NATIONAL BUREAU OF STANDARDS REPORT

4047

STABLE CARBON ISOTOPE ANALYSIS

BY OPTICAL SPECTROSCOPY

by

R# E# Ferguson and H. P. Broida

Progress Report

to

Atomic Energy Commission

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

0301-202691 April 1, I955

NBS REPORT

4047

STABLE CARBON ISOTOPE ANALYSIS

BY OPTICAL SPECTROSCOPY

by

R. E. Ferguson and H. P. Broida

Progress Report

to

Atomic Energy Commission

Washington 25 , D.C.

U. S, DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, re Approved for public release by the in part, is prohibited

unless permission! director of the National Institute of andards, Washington

25, D.C. Such pei

cal ly prepared If t Standards and Technology (NIST) on October 9, 2015

sport has been specifieport for its own use,
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FOREWORD

This report covers the initial phases of the project on

extending the optical spectroscopic method of stable isotope

determinations to analysis of C^3/cl2 mixtures.

Later, it is intended to publish this report in Analytical

Chemistry,

J}

F. G. Brickwedde, Chief

Heat and Power Division

A. V. Astin

Director
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STABLE CARBON ISOTOPE ANALYSIS BY OPTICAL SPECTROSCOPY

by

R. E. Ferguson and H. P. Broida

ABSTRACT

An optical spectroscopic technique for measuring

relative concentrations of the stable carbon isotopes,

C12 and C^-3, has been investigated. Acetylene prepared

from the sample to be analyzed is burned in a premixed flame

with air, and the relative intensities of Swan bands due to

isotopic C2 radicals are measured photoelectrically. The

adaptability and accuracy of the method are discussed.

I. INTRODUCTION

Previous successful applications of optical spectroscopy to routine

precision analysis of hydrogen isotope concentrations [1,2] have encouraged the study of such determinations for other stable isotopes.

This note describes a technique for the measurement of relative concentrations of the carbon isotopes, C^-2 and C^3

9 utilizing C2 radiation

from a flame.

The C2 bands have been used previously for the rough estimation of

carbon isotope ratios [6,73. The method to be described differs from

previous ones in that a flame is the light source, measurements are

made photoelectrically rather than photographically, and a calibration

curve obtained with standard samples is used. These changes result in

increased accuracy and a considerable decrease in the time required

for analysis.
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The carbon compound to be analyzed is converted by a generally

applicable method to acetylene which is burned with air on a small burner,

and the emission spectrum of the inner cone is observed photo electrically.

The relative intensities of the 1,0 band heads of the (Swan)

system due to C*^C^, C^C^, and in the region of 4735 bo

4755A are measured. The relative concentrations of the isotopes are

then determined from a calibration curve constructed from intensity data

obtained with acetylene samples of known content. By the use of a calibration curve, possible difficulties caused by small isotope effects

on reaction rates and differences in spectral characteristics of the

isotopic C2 radicals are avoided.

The 1,0 band was selected because of its relatively high intensity

in a flame, the low intensity of background radiation at the head of the

band, the relatively large separation of the band heads of the isotopic

C2 radicals (of the order of 7A) , and because in this band the C-^C^-3

12 12 head is clear of overlapping lines of the C C spectrum.

An acetylene flame was chosen because of the relatively high intensity of C2 emission from the inner cone. Air rather than oxygen was

used to facilitate adjustment of flow rates at low total flow through

the burner. With oxygen, it is difficult to avoid flashback and at

low flow rates the flame is inconveniently sensitive to adjustments.

II. EXPERIHENTAL

Acetylene - C^-3 was prepared by the carbide method [ 3 ] from

enriched BaCO^ (Eastman Kodak Company) • It was purified by three vacuum

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distillations from a trap at dry ice temperature into a trap at

liquid nitrogen temperature* Ordinary tank acetylene, freed of

acetone by vacuum disti Llation, was used to dilute samples of the

acetylene-cA^ for the preparation of mixtures for analysis. The

mixtures were prepared volumetrically on the vacuum line by first

measuring acetylene-(A3 and ordinary acetylene separately and then

condensing them together into a storage bulb. Volume measurements

were made at the same pressure to avoid the necessity of compressibility corrections.

