In determining the composition of complex mixtures that are subjected to chromato- graphic separation, the elemental composition of the fractions is of crucial importance.
First, it provides valuable information for identification. Second, it permits the quanti- tative analysis of unseparated zones of compounds that exhibit qualitative and even quantitative differences with respect to elemental composition.
In the 1960s, when selective elemental detectors were virtually unknown, elemental analysis could be performed only by using chemical methods. Today these methods do not seem as essential as they used to be. However, there are some very important reasons that prompt further work in this field: the chemical methods and equipment are simple; they allow the use of the simplest detectors, such as TCDs and plasma ion detectors; they allow the calculation of individual calibration coefficients; they are accurate; and they make it possible to determine the ratio between elements in the course of a single experiment.
In some instances other detectors may be preferred over TCDs. Windsor and Denton [ 1741 , e.g., applied inductively coupled plasma atomic emission spectrometry to the elemental analysis of GC effluents.
The elemental composition of a compound can be determined on the basis of its high-resolution mass spectrum [124-1261. McLafferty [ 1271 developed ways of using medium- and low-resolution mass spectrometry for the determination of elemental composition, All elements fall into groups (A), (A + 1) and (A + 2), and the grouping of elements is dependent of their isotopic composition, which may differ by one or two mass units. For example, chlorine, whose isotope masses are 35 and 37, belongs to group (A + 2). Group (A) includes hydrogen fluorine and phosphorus; group (A + 1) includes carbon and nitrogen; group (A + 2) includes chlorine, bromine, sulphur, silicon and oxygen. A special computer program was worked out t o facilitate calculations [128].
Cacace et al. [ 1291 devised a method for determining carbon and hydrogen in pure volatile compounds separated in a chromatographic column. Following the separation, individual fractions were transferred in a flow of carrier gas to a reactor filled with copper oxide and iron. The products of conversion, carbon dioxide and hydrogen, were separated in a column of length 4 m and I.D. 6mm containing acetylacetone and main- tained at 18°C. The method can be used to identify unknown compounds. However, the method suffers from a relatively low accuracy of the GC determination of the carbon to hydrogen ratio. The accuracy of a single determination is not higher than 3%.
Further development of the method has made it possible to apply it to the analysis of labelled compounds [130]. The conversion to hydrogen and carbon dioxide and separate determination of the activity of these chromatographic zones with the aid of flow-through counters have substantial advantages over the conventional method for measuring the activity of different compounds: (1) an important factor is the operational stability of the flow-through counter; (2) measurements can be made at room temperature, so that sophisticated equipment is not necessary; (3) it is possible to determine simultaneously the activity with reference to 14C and 3H in doubly labelled molecules.
REACTION GC METHODS OF ELEMENTAL ANALYSIS This is an example of how analytical reaction gas chromatography can be used for both elemental and isotopic analysis. The application of elemental analysis to radio- chromatography was dealt with in great detail by Roberts [131].
Skornyakov [ 1321 recommends that compounds separated in a gas chromatographic column should be identified by being converted on copper oxide and iron at 700-800°C.
The resulting carbon dioxide and hydrogen are separated in another column. The method makes it possible to determine the carbon to hydrogen ratio in sample molecules.
Revelsky and co-workers [ 133-1351 modified and improved the method developed by Cacace et al. [129], by selection of separated chromatographic zones for elemental analysis by means of a proportioning tap. Franc and Pour 11361 determined the C:H ratio on a difference basis, using selective absorbers. This method provides for the con- tinuous determination of C:H ratios using two TCDs; in other words, the method makes it possible t o record two chromatograms; the readings on the first chromatogram are proportional to the total of carbon and hydrogen, whereas the readings on the second chromatogram are proportional t o carbon or hydrogen alone. First the total of the reaction products (water and carbon dioxide) is measured, followed by the measurement of only one product.
The determination of the C:H ratio for identification purposes is practicable only if it is performed with a high degree of accuracy. It should be remembered for comparison that the accuracy of classical elemental analysis methods is about 0.3%, which is insuf- ficient for the identification of organic compounds. For example, an absolute error of 0.3% in the determination of hydrogen in heptane (16% theoretical) may be misleading, as one can identify hexane (16.3% of hydrogen) and octane (15.8% of hydrogen) at the same time.
