1251
Anal. Chem. 1991, 63, 1251-1256
Preparation of Polyester Samples for Composition Analysis G.William Tindall,* Randall L.Perry, James L. Little, and Arthur T.Spaugh, Jr. Research Laboratories, Eastman Chemical Company, Eastman Kodak Company, Kingsport, Tennessee 37662
I n t " o d y used methodsfor the analyslsofpdyerters, the polymer k hydrolyzed to its monomeric components, whkh are then determined by chromatography. The time it takes to hydrolyze the polyester k often considerably longer than tho t h e spent on wbmquont steps of the analysis. Furthermore, highly crystaiiine and iiquid-crystaiilne polyesters can be nearly Inert In the reagents used to hydrolyze other pdyeaters. I t would, therefore, be desirable to Improve the rate at whkh polyesters could be prepared lor analysis. I t was found that a mixture of hydroxkk, an alcohol, and certain polar aprotk mivents w#l hydrolyze polyesters at rates much faster than any reagent prevloudy described. Even highly cryslalilne polyestors can be quantItalively hy&olyzdto the& corresponding monomers In a few mlnutes with thls reagent. Three methods lor the analyrk of polyesters are described that use this reagent for the rapkl preparation of the polymer sample. Methods for the determination of acids, diethylene &col In poiy(ethybne temphthaiale), and Q)ycok and diackk by NMR are described. We conclude that this new reagent p"much faster rates for pdyesler hydrolyds than those previously dkckud. Thk reagent can be usad to prepare samples for analyses by chromatography or NMR.
INTRODUCTION Numerous methods have been described for the determination of components in polyester polymers. One common strategy is to convert the polyester to its monomeric components, which can then be determined by a number of techniques, for example, chromatography. Reactions that have been used fall into one of three classes-aminolysis, transesterification, and hydrolysis. Aminolysis. In aminolysis, the polyester is refluxed with a primary amine. The reaction products are the diamide of the diacid and glycol. Esposito (1) used aminolysis to identify the glycols in paint resins: ASTM Method D 2456 describes a similar method. Esposito (2)also described the use of benzylamine for the same application. Wittendorfer (3)used phenylethylamine for the aminolysis of polyester foam materials. Hydrazine may be used instead of a primary amine. Because it is a potent nucleophile, the reaction should proceed at a fast rate. Hovenkamp and Munting (4) determined diethylene glycol after refluxing poly(ethy1ene terephthalate) (PET) for 15 min with hydrazine (presumably hydrazine hydrate). Vink and van Wijk (5) described a similar method for methyl end groups, and Nissen et al. (6) used hydrazine in a mixture of dioxane and butanol to degrade PET for the determination of carboxyl end groups. Transesterification. In transesterification, the polyester is heated with excess alcohol; a catalyst may be added. The products are the diester of the diacid and glycol. Esposito and Swam (7)identified the carboxylic acids in polyester paint resins by gas chromatography after refluxing the resin in methanol with lithium hydroxide catalyst: ASTM Method D 2455 uses this approach. Percival(8) accomplished the same analysis with methanol and sodium methoxide after refluxing the sample for 18 h. Janssen et al. (9) determined diethylene glycol in PET after transesterification with ethanol at 250 "C 0003-2700/91/0363-1251$02.50/0
without a catalyst. Highly crystalline polyesters required up to 18 h to complete the reaction. Jankowski and Garner (10) described a novel transesterification reagent consisting of methanol, sodium methoxide, and methyl acetate. Refluxing for 1-2 h was sufficient for the polyester polymers they analyzed. Rawlinson and Deeley (11)used this mixture at 175 "C for polyester paint resins. They proposed a reaction mechanism involving methyl acetate in the transesterifhation. Perlstein and Orme (12)reported a similar method for polyester elastomers. The sample was refluxed with ethanol, sodium ethoxide, and ethyl acetate for 30 min. Hydrolysis. In hydrolysis, the polyester is heated in a solvent with a base. The products of the reaction are the glycol and diacid salt of the polyester. Kirby et al. (13)described the determination of diethylene glycol in poly(ethy1ene terephthalate). PET fiber was refluxed with KOH in ethanol. West (14) described a method to determine components in polyester paint where a solution of tetramethylammonium hydroxide in methanol was used. Ponder (15) hydrolyzed PET in a Paar pressure vessel at 230 O C with water. Allen et al. (16)used a mixture of KOH in 2-ethoxyethanolto quantitatively hydrolyze several polyester compositions. This method was more rapid than previous methods, but impending OSHA regulation of 2-ethoxyethanol makes its use unattractive in some laboratories. Haken and Rohanna (17) developed a novel approach to hydrolysis whereby the sample is fused with sodium hydroxide. The previously described methods are adequate for many kinds of polyesters, particularly when analysis time is not critical. But none of these methods are adequate to support modern polyester manufacturing, research, and development, particularly when highly crystalline and liquid-crystalline polyesters must be analyzed. The various aminolysis procedures are slow, and there is unpublished evidence for the degradation of some glycols during aminolysis. Transesterifications are similarly slow. However, transesterification can proceed under mild conditions, and the products can often be analyzed by gas chromatography without further derivatization. These