Anal. Chem. 1991, 63,2970-2973
2970
noise is primarily random in nature and can be diminished by the use of ensemble averaging, appropriate timing of data collection, and appropriate filtering. Electrodes of larger size should exhibit better signal-to-noise ratios at these scan rates; the signal increases with the electrode area, and thus the value of Rf can be decreased, diminishing the Johnson noise. However, for in vivo applications this will cause greater tissue damage, and very large electrodes cannot be used at high scan rates. Electrodes of smaller dimensions require larger values of Rfto obtain an equivalent signal output, and their band-pass may be limited by stray capacitance. Note that an individual cyclic voltammogram used to construct Figure 4B is a measure of the oxidation of 300000 dopamine molecules. In a solution of equivalent concentration, a smaller electrode will oxidize even fewer molecules, thus further confounding detection. However, in a heterogeneous environment, a smaller electrode can be placed closer to the source of the chemical species and, thus, may experience higher local concentrations.
LITERATURE CITED (1) (2) (3) (4)
(5) (6) (7) (8) (9) (10) (11) (12) (13) (14)
(15) (16) (17) (18)
Coor, T. J. Chem. Educ. 1988, 4 5 , A533-A544. Coor. T. J . Chem. Educ. 1988, 45, A583-A596. Hieftje, G. M. Anal. Chem. 1972, 44 (6),81A-88A. Hieftje, G. M. Anal. Chem. 1872, 44 (7), 69A-78A. Becker, E. D.; Farrar, T. C. Science 1972, 778, 361-368. Bracewell, R. N. The Fourier Transform and Its Applications. 2nd ed.; McGraw-Hill: New York, 1986. Horiick, G. Anal. Chem. 1971, 43, 61A-66A. Maimstadt, H. V.; Enke, C. 0.; Crouch, S. R. Electronics and Instrumentatbn for Scientists; BenjaminlCummings: Menlo Park, CA. 1981. Smlth, D. E. Anal. Chem. 1978, 48, 221A-240A. Smith, D. E. Anal. Chem. 1978, 48, 517A-526A. Nlelsen, M. F.; Laursen, S. A.; Hammerich, 0. Acta Chem. Scand. 1990, 44, 932-943. Rice, M. E.; Nicholson, C. Anal. Chem. W89, 6 7 , 1805-1810. Klssinger, P. T.; Hart, J. B.; Adams, R. N. Brain Res. 1973, 55, 209-213. McCreery, R. L.: Dreiling, R.; Adams, R. N. Brain Res. 1974, 73, 23-33. Adams, R. N. Anal. Chem. 1878, 48, 1126A-1138A. a n o n , F. 0.; Fombarlet, C. M.; Buda, M. J.; Pujol, J. F. Anal. Chem. 1981, 5 3 , 1386-1389. Wlghtman, R. M. Anal. Chem. 1981, 5 3 , 1125A-1130A. Gerhardt, 0.A.; We, A. F.; Nagy, 0.; Moghaddam, B.; Adams, R. N. Brain Res. 1984, 290, 390-395.