Scheil and Harris [ 1371 developed a system for the accurate determination of C:H ratios in chromatographically separated peaks. A proportioning tap feeds fractions subjected t o the analysis into a reactor fdled with copper oxide, where oxidation takes place at 700°C. The oxidation products are separated on Porapak N . The results of the analysis were evaluated by a computer. The method was applied to compounds conraining up to 12 atoms of carbon. If reference compounds of a similar structure are available, the method can reduce the mean error of determining C:H ratios to 0.2%, which corresponds to the absolute error of carbon and hydrogen determination (0.02- 0.04%).
hlethods for determining the ratio between carbon and elements other than hydrogen have also been developed. Reitseme and Allphin [59] proposed a method for determining nitrogen in the components of a chromatographically separable mixture. From the reactor, the oxidation products of the chromatographically separated fractions are transferred to a column containing silica gel in which carbon- and nitrogen-containing products are separated.
In the analysis of a multi-component mixture, the zone of one compound may be superimposed on the zone of another compound. It is necessary, therefore, to develop a method that would make it possible to record and measure only the nitrogen peak.
The area of this peak is proportional t o the content of nitrogen in a given component and in the mixture as a whole. This type of analysis is carried out with the use of carbon dioxide as the carrier gas [ S S ] . In this instance TCD is unable to detect the carbon
ELEMENTAL ANALYSIS OF SEPARATED FRACTIONS
90
80
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3 60-
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- 50-
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1 3 4 5
Fig. 7.5. Chromatogram of a mixture of hydrocarbons and nitrogencontaining compounds. Carrier gas: (A) helium; (B) carbon dioxide. Peaks: 1 = pyridine; 2 = xylene; 3 = picoline; 4 = methylnaph- thalene; 5 = quinoline. Reprinted with permission from ref. 59.
dioxide produced as a result of combustion or organic compounds. The method is suit- able for the analysis of the absolute nitrogen content of pure compounds. A complete analysis can be carried out within 2 min. The nitrogen content is measured by comparing the actual data with the results of an analysis of pure nitrogen-containing reference compounds, carried out under conditions similar to those of the experiment. The volume of compound subjected to analysis is about 0.001 ml. The relative error is 1-3%.
The foregoing method has been used to study the nature of nitrogen-containing compounds in complex mixtures. In the first analysis helium was used as the carrier gas and carbon dioxide in the second. Fig. 7.5 shows a chromatogram of the separation of a mixture of hydrocarbons and nitrogen-containing compounds [59]. The chromato- gram of the second analysis, during which carbon dioxide was used as the carrier gas, refers only to nitrogen-containing compounds.
A number of workers have studied the development of methods for the determination of nitrogen in compounds separated in a GC column. Coulson [138, 1391 devised a special coulometric detector that only detects ammonia. After separation, the compounds under investigation were hydrogenated in a flow of hydrogen over a nickel catalyst. As a result of hydrogenation, nitrogen-containing compounds formed ammonia, which was detected by the coulometric detector. A similar method was developed by Martin [ 1401
236
....
I;
2
U
Fg. 7.6. Diagram of a chromatographic assembly for determining C:H ratio in chromatographically separated fractions [ 142). See text for identification of components.
who used a special automatic titrator for the determination of ammonia. The above methods are concerned only with the determination of nitrogen content and not C:H ratio measurements. Apart from the above-mentioned paper by Reitseme and Allphin [ 5 9 ) , the experiments conducted by Liebman et al. 11411 seem t o b e unique in the sense that the oxidative degradation products of chromatographically separated compounds were separated in a second column, which made it possible to determine the C:N ratio Franc and Pour [ 142 J developed an original method for the continuous determination of the C :N ratio of chromatographically separated compounds. First, the compounds under investigation are separated in a flow of carrier gas in a chromatographic column, then they are transferred to a reactor to form, after combustion. carbon dioxide, water and nitrogen. This mixture is first driven through an absorber which absorbs water, then it is passed through a first detector which measures the total of carbon dioxide and nitrogen. Downstream of the first detector, the helium flow reaches a second absorber in which carbon dioxide is removed; finally, the helium flow reaches a second detector which measures the nitrogen content. The C:N ratio is determined by comparing the chromatograms obtained with both detectors.
The chromatographc equipment (Fig. 7.6) consists of a chromatograph equipped with a detection device having means for the combustion of separated compounds [143].