The starting sample ( 66.6 atom per cent (A3) and one diluted

sample (22.9 atom per cent (A^) were analyzed mass spectrometrically.

The other mixture compositions, 11.9, 6 . 3 , and 3*6 atom per cent (A-^,

were calculated from the results of these analyses and the volumetric

dilution measurements. The mass spectrometric analyses showed a maximum of 1 per cent impurity, most of which was ethylene.

The isotopic acetylene mixtures prepared by dilution of (A3

enriched acetylene with ordinary acetylene do not have the carbon

isotopes distributed randomly among the acetylene molecules. For

example, in the mixture containing 22.9 atom per cent (A3 , the isotopic

acetylenes HC-^C-^H, HC^=(A%, and HC^(A^H are present in the concentrations 69 : 16: 15 , respectively. If the acetylene sample had been

prepared from homogeneous BaCO^ containing 22.9 atom per cent (A3, the

respective concentrations would be approximately 59 • 35 : 5* The slight

effect of the isotope distribution in the acetylene upon that in the

C2 in the flame will be discussed below. -3-
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The acetylene mixtures were burned on a small welding torch with

an orifice diameter of 0.5 mm, using the delivery apparatus shown in

i Figure 1. The acetylene and air were premixed in the torch in proportions (somewhat less than the stoichiometric amount of air) which

gave the greatest intensity of the C2 bands. Control valves were set

using tank acetylene and were left in the same position for all mixtures

burned. The sample was delivered from a rubber gas expansion bag filled

on the vacuum line; constant delivery was achieved by surrounding the

bag with nitrogen at 5 psig from a pressure - regulated tank. The threeway stopcock shown in Figure 1 was used for flushing or evacuating the

upper part of the assembly between determinations. A stable and reproducible flame of sufficient intensity was obtained with an acetylene

flow of 6 cc/min. The samples ranged from 50 to 100 cc N.T.P. Ten

separate spectra could be obtained with a 100 cc sample in a burning

time of about 16 minutes.

The inner cone of the flame was focussed on the slit of a monochromator with photomultiplier detection and pen recording

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Recorded intensities are directly proportional to the emitted light

intensities. A slit height of 2 mm and a slit width of about 0.02 mm

were used; this permitted the clear resolution of lines 0.6A apart. For

the purpose of analysis considerably lower resolution would be adequate.

* Constructed by the Research Department, Leeds and Northrup Company

and loaned to the Rational Bureau of Standards on a field trial

arrangement

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Figure 2 shows recordings of the 1,0 band heads of the isotopic

Cg radicals obtained with three acetylene samples of different C^-3

content. The scanning rate was 12A/min. The first recording. A,

made with acetylene containing 22.9 atom per cent is shown to

illustrate the reproducibility of the relative band head intensities.

The dotted line indicates the point at which the direction of scanning

the spectrum was reversed and the tracing repeated as a mirror image.

Curve B is a recording of the same spectral region with a flame of

acetylene containing 6.3 atom per cent At this C^-3 concentration

the C^c^3 band head intensity is too low to stand out above the noise

12 12

level, although the C C J head can still be measured accurately.

Recording C was made with a flame of ordinary tank acetylene; the

cl2cl3 band head is due to the natural abundance of which is

about 1.1 atom per cent.

It can be seen from Figure 2 that the band heads are well separated

and that the intensities can be measured easily. Only the C-^C^ and

cl2cl3 band head intensities are needed for the determination of the

relative abundance of C^3 # A calibration curve using these two band

heads is shown in Figure 3> where the observed intensity ratio C^C^-3/

j_ s p]_0tted against the C J abundance, expressed in atom per cent,

in the acetylene burned. Each ratio plotted is the average for three

separate determinations. In all cases, the standard deviation of the

mean of the three measurements was less than 3 per cent of the mean value.

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The corresponding precision in the C1^ abundance, expressed in the

same way, did not exceed 3 per cent of the measured abundance*

With samples containing less than 15 atom per cent C*^, the

*10 to 3J2 13

and C C intensities were very nearly proportional to the

expected isotopic C2 concentrations calculated by assuming a random

distribution of the isotopes in C2; thus it is estimated that the

measured intensity ratio alone could be used directly to determine the

abundance to the nearest 1 per cent in this range. For greater

accuracy the use of calibration curve is desirable.

