(10) Gerischer, H., ANAL.CHEM.31, 33 (1959).
(13) Hodgman, C. D., ed., “Handbook of Chemistry and Physics,” 41st ed., Chemical Rubber Publishing Co., Cleveland, 1959. (14) Jahnke, E., Emde, F., “Tables of Functions,” 4th ed., Dover Publications, New York, 1945. (15) Kambara, T., Tachi, I., J . Phys. Chem. 61, 1405 (1957). (16) Kolthoff, I.,, M., Lingane, J. J., “Polarography, 2nd ed., Vol. I, Interscience, New York, 1952. (17) Koutecky, J., “Soviet Electrochem177, Consultant’s istry,” Vol. I, Bureau, New Yor!;1961.
(18) Koutecky, J., Cizek, J., Collection Czech. Chem. Commun. 22, 914 (1957). (19) Macero, D. J., Rulfs, C. L., J.Am. Chem. SOC.81, 2942 (1959). (20) Matauda, H., 2. Elektrochem. 61, 489 (1957); 62, 977 (1958). (21) Matauda, H., Delahay, P., J. Am. Chem. SOC.82, 1547 (1960). (22) Matauda, H., Oka, S., Delahay, P., Ibid 81, 5078 (1959). (23) &holeon, M. M., Ibid., 76, 2539 (1954). (24) Oldham, K. B., J . Electrochem. SOC. 107, 766 (1960). (25) Randles, .J; E. B., “Progress in Polarography, P. Zuman, ed., Vol. I, p. 123, Interscience, New York, 1962. (26) Randles, J. E. B., Trans. Faraday SOC.44, 327 (1948). (27) Reinmuth, W. H., AKAL. CHEM.33, 485, 1438 (1961).
(28) Ibid., p. 1793. (29) Reinmuth, W. H., J . A m . C h e m SOC. 79, 6358 (1957). (30) Reinmuth, W. H., J. Phys. Chem. 61, 1405 (1961). (31) Sevcik. A,. Collection Czech. Chem. ‘ commun.’l3, ’349 (1948). (32) Vernotte, P., “ThBorie et Practique des Shies Divergentes,” Publications Scientifique du Ministere l’air) KO.207, Paris, 1947. (33) Vielstich, W., Delahay, P., J . S m . Chem. SOC.79. 1874 (1957). (34) Wijnen, ~. D., ’Rec. T r m . Chim. 79, 1203 (1960). (35) Wilson, E. B., “An Introduction t o Scientific Research,” McGraw-Hill, Xew York, 1952. RECEIVEDfor review June 11, 1962. Accepted August 8, 1962.
Use of EIectricaIIy Excited Oxygen for the Low Temperature Decomposition of Organic Substances CHESTER E. GLEIT and WALTER D. HOLLAND Tracerlab, Division of Laboratory for Elecfronics, Richmond, Calif.
b A stream of oxygen excited by a radio frequency discharge can be used to decompose organic substances prior to trace element analysis. Biological tissue, graphite, filter paper, and ion exchange resin have been oxidized by this method. Rates of 1 gram per hour can be achieved with a 300-watt, 13.56-Mc. oscillator. Temperatures of less than 100’ C. can be maintained. Radioactive tracer studies demonstrate that 17 representative elements can be quantitatively recovered after complete oxidation of the organic substrate.
M
mployed t o decompose organic samples prior to elemental analysis ( 2 ) . Wet ashing, used extensively with mixtures of hot mineral acids, is generally tedious and potentially hazardous. The anions which are introduced tend to interfere with subsequent analyses. As impurities in reagents produce a significant problem in wet ashing, dry ashing is generally recommended (‘7) for trace element analysis. However, during dry ashing, which requires heating the specimen to temperatures in excess of 400” C., elements from the container and the atmosphere may be introduced into the sample. Certain classes of biological material tend to froth and char. Others, high in nitrogen, may ignite upon heating. hlany substances are converted into refractory, difficultly soluble compounds. Serious losses are caused by diffusion and volatilization. As vol1454
ANY TECHNIQUES are
ANALYTICAL CHEMISTRY
atility depends on the oxidation state of the element and the composition of the substrate, one cannot accurately predict which elements will be lost in a specific sample. Recently, Schoniger’s (6) modification of the Hempel oxygen flask technique (8) has been widely used for microanalysis. Because of the high pressure developed in the flask, this technique is limited t o small samples. An alternate method, described in this paper, employs a high frequency electromagnetic field t o produce a stream of reactive oxygen, which decomposes the organic substances. Superficially, the apparatus employed in this method is similar to high temperature dry ashing in an induction heated tube furnace. Both methods employ a n essentially closed system to minimize the introduction of atmospheric impurities and permit recovery of volatilized elements. However, induction heating units incorporate a metallic substance into the sample or combustion boat to heat the sample to high temperature and provide the requisite activation energy. On the other hand, in the radio frequency discharge method, electrical energy is transferred directly to a stream of low pressure gas, producing highly excited states of oxygen. The reaction of these species with the elements in the organic sample is selective, minimizing general heating. Because the sample temperature is low, volatility and diffusion losses are substantially decreased. Nonvolatile mineral constituents remain in the
