orders of magnitude greater than normal. H o w v c r , with the return of the sample flow, the instrument operation returned to normal. Precision and stability observations were made during the second phase of the testing, when the sensing unit was installed in the storage tank sampler system. Transfers into and out of the storage tank were infrequent and the concentration of the tank contents remained very constant a t about 400 grams of uranium per liter. During one 18-hour period m hile the instrument was automatically standardized every 15 minutes, 72 separate measurments of the uranium concentration were recorded. (A portion of this record is shown in Figure 6.) During this period the mean chart rcading 1w.s 50.63 divisions (100 divisions full scale), corresponding to 403 grams of uranium per liter, and the standard deviation was 1.03 divisions, corresponding to 3 grams of uranium per liter, or about 1% of the indicated uranium concentration. The calibration stability of the instrument is a function of the standardization system. Essentially it is the ability of the system to correct for conditions that tend to change the apparent intensity of the incident light over a prolonged period of time-for example, fouling of the viewing windows or tungsten condensation on the source lamp bulb. The calibration stability of the instrument was determined by comparing the calibration curve obtained
after a prolonged period of operation with its original calibration curve. Figure 7 shows the two calibration curves, the solid curve being the original calibration and the broken curve being a calibration obtained after about 5 months of steady oFeration. During the first calibration the instrument had clean viewing windows and an undarkened light source that operated a t about 6 volts t o give the required signal strength. Only a small amount of cell windoTv fouling was observed during the second calibration, but considerable light bulb darkening had occurred, necessitating operation a t about 8 volts t o give the required signal strength. (The source light bulbs are normally replaced when the operating voltage reaches approximately 7 volts, but in this instance it was allowed to continue operating to observe the shift in the calibration curve that might accompany the higher-than-normal bulb operating voltages and excessive bulb darkening.) The mavimum calibration drift under normal operating conditions can be espected to be less than 3% (Figure 7 ) . The temperature sensitivity of the instrument has not been investigated thoroughly, as all of the applications of the photometer have been on room temperature sample streams. However, one sensing unit (window spacing 0.075 inch) shoived a difference of about 4% between the calibration curves obtained at 18’ and 43’ C., throughout the range 0 to 140 grams of uranium per liter.
For a given instrument reading the 43” C. calibration curve indicated a lower uranium concmtration than the 18’ C. calibration. Wliile the calibration shift is reversible, the shift resulting from a n increase in sampli. tpmperature is completed in 30 minutes or so, but the shift resulting from a corres1 onding decrease in sample temperature requires several hours to complete. The addition of a xvater-cooling system to cool the phototube and the sample cell block reduced the calibration shift to less than 2% for sample temperature changes between 18’ and 43” C. LITERATURE CITED
(1) Pierce, C. E., Ind. Eng. Chent. 48,
KO.3, 77A (1956).
(2) Pleasance, C. L., IS.4 Journal 5 , NO. 6, 39-42 ( 195S).,, (3) Prohaska, C. A., Flow Colorimeter
for Measuring Uranium Concentration in Process Streams,” U. S. Atomic Energy Comm., Rept. DP-229 (1957). (4)Savitzky, A., ANAL.CHEY.30, No. 3, 17A (1958). ( 5 ) Siggia, S., “Continuous Analysis of Chemical Process Systems,” Wiley, New York, 1959. ( 6 ) Stelzner, R. W.,Oak Ridge.Nationa1 Laboratory, private communication.
RECEIVED for review June 8, 1959. Accepted November 2, 1959. Division of Analytical Chemistry, Beckman -4ward Symposium on Chemical Instrumentation Honoring Howard Cary, 135th Meeting, ACS, Boston, Mass., April 1959. Work performed under contract No. AT-(45-1)1350 for the U. S.Atomic Energy Commission.
Carbon-Hydrogen Determination by Gas Chromatography ALLEN A. DUSWALT’ and WARREN W. BRANDT Deportment o f Chemisfry, Purdue University, Iafayette, Ind.
A method developed for the determination of carbon and hydrogen b y gas-solid chromatography has a precision to 0.5% absolute for carbon and 0.1% for hydrogen. The time for combustion, separation, and recording of the chromatogram is 20 minutes. On a continuous basis, a new sample may b e started every 10 minutes. Compounds containing oxygen, nitrogen, halogens, or sulfur may b e run directly without difficulty. Sample size i s on the order of 2 to 6 mg. and might be considerably smaller with more sensitive equipment. The technique is simple and experimental manipulation is reduced to a minimum.
