The separation of the trace and main components can be substantially improved if the main component is converted into a highly volatile compound characterized by insignificant retention. Conventional chromatographic analysis of aqueous solutions is complicated by the fact that the chromatographic zone of water is usually diffuse and asymmetric and often masks the zones of other components. It is therefore advisable in some instances to convert water into compounds that are eluted from the column ahead of all sample components.
262
Time (min)
Fig. 8.4. Chromatograms of alcohol mixtures containing 90% water: (A) by using a calcium carbide reactor and (B) without a reactor. Peaks: (A) 1 = air; 2 = acetylene; 3 = methanol; 4 = ethanol;
5 = I-propanol; 6 = 2-methyl-1-propanof; 7 = lbutanol; (B) 1 = air; 2 = methanol; 3 = ethanol;
4 = water; 5 = 1-propanol; 6 = 2-methyl-I-propanol. Reprinted with permission from ref. 1 7 .
A method has been proposed [ 171 based on conversion of water into acetylene in a special reactor containing calcium carbide, which is located upstream of a chromato- graphic column. Water was converted into acetylene at 220°C in a Pyrex reactor (30 x
1.8 cm) containing a mixture of calcium carbide (30 mesh) and glass beads (0.5 mm in diameter) in a ratio of 1:2. This technique was successfully applied to the analysis of aqueous solutions of aldehydes, ethers and alcohols. Organic acids are retained in the reactor, which is why this method cannot be used for their analysis. Chromatographic separation was conducted at 74°C on a column (250 x 0.7 cm I.D.) containing the polar stationary phase Ucon 50HB-200 and a thermal conductivity detector was used for detection.
Fig. 8.4 shows two chromatograms of the analysis of aqueous alcoholic solutions, (A) with and (B) without a reactor. It can be seen that the use of a reactor containing calcium carbide permits, for example, the analysis of alcohols such as 2-methylpropanol-1 and butanol-1 , which cannot be analysed under such conditions without conversion of water.
If, as a result of a chemical reaction, a trace component forms a volatile compound that can be easily separated from the main component (or the product of its conversion), the trace component can be determined and the sensitivity can be increased (the con- centration of the components eluted from the column increases inversely with respect to their retention volume).
In view of the difficulties involved in the direct determination of water in various commercial solvents whose components exhibit retention times close to that of water, Bayer [20] proposed a method in which the sample passes in the carrier gas flow through a tubular reactor containing calcium carbide, upstream of a column. Acetylene formed
as a result of the reaction between calcium carbide and water is eluted in the form of a narrow symmetrical peak, at room temperature, much earlier than any of the components present in the complex mixture of solvents. The method is applicable to the deter- mination of water at concentrations down to 1 *
The carbide method for the determination of trace amounts of water was studied in detail by Knight and Weiss [72]. For the detection of the acetylene evolved they used a flame-ionization detector, which has made it possible to enhance considerably (by almost an order of magnitude) the sensitivity of the method. A similar technique for determining water in liquefied hydrocarbon gases was developed by Goldup and Westaway [73].
To eliminate the kinetic hindrances inherent in the reaction of water with calcium carbide, Guerrant [74] , who determined trace amounts of water in anhydrous ammonia, conducted the reaction for lOmin in a special reactor in which the sample reacted with calcium carbide. An attempt to conduct the reaction quantitatively in the flow through a column (30 x 0.3 cm I.D.) filled with calcium carbide failed because the reaction between water and the carbide is not fast enough.
Kaiser [75] has shown that by combining the reaction with calcium carbide and concentration one can attiin a detection limit of ppb (lo9). This method was used to determine water in polyorganosiloxanes [76], refrigerants [77] and other compounds.
In the reaction method use is also made of lead tetraacetate, which yields, during hydrolysis, two molecules of acetic acid per water molecule [78]. The reaction proceeds at 70-75°C in a tubular reactor (20-40 cmj containing lead tetraacetate. The method is applicable to the determination of water in hydrocarbons, alcohols, ethers and ketones.
Heterogeneous gas-solid and liquid-gas-solid reactions are complicated by a number of side-effects. Therefore, they can be recommended for the accurate qumtitative deter- mination only if calibration is effected by analysing samples with known water con- tents.
Of particular interest for the determination of water content is the use of a liquid reagent such as 2,2-dimethoxypropane, which yields liquid products (acetone and methanol) during hydrolysis [79-811. To accelerate the hydrolysis it is conducted on a water-bath with heating in the presence of methylsulphuric acid (catalyst). The hydrolysis lasts 1 min. The method was applied to the determination of water in organic solvents and crystal hydrates of salts.
To determine water in germanium hydride it has been proposed to use the chemical amplification method using a reactor containing layers of carbon (1000°C) and copper oxide (550°C) [82].
Drawert at al. [38] employed chemical reactions in the chromatographic deter- mination of water in alcohol solutions and of ethanol in blood. To determine water in alcohol solutions they used a reactor filled with a mixture of Sterchamol and calcium hydride (1:l). The hydrogen formed as a result of the reaction of water with calcium hydride was easily separated from all alcohols on a chromatographic column and detected as a narrow peak.
