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Method for the determination of the concentration and of the carbon and oxygen isotopic composition of atmosspheric carbon monoxide
fnternational Journal of %zss Spec?;ometry and Ion Physics ElsevierPublishing Company, Amsterdam - Printed in the Netherlands
265
METHOD FOR THE DETERMINATION OF THE CONCENTRATION AND OF THE CARBON AND OXYGEN ISOTOPIC COMPOSLTION OF ATMOSSPHERIC CARBON MONOXIDE*
C. M. STEVENS APUT LLOYD KROUT** Argonne Narional L&oratory,
Argonne, 111.60439 (US-A.)
ABSTRACT
A procedure is described by which carbon monoxide in the air at the lowest concentration levels is quantitatively removed from the air and converted to pure carbon dioxide. The original isotopic composition of both carbon and oxygen is preserved and determined by analysis with an isotope-ratio mass spectrometer. The problems associated with isotopic anaIysis of these samples is discussed.
iNTRODUCTION
The isotopic analysis of the stable isotopes of trace gas constituents of the atmosphere can be a useful tool in identifying the origin of these gases, whether frompollution or natural sources. This technique has been used in studies of atmospheric SO, l-‘. Until recently the origin of carbon monoxide in the world’s atmosphere was assumed to be emissions from internal combustion engines. Swiunerton et al.* and SeiIer and Junge’ have shown that the oceans are also a source of CO. If the isotopic composition of world-average engine CO is different from that emitted by natural sources, then the engine CO can be considered as an internal isotopic standard, the amount of which in the atmosphere world-wide can be estimated with reasonable accuracy6, and the extent of natural emissions might be determined from the atmospheric CO isotopic composition using the method of “isotopic dilution”‘. There are many reasons why the isotopic composition of CO in air cannot be- determined directly even with highest resolution mass spectrometers. Methods for the extraction and purification of CO utilizing differences in the physical pro* Dedicated to Prof. A. 0. C. Nier to commemorate his 60th birthday. ** Present address: 9721 Court, Oak Lawn, Ill. 60453 (U.S.A.). ht. J_ Mass Spectrom. Ion Phys., 8 (1972) 265-275
266
C. M. STEVENS,
L. KROUT
perties are also difficult. Based on experience at Argonne in the analysis of oxygen by reacting oxygen with hot graphite to produce CO which was then converted to Cot, a technique was developed by which CO is quantitatively removed from samples of air and converted to CO, of sufficient quantity and purity for isotopic analysis. Each sample is passed through a train that first removes moisture, atmospheric CO*, and N,O, next oxidizes the CO to COz with Schutze reagent (I,O,), then purities the CO-, of CO-origin by freezing with liquid nitrogen and pumping, and finally manometrically measures the CO2 . In the oxidation of CO by f;05 at room temperature, the original oxygen atom of CO is retained in the resulting CO2 and the other oxygen atom in CO2 (which is acquired from the 1205 in the oxidation process) has the same isotopic composition for each sample. Xf only the carbon isotopic composition were used, the differentiation between engin<-CO and CO emitted by natural sources might be difficult because of the variability in the ’ 'C : “C ratio in petroleum carbon’ and hence in the CO of engine. emissicns_The oxygen isotopic composition of engine CO could reasonably be expected to be independent of time and geography. Thus the use of the isotopic compositions of both carbon and oxygen might be of special diagnostic value because ol’ the different natural cycies. The common method of converting CO to COZ by the use of CuO at 450 OC was unsuitable, mainly because COZ is also produced from oxidation of hydrocarbons but also because the oxygen isotopic compositions of the CO is destroyed in this reaction_ &other objection to hot CuO is due to the cooling effect of the relatively fast flow of air sample (2 1min-I) through the oxidizer in order to process the large air samples (0.1 to 0.3 m3) in reasonable time.
SAMPLJZ PRJ%PARATlON The concentration of CO in unpolluted surface air varies from 0.04 to 0.3 p_p_m_6*g*l O, depending on season and latitude. In order to have an adequate amount of CO for isotopic analysis, our air samples were collected in 87-liter alumintun cylinders which were filled to a pressure of 30-40 Lb inchs2 using one or two small rubber-diaphragm compressors* operated from a 12 V automobile battery. The cylinders were evacuated beforehand with a mechanical pump acting through a liquid nitrogen trap. Comparison between CO concentrations in sampies collected with and without the compressors showed that the latter contributes tO.O1 p-p-m_ CO. The air sample is processed in the system shown in Fig. 1. The air sample is admitted to the system through a liquid nitrogen trap, a flow meter, a large molecular-sieve trap and a set of ten parallel capillary liquid nitrogen-cooled traps * Model 700 K manufadunxl ht.
J. Mz.ss Spectrum-
by Peters 2nd Russell, Inc,, Springfield, Ohio, U.S.A.
Zon PJtys., 8
(1972) 265-275
ISOTOPIC
COMPOSITION
OF
lo ItV4CLlUM
0
B
H%Ly
tqlELlUH SAMPLE AWNOh4ETER
MANOMETER UQ. t$ AIR SAMTLE
bY?r
Fig. 1. Apparatus for the purification, collection and measurement of atmospheric CO and its oxidation with 1,05 to CO2 _
T, which remove the CO?, H20, N,O, and heavier hydrocarbons but do not affect the CO. The air then is passed through a bed of Schutze reagent (i205) which oxidizes the CO to CO1 ’ 1lI2 and the resulting COZ is collected by another set of ten parallel capillary liquid nitrogen-cooled traps T2. When the air sample has al1 been drawn through the system, the traps containing the CO, from the oxidized CO are evacuated to a vacuum of t0.01 torr and then allowed to warm. Then by purging with a stream of helium, the COz is transferred to the accurately calibrated small-volume manometer system by way of a trap, T,, immersed in dry ice and acetone. The quantity of CO* (O.OI-O.l cm’ (STP)) is measured, and fina.lIy the COZ is distilled into a smaIi sample bulb cooled by liquid nitrogen. The flow rate of air into the apparatus is about 2 1 tin -‘. The initia! pressure of air in the aluminium cyiinders is measured with a standardized Bourdon gauge and the final pressure with a mercury manometer. The purpose of the first liquid nitrogen-cooled trap is to remove the bulk of the CO, and water vapor- Before this trap was added to the train, it was discovered that the effectiveness of the moIecular-sieve trap in retaining HzO, to the high degree required, was reduced for air samples collected on very humid days. AlI atmospheric water vapor and CO, must be removed prior to the oxidation process because moisture would hydrolyze the Schutze reagent and atmospheric COZ would interfere with the determination of the CO2 of CO origin. The molecular-sieve bed uses Linde 13X pellets cbntained in two 2f-inch-diameter glass tubes, 22-inches long, connected in parallel with fritted glass disks at the bottoms to support the molecular-sieve material. The molecular sieve is regenerated after every sample or after passage of no more than 300 1 (STP) of air by heating at Inr- J. Mass