Present address, Hercules Powder Co., Wilmington, Del. 1
272
ANALYTICAL CHEMISTRY
HE Pregl method for combustion T a n a l p i s , generally considered to be the standard method for carbonhydrogen microdeterminations, has excellent accuracy and precision ( 2 ) . It is, however, somewhat lengthy and a fair amount of personal error may be introduced. By utilizing some of the advantages of gas chromatography (1), a method has been developed for carbon-hydrogen determination which reduces these disadvantages. The time of analysis has been improved to about 20 minutes for a single determination and to about 10 minutes each for a continuous series of analyses including the area measurement. The organic sample is burned in a dry, carbon dioxide-free oxygen stream. The resulting carbon dioxide and water
vapor are passed through a calcium carbide tube which converts the n-ater vapor to acetylene. The gases are passed through a liquid nitrogen freeze trap to concentrate the materials to be analyzed. The carbon dioxide and acetylene are then vaporized and swept into the chromatographic system by the helium carrier gas. Separation and detection occur in the column and thermal conductivity cell, respectively, resulting in a typical chromatogram with symmetrical peaks. The areas of the carbon dioxide peak and the acetylene peak are proportional to the weights of carbon and hydrogen, respectively, in the original compound. APPARATUS AND MATERIALS
Chromatographic.
A
4-filament
2: G l
Figure 1 .
E. F. G.
l
C
J
Schematic diagram of apparatus
A. O x y g e n inlet 6. Combustion tube C. Tube furnace D.
U
Aluminum heat boffle Calcium carbide tube Freeze trap liquid nitrogen Dewar
Cow-Mac thermal conductivity cell. Model 9193, with power supply proved more reproducible t h a n t h e thermistor cells tried. Varian G-10 10-mv. recorder. Column. 3 feet of Tygon tubing, X 1/4 inch, insulated by 75-mm. glass tubing wrapped in glass wool. Packing, activated silica gel, 25 to 60 mesh (regenerated after 50 runs). Carrier, hclium, controlled b y a twostage reduction valve followed by two needle valves. One needle valve did not provide sufficient steadiness. Freeze Trap and Sample Inlet. The freeze trap u-as constmcted from 10mm. glass tubing modified b y pushing Vigreux-likr glass spikes torvard the tube center in a diredion against the intended carrier flow. Two three-way stopcocks permit openings to the vacuum. the combust Lon train, or the helium bypass to the column. The U part of the trap is approximately 8 inches long and fits into a closedmouthed. 1-pint thermos flask. Liquid nitrogen is used to freeze out carbon dioxide and acetylene from the conibustion stream. Combustion. Carl ier a n d combustion an., high pxrity oxuyqen. Drying agent, magnesium perchlorate-filled tube. 12 inches X 16 mm. A 12-inch tube furnace. A Pyrex rombustion tube 8 mm. in inside diameter X 5 2 em. Tyith side arm. Corning glass So. 8680. Sample \-aporizcr. 1.5 fret of Chrome1 n i r e S o . 24, n o u n d around t h e combustion tube to clover a length of 2.5 inchcs prior to th? furnace heated area. Combustion Tube Packing. Because of the comparatively rapid oxygen flow rate, the lengths of filling material are somen hat greater and the packing is tighter. The following materials are given in terms of length of calumn occupifd. From the postconibustion end, the tube contains: Silver (6 em.), needles 1 to 2 mm. long, gronn on copper screening from silver
nitrate solution. Copper oxide (18 cm.), obtained from oxygen passing over hot copper screening, 40 mesh, tightly rolled and inserted in 3cm. sections. Platinum (6-cm.) gauze, approximately 40 mesh tightly rolled.
Calcium Carbide Cartridge. A medicine dropper, 5 cm. X 8 nim. in outside diameter. proved t o be the most convenient size. Fresh cartridges n ere
