LITERATURE CITED
(I) Albert,
A., Biochem. J . 47,531 (1950);
50,690 (1952). (2) Baudet, P., Cherhuliez, E., Helu. Chim. Acta38,841(1955).
(3) Beauchene, R. E., Berneking, A. D., Shrenk, W. G., Mitohell, H. L., Silker, R. E., J . Rid. Chem. 214,731 (1955). ( 4 ) Bjerrum, J., Schwartzenhrtch, G., Sill&, L. G., “Stability Constants,” Pa3 1, “Organic Ligands,” Chemical Society, London, 1957.
(5) Blackhum, S., Rohson, A., Biochem. J . 54,295 (1953). ( 6 ) Blaedel, TV. J., Todd, J. W., ANAL. CHEM.30,1821 (1958). (7) Borchers, R., Ibid., 31, 1179 (1959). (8) Kolthoff, I. ,,M., Lingane, J. J., “Polarography, 2nd ed., p. 493,
Interscience, New York, 1952.
(9) Martin, A. J. P., Mittelmann, R., Bioehem. J . 43,353 (1948). (10) Meites, L., “Polnrogrsphic Techniques,” p. 135, Interscience, New York, 1955.
(11) Pope, C. G., Stevens, M. F., Biochem.
J . 33,1070(1939). (12) Schraeder, W. A,, Kay, L. M., Mills, R. S., ANAL.CHEM.22, i 6 0 (1950). (13) Spies, J. R., J . B i d . Chem. 195, 65
(1952). (14) Wirth, H. E., IND. ENG. CHEM., ANAL.ED. 14, 722 (1942). (15) U‘oiwod, A. J., Hiochem. J . 45, 412 (1949).
RECEIVED far review November 27, 1959 Accepted February 18, 1960.
Continuous Trace Hydrocarbon Analysis by Flame Ionization A. J. ANDREATCH and RAYMOND FEINLAND Central Research Division, American Cyanamid Co., Stamford, Conn.
F An inexpensive, rugged, and portable flame ionization detector has been designed for the continuous analysis of trace amounts of hydrocarbons. The detector is insensitive to inorganic gases but responds to hydrocarbons in proportion to the carbon atom content. A detectable limit of 1 p.p.b. of hexane is obtainable. As an automotive exhaust analyzer, the unit can first measure total unburned hydrocarbons, and then b y introducing an 8-inch silica gel column, the unit can determine methane, ethane, ethylene, acetylene, and propylene.
T
QUANTITATIVE estimation of trace amounts of hydrocarbons is of great importance in air pollution, oil exploration, oxygen manufacture, leak detection, and exhaust gas analysis. McWilliams and Dewar (7, 8) have recently reported a flame ionization instrument capable of detecting less than 1 p.p.h. of hydrocarbon. Similar detectors with lower sensitivity have been reported by Harley (4)and Thompson (9). The p r a y argon ionization detector has sufficient sensitivity hut does not respond to C, or C, hydrocarbons. The flame ionization principle was, therefore, investigated for continuous trace hydrocarbon analysis. The detector described is simpler than any reported in the literature, yet has a high sensitivity. It is also more versatile than the Perkin-Elmer continuous hydrocarbon detector and meets the requirements for either a continuous analyzer or a gas chromatography detector. The flame ionization detector makes use of the electron concentrations formed when a hydrocarbon is introduced into a hydrogen flame (1, 9, 10).
function. The thermionic work function for carbon is 4.35 electron volts (2) which is lorn enough for electron emission at flame temperatures. The fact that the conductivity appears t o he a function of the number of carbon atoms and is not a function of their nature supports this contention. APPARATUS
HE
Figure
1.