Spectrom. Ion Thys.;
8 (1972) 265-275
268
C. hi. STEVENS,
L. KROUT
250 “C for three hours while pumping with a mecba.nical pump through a liquid nitrogen-cooled trap. The molecular-sieve bed, which has a large unused capacity for CO, and water vapor, is needed to retain more than 99.99 % of the atmospheric N,O (about 0.3 p.p_rr~) in order to avoid introducing errors into :he isotopic analysis. As N,O and CO2 have the same molecular weight, the presence of even a very small amount of P&O causes the apparent 13C:12 C and %: 160 ratios to be too 10~‘~. ThiS occurred in samples analyzed in 1969 and early 1970 when a smaller capacity molecular-sieve bed was used which, although adequate for CO2 and water vapor, resulted in contamination of the CO2 of CO origin with atmospheric NzO. During this period the N20 impurity was determined with the Argonne IOO-inch mass spectrometer, which can resolve N20 and C02, and corrections were applied to the results of isotopic analysis. The conversion of CO to CO, is virtuaIIy quantitative as is shown by the first four results in Table 1. For these, measured amounts of CO were introduced into the conversion train by means of a caiibrated gas pipet and a stream of helium, TABLE
1
REAIX-IVITIES
GUS
OF co,
C&
Added 01 mole)
CO
co co co CH, CIt
CHs &He
6.50 6.50 IL98 1196 6.46 6.48 6.48 710
AND
&HI
TO
SCHUlZE
FtE4GEXT
Recocered as car (p mde)
Recocery CX)
6.51 6.40 12.84 12.95 0.02 0.00 0.00 0.15
100 98 99 100 0 0 0 0.02
AT
ROOM TEMPERATURE
and the amount of CO2 collected was measured with the manometer_ In another test, 0.107 cm3 (SW) of CO was added to air at 28 lb inch-’ in an aluminium cylinder and an identicai cylinder was filled with air to the same pressure at the sake time. The latter control sample of air contained 1.57 p.p.m_ CO and the spiked sample , 2.07 p-p-m. CO. This measured difference of 0.50 p.p.m. would have required the addition of 0.115 cm3 (SIP) or 7 oA more than the amount of CO actuaIly added. The resuh however feII within the overall measurement uncertainty of - t_3 9,;. Finally, comparison of analyses of the CO concentration in air samples cokcted on a sea voyage made by different analytical methods (Table 2) showed reasonabIe agreement between our method and the gas chromatographic methods used by Swinnerton e? al.I4 of the NavaI Research Laboratory and by Stevens, O’Keefe and Ortman l5 of the National Air Pollution Co~~troIAdministratiOTL
fnt. J. Mass Spectrom. Ion Phys_, 8 (1972) 265-275
ISOTOPIC
COMPOSITKON
OF ATMOSPHERIC
CARBON
269
MONOXIDE
TABLE 2 CO.HPARISoN OF AXi
MEASURED
GAS CNilOMATOGRAPHIC
TAICE?J AT 14”
VALXXS
OF ATMOSPHERIC
METHODS.
42’ w, 21 o 04’ N,
SAMPLES
JUXE 8,
co
CONCENl-IWTI0N.S
OF SURFACE
AIR
USING
f=Os
OVER THE PACIFIC
OXIDATION
OCEAX
‘WERE
1970 1500 h.
Method
CO concentration @.p.m.)
Oxidized by 1205 and collected as CO-. (Argonne) Gas chromatographic’ Gas ChromatographiZ
0.16 0.14 0.19
a Ref. 14. b Ref. IS.
Methane is present in the atmosphere at a concentration of about 1.5 p.p.m_ Tests were made to check for any CO, produced by oxidation of CH, by Schultze reagent. The results in Table 1 show that there was negligible reactivity of CH, with Schutze reagent_ Similarly, C,H,, although present in unpolluted air at less than IO3 p.p.m.6, did not show any significant oxidation. The possibility of any CO2 impurity being introduced by the train itself was established by blank runs of 20%300 i (STP) of N, from a tank which contained only 0.01 p.p.m. CO. Several other tanks of N2 tested were shown to contain OS-I.0 p.p.m. CO. Argon containing less than 0.01 p_p.m_ CO was adopted later as a CO-free purge gas. The conversion of CO to COz by oxidation with I,& does not impair the 160 ratio in the CO. This was established by two tests determination of the “0: using CO enriched in 180, the results of which are shown in Table 3. The CO, from the o_xidation of the CO contained all of the “0 in the latter. In a second test to check for any exchange of the oxygen in the CO with the walIs.of the aluminium cyIind.ers, a CO spike withO. oA Cl80 was introduced into a cylinder along with air at ambient pressure and allowed to stand for twelve days. Acontrol sample of air taken at the same time contained0.022cm3 (STP) CO with 6( 180:160) = + 8 “/o. cerszls SMOW (Standard Mean Ocean Water). The CO, from the sample with TAEKE 3 FWZOVERY
OF ‘*o
IN THE OXIDATION
OF EhTICHED
csso
Test 1
l*O: I60 ratio in CO IsO : I60 ratio in CO= produced by oxidation of CO with fZOs Amount of “0 spike recovered (%)
0.0143’ 0.0168” 103
BY I305
TO co2
Test 2
0.00334= 0.00545b 102
a Measured with Cl’%C. 21-620 mass spectrometer. a Measured with Consolidated-Nier isotope-ratio mass
spectrometer.
Int. J. .%fass Spectrom. Ion Phys., 8 (1972) 265~275
C. 3%. STEVENS,
270
I_. KROUT
the spike measured 0.305 cm3 (STP) and 6(‘*0: 160) = 313 o/oo bersus SMOW. The amount ofexcess Cl*0 was OBOO390cm3 (SIX?), of which 0_000382 cm3 (STP) was recovered- This implied that the exchange or loss of the CO was < 2 %.
IsarOPIC
ANALYSIS
The samples of CO2 of CO origin are isotopically analyzed by the highprecision McIGnney-Nier technique16~I’3 using a modified consohdated-Nier isotope-ratio mass spectrometer_ This instrument is the oldest mass spectrometer at Argome, and the spectrometer tube with ion source and col.Iector assemblies and the magnet are still the original units. The instrument has been modified with a small-vohune manifold, new gIass vaIves, compIeteIy new electronics units, and a semi-automatic ratio-measuring and data-cohecting system. In January 1971 the spectrometer tube, ion source, and collector were cleaned and new diffusion pumps installed The minimum volume of the sampIe manifotd is 4.4 cm3, and with a sample size of 0.05 cm3 (STI?) (0.2 p-p-m. CO from 250 I of air), the flow rate into the spectrometer is about 10e6 i-mm set- ‘. The mass-44 ion intensity is 5 x 10-l’ A for an electron emission current of 2 mA. The instrument CO,* background is
REFERENCE (ILL
UNKNOWlU WEST
ENGINE CO.)
CCAST,
AWSTRALIA.
