Preparation of Carbon Dioxide from Sulfates, Sulfur Dioxide, Air, and Water for Determination of Oxygen Isotope Ratio Ben D. Holt Chemical Engineering Division. Argonne National Laboratory, Argonne, Illinois 60439
To facllltate the study of mechanisms of formation of sulfates In the atmosphere by oxygen isotope ratio measurements, a group of procedures was developed for the converslon of oxygen in sulfates, sulfur dioxide, air, and water to carbon dloxide for mass spectrometric determination of isotope ratlos, using a single graphite furnace. A new procedure is described for preparlng sulfate samples for the hot graphlte reduction reaction.
T h e oxygen isotopy in atmospheric processes that lead to t h e formation of atmospheric sulfates from SO,, water, and oxygen in the air is uniquely dependent on the mechanism of formation. Two overall mechanisms of formation, each of which may encompass multiple reactions, are t h e oxidation of hydrolyzed SO2 and the hydrolysis of oxidized SO,. T h e oxygen isotope ratios, 180/160, of the sulfate products formed by the two mechanisms are not identical, probably because of differing fractionation effects as the SO2,HzO, and air enter by different sequences into the sulfation reactions ( I ) . Studies of t h e oxygen isotope ratios of the major components of the sulfation reactions can therefore help t o identify t h e predominant mechanisms of sulfate formation in a given geographical region and thus contribute t o the selection of effective and economical methods of control of atmospheric sulfates (1-3). This report describes techniques for using a single graphite reduction furnace for t h e conversion of all t h e major components of sulfation reactions (sulfates, SOz, water, and air oxygen) to C 0 2 for subsequent mass spectrometric analysis. A new procedure is also given for t h e preparation of B a S 0 4 samples for t h e hot graphite reduction. A procedure for t h e conversion of oxygen in B a S 0 4 to C 0 2 by hot graphite was described by Clayton and Epstein ( 4 ) in 1958, a n d variations of the method have since been used by others (5-8). Clayton a n d Epstein (4),Lloyd ( 5 ) ,and Longinelli and Craig (6) mixed the BaS04with graphite powder by mortar-and-pestle grinding and placed t h e mixture in a graphite crucible for t h e high-temperature reaction by induction heating. Rafter (7) placed the powdered mixture of BaS04 and graphite in a platinum boat, which was then placed in a quartz tube for heating in a resistance-wire furnace. Mizutani (8)used a small folded sheet of platinum to function both as a boat for containment of the powdered BaS04graphite mixture and as a heat source, by passing sufficient electrical current through the platinum to raise its temperature to 1000 "C. These methods all entail the multiple operations of filtering the BaS04,drying, batchwise mixing with graphite powder, and charging into a suitable container for the thermal conversion t o COz. By the technique t o be described, t h e B a S 0 4 is cofiltered with graphite powder directly into a graphite capsule, in which i t is subsequently heated to decomposition. For the determination of the oxygen isotope ratio in water, a widely used method of preparing COP is t o equilibrate a measured quantity of the water with a measured quantity of 1664
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C 0 2 of known isotopic composition (9). The equilibration is usually carried out by frequent agitation of the water and COS for about three days a t a constant temperature. With t h e application of an isotope separation factor for t h a t temperratios ature, a mass spectrometric comparison of the 180/160 in the C 0 2 before a n d after equilibration with the water is used to calculate the isotopic composition of the water oxygen. Majzoub (10) developed a method for converting all of t h e oxygen in microquantities (-4 pL) of water to C 0 2 for mass spectrometric analysis. The method consists of hot-graphite reduction of the water to CO and H2,separation of the H 2 from the CO through a palladium membrane, and conversion of the CO to CO, and carbon. We prefer a carbon-reduction method to t h e C 0 2 - H 2 0 equilibration method (9) because of t h e applicability to our graphite furnace. Furthermore, very small samples ( 2 pL) are required, and t h e long (three-day) equilibration requirement is avoided. For the conversion of oxygen in air to COz, a method described by Horibe e t al. (11)was also modified for use with our graphite furnace. EXPERIMENTAL Sampling. Sulfate aerosol samples were received for analysis as particulate material collected on 8 x 10 in. fiber-glass filter sheets in a high-volume (hi-vol) air sampler. Atmospheric sulfur dioxide samples were also received as sulfates, formed by air oxidation on 8 X 10 in. cellulose filter sheets (downstream from the aerosol filter) which had been pretreated with a solution of 25% K2C03and 10% glycerol (12). Atmospheric water vapor samples were received as condensate from a stream of air conducted through a cold trap at dry-ice temperature. Rain and snow samples ( - 2 L) were analyzed for oxygen isotope ratios in both the water and the dissolved sulfates. Sulfate samples generated from laboratory experiments were aqueous solutions; laboratory samples of sulfur dioxide, oxygen, and air were sampled as the respective gases. Preparation of BaSOl for Graphite Reduction. Sulfate samples were precipitated as BaS04 and cofiltered with graphite powder in porous graphite capsules. Each capsule was placed in an inductively heated graphite furnace for decomposition to CO and C 0 2 . Apparatus. Figure 1 shows the graphite capsule (made from a spectrographic cup 22 mm long, 6.15 mm o.d., with machine threads cut at the open end) mounted in a glass holder that extends into a 250-mL suction flask. A short segment of rubber hose seals the capsule to the holder and supports a glass funnel, into which the aqueous suspension of BaSO, and graphite powder is poured. Procedure. Use scissors to clip from the hi-vol filter sheet a sample aliquot which is estimated to contain about 2-5 mg SO:-. Shred the paper into a 250-mL beaker and add 50 mL water. (If the filter sheet is fiber glass, precede the water addition with the injection of a few milliliters of acetone to wet the fiber glass surfaces,) Heat to boiling (completely expelling the acetone, if added). Using methyl orange indicator, acidify by dropwise addition of 1-1 HC1; add 1 mL excess. Filter the solution through Whatman No. 42 paper, or equivalent, and precipitate BaSO, by a standard procedure (13). Allow the precipitate to age a t least 2 h, preferably overnight. For alkaline-treated filter sheets, determine the sulfite content by iodometric titration (14) before taking an aliquot for analysis.
