ON THE METHODOLOGY OF MASS-SPECTROMETRIC ANALYSIS OF THE ISOTOPIC COMPOSITION OF CARBON IN OILS AND GASES

N. K. GRIGOR’EV

(Presented by Professor A. V. Aksarin)

Beginning in January 1967, on the initiative of V. L. Kokunov, work was begun in the mass-spectrometric laboratory of the Geological Prospecting Faculty on the study of the isotopic composition of the carbon of oils, gases, condensates, and dispersed organic matter from deposits of Tomsk Oblast.

In organizing work to determine the isotopic composition of substances on mass spectrometers, a highly important factor is the proper choice of the method for preparing samples for analysis and of the method for introducing the prepared specimens into the ion source of the mass spectrometer.

Preparation of samples for analyses consists, above all, in isolating the substance under investigation from the sample and converting it into a form convenient for mass-spectrometric analysis. Carbon is usually isolated from samples by thermal treatment of the sample [6] or by treatment with acids [5]. The latter method is applicable only for isolating carbon from carbonates. There are various installations for isolating carbon dioxide from hydrocarbon samples by thermal treatment. In [9] an installation is described for obtaining carbon dioxide that constitutes a closed system directly connected to the inlet system of the mass spectrometer. Combustion of the sample takes place in a stream of oxygen in a quartz furnace at a temperature of 950°. The evolved carbon dioxide, purified of water vapors by traps with a mixture of dry ice and acetone, enters the inlet system for isotopic analysis.

E. M. Galimov obtains carbon dioxide by thermal treatment of a sample in a special vacuum installation, with subsequent evacuation of CO₂ into glass ampoules. The gas sample in the sealed ampoule is transferred to a device for opening the ampoules, from which the released gas enters the inlet system of the mass spectrometer.

Both installations described above possess a number of advantages: 1) the isolation of carbon dioxide from the sample takes place under high-vacuum conditions, thereby eliminating the possibility of contamination of the sample by atmospheric carbon; 2) complete liberation and capture of CO₂ are achieved, thereby excluding the process of isotope fractionation.

However, for obtaining carbon dioxide from gas samples these schemes are unsuitable, and for oil samples they are not entirely successful.

In oils and gases the content of hydrogen, oxygen, nitrogen, helium, and other inert gases reaches 12–20%; the boiling point of which—

is very low (about −200° and below). When samples of oil and gas are burned in closed systems, with the final product for analysis obtained in the form of CO₂, these elements are not trapped by traps containing a mixture of dry ice and acetone (boiling point −70°) and, together with the substance under investigation, enter the mass spectrometer and create a general background, which may lead to distortion of the analytical data. Another substantial shortcoming of this method is that, as one of the by-products of the combustion reaction, nitrous oxide is formed; it then enters the mass spectrometer together with carbon dioxide and is superimposed on one of the working masses of carbon dioxide ($\mathrm{N}^{14}\mathrm{N}^{14}\mathrm{O}^{16}=44$ and $\mathrm{C}^{12}\mathrm{O}^{16}\mathrm{O}^{16}=44$). In view of these shortcomings, we adopted a procedure (6) according to which the product of the combustion reaction—carbon dioxide—is converted into barium carbonate. In this case the final sample for analysis contains no extraneous components.

Combustion of hydrocarbons by this procedure is carried out in a quartz tube, one-third filled with copper oxide and placed in cylindrical heating furnaces. An oil sample of 0.04–0.05 ml is placed in a special boat, introduced into the quartz tube, and burned at a temperature of 950° in a stream of oxygen purified of foreign impurities by absorbers. The carbon dioxide evolved passes through a system of Drechsel bottles filled with a solution of barium hydroxide and is converted into barium carbonate. To prevent carbon dioxide from the air from entering the gas absorbers, a vessel with Ascarite is installed next to the latter. It can also serve as a control element for checking the completeness of absorption of the carbon dioxide obtained during combustion of the sample.

The same apparatus is used for combustion of samples of any hydrocarbons.

The barium carbonate is washed with distilled water, dried, and stored in sealed ampoules or in test tubes with a ground-glass stopper. In analyses, carbon dioxide is liberated from $\mathrm{BaCO_3}$ in the reactor of the inlet system by the action of orthophosphoric acid.

