per 40 monomer units (IO). Thus, the proteins move more and more slowly until they reach a certain relationship to the polymerized chains of the gel such that their further movement is linear with time at constant voltage. Therefore, “pore-limit” can be defined as the distance migrated from the original starting point in a specific gradient after which further migration occurs at a rate directly proportional to time (at a specific voltage). It also follows that when all the components in a mixture have reached their respective “porelimit” the time of migration becomes unimportant-oiz., further migration will not change their positions relative to each other. Furthermore, if two proteins of known molecular weight are included in the mixture as standards, then after the “pore-limit” has been reached, a simple log-log plot can be used to determine the molecular weight of each of the other components in the mixture. Obviously there are limitations to the determination of molecular weight by the method described here. First, a protein must carry enough net charge to move it at an initial rate which is greater than the rate it would have at its “porelimit.” It is also possible that very long and narrow proteins will migrate at slower rates at the “pore-limit” than globular proteins of equivalent weight; however, this remains to be determined. Also, the possibilities of association and disassociation are probably greater because the protein mixtures are in contact with the buffer for longer periods of time
than is usual in single concentration gels. However, this method has the advantage that weakly joined associations of molecules of different size may be separated, and the molecular size of the molecules comprising the association may be quickly determined. The experiments reported here were made using alkaline buffers ; however, gradients have been prepared with acid buffers as well, although these have not yet been studied extensively (11). The present method of separation and estimation of molecular size should have considerable value in studies on the association and dissociation of specific proteins. Furthermore, since considerable data are already available on migration rates in starch, agar, and polyacrylamide gel of one concentration, when these supports are combined with a gradient in a two-dimensional system, two different kinds of data will be available on a two-dimensional gel slab-Le., mobility and molecular size-or when isoelectric separation is used both isoelectric point and molecular size can be obtained. This ability to have simultaneously two means of identification for a substance plus the high resolution the system provides, should make these new two-dimensional systems useful tools for protein studies. RECEIVED for review February 28, 1969. Accepted May 2, 1969. Part of this work was supported by Grant No. NB3556 from the USPHS. (11) G. G. Slater, Neurobiochemistry Laboratory, Veterans Ad-
(10) M. L. White,J. Pl7ys. Chem., 64,1563 (1960).
ministration Center, Los Angeles, Calif., unpublished data.
An Oxyacidic Flux Method for Determination of Carbon in Sodium C. C. Miles Argonne National Laboratory, Idaho Facilities, P.O.Box 2528, Idaho Falls, Idaho
A novel combustion method is described for the determination of carbon in sodium with a useful range of 0-1000 ppm and a precision of i5% at carbon levels of 30 rg. Sodium is burned in an oxygen atmosphere inside a narrow-mouthed bottle containing potassium dichromate flux. Oxides of carbon are passed over platinum catalyst and cupric oxide for conversion to carbon dioxide which is cold trapped and determined chromatographically. During combustion, inrush of oxygen through the narrow bottle mouth prevents significant ejection of contents. All the sodium oxide is incorporated in the flux melt which forms at 398 O C . As the temperature i n the reaction bottle is raised, oxidation of all carbon occurs; carbon dioxide is displaced from the sodium oxide by the acidic flux; and finally the flux decomposes, producing oxygen which sweeps the oxides of carbon from the melt and out of the bottle. This method eliminates errors of carbon dioxide gettering, incomplete combustion, and high blanks. THEWORK described in this report was undertaken after the “round-robin” tests reported in 1964 ( I ) . The scattered results of the tests using available methods indicated a need for sampling, handling, and analytical techniques that eliminated errors found by other workers. A study of previous methods (2-11) revealed several areas where a different approach would eliminate problems. Broadly outlined, these areas are as follows: sampling and handling methods requiring minimum environmental contact;
reduction of the blank to very low values by the use of carbonfree or low-carbon materials in the apparatus; a combustion method which ensures that the sodium oxide produced cannot retain carbon combustion products and that all the carbon converts to CO,; and a method capable of reproducible measurements of the COa. With these four general requirements in mind, the method described was developed and tested. ~
~~~
(1) Results of the USAEC Round Robin Analysis for Carbon Impurity in Sodium, GEAP-4602, March 1964. (2) V. M. Sinclair, J. L. Drummond, and A. W. Smith, TRG Report
11850, February 1966. (3) M. S. A. Research Corporation, Third Quarterly Progress
Report, Contract AT(ll-1)-765,December 1965.
