Determination of Composition of Polyamide Resins by Trimethylsilylation and Gas Chromatography Sadao Mori and Motohisa Furusawa Laboratory of Chemistry, Faculty of Engineering, Yamanashi University, Kojiu, Japan
Tsugio Takeuchi Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Nagoya, Japan The trimethylsilylation of the diamine dihydrochlorides, the dibasic acids, and the w-amino acid hydrochlorides recovered from the acid hydrolysis of copolyamides was examined along with the effect of the hydrochloric acid liberated during silylation. A trimethylsilylation with bis(trimethylsilyl)acetamide in acetonitrile at 90 O C proved to be the simplest and to be suitable for the gas chromatographic analysis of the components of polyamides. The free hydrochloric acid disturbed the conversion of the components into the TMS derivatives and triethylamine which served as a catalyst for silylation was added as a hydrochloric acid scavenger. The TMS derivatives were separated on a 2-meter column packed with 5% neopentyl glycol succinate polyester coated on Celite 545. Appropriate homologs were used as internal standards. The method has an average relative error of &4.4% in accuracy and an average relative standard deviation of 1.1% in precision.
A TECHNIQUE for the analysis of copolyamides has been developed ( I ) . In the paper, diamines, dibasic acids, and wamino acids recovered from the hydrolyzed copolyamides were converted into the di-N,N'-trifluoroacetyl diamines, the dibasic acid dimethyl esters, and the N-trifluoroacetyl amino acid methyl esters by esterification and trifluoroacetylation, respectively. The method required two derivatization techniques. Trimethylsilylation is a general derivatizing agent for the analyses of materials with active hydrogen such as carboxylic acids (2-4, amines (9,and amino acids (4, 6, 7), as well as esterification and trifluoroacetylation. The silylating agents in general use are hexamethyldisilazane (HMDS), trimethylsilylalkylamines, or a mixture of HMDS and trimethylchlorosilane (TMCS) (7). Klebe, Finkbeiner, and White (4) reported that bis(trimethylsily1)acetamide (BSA) is a silylating agent superior, in many respects, to the presently used methods. Pierce and his coworkers (8) incorporated BSA into a procedure for the determination of tea flavanols. Richter and his coworkers (9) detected oxocarboxylic and dicarboxylic acid silyl esters by mass spectrometry. The method described in this paper concerns the trimethylsilylation and the gas chromatographic resolution of the di(1) S. Mori, M. Furusawa, and T. Takeuchi, ANAL.CHEM., 42, 138 (1970). (2) E. R. Blakley, A n d . Biochem., 15, 350 (1966). (3) . . J. P. Shyluk. C. G. Younns. - . and D. L. Gamb0ra.J. -. Chromatoar.. 26,268 (i967j. (4) . , J. F. Klebe. H. Finkbeiner. and D. M. White. J. Amer. Chem. Soc., 88, 3390 (1966). ( 5 ) L. Birkofer and M. Donike, J. Chromarogr., 26, 270 (1967). (6) L. Birkofer and A. Ritter, Chem. Ber., 93,424 (1960). (7) E. D. Smith and H. Sheppard, Jr., Nature, 208, 878 (1965). (8) A. R. Pierce, H. N. Graham, S. Glassner, H. Madlin, and J. G. Gonzalez, ANAL.CHEM., 41,298 (1969). (9) W. J. Richter, B. R. Simoneit, D. H. Smith, and A. L. Burlingame, ibid., p 1392. -
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amine dihydrochlorides, the dibasic acids, and the w-amino acid hydrochlorides recovered from hydrolyzed copolyamides. The dried hydrolysis product of the polymer is directly silylated with BSA in acetonitrile and gas chromatographed. The effect of hydrochloric acid for the trimethylsilylation is eliminated by the addition of triethylamine to the reaction mixture. The accuracy of this method is somewhat worse than the earlier technique (I); however, the derivatization procedure is simpler; in consequence, this method can reduce the time required for the analysis. EXPERIMENTAL Apparatus. A gas chromatograph, packed columns, and the setting of the detector current were the same as those used previously ( I ) . The injection port and the detector oven were maintained at 230 and 260 "C, respectively. A helium flow rate of 40 ml per minute was maintained. A series of the TMS derivatives was separated at the column temperature programmed from 120 to 220 "C or 160 to 220 "C at a rate of 4 "C per minute. A quantitative analysis of the composition of some polyamide resins was attained at isothermal conditions. Reagents. All diamines, dibasic acids, and w-amino acids were the same as those used before ( I ) . BSA, HMDS, TMCS, triethylamine (TEA), and bis-2-(2-methoxyethoxy) ethyl ether were purchased from the Tokyo Chemical Industry
