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Anal. Chem. 1981, 53, 164-167
Determination of Oxygen in Organic Substances by Reaction-Frontal Gas Chromatography Jitka Uhdeovii" and Vlastimll Rerl Institute of Analytical Chemistry, Czechoslovak Academy of Sciences, 6 I 142 Brno, Czechoslovakia
A method Is descrlbed for the elemental mlcroanalysis of up to 450 pg of oxygen In organic substances. The method Is based on the pyrolysis of the sample In a stream of hellum, conversion of the oxygen-contalnlng pyrolysls products to carbon monoxlde on a Nlcoated carbon packing at 1050 O C , and oxldatlon of the produced H2 and CO to H20 and COOon CuO at 650 O C . The lnterferlng elements (halogens, sulfur) are retalned on Ag. The homogenlzed mixture of He, N2, C02, and H20 Is analyzed by frontal GC on a column wlth Porapak 0, employing a thermal-conductlvlty detector. The complexity of the opposlng processes taklng place on the carbon packlng is reflected by the fact that the actual blank value differs from that measured when no pyrolysis products have passed through the carbon packing. Rellable results can be obtained when employing a standard, the qualltative composltlon of which Is the same as that of the substance under analysls.
The hitherto published detailed studies of frontal gas chromatography (GC) as a means of analysis of simple mixtures of gases have focused on the development of suitable instrumentation and, in its application, on the elemental analysis of C, H, and N (1-7),determination of hydrogen in and instrumental coupling metals (8)and oxygen in copper (9), with elution GC to provide a tool for the identification of GC eluates (10,11). The present study of the elemental analysis of oxygen links up with those on the analysis of C, H, and N; the aim was to minimize alterations in the instrumental section in order to facilitate the passage from the C, H, and N analysis to the analysis of 0 and vice versa. Therefore, attention was concentrated mainly to changes in the reaction section and to some peculiarities characteristic of the given instrumental setup with the use of reduction conversion on a carbon packing (12). The process taking place in the determination of oxygen by the carbon reduction method is determined by the following factors: (i) the composition of the pyrolysis products of the substance analyzed, (ii) the properties of the carbon packing, (iii) the reactions of the pyrolysis products with the carbon packing and with the reaction tube. With regards to (i), the pyrolysis of C, H, 0 compounds in an empty pyrolytic tube (13) resulted in CsHe, c02or co (and/or a mixture of both), CHI, Hz, H 2 0 (saccharides), and traces of acetylene, nitrogen-containing compounds yielded HCN and/or NO, and sulfur compounds produced HzS, CSZ, and COS (14-17). The pyrolysis on 50% platinized carbon (14) led to the same products (except for benzene and acetylene). With regards to (ii), it was proved that carbon-oxygen complexes, C,O, with 3c >> y, denoted C(0) hereafter, originated on the charcoal surface when reaction of COz, 0 2 , CO, and a mixture of CO and C02 with charcoal was carried out at temperatures of 650-1100 "C (18,19). The C(0) complexes originated preferentially from C02 and did not decompose under usual experimental conditions. They were decomposed 0003-2700/81/0353-0184$01.00/0
by CO only. The conversion to CO depends also on the catalyst-to-carbon ratio on the surface of the carbon packing. The specific surface area of the charcoal had no effect on the final yield of CO (20,21). With regards to (iii), the course of the reduction of COz on charcoal is described by two simultaneous balanced reactions (19,22)
c02 + c + 2co c02 + c + C(0)
+ co
(1) (2)
An analogous reaction course can be assumed in the reduction of oxygen-containing pyrolytic products on the carbon packing (20). Reaction 2 is considered to be one of the sources of the blank value, Le., the so-called "blank error". Also other pyHCN, "3, hydrogen halogenides, etc.) rolysis products (H2, may decompose C(0) complexes, thus increasing the contents of CO in the gaseous phase. Another source of the blank value may be the "side reaction error" (20),represented by the reaction SiOz + C ;=e CO + SiO, taking place on the surface of the quartz reaction tube or the quartz wool plugs. From the above data, the carbon reduction method of the microdetermination of oxygen suffers from a variable blank value depending on the composition of the pyrolysis produds, which can generally cause systematic errors. The aim of this paper is to provide practical conclusions that may help obtain correct results of the elemental analysis of oxygen.