H. 1. J. K. I.
Vacuum connection Chromatographic column Insulating jacket Reference side of defector Sensing side of detector M. Helium inlet N. Flowmeter and outlet
filled from a stoppered reagent bottle with a glass tube for pouring and stored over carbide. Calcium carbide, half-inch lumps (Union Carbide) , crushed and sieved to take out fines and hydrolyzed material. The 30- to 60-mesh portion ITas used. Care is necessary to prevent excrssive exposure of the freshly ground material to atmospheric moisture. Tubing Connections. Because rubher readily absorbs acetylene, all glassto-glass connections were made n-ith Tygon tubing. If rubber is exposed anywhere to acetylene, between the carbide tuhe and the freeze trap, a negative effect on the liydrogen value is readily apparent. Figure 1 is a diagram of the conibustion and chromatographic apparatus. PROCEDURE
Preparation of Apparatus. After arrangement of t h e apparatus, as in Figure 1, t h e furnace is alloired t o reach a temperature of 750" C. T h e three-n-ay stopcocks are positioned t o open t h e combustion tube t o t h e freeze t r a p a n d vacuum line. Helium is allowed t o flow through t h e bypass t o t h e chromatographic column, and the bridge current turned on. The helium flow is regulated to 60 ml. prr minute. Oxygen pressure is adjusted to 1 or 2 p s i . and the needle valve opened to obtain a flow of about 25
Table I.
ml. per minute. The vacuum pump is started and the pressure in the freeze trap reduced to around 110 mm. of mercury with the oxygen flowing through the system. Air is bled into the vacuum line to control the pressure in the trap. The adequacy of the vacuum in the trap is tested b y immersion of the trap in the liquid nitrogen bath for 5 minutes. If no liquid oxygen is condensed out of the trap at this time, the vacuum is sufficient. Heated oxygen is allorred to purge any residual nater vapor or carbon dioxide from the system. A freshly filled calcium carbide cartridge is inserted, after the combustion tube has been closed to the vacuum. The vacuum line is wopened and oxygen flushed through the cartridge to remove any residual acetylene. Analysis of Sample. T h e combustion tube is closed t o t h e vacuum b y turning t h e appropriate stopcock. T h e t r a p is immersed in t h e liquid nitrogen bath. After t h e flow of ovypen has eliminated t h e vacuum in t h e combustion tube (determined by the ease of removal of the cork), the cork is removed from the tube and the sample boat inserted. The boat is placed approximately in the centw of the sample of the vaporizing heater and the tube is corked. After the closed stopcock has been reopened, the wire sample vaporizer is heated with approxiniateiy 18 volts. Eight minutes are allowed for vaporization, combustion, and flushing through to the freeze trap. The trap is then isolated from the combustion system by the three-way stopcocks. By removing the liquid nitrogen, the frozen acetylene and carbon dioxide are quickly uaporized. The three-way stopcocks are opened to the chromatographic system and the bypass is closed, causing the helium carrier gas to flovv through the trap and sweep the sample gases into the silica pel column. T h e n the gases are completely removed from the trap, as evidenced by the appearance of the oxygen peak on the chromatogram, the helium can be rerouted through the bypass and the
Determinations of Carbon and Hydrogen in Compounds Containing Oxygen, Nitrogen, Halogen, and Sulfur
Compound hnalyzed Oxalic acid dihydrate
Theoretical, 7' C H 19.05 4.85
Obtained, 70 C H 18.9 4.68 19.1 4.64 19.3 4.62
Other Elements 0
3-Amino-acetophenone Benzoic acid Tartaric acid 8-Quinolinol
68.84 32.0 74.3
4.00
p-Toluenesulfonic acid dihydrate
44.1
5.28
4-Hydroxy-7-chloroquinoline
60.9
3.36
Thiourea
15.9
5.26
4.95 4.84
68.4 31.0 74.0 75.4 44.1 44.2 62.2 61.5 16.1 16 2
4.90 4.00 4.79 4.94 5.21 5.32 3.44 3.43 5.24 5.12
VOL. 32, NO. 2, FEBRUARY 1960
0 0 0, i\; 0, s
0, ii, c1
h->s
273
trap connected again to the conibustion system for a second analysis. While the gases from one combustion are still on the column, a second determination may be started within 10 minutes of the first. The time for completion of the chromatogram from the introduction of the gases on the column is about 8 minutes. K h e n the relatively insensitive 10mv. recorder was used, sample sizes of approximately 2 t o 6 mg. were generally very adequate for producing measurable peak areas for carbon dioxide. An attenuation of maximum was usually necessary to keep the acetylene peak on scale. A more sensitive recorder and one of the more sensitive detectors available today would allow analysis of niicrogramsize samples. RESULTS A N D DISCUSSION
I n Table I the results of carbon and hydrogen analysis by gas chromatography are given. Average areas of 3 square inches were determined with a planimeter. The results in Table I show the variety of compounds which were analyzed by this method. For these determinations, a n over-all absolute precision to 0.5y0for carbon and 0.1% for hydrogen was obtained. These materials, containing oxygen, nitrogen, halogen, and sulfur, were analyzed directly for carbon and hydrogen content. Peaks resulting from the oxygen and nitrogen constituents are well separated from those of carbon dioxide and acetylene and do not interfere with their determination. The combustion products of the sulfur and
halogen components are removed by a section of silver metal in the combustion tube. It was necessary to reduce the rate of flow of the oxygen carrier gas through the combustion tube from 25 to 15 ml. per minute to effect complete removal of the halogens and sulfur products. The time allowed for flushing the sample into the freeze trap was lengthened to 12 minutes. Incomplete removal of these materials from the carrier stream resulted in chromatographic peaks having the same retention time as carbon dioxide. The precision of the determination was obtained from 12 runs on reagent grade benzoic acid. The standard deviations were 0.36% C for carbon and 0.058% H for hydrogen. The improvement over Table I can be attributed to the greater purity of the burned material. The problems presented by the conversion of water to acetylene were overbalanced by the problems of chromatographing a mixture of carbon dioxide and water and working a t a n elevated temperature. The calcium carbide reaction proved convenient, workable, and accurate. The reaction of mater vapor with calcium carbide is as follows: CaCz
+ 2H,O
=
Ca(OH)?