features make transesterification procedures attractive for some kinds of nonroutine analyses. Hydrolysis has been the most practical approach for preparing samples for routine analysis by chromatography. However, recent trends in the manufacture of polyesters have resulted in the need to improve the rates of polyester hydrolysis. Even for easy to hydrolyze polymers, such as amorphous PET, the hydrolysis step is rate limiting for determining diethylene glycol and other components in this polymer (18). Furthermore, many highly crystalline polymers and liquid-crystalline polymers are nearly inert in the hydrolysis solutions used to analyze PET. More aggressive hydrolysis conditions are essential to prepare these kinds of polyesters for analysis. The most promising reaction to improve is hydrolysis. Hydrolysis, or alcoholysis when attack is by an alkoxide, proceeds by the following mechanism (19): Oe
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e
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Q 1991 Amerlcan Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 83, NO. 13, JULY 1, 1991
Reaction temperature, properties of the solvent, and the nature of the nucleophile, such as electron density, size, and concentration, could be expected to have a significant effect on the rate of this reaction. Systematic investigations of these variables have shown that they have orders of magnitude effects on hydrolysis rates of monomeric esters. However, investigation of these variables for polyester hydrolysis has not been reported even though there would appear to be several possible ways to improve the rate of reaction. One should not expect that what is known about low molecular weight ester hydrolysis will apply to hydrolysis of a polymer, which will typically be insoluble in the reaction mixture and will interact with solvents in ways unique to polymers. The objective of this work was to investigate the effects of reaction conditions on polyester hydrolysis. A further objective was to identify reaction conditions that provide faster rates than conditions currently used and to develop rapid methods for polyester analysis based on these conditions.
EXPERIMENTAL SECTION Procedure for Evaluating Degradation Rate. Unless noted, the polymer used was pelletized medium crystallinity poly(ethylene terephthalate). To measure the rate of degradation, 0.1 g of polymer was placed in a Coming 25- X 1 " m screw cap culture tube (No. 9826) with 5 mL of the solvent mixture to be teated and a small stirring bar. The cap was placed loosely on the tube. The mixture was heated and stirred with a Pierce Reacti-Therm I11 heating/stirring module. The time for the polymer pellets to disappear (hydrolyze) was recorded. Experiments were done at least in duplicate, and the average time was reported. Solvents were obtained from Burdick & Jackson and used without further purification. Procedure for Determining Acid Components. About 100 mg of accurately weighed polymer was placed in a Pyrex No. 9826 culture tube with a small stirring bar. Four milliliters of dimethyl sulfoxide (DMSO)and 1mL of 5 N sodium hydroxide in methanol were added. The tube was loosely capped and placed in a Pierce Reacti-Therm I11 heating/stirring module at 120 OC for 15 min. After 15 min, the tube was taken from the module and allowed to cool. When cool, 6 mL of water and 0.2 mL of phosphoric acid were added. A portion of the solution was put in a 1-mL autosampler vial and capped. As we will demonstrate in subsequent papers, this reagent will also attack hydrolyzable tissue components at enhanced rates. Therefore, care must be taken to avoid contact of the reagent with skin and eyes. Relevant MSDS documents should be consulted before handling methanol, DMSO, and the sodium hydroxide. Analyses were performed on a high-performance liquid chromatography system consisting of a Waters automated gradient controller, a Model 510 solvent delivery system, a Perkin-Elmer LC 600 autosampler, and a Waters Lambda-Max Model 481 LC spectrophotometer. The reversed-phase analytical separation was performed on a 150-mm X 4.6-mm-i.d. Spherisorb ODs2 column with 5 pm packing. The mobile phase was a mixture of 0.115 M phosphate buffer, pH 2.5, and methanol. A gradient program from 0 to 50% methanol was used. Sample injection was made by the autosampler with a 10-pL injection loop. Method for Determining Diethylene Glycol in PGT. About 250 mg of accurately weighed polymer was placed in a culture tube with a small stirring bar. Four milliliters of internal standard solution (6.25 mg/mL diphenyl ether in DMSO) and 1 mL of 5 N sodium hydroxide in methanol were added to the tube. The tube was loosely capped and placed in the heating module for 10 min at 120 OC. After 10 min, 2 mL of 2 N HCl in DMSO was added to the tube. The tube was removed from the heating module and the contents allowed to cool and settle. A 20-pL sample was silylated with 400 pL of silylating reagent in a 1-mL autosampler vial. The silylating reagent, N,O-bis(trimethy1sily1)trifluoroacetamide(BSTFA)with 1% trimethylchlorosilane, was obtained from Regis Chemical Company. Analysis by gas chromatography was performed on a PerkinElmer Sigma 3B gas chromatograph equipped with a flame ionization detector and modified with a J&W split/splitless capillary conversion kit. A PE Nelson Analytical Model 2600 data system
t 1.0 M KOH/VBut.nol
M KOHIButrnol c 1.O M KOHIN-Propanol M KOHIN-Propanol +1.O M KOW2-Ethoxy Ethanol .+.0.5 M KOHI2-Ethoxy Ethanol ..L.-0.5
-+--0.5
Hwlng Block Rmp.nturq°C
Hydrolysis times for PET in alcohols as a function of hydroxide concentration and temperature. Figure 1.