(19) ArmstrongJames, M.; Fox, K.; Kruk, 2. L.; Mlllar, J. J . Neurosci. Methods 1981, 4, 385-406. (20) Howell, J. 0.; Wightman, R. M. Anal. Chem. 1984. 5 6 , 524-529. (21) Mlllar, J.; Stamford, J. A,; Kruk, Z. L.; Wightman, R . M. Eur. J. PharIlMCOl. 1985, 709, 341-348. (22) Baur, J. E.; Kristensen, E. W.; May, L. J.; Wiedemann, D. J.; Wightman, R. M. Anal. Chem. 1988, 6 0 , 1266-1272. (23) Kuhr, W. G.; Wightman, R. M. Brain Res. 1888, 381, 168-171. (24) May, L. J.; Wightman, R. M. J. Neurochem. 1088, 51, 1060-1069. (25) Wightman, 769A-779A.R . M.; May, L. J.; Michael, A. C. Anal. Chem. 1988, 6 0 , (26) Ewing, A. G.; Bigeiow, J. C.; Wightman, R. M. Science 1983, 227, 169-1 70. (27) Suaud-Chagny, M. F.; Mermet, C.; Gonon, F. J. Neuroscience 1990, 3 4 , 41 1-422. (28) Williams, G. V.; Miliar, J. J. Neurosci. 1990, 39, 1-16. (29) Nicolaysen, L. C.; Ideda, M.; Justice, J. B.; Neill, D. B. Brain Res. 1988, 460, 50-59. (30) Kuhr, W. G.; Ewing, A. G.; Caudili, W. L.; Wightman, R. M. J. Neurochem. W84, 43, 560-569. (31) Wightman, R. M.; Amatore, C.; Engstrom, R. C.; Hale, P. D.; Kristensen, E. W.; Kuhr, W. G.; May, L. J. Neuroscience 1988, 25, 513-523. (32) Moghaddam, 6.; Bunney, 6. S. J. Neurochem. 1989, 5 3 , 652-654. (33) Reid, P. M. S.; Herra-Marshitz, M.; Kehr, J.; Ungerstedt, U. Acta Physioi. Scand. 1990, 140, 527-537. (34) Kelly, R. S.; Wightman, R. M. Anal. Chim. Acta 1988, 787, 79-87. (35) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T. Numerical Recipes in Pascal, the Art of Computing; Cambridge Press: New York, 1989; Chapter 12. (36) Howell, J. 0.: Kuhr, W. G.; Ensman, R . E.; Wightman, R . M. J. Electroanal. Chem. Interfacial Electrochem. 1988, 209, 77-90. (37) Kristensen, E. W.; Wilson, R. L.: Wightman, R . M. Anal. Chem. 1988, 54, 986-988. (38) St. John, P. A.; McCarthy, W. J.; Winefordner, J. 0. Anal. Chem. 1987, 39, 1495-1497. (39) Wlpf, D. 0.; Kristensen, E. W.; Deakin. M. R.; Wightman, R. M. Anal. Chem. 1988, 6 0 , 306-310. (40) Morgan, D. M.; Weber, S. G. Anal. Chem. 1984, 5 6 , 2560-2587. (41) Lasson, E.; Parker, V. D. Anal. Chem. 1890, 6 2 , 412-412. (42) Michael, A. C.; Wightman, R . M.; Amatore, C. A. J . Nectroanal. Chem. Interfacial Electrochem. 188% 267, 33-45. (43) Deakin, M. R.; Kovach, P. M.; Stutts, K. J.; Wightman, R . M. Anal. Chem. 1988, 5 8 , 1474-1480. (44) Mlllar, J.: Barnett, T. G. J . Neurosci. Methods 1988, 2 5 , 91-95. (45) Wightman. R. M.; Zlmmerman, J. B. Brain Res. Rev. 1990, 75, 135- 144.
RECEIVED for review July 15, 1991. Accepted September 25, 1991. This research was supported by NIH (Grant NS15841). K.T.K. is a DOE fellow, and R.T.K. is the recipient of an NSF fellowship.
Determination of Trace Iodine in Food and Biological Samples by Cathodic Stripping Voltammetry Shuxun Yang,* Shoujun Fu, and Minlu Wang Department of Chemistry, Zhengzhou University, Zhengzhou, Henan 450052, People’s Republic of China
This paper describes a SenSnlve and selectlve method for the determlnatlon of lodlne In food and blologlcal samples. The method Involves treatment of samples by combustion In an oxygen flask and determlnation of lodkle by cathodlc stripplng voltammetry of the solld phase formed with the quaternary ammonlum salt Zephlramlne as the Ionic associating agent; Br- Is used as the complexlng agent In the preconcentratlon process. We have studled the effect of concentration of Zephlramlne, Br-, I-, and some other elements presented, deposltlon potential, preelectrolysls time, and scan rate, on the strlpplng curve shape and maximum stripping current. Determlnatlons of trace lodlne In table salt, laver, and eggs were demonstrated as practical examples.
INTRODUCTION Recent developments in stripping voltammetry methodology have resulted in three different preconcentration procedures: (1)utilizing the association of ions to form insoluble compounds on the surface of the working electrode (1-3), (2) utilizing the adsorption of the analyte as metal chelates on the surface of the working electrode ( 4 ) ,and ( 3 ) utilizing the chemical-modified electrode to catch the determined ions on the surface of the working electrode (5-10). Compared with the procedure of ordinary electrolytical deposition, the above mentioned procedures offer advantages in terms of sensitivity and selectivity or broaden the range of detected elements.