(4) W. W. Sabol and R. W. Lockhart, AEC Research and De-
velopment Report, General Electric Contract AT(04-3)-189, September 1963. ( 5 ) AEC Research and Development Report, Pratt and Whitney Aircraft Contract AT (30-1)-2789,April 1965. (6) D. D. Van Slyke. ANAL.CHEM., 26. 1706 (1954). ( 7 ) G. I. Robertson,. L. M. Mett, and’L. Dorfrnan, ANAL.CHEM., 30. 132 (1958). (8) K.Y . Eng, R. A. Meyer, and C. D. Bingham, ibid., 36, 1832 (1964). (9) R. E. Fryxell, ibid., 30, 273 (1958). (10) R. D. Bardner and W. H. Ashley, AEC Report LA-3035, Los Alamos, N. M., March 1964. (11) H. Bradley and S. A. Meacham, AEC Report APDA-164, Detroit, Mich., November 1964. VOL. 41, NO. 8,JULY 1969
1041
FLOWMETERS
m
\
1 4 IN.COPPER TUBE JOINT 12/5
'1 1
VYCOR SPHER. JOINT l2/5
s
\QUARTZ WOOL
15CM
20CM
SLIDER VALVES
QUARTZ
DEWAR
32 MM O D QUART TUBE CHROMATOGRAPH
30 CM
JOINT 35/20
-
----.-A
IO CM
PYREX SPHER. JOINT 35/20
TO WASTE
Figure 2. Gas purification tube ENTRANCE LOCK
Figure 1. Block diagram of analytical system
The salient feature of the method is the confined combustion of the sodium in an acidic oxidizing flux contained in a narrow-mouthed reaction bottle. This feature prevents sodium oxide from depositing downstream of the reaction zone, prevents sodium oxide gettering of carbon dioxide, and ensures combustion of the sample and impurities through intimate contact with the molten flux. The result is quantitative oxidation of all susceptible elements present in the sodium sample. The oxidation is followed by complete release of gaseous combustion products owing to spontaneous high-temperature decomposition of the excess flux which provides oxygen that sweeps out both the flux-melt and the bottle. DESCRIPTION AND PREPARATION OF APPARATUS
The analytical equipment consists of the following six subsystems whose relationship is shown in the block diagram of Figure 1: (a) gas-supply and purification train, (b) sample handling system, (c) sample combustion train, (d) COS trapping system, (e) COZ measurement system, (f) train-gas composition and volume measurement system. Gas-Suppl y and Purification Train. The gas-supply and purification train system consists of gas bottles delivering helium and oxygen at a pressure of 10 psig through valvecontrolled flowmeters to a quartz tube containing oxidizers and absorbents. Both gases are purified by passing in suc10 OC, Ascarite, and BaO. cession over CuO heated to 600 To achieve the high-purity oxygen and helium necessary for a low blank, extreme care must be exercised in the preparation of the purification train; hence, a detailed description of this train is given even though the purification method is common for less stringent analytical methods. A free-drawn quartz tube (Figure 2) with standard Vycor or quartz joints is cleaned with detergents, rinsed, and then etched inside for 5 min at room temperature with reagentgrade 48 HF. (Quartz tubing formed on carbon mandrels has close-tolerance dimensions but gives higher blanks than
*
z
1042
ANALYTICAL CHEMISTRY
free-drawn tubing.) Care is taken to prevent etching of the standard ground joints. The HF is rinsed out with distilled water followed by reagent-grade acetone. The tube is then heated to red heat by using an oxyhydrogen (not oxygenhydrocarbon) flame applied to the outside. The tube is air cooled and thereafter extreme care is exercised to prevent introduction of organic vapors or particulates. A small piece of quartz wool, preignited in air at 900-1000 "C for 16 hr, is placed inside the large tube at the outlet end. The wool is packed by using a specially cleaned metal ram, which should be scoured with abrasive household cleanser followed by a water rinse, an acetone rinse, and gentle heating in an oxyhydrogen flame (not oxygen-hydrocarbon). The tube is clamped vertically, and sufficient BaO (10-20 mesh dehydrating agent, G. Frederick Smith Chemical Co., 867 McKinley Ave., Columbus, Ohio) is poured into the tube to give the bed length shown in the drawing. Only cleaned metal or glass funnels may be used for this operation. Lightly tapping the quartz tube with the fingers packs the BaO sufficiently to prevent channeling of the gas flow. If the BaO is packed too tightly, the tube may rupture after a few months of use. BaO is converted to BaC03 and Ba(OH)Z, both of which have a higher specific volume than BaO. Next, enough Ascarite (8-20 mesh COZabsorbent, Arthur H. Thomas Co.) is poured onto the BaO to give the bed length shown in the drawing. This bed is also packed by light tapping. A 1-in. barrier of preignited quartz wool is then packed onto the Ascarite. Next, pure copper (light turnings, reagent grade, J. T. Baker Chemical Co.) is packed into the tube by using the ram, clean tongs, and clean scissors. Commercial preparations of CuO cannot be used because they are much too high in carbon. After careful packing of the tube, the section containing the copper turnings is placed in a tube furnace and heated to 600 f 10 "C. A 50z oxygen-helium mixture is passed through the tube at 80-100 cc/min for 16-24 hr. After the CuO bed appears entirely black, the helium flow is stopped and pure oxygen is passed over the CuO for 8 hr more. Pure oxygen should not be used during the initial oxidation of the copper because overheating occurs, which causes the copper oxide and copper to melt into pools which quickly react with and destroy the quartz tube. The outlet of the purification train is connected to a flexible '/*-in. copper tube by either a graded-glass to metal seal or a
r”:D””
GAS INLET\ M
I
18MM I D
22MM O D
QUARTZ FRIT-=,+