EXPERIMENTAL SECTION Reagents. Technical grade helium of 99.9% purity (VEB Technische Gase, Leipzig, GDR) was purified by passing it over molecular sieve 5A, quartz wool, carbon black for microanalysis, copper wire, cuprous oxide, granulated silver (0.5-0.7mm), nickel nitrate, and Porapak Q (80-100 mesh) (Waters Associates, Inc., Milford, MA). The standard substances were of a purity for elemental analysis (Lachema N.E., Brno, Czechoslovakia) or microandytical-grade compounds (Carlo Erba, Milan, Italy). The boats for solid samples were prepared by folding sheets of silver foil (0.01 mm thick) with a special device (Laboratory Equipments, N.E., Czechoslovakia); the dimensions of the boats were 3.0 X 8.0 X 2.5 mm, and their weight was about 14 mg. Nickelized carbon packing was used (50 w t % of Ni, prepared from Ni(N03)2.6H20and carbon black). Apparatus. The measurements were carried out on a CHN-1 analyzer (3-5). A simplified diagram of the reaction train and analytical system is shown in Figure 1. The analysis is started with a diaphragm valve arrangement [13a, 14b, 15a] and the silver boat with sample is introduced by means of a sampler [2] into the pyrolytic section of the oxygen reactor [3]. The location of the packing in the reactor and the temperature profile in it are illustrated in Figure 2. H2and CO are oxidized to H20 and C02 over CuO (7). The pyrolytic products are carried by helium over the reactor packing and the effluent is collected in a dilution chamber [4], the piston of the latter being shifted from the right outset position into the left. As soon as the pressure within the chamber reaches a preset value of 0.078 MPa the chamber is closed [13b, 14b, 15b] and 1.5 min are allowed to achieve homogenization of the contents (He, Nz, C02, and H20) through diffusional equilibration under constant-state conditions. Then the piston 0 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981 ADSORPTION
0
1
2
!
165
DE SORPTION
4
#
6
7
8 MIN
Figure 3. Frontal chromatogram In the determination of oxygen.
35
4.
100
a
-5
a
9
7
I
5--+10mm
_I
e
5
7
5
5
6
-
4
_-
1
--
i"
-
4
starts pushing the contents of the dilution chamber [13b, 14a, 15b] into the chromatographic column [5], whereby developing a frontal chromatogram. After a plateau of the HzO front has been established, the remainder of the gas in the dilution chamber is exhausted to the atmosphere [13c, 14b, 15bI. The effluent from the column enters the measuring cell of a katharometer [6], and the recorder [8] provides the corresponding adsorption frontal chromatogram (Figure 3). The heights of the adsorption fronts are proportional to the concentrations of the components within the dilution chamber (a), which is the basis for the calculation of the results. After the adsorption frontal development has been completed and the dilution chamber emptied, further sample is pyrolyzed in the pyrolytic section [13a, 14b, E a ] while the chromatographic columin [5] is flushed with helium which gradually elutes the adsorbed components out of the column. The effluent again enters the measuring cell of the katharometer [6] and the recorder [8] provides the correspondingdesorption frontal chromatogram (Figure 3). The chromatographic column was made of a 130 cm long, 0.3 cm i.d. stainless-steel tube, packed with Porapak Q 80-100 mesh. The optimum column temperature was 65 "C. It is Nz, COz, and H2O only that enter the dilution chamber and are subject to frontal GC. Sulfur and halogen products of pyrolysis are bound on the silver in the oxidation section of the oxygen reactor.
