August, 19581 DETERMINATION OF SUGARS AND URONIC ACIDS 455 Applications of Gas - Liquid Chromatography The Examination of Solvents from Plastic Adhesives BY J. HASLAM AND A. R. JEFFS (Imperial Chemical Industries Ltd., Plastics Division, Welwyn Garden City, Herts.) Details of the methods that have been used in the examination of mixed solvents from plastic adhesives, spray laquers, etc. , are given. The isolation of the solvent, its gas - liquid chromatographic separation on polar and non-polar columns and the infra-red and chemical tests on the separated products are described. IN the analytical examination of adhesives, plastic paints, etc., containing various mixed solvents, it is often necessary to express opinions on the composition of the solvent mixture that has been used.Within the past 2 years we have examined a large number of materials, for example, adhesives for bonding plastics to plastics, plastics to metal and plastics to glass,456 HASLAM AXD JEFFS: APPLICATIONS OF [Vol. 83 as well as plastic spraying lacquers, polymer coating solutions and inks suitable for printing on plastic materials. We have found gas - liquid chromatography to be of great value in the examination of these preparations and the purpose of this paper is to give details of the methods that we have found to be most useful. It is first necessary to isolate the solvent mixture in a clean condition from the sample under test. The following method has given excellent results in the analysis of a varied range of preparations. METHOD OF ISOLATING THE SOLVENT MIXTURE FROM THE COMPOSITION UNDER TEST- It is a modification of the vacuum-depolymerisation apparatus originally used by Haslam and S0ppet.l Approxi- mately 6 g of the composition are introduced into the bottom of tube A.This tube is then placed in a solid carbon dioxide - methanol bath at -80” C and the open end, B, is then sealed in a flame. The open end, C, is now attached to a vacuum-pump and the apparatus is evacuated, tube A being kept at -80” C; with the Viicuum pump still running the apparatus is then sealed at the constriction, D, by means of a hand torch. An H-tube is constructed having the dimensions shown in Fig. 1. D ,I 0 rnni Fig. 1. Modified vacuum-depolymerisation Tube A is now removed from the solid carbon dioxide - methanol bath and is replaced by tube E.As tube A gradually attains room temperature, the solvent tends to be volatilised from A to E. This transfer is assisted by warming tube A in a heated water bath or even in a heated oil-bath if the solvent is not readily volatilised. I t is often useful to allow this recovery process to proceed overnight. The dimensions of tube A are made purposely large, as some compositions tend to froth during this recovery process. When the material in A is observed to be “dry,” the seal of the apparatus is broken at D and tube E is cut off below the connecting tube. The mixed solvent in a clean condition is now ready for the preliminary gas - liquid chromatographic test. It has been reported that some workers introduce the composition directly on to an asbestos pad at the top of a gas - liquid chromatographic column and allow the solvent to evaporate in the gas stream.Apart from the difficulty of introducing such viscous samples by syringe, our experience is that the drops of the composition “harden” on the outside and trap some solvent on the inside. The result is that the solvent dries out slowly and gives a trace that is not a chromatogram. Alternatively, if a high-temperature vaporiser is used, there is a tendency with some preparations to get a chromatogram of the solvent 9lus depoly- merisation products. We believe that our met hod, although more time-consuming, is to be preferred. PRELIMINARY GAS - LIQUID CHROMATOGRAPHIC TEST-- of the mixed solvent. u / -’ apparatus The purpose of this test is to obtain preliminary information about the general complexityAugust, 19581 GAS - LIQUID CHROMATOGRAPHY 457 The gas - liquid chromatographic test is carried out on 1 drop of the isolated solvent; the column is 6 feet long, of $-inch nominal bore and packed with 30 per cent.w/w of dinonyl phthalate