Front view
of
detector
Inorganic gases such as hydrogen, nitrogen, carbon dioxide, and steam have high ionization potentials (12 t o 16 electron volts) and are not ionized at flame temperatures. These gases do not interfere with the analysis. The mechanism of the ionization of the hydrocarbons in flames is complex and not completely understood. Hydrocarbons are cracked in the preheating zone of the flame t o produce acetylene and large unstable hydrocarbon molecules. These molecules form carbon radicals such as C2 and CH which are in highly activated electronic states. The carbon radicals are believed to react to form carbon nuclei which react with the original hydrocarbon and intermediates t o form carbon particles. The carbon particles, containing ahout 50,000 atoms (S), may then he assumed to behave similarly to solid carbon and to emit electrons due to their low thermionic work
Figures 1 and 2 illustrate the electronic and mechanical portions of the unit. One type of burner (Figure 2) is mounted on a %inch Teflon base through which the electrical and gas connections are made. The entire flame is enclosed in an aluminum cover ii.hich has a fine screen a t the top to permit the exhaust gases to escape. Air, filtered to remove dust particles, is supplied from a compressed air line at a rate of 1.5 t o 2 cubic feet per hour. The burner consists of a glass-insulated lead-through silver-soldered t o a 2-mm. stainless steel capillary tube. A 1-cm. stainless steel tube (the needle section of a S o . 21 hypodermic needle) is silversoldered in the tip of the insulator. A second type of burner that i? also satisfactory is made by pressing a hypodermic adapter into the ,Teflon base. Hypodermic jets of any size can he installed in this type of construction. A 300-volt direct current potential, supplied by a dry cell, is applied across the flame by making the burner positive and a nickel screen, placed 7.5 mm. above the jet, ncgative. Experiments with oxygen and hydrogen mixtures were not possible with the standard burner because of flash back into the burner and tubing. The burner vas redesigned so that the hydrogen and sample stream were mixed a t the burner orifice. Flash back was not observed with this type of construction and it was also possible t o VOL. 32, NO. 8, JULY 1960
1021
observe hydrogen and sample flow rates over a much wider range without extinguishing the flame. Sensitivity was equal t o that of the standard burner nhen the sample was introduced in either the hydrogen or nitrogen stream. The direct current amplifier uses a single Victoreen tube (VX55). The poiver supply for the filament is a 1.3volt hIallory cell 111th a 15-volt dry cell (two C batteries) to supply the plate voltage. The gain of the amplifier is adjusted by varying the input resistor of the tube in steps of 105, lo6, lo7, lo8. 109, 10'0, and 10" ohms. The output is observed on a 30-ma. meter and or a 10-mv. recorder. The recorder output is taken aero's the meter (2200 ohms resistance) and a 1100-ohm fixed resistor so that full scale recorder response is obtained with a 3-ma. flow through the meter. The recorder gain can be varied b j switching the recorder output across 1100, 2200, or 3300 ohms. Kithout a flame, no noise, zero signal, or drift is observed a t the highest gain setting. The signal lead from the grid of the s'X55 tube to the cathode must be well insulated to prevent leakage of current. To ignite the flame, a Nichrome 11ire is heated to red heat by a 6.3-volt alternating current transformer or from a battery. During the ignition home noise is introduced into the amplifier, but the noise disappears about 5 qeconds after the flame is ignited. The hydrogen flame produces a background signal which is 1.2 X lo-" ampere or 3.2 nir'. on the 10lO-ohm gain setting v i t h a hydrogen flow rate of 25 ml. per minute and a nitrogen flow rate of 30 nil. per minute. By decreasing the flow rates the background signal could be reduced to 1.6 X lo-'* ampere or 4 niv. on the 10"ohm gain setting. The signal is due t o trace impurities such as sodium and methane and to some ionization of hydrogen. The background signal can be subtracted in any case with the zero control on the amplifier. The sodium background can be observed by decreasing the nitrogen flow so that the flame burns closer to the jet. The tip of the jet becomes red hot. and the characteristic color of sodium emission is observed. The yellow color soon disappears. and the flame becomes colorless. Methane concentrations of several parts per million vere present in the laboratory air, compressed air. oxygen. aiid outside air. The presence of methane in the conipressed air would give a background signal. Noise on the 101C-ohm range is about . the zero drift is about 20 10 p ~ and pv. for a 2-hour period. The sensitivity increaseq rapidly as the direct current polarizing voltage is increased to 115 volts. Between 115 and 340 volts, the sensitivity is constant. The analyzer is relatively insensitive to vibra1022
ANALYTICAL CHEMISTRY
tion, tilt, and shock. It does not appear to be sensitive t o room temperature or humidity variations. The response time is less than 1 second for a sample injected at the detector. The distance between the jet and the collecting screen was varied from 2.5 to 15.0 mm. The sensitivity was constant over this range but the unit became noisy when the distance was less than 2.5 mm. PROCEDURE
Continuous Trace Hydrocarbon Gas Analysis. The flame ionization detector was first used in t h e analysis of total unburned hydrocarbons in automotive exhaust gas. I n this operation, exhaust gases were filtered with a small glass \vool plug t o remove dust particles, mixed with hydrogen, and continuously passed directly into t h e burner. Hydrogen and exhaust gas flows were adjusted by needle valves and flowmeters after passing through capillary tubing to provide additional resistance to flow. The total hydrocarbon contcnt \vas observed on the microammcter or the 10-mv. recorder. Gas Chromatography Analysis. I n this operation, nitrogen mas used as a carrier gas, and 1-to-10-ml. exhaust gas samples were injected ahcad of a n 8-inch silica gel column. Hydrogen was added after the column and the mixture passed into t h e burner. Davison KO. 912 silica gel n a s screened between 60 and 100 mesh prior to packing in 1 '4-inch stainless steel tubing. The peak height or area of the eluted gases was observed on n 10mv. recorder. For analysis in the parts per billion range, the millivolt signal was amplified 20 times nith a direct current amplifier and recorded on a IO-mi-. recorder.
