tor in the 90-volt supply to the bridge are used to produce a change in bridge current while making the compensating adjustments. Decade attenuation of the bridge output signal is accomplished with the switches marked x 10, x 100, and x 1000. It will be noted that these are the only moving contacts so located that thermal e.m.f.’s or contact resistance might affect the signal. No noise from this source has been observed, using Automatic Electric Type BQA relays. The potentiometer circuit, suggested by Hoe11 ( d ) , comprises a 1000-ohm Helipot and two 4G-ohm fixed resistors. The current source for the potentiometer is of relatively high impedance, so that noise generated a t the Helipot slider appears across the source impedance, rather than the potentiometer output. The potentiometer span is changed by the 50,000-ohm rheostat and the tap switch a t the bottom of the diagram. When operating a t high sensitivity levels, the bridge output does not stabilize until several seconds after a bridge balance adjustment is made. This lag is undesirable in an automatic process chromatograph, where rapid zero adjustment may be required. This difficulty was overcome in the following manner : The 1000-ohm potentiometer Helipot and a 10,000-ohm transmitting Helipot are driven by a 162-r.p.m. Brown balancing motor, at a maximum speed of about 0.5 second per revolution. One turn of the transmitting Helipot drives the recorder full scale. The output of the transmitting Helipot is fed through a capacitor to an electrometer cathode follower, which, in turn, drives the recorder. Momentary grounding of the electrometer grid provides practically instantaneous zeroing of the recorder,
Pi Determinatio ALFRED
M. VOGEL
so that the manual bridge-balancing adjustments need not be manipulated unless a positive or negative zero drift of more than about four times full scale occurs. A 100-volt supply to the transmitting Helipot provides a sufficiently high signal level to eliminate problems of stability in the cathode follower circuit. A finer bridge balance adjustment can, of course, be provided in place of this feature, where speed is not required. A standard Brown 40X servoamplifier was modified for use in this servopotentiometer. The necessary modifications include substitution of a 7500ohm input transformer for that furnished with the amplifier, and the incorporation of a feedback velocitydamping circuit. The latter circuit is that ordinarily used in the Brown 0.5-second recorder. An external amplifier gain adjustment mas added for use in the automatic process chromatograph. SENSITIVITY MEASUREMENT
The sensitivity of the detector was measured in the following manner: A flow sysLem was constructed equivalent to the conventional bypass sampling system, whereby a stream of helium could flow either directly through the detector or via a column packed with naphthalene crystals. The bridge m-as balanced with helium flowing through the detector. Then, after flowing the helium over the naphthalene until a steady reading was attained, the rise in detector output signal was noted. This test Fas repeated a t progressively lower flow rates, until further flow reduction made no change in the measured signal. It was assumed that, a t this point, the helium was saturated with naphthalene vapor. From the known vapor pressure of
naphthalene a t the operating temperature (approximately 25’ C.), the sensitivity of the detector was calculated. By extrapolation, the measured noise level of 0.3 pv. corresponded to a naphthalene concentration of 2 x lo-* mole per mole of helium. In the units proposed by Young (7), pQo = 8.7. For comparison, typical commercial thermistor detectors tested at this laboratory show a pQo of about 7.0, CONCLUSIONS
The major part of the noise evident in most gas chromatographs using thermistor detectors arises from sources external to the thermistors and can be eliminated by proper instrument design. Application of the techniques discussed in this paper produces an approximate 50-fold improvement in signal-to-noise ratio over thermistor detectors of conventional design. LITERATURE CITED
(1) Bennett, C. E., dal Kogare, S., Safranski, L. W., Lewis, C. D., A~YAL.
CHEY.30,898 (1958).
( 2 ) Cowan, C. B., Stirling, P. H., Intern.
Gas Chromatography Symp., Instrument Society of America, East Lansing, Mich., August 28-30, 1957. (3) Dimbat, M., Porter, P. E., Stross, F. H., ANAL.CHEM.28, 290 (1966). (4) Hoell, P. C., E. I. du Pont de Nemours & Co., Inc., private communication. ( 5 ) Jones, W. L., E. I. du Pont de Kemours & Co., Inc., private communication. ( 6 ) Stirling, P. H., Second Biannual Intern. Gas Chromatography Symp., Instrument Society of America, Eaet Lansing, Mich.. June 10-12. 1959, privatecommunication. ( 7 ) Young, I. G., Second Biannual Intern. Gas Chromatography Symp., Instrument Society of America, East Lansing, Mich., June 10-12, 1959. I
I
-
RECEIVEDfor review May 19, 1960. Accepted August 15, 1960.
graphic Metha rbon and Hydrogen
and JOSEPH J. QWATTRONE, Jr.
Chemistry Department, Adelphi College, Garden City, N. Y .
A rapid, easy method for the determination of carbon and hydrogen in organic compounds involves a bomb combustion of an 8- to 1 1 - W . Sample in an Oxygen atmosphere8 sampling the products of combustion, subjecting the sample to gas chromatography, evaluating carbon and hydrogen in terms of the planimeter-mgasured integrated ureas for water vapor and
1754
e
ANALYTICAL CHEMISTRY
carbon dioxide. Based on the results obtained from five organic compounds, the average deviation from the known values was 5.0 parts per thousand for carbon and 8.4 for hydrogen. The total time for a single run is 17 minutes. Triplicate results from a single combustion can be obtained in about 40 minutes.