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Figure 2. Apparatus for converting samples to CO,
Make the sulfite test on a solution produced by adding 1/16 of the filter sheet to 100 mL water. (Most SO2 samples collected on the alkaline filter sheets are already completely oxidized to sulfate after 24 h in the hi-vol air sampler.) Allow samples which still contain a significant fraction of total sulfur as sulfite to air-oxidize to sulfate before dissolution for isotopic analysis. Evaporate 2-L samples of rain or snow water to -50 mL; acidify (methyl orange indicator) with 1-1 HCl and add 1 mL excess. Filter and precipitate BaS04 as described above. Install the graphite capsule as shown in Figure 1 and wash the capsule a few times with reagent-grade acetone. Decant the supernatant liquid from the BaS04 precipitate ( - 5 mg Sod2-) into the funnel for filtration through the capsule. Add about 50 mg spectroscopic-grade graphite powder to the residual BaS04 precipitate and transfer the BaS04 and graphite powder (using hot water from a wash bottle) to the funnel for collection in the capsule. Wash the mixture of BaS04 and graphite powder with hot water until negligible chloride is detected in the filtrate by the silver nitrate test. Wash the funnel and upper edges of the capsuie 5-6 times with acetone. Remove the capsule, close it with a threaded graphite plug, and dry a t 110 "C. Preparation of C O P .The apparatus and procedures for the conversion of oxygen in barium sulfate, sulfur dioxide, air, and water to C 0 2 for mass spectrometric analysis are described in the following paragraphs. Barium Sulfate. The apparatus used for converting the oxygen in the BaS04 to COSis shown in Figure 2. The main components, connected in a loop arrangement, are the graphite furnace, a high-voltage discharge tube, cold traps, a Toepler pump, capillary manometers, gas sample bulbs, and a thermocouple gauge. The Toepler pump is used to circulate noncondensible gases through the loop, as described later. Vacuum ( - 5 X lo4 Torr) is provided
(through a liquid nitrogen cold trap) by a mechanical rotary pump. Purified tank helium is provided for flushing the loop during sample introduction. A detail of the crucible assembly in the water-cooled graphite furnace is shown in Figure 3. The inductively heated graphite crucible is supported upon the fused quartz holder by a short segment of platinum tubing. The CO and C 0 2 produced by the reaction of BaSOl with hot graphite diffuses through the porous walls of the graphite capsule into the analytical loop. Induction heating is supplied through a low-voltage, high-current, 4-turn work coil, 3 cm long. The procedure of operation follows. Outgas all the components of the C02-conversionapparatus. Arrange the stopcocks to allow introduction of a sample-loaded graphite capsule into the graphite crucible, with dry helium flowing through the loop and out the top of the furnace. Close the furnace a t the T 34/45 joint and evacuate; outgas the sample and capsule by heating the crucible to dull redness (-575 " C ) for 10 min. With the loop evacuated and isolated from the vacuum manifold, close stopcocks 7 and 13, and connect the graphite furnace to the high-voltage discharge tube. Immerse cold trap B in liquid nitrogen a t a level below the platinum electrodes; apply high voltage (-1 kV) to the electrodes; heat the crucible to -1150 "C for 20 min. Isolate the high-voltage discharge tube at stopcock 6; turn off the voltage; remove the liquid nitrogen coolant and heat the glass tube with a flame to desorb CO from the finely divided carbon deposits on the inner walls; recondense the C 0 2 with liquid nitrogen and reapply the high voltage briefly to disproportionate the desorbed CO. Discharge the negligible amounts of residual noncondensible gas from the loop to vacuum, and cryogenically transfer the COPfrom cold trap B through cold trap A (at the temperature of melting pentane, --132 OC) to the sidearm cold trap on capillary manometer I1 (at liquid nitrogen temperature). Close the manometer stopcock and warm the manometer cold trap to room temperature; read the pressure change on the precalibrated manometer and convert the reading to rmol C02. (Measurement of the C 0 2 provides an estimation of the amount of sulfate or SO2 in the sample.) Cryogenically transfer the C 0 2to the sample bulb (b) for transmittal to the mass spectrometer. Remove the discharge tube from stopcock 6 a t the ball joints. Remove the electrode holder a t the 3 34/28 joint (Figure 4) and heat the platinum electrodes to redness with a hand torch to burn off carbon deposits. Reassemble the unit; evacuate and outgas by heating to 100 "C. (Use a grease such as Apiezon-H on joints and stopcocks that are heated for