Critical Review 14
CO2 Measurement in Air: Literature Review and a New Sensitive Method
ANDREAS STORK,* RAPHAEL WITTE, AND FRITZ FU ¨ HR Institute of Radioagronomy, Forschungszentrum Ju ¨ lich GmbH, D-52425 Ju ¨ lich, Germany
A simple method has been developed for the measurement of low concentrations of 14CO2 in air. CO2 and 14CO2 are quantitatively absorbed by 2-methoxypropylamine in a newly developed intensive wash bottle with reflux cooling after intensive drying with silica gel and phosphorus pentoxide. The maximum air flow rate is 3.5 L min-1, which is relatively high as compared to the volume of absorbent (50 mL). A maximum of 10 m3 air can be sampled within a sampling period of 48 h. Aliquots of 10 mL of absorbent, corresponding to 2 m3 CO2 equivalent of air or about 0.4 g of CO2 carbon, can be measured in one sample by liquid scintillation counting. The sampling efficiency is better than 99.98%, and the detection limit is lower than the ambient 14CO2 background of 54 mBq m-3. The remarkable features of the new method are as follows: low limit of detection, easy sample preparation, and the possibility of short-term measurements. The new air sampler is currently used in windtunnel experiments, where 14CO2 is largely diluted, in order to quantify the mineralization of 14C-labeled environmental chemicals in agroecosystems.
Introduction Radiocarbon (14C) occurs naturally in the environment. In the stratosphere, neutrons are permanently generated by cosmic radiation, forming 14C (14N(n,p)14C reaction) (1-3). Oxidizing conditions in the atmosphere cause radiocarbon to predominate in the form of 14CO2 (4, 5). Similar reactions also lead to the formation of 14CO2 in nuclear reactors. Due to natural production, burning of fossil fuels poor in 14C (Suess effect) (6, 7), and 14C production through atomic bomb testing (4, 8-11), the ambient 14CO2 concentration is now about 54 mBq m-3 or 0.273 Bq (g of C)-1 (12, 13). The ambient CO2 (in this paper, CO2 is understood as the sum of 12CO2 and 13CO2) concentration is of importance for the design of a 14CO2 sampler, since the capacity of any sorbent will be mainly depleted by CO2. Until the beginning of the industrial revolution (about 1800), the average CO2 concentration in the atmosphere was about 280 µL L-1 (14-18). However, it is presently increasing by 1.8 µL L-1 annually (8) due to coal and mineral oil combustion activities. Recently, CO2 concentrations of 350-440 µL L-1 (noon/night) have been measured (19). The calculations in this paper are based on an assumption of 370 µL L-1 average atmospheric CO2. 14CO measurements are performed for different purposes 2 including (a) monitoring of emissions from nuclear power * Corrresponding author telephone: (+49)2461-615656; fax: (+49)2461-612518; e-mail:
[email protected].
S0013-936X(96)00580-9 CCC: $14.00
1997 American Chemical Society
plants and other nuclear facilities with typical concentrations from 1 to 2600 Bq m-3 in stack air (13, 20-23), (b) observation of ambient air and plant tissues near nuclear power plants (24-26), (c) age determination (14C dating) of prehistoric samples in archaeology and the geosciences (1), and (d) expiration analysis for the detection of internal 14C contamination (27). Investigations concerning the environmental fate and behavior of pesticides and other environmental chemicals are preferably done using 14C-labeled compounds. The advantages are low detection limits, quantification of nonextractable (bound) residues, andsin combination with chromatographysdetection and quantification of the unchanged test compound and its metabolites as well as visualization of uptake and transport in plants by autoradiography (28, 29). 14CO2 may be used as a parameter for mineralization since it is produced as the final metabolite by decomposition of the test compound (30, 31). In laboratory experiments, the determination of low 14CO2 concentrations is normally not necessary since 14CO2 is usually not diluted in large volumes of air but rather trapped using a slight air flow. The case is different in the new wind-tunnel experiments (32, 33). A high air exchange rate greatly dilutes 14CO2 in air. Thus a sensitive 14CO2 detection is required. Therefore, the aim of this investigation was to develop a method with high sensitivity, short sampling times, and simple sample preparation since no established procedure could be found. This paper deals only with the measurement of low concentrations of 14CO2, which require enrichment before actual measurement. The first part describes the theory of 14CO2 measurement. The second part gives a literature review. The third part describes and validates this new method.