sample contiiiner in a form amenable to subsequent quantitative determination. EXPERIMENTAL
Apparatus. Figure 1 illustrates schematically the principal apparatus employed in this study. Molecular oxygen (Linde commercial grade) passes through a flow meter and is admitted t o the system through a needle valve, A . T h e oxygen flows through a borosilicate glass reaction chamber, B , 30 cm. long and 4 cm. i n diameter. T o determine the effects of gas flow rate, pressure, and power, specimens were placed in a borosilicate glass boat in chamber B, 25 cm. from the oxygen inlet. Volatility and recovery experiments employed a 30-cm. long water-cooled chamber, C. At a point 15 cm. from chamber B the inside diameter of the tube was reduced from 4 cm. to 1.2 cm. To perform volatility studies, samples were placed in a n open glass boat 20 cm. from chamber B. To determine the types of materials nThich could be ashed by this technique, specimens were placed in chamber C in glass tubes 2.5 cm. in diameter. Exhaust vapors pass through a cold trap, D,surrounded by a mixture of dry ice and acetone. A general purpose mechanical vacuum pump (Kinney model KC-3) is attached a t E. To protect the Tygon tubing connections from the action of active oxygen, a small piece of platinum gauze is placed in tube E. A mercury manometer is attached to sidearm F . Power is supplied by a conventional radio frequency oscillator, G. The oscillator has a crystal-controlled driver
B
F
H
G
L-
Figure 1, Low temperature, radio frequency oxidation a p pa ratus
which operates a power stage employing an Eimac 4-125A power tetrode (EitelRlcCullough, Inc.). T o comply with Federal Communications Commission radiation requirements on Industrial, Scientific, and Medical equipment, with minimal shielding, a frequency of 13.56 M e . was chosen. Inductive coupling transfers power to the reaction chamber. Coil H consists of 18 turns of 1/4-inch copper tubing. The plate tank is tuned by means of a variable capacitor, 1. K i t h a coil having a n inductance of 10 phenries, a 50- to 150-picofarad air capacitor is satisfactory. PROCEDURE
Measurements of the effect of pressure, flow, and power on oxidation rate employed 0.25-inch lengths of 0.242 inch in diameter spectroscopic grade graphite electrodes weighing 300 mg. (Sational Carbon Co., KO. 3829). Graphite felt (National Carbon Co., Grade WDD) was used to obtain a larger surface to weight ratio. Specimens were weighed periodically to determine oxidation rate. A radioactive tracer, NaZ2,was added to several specimens to determine that the change in weight was not caused by graphitic particles being blown from the combustion boat. Power was calculated from the measured cathode current and plate voltage. Plate loss was calculated from known circuit parameters and agrees with visual observation of plate temperature. The majority of volatility experiments employed 1.0-ml. aliquots of whole human blood containing 20% acid citrate as an anticoagulant. Radioactive tracer, 100 pl., was added to the blood. After 24 hours, the samples mere dried in a desiccator containing CaC12, placed in chamber C and exposed to a stream of activated oxygen for 1.5 hours. -4pressure of 400 microns of mer. cury, a flow rate of 4 cc. per minute, and a delivered pon'er of 150 watts were employed. RIaximum sample temperature n a s less than 100' C. iifter ashing, the combustion boat was reweighed and counted in a Tracerlab SC-57 wellcounter. The contents of the boat were dissolved in a mineral acid and recounted in a conventional 4-ml. vial. All glassware was rinsed with an appropriate mineral acid, and the wash solutions were counted. A sample of alfalfa grown in soil enriched in Se76
m
o
\
g
2ol 0.5
I5
I
2
25
HOURS
Figure 2. rate
Effect of surface area on oxidation
Abscissa indicates weight of graphite oxidized during prior 30-minute interval
was ashed in a similar manner. The experiments with iodine employed W h ~ t r I mNo. 42 filter paper disks 8s the substrate. RESULTS
A wide variety of organic substances ashed, including muscle tissue, fat, fecal matter, ion exchange resin (Dowex AG50W, 200- to 400-mesh), cellulose and poly(viny1 chloride) filter paper, activated charcoal, and a 40-gram rat. I n all cases, the ash was completely soluble in mineral acid, and the weight of ash was the value anticipated from muffle furnace ashing and known mineral content. Figure 2 shows the reaction rates for graphite rod, carbon felt, and a graphite rod that had been shaved into small slivers. Oxidation rate is a function of exposed surface and decreases markedly as the surface becomes covered with a mineral residue. The change in decomposition rate of tissue samples with time is similar to that shown for carbon felt. Table I presents the effect of pressure, flow, and power on oxidation rate. Over the range of 100 to 400 cc. per minute (S.T.P.), the rate of decomposition is essentially independent of the oxygen flow rate (Figure 3). Rate increases linearly with power in the range of 120 to 300 watts. Doubling the power produces a 25y0 increase in oxidation rate. For specimens with larger surface area, the change in rate with increased power is greater. The discharge, which is characterized by an intense light blue glow, could not be maintained when less than 120 watts was delivered to the induction coil. The relationship between pressure and