The non-equilibrium composition of the isotopic acetylene

mixtures burned was referred to earlier. Another study [5] has shown

that the emission from the isotopic C2 radicals in the flame depends

to a slight extent upon the isotopic composition of the acetylene.

For example, if the acetylene has a deficiency (compared with the

statistical equilibrium concentration) of HC^C^H, the intensity will be lower than that calculated from the abundance alone.

The effect is as if about one tenth of the C2 comes directly from the

carbon-carbon pairs of the acetylene introduced into the flame. It is

necessary to take this effect into account if it is desired to make

accurate measurements in the range above 15 atom per cent on

samples prepared by dilution with ordinary acetylene. This can be done

by using a calibration curve obtained with acetylene samples similar in

composition to those to be analyzed. On the other hand, in undiluted

acetylene prepared from diluted CO2 or BaCO^, the isotopic species will

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be present in nearly equilibrium concentrations and the effect will

be absent.

IV. DISCUSSION

These results show that optical spectroscopy can be used for

rapid and reasonably accurate determinations of C^/C^ ratios. The

major disadvantages of this method are (l) that acetylene must be

prepared from the carbon-containing sample and (2) that the sample of

acetylene is destroyed by the analysis. These disadvantages can be

lessened by the following modifications of the procedures described

above

.

In actual application of the method, the carbon in the compound

to be analyzed can be oxidized to CO2 . The CO2 can then be converted

to acetylene by the method of Monat and Robbins [8] with 98 per cent

yield. This is a simple procedure and can be used for the preparation

of acetylene from any carbon-containing substance.

To reduce the size of sample required for burning, the acetylenec!3 can be diluted with ordinary acetylene, remembering that the

natural abundance of is 1.1 atom per cent. The extent of dilution

possible will depend upon the optical instrumentation used and upon the

accuracy desired. In addition, the dead space in the burner assembly

can be reduced and the rate of burning might also be decreased. It

should be possible, with care, to reduce the total sample to less than

10 cc of acetylene.

The results reported here do not give basis for a reliable estimate

of the accuracy possible with this method. In the determination of -7-
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hydrogen isotope ratios, using the same instrument, a precision of

0*1 per cent in the ratio of two intensities was obtained [1]* This

may be taken at present, with the instrument used, as a lower limit

to the precision which can be acheived in such measurements under

optimum conditions.

The technique described here does not compare favorably with the

mass spectrometric method in regard to the time required for sample

preparation (preparation of instead of CO2), size of sample used

(10 cc as compared to 0.1 cc) and in the stage of development of the

procedure. However, it may be found useful, especially in laboratories

i which do not have access to a mass spectrometer, but which are equipped

with an optical spectrometer of moderate resolving power.

literature cited

1. Broida, H. P., Morowitz, H. W. , and Selgin, M. , J. Res. NBS j>2,

293 (1954).

2. Broida, H. P., and Moyer, J. W., J. Opt. Soc. Am. ^2, 37 (1952).

3. Calvin, M. , et al.. Isotopic Carbon (John Wiley and Sons, Inc.,

New York, 1949) p. 205.

4. Fastie, W. G., J. Opt. Soc. Am. !£, 64I (1952).

5. Ferguson, R. E. , to be published elsewhere,

6. King, A, S. and Birge, R. T., Astrophys, J. f]2m9 19 (1930 )

.

7. McKellar, A., Publ, Dom. Astrophys. Obs. Ottawa 2, 395 (1949).

0, Monat, S., and Robbins, C., U, S, Atomic Energy Commission

Report AECU - 672.

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Figure 1.

Figure 2.

Figure 3 .

Burner assembly

Recordings of the 1, 0 band heads (^tT - ^Tf transition) of C2 in an acetylene-air flame.

A, acetylene - C^-3, 22.9 atom per cent C^-3.

B, acetylene - C^3, 6.3 atom per cent C^-3 # C, ordinary tank acetylene.

Plot of observed C^C^/C^C^ band head intensity

ratios against C^3 abundance in atom per cent.

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