4-
QUARTZ TUBE 32 MM O D
II
I
22CM
-I44
Pt on Si00
VYCOR. SPHERICAL JOINT 12/5
Figure 3. Combustion tube standard ground joint sealed with cupric phosphate cement. (The cement is made by triturating CuO powder in 8 5 % phosphoric acid for about 10 min to give a creamy black paste. Setting time is 24 hr.) The copper tube passes through the wall of the glovebox used for sample handling. Sample Handling System. The sample handling system consists of an inert-atmosphere glovebox with a vacuum lock and contains the following items: a balance with a doublewedge-shaped pan adapter; a sodium extrusion press; a supply of reagent-grade K2Cr207; a glass “powder horn” for loading the K2Cr207into the reaction bottle; a supply of pre-etched reaction bottles and shield-tubes in a clean metal or glass tray; a pair of tongs, tweezers, and a long, hooked rod to insert and remove reaction bottles; wedge-shaped storage racks for the handling tools; sodium samples in quartz vials or metal extrusion vessels; and a jar for waste. Carbon dioxide and water vapor are removed from the glovebox atmosphere by a single pass over Ascarite and magnesium perchlorate as the gas flows from the supply tanks to the glovebox (12). Volatile organic materials must be rigorously excluded from the glovebox atmosphere. An acceptable glovebox window cement is 3M Scotch Seal from Minnesota Mining and Manufacturing Co. Other preparations may contain organics which slowly volatilize from the cement for weeks after it has set up. Some organic materials cannot be avoided inside a glovebox; however, if the organics do not move about as particles or vapor, they are acceptable. Paper or cloth inside the glovebox must be used with care to avoid stirring up fine fibers which are readily attracted to reaction-bottle surfaces by electrostatic forces. The double-wedge-shaped pan adapter is used to prevent the reaction bottle from rolling and to prevent gross contact with the balance pan during weighing. The adapter is a widebase piece of stainless steel or aluminum constructed with two V-shaped notches in alignment about 2 in. apart. The bottle is placed horizontally in the notches for weighing operations. Reagent-grade K2Cr207contains from 5 to 15 ppm of carbon which is readily removed by an hour of ignition at 750 “C. (12) G. Goldberg, AEC Report ORNL-TM-1357, Oak Ridge, Tenn., December 1965.
X 3MM
MM OD
Figure 4. Reaction bottle, shield-tube
The K2Cr207is poured into a powder-horn loader for ease, neatness, and simplicity in loading the reaction bottle. This direct method of transfer gives no opportunity for additional carbon contamination of the flux charge. The tongs, tweezers, hooked rod, and any other handling tools must be cleaned with appropriate reagents and ignited in a hydrogen-oxygen flame until the metal is free of surface carbon. A light oxide discoloration is proof of sufficient ignition. The double-wedge storage racks for the handling tools are necessary to prevent surface contamination with organic materials. The handling tools should never be touched by anything at the ends which contact reaction bottles or sodium, and they should always be stored on the racks. The glovebox is integral with the purification and combustion trains. The line from the purification system enters through the wall of the glovebox and is attached to the entrance port of the combustion tube. The combustion-tube entry into the glovebox is made gas tight with a flat flexible silicone-rubber collar fitting tightly around the cool part of the combustion tube near the entrance port. A metal compression fitting holds this silicone-rubber collar to the metal glovebox wall. Sample Combustion Train. The sample combustion train (Figure 3) consists of a free-drawn quartz tube with an allmetal entrance port and a spherical ground joint at the exit. Within the quartz tube is a removable Vycor shield-tube containing a quartz reaction bottle (Figure 4) located in the reaction zone; next is a bed of platinized silica, followed by a final oxidizing bed of CuO. The port and port cover are made of Type 304 stainless steel. The carbon-free gastight seal is made of a wide, soft lead gasket soldered into and completely filling a machined groove in the port cover. A 45” knife-edge is machined into the port flange so that the knife-edge is concentric with the opening to the combustion tube. The port cover is held against the knife-edge on the port by four spring-loaded bolts, all tightened to the same tension by displacement limiting stops. This closure method prevents overtightening or undertightening, and extends the life of the lead gasket to several months of use. Organic sealing gaskets, greases, or cements must not be used in or between the purification and combustion tubes though they may be used elsewhere. The gas line enters the combustion tube through the entrance-port flange and is designed to flush the annular space between the port cover and the inside of the port. Within the quartz tube located in the reaction zone is a removable Vycor shield-tube containing a quartz reaction bottle (Figure 4). The shield-tube is necessary because the quartz reaction bottle may break, flooding the vicinity with molten K2C1-20,. If the molten K~Cr207touches the combustion-tube wall, the quartz at that point is seriously weakVOL. 41, NO. 8, JULY 1969
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~4 MM GLASS TUBE INLET
112 [
SILICONE "0" RINGSALIGNMENT C
IN. MOUNTING ROD
1/8 IN. COPPER /-TUBE OUTLET
THERMOCOUPLE^^ %TO
BRASS JAM NU 314 IN. 0 D BRASS BOD
VARIAC 6' IN.