+ C1H2
The build-up of calcium h>droxidc is undesirable because of its observed tendency t o retain water. For two or three runs, this retention effect is not significant. For a greater series of analyses, the calcium carbide cartridge is replaced after every three runs. A fresh cartridge was used TT-hen starting
up after any period of disuse over hour. No significant amount of error mas detected because of reaction of carbon dioxide with the small amounts of calcium hydroxide formed. Because of the reported tendency for solid acetylene to be highly explosive, the freeze trap was initially taped. After hundreds of analyses had been run without evidence of mishap, a study was made to determine whether the frozen carbon dioxide-acetylene mixtures could be induced to explode. Mechanical and thermal shocks were applied to the solid mixtures obtained from various analyses. An explosion could not be made to occur. Apparently the conditions within the trap, a partial vacuum (ea. 110 mm. of mercury) and dilution with carbon dioxide, render the material harmless. -4 small but definite variation in sensitirity was observed from day to day. This was attributed to inability to reproduce operating conditions closely, e.g., room temperature-and necessitated the running of a knonn standard to calibrate the apparatus before a series of analyses. LITERATURE CITED
(1) Keulemans. A. I. M.. Verver. G. C. \
I
ed., “Gas Chromatography,” 2nd ed., Reinhold, New York, 1959. ( 2 ) Steyermark, A., “Quantitative Organic Microanalysis,” Blakiston, Philadelphia, 1951. RECEIVED for review .lugust 10, 1959. Accepted November 12, 1950. From a thesis submitted by A. A. Duswalt to Purdue University in partial fulfillment of the requirements for the degree of doctor of philosophy, January 1959.
Application of Gas Chromatography to Microdetermination of Carbon and Hydrogen 0. E. SUNDBERG and CHARLES MARESH Research laboratory, American Cyanamid Co., Bound Brook, N. 1.
b The technique is based on the combustion of the organic compound to carbon dioxide and water by a mixture of copper oxide and copper in a helium atmosphere. The water is subsequently converted to acetylene by reaction with calcium carbide and carbon dioxide and acetylene are separated by elution through a silica gel column with helium. The separated gas components are detected by thermal conductivity and the concentrations recorded in terms of peak areas by a strip chart recorder. The analysis i s completed by comparing the measured 274
ANALYTICAL CHEMISTRY
carbon dioxide and acetylene peak areas with previously established calibration charts of peak areas v5. milligrams of carbon and hydrogen.
M
of the published information on elemental analysis is based on modifications of the original Pregl methods (1, 4 ) 7 ) . The work described in this paper is based primarily 011 the determination of carbon and hydrogen, with the ultimate objective the simultaneous determination of carbon, hydrogen, and nitrogen. OST
DEVELOPMENT OF METHOD
The chemical aspects of the investigation included the elimination of excess oxygen, the combustion of organic compounds with copper oxide in a helium atmosphere, the conversion of viater t o acetylene (3) by reaction with calcium carbide, and the possible retention of carbon dioxide by calcium hydroxide formed as a by-product during this conversion. There were two reasons for eliminating oxygen from the combustion procedure. If the method is to be economical, it should include the simul-