In the determination of ethanol in blood the ethanol was first converted into ethylene for better separation and higher sensitivity. The dehydration proceeded quantitatively at
200-300°C in a reactor filed with phosphoric acid on Sterchamol(1:2) and arranged upstream of the chromatographic column.
A method for the determination of water in liquids, based on GC analysis of the hydrogen evolved during the reaction of water with calcium hydride, was described by Starshov et al. [83].
Another method was developed [18, 191 for determining trace amounts of water in liquid hydrocarbons and some oxygen-containing compounds. It consists in the chromatographic determination of the amount of the hydrogen evolved as a result of the reaction of water present in the liquid sample with a solution of sodium aluminium hydride in the dimethyl ester of diethylene glycol. In these experiments use was made of a liquid bubbler, which is highly promising for reaction GC, incorporated into the chromatographic system. The employment of such a reactor considerably widens the range of the reactions that can be used in analytical reaction GC (in particular, the bubbler can be used to conduct liquid-phase reactions involving several reagents, at room temperature) and allows the initial sample size to be dramatically increased, and thus the sensitivity of analysis to be improved. The determination of water in organic compounds is an important analytical task [84]. The proposed method is characterized by a higher sensitivity and better reliability than others described in the literature.
A bubbler-type reactor is incorporated into the chromatographic system upstream of the column. The thoroughly dried carrier gas (high-purity argon containing about 0.003% of oxygen) is supplied into the reactor from the bottom, which ensures intimate agitation of the entire reaction mixture. The sample is released into the reactor by turning the tap of one of the calibrated burettes, the other being filled with a standard solution containing a known amount of water, which served for checking the calibration of the instrument.
The water content in the sample can be determined in two ways: by using a calibration graph or by the comparison method.
In polymer production the water content in the starting monomer and in solvents is of great importance with respect to product quality. The proposed method was used to determine water in some diene and olefin monomers (isoprene, styrene, heptene, vinylcyclohexane, etc.), w h c h are normally inert with respect to lithium aluminium hydride solutions. The method was also applied to the determination of water in ethers and cyclic esters.
A number of elegant experiments based on the conversion of involatile into volatile compounds have been carried out in order to determine trace components in inorganic materials (minerals, metals, etc.).
Carpenter [SS] used the method of conversion of a trace component into a volatile compound for the rapid determination of trace amounts of carbonates in inorganic materials and proposed a simple arrangement connected t o a standard chromatograph.
When the sample is treated with an acid (lOml of 3 N hydrochloric acid), the carbonate releases carbon dioxide and decomposes in a closed-loop system under a slight vacuum of about 0.5atm. During the reaction the sample is vigorously stirred by means of a magnetic stirrer for 5min. After decomposition of the carbonates the reaction flask is connected to a pre-evacuated calibration space, which also serves as the metering volume of the chromatograph. The gas sample emerging from the reaction flask contains
265 air, water and carbon dioxide. The water is absorbed in a drying tube containing magnesium perchlorate, inserted between the metering space and the chromatographic column. Air and carbon dioxide are separated at room temperature on a column con- taining silica gel (30cm) at a carrier gas (helium) flow-rate of 4cm/sec. The thermal conductivity detector used in the system permits the determination of 0.01 mmol of carbon dioxide in the sample. The method allows the analysis of samples containing 0.2
Independently of Carpenter, Jeffery and Kipping [86] proposed a method for deter- mining carbonates in rocks. In contrast to Carpenter’s method they proposed to use a dilute solution of orthophosphoric acid to decompose the carbonates. The same method of determining gaseous products by decomposing salt-like derivatives of phosphoric acid was also used for determining carbon dioxide and nitrogen oxide in aqueous solutions of monoethanolamine [87].
Some investigators successfully used the reaction chromatographic method in the analysis of boron hydrides [ S S ] , silicon and germanium hydrides [89], etc. Consider, for example, the determination of lithium carbide in lithium hydride [90]. The sample was decomposed using water, and the evolved gas was trapped in a measuring burette.
Then, by means of a gas metering valve, the gas sample was introduced into the chromato- graph where the concentrations of acetylene and hydrogen, proportional to lithium carbide and lithium hydride, respectively, were determined.
Nersesyants et al. 1911 were successful in using this method for the separate determi- nation of aluminium and aluminium carbide. A solid sample was treated with an acid, and the gas evolved was collected in a gas volume-measuring device. The gas composition was determined on a column containing molecular sieves by conventional gas chromato- graphy. The hydrogen content is proportional to that of the metal in the sample, and the content of the organic gaseous component corresponds to that of the carbide [91].
Hogan and Taylor 1921 proposed to determine magnesium, aluminium and other active metals using a GC method with measurement of the amount of the hydrogen evolved as a result of interaction of the metal with an acid.
A mixture of sulphides, sulphates and carbonates can be analysed 193, 941 after acid decomposition of the sample, releasing carbon dioxide, hydrogen sulphide and sulphur. The gas mixture evolved can be determined by GC. The acid decomposition method was used in the analysis of moon rock samples [95] .