was used for integration and reporting. The separation was performed on a J&W 30-m X 0.32-mm fused silica capillary column coated with a 0.25-pm f h of DB-5 stationary phase. One microliter was manually injected with a 50:l split ratio. High purity helium was used for the carrier gas. Components were separated isothermally at 100 OC. The gas chromatography/mass spectrometry analyses of the silylation artifacts were performed on a VG 7070-VSEQ mass spectrometer with injector and transfer lines at 275 O C and the source temperature at 200 OC. Accurate mass ammonia chemical ionization data were obtained at a resolution of 5000. Perfluorokerosene was used as the internal standard. The mass accuracy was f10 ppm. Gas chromatography/Fourier transform infrared data were obtained on a Mattson Cryolect instrument. Determination of Composition by NMR. About 100 mg of polymer was placed in a reactor constructed from a Wheaton Micro Kit 10-mLflask and an 80-mm water condenser. A small stirring bar was added to the flask along with 1.6 mL of deuterated DMSO and 0.4 mL of 5 M sodium hydroxide in deuterated methanol. The flask was placed in the heating module at 120 "C and refluxed until the polymer hydrolyzed, usually 5-30 min. After hydrolysis, the flask was cooled and about 4 mL of deuterated water was added to dissolve the salts. (The amount of water may vary depending on the acids used to prepare the polyester.) The resulting solution was added to a 5-mm sample tube with about 1mg of sodium 3-(trimethylsilyl)tetradeuteriopropionate. Proton NMR spectra were acquired on a JEOL Model GX-270 spectrometer operating at 270.05 MHz. RESULTS AND DISCUSSION The P E T polymer used for evaluating hydrolysis solvents represents a typical poly(ethy1ene terephthalate) in terms of molecular weight and crystallinity. The pellet form of this sample provides an unfavorable surface area for chemical attack. Current methods for hydrolyzing this material take from 1to 4 h. For convenience, we will refer to the reaction of base with the polyester to yield the salts of acids and glycols as hydrolysis; however, attack of the polymer ester group may also be made by the alkoxide. By calling these reactions hydrolysis, we do not mean to imply the details of the mechanism are known. Poly(ethy1ene terephthalate) is insoluble in the solvents used in this investigation. The polymer samples disappear by chemically degrading to monomeric components that are more or less soluble in the hydrolysis solution. Hence, the time it takes for pellets to disappear provides a quick and convenient means for comparing reaction rates in various solvents. Figure 1 compares the hydrolysis rates of PET in three alcohols at two concentrations of KOH. Table I shows the hydrolysis rates for some other solvent mixtures. Figure 1 shows that hydroxide concentration and temperature have a major effect on hydrolysis rate and that there are rate dif-
ANALYTICAL CHEMISTRY, VOL. 63,NO. 13, JULY 1, 1991
Table I. Hydrolysis Rate Data for Various Solvents solventa
block tamp, O C
base
time to hydrolyze sample
1-PrOH 100% 120 1MKOH l h 1-PrOH/water 95/5 120 1 MKOH 1.5 h 1-PrOH/water 90/10 120 1 MKOH >2 h 1-PrOH 100% 97 0.5M KOH 31/2 h 1-PrOH/anisole 75/25 97 0.5 M KOH 2h 1-PrOH/anisole 50/50 97 0.5 M KOH 1 l/, h EtOH 100% 100 0.5MKOH >2h 1-PrOH/TBAH 50/50 120 0.5MOH>2 h 50/50 120 0.5 M OH2-ethoxyethanol/ >2 h TBAH l-butanol/TBAH 50/50 120 0.5 M OH>2 h 50/50 120 0.5 MOH>2 h anisole/TBAH DMF/TBAH 50/50 120 0.5MOH>2 h 50/50 120 0.5 MOH7 min DMSO/TBAH DMSO/MeOH 80/20 120 0.5MNaOH 10min "AH = tetrabutylammonium hydroxide, Eastman 25% (1 M) in methanol. DMF = dimethylformamide. DMSO = dimethyl sulfoxide. ferences among solvents. (The curves for the more volatile alcohols