Improved Instrumental Determination of Oxygen in Organic Compounds by Pyrolysis-Gas Chromatography E. Pella and B. Colombo Istituto Carlo Erba per Ricerche Terapeutiche, Milan, Ztaly An improved method for the determination of oxygen in the range of 100-300 pg, present in organic substances is described. The analytical technique involves instantaneous pyrolysis of the sample in a stream of helium, quantitative conversion of the oxygen-containing gases to CO on contact with a special form of carbon at 1000 "C, and quantitative separation of the CO from the mixture by gas chromatography. Measurements are made by a thermal conductivity method. An improved packing for the reactor is proposed to eliminate the high blank found in the pyrolysis of chlorinated compounds. Positive pressure is established in the reaction zone to reduce back-sublimation and diffusion. To obtain the best separation of the mixture, which results after pyrolysis and conversion, a gas chromatographic system has been designed which completely eliminates dead volume. The nitrogen absorption on the carbon by the pyrolysis of nitrogen-containing compounds was studied in particular. In order to obtain active carbon, free from adsorption sites and without a blank value, a special carbon was prepared, coated with nickel (ca. 15%) and platinum (ca. 30%). I n the case of the pyrolysis of nitrogen-containing compounds, the oxygen/nitrogen ratio may be obtained from the ratio of the integrated values of the effluent CO and N2,without weighing the sample used for the analysis.
OVERTHE PAST TEN YEARS, many works have been published on the direct determination of oxygen, based on pyrolysis of the sample in an inert gas, contact of the pyrolytic gases with pelleted carbon at high temperature, and measurement by thermal conductivity of the C O separated by gas chromatography (1-9). The characteristics of the different methods proposed have been well summarized by Klesment (8). The determination of organic oxygen by pyrolysis-gas chromatography presents certain advantages : visualization of the effluent gases, a shorter analysis time, masking of the permanent blank value, and extension of the method to sulfurcontaining compounds; but it does suffer from several limitations. Ehrenberger (5) states that hydrogen is unsuitable as a carrier gas, and uses helium. However, helium is not the most suitable carrier since it does not promote the conversion of the pyrolytic gases to C O (IO); a higher temperature is required for the conversion (8) or pyrolysis is used in a static mode (11). The incorrect use of large carbon (1) A. Gotz and H. Bober, Fresenius' 2.Anal. Chem., 187,92(1961). (2) A. P. Terentev, A. M. Turkeltaub, E. A. Bondarevskaja, and L. A. Domotschkina, Dokl. Akad. Nauk (USSR), 148, 1316 (1963). (3) L. V. Kuznetsova, E. N. SotljaroLa, and S. L. Dobytschin, J. Anal. Chem. USSR,20, 836 (1965). (4) G. Pippel and S . Rorner, Mikrochim. Acta, 1966, 1036. (5) F. Ehrenberger and 0. Weber, ibid., 1967, 513. (6) W. Walisch and W. Marks, ibid., p 1051. (7) C. F. Meade, D. A. Keyworth, V. T. Brand, and J. R. Deering, ANAL. CHEM., 39, 512 (1967). (8) I. Klesrnent, Mikrochim. Acta 1969, 1237. (9) G. Dugan and V. A. Aluise, ANAL. CHEM., 41,495 (1969). (10) E. Pella, Mikrochim. Acta, 1968, 13. ( 1 1 ) G. Kainz and E. Wachberger, Fresenius' 2. Anal. Chem., 222, 278 (1966).
fillings (I, 4 , 7) comes from the use of helium as carrier and from the necessity of carrying out the pyrolysis in dynamic mode. The active, pelleted carbon, as usually used, has been shown (6)to be responsible for adsorption of nitrogen. Consequently the use of carbon coated with variable amounts of nickel has been suggested (2, 3, 6). Klesment (8) states that the gas chromatographic method has low sensitivity, while others (7) use it for determination of traces of oxygen. Other criticisms single out the intermittent, manual introduction of the sample, leading to trouble in the pressure-equilibrium of the system. Kainz (11) has directly questioned and refuted the possibility of separating gaseous pyrolytic products by gas chromatography. Difficulties arise with substances containing halogens and nitrogen (4), and this could limit the use of the gas chromatographic method t o substances containing only C-H-0. According t o our experience the method has the following drawbacks. First, very high blanks arise from the pyrolysis of substances containing hetero elements, especially chlorine, making the oxygen values unacceptable. Second, the adsorption of nitrogen on the pelleted carbon interferes with the quantitative separation of C O from