Theory of 14CO2 Measurement The following steps are distinguished to achieve a low detection limit: CO2 enrichment ) high CO2 saturation of sorbent; sample preparation ) conversion of a large amount of CO2 into a small volume; radioactivity measurement ) high 14C counting efficiency, low background count rate. CO2 Enrichment. CO2 and 14CO2 are only slightly different in their physicochemical properties. Thus, procedures for isotope enrichment are laborious as well as time-consuming and excluded from routine analysis (1). The natural concentration of 370 ppmv CO2 limits the sorption capacity with ambient measurements since 14CO2 and CO2 can only be trapped together from a gas mixture. Table 1 lists common CO2 enrichment procedures and differentiates between reactive and non-reactive procedures. Reactive procedures have been successfully used in the past (Table 2). Adsorption
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TABLE 1. Procedures for Enrichment of CO2 from Gas Mixtures (Extended after Ref 13) procedure
freeze-out diffusion/membrane permeation adsorption at molecular sieves
absorption with Ca(OH)2 granules absorption with soda lime/sodium asbestos absorption in alkaline solutions
absorption in solutions of organic amines
comments
sample preparation
Non-reactive large-scale, needs permanent control under development, promising for continuous measurements simple construction, relatively complicated sample preparation before measurement Reactive suitable for routine measurements; complicated handling because heating to 360 °C necessary (34) difficult because of high blank values of carbonate, even in p.a. quality relatively slow reaction, thus incomplete reaction with high flow rates,a complicated sample preparation before measurement or measurements of only small aliquots in LSCb losses of absorbent without cooling, drying of air sample necessary, very reactive, high flow rates possible, simple and sensitive measurement
warm up, introduce CO2 into absorbent or gas counting tube none, continuous measurements in gas counting tube thermal desorption, absorption of CO2 in Ba(OH)2 or organic amine
dissolve with HCl, absorption of CO2 in Ba(OH)2 or organic amine see above precipitate as carbonate through addition of BaCl2 or CaCl2, filtrate, dry, weigh, grind or measure directly in LSC none, direct measurement in LSC possible
a Increasing the flow rate from 0.5 L min-1 to 2.75 L min-1 reduced the sampling efficiency of CO from 90% to 50%, using 5 N NaOH and 2 conventional wash bottles (21). b LSC, liquid scintillation counter.
onto molecular sieves is a newer, commercially developed method. However, there is little experience with the diffusion method. Freeze-out of CO2 is maintenance-intensive and excluded from routine analysis. Sample Preparation. Many procedures require that the trapped CO2 be released again and converted to a measurable form. For the most part, the trapped CO2 gas is released either through thermal desorption from molecular sieves (13, 46), dissolution of soda lime in hydrochloric acid (31), or warming up of CO2 snow (26). CO2 gas is subsequently trapped either with Ba(OH)2 (forming BaCO3 precipitate) or with organic amines (ethanolamine, phenethylamine, 2-methoxypropylamine) forming carbamates. After trapping CO2 in alkaline solutions, a carbonate precipitate can be obtained by adding BaCl2 or CaCl2. The carbon concentration in CaCO3 is 12% and thus allows more sensitive measurements as compared to BaCO3 (6.1% C) (Tables 2 and 3). A very sensitive detection technique is required for environmental samples in order to determine a slight increase of natural 14C contents (near nuclear power plants) or a decrease of 14C (14C dating) in organic material (e.g., plant tissue). The samples are completely oxidized, and the resulting CO2 is trapped as described above. After release, the purified CO2/14CO2 gas is measured in a special CO2 counting tube. Often benzene, methane, or acetylene is synthesized before the actual measurement. This method, able to quantify the largest amounts of carbon (Table 3), requires the most laborious sample preparation and thus is not suitable for large numbers of samples. Organic amines and benzene can be measured directly in the liquid scintillation counter (LSC). Carbonate precipitates have to be filtered, dried, weighed, and ground prior to the measurement. Only the direct trapping of CO2 in organic amines and the diffusion method do not need any further sample preparation. 14C Measurement. Radiocarbon (14C) is a low-energy β-emitter (Emax ) 0.156 MeV). Several detectors can be used for its measurement (Table 3): (a) liquid scintillation counting (LSC), (b) low level counter (LLC) (end window counting tube, methane flow counter), and
