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Potentialities of infrared spectroscopy in determining traces of carbon or oxygen as carbon dioxide
Talmta,
1968, Vol. 15, pp. 1199 to 1203. Pergamon Press. Printed in Northern Ireland
POTENTIALITIES OF INFRARED SPECTROSCOPY IN DETERMINING TRACES OF CARBON OR OXYGEN AS CARBON DIOXIDE T. CARTERand H. I. SHALGOSKY Analytical Sciences Division, U.K.A.E.A. Research Group Atomic Energy Research Establishment, Harwell, Didcot, U.K. (Received 29 April 1968. Accepted 30 April 1968) Summary-The determination of traces of carbon dioxide by infrared spectroscopy has been investigated. For maximum sensitivity the absorption band at 2350cm-’ is measured, the total pressure being raised to one atmosphere by addition of an inert gas. With a micro gas-cell and a beam-condenser the limit of detection is 0.02 rg of carbon, and for 4 ,ug of carbon the coefficient of variation is 1.5 %. determination of traces of oxygen by the inert-gas fusion method, carbon monoxide is produced and may be measured directly. More commonly, the carbon monoxide is converted into carbon dioxide which is separated in a freeze-trap and measured manometrically .I Similar measurements of carbon dioxide are employed in the determination of traces of carbon by the combustion method. The manometric technique has proved satisfactory for as little as 10 pg of carbon or oxygen, but it is difficult to construct a combined freeze-trap and gauge with a volume smaller than O-5 ml, and for lower weights of the elements an alternative method of measuring the oxides of carbon was considered desirable. The results of an investigation into infrared spectroscopy are presented below; the potentialities of several other techniques have also been examined, and these will be compared in a subsequent publication.2 Gases which have molecules composed of just a few atoms have relatively simple infrared spectra, and the bands are often specific in experiments involving mixtures. This specificity is of course assured if a single absorbing gas is mixed with nonabsorbing gases such as monatomic or symmetrical diatomic species, The intensity-concentration relationships do not necessarily follow the LambertBeer law. Deviations are found, particularly among gases of low molecular weight, and are due in part to collision-broadening of the rotational fine structure of the bands. Broadening can be caused by the addition of an inert gas, and an effect commonly observed at low spectrometer resolution is a marked enhancement of the intensity of a band envelope as the pressure of inert gas is increased.3 This effect is of analytical value. The relative absorption intensities of many gases have been indicated in the literature.4 Carbon dioxide has a more intense absorption than carbon monoxide by virtue of the band at 2350 cm-l, and this gas was therefore selected for study. Weaker bands are to be found at 3720, 3610 and 670 cm-l. Collision-broadening effects are observed in carbon dioxide at relatively low pressures, and useful intensity enhancement factors can be obtained by raising the IN THE
1199
1200
T. CARTERand H. I. SHAL~OSKY
pressure of the system to one atmosphere with inert gas. It is convenient to use nitrogen or air for this purpose. Complete broadening of the rotational structure of carbon monoxide is only achieved at pressures of many atmospheres;s this is due to the wider spacing of the rotational lines, consequent on the smaller moment of inertia of the molecule. EXPERIMENTAL Apparatus Per kin-Elmer
mcdel 137G grating near infrared spectrophotometer.
Experiments with a standard IO-cm gas cell The gas was contained in a IO-cm path-length cylindrical Pyrex cell with entrance and exit ports. The spectrophotometer was open to the air and spectra were run with the standard slit programme. Streams of carbon dioxide and nitrogen from two cylinders were mixed and passed into the cell, and the flow-rates, which were shown to be reasonably constant, were measured at the exit port with a soap-bubble flow-meter. The system was assumed to be at atmospheric pressure, and the partial pressure of carbon dioxide was thus given by: CO8 flow-rate POO,
=
cco,
+
N,j
flow_rate
x
atmospheric
pressure.
The weight of carbon (or oxygen) corresponding to the amount of carbon dioxide in the cell could thus be calculated; in the results which follow, only weights of carbon are given. Care was taken to
'00
I
loo
I
I
200
300
Weigh
I
I
400
5wJ
of corbm,
1
6oo
I
7oo
pq
FIN. 1.-Determination of carbon dioxide in admixture with nitrogen at a total pressure of one atmosphere, using a standard IO-cm gas cell.