nitrogen. This would invalidate the gas chromatography method. Third, the diffusion phenomena in the pneumatic system reduce the conversion to CO, and increase peak tailing. However these drawbacks will be found in all methods which use granular carbon, according to Unterzaucher (12), and which measure the product of the conversion, CO, or its oxidized product, COz by thermal conductivity. In the study of the gas chromatographic determination of oxygen, a commercially available analyzer has been used because of its automatic sample introduction system and final digital print-out. To eliminate the drawbacks listed above, the gas chromatographic unit, reactor, and type of filling have been modified. All the improvements mentioned have been incorporated into the apparatus used, but they may also be applied t o other instruments which use the same method of analysis. EXPERIMENTAL
Reagents. The most important, original reagent used is the special carbon coated with Ni and Pt, described separately. The other reagents were: helium, G . C. grade; Pt wool, made from Pt wire (diam. 0.03 mm), C, Erba, Milan, Italy; Ag foil, for the containers, thickness 0.01 mm, purity 99.99 %, Degussa, Frankfurt, W. Germany; BTS catalyst, 7-35 mesh, B.A.S.F., LudwigshafenIRhein, Germany, activated and used at room temperature (13); Lithasorb, 16-25 mesh, Fisher, Fair Lawn, N.J., U S A . ; magnesium perchlorate, anhydrous, 7-16 mesh, G. Frederick Smith Chemical Co., Columbus Ohio, U S A . ; and Sicapent, 7-16 mesh, Schuchardt, Miinchen, W. Germany. The reference organic samples used were micro-analytical grade compounds C. Erba. Most of these were purified by (12) J. Unterzaucher, Ber Deut. Chem. Ges., 73, 391 (1940). (13) M. Schiitze, Angew. Chem., 70, 697 (1958).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
1563
+ oxi-circuit
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Figure 1. Line diagram of apparatus Pressure regulator Flow controller Purification tubes Sweeping valve Samples dispenser Water cooler OXY reactor Oven for OXY reactor Calibrated capillary tube Scrubbing tube Thermostated oven OXY chromatographic column Detector Recorder Digital integrator-printer
zone-refining. The purity of all reagents was checked by fusion curves (14) using a Perkin-Elmer DSC-1B calorimeter. Special Carbon CK3-Ni-Pt (Patent pending). Pelleted carbon, 25-120 mesh, type CK3 (Degussa, Frankfurt, W. Germany) was heated at 1100 "C for 8 hr in a hydrogen stream, then allowed to cool under hydrogen. It was then treated with a 10% solution of Ni(NO& with a nickel-to-carbon ratio of 1:4 by weight. The slush was put into a crucible and evaporated t o dryness with stirring. The dry material was then placed in a quartz tube, heated to 150 "C, and flushed with a hydrogen stream. The temperature was increased to 1000 "C over 2 hr. Between 300-500 "C, N H 3 is evolved; above 500 "C, nitrogen oxides are lost. The material was kept at 1000 "C for 30 min, then cooled to room temperature in a hydrogen stream. The nickel-coated carbon was then treated with a concentrated solution of H2PtCls,containing Pt at the ratio of 2 : 5 by weight with the nickel-coated carbon. The mixture was rapidly evaporated to dryness in a crucible with continuous stirring; it was then placed in a quartz tube at 200 "C and flushed with a hydrogen stream. The temperature was raised (14) A. P. Gray, "Thixmal Analysis Newsletters," No. 5 and No. 6,
Analytical Division, Perkin-Elmer Co., Norwalk, Conn. 1564
e
Figure 2. Pyrolysis reactor, filling and temperature distribu-
tion to 500 "C and maintained for 30 min. It was then increased to 1000 "C for 30 min. The reagent was cooled in a hydrogen stream. The resultant bimetallic carbon contained carbon, nickel, and platinum in the ratio of 57.1 :28.6: 14.3 by weight. It was a grey-black, granular material, with moderate mechanical resistance; when packed in the tube, it offered less resistance to the gas than the starting material. 4.5 grams are required for one packing, which was then used for cu. 1000 analyses without any noticeable decrease in activity, and showing only slight reduction in volume. Apparatus. An Elemental Analyzer, Model 1102 ( C . Erba) was used, consisting of a control unit, a n analytical unit, and a print-out integrator. The control unit includes all the controls necessary for the analytical program, operation of the detector bridge, temperature control, and recording. The analytical unit consists of two separate and independent analytical channels (Figure l), OXY and CHN, the first used for the determination of oxygen and the second for the determination of C-H-N in organic samples. One channel passes through one arm of the detector and is used as a reference while the other channel passes through the remaining arm and is used for the analysis. The two analytical channels d o not therefore operate simultaneously, but may operate alternately as analytical and reference circuits (Figure 1). This work does not describe the C H N circuit which is considered only as a reference circuit. The OXY circuit (Figure 1) consists of the carrier source, a system to control and purify