co. Procedure. PREPARATION OF DERIVATIVES. About a 10-mg portion of the sample was dissolved in 0.15 ml of acetonitrile in a 10-ml flask, 0.03 ml of TEA and 0.15 ml of BSA were added to the flask in this order. The flask was fitted with a reflux condenser and then the contents were heated in an oil bath for 30 minutes at 90 "C. Air moisture was kept out by means of a Drierite tube. Additional BSA and acetonitrile, 0.15 ml each, was added to the flask and the mixture was again heated for 30 minutes. The solution was then filled up to about 2 ml with acetonitrile, and 10 rl of the final solution were injected into the gas chromatograph. BSA, acetonitrile, and TEA should be added in proportion to the weight of the sample. ANALYSIS OF COPOLYAMIDES. The acid hydrolysis of copolyamides and the dryness of the hydrolyzate were carried out according to the method described previously ( I ) . The hydrolysis residue was divided; one portion was used for qualitative analysis, and the other for quantitative analysis. For the latter analysis, a 10-25 mg portion of the dried hydrolyzate was weighed into a 10-ml flask, and the appropriate internal standards were added quantitatively to the flask. The weights of the internal standards were approximately 5 mg in each case. After the reaction of the mixture with BSA and gas chromatography, the contents of each component were determined from the calibration curves. The construction of calibration curves and the selection of appropriate internal standards were similar to those of the previous report ( I ) . Effect of Free Hydrochloric Acid. A portion, about 10 mg, of adipic acid, hexamethylenediamine dihydrochloride, e-
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Derivative Diamine Tetramethylene diamine (C,) Pentamethylene diamine (C,) Hexamethylene diamine (Cs) Heptamethylene diamine (C7) Octamethylene diamine (C,) Nonamethylene diamine (C,) Decamethylene diamine (GO) Dodecamethylene diamine (Clz)
Table I. Retention Times for TMS Derivatives Programmed, 4 "C/min 120-220 OC 160-220 "C 160 "C (min) (min) (min) 8.0 10.1 12.5 15.0 17.2 19.5 21.4 24.8
2.8 3.7 5.2 6.8 8.3 10.1 11.7 14.8
Isothermal 200 "C (rnin)
3.1 4.5 7.1 11.0 16.1 23.5
Amino acid Glycine (C,) @-Alanine(C,) yAmino-n-butyric acid (CJ 6-Amino-n-valeric acid (C,) e-Amino-n-caproic acid (CS) 7-Amino-n-caprylic acid (C,) K-Amino-n-undecanoic acid (CII) A-Amino-n-lauric acid (ClJ
2.2 4.5 6.4 8.5 10.4 15.0 21.7 23.2
3.4 6.7 13.0 14.5
5.0 11.7
Dibasic acid Succinic acid (CJ Glutaric acid (C,) Adipic acid (Ca) Pimeric acid (C7) Suberic acid (Ca) Azelaic acid (C,) Sebacic acid (GO) Dodecadioic acid (CIZ)
5.3 7.1 9.4 11.9 14.0 16.7 18.2 22.2
1.8 2.5 3.2 4.6 6.3 7.9 9.8 13.3
1.8 2.6 3.9 5.8 8.9 13.5 20.8
220 "C (min)
1 .o 1.2 1.5 1.8 2.4 3.0 5.0
2.0 1.4 2.1 6.0 7.3
0.7 0.8 1 .o 1.5 3.5 4.2
1.4 1.8 2.3 3.9
Table 11. Effects of Hydrochloric Acid and Triethylamine Area ratios of TMS derivatives to internal standard, arbitrary unita Adipic acid Hexamethylene diamine Amino caproic acid Amino caprylic acid Case 1 1.16 0 0 0 1.50 0 Case 2 0 0 3.07 Case 3 0.55 0.58 0.49 2.87 After 1 day 0.52 0.59 0.52 3.10 0.57 0.49 After 6 days 0.57 1.67 1.21 1.05 Case 4 1 .oo 1.22 Case 5 1.88 1.45 1.14 1.22 After 1 day 1.83 1.44 1.06 0.98 1.36 0.90 After 6 days 1.70 a Area ratios were corrected in inverse proportion to each weight ratio concerned to make the weight ratio of corresponding derivatives equal in all cases, and, in consequence, the area ratios in this table give the silylation ratio concerned. amino caproic, and g-amino caprylic acid hydrochlorides, and bis-2-(2-methoxyethoxy) ethyl ether, which was used as an internal standard for this objective (IO), was weighed into a 10-ml flask, respectively. The reaction of the mixture with BSA proceeded through the following conditions. Case 1. BSA 0.3 ml, acetonitrile (AN) 0.3 ml, reaction time (RT) 30 minutes at 90 OC Case 2. BSA 0.3 ml, A N 0.3 ml, RT 1 hour Case 3. To the final solution of Case 1, 0.2 ml each of additional BSA and A N was added, and the solution was heated again for 30 minutes Case 4. BSA 0.3 ml, A N 0.3 ml, TEA 0.1 ml, RT 30 minutes Case 5 . The same treatment as in Case 3 was given to the final solution of Case 4 In Cases 1 to 3, the liberated hydrochloric acid was aspirated intermittently. The completeness of the derivatization reaction and the optimum reaction time for silylation were confirmed through gas chromatography at a column temperature of 160 "C. (10) G.C. Ongemach and Asa C. Moody, ANAL.CHEM.,39, 1005 (1967). 960
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RESULTS AND DISCUSSION Under the conditions of programmed column temperature or isothermal specified, retention times for the TMS derivatives were very reproducible and are given in Table I. Symmetrical peaks were obtained without tailing. Columns packed with 5 % SE-30 or 5 % Apiezon L on silanized Celite 545,which have generally been used for the separation of TMS derivatives (4, 7), failed to give recognizable peaks even after repeated injections. The diamines and u-amino acids liberated from the acid hydrolysis of copolyamides form their hydrochlorides. Those hydrochlorides generate free hydrochloric acid during the silylation. Quantitative silylation and the effect of free hydrochloric acid were substantiated in several ways. The stability of the TMS derivatives, which were stored in the tightly sealed bottles, was also examined by leaving the reaction mixture at room temperature for one to six days. The results are shown in Table 11. The free hydrochloric acid disturbs the conversion of the diamine and the amino acids into the TMS derivatives (Cases 1 and 2). The reaction rate in the presence of TEA was rapid, and even after heating less