flatten at higher block temperature because once the boiling point of the alcohol is reached further increases in block temperature do not increase solvent temperature.) While the effects of temperature and hydroxide concentrations are as expected, the effect of solvent is more complex. The boiling point of the solvent sets an upper limit on reaction temperature and the solubility of base in the reaction mixture sets an upper limit on its concentration. The solvent could enhance reaction rate by swelling the polymer and solubilizingreaction products. But the solvent may have its greatest effect on nucleophile activity. Solvation of the nucleophile affects its size. Therefore, the nucleophile will often have a higher activity in an aprotic solvent. Polar aprotic solvents are also known to enhance the rate of certain reactions, such as hydrolysis, that involve a charged transition state (19-21). In the case of alcohol solvents, the solvent can become the nucleophile by reaction with hydroxide. Among the aliphatic alcohols and hydroxide, methoxide is the optimum compromise between size and electron density (221, so its presence in the hydrolysis reagent could be beneficial. From the above discussion, it would appear that the ideal solvent for hydrolysis would have a high boiling point, would dissolve molar quantities of hydroxide or methoxide, and would be polar and aprotic to enhance the activity of the nucleophile and lower the energy of the transition state. In addition, the solvent should be nontoxic and compatible with liquid and gas chromatographic methods commonly used for analysis of the monomeric reaction products. A mixture of water, KOH, and dimethyl sulfoxide has been used for the rapid hydrolysis of monomeric esters (23).KOH is insoluble in DMSO, so water was added to solubilize the KOH. We found this mixture degraded PET very slowly. One reason was precipitation of reaction products on the polymer, but we suspect water has other deleterious effects in DMSO solvent systems (see below). However, as shown in Table I, a mixture of DMSO and tetraalkylammonium hydroxide hydrolyzed the test polymer very rapidly, and further work was done to optimize a DMSO solvent system. This work is summarized in Table 11. The main problem in developing a DMSO solvent system was finding ways to dissolve an adequate quantity of hydroxide in DMSO while preserving its desirable properties. Neither KOH, NaOH, nor NaOMe is very soluble in DMSO. Physical mixtures of these reagents with DMSO hydrolyzed PET but not at the rate of solutions of tetraalkylammonium hydroxides.
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Table 11. Hydrolysis Rate Data for DMSO Mixtures block temp, "C
solventa
time to hydrolyze sample
base
DMSO 120 1MKOH >2h 120 1 M NaOMe 30 min DMSO 50/50 120 (100) 0.5 M OH10 min (15 min) DMSO/ TBAH DMSO/ 50/50 120 (100) 1.25 M OH5 min (8min) TMAH 80/20 100 1MKOH 9min DMSO/ MeOH 50/50 100 IMKOH 13min DMSO/ EtOH 50/50 100 1MKOH 15min DMSO/ I-PrOH "TBAH = tetrabutylammonium hydroxide, Eastman 25% (-1 M) in MeOH. TMAH = tetramethylammonium hydroxide, Eastman 25% (-2.5 M) in MeOH. Table 111. Effect of Water on the Hydrolysis Time of Poly(ethy1eneterephthalate)" 0% water 1%water 2% water 5% water
hydrolysis time
5 min
6 min
7 min
17 min
'Conditions: 0.25 g of poly(ethy1ene terephthalate) in 1 mL of 5 M sodium hydroxide in methanol and 4 mL of dimethyl sulfoxide. Heat at reflux. Table IV. Hydrolysis Times Using Various Polar Aprotic Solvents' solvent
hydrolysis time, min
7 dimethyl sulfoxide 10 N-methylpyrrolidone 20 pyridine >120 dimethylformamide "Conditions: 0.25 g of poly(ethy1ene terephthalate) in 2 mL of tetramethylammonium hydroxide (25% in methanol) and 3 mL of solvent. Heat at reflux.