MA-f 1971)
Fig. 2 Plot of mass4 inteasity and I80 abundancecersu~time for CO+ of atmosphericCO origin for a Sample of air coikcted on the west coast of Austrak, May 1971, compared to a reference CO2 sample from engine CO. Int. 3. Mass Specfrom. Ion I’&.,
8 (1972) 265-275
ISOTOPIC
COMPOSITION
OF ATMOSPHERIC
CARBON
MONOXIDE
271
5x lo-” A_ Samples as small as 0.01 cm3 (STP) were analyzed, but with reduced ion current and fess accuracy and precision_ The ion currents from the twin coilectors16 are amplzed by two vibratingreed electrometers and the ratio of the output voltages is measured by converting the vohage signals to pulses with voltage-to-frequency converters and counting the number of pulses of the smaller isotope signal for a preset number of pulses of the mass44 peak by use of a preset counter and printer. A simple programming circuit provides for measuring the mass-44 intensity and then for measuring the ratio for a preselected time (usually about 20 set), as well as for a preselected numnumber (5) of these sets of intensity-ratio readings,.and f?naIly for sequencing the reference and unknown samples (3-5 of each)_ In Fig. 2, a typical set of measurements of the [12C’60180: r2C1”Q’60] ratio for a sample of CO2 of CO origin extracted from air (0.080 p.p.m.) collected on the west coast of Australia in May 1971 is compared with a reference sample of engine CO. The isotopic ratios were measured by comparison with standard reference samples, the two used in this work being secondary standards prepared from CO from engine exhaust (as described below) and calibrated by comparison with standard tank CO,. For carbon, the absolute 13C : 12C ratio was established by comparing the standard tank CO2 with CO2 prepared from NBS Solenhofen limestone. The carbon results are given as permil differences, 6R relative to PDB carbon isotopic abundance, R, using Craig’s value r8 for Solenhofen ~NXS~LS PDB carbon, where
x 1oOO+1.25-~(‘80:‘60)x0.0165 The second and third terms are correction factors for the difference in the I70 abundance between CO, of CO-origin and CO2 prepared from PDB carbonate by the usual method of using 100 oA H,PO,. The quantity + 1.26°/oo is the I70 correction for the oxygen isotopic composition of the oxygen reference sample which in this work is CO, prepared by 1205 oxidation of CO made from Standard Mean Ocean Water (SMOW)-one oxygen atom is from SMOW, A(l’O: 160) = -41°/oo ret-sus PDB and the other oxygen atom is from the 1205 oxidation, 160) = -335.2o/oo ~XEYZLY PDB. A(“O: 160 ratios, CO was first quantitatively To determine the absolute “0: prepared from SMOW by reaction with graphite at 1000 “C; then this CO was oxidized to COZ with 1205 by the same niethod as was used for the atmospheric and engine-CO samples, and the resulting CO= was compared with the secondary engine-CO standard. The CO2 from both the SMOW and secondary engine-CO standards, as well as atmospheric CO samples, have one atom of oxygen, O*, contributed by the 1205 with the same isotopic abundancet; hence, the absoIute t O* = 523 o/oo relative to SMOW
by coznparison
of
(R-r)-
Int. .T. Mass Spectrom.
with(R,&.DB. Ion Phys., 8 (1972) 265-275
272
C. M. STEVENS,
L.
KROUT
value of the %: I60 ratio in the original CO, expressed as per mil difference, Sao relative to SMOW oxygen, is
@coo*>x xlooo
S&=2 [
o?m*~~owl-1
where (Rcoo*)x
is the 46 : 44 ratio of the CO, from oxidation of the CO sample is the 46 : 44 ratio of CO2 prepared from oxidatzan with 120, and (&o&~ow of CO (with IzOs) prepared from SMOW. The two secondary standards of CO? used in this work were prepared by J,Os - oxidation of the CO in the concentrated exhaust gases from an automobile engine burning a mixture of gasolines marketed in Illinois by four major oil companies. The tit was prepared in Juiy 1969 and was used until January 1971; the second has been used since its pfeparation in January 1971. Two difficulties were encountered in using these-two standards. First, a difference of 0.6 per mil between the 13C-. I2 C ratios for tke two standards implied that the scurce of the petroleum for the gasoline marketed in this region had changed in the two years between the times the standards were prepared_ The same change in petroleum source was implied by the change in results for air with high CO concentration, collected in urban Chicago neighborhoods which occurred in the autumn of 1970. Limited data on the gecgraphic origins of the petroIeum used in refining gasoline soId in Illinois’” indicates there is not enough variation in the origins to account for the observed variation in the 13C: ‘*C ratio of the engine CO. Second, between July and December, 1970, the isotopic compositions of the carbon and oxygen in the first standard changed by a total o~E(~~C:~“C) = 1_6°/,o and6(‘80:160) = +2.0°/oo; the increments in the one-month intervals between successive comparisons with the standard tank CO, were about to.3 and +OS o/0,,, respectively. The cause is suspected to be that the standard was exchanged with or diIured by atmospheric CO, absorbed on the surface of the sampIe manifoId of the LOO-inchmass spectrometer when measurements were made with it. The standard was introduced into this mass spectrometer during this period to determine whether or not N20, NOz, or hydrocarbozts were present, The estimated uncertainty from this source is qrsc: l*C) = -c-O.3‘ioo and b(180 : 160) = &-0.5 o/oo for measurements during the period August-December, 1970. The manifold pressure of the reference sample in the isotope-ratio instrument was always adjusted so that the mass-& intensity was within + I oA of the mass-44 intensity for the unknown sample. This ehminated the need for any correction of the ratio for the “pressure effeci”18, which for this instrument, was about -0.1 o/oo per 1% change in mass-44 intensity. The leak fractionation bias was - 1.Oo/oo for the 45 : 44 ratio and -2.0 o/oo for the 46 : 44 ratio. Leakage of the ghiss valves during a measurement was 6 % until January 197I, at which time a new set having a leakage of only 1.5 oA was installed_ Int. J. Mass Spectrom.
Zon Phys., 8 (1972) 265-275
ISOTOPIC
COMPOSITION
OF ATMOSPHERIC
CARBON
MONOXIDE
273
Such smaI1 samples as are used in this work are espeeiatly vulnerable to errors from impurities and isotopic-exchange effects. The sample bulbs used before December 1970 were Pyrex glass, I I mm 0-d. by 7 cm long, and were not degassed except for the annealing process and pumping at room temperature. Because there was evidence of isotopic exchange of the oxygen, especially in instances of newlymade bulbs without annealing of the entire bulb (presumably due to absorbed water vapor on the walls of the bulb), we changed to smaller sample bulbs, 5 mm o-d. by 1 cm long, and flamed the bulb while pumping just before a CC& sample was transferred to it. The samples were stored in the dark and iLwtopicaIIy analyzed as soon as possible. The CC& sampIes of CO origin were carefully examined with the Argonne IOO-inch mass spectrometer for N,O, NOz, or hydrocarbons such as C&s and C2H,0H which might not have been effectively separated in the gas-extraction system and could cause appreciable errors even in very small concentrations. Except for the instances of N20 contamination found early in thz program, no other impurities were found greater than 10B5 of the C02, even in sampIes as small as 0.015 cm3 (STP). After the isotopic analysis, the CO2 samples were transferred from the sampIe-introduction manifold to the sampie bulb by cooIing with liquid nitrogen. Repeated analyses on samples smaIIer than about 0.05 cm” (STP) gave “0 : 160 ratios which were 0.5°/00 higher than the first analysis. This increase indicated some small exchange of the oxygen in the CO, with water vapor absorbed on the walIs of the manifold lines.
