H2 Ratios for Various Samples Heated for 2.5 h at 1140 O C
1 2 3 4
wt, mg
Newport pressure, torr
HD/H2 ratio
wt, mg
Tara pressure, torr
HD/H2 ratio
wt, mg
pressure, torr
HD/H2 ratio
7.3 6.0 4.9
349.2 284.8 213.6
44 529 42 772 40 766
7.4 5.2
376.8 237.1
65 134 62 628
5.8 7.6 7.4 5.7
261.8 321.6 308.3 253.7
41 873 45 260 44 975 43 306
wt, mg
pressure, torr
HD/H2 ratio
wt, mg
pressure, torr
HD/H2 ratio
wt, mg
4.8 6.3 7.2 4.5
215.1 288.4 317.6 209.7
43 791 46 607 48 068 43 541
6.6 6.2 4.8 5.5
292.6 285.8 224.4 244.4
48 151 50 105 47 071 47 487
6.9 6.5 6.0 4.1
AA-2-s
1 2 3 4
Madagascar
Candle Wax pressure, torr HD/H2 ratio
XB-3-S
HD/H2 ratios. The length of heating time has an effect on the slope of that relationship: shorter heating periods result in steeper slopes. Table I1 lists the slopes and intercepts of the various lines shown in Figure 2 as obtained by statistical linear regression. Also shown in Table I1 are correlation coefficients ( r ) for these lines. It can be seen that the best correlation coefficients, i.e., the least scatter around the straight lines, are achieved after 2 and 2.5 h of heating. This probably indicates the time a t which isotopic equilibrium is achieved between hydrogen bound to methane and free elemental hydrogen. The slope of the 6.5-h line suggests that the Newport samples are near isotopic equilibrium; however, loas of gas by diffusion probably resulted in the larger scatter and the different slope for the Tara samples. Also noticeable is the higher HD/H2 ratios of samples in the 6.5-h experiment. This indicates a preferential loss of H over D atoms, hence, supporting the suggested diffusion mechanism. [Although the exact diffusion mechanisms are not known, H2 molecules or H atoms diffuse faster than HD molecules or D atoms (5).] It is important to note in Figure 2 and Table 11that in four out of the six heating periods (0.5, 1.5,2.0, and 2.5 h), the pair of lines for Tara and Newport are parallel. Of particular importance is the fact that the slopes for Tara and Newport, after 2 and 2.5 h of heating are virtually identical (Table 11). This suggests that the HD/H2 ratio vs. pressure relationship can be calibrated by using one (or two) standards and that an appropriate correction can be applied to unknown samples. It is also important to note that at an arbitrary pressure (e.g., 200 torr) the difference between the HD/H2 ratios for Tara and Newport is essentially identical (see Table 11) after 1.5, 2, and 2.5 h. On the basis of the results of the optimization experiments, 2.5 h of heating at 1140 "C was chosen to be the standard time/temperature combination. Under these conditions four different samples (named Madagascar, AA-24, XB-34, and Candle Wax)and the Tara and Newport standards were analyzed. Results of the analysis are shown in Table III. Again, as in Table I, sample weights, pressures and HD/H2 ratios are shown. Data in Table I11 are shown in Figures 3 and 4. In Figure 3 the weight/pressure relationship for various samples at 2.5
150
200
2S0
330.0 318.5 280.8 201.4
300
350
65 180 64 537 63 513 60 543
YO0
YLO
PRESSURE. TORR
Flgure 2. HD/H2 ratlos vs. presswe for different heating times at 1140 OC.
h of heating show a similar trend as the two standards. In Figure 4 the HD/H2 ratios w.pressure relationship also show similar trends (dashed lines) as the Newport standard (solid line). This again indicates that the HD/H2 ratio vs. pressure
2036
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986
Table IV. Corrected HD/H2 Ratios ( R ) at 200 Torr' 0 TAR4
9 :
I!
.