Alcohols would solubilize KOH and NaOH in DMSO. Methanol was most effective followed by ethanol and 1propanol. Although these solvents lower the boiling point of the mixture, the mixture will still hydrolyze PET extremely rapidly. A mixture of 80% DMSO, 20% methanol, and 1M NaOH provided the best rates, and it was evaluated for the hydrolysis of other polyesters. A polyester with a sterically hindered ester bond, poly(l,4-cyclohexylenedimethylene terephthalate) modified with 30 mol % 1,4-cyclohexanedicarboxylate,was not hydrolyzed in boiling 1-propanol/KOH after 4 h. But this sample was hydrolyzed by the DMSO reagent in 15 min at reflux. Pellets of highly crystalline poly(ethy1ene naphthalenedicarboxylate) are not hydrolyzed in 1-propanol/KOH in 4 h, but in the DMSO reagent they are hydrolyzed in less than 30 min. As shown in Table 111, small amounts of water have a deleterious effect on the hydrolysis rate of PET. The effect is even more pronounced for difficult to hydrolyze polyesters. Dry DMSO is hydroscopic, and when exposed to the atmosphere for several hours, it can absorb enough water to more than double the reaction time for polyesters difficult to hydrolyze. Other polar aprotic solvents were evaluated, and these results are shown in Table IV. It would seem that the success of this reagent depends on the base, an alcohol, and a polar aprotic solvent. Any strong base that dissolves seems to work
1254
ANALYTICAL CHEMISTRY, VOL. 63,NO. 13. JULY 1, 1991
nma [min) 1 ) %sutfoisophthalic acid
2) tnmsltmc add 3) 8 4) phydroxybenzdc acld + e l l 1,4~ydohexxaned1cerboxylic acid
Flguv 2.
nm).
5)
B 6 ) tram-14-cydohaxansdicerboxyllc acid
+
acetic aad, 2,2'-(1 ,3~phenylene(wIoxv)) bis7) lerephihdic Bad 8) isophthalic aud a) 2,Cnapthalenedlcarboxylicacid
Chromatogram of various polyester diadds (detection at 220
equally well. Of the alcohols and glycols evaluated, methanol seems to work best even though it provides the solution with the lowest boiling point. It is likely that methoxide may be the attacking nucleophile. Methyl ester intermediates have been identified when some polyesters are hydrolyzed by the methanol/DMSO system, which supports this hypothesis. There are considerable differences among the polar aprotic solvents evaluated. For many polyesters, DMSO results in slightly faster rates than N-methylpyrrolidone. These solvents provide much faster rates than the others evaluated. APPLICATION Several methods were developed for the analysis of polyesters that relied on the DMSO reagent to rapidly prepare the sample. These are described below. Method for Acid Components by Liquid Chromatography. The DMSO/methanol reagent reads with polyasters to yield glycols and salts of the acids, which can be readily determined by liquid chromatography. It is essential to completely hydrolyze multicomponent polyesters to obtain an accurate analysis. The hydrolysis rate of the ester bond depends on the substituents. When there is more than one kind of ester in the polyester, the composition of the hydrolysis solution will be biased toward the constituents that are most easily hydrolyzed unless the hydrolysis is carried to completion. While 15 min is sufficient for many polyesters, some may hydrolyze much faster, while those with a more sterically hindered ester may hydrolyze slower. Some oligomers are soluble in the hydrolysis solution, so the time to quantitatively hydrolyze the polymer may be slightly longer than the time for the polymer sample to disappear. To determine the time for complete hydrolysis, a sample can be analyzed for the amount of acids formed as a function of hydrolysis time. Another approach is to hydrolyze the sample by using deuterated DMSO and methanol and to determine by NMR if any ester bonds remain (see below). The acid salta are usually insoluble in the reaction mixture. However, addition of water will solubilize any of the acids commonly used to prepare polyesters. This solution can be acidified to a pH of about 6-7, and the acids will remain in solution. The resulting solution is appropriate for injection onto reversed-phase columns. A 10-pL sample can be injected without peak broadening even when the DMSO concentration is very high. The reversed-phase column suggested is particularly good for the separation of aromatic acids. Figure 2 shows the separation of some typical acids used to prepare polyesters. Even sulfoisophthalic acid is retained. The phosphate buffer provides good separation for the acids investigated, but there is considerable opportunity to optimize the separation for a