ACCURACY
The estimated error in determining the CO concentration is +O.Ol p.p.m. or 2 3 % of the concentration, whichever is greater. For small CO concentlations, this is due to the error in measuring the pressure of the CO, of CO origin in the small-volume manometer shown in Fig. 1, and for high CO concentrations due to the error in measuring the total quantity of air. In the first version of the extraction train, the amount of atmospheric COB impurity in the CO, of CO origin corresponded to about 0.01 p.p.m of the total in air. It is estimated that after the molecular-sieve tra; was enlarged and the f&t liquid nitrogen-cooIed trap was added, the amount of this CO, contamination was less than 0.001 p.p_m Estimation of the total error in the isotopic ratios is -cult because of uncertainities due to wide ranges of concentration, the smallnest of the samples, and the frequent changes and improvements in both the gas-extraction and mass spectrometric procedures over 23 years. For air samples with the smailest CO concentrations (to.15 p-p-m.), the principal sources of error are probably contamination Int. J. Mass Spedrmn. Ion Phys., 8 (1972) 265-275
271
C. M. STEYENS,
L. KROUT
with atmospheric CO, -(+0.3 ‘foe) and small hydrocarbon impurities (< 1 ‘joO) For high CO concentrationsz the mass spectrometric internal error (f0.1 ‘;Iee) and the error from insufficient compensation for the “pressure effect” + (0.1 ‘foe) become sign&cant_ Analyses made during the early stages of the program undoubtedly had errors due to exchange of oxy,oen with water vapor absorbed on the walk of sample bulbs, uncertainty of reference standards, and mass spectrometer difficuhies. When the most recent and improved procedures and apparatus are used, analysis of duplicate air samples leads to an estimate that the overall error for the 13C -. 12C ratio is between kO.3 and 0.5 “joO and that for the “0: 160, ratio is in the range from f0.5 to &-l.O”~oo, depending on the sampie size.
DXSCUSSIOX
The method described herein is quantitative in the yield of CO oxidized to CO,, suffers no exchange of ‘*O in the oxidation procedure, does not produce CO+. from any other known atmospheric compounds containing carbon, and produces CO2 of CO origin free from impurities that could cause large errors in either the concentration or isotopic analyses. The isotopic composition of the CO in unpolluted atmospheres as measured by this procedure depends on the season; it ranges from - 18 to +5 o/oo for Al*0 : 160 and from -5 to i-5 o/oo for A.’ 3C : 12C relative to engine CO, The most significant feature of the isotopic studies is the depletion of ‘*O in atmospheric CO compared to engine CO. This depiction implies the existence of natural sources of CO many times greater than worId-wide engine emissions. The smallest CO concentrations have the greatest depletion in l*O; thus the I80 : I60 values cannot be low due to undetected impurities in the samples interfering with mass-46 measurements. Only an impurity whose ion currents would be collected on the wide collector plate that measures the mass-44 intensity, or an impurity at the mass-46 position for the reference CO=, could result in low ‘*O : 160 values, and we are satisfied that neither possibility occurs. On the other hand, if some of the CO, collected in the extraction process is not actualiy from the oxidation of CO (Le., if the oxidation of some compound in the air sample led to CO? that derives *both its oxygen atoms from the fiOS) then the resulting ‘*O : 160 ratio would be depleted with respect to engine CO; and the amount of the depletion wouId be greater for Iower CO concentrations if the concentration of the interfering substances remains constant_ In these air samples, however, the only carboncontaining constituents -with concentrations above 0.010 p_p.m.6 are CO2 and Cl-&. Tests have indicated no possible interference from these. The results of the atmospheric CO studies wilI be described in a future report, ht. 1. &iass Spectrom. -1011 P&s_, 8 (1972)
265-275
ISOTOPIC
COLMPOSITION
OF
ATMOSPHERIC
CARBON
MONOXIDE
275
ACICNOWLEDGE3lENTS
We are indebted to Dermis Walling for electronic servicing of the mass spectrometer, Frank Lenkszus, Art Metz, and Stanley Rudnick for design and construction of the mass spectrometer automatic programming circuit, Antoinette Engelkemeir for mass spectrometric gas analysis, the Argonne Chemistry Division Giass Shop for the glass apparatus, John Sevec for mechanical assembly, Robert CIayton for providing a sample of Standard Mean Ccean Water, Ben HoIt and Harmon Craig for helpful discussions, and Don Stewart and Bernard Weinstock for encouragement and support. This work was supported by Coordinating Researcti Council-U.S. Environmental Protection Agency, and U.S. Atomic Energy Commission.
REFERENCES
i W. D. TUCKER(Editor), The Atmospheric Diagnostic Program at Brookhacen Nationaikbora-
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
tory: Second Status Report, BNL-50206, 1969. H. L. DEQUAS~S;AK~ D. C. GREY, Stable isotopes applied to pollution studies, Amer. Lob., (Dec. 1970) 19. B. D. HOLT, A. G. ENGELKEYEIR AND A. VENTERS, Srdfur-34 Yariarions in Environmenrai Media near Chicago, submitted to Environ. Sci. Tech. J. W. SWIKDIEIYTOX, V. J. LINNEIKM am R. A. LAMONiAGUE, Science, 67 (1970) 984. W. SEILERAXD C. JLXGE, J. Geophys. Res., 75 (1970) 2217. E. ROBWSON AKD R. C. ROBBINS,Sources, Abundance, and Fate of Gaseous Atmospheric PoIWants, Final Report Stanford Research Institute, Project PR-6755, 1968. M. G. INGHR+.M,Ann. Rev. Nucl Sci., 4 (1954) 81. S. R. SILWWN AXE S. EPSTE~, Bull. Amer. Ass. Petrol. Geol., 42 (1958) 998. C. JUXGE, W_ SEILERAXD P. WARNECK, J. Geophys. Res., 76 (1971) 2866. C_ hl. STEVEXS,L. KROUT, D. WALLING, A_ VEA?? A. ENGELKE!!IR, to be published. W. G. S~LJX, Nucl. Sci. Abstr-, 3 (1949) 391. B. D. HOLT, Anal. Chem., 27 (1962) 751. H. CRAIG AXD C. D. KETEL~G,Gecchim. Cosmochim. Acta, 27 (1963) 549. J. W. SWINKERKIN, V. J. LIIWEX~~MAND C. H. CHECK,Limnol- Oceanogr., 13 (1968) 1931. R. K. SZVENS, A. E. O’UEFEAND 0. C. ORTMAN, Proceedings of the Eighth Conference on Air Pollution Coetroi, Purdue Urdv., Oct. 1969. k O_ NIER, Rev. Sci. Instrum., 18 (1947) 398. C. R. MCKINKXY, J. M. MCCRW, S. EPSTEIN,H. A. ALLEN AND H. C. UREY, Rev. Sci., Irstrum. 21 (1950) 724. H. CRAIG, Geochim. Cosmochim. Acta, 12 (1957) 133. R. B. JACOBS,personal communication.
ht.