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(c) gas counting tubes (methane counting tube, CO2 counting tube, proportional gas counter). These detectors differ considerably with respect to the amount of measured CO2 carbon, the counting efficiencies, and consequently, the achievable detection limits (Table 3). The LSC combines high counting efficiencies, simple sample preparation, and automated measurement and, thus, is the detector of choice for many applications. The LSC was developed for the measurement of low-energy radiation. A mixing of the liquid sample (e.g., organic amines, benzene) with a liquid detector (scintillation cocktail) minimizes energy transfer losses. Even the measurement of solids as a suspension (e.g., carbonate precipitates) is possible, and special scintillation cocktails allow measurement of up to 10 mL of an aqueous sample. Only the measurement of organic solvent and water mixtures is difficult or often not possible due to phase separation. Detection Limit. The detection limit of a radioactivity counter is an important parameter since it decisively determinines the sensitivity of the total analytical procedure. Here the sensitivity of a radioactivity counter is defined as the lower limit of detection (LLD). This is the smallest amount of radioactivity that is statistically significantly different from the background (47). The LLD can be calculated from counting time and background counting rate, assuming equal measuring time of the blank and sample (eq 1 modified after ref 47):
LLD )
(kR + kβ)x2xA0 tη
(1)
where LLD is the lower limit of detection (Bq), kR is the onesided significance level, equivalent to 1 - R (R ) type 1 error), kβ is the one-sided significance level, equivalent to 1 - β (β ) type II error), A0 is the background (counts), t is the measuring time of sample and blank (s), and η is the counting efficiency of equipment (0 e η e 1). More significant for the comparison of different methods is the LLD′ which gives the minimum detectable radioactivity per gram of carbon (eq 2):
LLD′ ) LLD/C
(2)
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emissions from a heavy water reactor
emissions from various nuclear plants
emissions from various nuclear plants
emissions from nuclear reactor
2
and 3H monitor
and 14CO2 emissions from various nuclear plants
14C
14C
0.2
0.08
0.1
(diffusion only)
3.5
12
1
max 0.5
1
0.2
20
≈0.02
0.78
0.2
0.01
method of 14CO2 measurement
absorption in 300 mL of 2 N NaOH, precipitate as BaCO3, measure 2 g as suspension absorption in 3 M NaOH, precipitate as CaCO3, measure 8.3 g as suspension absorption in 200 mL of 1 N NaOH, precipitate as BaCO3, measure 1-2 g as suspension absorption in 200 mL of 5 N NaOH, precipitate as BaCO3, measure 2 g with LLC
0.4
0.3
4.0
nda
70.0
adsorptive drying of air sample, CO2 absorption in 25 g of ascarite (contains NaOH), release of 14CO2 through acidification, freeze-out, measurement of CO2 gas in gas counting tube adsorption on 250 g of molecular sieve, thermal desorption and freeze-out, absorb aliquot in ethanolamine and measure adsorption on 500 mL of molecular sieve, thermal desorption, precipitation with Ba(OH)2, measure 3 g as suspension
Solid Adsorbents absorption in 10 g of soda lime, dissolve in 6 N HCl, introduce CO2 in ethanolamine/methanol (30:70)
direct introduction in 50 mL of 2-methoxypropylamine, measure 10-mL aliquots
Liquid Absorbents: Organic Amines 40.0 direct introduction in 300 mL of ethanolamine solution, measure 5-mL aliquots
5.0
2.5
nda
0.7
Liquid Adsorbents: Alkaline Solutions ndae absorption in 0.2 N NaOH, release of 14CO2 through acidification, absorb in liquid scintillator nda absorption in LiOH absorbent, release of 14CO2 with HCl, re-absorption in NaOH, precipitate as BaCO3, release of 14CO2 with HCl, absorb in liquid scintillator 3.1 absorption in 250 mL of 4 N NaOH, precipitate as BaCO3, release of 14CO2 through acidification, aborption in ethanolamine soln. 0.1 absorption in 200 mL of 1 N NaOH, precipitate as BaCO3, measure 1 g as suspension 200.0 absorption in 100 mL of 1 N CsOH, measure small aliquot
specific flow rateb (min-1)
183
81
27
240f
400
172
122
122
1000
122
72
61
24
nda
20
C in counterc (mg of C)
0.69
0.77
2.32
0.26
0.16
0.36
20.58
1.03
0.13
1.03
0.87
2.06
2.61
nda
3.14
sensitivity (LLD′)d (Bq (g of C)-1)
13
46
45
30, 31
this paper
44
21
22, 23
41-43
20
40
38, 39
37
36
35
ref
a 14C measurement is performed with LSC, if not stated otherwise. b Specific flow rate (mL min-1 per mL sorbent). c Amount of carbon in radioactivity counter (mg), calculated with information from literature and assuming 370 µL L-1 ambient CO2 concentration. d Calculated lower limit of detection (dpm (g of C)-1) (eq 2). e No data available. f Assuming a 80% saturation of ethanolamine.