more limited set of possibilities. Both pH and ionic strength have a major effect on retention times, and they can be varied to obtain rapid separation of selected acids. This separation has been used to quantitatively determine minor and major component acids in polyester formulations with a typical coefficient of variation of 1-2 % . Method for Diethylene Glycol in PET. Diethylene glycol is formed in small amounts during the manufacture of poly(ethyleneterephthalate). The presence of this compound has a significant effect on the properties of the polymer, so its concentration must be carefully controlled. Methods to determine diethylene glycol must be precise and rapid. Suitable methods have been reported (5);however, advances in the automation of this analysis (18)have resulted in the need for even faster methods of sample preparation. By using traditional methods, PET pellets of average crystallinity are quantitatively hydrolyzed in 60 min. Hydrolysis with the DMSO reagent takes only 10 min, which can more than double the rate at which a control lab can analyze samples for diethylene glycol. Short-term precision of the method is kO.01 at the 2 w t 90level. There was no significant bias between this method and traditional methods. The method can be used for other glycols, but there is a potential problem. With time, new peaks appear in the chromatogram that result from the reaction of DMSO with the silylating reagent (see Figure 3). Substitution of N-methylpyrrolidone for DMSO will eliminate this source of potential interference, but impurities in the N-methylpyrrolidone are themselves a potential source of interference for some glycols. While the silylation artifacts observed in DMSO were not a problem for the determination of diethyleneglycol in PET, they could limit the application of this method for other glycols as well as the use of DMSO as a solvent for the preparation of silylated samples. Therefore, it was desirable to understand the origin of these artifacts with the hope of preventing their formation. Electron impact mass spectra and infrared spectra of these compounds were obtained after separation by gas chromatography. Molecular weights and formulae were determined by accurate mass ammonia chemical ionization data (24). Structures were proposed that are consistent with the mass spectral and infrared data. The proposed structures are probably formed by the following reaction sequence, which is similar to the Pummerer reaction sequence (25,26): OSi(CH&
I
w3c=
0
I1
NSi(CH&
+
CH3SCH3
__*
L
DMSO
BSTFA
J 1
+ 0 II CF+XHSi(CH&
Byproducts from the silylation reaction react with intermediate I to yield silylation artifacts II and 111as shown below: 0 II
m2sa3
I
CF3CNHSi(CH3h
CF3C= NSi(CH3h +
I -
n
+
HOSi(CH3h
O II
m Many other artifacts were formed in lower concentrations by the reaction of intermediate I or silylation artifacts I1 and I11 with various other nucleophiles. These "acetal-like" sily-
ANALYTICAL CHEMISTRY, VOL. 63, NO. 13, JULY 1, 1991 TIC (+RP)
25':W
15:w
1010
Retention mme
Figure 3. Chromatogram of sllylated dimethyl sulfoxide after 2 h of
reaction.
lation artifacts are formed by the general reaction mechanism noted below: &NH RNHCHZSCH, &NCHzNHR CHBSH
+
+
+
The acetal-like artifacts could be formed from methanol instead of DMSO. However, this was ruled out by utilizing deuterium-labeled reagents. Structures proposed from the spectral data for the acetal-like silylation artifacts are shown below in their amide form: 0
II
0 ii
C!F&N?iC!H~N
WSi(c€l~,h
0ccp3
'
m3!d W ' CIE,
a 2 s a 3
lv
V
m3fwsm3 0
0
w
w
However, for compounds 111-VII, the spectral data cannot rule out the possibility that the compounds contain an imino-ether structure or the amide structure. So, for example, the structure of VI could just as well be 0
II CF3C"@Si(CH3),
I
Isomcr of VI
By understanding the mechanisms for the formation of silylation artifacts (27), reaction conditions can often be modified to avoid their formation. The artifacts in this instance are formed from the presence of DMSO and BSTFA. BSA, N,O-bis(trimethylsilyl)acetamide,a weaker silyl donor than BSTFA (281, should retard the formation of intermediate I and, thus, the formation of artifacts. Indeed, all detectable artifacts are avoided by substituting BSA for BSTFA. Preliminary work shows that diethylene glycol and other glycols in hydrolysis mixtures can be successfully derivatized with
1255
BSA. Work in progress suggests BSA can be used instead of BSTFA in methods for the determination of glycols. Analysis of Polyesters by NMR. NMR provides a rapid means of determining the composition of polyesters. The sample is dissolved in a suitable solvent and a spectrum recorded. Integration of the various proton signals enables the molar ratios of the components to be determined. Unfortunately, suitable solvents are not known for many kinds of polyesters. However, it is irrelevant whether the NMR measurement is made on the polymer or its corresponding monomeric components. In fact, for some very high molecular weight polymers or exceptionally rigid ones and polymers prepared from asymmetric monomers, it would be advantageous to make the NMR measurements on representative monomeric samples. Hence, if the polyester can be converted to its monomeric components and they are soluble in a suitable solvent, the polymer composition can be determined by using the same NMR approach used for polymer solutions. A mixture of deuterated DMSO, deuterated methanol, and sodium hydroxide has successfully been used to prepare many kinds of intractable polyesters for analysis by NMR. The salts of the aromatic diacids used are generally insoluble in the DMSO-methanol mixture, but the addition of D20 will solubilize them. The amount of D,O will vary depending on the acids present. Volatile glycols, such as ethylene glycol, are lost if prolonged heating is necessary to react the polymer; however, a short water-cooled condenser will prevent such losses. Blanketing the reaction with argon will prevent decomposition by oxidation. Even hydroquinone can be quantitatively recovered when an argon blanket is used.