Il 1
FINE GLASS WOOL COPPER-CONSTANTAN THERMOCOUPLE
Figure 6. COStrap, internal detail Figure 5. COz trap, external detail ened and cracks to destruction upon cooling. Vycor is adequate for the shield-tube because it must be thrown away in any event if flux touches it during a run. Quartz is used for the reaction bottles because Vycor was found to have a breakage rate nearly ten times that of quartz. About 1 in 15 quartz reaction bottles breaks and allows flux to flood the shield-tube. The combustion tube with its beds of platinized silica and cupric oxide must be prepared with the same extreme care taken in the preparation of the purification system. The quartz tube with the metal entrance port attached is etched for 5 min at room temperature with 48% HF. The H F is not allowed to touch the metal, the graded-glass seal or the ground surface of the outlet. The H F is rinsed out with distilled water, followed by reagent-grade acetone. The tube is then clamped vertically and the quartz portion heated to red heat; the metal port and graded-metal to quartz seal should only be warmed very gently. The tube is cooled in air. Sufficient copper turnings (the same material used in the purification train) are packed into the tube to give the bed length shown in Figure 3. The platinized silica is made by mixing 150 g of 10-30 mesh silica gel (preignited in air for 16 hr at 1000 "C) with 28 g of chloroplatinic acid in 100 ml of distilled water. The mixture is dried with occasional stirring to break up lumps and is then ignited in air for another 16-hr period at 1000 "C. Enough of the platinized silica is then poured into the tube and packed lightly by tapping to give the bed length shown in Figure 3. A preignited quartz frit is then placed on the platinized-silica bed and set in place by dimpling the combustion tube. The use of a platinized-silica bed is based on the fact that this substance catalyzes the reaction between carbon and traces of oxygen (13) and hence may also catalyze the reaction between oxygen and traces of carbon. The necessity for the catalyst has not been established for the method reported here. Since the method works very well, the oxidizing efficiency of the individual beds in the combustion train has not been determined. After the combustion tube is prepared, it is positioned in the glovebox and split tube-furnaces. The copper is converted to copper oxide as described for the purification train. (13) I. J. Oita and H. S. Conway, ANAL.CHEM., 26,600 (1954). 1044
ANALYTICAL CHEMISTRY
The reaction-zone furnace is a 5-in. "Hevi-duty" multiple unit rewired with Nichrome to give 750 W. All other tube furnaces are standard. The rewired furnace burns out in about three months. This disadvantage is compensated by the fact that a standard furnace has such a slow heat-up rate that only one carbon analysis per day can be made, whereas with the rewired furnace, two per day can be made easily. COZTrapping System. The COZtrapping system consists of a small-volume trap (Figures 5 and 6) with a temperaturecontrol arrangement and a 0-100 cc/min flow indicator to monitor the entering gases. The flow indicator shows whether the trap is functioning or plugged, and, when compared to the flow indicators on the purification train, shows the presence of major leaks in the train system, such as may occur through improper closure of the combustion-tube port or through a leaky fitting. The trap is designed to trap C 0 2 quantitatively from a pure oxygen stream without simultaneously condensing the oxygen to liquid at the pressure in the train (640 f 20 mm at this altitude). To accomplish this, the temperature of the trap is maintained at -172 "C. The temperature is indicated by a copper-constantan thermocouple silver-soldered into a well at the bottom of the trap. The temperature of the trap is controlled by the heater and by the use of liquid argon (liquid nitrogen may be used) in a Dewar cup surrounding the lower part of the trap. The boiling temperature of liquid argon is -185.8 "C, but enough heat is added to the trap by conduction through the mounting and by the helium flowing up inside the heat barrier to keep the temperature at -172 "C. The heat barrier prevents the inside trap temperature from dropping to -185.8 "C. The helium purge (at 200 cc/min) is used to prevent moisture from condensing in and shorting out the heater coils. Shorting occurs readily if an unpurged heat barrier is used. For economy the chromatograph exhaust furnishes the purging helium. Under these conditions and with no heater current, the temperature distribution inside the trap is: top, - 108 "C; center, -153 "C; bottom, -172 "C. The heater is made of No. 30 Nichrome wire insulated with fiberglas sleeving and wound in two layers around the body of the trap. The resulting resistance is -95 Q. Eighty volts applied for 30 sec flashes the COZinto the chromatograph, giving a sharply peaked chromatogram. To avoid constant attendance to control the liquid-argon level in the Dewar cup, a large-capacity Dewar flask may be used to feed the smaller vessel. Control is easily maintained
fim I
I
Y
YlQ
f
TO WASTE
/-PNEUMATIC SLID: YALVE
COMBUSTION--L,
HELIUM CARRIER C A C
4
llcoLfl )I FROM SLIDER VALVE NO. I
Figure 8. Train-gas composition and volume measurement system
FLOWMETER
I*
The wet-test meter measures the volume of gas which passes through the train during a run and enables an accurate blank to be computed for a run if the previously measured blank is in terms of micrograms of carbon per liter of oxygen. ANALYTICAL PROCEDURES
P
STD. CO2 SUPPLY
\-PNEUMATIC
SLIDER VALVE NO 2
Figure 7. CO, measurement system by using a pressure feed, a solenoid valve for pressure bypass, and a float-microswitch control for the solenoid valve. C o n Measurement System. The COZmeasurement system consists of a chromatograph equipped with pneumatically operated slider valves. The arrangement (Figure 7) allows the COS trap to be switched quickly from the combustion train to the chromatograph. The arrangement also allows calibration of the chromatograph by using the calibrated loop to inject a definite volume of standard gas into the chromatograph, by using the loop to inject one or more volumes of standard gas into the trap from which the trapped C o n may be subsequently flashed into the chromatograph, or by insertion of a standard volume between the end of the combustion tube and the inlet to slider valve No. 1. The chromatograph uses a 12 ft X in. 0.d. aluminum tube packed with 30-60 mesh silica gel maintained at 145 "C, helium carrier gas flowing at 100 ccimin, a standard twofilament thermal conductivity detector, and a Speedomax H recorder (-0.1 to +1.0 mV, 0.025% repeatability). In conjunction with the COS trap, this arrangement has demonstrated a sensitivity of 0.12 hg of C as COS. Train Gas Composition and Volume Measurement. The train gas composition and volume measurement system (Figure 8) consists of a Beckman Model 778 polarographic oxygen sensor in a glass adapter; a COSand water absorption tube containing anhydrous MgC104 followed by Ascarite; and a Parkinson-Cowan wet-test meter, 0.1 % precision. The oxygen analyzer monitors the train-gas composition which must be known before combustion can be started, before the combustion-tube port can be opened, before the CO, trap can be switched from the combustion system to the chromatograph, and during oxidation of the copper oxide beds. The absorption tube prevents moisture and atmospheric CO, from being drawn into the CO, trap. During the burning of the sodium sample, the consumption of O2 is so rapid that the internal train-pressure drops below atmospheric pressure, causing backflow of gas downstream of the reaction zone.