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Figure 1. Gas chromatograms of TMS derivatives of hexamethylene diamine, adipic acid, e-amino caproic acid, and internal standards Column temperature: 160 "C 1. Acetonitrile, 2. BSA, 3. TMSA, 4. Adipic acid, 5. c-Amino caproic acid, 6. Hexamethylene diamine, 7. Suberic acid, 8. qAmino-+caprylic acid, 9. Octamethylene diamine c
3,
-1
,
I
I
O
5
10
I
I
I5
20
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than 30 minutes, a great quantity of the derivatives was observed. TEA might serve as the catalyst for a TMS derivatization as well as a hydrochloric acid scavenger. TEA would also shift the equilibria between the BSA and the TMS derivatives: the equilibrium between the BSA and the TMS derivative of adipic acid shifted on the free acid side, of the diamine and the amino acids on the product side, in comparison with Case 3. On addition of a 2.4-fold excess of BSA in Case 5, the equilibria moved far on the product side. Moreover the use of a slight excess of BSA caused the formation of monosilylated derivatives. A large peak of BSA and a small one of trimethylsilylacetamide (monosilylamide which was formed as a by-product) on the gas chromatograms are indispensable to obtain the equilibrium far on the product side. The per cent silylation was not obvious, but the reproducibility of the derivatization was good (Table 111), indicating that the derivatization was essentially quantitative. The stability of the TMS derivatives was good, even after six days no additional derivatives were formed, nor had considerable desilylation occured. Prolonged heating over one hour caused the decomposition of some TMS derivatives. It is not necessary to remove the TMSA and TEA-hydrochloride before the injection of the solution to the gas chromatograph. HMDS, TMCS, and a mixture of them were tested for the silylation and produced results dissimilar to that of BSA for this application. Even after the reaction of the amino acids or the diamines with the TMS reagents at 90°C for one hour, disilylated products were less than one tenth in comparison with BSA, and many mono-silylated products were formed, Because of this incomplete conversion into the fully silylated TMS derivatives, BSA was adopted in our study. Other TMS reagents available have not been evaluated. Flame ionization detector was examined extensively, but incomplete detection was observed. FID detected TMSA well, but BSA and the TMS derivatives incompletely. This imperfect detection might result from the incomplete ionization due to the presence of two trimethylsilyl groups on the both sides of the chain of the derivatives. For the analysis of Nylon 6, 66 or Nylon 6, 66, 610 copolyamides, octamethylenediamine dihydrochloride, suberic acid, and q-amino-n-caprylic acid hydrochloride were selected
Table In. Results of Recovery Experimentso Added, mg Found: mg Re1 error,
Mixture 1. Cediamine C6dibasic acid Cgamino acid 2. Cgdiamine C6dibasic acid C,amino acid 3. Ce diamine C e dibasic acid Ceamino acid
9.4 9.1 10.4 20.2 5.5 15.3 4.5 19.4 6.0
9.1 9.0 10.7 18.0 5.4 14.9 4.7 20.3 6.5
-3.1 -1.1 +2.9 -10.9 -1.9 -2.5 +4.5 +4.6 $8.3 Av 9 4 . 4
Column temperature: 160 OC. b Average of three injections. 5
as the internal standards, and the gas chromatographic column temperature was isothermal at 160 "C. For the analysis of Nylon 6, 12 copolyamide, q-amino-n-caprylic acid hydrochloride was used, and the column temperature was isothermal at 180 "C, or programmed from 160 to 220 "C at a rate of 4 "C/min. Application of the homologs to the internal standards was effective in reducing the error caused by silylation to a minimum. The gas chromatograms for the analysis of Nylon 6,66 copolyamide was shown in Figure 1. The recovery data are shown in Table 111. The precision of the gas chromatographic method was determined at 160°C of column temperature by injecting the same sample solution five times, in which diamine dihydrochlorides (C, and Cs), dibasic acids (C,and C,),and w-amino acid hydrochlorides (C, and C,) were contained. A relative standard deviation ranged from 0.4 to 1SZ and an average relative standard deviation of 1.1 Z was obtained. ACKNOWLEDGMENT
The authors were indebted to Miss Kikue Kobayashi for her technical assistance. Received for review February 2, 1970. Accepted May 4, 1970.
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