Potentialities of infrared spectroscopy
1201
ensure adequate purging of the cell at each level of concentration. The cell was sealed off and a spectrum recorded; a compensation cell in the reference beam was filled with nitrogen. Results. The fine structure of the bands was not observed; only the envelopes could be seen. The maximum absorptions of the bands at 372Ocn+ and 235Ocn+ were determined as the carbon content of the cell was varied from 23 mg down to 21 pg (290 to 0.27 mm Hg pressure of carbon dioxide). The results for the 2350 cm-l band are shown in Fig. 1 as a semi-log of transmittance trs. weight of carbon in the cell. Experiments using a micro gas-cell The use of a cylindrical gas cell is wasteful, and if the quantity of gas available is small, it can be
measured more efticiently in a minimum-volume cell with dimensions tailored to those of the spectraphotometer beam. The absorbance of a given quantity of gas increases as shorter cells are used; the reduction in path-length is more than compensated for by the increase in pressure because of the tapered construction of the cells. The efficiency is increased still further if a beam-condenser is used to concentrate the light from the source on to the sample contained in a very small volme. A micro gas-cell, with a path-length of 8 mm and a volume of @15 ml, was used in conjunction with a beam-condenser fitted with potassium bromide lenses. The transmission of light through the cell was about 60% and a wire gauxe attenuator was placed in the reference beam. The cell was attached to a vacuum line and carbon dioxide was admitted at various pressures. The absorption was measured, and again after admission of air to raise the pressure to atmospheric. In the latter case it was necessary to allow about 10 min for complete mixing of the gases within the cell, the absorption then being seen to reach a maximum. The sensitivity of the measurement was improved by increasing the slit width, and thus the available energy, this giving a more accurate pen response; it was further improved by purging the spectraphotometer with nitrogen to reduce absorption by atmospheric carbon dioxide,
Fro. [L.-Determination
of carbon dioxide, using a micro gas-cell and beam-condenser. (1) carbon dioxide alone; (2) carbon dioxide mixed with air at a total pressure of one atmosphere.
1202
T. CARTERand H. I. SEL~LCKBKY
Results. The maximum absorption of the band at 2350 cm-’ was determined as the carbon content of the cell was varied from 9.8 rug down to 0.26 fig (93 to 2.5 mm Hg pressure of carbon dioxide). The results are shown in Fig. 2. The reproducibility was assessed by measuring the absorption when the micro-cell was repeatedly filled with a carbon dioxide~nitrogen mixture from a flowing gas stream of constant composition. At the 40 % transmittance level, which corresponds to 4 ,ug of carbon, the coefficient of variation was l-5 % (10 readings). The limit of detection was about 0.02 ,ug of carbon. This was taken as 242 x the standard deviation of the blank, which was measured with the cell filled with air (10 readings).
DlSCUSSION
The standard IO-cm gas cell could conveniently be used to determine quantities of carbon in the 10-750 rug range, by measuring the band at 2350 cm-l. It would be possible to extend the measurements to significantly higher weights of carbon by using the band at 3720 cm- l; here the peak absorption was only 5 % of that at 2350 cm-l, and with 20 mg of carbon in the cell the transmittance was still 20%. The calibration curve for this band is not illustrated, but is similar in form to that shown in Fig. 1. A micro gas-cell and beam-condenser enabled the O-10 ,ug range to be studied, and the limit of detection was 0.02 ,ug of carbon. Lower limits could doubtless be achieved if ordinate expansion, greater source energy and better purging facilities were available. With a given sample it would be important to introduce as much as possible of the carbon dioxide into the cell in order to achieve the high sensitivity of which the method is capable. A convenient experimental assembly might embody a Toepler pumping system by which the same fraction of gas could be transferred in each determination. No effects were observed which could be attributed specifically to adsorption of carbon dioxide on the ceI1 walls. Any such effects may be minimized by mixing the carbon dioxide with the inert gas before admission to the ~ell,~ or by coating the metal surfaces of the cell with a suitable varnish.6 The large difference between the curves for carbon dioxide alone and carbon dioxide in admixture with air (Fig. 2) may be attributed to pressure broadening effects. Although the latter curve is almost linear, the former shows a progressive intensification of the absorption as the pressure of gas is increased. No previous report of this effect has been found for carbon dioxide, although it has been observed in nitrous oxide at low pressures;’ some theoretical justification for it has also been presented.3 The method is rapid; once the cell has been filled with gas it is a simple matter to position it in the spectrophotometer and to scan through the 2350 cm-l band. Care must be taken, however, to minimize any unwanted changes in absorption due to temperature and pressure fluctuations, and to introduce the carbon dioxide reproducibly into the cell. A recent method for the determination of traces of acetylene, carbon monoxide and nitrous oxide* used a versatile spectrophotometer equipped with ordinate expansion. The gas was contained in a 1-metre cell. Such long path-length cells are we11suited to the study of trace concentrations of gases in mixtures where the sample volume is not limited, as for example, in the field of air pollution. If the trace constituent can readily be separated then a method at least as sensitive can be achieved by the use of a micro gas-cell and beam-condenser.