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
the carrier, a vertical reactor fitted with an automatic sampling device and heated by an independent oven, a calibrated capillary restriction, a scruhkr to remove the acidic gaseous products, a chromatographic column, and a detector both thermostated at 120 -C. The detector signal feeds a conventional potentiometric recorder and i n parallel a dimtal electronic integwtor with print-out. Control and Purification ofthc Carrier Gas. After pressure control, the ciirrier gas i s purified i n two scrubbing tubes (height 270 mm, diam 32 X 28 mm). The first contains BTS catalyst (8-25 mesh); the second a mixture o f Arcariteactii~ Lithasorh (2 I vol. 80 rnm), magnesium perchlorate (80 mm), and Sicapent (80 mm). Attached to the reactor i s a calibrated capillary restriction (length I S 0 mm, diam 2 X 0.25 mm). which cau$es an 0.2 k g mi. pressure drop. The chromatographic column also causes a pressure drop of0.2 kg cmr and the carbon packing a further drop o f 0.05 kg~cm'. Therefore with a flow rate of 25 ml min, the pressure on the reactor inlet i s 0.45-0.5 k g cm2. Reactor. The OXY reactor i s made of a quartz tuhe, as shown i n Figure 2, designed to eliminate any dead volume. The upper part, or pyrolysis-reaction wction, i s hlown from pure q u a r k The lower part, inner diameter 1 mm, i s made of normal quartz. The tuhe i s heated :o that there are two sections of uniform constant temperature for the pyrolysis and the reaction zones. The pyrolysis 7one i s maintained 100 'C higher i n order to ensure ready fusion of the A g containers and instantaneous pyrolysis o f the sample. The temperature at the two ends o f t h c tube i s maintained at 40-50 OC. T h e tuhe, pretreated with 4 0 x HF for 60min. i s packed only with the special carbon CK3-Ni-Pt. held in place by two pt wool plug>. A pure quartz crucible i s placed ahovr the upper plug (Figure 2) to collect the containers ior ahout 200 analyws. Without thecollecting crucible, the Ag containers would make an alloy with thc metals coating the carbon. I t i s advisable to u w i( 0.1-mm thick 1'1 cylinder in the pyrolysis zone (Figure 2). The tub? ends are connected to the chromatographic circuit with nuts and Viton O-rings. Autnmatic Sampler. The sample dispenser, shown i n Figure 3 niakes i t posihle to introduce 23 samples of solids or liquids automatically without interrupting the gas flow or operation of the cycle. The sample dispenser i s kept at room temperature by a cooling coil fitted to the sampler supports. Scrubber. A glass scrubbing tube containing alkaline adsorbents and drying agents i s used to remove the acidic pyrolytic g a m . I t i s tilled with a mixture o f Ascarire-Lithasorb (2 1 VOI,50 mm), and anhydrous magnesium perchlorate (50 mm). 'The packing should he renewed monthly. Chromatographic Column. A column of staides) steel i s used, length 12Orl mm, (ham 6 X 4 mm, filled with Molecular Sieve SA (30-50 mesh). Refore use, the SA sieve i s washed repeatedly w i t h distilled water until the powder had k e n removed, oven-dried at 1 5 0 'C for 4 hours, then heated at 450 ' C i n a stream of dry nitrogen (50 ml min) for 6 hr and cooled under nitrogen. Operating at 120 "C with a carrier gas flow o f 25 ml/min, this column gives the following approximate retention times: hydrogen, I min 30 sec; nitrogen, 2 min 15 sec; methane, 3 min; carbon monoxide, 6 min. Detection and Measuring System. The detector used i s a conventional catharorneter with four WX Rhenium-Tungsten 9225 tilamentsas sensing elements (Gow-Mac InstrumentsCo. Madison, N.J. 07Y40). The potentiometric recorder has the following features: sensitivity I 2.5 mV full-scalc, pen speed I-sec iull-scale, chart speed 30 in. hr. The electrical signal also feeds an electronic digital integrator with data print-out (Figure 4). in parallel with the recorder, which can integrate well-resolved peaks from a steady and relatively drift-free base line. The integrator i s automatically zeroed to the base line o f the recorder at the beginning of the analysis. T h e maximum
Figure 3.