Procedure. The sampler with 17 samples (in amounts corresponding to as much as 450 pg of oxygen) is inserted into the cold section of the oxygen reactor being purged by hellium streaming through at a rate of 90 mL/min. As soon as the absolute b h k s become steady,the f i t sample is introduced and pyrolyzed at a temperature of 1050 "C in the pyrolytic section of the oxygen reactor. The oxygen pyrolysis products are converted to CO over the carbon packing kept also at 1050 "C, CO is oxidized to COz over CuO at a temperature of 650 OC, and COz, Nz, and H20 are transported by the carrier gas into the dilution chamber. The initial pressure in the oxygen reactor, given by the hydraulic resistance of the carbon packing and the piston in the dilution chamber, reaches to about 0.009 MPa. A pressure of 0.048 MPa, which proved experimentally to be the maximum pressure foir the conversion of oxygen-containing pyrolysis products to be complete (23),is established in the oxygen reactor within about 3 min after the analyzed sample has been pyrolyzed; in the final stage of f i h g the dilution chamber, the pressure rises up to 0.078 m a . It takes about 4 min to fill the dilution chamber at a carrier gas flow rate of 90 mL/min. This time is sufficiently long for all the reaction producb to be practically purged out of the carbon packing: (7). Hence, the role of the adsorption of the reaction products on the avbon produced by the pyrolysis of organic substances in the inert gas and/or on the carbon packing is negligible. From this point ot view, the time of the pyrolysis of the sample is not critical. The diffusional equilibrium within the dilution chamber is established in 1.5 min and the adsorption equilibirum in the chromatographic column is complete in 4.5 min. One determination of oxygen takes about 11min. When not in use, the carbon packing is kept under a stream of helium. Calculation. Numerical values of the front heights can be directly obtained by means of a digital voltmeter connected in parallel to the output of a standard CHN-1 analyzer. The spstem ifi calibrated by a standard compound containing oxygen. The response factor of oxygen, f, is calculated by f = [% oxygen (theor) X wt of standard compound (mg)]/[adsorption front height of COz (mV) - absolute blank value (mV)] and the content of oxygen in unknown substance is then calculated as % oxygen = cf X adsorption front height of COz (mV) absolute blank value (mV)]/[wt of sample (mg)]
RESULTS AND DISCUSSION Carbon Packing. Different forms of carbon are employed for the determination of oxygen by the pyrolysis of sample in an inert gas and conversion of the pyrolysis products on a carbon packing. Amorphous carbon and platinum-coated
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
Table I. Determination of Oxygen compound analyzed calcium carbonate
theoretical content of 0, % 31.97
sucrose ( C l ~ H ~ z O l l )
51.42
palmitic acid (CI,H310,)
12.48
cyclohexanone 2,4-dinitrophenylhydrazone
23.00
(C,2H14N404)
p-nitroaniline (C,H,N,O,)
23.17
calciferol 3,5-dinitrobenzoate
16.25
(C35H46N206)
acetanilide (C,H,NO)
11.84
phenacetin (Cl,,Hl3NO,)
17.85
sulfanilamide (C,H,N,O,S)
18.58
4-@-bromobenzenesulfonamide)anisole
14.03
(C13H12N03BrS) p-bromoacetanilide (C,H,NOBr) l-chloro-2,4-dinitrobenzene (C,H3N,0,Cl)
7.47 31.60
sample wt, mg 1.422 1.369 1.392 0.6786 0.7176 2.450 2.284 1.346 1.435 1.346 1.424 1.322 2.096 2.152 2.755 2.588 1.851 1.821 1.635 1.609 2.018 2.124 4.476 4.484 4.525 1.084 1.159 1.171
errora (% of 0) dl
d2
t 0.14
-0.10 -0.04 + 0.07 0.00 t 0.08 -0.10 t 0.55 t 0.57 t 0.62 t 0.52 + 0.67 t 0.15 t0.16 t 0.37 t 0.48 + 0.52 t 0.52 + 0.41 t 0.49 t 0.49 t 0.38 t 0.29 t 0.22 + 0.23 -0.28 -0.23 -0.24
-0.03 -0.01 t 0.04 -0.06 t 0.08 -0.25 -0.25 t 0.07 + 0.18 t 0.07 t 0.07 -0.06 t 0.02 t 0.13 t 0.02 to.10 t 0.03 + 0.04
a Error is the difference between the found and theoretical oxygen contents. d , and d , refer to CaCO, and cyclohexanone-2,4-dinitrophenylhydrazoneas standards for the calculation of the response factor of oxygen, f , respectively; f was an average value from three determinations, the absolute blank value subtracted was measured after the completion of the whole series of analyses; the carbon packing was 50 wt % of Nilcarbon. For the pyrolysis and reaction temperature, see Figure 2.