0003-2700/91/0363-2970$02.50/00 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 03, NO. 24, DECEMBER 15, 1991
Cathodic stripping analysis of iodine has been reported in 1966 and 1982 (1,2).The preconcentration of iodine on the surface of the electrode took place only in the presence of a dye-producing organic cation in the solution and of C1- ions. I- was oxidized and interacted with Cl- and triphenylmethane dye (as methyl violet) or rhodamine C (R) to form ion-pair compound RIzCl and deposited on the surface of the electrode. Recently, an improvement of cathodic stripping analysis of iodine has been proposed (3),using butyl-rhodamine B as the association agent (R) and Br- as the complexing agent, to form 12Br- and finally R12Br. But the methods have lower sensitivity and are subject to interference of other ions, such as S032-, S2-, S2032-, and SCN-. This paper reports the development of a new cathodic stripping voltammetry method for determination of trace iodine by use of Zephiramine (Zeph) as an ionic associating agent. The reactions involved are 21Iz 2e-
+ + + + - + Iz
IzBrZephIzBr
Br-
IzBr-
Zeph+
2e-
Zeph1,Brl
21-
Br-
+ Zeph+
The preelectrolysis is done in an acidic solution containing 0.1-20 ng/mL I-, 4 X 104-1 X mol/L Br-, and ca. 0.5 X lo4 mol/L Zeph. I- is oxidized at +0.95V (vs Ag/AgCl), then combines with Br- to form 1,Br- (3),and finally associates with Zeph to deposit (Zeph12Br) on the working glassy-carbon electrode (preelectrolysis period is 2 min). A stripping scan is carried out from +0.95 to 4 . 2 V (vs Ag/AgCl). At about +O.lV (vs Ag/AgCl), the peak current is rictilinearly related to the concentration of I- ions in the range 0.1-20 ng/mL. The method has been used for determination of iodine in table salt, laver, and eggs with satisfactory results.
EXPERIMENTAL SECTION Apparatus. Voltammetric measurements were made with a voltammetric analyzer type 79-1 (Jinan Instrumental Co., P. R. China) and fast scan recorder Type 3033A3 (Sichuan Instrumental Co., P. R. China). The instrument was fitted with a glassycarbon-dwk working electrode, 2.8 mm in diameter, corresponding to an area of 6.2 mm2, a platinum counter electrode, and a Ag/AgCl (saturated KCl) reference electrode. Before use, the working electrode was polished with a polisher, degreased with acetone, and rinsed with water. Polishing wm not necessary before each measurement, and 10 or more experiments without polishing gave high reproducibility. Reagents. All reagents were analytical pure or suprapure grade. Water waB redistilled. The potassium iodide standard solution was 2 mg/L, the potassium bromide solution was 7.5 X 10-3mol/L, sulfuric acid was 1 mol/L, and the Zephiramine (tetradecyldimethylbenzylammonium chloride, made by Japan Dojindo Labmol/L. oratories) solution was 1 x Procedure. A certain volume of test solution was transferred mol/L Zeph, 2.5 to a 50-mL volumetric flask, 2.5 mL of 1 X mol/L KBr, and 7.5 mL of 1 mol/L HzSO4 mL of 7.5 X solution were added, and then the solution was diluted to the mark with water. A 5-mL solution was pipetted into the electrolytic cell, the electrodes were immersed and conditioned at +0.95 V (vs Ag/AgCl) for 2 min with stirring, and iodine was deposited. The stirring was stopped, and after 30 s of quiescent time, the stripping scan was carried out from +0.95 to -0.2 V at 250 mV/s. The peak potential was at about +0.1 V. It was maintained at -0.2 V for at least 1 min for stripping residual deposition. The sample solution was measured by the standard addition method, by adding a very small volume of relatively concentrated iodide standard solution, so that the dilution could be neglected in the calculation. RESULTS AND DISCUSSION Effect of Different Quaternary Ammonium Salts. Some long-chain quaternary ammonium salts such as Zeph,
2971
34-
E E v
-
32
-
30
-
28-
26
-
24-
,J ,
,
,
,
,
,
,
Concentratlon of HS0, (mollL)
+
Flgure 1. Effect of acidity on i,. Test solution: 10 ng/mL I0.4 X mol/L Zeph 7.5 X lo4 mol/L KBr 4- X mol/L H,SO,. Conditions of apparatus: sensitivity = 0.05, record x = 500 mV/cm, y = 10 mV/cm, deposition potential = 4-0.95 V, prselectrolysis time = 2 min, scan rate = 250 mV/s.