The analytical procedures involve the following steps : (a) chromatograph calibration, (b) blank determinations, (c) preignition of reaction bottle and flux, (d) sample preparation and addition, (e) combustion and COZtrapping, and (f) COz measurement. Carbon recovery tests involving the listed steps were made as proof of method. Chromatograph Calibration. A glass calibration vessel is filled with standard gas (0.25% COn in He, Matheson Co., Inc.) at known temperature and pressure. The vessel is placed in the train between the combustion tube exit and slide valve No. 1 (point S in Figure 7). The COZ trap is cooled to approximately -172 "C, and the standard gas is passed through the trap by using helium at 100 cc/min. After trapping, the COztrap is heated by removing the coolant and turning on the heater for 30 sec. The heater is then turned off to avoid burnout, and the COS chromatogram is obtained. Repetitions of this procedure, accompanied by chromatograph-attenuator adjustments, are made until the desired calibration is achieved. To check the efficiency of the COS trap, especially for temperature adjustment, the standard loop is used, since it will give a chromatogram similar in shape to the peak obtained when C o n is flashed from the trap. Thus, a peak is first obtained from a loop-volume of standard gas flushed directly into the chromatograph. Then a peak is obtained by trapping the C o n from the same loop volume and flashing this into the chromatograph. Trap temperature is adjusted until a COZpeak from the loop matches a CO? peak from the trap. Blank Determination. The train is purged with helium at 100 cc/min. The tube furnaces are adjusted to the following temperatures : both copper oxide heating furnaces, 600 10 "C; platinized-silica heating furnace, 950 + 50 " C ; reaction zone furnace, as required. Shield-tubes and reaction bottles are etched by immersion in 48% H F for 5 min at room temperature. The H F is rinsed off with distilled water followed by reagent-grade acetone. The tubes and bottles are dried for 15 min at 120 "C. A shield-tube containing a reaction bottle is placed in the reaction zone. The combustion-tube port is closed and the reaction-zone furnace is set at maximum-Le., 980 + 10 "C. The helium flow is replaced by oxygen flow at 100 cc/min to waste through pneumatic slider valve No. 1. After ignition for 1 hr at 980 =t10 "C,the oxygen flow is replaced by helium flow at 100 cc/min and the reaction-zone furnace is turned off and opened to cool down. As soon as the furnace has cooled to 5 150 "C and the oxygen analyzer indicates 5 0 . 5 % 0 2 ,
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VOL. 41, NO. 8, JULY 1969
1045
the reaction bottle is removed, leaving the shield-tube undisturbed. The reaction bottle is placed on the double-V pan adapter on the balance pan and allowed to reach temperature equilibrium. The bottle is weighed to the nearest 5 mg, then 4 3 g of K2Cr2O7is poured into the bottle mouth from the powder horn and the bottle with K2Cr2O7 is reweighed. A minimum effective ratio by weight of flux to sodium has not been determined. However, in all tests providing the data of this report, the ratio of flux to sodium was never less than 8:l. The bottle with flux is replaced in the reaction zone and heated in oxygen for 1 hr at 750 f 10 "C. After the 1-hr cleanup ignition, the gas flow is diverted from waste to the COStrap through valve No. 1, the trap temperature is lowered to -172 "C,and the wet-test meter is read. The reactionzone temperature is then raised to 980 f 10 "C and heating is continued for 1 hr while trapping the COBfrom the oxygen stream. After an hour the oxygen flow is replaced by helium and the reaction zone is cooled to $150 "C. The wet-test meter is read when the train-gas composition is 0.5% oxygen and the trap is switched to the chromatograph. The amount of COZobtained from the run is measured and the blank is expressed as pg C/liter 02. Since the oxygen concentration increases and decreases at the beginning and end of a run, respectively, the percentage of Oz at which the analyst reads the wet-test meter to get oxygen volume is strictly arbitrary as long as the same concentration is always used. However, to avoid burning up a filament in the chromatograph detector or changing the calibration by partial oxidation, the trap is not switched to the chromatograph until the train-gas OZ concentration is j0.5 %. Likewise, the combustion-tube port should never be opened if the train-gas composition is above 1 oxygen to avoid oxygen contamination of the glovebox atmosphere. The flux must be cooled below 500 "C before liberation of oxygen stops or the train-gas composition will remain above 1 Oz at a helium flow of 100 cc/min for an unacceptable period of time. To save time it is recommended that the reaction-zone furnace be cooled with a fan. Preignition of Reaction Bottle and Flux. This step is described in Procedure (b). For an analytical run, the reaction bottle and flux are cooled to glovebox temperature before reweighing and sodium sample addition. Sample Preparation and Addition. Sodium samples, collected in preignited quartz vials of 1-g capacity or in extrusion vessels of 30-g capacity, are transferred to the inert-gas glovebox. The reaction bottle containing preignited flux is taken from the balance and a vial sample is dropped in, or a sodium slug is extruded and inserted into the bottle. If extrusion aliquoting is