on Celite 545. The Celite 545 is graded by elutriation in the manner described by James and Martin.2 The temperature of the column is maintained at 100' C. Fig. 2. Gas -liquid chromatograms on a 6-foot column of 30 per cent. w/w of (a) Light petroleum, boiling range below (c) Light petroleum, boiling dinonyl phthalate on Celite 545 a t 60" C. 40' C. range 60" to 80' C . (b) Light petroleum, boiling range 40" t o 60' C. (d) Light petroleum, boiling range 80" to 100' C Sample Fig.3. Gas -liquid chromatograms on a 6-foot column of 30 per cent. wjw of dinonyl phthalate on Celite 545 a t 100°C. (a) Light petroleum, boiling range 60" to 80°C. (b) Light petroleum, boiling range 80' to 100" C. (c) Light petroleum, boiling range 100" to 120' C. (d) Light petroleum, boiling range above 120' C The exit pressure is adjusted to 150 mm of mercury and, with a rate of flow of 2.0 litres of nitrogen per hour, the pressure drop along the length of the column is approximately 450 mm of mercury. A katharometer, at room temperature, is used as sensing mechanism.458 HASLAM AND JEFFS : APPLICATIONS OF [Vol. 83 Visual examination of the chromatogranl will quickly indicate the general boiling range of the solvent mixture. If components are present that are rapidly eluted from the column at 100" C, it is desirable to repeat this preliminary separation, but at a column temperature of 50" C.This test is particularly valuable for ascertaining the presence or absence of petroleum fractions and solvent naphtha. These solvents show characteristic patterns, examples of which are given in Figs. 2, 3 and 4. Fig. 2 shows chromatograms of the lower boiling petroleum fractions at a column temperature of 50" C. Figs. 3 and 4 (a) show chromatograms of higher boiling petroleum fractions and Fig. 4 (b) the chromatogram of a typical solvent naphtha carried out at a column temperature of 100" C. If the very high-boiling petroleum fractions are encountered as solvents, e.g., petroleum distillate, boiling range 190" to 275" C, and kerosine, boiling range 210" to 250" C, some difficulty wiU be experienced.I t can be seen from Fig. 4 (a) that, at a column temperature of 100" C, the chromatogram of white spirit takes 100 minutes to complete. The chromato- gram of kerosine, for instance, looks similar to that of white spirit, but the former contains many higher boiling constituents that may not be eluted from the column at 100" C. In any case of doubt, the column temperature is raised to 130" C (the maximum permissible with this stationary phase) and the flow rate is greatly increased to facilitate the removal of such constituents. min. min. (b) Fig. 4. If the preliminary test indicates that solvents are present that boil at temperatures of the order of 140" C or less, it is desirable t o proceed to the gas - liquid chromatographic test on polar and non-polar columns.In our experience in this type of work, solvents boiling above 140°C will normally, at this stage, have been identified as particular high-boiling petroleum fractions. An occasional instance of a composition containing fl-butyl lactate, b.p. 188" C, was encountered. GAS - LIQUID CHROMATOGRAPHIC TESTS ON POLAR AND NON-POLAR COLUMNS- The purpose of this test is to separate, as far as possible, the individual components of a mixed solvent on columns of quite different character, i.e., on (a) a non-polar column containing paraffin wax as stationary phase, and (b) a polar column containing tritolyl phosphate as stationary phase. The idea underlying this test was first put forward by James and Martin? Gas - liquid chromatograms on a 6-foot column of 30 per cent.w/w of dinonyl phthalate on Celite 545 at 100" C. (a) White spirit. (b) Solvent naphthaAugust, 19581 GAS - LIQUID CHROMATOGRAPHY 459 Further, in the course of this test the corrected relative retention times of the individual components, i.e., relative to pure benzene, are determined on both columns. The figures are used in the identification of the separated constituents. The paraffin wax column is 12 feet long, of $-inch nominal bore and packed with 33.3 per cent. w/w of paraffin wax (congealing point 54" C) on 