and/or nickel-coated carbon are used most often. In practice the latter two are preferred, mainly because they require lower reaction temperatures (900-1050 "C) and show a lower sorptive capability toward the reaction products; this is rather important with such systems in which the separation section (elution GC) is directly linked in an on-line arrangement to the reaction section. Nickel-coated carbon is cheaper and yields more consistent results with nitrogen compounds, as compared with platinized carbon. Its conversion capability is sharply decreased with chlorinated compounds, but the same phenomenon can be observed also with platinized carbon. Frontal GC as a Separation Method. The system of reaction-frontal GC in the arrangement described has some advantages in comparison with systems utilizing reactionelution GC: (1)The use of the dilution chamber as an interface between the reaction and the separation sections makes the functions of these sections mutually independent, which has the following consequences: (a) there is no problem with the anomalous katharometer response to hydrogen (24) produced by pyrolysis, (b) the reaction proceeds practically a t an atmospheric pressure (cf. the dependence of the degree of conversion on pressure, ref. 23), (c) pressure and/or volume changes taking place during the pyrolysis of the sample do not influence the process of separation, (d) the separation is not influenced by adsorption phenomena occurring in the carbon packing, (e) in the given arrangement the carbon packing is purged by about 350 mL of carrier gas. (2) The dilution chamber effectively assumes the function of an integrator. (3)Diluting the gaseous reaction products to concentrations of 5 1 % virtually eliminates the problems associated with the nonlinearity of the katharometer response (25).
(4) For each sample there are obtained two analytically utilizable records-an adsorption and a desorption one. (5) It is possible to perform the simultaneous analysis of 0 and H and, consequently, to accomplish the C, H, N, 0 analysis without weighing the sample in many cases (7). To make the account complete, it is necessary to mention that arrangement described provides for the probability of analyzing oxygen also in the form of CO. In this case, it is necessary to use molecular sieve 5A as the packing of the chromatographic column and to modify the reaction packing (carbon, Cu). The interfering elements (halogens, sulfur) are captured by the molecular sieve in the chromatographic column and oxygen, if any, is separated from the other components. The analytical utilization of frontal chromatography is limited by the extent of the concentration changes which occur in the adsorption part of analysis (this does not apply to the desorption course) due to the sorption of the componenb (6), similarly as with systems utilizing selective absorption (26). However, these effects are negligible at low solute concentrations and need not be considered in our case. Absolute Blank Value. Reaction-frontal GC, in the instrumental arrangement represented by the CHN-1 analyzer, makes it possible to measure absolute 0 values in the form of COz. The absolute blank is the amount of osygen released from about 4 cm3of carbon packing in about 4 min at a helium flow rate of 90 mL/min. With a fresh packing the blank values continually decrease with time and asymptotically approach zero. The elemental analysis is started after the blank value has been stabilized. The packing used in this work (50% Ni/carbon) rendered absolute blank values of about 20 pg of 0, and these values were not changed on replacing the quartz wool situated at the beginning of the carbon packing by a platinum gauze. The standard deviation of these blanks,
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
which expresses the variability of the latter within a course of many days, was 3 pg of 0. Actual Blank Value. The results of the determination of oxygen were processed by linear regression. The dependence of the katharometer response (in microvolts) on the amount of the substance analyzed (in micrograms of 0) is linear within the concentration limits employed. The intercept cut out by the straight-line on the ordinate represents the analytical blank value and can be called “actual blank value”. As the pyrolysis products of different substances analyzed are generally different and the concentrations of the former vary depending on the weighed sample amount, the amount of 0 by which the surface oxides C(0) and/or other side reactions contribute to the final results is also variable. Consequently, different substances render different actual blank values. With the same carbon packing, the actual blank values for oxygen compounds containing N, Br, and C1 are higher as compared with those for C, 8 , O compounds, and all the actual blank values differ significantly from the absolute blank values measured. The differences between the absolute and actual blank values can cause systematic errors in magnitude which depend on the composition of the standard used. Experimental Determination of t h e Blank Value. A basic prerequisite for obtaining correct results is that the decrease of the blank value during the period of measurement (e.g., 1 day) be kept as low as possible. Before starting the analyses of a series of igamples, it is necessary to wait until constant blank values have been established. The quickest way to attain this state is to pyrolyze several samples of an oxygen-containing compound. The blank value to be used for calculating the results i s that one which is determined after the above series of analyses has been finished. Microdetermiqation of Oxygen in Organic Substances. It is all the pyrolysis products of the substance analyzed, both oxygen containing and aonoxygen ones, that participate in the opposing reactions taking place on the carbon packing. The amount of determined oxygen depends on the concentration of the pyrolysis products in the carrier gas, which is given by the tyRe and weighed amount of the substance analyzed, on the ratio of CO to COz in the pyrolytic products (cf. reaction 2), on the hydrogen and water contents in the pyrolysis products and/or on the amount of hydrogen sorbed on the carbon packing surface, and on the presence of nonoxygen pyrolysis products (Nz, HCN, NH3, H2S, CSz, HCl, HBr, etc.). The “apparent oxygen content” of nonoxygen substances is attributed to reactions of the pyrolysis products of such substances with the surface oxides C(0) and/or to other side reactions. For example, 2-mg amounts of pyrene, azobenzene, and diphenylamine showed “apparent oxygen contents” of 0:51,0.24, and 0.53%,respectively. Elemental nitrogen did not show any significant “apparent oxygen content”. The analysis of 1.687 mg of dibromobenzene and 1.472 mg of hexachlorobenzene yielded 0.71 and 7.24% 0, respectively.