Potentialities
of infrared spectroscopy
Acknowledgement-We are grateful to Research and Industrial loan of a micro gas-cell and beam-condenser.
Instruments
1203 Company Ltd. for the
Zusammenfassung-Die Bestimmung von Kohlendioxidspuren durch Infrarotspektroskopie wurde untersucht. Fur maximale Emptindlichkeit wird die Bande bei 2350 cm-l gemessen, wobei durch Zugabe von Inertgas der Gesamtdruck auf eine Atmosnh&e gebracht wird. Mit einer’ Mikro-Gasktlvette und einem Strahlkondensor ist die Nachweisgrenze 0,02 pg Kohlenstoff, bei 4 pg Kohlenstoff der VariationskoetBzient 1,s %. RCurn&On a Btudib la determination de traces de gaz carbonique par spectroscopic infra-rouge. Pour la sensibilitt maximale, on mesure la bande d’absorption a 2350 cm-i, la pression totale etant &levee a une atmosphere par addition d’un gaz inerte. Avec une microcellule a gaz et un condenseur de faisceau, la limite de detection est de 0,02 pg de carbone, et pour 4 ,ug de carbone le coefficient de variation est de 195%. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
W. G. Smiley, Anal. Chem., 1955,27,1098. A. P. Mead, A.E.R.E.. Harwell. To be nublished. J. R. Nielsen, V. Thornton and E. B. Dile. Rev. Mod. Phys., 1944, 16, 307. R. H. Pierson, A. N. Fletcher and E. St. C. Gantz, AnaLVChem., 1956,28, 1218. S. S. Penner and D. Weber. J. Chem. Phvs.. 1951.19.807. L. D. Kaplan and D. F. Edgers, ibid., 1656; 25, 876.. D. E. Burch and D. Williams, Appl. Optics, 1962, 1,473. G. Kemmner, G. Nonnenmacher and W. Wehling, Instrument News (Perk&Elmer 1967,17, No. 3 ; 2. AnaI. Chem., 1966,222,149.
Corporation),
1968, Vol. 15, pp. 1199 to 1203. Pergamon Press. Printed in Northern Ireland
POTENTIALITIES OF INFRARED SPECTROSCOPY IN DETERMINING TRACES OF CARBON OR OXYGEN AS CARBON DIOXIDE T. CARTERand H. I. SHALGOSKY Analytical Sciences Division, U.K.A.E.A. Research Group Atomic Energy Research Establishment, Harwell, Didcot, U.K. (Received 29 April 1968. Accepted 30 April 1968) Summary-The determination of traces of carbon dioxide by infrared spectroscopy has been investigated. For maximum sensitivity the absorption band at 2350cm-’ is measured, the total pressure being raised to one atmosphere by addition of an inert gas. With a micro gas-cell and a beam-condenser the limit of detection is 0.02 rg of carbon, and for 4 ,ug of carbon the coefficient of variation is 1.5 %. determination of traces of oxygen by the inert-gas fusion method, carbon monoxide is produced and may be measured directly. More commonly, the carbon monoxide is converted into carbon dioxide which is separated in a freeze-trap and measured manometrically .I Similar measurements of carbon dioxide are employed in the determination of traces of carbon by the combustion method. The manometric technique has proved satisfactory for as little as 10 pg of carbon or oxygen, but it is difficult to construct a combined freeze-trap and gauge with a volume smaller than O-5 ml, and for lower weights of the elements an alternative method of measuring the oxides of carbon was considered desirable. The results of an investigation into infrared spectroscopy are presented below; the potentialities of several other techniques have also been examined, and these will be compared in a subsequent publication.2 Gases which have molecules composed of just a few atoms have relatively simple infrared spectra, and the bands are often specific in experiments involving mixtures. This specificity is of course assured if a single absorbing gas is mixed with nonabsorbing gases such as monatomic or symmetrical diatomic species, The intensity-concentration relationships do not necessarily follow the LambertBeer law. Deviations are found, particularly among gases of low molecular weight, and are due in part to collision-broadening of the rotational fine structure of the bands. Broadening can be caused by the addition of an inert gas, and an effect commonly observed at low spectrometer resolution is a marked enhancement of the intensity of a band envelope as the pressure of inert gas is increased.3 This effect is of analytical value. The relative absorption intensities of many gases have been indicated in the literature.4 Carbon dioxide has a more intense absorption than carbon monoxide by virtue of the band at 2350 cm-l, and this gas was therefore selected for study. Weaker bands are to be found at 3720, 3610 and 670 cm-l. Collision-broadening effects are observed in carbon dioxide at relatively low pressures, and useful intensity enhancement factors can be obtained by raising the IN THE
1199
1200
T. CARTERand H. I. SHAL~OSKY
pressure of the system to one atmosphere with inert gas. It is convenient to use nitrogen or air for this purpose. Complete broadening of the rotational structure of carbon monoxide is only achieved at pressures of many atmospheres;s this is due to the wider spacing of the rotational lines, consequent on the smaller moment of inertia of the molecule. EXPERIMENTAL Apparatus Per kin-Elmer
mcdel 137G grating near infrared spectrophotometer.