Automatic sampler
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Diagram of integrator
capacity i s 12.000 counts per minute, F.S. at detector attenuation 8. Procedure. The solid organic samples are weighed into the Ag containers in amounts not exceeding 300 pg of oxygen. For weights 5 1.5 mg, a Cahn-Gram electromagnetic balance i s used; for greater weights, normal microbalances are used. The containers are then closed manually. The containers, made o f Ag foil (height 6 mm, diam 4 mm, weight 15-17 mg), are washed with acetone, chloroform, ovendried at 120 "C and purged with hydrogen at 300 "C. After this treatment, the blank value i s only 0.2 *g o f oxygen, provided they are stored under an inert gas. Liquid samples are weighed into pretreated Ag capillary tubes (height 6 mm, diam 2 X 1 mm), which are closed at one end. The open ends of the tubes are pressure sealed and the tubes are weighed. T h e containers are loaded in the numbered holes of the sampling device drum. Between the unknown substances, some weighed CHNO standards are inserted for calibration. The drum i s then placed in the sampler. The cover i s closed, and the sampler i s flushed for 2 min at approximately 200 ml/min helium flow. The sweeping valve i s closed and the pressure in the circuit inlet i s restored t o the previous value. The carrier gas flow i s adjusted to exactly 25 ml/min.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9. AUGUST 1972
1565
Table I. Oxygen Analysis of Standard Compounds Compound Cholesterol 2-Bromo-4-chloroacetophenone 1-Hydroxy-4-phenyltetrazole
Acetanilide Sulfapyridine Chloranil Anthraquinone p-Fluorobenzoic acid Cyclohexanone-2,4-dinitrophenylhydrazone 9-Fluoro-16-hydroxyprednisolon Benzoic acid Urea
Sulfonal l-Chloro-2,4-dinitrobenzene Phthalic anhydride p-Nitrobenzoic acid Adipic acid Picric acid n = number of runs. X = theoretical oxygen. f = average found. 3 - X = error. S = standard deviation
Weight, I*g 2000-5000 1000-4000 1000-3000 1000-3000 1000-3000 1000-3000 1000-3000 500- 1500 500-1500
n
3 - x
S
3 2 3 5 10 3 2 4 10
4.14 6.85 9.81 11.84 12.86 13.01 15.37 22.84 23.00
4.05 6.91 9.76 11.90 12.81 13.13 15.48 23.02 23.01
-0.09 +O. 06 -0.05 +0.06 -0.05 +o. 12 +o. 11 $0.18 +0.01
0.094
500-1500 500-1500 500-1500 500-1 500 500-1000 500-1000 500-1000 500-1000 500-1000
2 12 3 3 10 4 12 3 3
24.34 26.20 26.64 28.03 31.59 32.41 38.29 43.79 48.88
24.51 26.18 26.42 27.90 31.61 32.32 38.23 43.50 48.70
+O. 17 -0.02 -0.22 -0.13 +0.02 -0.09 -0.06 -0.29 -0.18
0.175 0.111 0.079 0.137
x
The detection system is switched on and the programmer, set up for 7-min analysis time, is actuated. Calculations and Results. As already stated, the detector signal is fed t o a recorder where peaks corresponding to the effluent gases are drawn on a chart. The traditional method of measuring the peaks war rejected because it is tedious and subjective (15); it also offers poor precision and is not applicable to peaks off the scale. Therefore, it is better t o use a simple automatic, electronic integrator with data print-out, which receives the signal directly from the detector and not from the recorder, in order to reduce the errors and increase the dynamic range. The system is calibrated with oxygen-containing standard compounds. The oxygen response factor is calculated as follows : oxygen (theor.) X standard weight (pg) f = integral counts for C O The oxygen per cent for the unknown may then be calculated:
f X integral counts for CO sample weight (pg) Correction for blank is not necessary in the procedure described ; it becomes necessary in oxygen determinations under 100 pg. The blank value is then 0.2-0.3 pg of oxygen. The oxygen results for standard compounds are shown in Table I. The accuracy of the results is independent from the functional group in which the oxygen is bound, because of the high reducing activity of the bimetallic carbon used. Above 300 pg of oxygen, the accuracy decreases for compounds containing oxygen in the form of -COOH group. It remains good up to 600 pg of oxygen for compounds without this group. Compounds containing halogens, sulfur, and nitrogen gave no problems. Compounds with a low fluorine percentage may also be analyzed. oxygen
=
DISCUSSION
Study of the Blank Value. Instantaneous pyrolysis of a n organic compound has one intrinsic drawback-Le., a n oc(15) J. H. Graham, Microchem. .I., 13, 327 (1968). 1566
casional strong blank value associated with the pyrolysis of compounds containing hetero elements, especially chlorine. The possibility of such errors is seldom mentioned in the literature (4,16), perhaps because gradual pyrolysis shows less striking effects. However, it is well known (17) that there is a considerable difference in the blanks obtained if organic samples are pyrolyzed or not. When thermal conductivity measurements are used, the instantaneous pyrolysis of a sample, oxygen-free but with a high chlorine content, gives a peak which has the same retention time as CO. Compounds with high sulfur and nitrogen content but free from oxygen behave similarly but t o a lesser extent. If the compounds contain both oxygen and chlorine, much too high oxygen values are obtained. The phenomenon is more evident with new quartz tubes and fresh fillings. The influence of the filling materials on the blank value was also examined. Organic compounds, purified by zone-refining, were used as “cleaving” agents. The molar purity of these compounds, measured by a DSC technique (14), was generally greater than 99.9%. Oxygen from impurities was therefore excluded. They were stored under a n inert gas t o exclude free molecular oxygen (18). Since external oxygen is not present, the blank value can arise only from the interaction between the pyrolysis gases and the filling materials. From the values in Table 11, it may be deduced that the quartz wool is the material most responsible for the occasional high blanks. During the instantaneous pyrolysis of organic compounds containing hetero elements, relatively high concentrations of acidic gases are produced. These are very reactive in a reducing atmosphere (cf. 19), and attack the surrounding materials, of which the quartz wool is the most easily etched because of its large surface and its flux content. (16) D.Fraisse and R. Levy, Bull. SOC.Chim. Fr., 1968,445. (17) G. Ingram, “Methods of Organic Elemental Analysis,” Chapman & Hall, London, 1962,p 97. (18) P. Gouverneur and A. C. Bruijn, Tulurzta, 16,827 (1969). (19) L. Blom and M. H. Kraus, Frese,zius’Z. Anal. Clzem., 205, 50 (1964).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
Table 11. Influence of Filling Materials on Blank Value.