J. Mass Spectrom. Ion Phys., 8 (1972) 265-275
265
METHOD FOR THE DETERMINATION OF THE CONCENTRATION AND OF THE CARBON AND OXYGEN ISOTOPIC COMPOSLTION OF ATMOSSPHERIC CARBON MONOXIDE*
C. M. STEVENS APUT LLOYD KROUT** Argonne Narional L&oratory,
Argonne, 111.60439 (US-A.)
ABSTRACT
A procedure is described by which carbon monoxide in the air at the lowest concentration levels is quantitatively removed from the air and converted to pure carbon dioxide. The original isotopic composition of both carbon and oxygen is preserved and determined by analysis with an isotope-ratio mass spectrometer. The problems associated with isotopic anaIysis of these samples is discussed.
iNTRODUCTION
The isotopic analysis of the stable isotopes of trace gas constituents of the atmosphere can be a useful tool in identifying the origin of these gases, whether frompollution or natural sources. This technique has been used in studies of atmospheric SO, l-‘. Until recently the origin of carbon monoxide in the world’s atmosphere was assumed to be emissions from internal combustion engines. Swiunerton et al.* and SeiIer and Junge’ have shown that the oceans are also a source of CO. If the isotopic composition of world-average engine CO is different from that emitted by natural sources, then the engine CO can be considered as an internal isotopic standard, the amount of which in the atmosphere world-wide can be estimated with reasonable accuracy6, and the extent of natural emissions might be determined from the atmospheric CO isotopic composition using the method of “isotopic dilution”‘. There are many reasons why the isotopic composition of CO in air cannot be- determined directly even with highest resolution mass spectrometers. Methods for the extraction and purification of CO utilizing differences in the physical pro* Dedicated to Prof. A. 0. C. Nier to commemorate his 60th birthday. ** Present address: 9721 Court, Oak Lawn, Ill. 60453 (U.S.A.). ht. J_ Mass Spectrom. Ion Phys., 8 (1972) 265-275
266
C. M. STEVENS,
L. KROUT
perties are also difficult. Based on experience at Argonne in the analysis of oxygen by reacting oxygen with hot graphite to produce CO which was then converted to Cot, a technique was developed by which CO is quantitatively removed from samples of air and converted to CO, of sufficient quantity and purity for isotopic analysis. Each sample is passed through a train that first removes moisture, atmospheric CO*, and N,O, next oxidizes the CO to COz with Schutze reagent (I,O,), then purities the CO-, of CO-origin by freezing with liquid nitrogen and pumping, and finally manometrically measures the CO2 . In the oxidation of CO by f;05 at room temperature, the original oxygen atom of CO is retained in the resulting CO2 and the other oxygen atom in CO2 (which is acquired from the 1205 in the oxidation process) has the same isotopic composition for each sample. Xf only the carbon isotopic composition were used, the differentiation between engin<-CO and CO emitted by natural sources might be difficult because of the variability in the ’ 'C : “C ratio in petroleum carbon’ and hence in the CO of engine. emissicns_The oxygen isotopic composition of engine CO could reasonably be expected to be independent of time and geography. Thus the use of the isotopic compositions of both carbon and oxygen might be of special diagnostic value because ol’ the different natural cycies. The common method of converting CO to COZ by the use of CuO at 450 OC was unsuitable, mainly because COZ is also produced from oxidation of hydrocarbons but also because the oxygen isotopic compositions of the CO is destroyed in this reaction_ &other objection to hot CuO is due to the cooling effect of the relatively fast flow of air sample (2 1min-I) through the oxidizer in order to process the large air samples (0.1 to 0.3 m3) in reasonable time.
SAMPLJZ PRJ%PARATlON The concentration of CO in unpolluted surface air varies from 0.04 to 0.3 p_p_m_6*g*l O, depending on season and latitude. In order to have an adequate amount of CO for isotopic analysis, our air samples were collected in 87-liter alumintun cylinders which were filled to a pressure of 30-40 Lb inchs2 using one or two small rubber-diaphragm compressors* operated from a 12 V automobile battery. The cylinders were evacuated beforehand with a mechanical pump acting through a liquid nitrogen trap. Comparison between CO concentrations in sampies collected with and without the compressors showed that the latter contributes tO.O1 p-p-m_ CO. The air sample is processed in the system shown in Fig. 1. The air sample is admitted to the system through a liquid nitrogen trap, a flow meter, a large molecular-sieve trap and a set of ten parallel capillary liquid nitrogen-cooled traps * Model 700 K manufadunxl ht.
J. Mz.ss Spectrum-
by Peters 2nd Russell, Inc,, Springfield, Ohio, U.S.A.
Zon PJtys., 8
(1972) 265-275
ISOTOPIC
COMPOSITION
OF
lo ItV4CLlUM
0
B
H%Ly
tqlELlUH SAMPLE AWNOh4ETER
MANOMETER UQ. t$ AIR SAMTLE
bY?r
Fig. 1. Apparatus for the purification, collection and measurement of atmospheric CO and its oxidation with 1,05 to CO2 _
T, which remove the CO?, H20, N,O, and heavier hydrocarbons but do not affect the CO. The air then is passed through a bed of Schutze reagent (i205) which oxidizes the CO to CO1 ’ 1lI2 and the resulting COZ is collected by another set of ten parallel capillary liquid nitrogen-cooled traps T2. When the air sample has al1 been drawn through the system, the traps containing the CO, from the oxidized CO are evacuated to a vacuum of t0.01 torr and then allowed to warm. Then by purging with a stream of helium, the COz is transferred to the accurately calibrated small-volume manometer system by way of a trap, T,, immersed in dry ice and acetone. The quantity of CO* (O.OI-O.l cm’ (STP)) is measured, and fina.lIy the COZ is distilled into a smaIi sample bulb cooled by liquid nitrogen. The flow rate of air into the apparatus is about 2 1 tin -‘. The initia! pressure of air in the aluminium cyiinders is measured with a standardized Bourdon gauge and the final pressure with a mercury manometer. The purpose of the first liquid nitrogen-cooled trap is to remove the bulk of the CO, and water vapor- Before this trap was added to the train, it was discovered that the effectiveness of the moIecular-sieve trap in retaining HzO, to the high degree required, was reduced for air samples collected on very humid days. AlI atmospheric water vapor and CO, must be removed prior to the oxidation process because moisture would hydrolyze the Schutze reagent and atmospheric COZ would interfere with the determination of the CO2 of CO origin. The molecular-sieve bed uses Linde 13X pellets cbntained in two 2f-inch-diameter glass tubes, 22-inches long, connected in parallel with fritted glass disks at the bottoms to support the molecular-sieve material. The molecular sieve is regenerated after every sample or after passage of no more than 300 1 (STP) of air by heating at Inr- J. Mass
