Anal. Chem. 1996, 68, 1685-1687
Quantitative Determination of Completely Acetylated Pentaerythritol and Pentaerythritol Oligomers by High-Performance Liquid Chromatography Mark A. Tapsak, Guo Hong Wang, Krist Azizian, and William P. Weber*
Donald P. and Katherine B. Loker Hydrocarbon Research Institute, Department of Chemistry, University of Southern California, Los Angeles, California 90089-1661
Analysis of pentaerythritol (PE) and PE oligomer mixtures has been achieved. PE oligomer mixtures are completely and rapidly (15 min) acetylated using acetic anhydride and a catalytic amount of 4-(dimethylamino)pyridine. A Whatman Partsil-10 C8 HPLC column resolves acetylated PE oligomers, which include acetylated penta-PE, with a mobile phase of acetonitrile/water (1:1 v/v). The oldest method for analyzing pentaerythritol (PE) in the presence of PE oligomers is a gravimetric technique reported by Kraft.1 The dibenzylidene acetal of pentaerythritol is produced exclusively by treatment of PE and its oligomers with hydrochloric acid and benzaldehyde in water/ethanol (1:1 v/v). Dipentaerythritol (di-PE) and tripentaerythritol (tri-PE) do not interfere with the PE determination if their sum is less than 20% of the mixture. Wyler reported a method of analysis of PE and di-PE based on differences in their solubilities in water.2 However, tri-PE, tetrapentaerythritol (tetra-PE), and pentaerythritol (penta-PE) formals interfere with the accuracy of the analysis. Similarly, an infrared method developed by Jaffe and Pinchas, which is based on the intensity of the band (1115 cm-1 in CCl4) due to the ether linkage of the PE oligomers, is not able to distinguish between di-PE, triPE, and tetra-PE.3 Many chromatographic methods have been reported for the quantitative analysis of PE oligomer mixtures. Gas chromatography (GC) has been widely used.4 However, the low volatility of PE, di-PE, and the higher molecular weight PE oligomers does not permit their direct analysis. To utilize GC analysis, it is necessary to convert the polyhydroxylic PE oligomers into lower boiling esters or ethers by acetylation or silylation. Wiersma, Hoyle, and Rempis have reported the acetylation, separation, and identification of PE oligomers by gas chromatography.5 By refluxing a mixture of PE and PE oligomers for 2.5 h with acetic acid anhydride, they obtained an acetylated mixture which was injected directly without purification. Tri-PE octaacetate was the highest molecular weight derivative observed. p-Toluenesulfonic acid-catalyzed acetylation of PE oligomers has been studied. (1) Kraft, M. Y. J. Chem. Ind. 1931, 8, 507. (2) Wyler, J. A. Ind. Eng. Chem., Anal. Ed. 1946, 18, 777. (3) Jaffe, H.; Pinchas, S. Anal. Chem. 1951, 23, 1164. (4) Smith, B.; Tullberg, L. Acta Chem. Scand. 1965, 19, 605. Repasova, I.; Vanko, A.; Dykyj, J. J. Chromatogr. 1974, 91, 741. Simon, C. D.; Comninos, H.; Cornish, L.; Liebenberg, D. D.; Nilson, D.; Shaw, P. J. Chromatogr. Sci. 1993, 31, 237. (5) Wiersma, D. S.; Hayle, R. E.; Rempis, H. Anal. Chem. 1962, 34, 1533. S0003-2700(95)01195-4 CCC: $12.00
© 1995 American Chemical Society