HE standard method used for carbon and hydrogen determination in organic compounds, that of Pregl (W), requires 135 to 145 minutes for a duplicate determination. The method requires training of a high order and meticulous technique. Some combustion procedures of 15 to 20 minutes’ duration per determination have been described,
Figure 2. A. B.
C.F.L. 0:
E. G. H. 1.
1.
K. M. N.
Figure 1.
Diagram of apparatus
Oxygen tank and pressure gages Pressure gage, 0 to 100 p . 4 . Hand valves Solenoid valve Combustion bomb Pressure gage, 0 to 15 p.s.i. Absorption tube Sampling tube and injector To chromatograph Hand vent valve from sampling tube Oxygen carrier gas to chromatograph Constant temperature box
Chromatograph of typical run Time units in minutes
Sundberg and Maresh (4) and Duswalt and Brandt ( 1 ) describe carbon and hydrogen determinations which partially alleviate objectionable characteristics of the Pregl method by combining this method. or the method of Dumas, with gas chromatography. The method described here further simplifies the technique and eliminates the need for isolating the products of combustion from the combustion atmosphere. The combustion is carried out instantaneously in a specially designed bomb. The products of combustion are not separated from the oxygen itself. Instead, oxygen is used as a carrier gas in the chromatographic procedure to eliminate the need for removing it. The use of oxygen also allows a better evaluation of carbon dioxide and water on the resulting chart record because of the negative dip for water as compared to carbon dioxide (Figure 1) and eliminates the need for purifying the oxygen, since the sensing system is a differential one. Figure 1 is a chromatograph of a typical run. The chromatographic column material and the conditions for optimum results were determined empirically. All standard materials thought capable of separating HzO and COZ were tried. Of these, the commercial 2-meter Perkin-Elmer A column (dodecyl phthalate on diatomaceous earth) gave the best results. A column temperature of 104” C., a flow rate of oxygen of 4.2 units on the flow gage, a, voltage setting of 8 volts, and a sensitivit setting of 1 were used on a Perkin-Elmer 154-B Vapor Fractometer. APPARATUS AND MATERIALS
I n addition to the Vapor Fractonieter, whose output was recorded on a Bristol
5-mv. 1-second potentiometer, the apparatus consisted of a combustion and sampling train in a constant temperature box as blocked out in Figure 2. The oxygen was specified as “extra dry’’ by The Matheson Co., Inc., and was used without further purification. Copper tubing (l/d-inch) connected the reducing valve of the oxygen tank to a T junction. One arm of the T led to the .gas chromatograph, supplying the carrier gas, through 1/2-inch rubber tubing. The other arm led to the inlet of the combustion bomb by l/s-inch copper tubing. I n this train there were, successively, a James P. Marsh Corp. oxygen test gage calibrated from 0 to 100 p.s.i., with each division equivalent to 0.5 p.s.i., a manual hand valve, and a two-way solenoid valve attached as closely to the bomb as possible to eliminate dead space. Combustion Bomb. The bomb as shown in Figure 3 was made of a brass cylinder with 1/4-inch walls. The inside diameter was 2’/16 inches and the height was 2 1 i 2 inches. The base to which the cylinder was fused was made of 1/2-inch thick brass. A circular cup l/r-inch deep was cut into the base to hold a magnetic stirring bar. Around the upper rim of the cylinder was a brass lip 3,’8-inch wide and ‘/pinch thick. Six 1/4-inch bolts, 1-inch long, were anchored on to the lip at equal intervals. Six holes were drilled along the periphery of the cover (to correspond with the position of the bolts), so that the cover could be bolted to the lip securely with wing nuts. A groove was cut along the lip to hold a rubber O-ring to act as a gasket for sealing the cover to the bomb. The entire bomb rested on a magnetic stirrer. The combustion boat consisted of a circular fused quartz disk l o / l ~ inch in diameter with perforations through which was interwoven 28-gage platinum wire. It was attached to two small metal pegs on the cover of the combustion bomb by platinum wire. One peg passed through
an insulated hole in the cover. The other was attached directly to the brass cover (Figure 4). The combustion of the organic compound takes place on the combustion boat when a current of 8 amperes a t 15 volts is passed through the platinum wire for 10 seconds. Sampling Train. A sampling train was used to remove interfering products of the combustion and to inject a fixed sample into t h e chromatograph. First, a hand-controlled reducing valve, operating through a hole in the constant temperature box, \\-as placed as close t o the bomb as possible. This was follomed b y a 9-inch absorption tube, which was used to remove the oxides of sulfur and nitrogen. This tube was made from 1/2-inch copper tubing and was filled with a mixture of equal parts of 20- and 40-mesh zinc. Glass wool plugs were uscd a t both ends of the absorption tube. This in turn was followed by an oxygen pressure gage made by the James P. Marsh Corp. This gage covered the range 0 to 15 p.s.i., with each division equivalent to pound. From this gage 1/8-inch copper tubing led to the Perkin-Elmer gas-sampling valve provided with a 25ml. sampling tube. The combustion products passing through the sampling valve leave through a copper tube provided at the end with a hand valve to regulate the flow. The constant temperature box was 12 x 12 X 12 inches, made of aluminum, and insulated by l/z-inch felt. The box was provided with a number of heating elemenls, two blowers, and a thermostat. The temperature was maintained a t 70’ =k 0.5’ C. The box top was attached by piano hinges to give ready access to the bomb. Experimental. Thin wafers of the organic compounds t o be analyzed are made, using standard potassium bromide pellet maker, and are dried in a desiccator for 24 hours. These wafers are approximately 0.03 inch thick and are broken t o yield masses of about 9 mg. An 8- to 11-mg. VOI. 32, NQ. 13, DECEMBER 1960
@
1755
sample of the pellet, weighed to 1 0 . 0 1 mg., is placed in the combustion boat. The cover of the combustion bomb is then bolted down and oxygen flushed through the system for 2 to 3 minutes. The valve between the bomb and absorption tube is then closed. When the system from the oxygen tank to the bomb, inclusive,
--T
Table I. instrument Calibration Using Benzoic Acid as Standard
Benzoic iirea of Acid, COZ, Sample Mg. Sq. Cm. For Carbon (Known carbon A 10.73 32.9 B 6.43 19.9 C 8.79 27.1 D 9.64 29.8 E 7.03 21.7 Av.