+
30
'
26
'
E
g
22
-
'
D
"l 14'
i
4
'
7
5
9
11
13
15
Concentratlon of Br' (ml/L) (x 10 )
Fi@we2. Effect of concentration of Br on i,. Test sdutkn: 10 n g / d I0.4 X mollL Zeph X moilL KBr 0.15 mollL H,SO,.
+
+
+
CTMAB, and TPC were used as the ion-associating agents individually. The results show that the measuring sensitivity and reproducibility of Zeph were better than those of others. The relative standard deviation for eight runs is 3.5%. Effect of Acidity of Solution. The result is shown in Figure 1. The acidity of the solution was adjusted with H#O& The dependence of the peak current on the concentration of HzS04was varied as a parabola, as in Figure 1; the peak current increased with the increase of acidity, and the maximum of the peak current corresponds to that of the concentration of H2S04at 0.15 mol/L. If H2S04 was substituted by HC1, the peak current lowers about one-fourth on the other hand, if it is replaced by HC104, a white precipitate is present in the solution and the peak current vanishes. Zeph-C104 may be formed and thus prevents Zeph-IzBr from depositing on the working electrode. Effect of Concentration of Bromide. C1- or Br- was considered to form a complex anion with iodine (IzCl-or 1,Br-I ( 1 , 3 ) ,which could further associate with Zeph, as ZephIzC1 or ZephIzBr, to deposite on the working electrode. But the peak current for the Br- complex ion was 4 times higher than that of the C1- complex (3). The effect of concentration of Br- on peak current is shown in Figure 2. When the bromide concentration was smaller than 3.75 X IO4 mol/L, the peak current decreased quickly with the decrease of bromide concentration; when the bromide concen-
ANALYTICAL CHEMISTRY, VOL. 63, NO. 24, DECEMBER 15, 1991
2972
Table I. Tolerable Amount of Other Ions' 30
-
E E .s
ion, A
max tolerable amt, pg/mL
tolerable ratio, A:I
Nat NH,+
73 50 10 12.5 10 5 5 5 1 0.5 0.05 50 8 5 5 1.3 200 200 100 5 0.025 0.025 0.025
7300 5000 lo00 1250 1000 500 500 500 100 50
Zn2+
Pb2+ cu2+ Mg2+
20
Ba2+ Cd2+ Fez+ Ca2+ Hgz+ 10
AP+
d 0.3
0.1
0.5
0.7
1.1
OS
Concentrationof Zeph (mollL) ( x
1.3
1.5
lo4)
Flgwe 3. Influence of concentration of Zeph on i,. Test solution: 10 ng/mL I- +- XmoVmL Zeph +- 3.75 X lo4 mol/L KBr 0.15 mol/L H2SO4.
+
Bi3+ Sb3+ Fe3t Cr3+ N0c NO2c1SO?-
SCNS2-
s20t-
5 5000 800 500 500 130 zoo00 zoo00
loo00 500 2.5 2.5 2.5
"The test solution contained 10 ng/mL I-. Table 11. Determination of Iodine in Food and Biological Samples
€0
name of sample
I 4
I
1
1
I
i
8
12
16
20
24
I
Concentration of I' (ng/ml)
Figure 4. Relationship between concentration of I- and i Test solution: Xng/mL I- +- 0.5 X lo4 moi/L Zeph + 3.75 X lo5 moVL KBr 4- 0.15 mol/L H2S0,. tration was in the range 3.8 X lO*l.l X mol/L, the peak current remained almost constant. Effect of Concentration of Zeph. Figure 3 shows that when the concentration of Zeph was less than 0.1 X mol/L, there was fundamentally no peak current; however the peak current increased linearly with the concentration of Zeph in the range 0.1 X 104-0.5 X lo-' mol/L until the maximum value was reached and then fell slowly. Zeph itself may be adsorbed onto the working electrode and affects the deposition of ZephIzBr as the concentration of Zeph over 0.5 X mol/L. Effect of Instrumental Parameters. The variations in peak current with deposition potential, preelectrolysis time, and scan rate were investigated. Effect of Deposition Potential. The peak current increased with increasing deposition potential up to a limiting value at about +1.0 V. A sharp decrease in peak current is evident with deposition potentials greater than the deposition potential of maximum response. This is probablly due to the formation of Br3- on the electrode, which will affect the deposition of Zeph-12Br. The deposition potential of +0.95 V was used for optimum reproducibility. Effect of Preelectrolysis Time. The peak current also increased with preelectrolysis time until the surface of working electrode was saturated. It follows a linear relationship with preelectrolysis time in the range 1-5 min.
