The first step in the GC determination of carbon and hydrogen is quantitative oxidation of the sample organic compounds, usually with a catalyst. Copper oxide is commonly used, but the reaction is relatively slow and elevated temperatures of about 900°C are required. With silver permanganate the reaction temperature is reduced to 550°C [34, 351 and with cobalt oxide to 750°C [36, 371; both compounds provide a shorter oxidation time. Other catalytic oxidizing agents, such as nickel oxide [38, 391 and ceriuni(1V) oxide [40], have been found promising. Platinum can also be used, especially when it is necessary to avoid the retention of any oxidation products by the solid catalyst.
Some workers have experimented with oxidation under static conditions. According to Rezl and Jan6k [ 121 , good results can be obtained by combining the static and dynamic methods. They recommend that the chemical decomposition should take place in an inert atmosphere under static conditions and that the sample should have a layer of a catalyst on it; final oxidation takes place in a flow-through reactor in a layer of oxidation catalyst.
Oxygen is used with catalysts such as copper oxide, mixed oxides of cobalt and silver permanganate. Different techniques are used to separate water from carbon dioxide.
One of the first publications dealing with GC elemental analysis [31] described the oxidation of organic compounds by copper oxide at 750°C in a flow of high-purity oxygen. A sample of 2-6 mg is burnt in a trough in a Pyrex tube (52 cm x 8 mm I.D.). A tubular furnace 30 cm long is used for heating. Oxygen is fed through a branch pipe in the inlet portion of the Pyrex tube. To ensure rapid evaporation of the sample, a special elastic heater element is wound around the quartz tube at a distance of 6 cm from the oxygen intake point. The contents of the next reactor, intended for volatile oxidation products, are as follows: 6 cm of platinum mesh; 18 cm of copper oxide, the layer con- sisting of six 3-cm sections; and 6 cm of silver-plated copper mesh. The catalyst layer is reinforced with a platinum mesh of mesh size of 0.39mm. In the case under review, copper oxide was obtained by oxidizing a copper mesh of mesh size of 0.39mm. Silver nitrate solution was used for silver plating of the copper mesh; the process resulted in
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crystalline silver needles 1-2 mm long. The purpose of the silver layer is to absorb some oxidation products resulting from the combustion of halogen- and sulphur-containing organic compounds. According to Duswalt and Brandt [31], incomplete removal of such products lead to additional peaks whose retention time is equal to that of carbon dioxide.
In the experiment under review, the carrier gas was oxygen, which was thoroughly dried in advance in a tubular absorber (30cm x 16 mm I.D.) containing magnesium perchlorate.
The products of oxidation of the organic compounds, i.e., water and carbon dioxide, were transported by the carrier gas flow to a reactor of length 5 cm containing calcium carbide of grain size 0.5-0.25mm, in which steam was quantitatively converted into acetylene. A fresh filling of calcium carbide could be used for two or three analyses.
To concentrate the acetylene and carbon dioxide resulting from the combustion of the sample, the gas flow was passed through a U-shaped glass trap cooled with liquid nitrogen.
To avoid condensation of oxygen, the pressure in the trap was maintained at 1 lOmm Hg.
After the combustion of the sample, which took about 8 min, and concentration of the products, the trap was connected to a GC system. The Dewar flask containing liquid nitrogen was removed and the temperature in the trap rapidly increased. The condensed products were then introduced in a flow of helium into a chromatographic column of length 1 m and I.D. 6 mm. The separation of acetylene from carbon dioxide was carried out on activated silica gel with a grain size of 0.5-0.25 mm, which was regenerated after 80 experiments. Oxygen and nitrogen produced in the course of combustion were easily separated from carbon dioxide and acetylene in the chromatographic column. The peak areas of carbon dioxide and acetylene were proportional to the concentrations of carbon and hydrogen, respectively, in the sample compound. The instrument was re-calibrated before each series of experiments.
The method described above has been used successfully for the determination of com- pounds such as 3-aminoacetophenol, 8-hydroxyquinoline, 7-hydroxy-7-chloroaniline and benzoic acid, etc. The absolute error was f 0.5% for carbon and k 0.1% for hydrogen. The reproducibility was checked on the basis of benzoic acid analyses. For carbon, deviations from the mean value were k 0.58%. The total duration of the analysis, including com- bustion of the sample. separation and chromatography, is 20 min per sample. Working continuously, one can introduce a fresh sample into the combustion tube every 10min.
A similar method for the determination of carbon and hydrogen was developed by Sundberg and Maresh [ 3 2 ] . Unlike Duswalt and Brandt [31]. they use helium instead of oxygen for combustion, which has made it possible to simplify the equipment and employ the method for nitrogen determination. The Dumas combustion method was used in a flow of helium with copper oxide as the oxidizing agent (750°C) and metallic copper to reduce nitrogen oxides. To ensure complete oxidation of carbonizable substances, such as saccharose. such substances are placed in a trough prior to combustion and mixed with finely ground copper powder. The combustion is carried out for at least 26 min. Steam is quantitatively converted i n t o acetylene following its reaction with calcium carbide in a reactor 20 cm long and I 5 nim in diameter. Carbon dioxide and acetylene are captured in a trap cooled with liquid nitrogen. Carbon dioxide is separated from acetylene in a column 0.9 long and 6 mm in diameter containing silica gel with a grain size of 0.5-0.25 mm.