Experiments with a standard IO-cm gas cell The gas was contained in a IO-cm path-length cylindrical Pyrex cell with entrance and exit ports. The spectrophotometer was open to the air and spectra were run with the standard slit programme. Streams of carbon dioxide and nitrogen from two cylinders were mixed and passed into the cell, and the flow-rates, which were shown to be reasonably constant, were measured at the exit port with a soap-bubble flow-meter. The system was assumed to be at atmospheric pressure, and the partial pressure of carbon dioxide was thus given by: CO8 flow-rate POO,
=
cco,
+
N,j
flow_rate
x
atmospheric
pressure.
The weight of carbon (or oxygen) corresponding to the amount of carbon dioxide in the cell could thus be calculated; in the results which follow, only weights of carbon are given. Care was taken to
'00
I
loo
I
I
200
300
Weigh
I
I
400
5wJ
of corbm,
1
6oo
I
7oo
pq
FIN. 1.-Determination of carbon dioxide in admixture with nitrogen at a total pressure of one atmosphere, using a standard IO-cm gas cell.
Potentialities of infrared spectroscopy
1201
ensure adequate purging of the cell at each level of concentration. The cell was sealed off and a spectrum recorded; a compensation cell in the reference beam was filled with nitrogen. Results. The fine structure of the bands was not observed; only the envelopes could be seen. The maximum absorptions of the bands at 372Ocn+ and 235Ocn+ were determined as the carbon content of the cell was varied from 23 mg down to 21 pg (290 to 0.27 mm Hg pressure of carbon dioxide). The results for the 2350 cm-l band are shown in Fig. 1 as a semi-log of transmittance trs. weight of carbon in the cell. Experiments using a micro gas-cell The use of a cylindrical gas cell is wasteful, and if the quantity of gas available is small, it can be
measured more efticiently in a minimum-volume cell with dimensions tailored to those of the spectraphotometer beam. The absorbance of a given quantity of gas increases as shorter cells are used; the reduction in path-length is more than compensated for by the increase in pressure because of the tapered construction of the cells. The efficiency is increased still further if a beam-condenser is used to concentrate the light from the source on to the sample contained in a very small volme. A micro gas-cell, with a path-length of 8 mm and a volume of @15 ml, was used in conjunction with a beam-condenser fitted with potassium bromide lenses. The transmission of light through the cell was about 60% and a wire gauxe attenuator was placed in the reference beam. The cell was attached to a vacuum line and carbon dioxide was admitted at various pressures. The absorption was measured, and again after admission of air to raise the pressure to atmospheric. In the latter case it was necessary to allow about 10 min for complete mixing of the gases within the cell, the absorption then being seen to reach a maximum. The sensitivity of the measurement was improved by increasing the slit width, and thus the available energy, this giving a more accurate pen response; it was further improved by purging the spectraphotometer with nitrogen to reduce absorption by atmospheric carbon dioxide,
Fro. [L.-Determination
of carbon dioxide, using a micro gas-cell and beam-condenser. (1) carbon dioxide alone; (2) carbon dioxide mixed with air at a total pressure of one atmosphere.
1202
T. CARTERand H. I. SEL~LCKBKY
Results. The maximum absorption of the band at 2350 cm-’ was determined as the carbon content of the cell was varied from 9.8 rug down to 0.26 fig (93 to 2.5 mm Hg pressure of carbon dioxide). The results are shown in Fig. 2. The reproducibility was assessed by measuring the absorption when the micro-cell was repeatedly filled with a carbon dioxide~nitrogen mixture from a flowing gas stream of constant composition. At the 40 % transmittance level, which corresponds to 4 ,ug of carbon, the coefficient of variation was l-5 % (10 readings). The limit of detection was about 0.02 ,ug of carbon. This was taken as 242 x the standard deviation of the blank, which was measured with the cell filled with air (10 readings).