The carrier gas (helium) is fed from a bottle ( I ) through a drier (2), manostat (3) and manometer (4) t o comparison chambers of two TCDs (7 and 8). It is then passed through a sample injection device (5) t o a column (6), consisting of a quartz reactor (34 x 0.5 cm) filed with copper oxide, a first absorber (10 x 0.4cm) (10) filled with magnesium perchlorate, the measuring chamber of the ftrst TCD (7), a second absorber (10 x 0.4 cm) ( 1 1 ) filled with alkali asbestos, and finally to the measuring chamber of the second detector (8). The carrier gas flowqate is measured by a soap-bubble flow meter (13).
Both detectors are connected to a double-beam recorder. Compounds of known compo- sition, such as benzene and pyridine, are used to calibrate the ( C 0 2 + N2):N2 ratio (see Fig. 7.7). The method makes it possible to determine to a high degree of accuracy the C:N ratio of compounds subjected to analysis. For example, the C:N ratio is 3.0:0.96 1141 J .
2 3 1
Fig. 7.7. Chromatogram of separation of benzene (1) and pyridine (2) [ 1421. The larger peaks corre- spond to the concentration o f carbon dioxide and nitrogen (oxidation products) in the carrier gas;
the small peak (3) relates t o the concentration in the carrier gas of nitrogen formed from pyridine.
for dimethylformamide (the theoretical value is 3.0: 1 .O), 5.0: 1.02 for pyridine (theo- retical 5.0: l.O), 9.0: 1.01 for 1-aminopropane (theoretical 9.0: 1 .O) and 2.0:0.94 for acetamide (theoretical 2.0: 1 .O).
The method under review is particularly convenient for determining the elemental composition of compounds that contain a heteroatom. According to Berezkin and Tatarinsky 11441, t h e method has been used to determine C:H:N:O ratios with a view to identification of chromatographic peaks.
In many instances purely chromatographic methods of identification, based on measuring retention values, are unable to ensure a rapid and unambiguous determination of the qualitative composition of a mixture. Prior knowledge of the elemental compo- sition of components under investigation can substantially simplify the identification.
Elemental analysis has been used by a number of workers for chromatographic sepa- ration, but usually it has been confined to the determination of one or two elements.
These drawbacks were eliminated in a method for determining the elemental composition of chromatographic zones produced as a result of separation of a mixture in a chromato- graphic column for four elements: carbon, hydrogen, nitrogen and oxygen [ 1441.
With heteroatomic organic compounds, elemental analysis enables one not only to establish the presence of heteroatoms, but also to determine their number. In addition, the availability of chromatographic zones of two compounds with different elemental compositions (with respect to heteroatoms) makes it possible to determine the presence of these compounds in a mixture even if they cannot be separated chromatographically.
Qualitative identification makes use of both Chromatographic data and elemental analysis
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data. The outlet of the working cell of the katharometer is connected to a metering tap which makes it possible to select and direct the peak of the component undergoing identification, or a part o f that peak, to the elemental analysis system. If the sample loop is filled with an appropriate sorbent, one can accumulate the compound under investigation, collecting it from several samples, which makes it possible to perform elemental analysis on the microscale.
The metering tap divides the chromatographic zone of the sample into two flows.
The first is directed to a quartz reactor 4 0 c m long, fdled with copper oxide (20cm) and reduced copper oxide (20cm). In this reactor, the sample compound is burned at 800°C and the combustion products are converted into nitrogen, carbon dioxide and water, The carrier gas transfers the conversion products t o a chromatographic column (200 x 0.4 cni I.D.) fdled with Chromopor; the column is installed in the thermostat of an LHM-7A chromatograph and maintained at 95°C.
The second flow is driven t o a system for the elemental analysis of nitrogen and oxygen, consisting of a quartz reactor f d e d with a 25 cm layer of platinized anthracene black (500/0 of platinum) maintained at 900°C. The conversion products, i.e., nitrogen and carbon oxide, are transferred to a chromatographic column (100 x 0.4cm ID.) containing 5 A molecular sieves maintained at room temperature.
From both systems, the gas flows are directed to a thermal conductivity detector.
Both cells of the detector are working cells. The ratio between the rates of the two gas flows is selected so that n o overlapping of the chromatographic peaks takes place.