Preparation of Gas and Vapor Standards. Gas standards were prepared by mising gases in 1-liter glass flasks. The flask n a s evacuated and filled n-ith nitrogen t o atmospheric pressure. An injection port, sealed with a rubber stopple, was placed in the side of t h e flask. X 1000-p.p.m. standard was prepared by injecting a 1-ml. gas sample of the 1007, gas into t h e flask with a syringe and mixed by shaking n-ith 5-mm. glass beads whose total volume n a s 10 ml. Samples were taken from t\vo different portions of the flask and analyzed for complete mixing. -4 second 1-liter flask was evacuated and filled nith nitrogen. A 1-ml. sample from the 1000-p.p.m. flask was then injected into the second flask to obtain a 1-p.p.m. standard. Two, three, or more parts per million standards can be prepared by injecting the correct number of milliliters from the 1000-p.p.m. standard. Mixing was again checked, taking samples from different portions of the flask. Gas standards prepared by the Matlieson Co. nere also used. For liquids, a glass hypodermic syringe was inserted into the vapor above the liquid and a sample obtained. The concentration of the vapor was calculated from the vapor pressure of the liquid a t room temperature. The syringe was cleaned n-ith acetone and compressed air after each liquid tested. The blank syringe was then checked for contamination. RESULTS
Optimum Gas Flow Rates. Figure 3 illustrates t h e variation in sensitivity for increasing sample flow rates a t constant hydrogen flow rates for a 1000-p.p.m. methane in nitrogen standard. The sensitivity appears t o increase linearly and finally ap-
i
x
5 5-
\
i 4-
Figure 3. Sensitivity for various sample flow rates
-25
\
\
, 'x
= 3-
-2c
,
3
d
2
1 5
8
2-
-10
c
1-
-5
x 0 / 0
2C
C
30
-
X
40
IYZRffiIEU FLOW
--Ac 6c 70 80
C C / ~n
Figure 4. Sensitivity for various hydrogen flow rates
pioaches a saturation value a t high sample rates. The effect of varying the hydrogen flow rate while maintaining a constant flow of 1000-p.p.iii. methane in nitrogen is shonn in Figure 4. The recorder response is shonn on the left side of the ordinate. =2s the hydrogen flow rate is increased, the sensitivity increases to n maximum and then decreases. To simulate the elution curve obtained with a gas chromatography column, a 0.15-ml. methane sample was eluted through a 25-nil. mixing chamber a t a constant nitrogen flow rate. The hydrogen flow rate was varied and the response was measured as the area under the elution curve. The area response is plotted on the right side of Figure 4. I n grneral, the same type of responsc curve was obtained for the eluted sample as for a continuous methane qaiiiple. The maiimuni response occurs a t a hydrogen-nitrogen ratio of about 0.85 to 0.9 n i t h a platelau betueen 0.7 and 1.2. l'heqe results indicate that control of -,
(1906).
RECEIVED for review January 4, 1960. -4ccepted .Ipril 11, 1960. Pittsburgh Conference on Analytical Chemistry and A4pplied Spectroscopy, Pittsburgh, Pa., Fehruary 1960.