Area of COz per Wt. of C [ K J ,Sq. Cm./Mg.
S C A L E I"
TOP V I E W
= 68.84%)
4.45 4.50 4.48 4.49 4.49 4.48 & 0.01
For Hydrogen (Known hydrogen = 4.95%) A 10.73 20.7 39.0 B 6.43 12.4 39.0 C 8.79 17.0 39.1 D 9.64 18.6 39.4 E 7.03 13.5 38.7 Av. 39.0 f 0.16
Table
I!.
Figure 3.
Combustion bomb
A. Bolts for cover
B. Groove for O-ring C. Oxygen inlet
D. E.
Sampling outlet Magnetic stirrer well
Determination of Carbon and Hydrogen in Five Compounds by Combustion-Fractometer Method
Av. Dev. from Known
Benzoic acid
Cystine
Dextrose Glycine 2-Naphthaienesulfonic acid
%C 68.44 69.08 68.81 69.00 168.90 29.90 30.04 30.80 37.02 39.40 39.82 39 * 77 32.42 31.87 32.16 57.71 57.53 57.32
%" 4.97 4.95 4.96 4.97 4.93 5.10 5.01 5.27 5.00 6.98 6.72 6.67 6.84 6.67 6.97 3.84 4.66 3.82
Dev. from Known, yo 70, Parts/1000 c H C H -0.40 + O . 24 -0.03 $0.16 +0.06 -0.09 $0.05 +0,51
Omit
-0.59 -0.17 -0.12 $0.43 -0.12 $0.17 +0.03 -0.15 -0.36
$0.02 0.00 $0.01 $0.02 -0.02 +0.07 -0.02 $0.24 -0.03 $0.27 fO.O1
-0.04 $0.13 -0.14 $0.26 -0,03
2.6
has reached equilibrium a t 55 pounds of pressure, the solenoid valve is closed. Current is then passed through the platinum wire for 10 seconds. Upon completion of the combustion process, the magnetic stirrer is turned on. Two to 4 minutes are required to allow the system to come to equilibrium a t the constant temperature of 70" C. In sampling the combustion products, the valve a t the exit of the combustion bomb, as well as the valve a t the vent of the Perkin-Elmer gas sampling unit, is opened slightly to allow the combustion products to flush the sampling system. Throughout this entire operation the gas-sampling valve which regulates the injection of the sample into the gas chromatograph is set in the normally closed position. After 35 to 50 ml. of the gaseous products have passed through the system, the exit valve is closed. Pressure is allowed to build up in the sampling system until 14-pound pressure is indicated on the gage. The hand valve at the combustion bomb is then closed and the exit valve opened slightly, allowing the pressure to drop slowly to 8 pounds. The exit valve is then closed and the gas-sampling valve is opened, allowing the gaseous combustion product contained in the 25ml. constant volume tube to pass into the gas chromatograph. Reagents. Benzoic acid, r a t i o n a l Bureau of Standards, microchemical standard 140-a. Crstine. National Bureau of Standards," micrbchemical standard 143-a. Dextrose, National Bureau of Standards, standard sample 41. Glvcine (Fisher Scientific Co.). chemic& puie. used as received. 2: Xaphthalenesulfonic acid (Matheson Coleman and Bell) used as receired.
2.0
8.0
1.8
7.5
19.4
6.0
21.0
3.1
2.6
RESULTS
The instrument was calibrated using benzoic acid as a standard (Table I). In every case, each value of the area under the curve is the average of three separate samplings taken from each combustion. For carbon the average deviation for the three samplings was
* ,
Omit
-0.03
I'
I
Y Table 111.
Comparison of Results
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