used, four or five very short sodium wafers are cut off and discarded to clean the cutoff knife and the handling tweezers. A sodium slug of the desired length (1 in. equals ~1 g at 0.27 in. 0.d.) is then extruded, cut off, and inserted into a reaction bottle. Both types of sample are placed as far as possible from the mouth of the bottle by holding the bottle vertically during insertion. The reaction bottle and contents are weighed to 5 mg and the sample is determined by difference. The reaction bottle is placed in the reaction zone so that it is centered with respect to the furnace, and then is turned to position the solid flux above the sodium sample. As the flux melts, it washes down the interior of the bottle to include all the NazO produced by the combustion of the sodium. If the solid flux is allowed to remain on the bottom, in contact with the sodium, two deleterious processes may occur. Sometimes the helium forms a gas pocket in the upper space of the bottle so that during combustion it expands and blows some flux and burning sodium out of the reaction bottle. Also, the burning sodium reacts sometimes quite vigorously with the flux and again material is blown from the reaction bottle. After the bottle is properly positioned, the port is closed and the train-gas flow is switched to the C 0 2 trap. 1046
ANALYTICAL CHEMISTRY
Combustion and C 0 2Trapping. The trap is cooled to - 172 "C and helium is replaced by oxygen at 100 cc/min. After the oxygen flow is started, an initial wet-test meter reading is obtained. As soon as the train gas is 95 % Oz, the reaction zone is heated to 200 "C. At some point during heat-up, the sodium will ignite and burn completely to Na20. If the train gas is less than 95% 0 2 , combustion will be delayed or be erratic, accompanied by ejection of sodium oxide smoke, molten flux, and bits of burning sodium. If this occurs, one should reject the sample and start over. If the sodium burns without significant ejection of material from the reaction bottle, the reaction zone is heated to 980 10 "C and maintained at this temperature until 8-10 liters of oxygen have passed through the train. The reaction-zone furnace is cooled down, oxygen is replaced by helium, and a wet-test meter reading is taken. When the O2concentration has dropped to S0.5%, the COZtrap is switched to the chromatograph. COZ Measurement. As soon as the chromatograph has stabilized with the trap in the flow path, the coolant is removed and the heater turned on for 30 sec. The heater is switched off after 30 sec, the chromatogram is obtained, and the C o n trap is switched back to the combustion-train flow path. Carbon recovery tests were made by the combustion of carbon-containing compounds by different methods of standard addition. The first approach with a compound was to determine whether or not the KzC1-207flux would completely oxidize it. The compound was then used as a standard with sodium present but not mixed with the standard. Finally, if recoveries were near loo%, some of the compounds were placed in intimate contact with the sodium in a quartz vial and were then taken through the analysis. Water-soluble organics, stable metal carbides, and elemental carbon were tried. The detailed procedure for these tests follows: Micropipets, spot plates, volumetric flasks, and any other apparatus used to contact standard solutions are boiled in a 1 :3 HN03-HC104 mixture. An all-glass distillation apparatus is set up without the use of any organic materials. A mixture of HN03-HC104 (1 :3) is distilled through the apparatus which is heated until fumes of HClOl pass copiously from the uncooled condenser. After 20 min of fuming, the apparatus is cooled, and the boiler only is rinsed. A solution of basic K M n 0 4 (1.ON NaOH, 0.25N KMn04) is added to the prepared still-boiler, a few hundred milliliters of water are distilled to waste, and carbon-free water is then distilled directly into the volumetric flask used to prepare the water-soluble standards. After completion of Procedure (c), the water-soluble organic standard is micropipetted into the bottom of the reaction bottle, as far as possible from the mouth. The micropipet is rinsed into the reaction bottle which is then placed in the reaction zone of the combustion tube. The outlet of the combustion tube is disconnected from slider valve No. 1, the helium flow adjusted to 100 cc/min, and the reaction-zone temperature controlled to 85 =t 10 "C. Evaporation is usually complete in 3 hr, as indicated by lack of condensation inside the outlet tube. The CO2 trap becomes plugged during the combustion step if evaporation is not complete. After evaporation, the combustion tube is reconnected to the chromatograph, and the combustion step is carried out as described in Procedure (e) but without sodium present. To carry out a similar recovery test with sodium present, a sodium sample is placed in the reaction bottle as described in Procedure (e). Recovery tests with sodium in intimate contact with the standard are carried out in quartz vials. Small quartz vials (1 in, long X 0.26 in. o.d.), having a capacity of -0.8 g of Na, are etched in 4 8 x H F until their weights average 0.5 g. They are then rinsed and ignited in air for 16 hr at -1000 "C. The vials are weighed, and standards are added by solution transfer, or by direct weighing of solids with a Cahn Electrobalance. The solids, either from evaporation or direct