Spectrom. Ion Thys.;
8 (1972) 265-275
268
C. hi. STEVENS,
L. KROUT
250 “C for three hours while pumping with a mecba.nical pump through a liquid nitrogen-cooled trap. The molecular-sieve bed, which has a large unused capacity for CO, and water vapor, is needed to retain more than 99.99 % of the atmospheric N,O (about 0.3 p.p_rr~) in order to avoid introducing errors into :he isotopic analysis. As N,O and CO2 have the same molecular weight, the presence of even a very small amount of P&O causes the apparent 13C:12 C and %: 160 ratios to be too 10~‘~. ThiS occurred in samples analyzed in 1969 and early 1970 when a smaller capacity molecular-sieve bed was used which, although adequate for CO2 and water vapor, resulted in contamination of the CO2 of CO origin with atmospheric NzO. During this period the N20 impurity was determined with the Argonne IOO-inch mass spectrometer, which can resolve N20 and C02, and corrections were applied to the results of isotopic analysis. The conversion of CO to CO, is virtuaIIy quantitative as is shown by the first four results in Table 1. For these, measured amounts of CO were introduced into the conversion train by means of a caiibrated gas pipet and a stream of helium, TABLE
1
REAIX-IVITIES
GUS
OF co,
C&
Added 01 mole)
CO
co co co CH, CIt
CHs &He
6.50 6.50 IL98 1196 6.46 6.48 6.48 710
AND
&HI
TO
SCHUlZE
FtE4GEXT
Recocered as car (p mde)
Recocery CX)
6.51 6.40 12.84 12.95 0.02 0.00 0.00 0.15
100 98 99 100 0 0 0 0.02
AT
ROOM TEMPERATURE
and the amount of CO2 collected was measured with the manometer_ In another test, 0.107 cm3 (SW) of CO was added to air at 28 lb inch-’ in an aluminium cylinder and an identicai cylinder was filled with air to the same pressure at the sake time. The latter control sample of air contained 1.57 p.p.m_ CO and the spiked sample , 2.07 p-p-m. CO. This measured difference of 0.50 p.p.m. would have required the addition of 0.115 cm3 (SIP) or 7 oA more than the amount of CO actuaIly added. The resuh however feII within the overall measurement uncertainty of - t_3 9,;. Finally, comparison of analyses of the CO concentration in air samples cokcted on a sea voyage made by different analytical methods (Table 2) showed reasonabIe agreement between our method and the gas chromatographic methods used by Swinnerton e? al.I4 of the NavaI Research Laboratory and by Stevens, O’Keefe and Ortman l5 of the National Air Pollution Co~~troIAdministratiOTL
fnt. J. Mass Spectrom. Ion Phys_, 8 (1972) 265-275
ISOTOPIC
COMPOSITKON
OF ATMOSPHERIC
CARBON
269
MONOXIDE
TABLE 2 CO.HPARISoN OF AXi
MEASURED
GAS CNilOMATOGRAPHIC
TAICE?J AT 14”
VALXXS
OF ATMOSPHERIC
METHODS.
42’ w, 21 o 04’ N,
SAMPLES
JUXE 8,
co
CONCENl-IWTI0N.S
OF SURFACE
AIR
USING
f=Os
OVER THE PACIFIC
OXIDATION
OCEAX
‘WERE
1970 1500 h.
Method
CO concentration @.p.m.)
Oxidized by 1205 and collected as CO-. (Argonne) Gas chromatographic’ Gas ChromatographiZ
0.16 0.14 0.19
a Ref. 14. b Ref. IS.
Methane is present in the atmosphere at a concentration of about 1.5 p.p.m_ Tests were made to check for any CO, produced by oxidation of CH, by Schultze reagent. The results in Table 1 show that there was negligible reactivity of CH, with Schutze reagent_ Similarly, C,H,, although present in unpolluted air at less than IO3 p.p.m.6, did not show any significant oxidation. The possibility of any CO2 impurity being introduced by the train itself was established by blank runs of 20%300 i (STP) of N, from a tank which contained only 0.01 p.p.m. CO. Several other tanks of N2 tested were shown to contain OS-I.0 p.p.m. CO. Argon containing less than 0.01 p_p.m_ CO was adopted later as a CO-free purge gas. The conversion of CO to COz by oxidation with I,& does not impair the 160 ratio in the CO. This was established by two tests determination of the “0: using CO enriched in 180, the results of which are shown in Table 3. The CO, from the o_xidation of the CO contained all of the “0 in the latter. In a second test to check for any exchange of the oxygen in the CO with the walIs.of the aluminium cyIind.ers, a CO spike withO. oA Cl80 was introduced into a cylinder along with air at ambient pressure and allowed to stand for twelve days. Acontrol sample of air taken at the same time contained0.022cm3 (STP) CO with 6( 180:160) = + 8 “/o. cerszls SMOW (Standard Mean Ocean Water). The CO, from the sample with TAEKE 3 FWZOVERY
OF ‘*o
IN THE OXIDATION
OF EhTICHED
csso
Test 1
l*O: I60 ratio in CO IsO : I60 ratio in CO= produced by oxidation of CO with fZOs Amount of “0 spike recovered (%)
0.0143’ 0.0168” 103
BY I305
TO co2
Test 2
0.00334= 0.00545b 102
a Measured with Cl’%C. 21-620 mass spectrometer. a Measured with Consolidated-Nier isotope-ratio mass
spectrometer.
Int. J. .%fass Spectrom. Ion Phys., 8 (1972) 265~275
C. 3%. STEVENS,
270
I_. KROUT
the spike measured 0.305 cm3 (STP) and 6(‘*0: 160) = 313 o/oo bersus SMOW. The amount ofexcess Cl*0 was OBOO390cm3 (SIX?), of which 0_000382 cm3 (STP) was recovered- This implied that the exchange or loss of the CO was < 2 %.
IsarOPIC
ANALYSIS
The samples of CO2 of CO origin are isotopically analyzed by the highprecision McIGnney-Nier technique16~I’3 using a modified consohdated-Nier isotope-ratio mass spectrometer_ This instrument is the oldest mass spectrometer at Argome, and the spectrometer tube with ion source and col.Iector assemblies and the magnet are still the original units. The instrument has been modified with a small-vohune manifold, new gIass vaIves, compIeteIy new electronics units, and a semi-automatic ratio-measuring and data-cohecting system. In January 1971 the spectrometer tube, ion source, and collector were cleaned and new diffusion pumps installed The minimum volume of the sampIe manifotd is 4.4 cm3, and with a sample size of 0.05 cm3 (STI?) (0.2 p-p-m. CO from 250 I of air), the flow rate into the spectrometer is about 10e6 i-mm set- ‘. The mass-44 ion intensity is 5 x 10-l’ A for an electron emission current of 2 mA. The instrument CO,* background is
REFERENCE (ILL
UNKNOWlU WEST
ENGINE CO.)
CCAST,
AWSTRALIA.