52 to 60-mesh Johns Manville Silocel C22 firebrick; the column is heated by means of a steam jacket at 100" C. The exit pressure of the column is adjusted to 150 mm of mercury and the inlet pressure controlled so as to give a flow rate of 2.0 litres of nitrogen per hour through the column.The tritolyl phosphate column is similar to the paraffin wax column, except that 33.3 per cent. w/w of tritolyl phosphate is substituted for the paraffin wax. This column is enclosed in the same steam jacket as the paraffin wax column. A portion of the sample is first diluted with about one-fifth of its volume of pure benzene and four chromatograms are run, i.e., with and without benzene on each of the columns. Visual examination of the four chromatograms will indicate whether or not benzene is present in the mixed solvent under examination. The corrected relative retention times are now calculated for the individual components separated on each column.The method of calcu- lation is illustrated by the example given in Fig. 5 , which represents the chromatogram on the tritolyl phosphate column of a mixed solvent containing ethanol, ethyl acetate, rt-butyl alcohol, n-butyl acetate and added benzene. Comparison of the results obtained on the two columns with the calibration charts shown in Table I will then indicate the composition of most mixed solvents. This Table has been prepared by the examination of known mixtures containing added benzene. Certain solvents, on a given column, were found to have cor- rected relative retention times similar to that of benzene. Such solvents were examined on their own, pure benzene being run immediately before and after the test substance; the average value for benzene was then used in computing the value for the test substance.It was noted that the alcohols did not give very reproducible results on the paraffin wax column and the values given are the average of several results. Calculation of corrected relative retention times- (24'7-2.1) = 0.52 (32'7-2.1) - 0.71 (107.4-2.1) Peak A (45.5-2.1) Peak (45.5-2.1) - Peak c (45.5 - 2.1) = 2*43 (123.8-2.1) - 2.80 (45.5 - 2.1) - PeakiD Solvent mixture sample identified as- A = Ethanol B = Ethyl acetate C = n-Butyl alcohol D = n-Butyl acetate Fig. 5. Specimen gas - liquid chromatogram of a mixed solvent on a 12-foot column of 33.3 per cent. wjw of tritolyl phosphate on Silocel C22 for determining the corrected relative retention times Apart from this anomaly, we find in practice that the reproducibility of the test is of the order of +0.02 to 0.03 units up to a corrected relative retention time of 1.0, f0.04 to 0.08 units for 1.0 to 3.5 and k0.1 to 0.15 units for above 3.5.It may be necessary, therefore, that, in order to obtain two peaks in a chromatogram from a mixture of two substances, their corrected relative retention times should differ by as much as 0.08 units up to a corrected relative retention time of 1 and 0.15 to 0.25 units for a corrected relative retention time above 1. It will be realised from these figures that difficulties may arise because of the overlap of peaks, and for this and other reasons we find it invaluable when there is any doubt to carry out separations on columns with isolation of the separated components and infra-red examination of the separated products.For example, in our experience a mixture of methyl acetate and acetone gives a single peak on both columns, but such a mixture would not460 HASLAM AND JEFFS: APPLICATIONS OF [Vol. 83 deceive an infra-red spectroscopist. A corresponding instance would be that of a mixture of m-xylene and 9-xylene. TABLE I CALCULATED RELATIVE RETENTION TIMES FOR VARIOUS SOLVENTS ON 12-FOOT COLUMNS O F 33.3 PER CENT. OF PARAFFIN WAX ON SILOCEL c22 AND 33.3 PER CENT. OF TRITOLYL PHOSPHATE ON SILOCEL C22 Solvent Parafin wax column- Acetaldehyde . . .. Methyl formate . . .. Methanol * . .. Ethanol . . .. * . Acetone . . .. .. Methyl acetate . . .. isoPropyl alcohol . . Ethyl formate . . .. Allyl alcohol . . .. Diethyl ether .. . . Methylene dichloride . . tert.-Butyl alcohol . . n-Pentane . . .. n-Propyl alcohol . . Ethyl methyl ketone . . Ethylidene dichloride . . sec.