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Table I. Summary of Some Carbon Values on EBR-I1 Sodium by Oxyacidic Flux Method Primary sodium Secondary sodium Sample No. of Carbon, Sample No. of Carbon, date aliquots ppm date aliquots ppm 8-24-67 9-20-67 9-21-67 4-30-68 6-26-68
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2 4 6 8 IO 12 1 4 1 6 MICROGRAMS OF CARBON FOUND
Figure 9. Extrapolated blank for carbon in sodium
weighing, are confined to the bottom tenth of the vial. With the use of specially designed apparatus, liquid sodium a t -280 "C is poured into the vial to completely cover the solid standard. The prepared vial is then placed in a reaction bottle and the analysis performed as in Procedure (e). RESULTS AND DISCUSSION
Blank runs, made routinely once a week over a period of two years, have varied between 0 and 0.3 pg of C per liter of oxygen. Exhaustion of the gas purification train was arbitrarily determined as that point at which a blank of 0.3 pg C/liter of 0, was found. A properly prepared purification and combustion train always resulted in blanks of 0-0.1 pglliter. The blanks remained low until the BaO absorbent had partially broken up into a band of fines. The blank began to exceed the 0.1 pg/liter figure when the band of BaO fines approached 1 in. in length, measured from the junction with the Ascarite. At about 2 in., the blank was usually unacceptably high. Carbon recovery tests were necessary as final proof that the method gave valid results for carbon in sodium. However, a dilemma was presented by the lack of carbon-free sodium or sodium containing known carbon concentrations as well as nonavailability of a carbon analytical method which could be used for comparison purposes. The approach taken involved three steps. First, six runs were made on carbon standards without sodium present. With sucrose and potassium acid phthalate at a carbon level of 25-27 pg, a recovery of 99.4 i 2.3% was demonstrated. Second, four sodium samples were obtained from a large bulk source which was not expected to show short-term carbon-concentration changes. Twenty analyses were made on the four sodium samples, giving an average value of 14.7 =t1.6 ppm of carbon. Third, sodium from the large bulk source was again obtained, potassium acid phthalate standards were prepared, and recovery tests in the presence of sodium were performed using the 14.7 f 1.6 value. At the 150- and 160-pg level of carbon, seven runs gave a mean value of 99.2 f 1.7% recovery. Next, as proof that a zero blank could be obtained in the
12 5
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7.0 f 1 . 7 5.6 f 0.7 4 . 9 10 . 8 3 . 7 1. 0 . 6 4.8 & 1.8
11-8-67 11-16-67 12-4-67 12-12-67 2-1-68 6-4-68 7-22-68
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4.8 i0.5 3 . 5 10 . 6 4.8 f 3.5 4 . 0 10 . 7 3 . 4 1 0.6 4.41.0.5 3 . 3 1. 0 . 6
presence of sodium containing no carbon, a series of analytical runs was made on sodium with sample weights from 0.2 g to 1.2 g. The plotted data (Figure 9) gave an extrapolated blank of essentially zero within the limits of error. A basic weakness of the recovery tests performed by evaporating a solution directly on the flux inside the reaction bottle was that the most favorable condition possible was thereby provided for oxidation of the standard and escape of the COz. A more realistic approach for recovery tests would have been to add the standard to the molten sodium, blend for homogeneity, and aliquot the resulting solution. Although ideal in theory, this method of standard preparation could not be achieved because we did not have carbon-free sodium and because knowledge was lacking on the behavior of compounds in molten sodium. The approach taken was to use the quartz-vial method previously described. By isolating the standard between the vial wall and the sodium, the most unfavorable method practical was provided for oxidizing and recovering the carbon. With the vial technique, four recovery tests were made on benzene hexol standards. Benzene hexol was chosen because it is stable in sodium at moderate temperatures (14). In addition, four recovery tests were made with benzene hexol standards evaporated on the flux and oxidized in the presence of sodium. The carbon recovery from the vial tests was 98.4 13.7% at a carbon level of 40 pg. The carbon recovery from the tests in which the flux contacted the standards was 102 12,0y0 at a carbon level of 140 pg. Both sets of tests gave essentially 100% recovery and support the validity of recovery tests in which the standard is placed on the flux and oxidized in the presence of sodium. Carbon compounds containing single-bond carbon-nitrogen and double-bond carbon-nitrogen links were used as standards and oxidized on flux in the presence of sodium. Three tests using cyanuric acid at the 89-pg level gave a recovery of 98.1 i 3.47,, and three tests using ethylene diamine tetracetic acid at the 26-pg level gave a recovery of 107 f 3.6%. The latter high recovery reflects merely the difficulty in preparing an accurate standard at low carbon levels. Elemental carbon as pure graphite was tried by all three methods. In each case the recovery was as follows: On flux without the presence of sodium 96 f 3.1%; on flux in the presence of sodium 100 11.8%; in vials filled with sodium 99.6 i 1.3%. The level varied from 42 to 152 pg of carbon for a total of twelve tests. Zirconium carbide in three tests a t the 60-100 pg level gave 99.5 f 2.4y0 recovery.