MA-f 1971)
Fig. 2 Plot of mass4 inteasity and I80 abundancecersu~time for CO+ of atmosphericCO origin for a Sample of air coikcted on the west coast of Austrak, May 1971, compared to a reference CO2 sample from engine CO. Int. 3. Mass Specfrom. Ion I’&.,
8 (1972) 265-275
ISOTOPIC
COMPOSITION
OF ATMOSPHERIC
CARBON
MONOXIDE
271
5x lo-” A_ Samples as small as 0.01 cm3 (STP) were analyzed, but with reduced ion current and fess accuracy and precision_ The ion currents from the twin coilectors16 are amplzed by two vibratingreed electrometers and the ratio of the output voltages is measured by converting the vohage signals to pulses with voltage-to-frequency converters and counting the number of pulses of the smaller isotope signal for a preset number of pulses of the mass44 peak by use of a preset counter and printer. A simple programming circuit provides for measuring the mass-44 intensity and then for measuring the ratio for a preselected time (usually about 20 set), as well as for a preselected numnumber (5) of these sets of intensity-ratio readings,.and f?naIly for sequencing the reference and unknown samples (3-5 of each)_ In Fig. 2, a typical set of measurements of the [12C’60180: r2C1”Q’60] ratio for a sample of CO2 of CO origin extracted from air (0.080 p.p.m.) collected on the west coast of Australia in May 1971 is compared with a reference sample of engine CO. The isotopic ratios were measured by comparison with standard reference samples, the two used in this work being secondary standards prepared from CO from engine exhaust (as described below) and calibrated by comparison with standard tank CO,. For carbon, the absolute 13C : 12C ratio was established by comparing the standard tank CO2 with CO2 prepared from NBS Solenhofen limestone. The carbon results are given as permil differences, 6R relative to PDB carbon isotopic abundance, R, using Craig’s value r8 for Solenhofen ~NXS~LS PDB carbon, where
x 1oOO+1.25-~(‘80:‘60)x0.0165 The second and third terms are correction factors for the difference in the I70 abundance between CO, of CO-origin and CO2 prepared from PDB carbonate by the usual method of using 100 oA H,PO,. The quantity + 1.26°/oo is the I70 correction for the oxygen isotopic composition of the oxygen reference sample which in this work is CO, prepared by 1205 oxidation of CO made from Standard Mean Ocean Water (SMOW)-one oxygen atom is from SMOW, A(l’O: 160) = -41°/oo ret-sus PDB and the other oxygen atom is from the 1205 oxidation, 160) = -335.2o/oo ~XEYZLY PDB. A(“O: 160 ratios, CO was first quantitatively To determine the absolute “0: prepared from SMOW by reaction with graphite at 1000 “C; then this CO was oxidized to COZ with 1205 by the same niethod as was used for the atmospheric and engine-CO samples, and the resulting CO= was compared with the secondary engine-CO standard. The CO2 from both the SMOW and secondary engine-CO standards, as well as atmospheric CO samples, have one atom of oxygen, O*, contributed by the 1205 with the same isotopic abundancet; hence, the absoIute t O* = 523 o/oo relative to SMOW
by coznparison
of
(R-r)-
Int. .T. Mass Spectrom.
with(R,&.DB. Ion Phys., 8 (1972) 265-275
272
C. M. STEVENS,
L.
KROUT
value of the %: I60 ratio in the original CO, expressed as per mil difference, Sao relative to SMOW oxygen, is
@coo*>x xlooo
S&=2 [
o?m*~~owl-1
where (Rcoo*)x
is the 46 : 44 ratio of the CO, from oxidation of the CO sample is the 46 : 44 ratio of CO2 prepared from oxidatzan with 120, and (&o&~ow of CO (with IzOs) prepared from SMOW. The two secondary standards of CO? used in this work were prepared by J,Os - oxidation of the CO in the concentrated exhaust gases from an automobile engine burning a mixture of gasolines marketed in Illinois by four major oil companies. The tit was prepared in Juiy 1969 and was used until January 1971; the second has been used since its pfeparation in January 1971. Two difficulties were encountered in using these-two standards. First, a difference of 0.6 per mil between the 13C-. I2 C ratios for tke two standards implied that the scurce of the petroleum for the gasoline marketed in this region had changed in the two years between the times the standards were prepared_ The same change in petroleum source was implied by the change in results for air with high CO concentration, collected in urban Chicago neighborhoods which occurred in the autumn of 1970. Limited data on the gecgraphic origins of the petroIeum used in refining gasoline soId in Illinois’” indicates there is not enough variation in the origins to account for the observed variation in the 13C: ‘*C ratio of the engine CO. Second, between July and December, 1970, the isotopic compositions of the carbon and oxygen in the first standard changed by a total o~E(~~C:~“C) = 1_6°/,o and6(‘80:160) = +2.0°/oo; the increments in the one-month intervals between successive comparisons with the standard tank CO, were about to.3 and +OS o/0,,, respectively. The cause is suspected to be that the standard was exchanged with or diIured by atmospheric CO, absorbed on the surface of the sampIe manifoId of the LOO-inchmass spectrometer when measurements were made with it. The standard was introduced into this mass spectrometer during this period to determine whether or not N20, NOz, or hydrocarbozts were present, The estimated uncertainty from this source is qrsc: l*C) = -c-O.3‘ioo and b(180 : 160) = &-0.5 o/oo for measurements during the period August-December, 1970. The manifold pressure of the reference sample in the isotope-ratio instrument was always adjusted so that the mass-& intensity was within + I oA of the mass-44 intensity for the unknown sample. This ehminated the need for any correction of the ratio for the “pressure effeci”18, which for this instrument, was about -0.1 o/oo per 1% change in mass-44 intensity. The leak fractionation bias was - 1.Oo/oo for the 45 : 44 ratio and -2.0 o/oo for the 46 : 44 ratio. Leakage of the ghiss valves during a measurement was 6 % until January 197I, at which time a new set having a leakage of only 1.5 oA was installed_ Int. J. Mass Spectrom.
Zon Phys., 8 (1972) 265-275
ISOTOPIC
COMPOSITION
OF ATMOSPHERIC
CARBON
MONOXIDE
273
Such smaI1 samples as are used in this work are espeeiatly vulnerable to errors from impurities and isotopic-exchange effects. The sample bulbs used before December 1970 were Pyrex glass, I I mm 0-d. by 7 cm long, and were not degassed except for the annealing process and pumping at room temperature. Because there was evidence of isotopic exchange of the oxygen, especially in instances of newlymade bulbs without annealing of the entire bulb (presumably due to absorbed water vapor on the walls of the bulb), we changed to smaller sample bulbs, 5 mm o-d. by 1 cm long, and flamed the bulb while pumping just before a CC& sample was transferred to it. The samples were stored in the dark and iLwtopicaIIy analyzed as soon as possible. The CC& sampIes of CO origin were carefully examined with the Argonne IOO-inch mass spectrometer for N,O, NOz, or hydrocarbons such as C&s and C2H,0H which might not have been effectively separated in the gas-extraction system and could cause appreciable errors even in very small concentrations. Except for the instances of N20 contamination found early in thz program, no other impurities were found greater than 10B5 of the C02, even in sampIes as small as 0.015 cm3 (STP). After the isotopic analysis, the CO2 samples were transferred from the sampIe-introduction manifold to the sampie bulb by cooIing with liquid nitrogen. Repeated analyses on samples smaIIer than about 0.05 cm” (STP) gave “0 : 160 ratios which were 0.5°/00 higher than the first analysis. This increase indicated some small exchange of the oxygen in the CO, with water vapor absorbed on the walIs of the manifold lines.