DlSCUSSION
The standard IO-cm gas cell could conveniently be used to determine quantities of carbon in the 10-750 rug range, by measuring the band at 2350 cm-l. It would be possible to extend the measurements to significantly higher weights of carbon by using the band at 3720 cm- l; here the peak absorption was only 5 % of that at 2350 cm-l, and with 20 mg of carbon in the cell the transmittance was still 20%. The calibration curve for this band is not illustrated, but is similar in form to that shown in Fig. 1. A micro gas-cell and beam-condenser enabled the O-10 ,ug range to be studied, and the limit of detection was 0.02 ,ug of carbon. Lower limits could doubtless be achieved if ordinate expansion, greater source energy and better purging facilities were available. With a given sample it would be important to introduce as much as possible of the carbon dioxide into the cell in order to achieve the high sensitivity of which the method is capable. A convenient experimental assembly might embody a Toepler pumping system by which the same fraction of gas could be transferred in each determination. No effects were observed which could be attributed specifically to adsorption of carbon dioxide on the ceI1 walls. Any such effects may be minimized by mixing the carbon dioxide with the inert gas before admission to the ~ell,~ or by coating the metal surfaces of the cell with a suitable varnish.6 The large difference between the curves for carbon dioxide alone and carbon dioxide in admixture with air (Fig. 2) may be attributed to pressure broadening effects. Although the latter curve is almost linear, the former shows a progressive intensification of the absorption as the pressure of gas is increased. No previous report of this effect has been found for carbon dioxide, although it has been observed in nitrous oxide at low pressures;’ some theoretical justification for it has also been presented.3 The method is rapid; once the cell has been filled with gas it is a simple matter to position it in the spectrophotometer and to scan through the 2350 cm-l band. Care must be taken, however, to minimize any unwanted changes in absorption due to temperature and pressure fluctuations, and to introduce the carbon dioxide reproducibly into the cell. A recent method for the determination of traces of acetylene, carbon monoxide and nitrous oxide* used a versatile spectrophotometer equipped with ordinate expansion. The gas was contained in a 1-metre cell. Such long path-length cells are we11suited to the study of trace concentrations of gases in mixtures where the sample volume is not limited, as for example, in the field of air pollution. If the trace constituent can readily be separated then a method at least as sensitive can be achieved by the use of a micro gas-cell and beam-condenser.
Potentialities
of infrared spectroscopy
Acknowledgement-We are grateful to Research and Industrial loan of a micro gas-cell and beam-condenser.
Instruments
1203 Company Ltd. for the
Zusammenfassung-Die Bestimmung von Kohlendioxidspuren durch Infrarotspektroskopie wurde untersucht. Fur maximale Emptindlichkeit wird die Bande bei 2350 cm-l gemessen, wobei durch Zugabe von Inertgas der Gesamtdruck auf eine Atmosnh&e gebracht wird. Mit einer’ Mikro-Gasktlvette und einem Strahlkondensor ist die Nachweisgrenze 0,02 pg Kohlenstoff, bei 4 pg Kohlenstoff der VariationskoetBzient 1,s %. RCurn&On a Btudib la determination de traces de gaz carbonique par spectroscopic infra-rouge. Pour la sensibilitt maximale, on mesure la bande d’absorption a 2350 cm-i, la pression totale etant &levee a une atmosphere par addition d’un gaz inerte. Avec une microcellule a gaz et un condenseur de faisceau, la limite de detection est de 0,02 pg de carbone, et pour 4 ,ug de carbone le coefficient de variation est de 195%. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
W. G. Smiley, Anal. Chem., 1955,27,1098. A. P. Mead, A.E.R.E.. Harwell. To be nublished. J. R. Nielsen, V. Thornton and E. B. Dile. Rev. Mod. Phys., 1944, 16, 307. R. H. Pierson, A. N. Fletcher and E. St. C. Gantz, AnaLVChem., 1956,28, 1218. S. S. Penner and D. Weber. J. Chem. Phvs.. 1951.19.807. L. D. Kaplan and D. F. Edgers, ibid., 1656; 25, 876.. D. E. Burch and D. Williams, Appl. Optics, 1962, 1,473. G. Kemmner, G. Nonnenmacher and W. Wehling, Instrument News (Perk&Elmer 1967,17, No. 3 ; 2. AnaI. Chem., 1966,222,149.
Corporation),