emissions from various nuclear plants
14C
from degradation of 14C-labeled pesticides (biometer flasks)
2
and 14CO2 monitor in wind-tunnel experiments
14CO
14C
from degradation of 14C-labeled pesticides in plant growth chamber
2
14CO
2
2
14CO
14CO
2
14CO
14CO
monitor
and
2
14C
14CO
emissions from nuclear reactor
emissions from nuclear plant
14C
2
emissions from nuclear plant
14C
14CO
and 3H monitor
14C
measurement of
flow rate (L min-1)
TABLE 2. Compilation of Different Methods for Measurement of 14CO2a
TABLE 3. Measurable Amount of Carbon and Achievable Detection Limits with Different Methods of 14CO2 Detection (Modified after Ref 43)a LSC (40-90%)
(typical counting efficiencies) (g of C) solutions of organic amines carbonate precipitates BaCO3 CaCO3 benzene CO2, methane or acetylene a
0.65 0.5 1.0 10.0 -
LLC (2%)
(mBq m-3) 16
-
30 14 1.7 -
0.13 0.25 -
(mBq m-3) 1200 800 -
(g of C)
(mBq m-3)
-
-
-
-
3.0
2.8
Assuming a standardized counting time of 200 min, which could be extended in practice. (-), no measurement possible.
where LLD′ is the lower limit of detection per mass of carbon (Bq (g of C)-1) and C is the mass of carbon (g). The appropriate statistical significance level can only be defined arbitrarily. Often 5% is assumed to be useful (kR ) kβ ) 1.65) (47-49). The LLD can be lowered by increasing the counting time as well as by reducing the radioactivity background (e.g., shielding, use of special low-level or polyethylene counting vials instead of ordinary glass counting vials). Review of Published Methods of 14CO2 Measurement in Air. Table 2 shows a comprehensive compilation of published methods for the measurement of 14CO2 in air. Each method is a special combination of CO2 enrichment (trapping), sample preparation, and radioactivity measurement. The methods have been compared in terms of the amount of carbon that can be measured at one time in the radioactivity monitor as well as the LLD′ (eq 2), assuming a radioactivity counter with the same efficiency and background count rates. [Assumptions: background (Z0) ) 25 cpm (0.417 Bq); measuring time (t) ) 1 h; counting efficiencies (η), organic solvents (LSC) ) 0.8, suspensions (LSC) ) 0.4, solids (LLC) ) 0.02, gas counting tube ) 0.8. The LLD was calculated as follows: LLD (Bq) ) 0.0502η-1 (eq 1), thus LLDsolv ) 0.0628 Bq, LLDsusp ) 0.1255, LLDsolid ) 2.5104, LLDgas ) 0.0628.] The sensitivity of a method increases (decreasing LLD′) with an increasing amount of measured carbon. Having a given amount of carbon, the LLD′ is only dependent on the properties of the radioactivity monitor (radioactivity background, counting efficiency). Note that the counting efficiency of the low level counter is about 25-40-fold lower as compared to the LSC (Table 3). Most methods trap 14CO2 in conventional wash bottles filled with alkaline solutions (KOH, NaOH, Ba(OH)2), thereby being limited to low flow rates only. All of the methods described so far are limited by one or more of the following factors: sensitivity, low flow rates if long-term sampling is done, and laborious analytical procedures. Thus, a new sensitive method had to be developed fulfilling the following demands: low detection limit since 14CO2 is greatly diluted in large volumes of air (32, 33); comparatively short sampling intervals (48 h) in order to observe the kinetics of 14CO2 evolution in detail; easy sample preparation due to a large number of samples. Conception of an Improved Method. Due to easy sample