However, this catalyst also caused the hydrolysis of pentaerythritol formals. Relative response factors with a thermal conductivity detector were found to have errors as high as 6.50% when significantly different relative amounts of PE, di-PE, and tri-PE were analyzed. Suchanec reported that trimethylsilyl ether derivatives of PE and PE oligomers are advantageous because of their ease of preparation as well as the ability to resolve trimethylsilyl derivatives of tetra-PE and penta-PE by gas chromatography.6 In addition, the high volatility of the trimethylsilyl derivatives facilitates the use of lower column temperatures. A mixture of pyridine, hexamethyldisilazane (HMDS), and trimethylchlorosilane (TMCS) was used to achieve trimethylsilylation of PE and PE oligomers. Upon cooling, both ammonium chloride and pyridinium chloride precipitate. No further purification of the supernatant liquid is necessary before analysis. The susceptibility of trimethylsilyl ethers to hydrolysis presents a problem, particularly when derivatized sample solutions must be stored for future reference. A direct assay of PE and PE oligomers would be ideal in order to circumvent the inherent uncertainties in derivatization. Indeed, high-pressure liquid column chromatography (HPLC) has been utilized for the separation of various polyalcohols,7 including mixtures that contain PE and di-PE.8 HPLC was used because of its speed and high-resolution capability. An aqueous mobile phase can be particularly advantageous when using a refractive index detector due to the relatively low refractive index of water.9 This provides good detector sensitivity, while the high viscosity and heat capacity of water produce excellent baseline stability. In addition, the short-time noise and long-range drift are minimal. Callmer has also reported that, for PE oligomer mixtures, there exists a correlation between solubility in water and retention time on a µBondapak C18 HPLC column.9 The solubilities of PE and di-PE at 20 °C are 7.2 and 0.22 g/100 of water, respectively.10 TriPE is practically insoluble at ambient temperature and only slightly soluble at 100 °C, 0.5 g/100 g of water. Thus, the relative retention (6) Suchanec, R. R. Anal. Chem. 1965, 37, 1361. Current modification of this method involves the use of N,O-bis(trimethylsilyl)trifluoroacetamide and a 0.25 mm film thickness DB-1 [cross-linked 100% poly(dimethylsiloxane)] capillary column. (7) Jandera, P.; Churacek, J. J. Chromatogr. 1974, 93, 55. (8) Clark, R. T. Anal. Chem. 1958, 30, 1676. Spencer, N. J. Chromatogr. 1967, 30, 566. Belue, G. P. J. Chromatogr. 1974, 100, 233. (9) Caller, K. J. Chromatogr. 1975, 115, 397. (10) Berlow, E.; Barth, R. H.; Snow, J. E. The Pentaerythritols; Reinhold: New York, 1958.
Analytical Chemistry, Vol. 68, No. 10, May 15, 1996 1685
Figure 1. HPLC chromatograph of a mixture of PE, di-PE, tri-PE, tetra-PE, and penta-PE, 6.9%, 9.2%, 65.4%, 15.8%, and 2.8% respectively.
time of PE, di-PE, and tri-PE increases with a decrease in their aqueous solubility. Unfortunately, the higher molecular weight PE oligomers such as tetra-PE and penta-PE cannot be directly analyzed by HPLC using water as a mobile phase because they are virtually insoluble. In the present work, PE and PE oligomer mixtures were separated by HPLC using a stationary C8 phase column and a refractive index detector. PE, di-PE, and tri-PE were analyzed on a C8 HPLC column using N-methyl-2-pyrrolidinone as the mobile phase at 75 °C. Unfortunately, this procedure did not allow analysis of either tetra-PE and penta-PE. On the other hand, acetylated PE oligomer mixtures are sufficiently soluble in acetonitrile/water (1:1 v/v) that PE, di-PE, tri-PE, tetra-PE, and penta-PE can be easily analyzed. We have developed a new rapid (15 min) and quantitative acetylation procedure using the 4-(dimethylamino)pyridinecatalyzed reaction of PE and PE oligomers with acetic anhydride. In certain circumstances, this method is preferable to the