(14) V. M. Sinclair, R. A. Davies, and J. L. Drummond, TRG Report 1381 (D), March 1967. VOL. 41, NO. 8, JULY 1969
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Boron carbide B4C and chromium carbide Cr3C2 gave very poor recoveries over a long ignition period. Neither compound gave up over 86% of its carbon. If carbon should be present in sodium in these forms, one should expect low results by this method; however, both carbides were prepared at over 1600 “C yielding a very refractory crystalline form unlikely to be found in any sodium system. The method described in this paper has been in use at EBR-I1 for two years. The precision which can be expected at the low carbon levels found in EBR-I1 sodium is shown in Table I which gives some typical data.
(EBR-11-ExperimentalBreeder Reactor No. 11, is operated by Argonne National Laboratory for the U. S. Atomic Energy Commission in Idaho.) ACKNOWLEDGMENT
The author thanks Mr. Elon Wood for his excellent support in the experimental work and Mr. E. R. Ebersole for many helpful suggestions in the preparation of this paper. RECEIVED for review March 24, 1969. 1969.
Accepted May 8,
The Palladium Transmodulator: A New Component for the Gas Chromatograph J. E. Lovelock,’ K. W. Charlton, and P. G . Simmonds Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grooe Drive, Pasadena, Gal$ 91103 This paper introduces a new component for the gas chromatograph, the “gas transmodulator”; this component functions by transferring the separated components from the column carrier gas to a second carrier gas which is chosen to provide optimum performance from the detector. The construction and use of a practical gas transmodulator is described. It consists of a palladium silver alloy tube, and with it a gain in sensitivity of at least 40-fold isdemonstrated in analyses using thermal conductivity and ionization cross section detectors. I n addition, the device enables these detectors to be used under conditions where the column carrier gas flow is changing-such as in analysis with flow programming-which are otherwise difficult to conduct without loss of accuracy or performance.
of this new component is to transfer the gas or vapor components, after their separation on the column, from the potentially variable flow of column carrier gas to a second carrier stream which is kept flowing at a rate which is both constant and chosen to be optimum for the performance of the detector in use. The use of two carrier gas streams each of which can be optimized for the needs of the column and detector, respectively, not only improves analytical accuracy; but also, in those circumstances where the column and detector flows are much mismatched, may provide a great increase in detectivity. One practical version of this new component comes from the proposal of Lucero and Haley (1) that a tube of palladium silver alloy would serve as a useful enrichment device between a gas chromatograph column and a mass spectrometer inlet. There follows a discussion of the theory of transmodulation in gas chromatography; the description of the development and performance of a practical transmodulator based on the palladium silver alloy tube and examples of its use in gas chromatographic analysis. Theoretical Aspects. With a conventional chromatograph incorporating a colligative property detector, both the peak area ( A ) and the peak height ( H ) corresponding to a given mass of test substance are functions of the carrier gas flow rate (F)as follows:
A PARADOX in gas chromatography is that those detectors such as the gas density balance, the thermal conductivity, and the ionization cross section, which in isolation are reliable and accurate, do not easily yield accurate analyses when incorporated as components in a gas chromatograph. The explanation of this inconsistency is to be found in the fact that the detectors mentioned give a signal related to the concentration of sample gas or vapor in the carrier gas; consequently, any variation in the rate of flow of carrier gas after the injection of a sample will be followed by a corresponding change in the sample concentration. The detector will faithfully follow these changes in sample concentration but the time integral of its signal will not be an accurate measure of the quantity injected. The conditions for accurate analysis with these otherwise excellent detectors are therefore limited to those in which the carrier flow rate can be maintained strictly constant. This means in practice that convenient procedures such as temperature programming of the column, which almost inevitably are accompanied by some change in column flow rate, attract a penalty in the form of a reduced accuracy of analysis. With these concentration sensitive detectors, flow programming and accurate analysis are essentially incompatible. This paper, concerned with a possible resolution of this paradox, introduces a new component between the column and the detector; the “gas transmodulator”. The function
where M and Z are the mass and molecular weight of the test substance, R, T , and P are the gas constant and ambient gas conditions of the detector, and K is the detector constant, respectively. The peak height is determined by a more complex function since the properties of the column are also involved; it is, nevertheless, inversely related also to the carrier gas flow rate, since the peak height is a direct function of the concentration of test substance entering the detector and this in turn is a function of the mass injected divided by the volume of gas in which it emerges from the column. Thus, with the conventional gas chromatograph both peak
Present address, Bowerchalke, Nr. Salisbury, Wilts, England
(1) D. P. Lucero and F. C . Haley,J. Gas Chromatogr.,6,477 (1968).
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ANALYTICAL CHEMISTRY
MRT A = 1/FXKPZ