-Butyl alcohol . . Diisopropyl ether . . Methyl propionate . . isoPropyl acetate . . Chloroform . . .. Tetrahydrofuran . . Ethylene dichloride . . Methyl n-propyl ketone n-Propyl acetate . . Ethyl propionate . . Carbon tetrachloride . . Methyl n-butyrate . . Dioxan . . .. .. Propylene dichloride . . isoPropyl propionate . . isoButyl methyl ketone Trichloroethylene . . isoOctane * . .. n-Heptane . . * . isoButyl acetate * . Methylcyclohexane . . n-Butyl acetate .. Toluene . . .. .. n-Octane .. .. Ethylbenzene . . .. Di-n-butyl ether .. p-Xylene . . .. .. o-Xylene .. .. Vinyl acetate . . .. Ethyl acetate . . .. isoButyl alcohol * .n-Butyl alcohol .. cycloHexane . . .. m-Xylene . . .. Bo i 1 in g - point, "C 20.2 31.5 64.6 78.3 56.0 57.1 82.4 54.2 97.1 34.6 40.1 82.5 36.1 77.0 97.2 80.0 77.2 57.3 99.5 67.5 79.9 108.1 88.9 61.2 65.0 83.5 102.3 118.0 101.6 99.1 76.7 102.3 101.4 80.8 96.4 111.3 116.8 86.7 99.2 98.4 117.2 100.8 126.2 110.8 125.6 136.2 142.4 138.4 139.3 144.0 Calculated relative retention time 0.09 0.11 0.13 0.17 0.18 0.23 0.23 0.23 0.25 0.26 0.27 0.28 0.29 0.35 0.36 0.40 0.43 0.44 0.50 0.52 0.54 0.55 0.58 0.60 0.62 0.72 0.81 0.83 0.93 0.94 1.06 1.09 1.12 1.14 1.17 1-25 1.28 1.35 1-31 1.40 1.51 1-91 2.13 2.32 3.10 4.6 4.8 5.2 5.3 6.1 Solvent Trifolyl phosphate column- n-Pentane .. .. Diethyl ether . . .. Acetaldehyde . . .. Methyl formate . . .. Diisopropyl ether . .Methanol .. .. isoOctane * . .. Ethyl formate . . .. Methyl acetate . . .. n-Heptane .. .. cycZoHexane . . .. Acetone . . .. .. Methylene dichloride . . Ethanol . . .. .. tert.-Butyl alcohol . . Ethylidene dichloride . . isoPropy1 alcohol . . Methylcyclohexane . . Carbon tetrachloride . . Methyl propionate . . isoPropy1 acetate . . %-Octane .. . . Tetrahydrofuran . . Ethyl methyl ketone . . Chloroform . . .. sec.-Butyl alcohol . . n-Propyl alcohol . . Ethyl propionate . . Trichloroethylene . . n-Propyl acetate . . Ethylene dichloride . . Methyl n-butyrate . . ZsoPropyl propionate . . Methyl n-propyl ketone Propylene dichloride . . Dioxan .. .. I . Toluene . . .. . . isoButyl methyl ketone Vinyl acetate . . .. Ethyl acetate . . .. Allyl alcohol . . .. isoButyl alcohol ..isoButyl acetate . . n-Butyl alcohol . . .. Di-n-butyl ether .. n-Butyl acetate .. Ethylbenzene . . . . p-Xylene . . .. .. m-Xylene . . .. o-Xylene .. .. Boiling- point, "C 36.1 34.6 20.2 31.5 67.5 64.6 99.2 54.2 57.1 98.4 80.8 56.0 40.1 78.3 82.5 57.3 82.4 77.0 100.8 77.2 76.7 79.9 88.9 126.6 65.0 80.0 61.2 99.5 97.2 99.1 86.7 97.1 101.6 83.5 102.3 111.3 102.3 108.1 96.4 117.2 101.4 110.8 116.8 11 8.0 142.4 126.2 136.2 138.4 139.3 144.0 Calculated relative retention time 0.11 0.17 0.19 0.22 0.29 0.36 0.37 0.38 0.40 0.42 0.45 0.46 0.47 0.52 0.53 0.57 0.58 0.59 0.66 0.70 0.76 0.77 0.79 0.85 0.86 0.89 0.99 1.14 1.15 1.24, 1.26 1.26 1.35 1.37 1.46 1.48 1.55 1.85 1.76 1-93 1.95 2.10 2.12 2.41 2.41 2.78 4.1 4.2 4.4 5.3 CHROMATOGRAPHIC SEPARATION AND INFRA-RED :EXAMINATION OF SEPARATED PRODUCTS- The great advantage of this method is that, idthough the column may have to be loaded with a comparatively large amount of sample, and hence there may be a considerable loss in column efficiency, once the products have been.separated their infra-red identification is usually unequivocal.August, 19581 GAS - LIQUID CHROMATOGRAPHY 461 The principle of this method, which involves the trapping of the separated products after passage through the column, has been described previously? In the interval, the trapping system has been modified and reduced in size. Its dimensions are shown in Fig. 6. Nichrome wire covered with Electrical leads to variable transformer Fig. 6. Modified trapping apparatus The choice of column, load and conditions of separation in this test are largely governed by the boiling range of the mixture as indicated by the preliminary chromatogram. The column temperature is adjusted so that complete separation between the peaks, with a suitable load, is indicated by the record.I t must be borne in mind, however, that it is usually necessary to separate at least 0.05 ml of a particular constituent in order to obtain a satis- factory