usual trimethylsilylation/GC analytical procedure. Acetate derivatives of PE and PE oligomers are less susceptible to hydrolysis and are more stable than the corresponding trimethylsilyl ether derivatives. This is important if sample storage is required. In addition, acetylated and partially acetylated PE derivatives are important commercial products in their own right. Analysis of such mixtures by the trimethylsilylation/GC method requires initial hydrolysis of the acetylated and partially acetylated PE and PE oligomers by treatment with aqueous tetramethylammonium hydroxide to yield PE and PE oligomers, which can then be silylated. Complete acetylation catalyzed by 4-(dimethylamino)pyridine is such situations permits more direct analysis. Comparison of the analyses of identical PE and PE oligomer mixtures by both methods reveals that both are accurate. The choice between these methods will depend on the particular situation and requirements. 1686
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Safety: While only a catalytic amount of 4-(dimethylamino)pyridine is utilized, it should be noted that it is not only corrosive but also highly toxic. In particular, appropriate rubber gloves must be worn to avoid any skin contact. Consult MSDS. Trimethylchlorosilane, hexamethyldisilazane, and N,O-bis(trimethylsilyl)trifluoroacetamide are all flammable and corrosive liquids. EXPERIMENTAL SECTION Apparatus. All experiments were performed on a Waters HPLC apparatus comprised of a 510 HPLC solvent delivery system pump, a U6K injector, a R401 refractive index detector, and a Maxima Model 820 control system. A Whatman Partisil-10 C8 column (250 mm × 4.6 mm i.d.) was used for analysis. The flow rate for the mobile phase was 0.40 mL/min. The mobile phase consisted of distilled water and HPLC grade acetonitrile (Aldrich) (1:1 v/v). Reagents and Solutions. Reagent grade PE, acetic anhydride, and 4-(dimethylamino)pyridine were purchased from Aldrich and were used without further purification. PE mixtures of known composition were obtained from Hercules Inc. Anhydrous N-methyl-2-pyrrolidinone (Aldrich) was used directly from a Sure/ Seal bottle. Acetylation Procedure. In a typical procedure, a mixture of PE oligomers (0.50 g), acetic anhydride (6.00 g), and 4-(dimethylamino)pyridine (0.05 g) and a Teflon-covered magnetic stirring bar are placed in a one-neck 25 mL round-bottomed flask equipped with a reflux condenser. The mixture is heated at 120 °C for 15 min. At this point, the volatile solvents are removed by evaporation under reduced pressure. The residue is dissolved in 3 mL of acetonitrile. The derivatized mixture is filtered through a Spartan-25 syringe filter, 0.45 µm porosity nylon membrane filter having a diameter of 25 mm, just prior to injection on the HPLC apparatus.
Table 1. Comparison of HPLC Analysis of PE and PE Oligomer Mixtures to the GC Method sample 1
PE di-PE tri-PE tetra-PE penta-PE
sample 2
sample 3
sample 4
sample 5
GC
HPLC
% diff
GC
HPLC
% diff
GC
HPLC
% diff
GC
HPLC
% diff
GC
HPLC
% diff
55.52 27.94 12.86 3.32 0.36
55.19 28.35 13.2 3.3 0.35
0.59 1.47 2.64 0.6 2.78
53.24 28.53 13.01 3.98 1.23
54.73 28.21 12.76 4.08 1.2
2.8 1.12 1.92 2.51 2.44
74.89 14.32 7.13 2.56 1.11
75.25 14.73 7.24 2.61 1.13
0.48 2.86 1.54 1.95 1.8
6.91 9.16 65.43 15.75 2.75
6.99 9.41 64.93 16.02 2.72
1.16 2.66 0.77 1.69 1.1
80.95 5.12 10.91 1.47 1.55
80.83 5.23 10.83 1.51 1.51
0.15 2.15 0.73 2.72 2.58
Figure 2. GC chromatograph of the PE, di-PE, tri-PE, tetra-PE, and penta-PE mixture analyzed by HPLC in Figure 1.