CONCLUSIONS A mixture of DMSO, methanol, and a base can be used to prepare polyester samples for a variety of useful analyses. Its advantage over traditional methods is the improved rate a t which it can convert polyesters to their respective acid salts and glycols. Its use in methods that call for the basic hydrolysis of a polyester has resulted in improved productivity of these methods and, in some cases, enables the analysis of polyesters that cannot be analyzed by conventional means. Registry No. DMSO, 67-68-5; i-PrOH, 71-23-8;N+Bu,.DH-, 2052-49-5; MeOH, 67-56-1; EtOH, 64-17-5; NaOH, 1310-73-2; N-methylpyrrolidone, 872-50-4; pyridine, 110-86-1; 1-butanol, 71-36-3; (1,4-cyclohexanedicarboxylicacid)(l,4-~yclohexanedimethanol)(terephthalicacid) (copolymer), 29089-14-3; 2-ethoxyethanol, 110-80-5;poly(ethy1ene naphthalenedicarboxylate) (copolymer), 9020-32-0; poly(ethy1ene naphthalenedicarboxylate) (SRU),9020-73-9;polyethylene terephthalate (SRU),25038-59-9. LITERATURE CITED (1) (2) (3) (4) (5) (6)
Esposito, G. G. Anal. Chem. 1961, 33, 1854. Esposito, G. G. Anal. Chem. 1962, 34, 1173. Wittendorfer, R. E. Anal. Chem. 1964, 3 6 , 930. Hovenkamp, S. 0.; Munting, J. P. J. Polym. Sei. 1970, 8, 879. Vink, D.; van Wijk, R. 2.Anal. Chem. 1973, 264, 293. Nissen, D.; Rossbach, V.; Zahn, H. J . Appl. Polym. Scl. 1974, 18, 1 .*" Q W
(7) Esposko, G. G.; Swann, M. H. Anal. Chem. 1962, 3 4 , 1048. (8) Percival. D. F. Anal. Chem. 1963, 35, 236. (9) Janssen, R.; Ruysschaert, H.; Vroom R. Mekromol. Chem. 1964, 77, 153. (10) Jankowski, S. J.; Garner, P. Anal. Chem. 1965. 37, 1709. (11) Rawlinson, J.; Deeley, E. L. J. OIlCd. Chem. Assoc. 1967, 50, 373. (12) Perlstein, P.; Orme, P. J. Chromatogr. 1986, 351, 203. (13) Kirby, J. R.; Baldwin, A. J.; Heidner, R. H. Anal. Chem. 1966, 3 7 , 1308. (14) West, J. C. Anal. Chem. 1975, 47, 1708. (15) Ponder, L. H. Anal. Chem. 1966, 40, 229. (16) Allen. B. J.; Eisea, 0.M.; Keiler, K. P.; Kinder, H. D. Anal. C h m . 1977, 49, 741. (17) Haken. J. K.; Rohanna. M. A. J . Chrometogr. 1964, 298, 263. (18) Oestreich, 0. J. J . Chrometogr. Sc/. 1967, 2 5 , 214. (19) Parker, A. J. J . Chem. Soc. 1961, 1328. (20) Roberts, D. D. J . Org. Chem. 1964, 2 9 , 2039. (21) Roberts, D. D. J . &g. Chem. 1964, 2 9 , 2714. (22) Isaacs. N. S. phvslcal Organic Chemlstry; John Wlley: New York, 1987; 485ff. (23) Vinson, J. A.; Fritz, J. S.; Kingsbury, C. A. Talents 1966, 13, 1673.