In the experiments under review, this condition was satisfied with equal rates of the two gas flows; the absolute flowrate was 30cm3/min. The method was checked exper- imentally for determining the elemental composition of a mixture containing compounds of different classes, such as diethyl ether, methanol, nitromethane, benzene and ethyl- enediamine. The separation was carried out in an LHM-8M chromatographic column (200cm x 0.4 cm I.D.) filled with 10% of PEG-400 on Chromosorb W. The carrier gas was helium at a flow-rate of 50cm3/min and the separation temperature was 100°C.
The chromatogram for the separation of a model mixture is presented in Fig. 7.8, which also shows separation chromatograms of the compounds involved in the analysis. The results of composition are calculated on the basis of the chromatograms. Clearly, the system can considerably facilitate the identification of unknown components of sample mixtures. For example, peak 111 in Fig. 7.8 corresponds to a relative retention time (benzene = 1.00) of 1.15. Judging from data in the literature [145], this compound may be diethyl ketone (1.18), methanol (1.1 1) or 3-methylbutanol-2 (1.08). The C:H:O ratios in the molecules of these compounds are 5:10:1, 1 :4:1 and 5:13:1, respectively.
The data obtained from elemental analysis (1.00:4.02: 1.02) unambiguously indicate that peak I11 ccrresponds t o methanol. The system discussed above can be used as an independent attachment t o a chromatograph.
in many instances it is important to determine the sulphur content in chromato- graphically separable compounds. Klaas [ 1461 developed a method for the identification and determination of trace amounts of sulphur-containing compounds in ligroins. The method consists in selective chromatographic separation, subsequent oxidation and deter- mination of the resulting sulphur dioxide by titration. Huyten and Rinders [147] use hydrogenation to identify the components of a mixture subjected to analysis following
ELEMENTAL ANALYSIS OF SEPARATED FRACTIONS
Time (rnin)
Fig. 7.8. Chromatograms of separation of model mixture and elemental analysis of isolated peaks.
I = Diethyl ether; I1 = benzene; 111 = methanol; IV = ethylenediamine; V = nitromethane; 1 = nitrogen; 2 = carbon dioxide; 3 = water; 4 = carbon monoxide. From ref. 144.
GC separation. The sulphur-containing compounds were identified on the basis of hydrogen sulphide peaks; oxygen-containing compounds were identified on the basis of water, etc.
Franc and Pour [ 1481 developed a method for the continuous determination of the C:S ratio in chromatographically separated compounds with a view to obtaining data
vital for identification purposes. Components separated in a chromatographic column were carried in a flow of hydrogen to a reactor where they were subjected to destructive hydrogenation on platinum mesh at 800"C, forming methane, hydrogen sulphide, ammonia and water. Ammonia and water were absorbed in 10 x 0.4cm absorber, half of which was filed with magnesium perchlorate and the other half with phosphorus pentoxide. The mixture of methane and hydrogen sulphide was then carried in the flow of hydrogen t o a first TCD for measurement; the mixture was subsequently passed through an alkali absorber where the hydrogen sulphide was removed; methane was measured by a second detector. A double-beam potentiometer recorded the signals arriving from both detectors. One of the resulting chromatograms contained data on the total carbon and sulphur content; the second chromatogram corresponded to the carbon content only. The C : S ratio in the compounds under investigation was calculated on the basis of a comparison of the two chromatograms. The results of the experiments show that the method is practicable. For example, the following C : S ratios were obtained: 8.0: 1.3 for dibutyl sulphide (the theoretical value is 8.0: 1 .O), 3.0: 1.03 for phenyi propyl sulphide (theoretical 3.0: 1 .O), 2.0:0.93 for cresyl methyl sulphide (theoretical 2.0:l .O) and 0.5:0.98 for hydrogen sulphide (theoretical 5.0:l .O).
An original method for determining the C : O ratio in organic compounds subjected to GC separation was proposed by Khripin and Pankov [149], based on destructive hydrogenation of separated compounds in a flow of hydrogen with the use of a nickel catalyst. The process is carried out at 300°C and results in a mixture of methane and water. The water is determined by a coulometric moisture gauge. The methane is measured by a plasma ion thermal conductivity detector. The moisture gauge and methane detector are placed in series. The signals of both detectors are independently recorded by a double-beam potentiometer or by two potentiometers. On the basis of chromatograms thus obtained and on the basis of calibration with reference compounds, the C:O ratio can be determined t o a sufficiently high degree of accuracy.