preparation, direct trapping of 14CO2 in liquid absorbents was favored. The absorbent had to have the following characteristics: high reactivity since sampling intervals are short and flow rates are high, low viscosity for a good dispersion of the gas sample for a quantitative reaction, good miscibility in a scintillation cocktail in order to measure a large amount via LSC. Fulfilling these requirements, 2-methoxypropylamine (2MP, Carbosorb E, registered trademark of Packard, Canberra Co.) was selected. [Caution: 2-Methoxypropylamine is very irritating to eyes and skin. Protective clothes, gloves, and goggles are recommended!] Preliminary experiments showed that even a saturation of about 80% did not cause a breakthrough of CO2. At room temperature, 2MP is highly volatile, so intensive cooling is necessary in order to
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(g of C)
gas counting tubes (80%)
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FIGURE 1. Schematic diagram of the medium-volume sampler (MVS) for the sensitive measurement of 14CO2. I, air inlet; X1/2, XAD cartridges; D1, drying stage 1 (silica gel); D2, drying stage 2 (phosphorus pentoxide); CIW, cooled intensive wash bottle with 2-methoxypropylamine; A, activated charcoal filter; D, dust filter; PG, pressure gauge; B, bypass with control valve; P, metal bellows pump; G, gas meter; F, flow meter; O, air outlet. avoid vaporization during sampling. Thus, a special cooled wash bottle was designed. Furthermore, intensive drying of the air sample before it enters the wash bottle is necessary to avoid water enrichment in the absorbent. This would unnecessarily dilute the absorbent and reduce its sorption capacity, thereby complicating the miscibility with scintillation cocktails. Preliminary experiments verified the observation (21) that 6 N NaOH was too slow to react at high flow rates, causing an early CO2 breakthrough. Ethanolamine has to be diluted with solvents due to its high viscosity and further increases in viscosity with increasing CO2 saturation. Due to autoxidation, ethanolamine turns brown, reducing the counting efficiency in the LSC (quench effect). It takes about 50 mL of absorbent to perform a multiple measurement in the LSC, using 10 mL of sample plus 10 mL of scintillation cocktail (see below). Since the CO2 capacity of 2MP is 4.8 mol L-1, 50 mL of 2MP can theoretically absorb 192 mmol of CO2 (at 80% saturation), which is contained in 11.55 m3 ambient air (at 370 µL L-1).
Materials and Methods Liquid Scintillation Counter. A Packard Tri-Carb 2500 TR (a registered trademark of Packard, Canberra Co.) was used. The counting efficiency (η) was 0.78 on average, using a Carbosorb E: PermafluorE+ mixture (Canberra-Packard products) of 10:10 mL v/v. The background was 25.5-26.5 cpm using ordinary glass counting vials. The LLD was calculated to be 36.6 mBq (at 200 min counting time and 5% significance level) (eq 1). Description of the New Method. The technical diagram of the medium-volume sampler (MVS) is given in Figure 1. In order to ensure a sampling of 14CO2 only, volatile organics that might be present in the air sample are trapped with two cartridges (25 mm i.d., 168 mm high). These are filled with
FIGURE 3. Schematic diagram of the 14CO2 generator. TF, tapered flask (containing Na214CO3); HP, hose pump; T, timer; IA, inlet air; PP, phosphorus pentoxide; TNF, three-necked flask; GW, glass wool; OA, outlet air (containing 14CO2).