Variation of filling in the reactorb B = carbon CK3, treated in hydrogen for 8 hr at 1100 "C Temp. = 1120 "C
Cleaving agent Weight, r g Hexachlorobenzene 4560 Hexachlorobenzene 3520 Hexachlorobenzene 2560 Hexachlorobenzene 2456 Imidazole 1860 Imidazole 1118 Thiourea 2090 Thiourea 2106 Hexachlorocyclohexane 2260 Hexachlorocyclohexane 2231 Hexachlorobenzene 2678 B = carbon treated with chlorine Hexachlorobenzene 2581 for 2 hr at 1000 "C Hexachlorobenzene 2647 Temp. = 1120 "C Hexachlorobenzene 2655 B = carbon CK3 10% Ni Hexachlorobenzene 2543 Temp. = 1000 "C Hexachlorobenzene 2585 Thiourea 2017 Thiourea 2064 Hexachlorobenzene 2553 B = carbon 50% Pt Hexachlorobenzene 2580 Temp. = 1000 "C Hexachlorobenzene 2601 Hexachlorobenzene 2108 B = quartz chips washed with 40% HF Hexachlorobenzene 2675 (carbon eliminated) Hexachlorobenzene 2654 Temp. = 1100 "C Hexachlorobenzene 2519 Hexachlorobenzene 2456 A = Pt WOOI Hexachlorobenzene 2513 B = carbon CK3 Hexachlorobenzene 2640 c = Pt wool Hexachlorocyclohexane 2118 Temp. = 1120 "C Hexachlorocyclohexane 2441 Hexachlorocyclohexane 2550 Imidazole 2815 Imidazole 264 Thiourea 2131 Thiourea 2226 Anthracene 1890 Anthracene 2132 Phenanthrene 3300 Phenanthrene 1640 2-Chloronaphthalene 3446 2-Chloronaphthalene 2456 4 General experimental conditions: tube in Brazilian quartz, instantaneous pyrolysis, carrier gas at 25 ml/min. Filling (A + B + C): A = 20 mm quartz wool, B = 140 mm active carbon, C = 10 mm quartz wool.
+
+
Thus oxygen is released and converted t o CO. If the quartz wool is removed, the blank value is drastically eliminated. A very small blank remains, due to the partial attack on the quartz walls by the pyrolysis gases. This value disappears in the case of hydrocarbons, which cannot produce acidic gases. Lack of knowledge has up to now prevented the adoption of remedies such as the replacement of the quartz wool in the filling by Pt wool, and lining the walls of the tube in the pyrolysis zone with a thin cylinder of Pt. Adsorption Phenomena. The prerequisite for the application of gas chromatography to the determination of oxygen is the quantitative separation of the pyrolysis gases. The gases are eluted from the column in the following order: Hz, Nz, CH4, and CO. Using our special carbon, C H a is seldom present except in small amounts. Hydrogen produces only slight effects on the base line (round top and/or negative peaks). The main effluents are therefore nitrogen and CO. However, using untreated carbon, it is impossible to completely separate nitrogen from C O because of the nitrogen adsorption on the carbon. Partial adsorption is also caused by the carbon formed during the thermal degradation of the sample. The nonlinear desorption of nitrogen can be detected experimentally by comparing the chromatograms obtained using
Blank pg oxygen 190 97 63 55 22 17 35 30 16 65 14 65 64 105 94 78 45 39 110 72 63 154 93 67 80 3 2 2 1 3 0.1 2
3 3.6 3 0.1
1 1
0.5 0.6 0.5
untreated carbon and adsorption-free carbon, like the special carbon CK3-Ni-Pt. The difference in behavior is striking (see Figure 5 , A and B). When untreated carbon is used, there are certain undesirable effects such as: (1) The nitrogen desorption isotherm is not linear and the N$ partially overlaps the C O peak. The oxygen values are too high. (2) Compounds with low or medium nitrogen content give very small or no nitrogen peaks. (3) Compounds with high or medium nitrogen content give a much smaller peak than expected. (4)The difference in the response factor, detectable using special and untreated carbon, would also suggest temporary adsorption of C O on untreated carbon. Tests with various types of carbon are summarized in Table 111. The adsorption is given in percentages for three typical standards, comparing the CO/Nz ratio with the theoretical' ratio. The use of carbon different from CK3 in origin or structure does not change the adsorption values; treatment of CK3 with acidic gases at high temperature modifies the adsorption values very little. Coating the carbon with metals gives remarkable advantages, but causes serious disadvantages at the same time. By coating the carbon up t o 50% with Pt, the adsorption is reduced, but large amounts of C H I form and the results are
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
1567
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Figure 5. Analysis chromatograms obtained using untreated carbon ( A ) and special carbon ( B )