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Anal. Chsm. 1991, 03, 1256-1261
(24) Haddon, W. F.; et ai. A General Method for Accurate Mass Measurement of Ammonia CA, Spectra Udng PFK Internal Reference. Roof the 36U1Annual c O n t ” e on Mass Spscbwneby and A M Topics; San Francleco, CA, 1988 pp 1396-1397. (25) Oar, S.; et al. T o t ” lBbS, IS, 817-820. (28) The kkKck I d x , 8th d.;Stecher. P. G., Ed.; Merck 6. Co.,Inc.: 1968; p 1204. (27) ~ m kJ., L. hmspectralI h - h of Artwe* Formed in Simtlon Reactions. Procwdhgs of Ihe 34th Annual Conference on Mess
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RECIEVED for review November 20,1990. Accepted April 2, 1991.
Pattern Recognition of Jet Fuel Chromatographic Data by Artificial Neural Networks with Back-Propagation of Error James R. Long,*Howard T. Mayfield, and Michael V. Henley Air Force and Engineering Services Center, Tyndall AFB, Florida 32403-6001 Paul R. Kromann Department of Chemistry, Fort Valley State College, Fort Valley, Georgia 31030
The purpose of this article b to demonstrate the appllcatlon of artllclal neural networks as a pattern recognttlon tool for chromatographk data. MultUayef fedforward netwoks mhg back-propagatlon and the generaked detta rule were slmulated on a mlcrocomputer. Network parameters and archltectures were optlmlred to glve maxlmum network classllcatlon performance. The chromatographlc data for seven different c l a w s of jet fuels were collected by GC and GC/ MS. Blnary patterns were used to represenl various classes. The technique was tested and compared to Knearest neighbor, KNN, and soft Independent modellng of class analogy, SIMCA.
INTRODUCTION Artificial neural networks have been the focus of a tremendous amount of interest with a great deal of the attention having been centered around speech and image recognition (1-3). A general approach to this type of pattern recognition has been to represent symbolic information as binary patterns that are used as input and output for the neural networks. An overview of neural networks and its applications has been presented by Lippmann (4). Within the field of chemistry, various applications have already been published. Thomsen and Meyer demonstrated the use of neural networks as an NMR spectral classifier (5). Proton NMR spectra from six different sugar alditols were used to train a neural network to recognize which class of alditol a specific spectrum represented. The paper went on to demonstrate the fault tolerance capabilities of neural networks by adding various distortions to the spectra and then attempting to reclassify the spectra with a previously trained neural network. Long et al. demonstrated the use of neural networks as a nonlinear spectral calibration tool (6). Near-IR and UV spectra were calibrated and used to quantitate multicomponent mixtures, with the results being compared to principal component regression. The purpose of this paper is to demonstrate the use of artificial neural networks as a tool for the classification of chromatographic data. By using binary patterns to represent
* Corresponding author. 0003-2700/91/0363-1256$02.50/0
various classes of jet fuel, neural networks are trained to associate the patterns with their respective chromatographic profile. The results of the neural network classifications are benchmarked against the results from classification by K nearest neighbor, KNN, and soft independent modeling of class analogy, SIMCA.
THEORY Artificial neural networks consist of multiple layers of highly interconnected and massively parallel processing elements known as nodes (Figure 1). These nodes are patterned after biological neurons, henceforth the name artificial neural network. Each node acts as a summing point for either external signals or weighted outputs passed on by nodes in a preceding layer. The sum of the weighted inputs is passed through a transfer function that may be linear or nonlinear. The traditional nonlinear transfer function is the sigmoid function that has a range of 0-1. The sigmoidal output of node j , oj, which is connected to Z nodes in the previous layer, is given by
with
=
cwijoi + e, 1
i=l
where is a bias term, oiis the output from the ith node of the previous layer, and wij represents the weight, or synaptic strength, between node i and node j . The value of the bias term, 0,: is equal to 1.0 multiplied by a weight. The bias is responsible for accommodating non-zero offsets in the data. In eq 1,y is known as the gain. By adjusting this parameter, one can alter the slope of the sigmoid transfer function. Feedforward networks consisting of two and three layers of nodes were constructed (Figure 2). The f i t layer, or input layer, consisted of nodes that functioned as an input buffer for the data. Signals introduced to the network, with one node per element in the sample data vector, passed through this layer of nodes, were weighted, and continued on to each node in the following layer. The following layer, or hidden layer, receives the weighted outputs from the previous layer of nodes. The hidden layer then sums the inputs at each node and passes the sum through a transfer function. The output of 0 1991 American Chemical Society