mean value of iodine in sample, ng/g iodine added in sample soln, ng/mL mean value of iodine recovery, ng/mL mean value of recovery percentage, % re1 std dev. %
table salt
laver
eggs
1.01
28.11
4.75
6.00
4.00
4.00
6.15
3.78
3.74
102.3
94.8
93.8
13.5 (n = 6) 10.0 (n = 9) 4.5 (n = 7)
Effect of Scan Rate. The effect of scan rate on peak current was also examined by varying the scan rate for fixed values of other parameters. The peak current is directly proportional to the scan rate up to 250 mV/s. Then it gradually deviated from linearity. Relationship between Peak Current and Concentration of I-. As shown in Figure 4, under the optimum condition, the peak current is rectilinearly related to the concentration of I- in the range 0.1-20 ng/mL and the limit of determination is 0.1 ng/mL. Influence of Other Ions. The experimental data are listed in Table I: 1000-fold and more amounts relative to I- of Na+, NH4+,Zn2+,Pb2+,Cu2+,A13+,NO3-,NO2-, and C1-; 500-fold amounts of Mg2+,Ba2+,Cd2+, Bi3+, Sb3+,Fe3+, and SO3*-; 100-fold amounts of Fe2+and Cr3+;and 50-fold amounts of Ca2+can be tolerated. Only a few kinds of ions such as Hg2+, S2-,S2032-, and SCN- affect the determination of I-. Fortunately, Hg2+is absent in ordinary cases; S2-,Sz032-,and SCNcan be oxidized to convert into SOS2-,and thus the effect is eliminated. Determination of Trace Iodine in Table Salt, Laver, and Eggs. The sample treatment of table salt is very simple because it is soluble in water. Laver, eggs, and other organic or biological samples must be treated by the method of oxygen flask combustion (11)using 2% acidified sodium formate as absorbent. The results of determination are given in Table 11. The relative standard deviations for measurement of table salt, laver, and eggs are 13.5%, 10.070, and 4.5%, respectively, and the recovery percentages are 83-110% for table salt,
Anal. Chem. 1091, 63, 2973-2978
83-107% for laver, and 83-11570 for eggs, --
CONCLUSION A new cathodic stripping voltammetry for determination of trace iodine has been investigated. The quaternary ammonium salt Zeph has been used as ionic associating agent to deposite IzBr- which is produced by oxidizing and then coordinating on the working electrode. The base solution contains 0.15 mol/L H2S04, 3.75 X mol/L KBr, and 0.5 X mol/L Zeph. The deposition potential is at +0.95 V (vs Ag/AgCl), the preelectrolysis time is 2 min, and the scan rate is at 250 mV/s. Under the optimum conditions, the peak current is linearly related to the concentration of I- in the range 0.1-20 ng/mL. The determination limit is 0.1 ng/mL. This method exhibits two advantages: (1) high sensitivity (subnanograms per milliliter range of I- can be determined; it is about 10 times more sensitive than the methods described in refs 1 and 3); (2) high selectivity (only a few kinds of ions containing sulfur interfere with the measurement, but they can be eliminated easily during sample treatment). Therefore,
2973
this procedure is suitable for determination of trace iodine in biblogical samples and has superiority over the methods using triphenylmethane dyes or rhodamine-derivated compounds as associating agents.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8)
Brainina, Kh. 2.; Sapozhnikova, Eya.Zh. Anal. K h h . 1988, 2 1 , 1342. Brainina, Kh. 2. Freseuius' Z . Anal. Chem. 1982, 312, 428. Yang, S.; Ye, B.; Wang, M. Anal. Lab. 1991, 10, 13. Wang, J. Am. Lab. 1985, 5 , 41. Oyama, N.; Auson, F. C. J . Am. Chem. SOC. 1979, 101, 3450. Oyama, N.; Auson, F. C. J . Nectrochem. Soc. 1980, 127, 247. Li, 2. M.; Thomas, E. J . Phys. Chem. 1987, 91 (3),643. Mukherljce, R.; Goswami, S.; Chakravorty, A. Inwg Chem. 1985, 24 (26), 4528. (9) Itaya. K.; Chang, H. C.; Uchide. I. Inwg. Chem. 1987, 26 (4), 624. (10) . . Hernandez. P.: Aida, E.: Hernandez, L. Fresenhs' Z . Anal. Chem. 1987, 327 (7), 676. (11) ChMs, C. E.; Meyers, E. E.; Cheng, J.; Laframboise, E.; Baiodis, R. B. Microchem. J . 1963, 7 , 266.