9 :-
Madagas-
0 NEWPORT 0 MADAGASCAR
Newport
I A A 2 S I XB-3-5 * C A N D L E WAX
40387 40418 40388
1 2 3 4
av 40348 f 17
Tara 61059 61598
car '
61329 f 270
Candle
AA-2-S XB-3-S
40157 43371 41885 44153 41969 44804 41815 43372 41456 43900 f 869 f 720
45581 47723 46394 46254 46488 f 897
Wax
61571 61247 61270 60504 61148 f 454
L1 Parameters for calibration line (linear regression through the Newport data): slope = 27.76, intercept = 34 846.5, correlation coefficient r = 0.9999.
Table V. Calculated 6 Values (760)
Madagascar 1 2 Figure 3. Weight-pressure of heating.
relationship for various samples at 2.5 h
3 4
av
t
flu
_si.
0 TARO
60
\u P*
NEWPORT MADAGASCAR AA-2-5
Candle Wax
AA-2-S
XB-3-S
-158.4 -152.4 -152.1 -152.7
-147.3 -144.6 -142.4 -147.6
-139.7 -132.3 -137.0 -137.4
-84.7 -85.8 -85.7 -88.4
-153.9 f 3.0 -155.1 f 0.8
-145.5 f 2.5 -145.9 f 0.7
-136.6 f 3.1 -137.7 f 0.5
-86.1 f 1.6
'6 by oxidation/reduction.
resent Newport, Tara, and unknown, respectively. This equation is identical with eq 8 in ref 1. For the particular set of data shown in Table IV and Figure 4,this expression takes the form
6, = (3.44
IS0
ZOO
IS0
300
150
YO0
US0
PRESSURE, TORR
Flgure 4. HD/H, ratios vs. pressure for various samples after 2.5 h
of heating. calibration and correction can be performed. The HD/H2 ratio vs. pressure calibration and correction was done in the following manner: (1) a best fit calibration line was calculated from the three Newport data points; (2) the value of the slope of this line was multiplied by the excess pressure (pressure over 200 torr) of each sample; (3) the product of (2) was subtracted from the measured HD/H2 ratios given in the Table 111. Results of this calculation are given in Table IV. Also given in Table IV are the averages of the corrected HD/H2 ratios. Once the corrected HD/H2 ratios are obtained, they can be converted into 6 values by using the corrected average HD/H2 ratios for the Tara and Newport standards in combination with their known 6 values. The mathematical expression for this is
X 10-3)(R,) -
296.5
(4)
6 values calculated with this equation are given in Table V. As can be seen, there is an excellent agreement between the 6 values obtained by the pyrolysis method and the slow oxidation/reduction method. The difference between the two methods is only reflected by the better reproducibility (as expressed by the standard deviation u) that the slow method offers. Still, the great advantage in the speed of the pyrolysis method makes it worthwhile to sacrifice some of the reproducibility; the precision of the pyrolysis method, however, is expected to be adequate for geochemical interpretations.
ACKNOWLEDGMENT The authors wish to thank John Zumberge, Won Park, Chris Sutton, Ted Frankiewiz, and Fred M e r for their critical reviews and Carol Kandall and Taylr Coplen (U.S. GS Reston, VA) for their help in analyzing the hydrogen isotopic composition of combusted organic samples. Registry No. H2, 1333-74-0; D2, 7782-39-0. LITERATURE CITED Sofer, Z. Anal. Chem., preceding paper in this issue. (2) Sofer, Z. Anal. Chem. 1980, 52, 1389-1391. (3) Des Marias, D. J.; Hayes, J. M. Anal. Chem. 1976, 4 9 , 1651-1642. (4) Weaver, E. A,; Heckman, R. W.; Williams. E. L. J . Chem. Phys. 1987, (1)
4 7 , 4891-4895. (5) Popov, E. V.; Kupryazkin. A. Ya. Russ. J . Phys. Chem. (Engl. Trans/.) 1879, 53, 571-571.
RECEIVED for review January 31, 1986. Accepted April 18, where R is the corrected HD/H2 ratio and N, T, and u rep-
1986.