affected by temporary adsorption (see Figure 6). Nickel coating even up to 1 5 % eliminates this, but there are increasing interactions between the metal and the quartz of the tube (Table 11). These interactions cause a standing blank, also mentioned in the literature (20), which is higher in the case of chloro compounds. This arises from attack on the walls of the tube by the metal and NiClr. To make the nickel-coated carbon inert, the carbon must contain an additional 30% of platinum. It then fulfills' the requirements of absence of adsorption sites, chemical inertia, strong reducing activity, and capacity to lower the C H 4 in the pyrolytic gases. It is also resistant to prolonged use, and has reasonable mechanical resistance. The presence of Pt has the effect of forming small amounts of unsaturated hydrocarbons. This has been proved (20) E. A. Bondarevskaja and M. Korshun, J. Anal. Chem. USSR, 18, 644 (1963). 1568
by oxidizing the gases eluted after the pyrolysis of compounds containing only C-H, and measuring the COz obtained. The special carbon must be protected with a quartz crucible (Figure 2) from contact with the Ag from the containers. The best temperature is 1000 OC but temperatures of 1060 or 1100 "C have been maintained for several hours without damage to the carbon. The use of special carbon does not altogether prevent partial adsorption in the pyrolysis zone due to the carbon formed at themoment of thermal degradation of the samples. However, the use of a Pt cylinder in the pyrolysis zone already described minimizes local absorption. It is nevertheless advisable periodically to clean the pyrolysis zone of the carbon formed with frequent runs of Ag20 (4-6 mg). GC Separation. Together with the study of adsorption, the factors on which the gas chromatographic separation of the effluent gases depends were also studied and improved.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
Table III. Study of Adsorption on Various Types of Carbon. Adsorption of nitrogen, % Cyclohexanone, Temp. of 2,4-dinitroCarbon used carbon "C Urea phenylhydrazone Acetanilide Type CK3 treated with Hz at 1000 "C 1120 45-47 42-46 80-85 Carbon CK3, as above, but treated with 1120 36-40 36-38 70-75 C12at lo00 "C for 2 hr Carbon wool (elementary fibers 0.101100 50-55 46-50 80-85 0.15 mm diam.) Type Spheron 9 1120 33-35 46-48 79-8 1 Carbon CK3 20% Pt 1000 20-22 22-24 48-50 1000 6-7 12-14 28-30 Platinized carbon (50% Pt) Carbon CK3 1% Ni 1000 20 31 47-50 1000 16 Carbon CK3 5 % Ni 17-18 28-30 1000 7-8 Carbon CK3 10% Ni 8-10 20-22 Carbon CK3 15% Ni 1000 Practically absenth Carbon CK3 25% Ni 1000 Practically absentb Carbon CK3 7% Ni 10% Pt 1000 4-5 5-7 14--16 1000 Carbon CK3 15% Ni 30% Pt Practically absenth 4 General experimental conditions: tube in Brazilian quartz, instantaneous pyrolysis, carrier gas at 25 ml/min., positive Dressure of 0.5 kg/cm2. Filling (A B C): A = 10 mm Pt wool, B = reducing layer, C = 0.5 mm Pt wool. Pelleted carbon used at 25-70 mesh. * The absorption of nitrogen on contact carbon is taken to be zero; however, from the carbon formed during the pyrolysis, there remains a minimum adsorption (urea 2%, cyclohexanone dinitrophenylhydrazone 4 z , and acetanilide 9-1 1 %).
+ + + + + + + +
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Figure 6. Analysis chromatograms obtained using platinized carbon (50x:)
The first condition, simultaneous injection of the gases, is ensured by the pyrolysis being instantaneous and by the absence of adsorption. If the pyrolysis temperature is too low, delay in the elution, and therefore tailing, may occur. A too small positive pressure in'the pyrolysis zone enhances the diffusion and increases tailing. The peak shape is also affected by the dead volumes in the system. To prevent these effects, a reaction tube free from dead volume is used, and unnecessary packing materials are avoided. Temporary adsorption may occur in the scrubbing tube through the use of unsuitable materials, and on the molecular sieve if not properly treated. Effects of Internal Pressure. As in all analytical systems using a gas chromatography column, there is a pressure difference between the inlet and exit in the column. In order to increase this pressure, a capillary restriction is inserted between the reactor and scrubbing tube (Figure l), this results in a positive pressure of 0.4-0.5kg/cmZ a t the circuit inlet.