The percentage of carbon and hydrogen in the samples was calculated from the following equations:
C (mg) 100 - Kco, Sco, * 100
For carbon: -
sample weight (mg) H (mg) . l o 0 sample weight (mg)
sample weight (mg) Kc, H, SC, H, * 100 sample weight (mg)
For hydrogen: = -
(7.3)
(7.4) where SCO, and SC,H, are the chromatographic peak areas of carbon dioxide and acetylene, respectively, and K c O z and KCzHz are the tangents of the angles of inclination of the calibration graph for carbon dioxide and acetylene, respectively. The peak areas were measured with the aid of a planimeter.
The two techniques discussed above indicate that the combined application of GC and chemical methods of analysis is promising and can be used to develop commercial instrumentation. It must be pointed out, however, that these methods are not free from disadvantages: (1) water is converted into acetylene, which complicates the analysis and is a source of unnecessary errors; (2) the use of a nitrogencooled trap to collect and concentrate volatile products resulting from oxidation and other chemical con- versions also complicates the analysis and is a source of unnecessary errors; (3) the dynamic conditions of combustion necessitate a rapid and complete oxidation reaction.
The chemical aspects of combustion under dynamic conditions have been discussed [41,42] .
Vogel and Quattrone [ I ] proposed a completely different method for the deter- mination of carbon and hydrogen, in which oxidation of organic compounds takes place under stationary conditions in an oxygen-filled cylinder instead of a gas flow. A sample of 8-1 1 mg was oxidized in oxygen at a pressure of 3.3 atm; the process was accompanied by rapid heating of platinum wire and wound round the trough containing the sample, for which purpose 120V was applied to the wire. After the combustion, part of the combustion products was transferred for analysis from the cylinder into a special 25-ml gas sampler attached to the chromatograph. Steam and carbon dioxide were analysed in a 2-m chromatographic column containing dodecyl phthalate at 140°C with dry oxygen as the carrier gas.
A typical chromatogram is presented in Fig. 7.1. The peaks are asymmetric, so the peak areas were measured with the aid of a planimeter*. The hydrogen and carbon contents were calculated from the peak areas.
In the analysis of benzoic acid, cystine, dextrose, glycine and 0-naphthalenesulphonic acid, the accuracy for the carbon content was 0.5% and for hydrogen 0.8%. Analyses were carried out over a long period, but the results were calculated on the basis of the original calibration. A single analysis never took more than 17min. With a singte com- bustion step, it takes 40min to perform three successive determinations. In comparison, repeated carbon and hydrogen determinations by the Pregl method take as long as 135-145 min and require great skill on the part of the operator and very stable exper- imental parameters [43].
Nowadays, the application of computers facilitates easier data processing and calculation of results [165-1671.
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Time
Fig. 7.1. Chromatogram of products resulting from combustion of an organic compound in oxygen contained in a gas cylinder. The chromatogram was obtained in a column of dodecyl phthalate main- tained at 104°C. Peaks: 1 = carbon oxide; 2 = water. Reprinted with permission from ref. 1.
TABLE 7.2
COMPARISON OF GAS CHROMATOGRAPHIC AND PREGL METHODS Reprinted with permission from ref. 33.
Element determined Relative standard deviation (%)
Gas chromatography Pregl method (all samples)
Benzoic acid Ephedrine
Carbon 0.5 0.29 0.35
Hydrogen 0.84 3.3 2.1
Table 7.2 [33] presents comparative results of elemental analyses carried out by GC and by the Pregl gravimetric method [44]. It can be seen the relative standard deviations for carbon determined by GC are about double those for the Pregl method, but GC is more accurate in the determination of hydrogen.
Other workers [45-471 have further developed the method based on combustion of the sample compound in an oxygencontaining cylinder and subsequent GC analysis of the combustion products.
An original version of the static method was proposed by Luskina et al. [48], using a simple instrument. Sample compounds were oxidized in a sealed tube in the presence of copper oxide at a residual pressure of 10 mmHg. This method of combustion helps to eliminate the disadvantages lnherent in the conventional methods of oxidation, namely, the admission of unburned products into the analysis zone and flammability and explosion hazards. All of the combustion products were forced by a flow of helium from the combustion tube into the chromatographic column where separation took place; thus the method makes it possible to dispense with special sampling devices. The separation of water from carbon dioxide was effected in a flow of helium in a column o f tricresyl phosphate (0.6 m x 3 mm I.D.).
The carbon and hydrogen contents were calculated on the basis of calibration con- stants and chromatographic peak areas. It can be inferred from the results [48] that the accuracies were 0.3% for carbon and 0.15% for hydrogen.