Bergmann and Martin [96] developed a reaction GC method for determining chlorine and bromine ions. Chlorine and bromine were converted into the corresponding hydrogen halides by acidifying the sample solution with 88% sulphuric acid. Gaseous hydrogen chloride and bromide were desorbed from the acidic solution by a flow of helium and concentrated in a trap cooled with liquid nitrogen. The concentrated hydrogen halides were separated at -78°C on a column packed with FTFE with a mixture of toluene and n-heptane on its surface as the liquid stationary phase. The release of hydrogen halides from the acidic solution took about 20min, the duration of analysis being 10min.
Studies of the effect of various cations have shown that most of them, except cobalt, do not influence the quantitative results. This method is interesting from the viewpoint of trace analysis. At present other more suitable methods can be used for the separation of hydrogen halides [97,98].
of carbonates. The analysis time is 10min.
266
Aleinikov et al. 1991 proposed a method for determining sulphur, selenium and tellurium in various samples by first obtaining volatile fluorides and then analysing them by GC. The sample was fluorinated with xenon difluoride in a reactor under static conditions at 150°C. After the reaction the volatile products were blown into a poly- fluoroethylene chromatographic column filled with 20% of Kel-F on Polychrome-1 and analysed at - 78°C.
Previously, another method of preliminary fluorination was used by Juvet and Fisher [ 1001 for the analysis of metal oxides, alloys, carbides and salts.
Also proposed was a method of determining diisobutylisobutoxyaluminium and isobutyldiisobutoxyaluminium in triisobutylaluminium from the hydrolysis product, namely isobutyl alcohol [ l o l l . Many other examples of the analysis of organometallic compounds from the products of their hydrolysis can be found in an excellent review by Ivanova and Frangulyan [98].
In many instances unstable compounds can also be analysed by being decomposed to give stable products. For example, to analyse solutions of hydrogen peroxide Greiner 11021 first quantitatively decomposed it on a platinum gauze at 150°C giving oxygen, w h c h was separated chromatographically from other components and detected with a thermal conductivity detector.
Nelsen and Groenning [ 1031 introduced a method for determining carbonaceous compounds plus hydrogen peroxide. The resulting oxygen and carbon dioxide were separated on a column of silica gel.
Methods based on the same principle are employed in ultimate analysis for deter- mining trace components in involatile compounds. For example, Juranek and Ambrova
11041 applied reaction chromatography to the determination of carbon in the presence of sulphur in iron, its alloys and other materials by burning the sample in a flow of oxygen. The detection limit was Methods for determining nonmetals in metals were developed by Stuckey and Walker [105], Mungall and Johnson [1061, Sukhorukov and Zhukhovitsky [ 1071 and Sukhorukov and Ivanova [I081 .
A method for determining sulphur in organic compounds was elaborated by Okuno et ai. [109]. It is based on catalytic destructive hydrogenation of the sample and chromatographic determination of hydrogen sulphide. The detection limit is
The method can be further developed to determine trace amuunts of sulphurous com- pounds in petroleum at levels down to
In some instances it is advisable to convert chemically active trace components capable of reacting with the liquid stationary phase, trace components in the carrier gas, etc., into less active compounds in order for them to be adequately adsorbed on standard solid supports.
Sulphur trioxide is difficult t o analyse directly by GC because it is highly reactive and is eluted from the column in an asymmetric zone when separated directly [110].
Bond et al. [ill] proposed to convert sulphur trioxide, prior to chromatographic separation, into an equimolecular mixture of carbon monoxide and dioxide by reaction with oxalic acid. Nebbia and Belloti [I121 offered a different solution to the same problem, viz., conversion of sulphur trioxide into carbon monoxide by reaction with sodium formate at 120°C. Diedrich et al. [ I 131 converted reactive ammonia into nitrogen in a reactor containing platinum at 700°C. The resulting nitrogen was separated at 30°C on a column of molecular sieves 5A.
[109].
A highly sensitive method for determining hydrogen using helium as the carrier gas was proposed earlier [21]. It is well known [97] that helium is not recommended for use as the carrier gas in hydrogen determinations because the accuracy is reduced as helium and hydrogen have almost identical thermal conductivities. In earlier work [21]
two tasks were accomplished: separation of hydrogen from extraneous gases and improve- ment of the sensitivity of its determination. A mixture of gases formed during vapour- phase ammonolysis of heterocyclic compounds was subjected to analysis. The mixture was first separated on a column (100 x 0.3 cm I.D.) filled with aqueous silicic acid activated at 200°C. On the same column carbon dioxide was separated from a group of permanent gases, including hydrogen. Then the mixture separated on the first column was carried by helium into a reactor filled with copper oxide and heated to 850°C. In this reactor hydrogen was quantitatively converted into water, which was separated from the mixture of permanent gases on a second column (50 x 0.3 cm I.D.) filled with 1% of glycerol on sodium chloride, The detector successively senses (1) the total permanent gases (first peak), (2) carbon dioxide and (3) water, whose concentration is proportional to that of hydrogen in the sample.
Thus, the conversion of trace components into volatile compounds which lend them- selves more readily to gas chromatography permits more selective and sensitive analyses of both volatile and involatile trace components in various samples, including reactive compounds.