When carrying out the combustion reaction and obtaining carbon dioxide from samples, appropriate measures must be taken to prevent processes of fractionation of carbon isotopes. In principle, we must obtain carbon dioxide whose carbon isotopic composition would fully correspond to the isotopic composition of the carbon of the sample. However, it is known that in reality any process of chemical interaction or transformation must be accompanied by separation of isotopes, caused by certain differences in the physicochemical properties of the isotopes.

The rate constant of the reaction with molecules containing a light isotope is greater than the corresponding rate constant of the reaction for molecules containing heavy isotopes of the given element. Therefore, in an incomplete chemical reaction the reacted product should be enriched in the light isotope, and the unreacted residue in the heavy isotope of the element. If the reactions proceed to completion, the isotopic composition of the reaction product will be equal to the isotopic composition of the starting substance.

Hence the necessary practical conclusion follows: when obtaining carbon dioxide from samples, it is necessary to achieve complete combustion of the sample and complete trapping of the carbon dioxide.

The next important point in organizing work on isotopic analysis is the choice of a method for introducing the substance under investigation into the ion source of the mass spectrometer. In practice this amounts to choosing a suitable scheme and fabricating, according to it, an inlet system,

which would make it possible to carry out isotopic analysis by the most accurate, precision method.

The factory inlet system supplied as a set with mass spectrometers is unsuitable for carrying out analyses by a precision method. Therefore, researchers are forced to design and manufacture for themselves an inlet system that satisfies the requirements of precision analysis.

As V. I. Ustinov and V. A. Grinenko [10] note, the most practical is a metal inlet system made of stainless steel. Such an installation would possess high operational reliability, good tightness, and would make it possible to eliminate completely the “memory” effect.

The technical difficulties of manufacturing a metal inlet system under laboratory conditions compel experimenters to resort to constructing the inlet system from molybdenum glass. An inlet system made of glass almost completely meets the requirements of precision analysis and is at present widely used [1, 4, 10, 11]. However, it is not without a number of substantial shortcomings: the sealing grease of the numerous vacuum stopcocks and ground joints (about 20 in number) used in this system may contaminate the substance under study with undesirable components; it is unreliable in operation, and it is difficult to obtain good tightness with it.

In view of this, and despite the technical difficulties, we designed and manufactured a metal inlet system based on supply and connecting tubes of stainless steel and Kovar, using vacuum valves from the factory set of the mass spectrometer. This made it possible to dispense completely with the use of glass vacuum stopcocks and ground joints in the inlet system. The use of other glass parts was reduced to a minimum (Fig. 1).

For alternate introduction into the ion source of the mass spectrometer of the working standard and the sample under study, a system of magnetically actuated valves was built into the scheme; these valves are put into operation by an automatic device with a time relay. The entire inlet system is evacuated independently to high vacuum by fore-vacuum and diffusion mercury-vapor pumps.

When analyzing barium carbonate, it is placed in a nickel cup in the reaction flask [8], and the entire system is evacuated to high vacuum. After this, the reactor valve is closed. Carbon dioxide gas is released from barium carbonate by the action of 95% orthophosphoric acid. For this purpose, with the aid of a magnet the nickel cup is tipped over, and BaCO₃ is poured into the test tube with orthophosphoric acid.

After the reaction is complete, the liberated carbon dioxide gas is transferred into one of the storage ampoules [4]. To purify the carbon dioxide from water vapor, traps (7) are provided, cooled with a mixture of dry ice and acetone.

Upon attaining the lowest background at the working masses (for CO₂—44 and 45), the gas under study is transferred from the storage ampoules into one of the two cylinders (2) and, through a capillary and magnetically actuated valves, enters the ion source for isotopic analysis.

The total amount of gas and the gas transferred into the cylinders is monitored by a mercury manometer (5) and a thermocouple lamp.

In conclusion, it should be pointed out that the design and manufacture of an inlet system for precision analysis requires a rather considerable amount of time. Therefore, simultaneously with the manufacture of the metal inlet system, on the same mass spectrometer we produ-

analyses of the isotopic composition of carbon in oils, gases, and bitumens were carried out by the single-beam method. For this purpose, a very simple inlet system was made, consisting of a reactor and two inlet cylinders with a capacity of about 3 liters each. Introduction of the gas under study into the ion

Fig. 1. Schematic diagram of the inlet system: 1—system of magnetically actuated valves, 2—cylinders, 3—capillaries, 4—sample-storage ampoules, 5—mercury manometer, 6–7—traps, 8—reactor

source was carried out by means of the metering valves of the standard mass-spectrometer set. The substance under study was introduced into the inlet system in the form of barium carbonate. Carbon dioxide was obtained in the reactor

by treatment with barium carbonate using orthophosphoric acid according to the method described above.