ACCURACY
The estimated error in determining the CO concentration is +O.Ol p.p.m. or 2 3 % of the concentration, whichever is greater. For small CO concentlations, this is due to the error in measuring the pressure of the CO, of CO origin in the small-volume manometer shown in Fig. 1, and for high CO concentrations due to the error in measuring the total quantity of air. In the first version of the extraction train, the amount of atmospheric COB impurity in the CO, of CO origin corresponded to about 0.01 p.p.m of the total in air. It is estimated that after the molecular-sieve tra; was enlarged and the f&t liquid nitrogen-cooIed trap was added, the amount of this CO, contamination was less than 0.001 p.p_m Estimation of the total error in the isotopic ratios is -cult because of uncertainities due to wide ranges of concentration, the smallnest of the samples, and the frequent changes and improvements in both the gas-extraction and mass spectrometric procedures over 23 years. For air samples with the smailest CO concentrations (to.15 p-p-m.), the principal sources of error are probably contamination Int. J. Mass Spedrmn. Ion Phys., 8 (1972) 265-275
271
C. M. STEYENS,
L. KROUT
with atmospheric CO, -(+0.3 ‘foe) and small hydrocarbon impurities (< 1 ‘joO) For high CO concentrationsz the mass spectrometric internal error (f0.1 ‘;Iee) and the error from insufficient compensation for the “pressure effect” + (0.1 ‘foe) become sign&cant_ Analyses made during the early stages of the program undoubtedly had errors due to exchange of oxy,oen with water vapor absorbed on the walk of sample bulbs, uncertainty of reference standards, and mass spectrometer difficuhies. When the most recent and improved procedures and apparatus are used, analysis of duplicate air samples leads to an estimate that the overall error for the 13C -. 12C ratio is between kO.3 and 0.5 “joO and that for the “0: 160, ratio is in the range from f0.5 to &-l.O”~oo, depending on the sampie size.
DXSCUSSIOX
The method described herein is quantitative in the yield of CO oxidized to CO,, suffers no exchange of ‘*O in the oxidation procedure, does not produce CO+. from any other known atmospheric compounds containing carbon, and produces CO2 of CO origin free from impurities that could cause large errors in either the concentration or isotopic analyses. The isotopic composition of the CO in unpolluted atmospheres as measured by this procedure depends on the season; it ranges from - 18 to +5 o/oo for Al*0 : 160 and from -5 to i-5 o/oo for A.’ 3C : 12C relative to engine CO, The most significant feature of the isotopic studies is the depletion of ‘*O in atmospheric CO compared to engine CO. This depiction implies the existence of natural sources of CO many times greater than worId-wide engine emissions. The smallest CO concentrations have the greatest depletion in l*O; thus the I80 : I60 values cannot be low due to undetected impurities in the samples interfering with mass-46 measurements. Only an impurity whose ion currents would be collected on the wide collector plate that measures the mass-44 intensity, or an impurity at the mass-46 position for the reference CO=, could result in low ‘*O : 160 values, and we are satisfied that neither possibility occurs. On the other hand, if some of the CO, collected in the extraction process is not actualiy from the oxidation of CO (Le., if the oxidation of some compound in the air sample led to CO? that derives *both its oxygen atoms from the fiOS) then the resulting ‘*O : 160 ratio would be depleted with respect to engine CO; and the amount of the depletion wouId be greater for Iower CO concentrations if the concentration of the interfering substances remains constant_ In these air samples, however, the only carboncontaining constituents -with concentrations above 0.010 p_p.m.6 are CO2 and Cl-&. Tests have indicated no possible interference from these. The results of the atmospheric CO studies wilI be described in a future report, ht. 1. &iass Spectrom. -1011 P&s_, 8 (1972)
265-275
ISOTOPIC
COLMPOSITION
OF
ATMOSPHERIC
CARBON
MONOXIDE
275
ACICNOWLEDGE3lENTS
We are indebted to Dermis Walling for electronic servicing of the mass spectrometer, Frank Lenkszus, Art Metz, and Stanley Rudnick for design and construction of the mass spectrometer automatic programming circuit, Antoinette Engelkemeir for mass spectrometric gas analysis, the Argonne Chemistry Division Giass Shop for the glass apparatus, John Sevec for mechanical assembly, Robert CIayton for providing a sample of Standard Mean Ccean Water, Ben HoIt and Harmon Craig for helpful discussions, and Don Stewart and Bernard Weinstock for encouragement and support. This work was supported by Coordinating Researcti Council-U.S. Environmental Protection Agency, and U.S. Atomic Energy Commission.
REFERENCES
i W. D. TUCKER(Editor), The Atmospheric Diagnostic Program at Brookhacen Nationaikbora-
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
tory: Second Status Report, BNL-50206, 1969. H. L. DEQUAS~S;AK~ D. C. GREY, Stable isotopes applied to pollution studies, Amer. Lob., (Dec. 1970) 19. B. D. HOLT, A. G. ENGELKEYEIR AND A. VENTERS, Srdfur-34 Yariarions in Environmenrai Media near Chicago, submitted to Environ. Sci. Tech. J. W. SWIKDIEIYTOX, V. J. LINNEIKM am R. A. LAMONiAGUE, Science, 67 (1970) 984. W. SEILERAXD C. JLXGE, J. Geophys. Res., 75 (1970) 2217. E. ROBWSON AKD R. C. ROBBINS,Sources, Abundance, and Fate of Gaseous Atmospheric PoIWants, Final Report Stanford Research Institute, Project PR-6755, 1968. M. G. INGHR+.M,Ann. Rev. Nucl Sci., 4 (1954) 81. S. R. SILWWN AXE S. EPSTE~, Bull. Amer. Ass. Petrol. Geol., 42 (1958) 998. C. JUXGE, W_ SEILERAXD P. WARNECK, J. Geophys. Res., 76 (1971) 2866. C_ hl. STEVEXS,L. KROUT, D. WALLING, A_ VEA?? A. ENGELKE!!IR, to be published. W. G. S~LJX, Nucl. Sci. Abstr-, 3 (1949) 391. B. D. HOLT, Anal. Chem., 27 (1962) 751. H. CRAIG AXD C. D. KETEL~G,Gecchim. Cosmochim. Acta, 27 (1963) 549. J. W. SWINKERKIN, V. J. LIIWEX~~MAND C. H. CHECK,Limnol- Oceanogr., 13 (1968) 1931. R. K. SZVENS, A. E. O’UEFEAND 0. C. ORTMAN, Proceedings of the Eighth Conference on Air Pollution Coetroi, Purdue Urdv., Oct. 1969. k O_ NIER, Rev. Sci. Instrum., 18 (1947) 398. C. R. MCKINKXY, J. M. MCCRW, S. EPSTEIN,H. A. ALLEN AND H. C. UREY, Rev. Sci., Irstrum. 21 (1950) 724. H. CRAIG, Geochim. Cosmochim. Acta, 12 (1957) 133. R. B. JACOBS,personal communication.
ht.
J. Mass Spectrom. Ion Phys., 8 (1972) 265-275