FIGURE 2. Cooled intensive wash bottle (CIW) for the quantitative absorption of CO2/14CO2 in volatile absorbents with high flow rates. (a, left panel) Schematic diagram. AI, air inlet; AO, air outlet; B, baffles for air bubbles; CI, coolant inlet; CJ, cooling jacket (-40 °C); CO, coolant outlet; CS, cooled spiral; DP, down pipe; EB, expansion bellows; F, funnel (for rinsing the spiral); GF, glass frit (porosity D1); GJ, ground-glass joint for dropping funnel); TC, Teflon discharge cock; TWC, three-way cock; VI, vacuum isolation. (b, right panel) View of gas dispersion at glass frit. about 55 mL of polymer resins (about 20 g), which have a high affinity to volatile organics (e.g., XAD-4, XAD-7, registered trademarks of Amberlite), or activated carbon. The volume of the cartridges was designed according to detailed investigations (50) concerning the retention volumes of polymer resins for organics. As long as no 14C is detected in the second cartridge, no breakthrough of 14C-labeled organic compounds is assumed. Moisture contained in 10 m3 of air should produce about 13-15 mL of water to be trapped in the cooled intensive wash bottle (CIW), which would freeze and clog the downpipe (Figure 2, DP). This is avoided by intensive drying of the air sample with about 3.5 L of silica gel (Figure 1, D1) and 0.8 L of phosphorus pentoxide (Figure 1, D2). Silica gel, which can be reused after drying at 110 °C, ensures that the phosphorus pentoxide, which cannot be regenerated, lasts longer. In our investigations, phosphorus pentoxide provided with a color indicator (Sicapent, Merck) was used, indicating water saturation and the need for replacement. Inside the CIW (Figure 2), CO2/14CO2 quantitatively reacts with 2MP forming a non-volatile carbamate. After this reaction, the air sample is CO2-free but saturated with 2MP, which is condensed at -40 °C in the cooled spiral (Figure 2a, CS) and drips back to the reservoir. Residues of 2MP are adsorbed with activated carbon (Figure 1, A) in order to protect the pump. The actual apparatus achieves a sampling rate of 10 m3 in 48 h (3.5 L min-1), which includes a safety margin (actual
CO2 saturation only 70%). Three to four 10-mL aliquots of absorbent are measured in the LSC, corresponding to a CO2 equivalent of about 2 m3 air (≈400 mg of C). Considering the detection limit of a conventional LSC (about 40 mBq), 20 mBq m-3 should be detectable. This is about one-third of the ambient 14CO2 concentration (54 mBq m-3; 12, 13). The cooled intensive wash bottle (CIW) (Figure 2a) has the following characteristics: (a) capacity for 60 mL of absorbent, (b) maximum air flow of 3.5 L min-1, (c) intensive dispersion of the air sample, (d) intensive cooling of outlet air for condensation of the volatilized absorbent, and (e) recoil of condensed absorbent back to the place of reaction. The air sample enters the CIW via a downpipe (Figure 2a, DP) and flows under the glass frit (porosity D1, Figure 2a, GF). During operation the absorbent remains above the glass frit via suction at the air outlet. The air sample is intensively dispersed at the glass frit, foaming the absorbent (Figure 2b). The length of the lower part of the glass body, provided with baffles, prevents the foamed absorbent from entering the glass spiral at the maximum flow rate. The baffles improve CO2 absorption by extending the dwell time of the gas bubbles in the absorbent. After CO2 absorption, the air sample is saturated with 2MP, which is condensed in the glass spiral. For this purpose, ethanol from a connected bath at -40 °C continuously sweeps the glass spiral (Figure 2, CS) of the CIW. The internal diameter of the spiral (7 mm) and its slope (8-12°) allows the condensate to flow back to its source and not be taken upwards with the air flow.
Results and Discussion: Performance of the New Method Sampling Efficiency for 14CO2. In order to determine the sampling efficiency, a known amount of 14CO2 gas was generated (Figure 3). 14C-labeled sodium carbonate solution (1 mmol L-1 Na214CO3, 18.5 MBq L-1) was slowly pumped (about 1 mL h-1) into a three-necked flask filled with phosphorus pentoxide. The evolving 14CO2 gas was quantitatively fed to the MVS. In order to observe a breakthrough of 14CO2, an additional CIW was installed behind the first one. After an air sampling period of 48 h at 3.5 L min-1, the radioactivity in the absorbent solutions was determined via LSC. NOTE: The measurement of the gross radioactivity (Na214CO3 solution) requires some precautions. 14C losses of 10-20% have been observed if scintillation cocktails are added directly to the Na214CO3 sample, possibly due to degassing of 14CO at a low pH. This was avoided by adding 300 µL of 6 2 N NaOH to each sample (22-mL vial) before adding the scintillation cocktail. Table 4 summarizes the results of three sampling efficiency experiments. On average >96% of the applied radioactivity (AR) was recovered. Considering analytical errors, a loss of radioactivity can be excluded, even in the adsorption and
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TABLE 4. Sampling Efficiency of Medium-Volume Sampler for Generated 14CO2 Gasa recovery of applied radioactivity (%) expt no.
trap 1
trap 2
trap1 rinsate
trap 2 rinsate
total
1 2 3 average CV (%)b
94.46 95.74 95.23 95.14 0.68
0.01