The positive pressure in the reactor eliminates the pressure variations arising from the sudden thermal degradation of the organic sample, and improves the catharometer operation. Moreover, the positive pressure hinders back-sublimation and reduces diffusion, which are the main disadvantages of using helium as the carrier in the determination of oxygen (10). Against these positive effects, there are also some negative ones. All reactions converting H 2 0 , C 0 2 ,NO, and SOz t o CO involve an increase in volume. A pressure increase in the system should change the equilibrium in the direction opposite t o the formation of CO. This is confirmed by plotting CO conversion percentage L'S. positive pressure in the reactor (Figure 7). For pressures of 0.5 kg/cm2, the conversion to CO was measured by a n absolute method, i.e., oxidation of the eluted CO t o COz and measurement of this; for higher pressures, values were calculated from the variation in the factor response. Figure 7 shows that the decrease in conversion is negligible up to 0.5 kg/cmZ; it is 4% at 1 kg/cm2,and as high as 14% at pressures of 2 kg/cm2. This means the maximum positive pressure to be applied is in the order of 0.5 kg/ cm z. Analytical Possibilities of the Method. The chromatographic method of oxygen determination makes it possible to determine the per cent of nitrogen (2,6), provided that adsorp-
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Table IV. Determination of
Compound Hydrastine Acetylaminophenyl salicylate p-Nitrobenzoic acid Atropine Hippuric acid Benzylisothiourea phosphate Picric acid l-Chloro-2,4-dinitrobenzene Phenacetin Nicotinic acid Isatin p-Aminobenzoic acid Cytidine Barbituric acid Research comp. Picrolonic acid Cyclohexanone-2,4-dinitrophenylhydrazone Acetanilide Benzarnide a-Benzyldioxirne Adenosine Research comp. Sulfapyridine Research comp. Urea Nicotinamide Isonicotinylhydrazide 1-Hydroxy-4-phenyltetrazole.
of Nitrogen and the COiN Values Ratio Nitrogen n X f a - X 2 3.65 3.78 $0. 13
Formula GiHziNOa CiaHiaNOa CiHsNOa CiiHz3NOs CgHgN03 C9HirClNzOeSP C6H 3N 3 0 7 C~HH,CINZO~ CioHuN02 C6HjNOz CsHbNOz C7HiNOz CgHi3N30s C&LNZOI CsH2NrOjS CioHsNaOs CizHirNaO4
2 2 4 20
5.16 8.38 4.84 7.82 8.13 18.34 13.88 7.82 11.38 9.52 10.21 17.28 21.87 23.14 21.21 20.14
CsHgNO CiH7NO CiaHi2NzOz CioHi3N~Oa CioHi~Nao~ CiiHiiN3OzS CsHjNj03S CHrNzO C6HeNzO C~H~HPO CiH7NaO
4 2 3 2 4 5 2 20 2 3 2
10.36 11.56 11.66 26.21 23.32 16.86 27.88 46.65 22.94 30.64 34.34
3 3 5 2 2 3 10 6
2 4 4 3
4.66 7.46
-0.50 -0.92 -0.23 -0.41 +0.77
4.61 7.41 8.90 18.30 13.64 8.04
-0.24 $0.22
9.12 10.35 17.11 21.85
-0.40 $0.14 -0.17 -0.02
20.86 20.08
-0.35 -0.06
10.24 11.29 11.21 25.35 23.01 16.65 28.14 46.30 22.24 31.10 33.82
-0.12 -0.27 -0.45 -0.86 -0.31 -0.21 +0.26 -0.35 -0.70 +O. 46 -0.52
-0.04
Ratio O/N 6 4 4 3 3 3 2.333 2 2 2 2 2 1.666 1.5 1.25 1.25 1 1 1
1 0.80 0.75 0.666 0.6 0.5 0.5
0.33? 0.25
Ratio CO/N 10.40 10.23 7.34 7.62 6.95 4.81 4.66 4.60 4.75 4.72 4.61 3.50 3.22 2.80 2.88 2.24 2.35 2.31 2.36 1.84 1.75 1.64 1.49 1.08 1.13 0.92 0.58
.-0
3In Figure 8. Plot of the ratios of the COinitrogen values us. the oxygen/ nitrogen atomic ratios
2 2
Atomic Ratio O/N b-,
tion of nitrogen is avoided and that the separation of nitrogen from hydrogen and methane is achieved. However, under normal conditions the nitrogen determination is not accurate enough because of local adsorption phenomena and the occasional formation of HCN (21, 22). The per cent of nitrogen in standard compounds containing nitrogen and oxygen in different ratios has been determined (21) F. Saker, Mikrochim. Acta, 1962,835. (22) R. Belcher, D. H. Davies, and T. S. West, Tuluntu, 12,43 (1965). 1570
(Table IV). A calibration factor obtained in the same way as for oxygen is used t o calculate the per cent of nitrogen. From Table IV it can be seen that the nitrogen values for high and medium contents are accurate enough, but less so for low contents. The main cause of error is obviously the partial adsorption on the carbon formed during the pyrolysis. The formation of HCN appears to be negligible, since compounds such as benzyldioxime, benzarnide, p-aminobenzoic acid, known t o produce high amounts of HCN (23), d o not give (23) R. Belcher, G. Ingram, and J. R. Majer, Mikrochim. Acta, 1968,418.