infra-red spectrum, although work is being carried out to extend the range of the tests so that much smaller amounts of substance can be dealt with. With very volatile constituents it may be necessary to separate rather more than this amount. We have pointed out previously4 that a single peak in a chromatogram does not always indicate the presence of a single substance.Recently, a spray lacquer was examined; this contained 16 per cent. of poly(viny1 chloride - vinyl acetate) copolymer, 9 per cent. of dioctyl phthalate plasticiser and 75 per cent. of mixed solvent. The preliminary chromatogram of the mixed solvent showed four quite separate peaks. The constituents corresponding to these four peaks were condensed in separate cold traps. Infra-red examination indicated that trap 1 contained a mixture of methyl acetate and acetone (a mixture difficult to resolve on many columns). Trap 2 contained tetrahydrofuran and traps 3 and 4 contained benzene and toluene, respec- tively. The mixture in fact contained five components, although only four were indicated in the chromatogram. I t is often desirable to ascertain the approximate composition of the solvent mixture and this is carried out by the method described below.DETERMINATION OF THE APPROXIMATE COMPOSITION OF SOLVENT MIXTURES- From the chromatograms obtained as described it is usually possible to estimate the approximate composition of an unknown solvent mixture. One or two synthetic mixtures similar to the estimated composition of the sample are examined under the precise conditions of test as with the unknown mixture. The composition of this latter mixture is then deduced by inspection of the chromatograms. With experience, the accuracy is normally quite adequate for most work of this type. If at any time greater accuracy is required, a calibration for each component with an internal marker is necessary when nitrogen is used as the carrier gas in conjunction with a katharometer detector. Finally, on occasion, use may be made of chemical tests on separated products.462 CROSSLEY AND THOMAS: THE SEPARATION OF SOME [Vol.83 In addition to the infra-red identification, it is possible to prepare chemically suitable derivatives of the isolated products. Moreover,, certain tests may be made directly on the separated products (diluted with carrier gas) as they leave the gas - liquid chromatographic column. For example, the presence of ketones may be readily proved by bubbling the exit gases through a trap containing 2 : 4-dinitrophenylhydrazine reagent .6 This reagent is prepared by dissolving 1 g of 2 : 4-dinitr0pheny:lhydrazine in 15 ml of concentrated sulphuric acid and diluting to 400 ml with water. The solution is set aside overnight in a refrigerator, after which it is filtered from the excess of reagent. The filtrate is then diluted to 500ml with water, 0.05 to 0.1 pl of acetone will produce a visible turbidity with 0.5 ml of this reagent. Even smaller amounts of acetone, i.e., of the order of 0.01 pl, may be detected by extraction of the reaction product with 1 ml of spectroscopically pure cyclohexane; the extract is examined spectrophotometrically against a corresponding extract of the reagent. Then again, we have shown that chemical tests may be applied in order to decide whether, for example, formaldehyde is produced from methanol under specific conditions of chromatographic test. For this test the exit gases were passed through phenylhydrazine reagent, after which the principle of Schryver’s test was applied in a very sensitive test for formaldehyde. CHEMICAL TESTS ON SEPARATED PRODUCTS- It seems to us that there are many other possibilities for this form of test. We thank Mr. H. A. Willis for his valuable assistance in the infra-red work and Mr. M. Green for his general assistance in the development of these methods. REFERENCES 1. 2. 3. 4. 6. Haslam, J., and Soppet, W. W., Analyst, 1950, 75, 63. James, A. T., and Martin, A. J. P., Biochem. J , 1952, 50, 679. -,- , Analyst, 1952, 77, 915. Haslam, J., and Jeffs, A. R., J . Apfil. Chern., 1957, 7 , 24. Perkin - Elmer Instrument News, 1957, 8, No. 4. Received Febvuary 25th, 1958