167
The pyrolysis products of a given amount of different, C, H, 0 substances differ in their cohcentration, in the gas entering the packing in the ratio of CO to C02, and in ,the contents of H 2 0 or hydrogen. Therefore, the actual blank values of different C, H, 0 substances also are generally different. However, when using a C, H, 0 compound as a standard, the systematic errors of the analytical results do inot exceed 0.2 % The errors can be larger with samples containing less than 100 pg of 0. C, H, N, 0 substances show higher actual blank valuee as compared with those displayed by C, H, 0 substances. Thus, if referred to a C, H, 0 standard, nitrogen compounds exhibit positive systematic errors (Table I); these errors usually exceed 0.3% abs. With fresh 50 w t % of Ni/carbon the actual blank values of C, H, N, 0 substances approach those of C, H, 0 substances, but with increasing age of the packing the situation is established as described above, though the degree of O/CO conversion remains unchanged. It can be supposed that HCN and NH3, produced by pyrolysis of nitrogen compounds, decompose on fresh nickel. If referred to a C, H, N, 0 standard, the accuracy of the results obtained for substances containing nitrogen and bromine or sulfur lies within the standard limits (Table I). In the case of halogen compounds, especially with thtose containing chlorine, the above-discussed phenomena occurring on the Ni-coated carbon packing are further complicated by an inhibition of the activity of this packing (cf. also ref 2!7),
.
LITERATURE CITED Rezl, V. Microchem. J. 1970, 75, 361. Rezl, V. Czechoslovak Patent 140342, 1971. Rezl, V. Czechoslovak Patent 157857, 1974. Rezl, V.; KaplanovP, B. Mlkrochlm. Acta 1075, 493. Rezl, V.; UhdeovP, J. Int. Lab. 1976, 11; Am. Lab. (Falrflekl,Conn.) 1976, 13. Rezl, V.; JanHk, J. J. Chromatogr. 1073, 87, 233. Rezl, V. Mikrochim. Acta 1078, 493. Rezl, V.; KaplanovP, B.; JanPk, J. Anal. Chem. 1975, 47, 159, KaplanovP, B.; Rezl, V. Hutn. Lisfy 1975, 70, 742. Rezl, V.; KaplanovP, B.; Janik, J. J. Chromatogr. 1972, 65, 47. Rezl, V.; Bursa, J. J. Chromatogr. 1976, 726, 723. Unterzaucher, J. Chem. Ber. 1940, 73, 391. Belcher, R.; Ingram, G.; Majer, J. R. Mlkrochim. Acta 1968, 418. Belcher, R.; Davies, D. H.; West, T. S. Talanta 1965, 72, 43. Maylott, A. 0.; Lewis, J. B. Anal. Chem. 1950, 22, 1051. Dundy, M.; Stehr, E. Anal. Chem. 1951, 23, 1408. Haraklson, L. Mikrochim. Acta 1962, 650. Rhead, T. F. E.; Wheeler, R. V. J. Chem. SOC. 1910, 2178; 11112, 831. Harker, H.; Marsh, H.; WynneJones, W. F. K. Ind. Carbon GrapMte. Pap. Conf. 1957, 291. Belcher, R.; Ingram, G.; Majer, J. R. Taknta 1969, 78, 681. Belcher, R.; Ingram, G. Mlcrochem J. 1966, 1 7 , 350. Bonner, F.; Turkevich, J. J. Am. Chem. SOC.1951, 73, 561. Pella, E.; Colombo, B. Anal. Chem. 1972, 44, 1563. Purcell, J. E.; Ettre, L. S. J. Gas Chromatogr. 1965, 3, 69. Clerc, J. T.: Simon. W. Microchem. J. 1963. 7 . 422. Clerc, J. T.; Dohner, R.; Santer, W.; Simon, W. Hehr. Chim. Acta 1963, 46, 2369. Pella, E.; Andreonl, R. Mikrochim. Acta 1976, 175.
RECEIVED for review August 22, 1980. Accepted October 20, 1980.