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large deviations. The high pyrolysis temperature ( 1100 “C), may explain the absence of interference of HCN. The major potential application of the method, however, consists not only in the semi-quantitative determination of nitrogen, but also in the information on the oxygen/nitrogen atomic ratio or vice versa, which is also obtainable without weighing the sample. In fact it may be postulated that, for pure samples, the integrated values or other parameters relative t o the peaks of the main effluent gases, C O and Ne, show a ratio proportional t o the ratio of the oxygen and nitrogen atoms present in the molecule. Table IV gives the ratios obtained from the integrated values for the C O and nitrogen for each compound. These are plotted (Figure 8) against the atomic ratios oxygen/nitrogen. The COlnitrogen values, deduced from the theoretical detector response, are also included and are plotted against the atomic ratios (sketched line). The CO/nitrogen values found experimentally agree substantially with the theoretical values. Deviations are shown
only by compounds with a low nitrogen content although the values found may be plotted in a regular curve. From the diagram it is possible t o obtain the oxygen/ nitrogen atomic ratio directly from measurements of signals or areas of peaks without weighing the sample or calculating percentages. The integrated CO values, based on the CO/nitrogen value, may be correlated with the integrated values for C 0 2 , HzO, and Nz, obtained by combustion of the same sample in a different analytical circuit and measured with the same catharometer in the same carrier gas. It is therefore possible t o obtain four integrated values proportional t o carbon, hydrogen, nitrogen, and oxygen, by analyzing the same sample, without weighing, in two different analytical circuits. By using a simple mathematical calculation, the atomic ratio C/H/N/O may be found from this data, and hence the empirical formula for compounds containing only these four elements. RECEIVED for review September 20,1971. Accepted March 8, 1972.
Determination of Alkoxy and Vinyl in Siloxane Materials Using Alkali Fusion Reaction and Gas Chromatography Carl L. Hanson and Robert C. Smith Analytical Sercices Department, The Dow Corning Corporation, Midland, Mich.48640
Determination of alkoxy and vinyl substituents on silicon at concentrations 0.001 to 1% in fluid, gum, and resin samples was accomplished using fusion with potassium hydroxide and gas chromatographic analysis of alcohols and ethylene. Flame ionization detection was employed. Alcohols were trapped in water and analyzed using a 80- to 100-mesh Porapak Q column. Type of alkoxy present was identified by observation of the corresponding C1 to C, alcohol. Ethylene was collected in a gas bag and was resolved from the major product, methane, with a 60- to8O-mesh activated alumina column. Gas bag volume was determined by employing butane as a reference standard. Relative standard deviation was generally less than 20% in the alkoxy analysis and within 10% for the vinyl method. Alkoxy results were always low but within about 90% of expected values. Analysis of standard vinyl-containing materials indicated a relative error of -10% or less in most cases.
SODIUM OR POTASSIUM HYDROXIDE has been employed for rapid fusion of siloxane polymers t o produce soluble silicates for total silicon analysis ( I , 2). The fusion reaction cleaves alkyl, aryl, and vinyl substituents to produce their corresponding saturated or unsaturated hydrocarbon. Alkoxy groups are converted t o alcohols. The usefulness of the reaction for qualitative identification of groups attached t o silicon has been reported ( I ) . Quantitative determination of alkyl content in polysiloxanes and functional silicon materials using gasometric measurements has been described (1) W. H. Grieve and K. F. Sporek, Winter ACS Meeting, Phoenix, A r k , January 17-21, 1966, paper No. 43. (2) J. H. Wetters and R. C. Smith, ANAL.CHEM., 41, 379 (1969).
(3). This paper describes methods to quantitatively determine alkoxy ( r S i O R ) and vinyl (=SiCH=CH2) a t 0.001 t o 1 %, employing alkali fusion reactions. Samples are decomposed with potassium hydroxide in Hastelloy crucibles contained within a quartz tube purged with nitrogen. Alcohols are collected in a trap containing water. Ethylene, produced from vinyl groups, is swept into a gas bag. The amount of alcohol or ethylene collected is determined by gas chromatography (GC). Analysis of alkoxy in polysiloxanes using acetylation (4, 5 ) or modified Zeisel method (6) is generally limited t o concentrations of 1% or greater. Variations of the Martin (7) mercuric acetate procedure and the iodine monochloride (8) reaction method for unsaturation are generally applicable with 1 % or greater vinyl present. Solubility problems are sometimes encountered with high molecular weight materials as gums and with certain resins and gels. Highly crosslinked polymeric siloxanes d o not readily dissolve in solvents and are not easily decomposed using acid ashing procedures. Alkaline fusion reactions provide the only practical dissolution method for these materials (I). The methods reported in this paper also complement existing alkoxy and vinyl pro(3) M. G. Voronkov and V. T. Shemyatenkova, Izc. Akad. Nauk SSSR, Ord. Khim. Nauk, 1961, 178; C.B. Trans. 164-5; Cliem. Abstr., 55, 162856 (1961). (4) J. A. Magnuson, ANAL.CHEM.,35,1487 (1963). (5) J. A. Magnuson and R. J. Cerri, ibid., 38, 1088 (1966). (6) “Analytical Chemistry of Polymers,” G. M. Kline, Ed., Interscience Publishers, New York, N.Y., 1959, p 372. (7) R. W. Martin, ANAL.CHEM.,21, 921 (1949). (8) “ASTM Standards, 1971,” Part 20, D 1959, p 867.
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