outgassing.) Sulfur Dioxide a n d Oxygen. The vacuum loop (Figure 2) is also used for converting oxygen in sulfur dioxide and tank oxygen to C 0 2 for " 0 analysis. The procedure follows. Use the Toepler pump to transfer -2 mL (STP) of the gas through stopcock 3, into capillary manometer I for measurement. Isolate the loop a t stopcock 4. Heat the empty graphite crucible to -1200 "C and, with the stopcock in the loop appropriately arranged, use the Toepler pump to circulate the gas through the loop for 20 min, quantitatively converting the oxygen of the sample to CO. Collect and measure the CO in manometer I to verify that its quantity is two times that of the starting gas (SO2 or 02). Circulate the CO through the high voltage discharge tube,
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Figure 4. High voltage discharge tube
converting it to C 0 2 as in the procedure for BaSO?. Measure the COS. (Its quantity should equal that of the original SO2or 02.) Transfer to a sample bulb for subsequent mass spectrometric analysis. Oxygen in Air. The procedure described for sulfur dioxide and pure oxygen is not applicable to oxygen in air because of the presence of nitrogen which forms nitrogen oxides in the highvoltage discharge. To convert air oxygen to COP,cover the graphite crucible with platinum gauze and operate at -600 "C (11). (Under these conditions, the oxygen is converted directly to C 0 2 with negligible formation of CO.) Collect the COPin cold trap B at -196 "C with no voltage applied to the electrodes. Transfer the C 0 2to manometer I1 for measurement and then to a sample bulb. Water. The oxygen in water is converted to C 0 2by reaction of the vaporized sample with the hot graphite crucible (Figure 3) to produce H2 and CO. The H2 is discarded through a heated palladium membrane to the vacuum manifold and the CO is converted to COz. The graphite-furnace unit, modified for water analysis, is shown in Figure 5 . A palladium-tube assembly is added a t the 34/45 joint and connected to the main loop through stopcock 16. The cooling water for the quartz chamber of the graphite furnace is confined to a closed system so that it is maintained at boiling-water temperature during furnace operation. (Apiezon H stopcock grease, or equivalent, is used in the heated standard-taper joints.) The palladium tube is heated by a heater tape maintained a t 475-490 "C on the exterior walls of the palladium-valve unit. The stopcock assembly (stopcocks 14 and 15) is also maintained a t -110 "C by a heater tape. The only change necessary for reconverting the apparatus to BaSO, analysis, after a water analysis, is to substitute the closed-end 34/45 joint (Figure 2) for the palladium-tube unit at the top of the furnace. After thoroughly outgassing the apparatus, manipulate the stopcocks t o allow dry helium to flow through the loop and out of the water sampler. Deposit a 2-pL sample of water from a micropipet into the lower end of the sampler tube. Emplace the 12/18 cap, close stopcock 3, and rotate stopcock 4 one-quarter turn. Immerse both the sampling tube and the adjacent U-tube in liquid nitrogen; evacuate the entire system through stopcock 17 to -5 X 10 Torr. Rotate stopcock 14 one-quarter turn to isolate the graphite-furnace loop from the main loop. Transfer the water sample cryogenically from the sampler to the chilled surface of the liquid nitrogen reservoir on the palladium tube unit and rotate stopcock 15 one-half turn. With the palladium tube connected to the vacuum system through stopcock 16, energize the induction heating coil to heat the graphite crucible to 1300 "C for 20 min. (The water vapor, confined to the heated furnace loop (1100 "C) is converted to H2and CO; the H2diffuses
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through the palladium wall a t 490 "C and is discarded through the vacuum manifold.) Rotate stopcock 17 one-half turn to isolate the evacuated main loop. Rotate stockcock 16 one-half turn, and close stopcocks 7 and 13. Immerse cold trap B in liquid nitrogen, as in the procedure for BaSO, analysis, and turn on the high voltage. Slowly rotate stopcock 14 to allow the CO to flow into the high voltage discharge unit. Carry out the conversion of CO to C 0 2 and subsequent operations as in the BaS04 procedure.
RESULTS AND DISCUSSION Isotopic results for several field samples and for components of sulfation reactions in laboratory experiments, using t h e single graphite furnace and associated procedures, are reported elsewhere (1-3). These results, however, are not necessarily useful in establishing t h e reliability of the methods. Since reliability depends upon the adequate control of experimental errors which may result from isotopic fractionation accompanying incomplete recoveries (