'Elemental' conversions of compounds are also of interest as a method for the deter- mination of separated compounds. The conversion of organic compounds to carbon dioxide was first used by Martin and Smart [ l 5 0 ] and has found extensive application in chromatographic analysis (see, for example, refs. 151-153). A reactor containing copper oxide or. in some instances, chromium oxide and heated t o 600-900°C is placed close to a chromatographic column. A water absorber is installed next to the reactor in order t o prevent ingress of moisture into the detector. The use of simple and reliable TCDs as GC detectors is essential; however, the use of TCDs for high-accuracy qualitative measurements necessitates complicated measurements in order to determine independent calibration coefficients (see, for example ~ refs. 154 and 155). Preliminary conversion of organic compounds into simple products, such as carbon dioxide, paves the way t o 'elemental' detection which has a number of advantages: (1) the calculation of individual calibration coefficients does not require any laborious experimental measurements [1561 ; ( 2 ) the sensitivity of detection is improved owing to chemical amplification, bearing in mind that the number of moles of the compound being detected increases as a result of combustion which leads t o the formation of carbon dioxide and water;
( 3 ) the detector can be maintained at room temperature, which is another factor that accounts for improved sensitivity.
According to Simmons et al. [157] , the sensitivity of detection with a TCD following combustion, provided that the carrier gas is air, nitrogen or argon, is identical with the detection sensitivity observed when the carrier gas is helium if the number of carbon atoms in the molscule of the organic compound is greater than or equal to five. If nitro- gen is used as the carrier gas, the detection sensitivity can be improved by using double- stage conversion whereby, following the combustion of an organic compound, water is reduced to hydrogen by iron [158]. However, this method has been found impractical because the conversion of water into hydrogen is not a quantitative reaction [ 153 , 1591 .
Chromatographs provided with converters of compounds to carbon dioxide and water are used for research, The Central Designing Bureau operating under the Ministry of the Petrochemical Industry of the U.S.S.R. has developed a commercial chromatograph featuring an oxidation converter [160].
Stepanenko and Kolodii [171]’ used a flame-ionization detector for the detection of halogenated hydrocarbons, these compounds are converted to hydrocarbons by pyrolysis at 1000°C in the presence of hydrogen.
Elemental analysis provides simple solutions to many technological and production problems, as was first suggested by Pankov and Khripin [ 1601 .
Most chromatographic analyses in industry are intended for monitoring of production equipment, such as reactors and columns. A complete analysis of the composition of raw materials and products resulting from chemical reactions is, in fact, the only way of determining mass concentrations and thus of evaluating the performance of the equip- ment. This requires much time, bearing in mind that at least two analyses of raw materials and end products have to be performed. Such methods are of no use for automatic control of production lines and processes. The latter problem is solved either by resorting to simplifications and approximations or by assuming that a change of a component’s peak with no change in the volume of the sample corresponds to a change in the weight concentration of the component in the flow of a product subjected to monitoring.
An example is the method for controlling the catalytic decomposition of ethanol to butadiene [161]. There have also been attempts to solve the probelm by finding corre- lations between peak parameters of components and the concentrations of key com- ponents of mixtures [162]. The latter method requires extensive preliminary work to establish correlations, and such correlations need frequent checking.
The use of internal standards makes it unnecessary to analyse all of the components of a mixture when evaluating process parameters, as it is sufficient to establish relative concentrations of the components and the standard. However, the introduction of a standard into the mixture being analysed by gravimetric techniques leads to serious difficulties in laboratory analyses, especially in the analysis of gases; clearly, such a solution is inapplicable to automatic production control. The standard may be a com- pound inert to a given reaction and incorporated in both the raw materials and the end products. Naturally, such compounds can be found only for a limited number of pro- cesses. For example, nitrogen contained in the air used for oxidation has been used as a standard for the oxidation of butane in a fluidized bed of a catalyst [163].
In an attempt to find a universal solution, it has been suggested that the internal standard should be a chemical element contained in the compounds subjected to analysis, such as carbon, nitrogen, hydrogen or chlorine [164]. The mass of the compound and,