RESULTS AND DISCUSSION PE, di-PE, and tri-PE may be analyzed without derivatization. HPLC of such mixtures, using N-methyl-2-pyrrolidinone as the mobile phase and a Whatman Partsil-10 C8 HPLC column at 75 °C, has been investigated. Regrettably, this method is limited by the solubility of the higher PE-oligomers. At ambient temperature, only PE and di-PE are detected. By raising the column and solvent temperature to 75 °C, tri-PE can also be detected; however, neither tetra-PE nor penta-PE could be observed. A more accurate method to analyze PE oligomer mixtures, which permits tetraPE and penta-PE to be separated and detected along with the lower molecular weight PE oligomers, is needed. The acetylation of amines, alcohols, and phenols with acetic anhydride11 or acetyl chloride12 in the presence of pyridine is well known. Unfortunately, acetylation of PE and its oligomers is a slow process. In the late 1960s, it was found independently by two groups that 4-(dialkylamino)pyridines are extremely efficient acylation catalysts compared with pyridine.13 The use of 4-(dimethylamino)pyridine and its 4-(dialkylamino)pyridine analogs as acetylation catalysts has been reviewed.14 In this study, we have found that a catalytic amount of 4-(dimethylamino)pyridine and acetic anhydride results in complete acetylation of PE and PE (11) Verley, A.; Bolsing, F. Ber. Dtsch. Chem. Ges. 1901, 34, 3354. (12) Einhorn, A.; Hollandt, F. Liebigs Ann. Chem. 1898, 301, 95. (13) Litvinenko, L. M.; Kirichenko, A. I. Dokl. Akad. Nauk SSSR, Ser. Khim. 1967, 176, 97. Steglich, W.; Hofle, G. Angew. Chem. 1969, 81, 1001; Angew. Chem., Int. Ed. Engl. 1969, 8, 981. (14) Scriven, E. F. V. J. Chem. Soc. Rev. 1983, 12, 129.
oligomers at 120 °C in only 15 min. The addition of either pyridine or triethylamine to the acetylation mixtures did not decrease the reaction time required for complete acetylation. By acetylating all of the hydroxyl groups of PE and its oligomers, the compounds’ solubilities in acetonitrile/water (1:1 v/v) are greatly increased. In this way, PE, di-PE, tri-PE, tetraPE, and even penta-PE can be separated and detected using acetonitrile/water (1:1 v/v) as the mobile phase with a Whatman Partsil-10 C8 HPLC column. To determine the response factors for the pentaerythritol compounds, reagent grade PE was acetylated. PE-tetraacetate was purified by distillation at 165 °C and a pressure of 0.05 mmHg. The response for PE-tetraacetate from the apparatus described above is 5000 V min mol-1. Integration of HPLC peaks was carried out by drawing tangent lines to the peak going through the base line. The area of the peak was calculated as half the product of the base times the peak height (see Figure 1). The relative response factors for di-PE, tri-PE, tetra-PE, and penta-PE were calculated from the detector response for PE-tetraacetate and acetylated PE oligomer mixtures. The response factors for di-PE, tri-PE, tetra-PE, and penta-PE are 0.62, 1.17, 0.96, and 0.29, respectively, given the response of PE as a value of 1.0. Figure 1 shows a typical HPLC chromatograph of a PE oligomer mixture. The peaks at 7 min are those for acetic anhydride and acetic acid. Peaks corresponding to PE, di-PE, triPE, tetra-PE, and penta-PE have retention values of 13, 18, 25, 37, and 55 min, respectively. These mixtures were also analyzed by trimethylsilylation and gas chromatography (Figure 2).6 It should be noted that the current modification of this GC method involves the use of N,Obis(trimethylsilyl)trifluoroacetamide and a 0.25 mm film thickness DB-1 [cross-linked 100% poly(dimethylsiloxane)] capillary column. Table 1 permits comparison of the analytical results obtained from the acetylation HPLC method reported herein with those acquired by the silylation GC method of Suchanec of the same PE oligomer mixtures.6 Good separation and detection of PE and PE oligomers has been achieved. PE oligomer mixtures are completely and rapidly (15 min) acetylated using acetic anhydride and 4-(dimethylamino)pyridine. A Whatman Partsil-10 C8 HPLC column resolves acetylated PE oligomers, which include acetylated penta-PE, with a mobile phase of acetonitrile/water (1:1 v/v) in less than 1 h. Received for review December 11, 1995. Accepted March 6, 1996.X AC951195+ X
Abstract published in Advance ACS Abstracts, April 1, 1996.
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