The techniques used by VeEefa [49] for the automatic microdetermination of carbon and hydrogen in organic compounds d o not differ from the chromatographic method, except that chemical absorbers are used instead of the chromatographic column; as a result, it is possible to detect only one product. An original sample of 1.0-1.6mg was mixed with 30-40mg of mixed oxides of cobalt and rapidly combusted in a quartz tube containing an appropriate catalyst.
In the microdetermination of carbon, the combustion took place in a flow of oxygen.
The primary combustion products completely oxidized in a layer of mixed oxides of cobalt on corundum. The halogens were absorbed by a layer of silver; water was absorbed by magnesium perchlorate; nitrogen oxides were absorbed by manganese dioxide. The carbon dioxide peak was measured with a TCD. For the determination of hydrogen, the sample and mixed oxides of cobalt were combusted in a flow of nitrogen. To ensure complete oxidation, the flow of nitrogen plus the combustion products was passed successively through a layer of copper oxide and mixed oxides of cobalt and through a layer of copper and iron. The iron was used to reduce water to hydrogen. After carbon dioxide had been absorbed by an absorber containing anhydrone and soda asbestos, a TCD was used to measure the amount of hydrogen. The contents of carbon and hydrogen were calculated on the basis of the peak areas with the due regard for the weight of the samples and the calibration graph. The relative standard deviations were 0.45% for carbon and 0.16% for hydrogen. It took 4-6min to perform one analysis, including the combustion and peak recording.
Today's commercial analysers are capable of determining three (carbon, hydrogen and nitrogen) or more elements. It is natural that one should employ such analysers for carbon and hydrogen determinations and a description of these instruments is given in the following chapter. At this point let us consider, as an example, a method for the separate determination of organic and inorganic carbon in soil and rock samples.
The determination of the total carbon and organic carbon contents is an important part of the analysis of bituminous shale. Normally, the organic carbon content is deter- mined indirectly, as the difference between the total and inorganic carbon contents [50].
One paper [51] described a method for the direct determination of organic carbon in rock samples. The method presupposes the use of standard equipment, namely, the Perkin-Elmer Model 240 analyser, but the combustion takes place at 450 f. IO'C, bearing in mind that inorganic carbonates d o not decompose at this temperature. The combustion takes 5 min. The accuracy of the determination of organic carbon is * 0.10%.
A similar method for the determination of organic and inorganic carbon can be carried out with the aid of a CHN analyser manufactured by Carlo Erba [52].
GC techniques are also used for the determination of admixtures of elements, e.g., the determination of trace amounts of carbon in waste water. A very convenient solution to this problem was offered by Ehrenberger [53]. Carbon is oxidized in a flow of oxygen, halogens and sulphur are bound by silver and nitrogen oxides are reduced by copper, which also binds the excess of oxygen. The gases thus produced are entrained in a flow of helium, which is passed through concentrated sulphuric acid, magnesium perchlorate
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and phosphorus pentoxide and then through a trap containing molecular seives, where carbon dioxide is adsorbed. The trap containing molecular sieves is then heated and the amount of carbon dioxide is determined with the aid of a TCD.
Ehrenberger also developed techniques for the determination of trace amounts of carbonates in liquid and solid waste products of the chemical industry [54]. For this purpose, a sample is introduced into diluted sulphuric acid, then the carbon dioxide is driven with a flow of helium to a trap column containing molecular sieves. The column is heated to desorb the carbon dioxide, then a TCD is used to determine thc amount of carbon dioxide.
In some instances the method for carbon determination has to be modified, e.g., the determination of trace amounts of what is referred to as “dissolved organic carbon”
in water after inorganic carbon has been removed. This type of carbon determination involves wet oxidation activated by silver ions in a solution of potassium persulphate in sulphuric acid. The oxidation of organic compounds gives carbon dioxide, which is adsorbed by molecular sieves. The molecular sieves are then heated in a flow of helium to desorb the carbon dioxide, the amount of which is measured by a TCD. The lowest concentration of organic carbon that can be measured in water is 0.2-2ppm [55] .
The application of chromatographic elemental analysis to the determination of the total carbon content in water has been described [56].
Nelsen and Groennings [57] developed a method for the determination of trace amounts of organic compounds in hydrogen peroxide, which is a neccssary step for checking the quality and stability of hydrogen peroxide. Their method consists i n thermal decomposition of hydrogen peroxide, accompanied by oxidation of organic compounds to carbon dioxide.
Ivanova et al. [169] described the determination of the carbon content of hydrides of Si, P, As, B and some gases. They could determine down to 1 I carbon in the sample. GC elemental analysis and the associated instrumentation are very suitable for the determination of gas-forming elements in metals. However, this chapter does not consider such methods, because they were adequately dealt with by Wasserman et al.
1581.
Today many companies manufacture CHN analysers for the simultaneous determi- nation of carbon, hydrogen and nitrogen. Let us now consider in more detail the methods for determining these three elements.