rate is more complex. For each reaction tube, a distinct optimum pressure was found, The maximum occurs a t a pressure of 1.5 mm. of mercury for the 30 cm. X 4 cm. chamber. The relationshir, between oxidation rate and sample position is strongly dependent on flow and pressure. The highest rates were recorded for samples mounted axially within several centimeters of the downstream end of the coil. At the upstream end of the coil, rates were approximately 30y0lcwer. At a power of 250 watts, pressure of 1.0 mm. of mercury, and flow rate of 10 cc. per minute, graphite rods could be oxidized at a rate of 35 mg. per hour, in a trap surrounded by dry ice 40 cm. downstream of the coil. I n the absence of the dry ice coolant, a rate of only 3
Table 1. Effect of Flow, Pressure, and Power on Oxidation Rate of Spectroscopic Grade Graphite Rods
Oxide Flow
rate,
cc./min. 10 18
25 50
100
200 300 400
70 70
70 70 70 70 70
70
Pressure, mm. of, Power, Hg wattts 5 .O 250 5.0 250 5.0 250 5.0 250 250 5.0 250 5.0 250 5.0 5.0 250 120 5.0 5.0 165 5.0 230 5.0 300 0.8 250 1.3 250 3.3 250 5.3 250
VOL 34, NO. 1 1 , OCTOBER 1962
tion
rate, mg:/30 min.
25 45 86 94 114
116 120 109 75 95 100
122 165 213 135 123 1455
mg. per hour could be attained a t this position. Table I1 presents the results of the volatility studies. For comparison, the data obtained in a recent study (4) of the retention of trace elements in muffle furnace ashing are given. Of the 17 elements tested, only four were partially volatilized. The mercury, silver, and gold activities were located by a r-ray sensitive survey meter on the vessel walls a short distance on both sides of the combustion boat. Between 1 and 9% of the iodide activity was deposited on the walls of the reaction vessel. The remainder was found in the cold trap, which had been filled with activated charcoal for the iodine experiments. Large variations in the distribution of radioactive tracer between the sample container and other parts of the system occurred in replicate experiments with the four volatile species, Agf, Auf3, Hgf2, and I-. Average values are given in Table I1 for these species. Volatilization loss occurred concurrently with sample oxidation. In a typicalexperiment with a silver chlorideblood mixture, oxidation was 96% complete after 10 minutes. During this period, 26% of the silver activity had been volatilized. I n the following 120 minutes less than 1% of the Agllo remaining in the combustion boat was transported to other parts of the system. DISCUSSION
The radio frequency discharge technique offers a convenient method of decomposing a wide range of organic substances. No variation of oxidation rate with type of organic compound was noted, indicating that the metastable oxygen species formed by the discharge has sufficient energy to rupture all C-C and C-H bonds. Therefore,
Table II.
I
I
I
I
I
I
I
+
+ ++
1456
ANALYTICAL CHEMISTRY
1
I
I
20
0
1
0
100
I I I 200 300 FLOW R A T E , C c l n i n .
I
I 400
Figure 3. Effect of oxygen flow rate on the oxidation rate of graphite rods at a constant pressure of 5 mm. of mercury and a delivered power of 250 watts
fractionation due t o a portion of the sample remaining in a partially oxidized, carbonaceous residue should be obviated. The data on oxidation rate have a precision of 4%, as determined from replicate experiments. The slow rise in rate with power suggests that the arrangement employed does not fully utilize the excited oxygen species. That the intensity of the blue glow is not appreciably decreased when the gas passes over small organic samples supports this conclusion. Higher rates can be achieved without increase in power by a more complex electrical generator and reaction chamber. Because the organic sample does not appreciably load the plate circuit, returning the capacitor during ashing is not required and unattended operation is feasible. The maxima observed in the studies of the effect of flow rate and pressure
Recovery of Radioactive Tracers in Dry Ashing
+ +++ + + ++
I
0
(Tabulated values are per cent recovered from vessel) R.f. discharge, 1.5 hr. Muffle furnace Trap and 24 hr., 3 hr., Sample Boat chambers 400” C. 900’ C. 0 67 9 SbC4 blood 99 0 23 0 100 HAEO~ blood 0 100 CsCl blood ... 30 0 102 COC12 blood 98 CuClz blood 0 100 58 101 CrCla blood 0 99 56 100 19 0 AuCL blood 70 30 .. 69 NaI filter paper 31 ... 0 .. NaI03 filter paper 100 ... 0 27 86 FeC13 blood 101 103 0 13 100 Pb(N0a)z blood 0 99 MnClz blood 79 99 0 Hg(N0a)z blood 8 92