.
RECEIVED for review April 3, 1991. Accepted September 25, 1991.
Chemical and Carbon Isotopic Alteration of Organic Matter during Stepped Combustion Ben D. Holt* and Teofilo A. Abrajano, Jr. Geoscience Group, Chemical Technology Division, Argonne National Laboratory, Argonne, Illinois 60439
Stepped combustion was examined for Its appllcablllty to the resolution of Isotopically dlstlnct carbon components of c o m plex organlc matter such as kerogen. A kerogen (Isolated from Green Rlver shale), a standard oll (NBS-22), and two single-component, pure, organic compounds (sucrose and pentadecane) were subjected to a modified procedure of stepped combustion. The modifled procedure was deslgned to test for completeness of reactlon at an arbltrarlly selected relatlvely low combustion temperature (225 "C)In repeated perlods of heating. I n sequential periods at 225 O C , the rate of carbon release as COz and CO generally dlmlnlshed from several percent C per hour to near-zero percent C per hour, over a comblned combustion tlme of up to 88 h. Nearly half of the carbon In all four materlals remained as charred, oxldatbn-reslstant residues that were readlly combusted at 500 O C . The carbon fractlons released at 225 and 500 O C , respectlvely, were lsotoplcally relatable to the chemlcal alteratlon of the organlc substances that occurred during the analytlcal process, rather than to lsotoplcally dlstlnct components In the orlglnal materials.
INTRODUCTION The analytical technique of stepped combustion has been applied to carbonaceous meteorites (1-5)and to sedimentrary rocks (6) in quest for isotopic and/or structural information on the complex organic components in these materials. According to reported procedures, the material to be analyzed is confined to a combustion chamber with a large excess of pure oxygen. In each of a series of combustion steps, the sample is heated for an equal length of time (e.g., 2 h by Kerridge (2) and 0.5 h by Gilmour and Pillinger (6)) at in0003-2700/91/0363-2973$02.50/0
creasingly higher temperatures. At the end of each combustion step, the excess O2is separated from the gaseous producta of the combustion (C02, CO, HzO, Nz, and other minor components), the CO is oxidized to C02, and the combined COz is measured and isotopically analyzed by mas8 spectrometry. Isotopic analyses may also be obtained for N2 and Hzby procedures used by Kerridge (2). Kerridge (2) reported application of stepped combustion with the objective of breaking down kerogen-like organic matter in meteorites in such a way as to resolve isotopically distinct Components. Kerridge et al. (3) later interpreted results obtained by this analytical technique as evidence of two organic components in meteoritic material. Swart et al. (1)described the apparatus and the procedure which they used for stepped combustion to resolve indigenous species of carbon in extraterrestrial samples from terrestrial carbon that had contaminated the samples during procurement and handling. The contaminating carbon was considered to burn at lower temperatures than the indigenous phases of carbonaceous chondrites. Gilmour and Pillinger (6) extended the stepped-combustion technique of Swart et al. (I) to the isotopic analysis of carbonaceous sediments. Specifically, they analyzed samples of Green River shale and two coals, torbanite and anthracite. They demonstrated the possibility of isotopic resolution of carbonate carbon from organic carbon in the shale by stepped combustion by showing that the organic carbon was released by oxidation before the carbonate carbon was released by thermal decomposition (4,5). Thus, the isotopic analysis of organic carbon in Green River shale could be made without having first to remove the carbonate carbon by acid treatment. Their results on torbanite and anthracite coals, however, suggest that carbon in organic phases at higher levels of maturity may not be resolvable from carbonate carbon by 0 1991 American Chemical Society