The mass spectrometer was adjusted and standardized before analyses using a working standard, for which we adopted a belemnite of Jurassic age, referred to by us as “Volga belemnite.” The carbon isotopic composition of the latter was determined by repeated analyses \((C^{12} = 98.804\%,\ C^{13} = 1.196\%,\ C^{12}/C^{13} = 89.68)\), (Table 1). Subsequently the mass spectrometer was standardized with a working standard \((\mathrm{CaCO}_3)\), accepted in the mass-spectrometric laboratory of VNIIYaGG and kindly supplied to us by V. S. Lebedev. The mean value of this standard, determined by precision analysis relative to PDB (Belemnitella americana), is: \(C^{13} = -0.85 \pm 0.1\%\) \((C^{12}/C^{13} = 89.65)\).

It is evident from Table 2 that the mean deviation of the isotope ratio \(C^{12}/C^{13}\), determined by the single-beam method, differs from the value obtained by the double-beam (precision) method by \(\pm 0.36\) (0.4%), which at first quite satisfied us. When sufficient statistics are accumulated (from 7–8 analyses of the same sample), the accuracy of determining the carbon isotopic composition by the single-beam method increases substantially. Thus, for example, the isotope ratio of carbon from \(\mathrm{CaCO}_3\), determined in our laboratory by the single-beam method, is \(C^{12}/C^{13} = 89.59\); the same value obtained by V. S. Lebedev by the precision method is 89.65; for “Volga belemnite,” respectively, \(C^{12}/C^{13}\)—89.68 and 89.61, Tables 1 and 3.

However, the accumulation of sufficient statistics for determining the carbon isotopic composition requires much time and increases the cost of the analysis.

REFERENCES

1. Aldberg A. M. Precision method for determining isotope ratios of light elements. Geochemical Collection, No. 8, 1963.

2. Vetshtein V. E. Ion receivers for analyzing the isotopic composition of boron, argon, carbon, and oxygen by the double-beam method in a mass spectrometer. Methodological Problems of Isotope Geology. Publishing House “Nauka,” 1965.

3. Vinogradov A. P., Kropotova O. I. On isotopic fractionation of carbon in geological processes. Izv. AN SSSR, Geological Series, No. 11, 1967.

4. Galimov E. M., Grinenko V. A., Ustinov V. I. On the selection of inlet parameters for a precision mass spectrometer. Instruments and Experimental Techniques, No. 3, 1965.

5. Galimov E. M. Methodology of mass-spectrometric analysis of the isotopic composition of carbonates. Proceedings of the Gubkin MINH and GP, No. 5, 1966. Collection “Geophysical Methods of Borehole Investigation.”

6. Grinberg I. V., Petrikovskaya M. E. Study of the content and ratio of hydrogen \((\mathrm{H}/\mathrm{D})\) and carbon \((C^{12}/C^{13})\) isotopes in hydrocarbons of the petroleum and coal series. Collection “Petroleum Geology,” Moscow, “Nauka,” 1964.

7. Golubchina M. N. Manual for Isotopic Analysis of Carbon on a Mass Spectrometer. Moscow, Gostoptekhizdat, 1956.

8. Dakhnov V. N., Galimov E. M. Application of mass spectrometry to the study of carbonate reservoirs. Soviet Geology, No. 9, 1965.

9. Lebedev V. S. Isotopic composition of carbon in oil and natural gas. Geochemistry, No. 11, 1964.

10. Ustinov V. I., Grinenko V. A. Precision mass-spectrometric method for determining isotopic composition. Series published by “Nauka,” Moscow, 1965.

Table 1

Reproducibility of the analysis in determining the isotopic composition of carbon in CaCO₃
(the working standard of the mass-spectrometric laboratory of VNIIYaGG)

C¹²/C¹³ = 89.59

Table 2

Comparative data of isotopic analysis of the carbon of oils by the precision
and single-beam methods

Table 3

Reproducibility of analyses in determining the isotopic composition of carbon
of “Volga belemnite”

C¹²/C¹³ = 89.68

NOTED MISPRINTS