Gel permeation chromatography of low molecular weight materials

Krishen, and Ralph G. Tucker. Anal. ... High-Performance Gel Permeation Chromatography Characterization of Oligomers Used in Coatings Systems .... Ann...
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Gel Permeation Chromatography of Low Molecular Weight Materials with High Efficiency Columns Anoop Krlshen” and Ralph G. Tucker The Goodyear Tire & Rubber Company, Research Division, Akron, Ohio 44316

Recent improvements In the efflclency of small pore packing materials and column preparation have advanced the speed and convenlence of gel permeation chromatography to that of gas chromatography and hlgh speed ilquid chromatography. Lack of volatility or the absence of slgnlflcant differences in polarity, solubility, or ionic characteristics, do not pose problems in this technique. A single column, 610 mm X 8 mm, showed a theoretical plate count of 16000. Uslng tetrahydrofuran as the eluant at a flow rate of 0.5 mL/mln, separations in the molecular weight range of 100-2000 were achleved in less than 30 min. A difference of one carbon atom was sufficient for satisfactory resolution of components in the lower molecular weight range. This technlque was operated wlth a dual detection system-differential refractlve index and UV absorptlon at 254 n m - t o provlde additional information. I t was applied to low molecular weight entitles encountered In the analysis of plastlclrers, antioxidants, various condensatlon products, and other ollgomerlc species.

The basic goal of chromatographic techniques is to achieve an appropriate separation and identification of components in a reasonably short time. Gas chromatography and high pressure liquid chromatrography have both been eminently successful in this endeavor, and this has resulted in their utility for a wide variety of organic materials. Each of these techniques has some fundamental limitations-thus volatility is essential for gas chromatography while sufficient differences in solubility, polarity, adsorption, or ionic characteristics are required for application of high pressure liquid chromatography. Another chromatographic technique, gel permeation chromatography, was developed by J. C. Moore (1). It has found extensive use in determining the molecular weight distribution of high molecular weight polymers. The great volume of literature on this application has been adequately covered in a number of review articles (2-4). Gel permeation chromatography is essentially a form of liquid chromatography in which solute molecules are selectively retarded as a result of their permeation into solvent filled pores in the column packing. Larger molecules, excluded from all or a portion of the pores by virtue of their physical size, elute from the column before the smaller molecules, thereby providing a separation based on the molecular size in solution. Lack of volatility or absence of differences in solubility, polarity, adsorption, or ionic characteristics do not constitute limitations for application of this technique. In view of these advantages, various studies have been made to extend this technique to lower molecular weight compounds. It has been used for the separation of surfactants ( 5 ) ,triglycerides (6), tall oil components (7), fatty acids (8), polyethylene glycol ( 9 ) ,and various other materials (10-14). In most of these cases the columns used were of considerable length and retention times were usually very long. Recent developments have resulted in the preparation of gels with small pore and particle sizes which permit high 898

ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977

resolution (1516). Specially packed columns using these gels have been shown to provide good resolution in a very short time (17). The chromatograms obtained show well defined distinct peaks for individual components as in gas chromatography or high performance liquid chromatography. The basic separation is, no doubt, based on the size of the molecule in solution, and this factor has been used in formulation of the various theories for interpretation of the data. The pioneering work of Hendrickson and Moore (18) correlated the chain length and carbon number with the retention volume. They also formulated rules to establish the effective carbon number for different functional groups and molecular structures. Later Lambert (19, 20) elucidated the molar volume concept for interpretation of GPC data. The simpler concept of utilizing the molecular weights has been in use all along as well. Although the molar volume is utilized widely for GPC work, the type of calibration curve is dictated by the end use or application. As in gas chromatography, straight lines may be obtained when the retention volume is plotted vs. the logarithm of the molecular weights or molar volumes. Components of a series of oligomers can be expected to behave in a similar manner. The utility of any technique depends on its ability to predict the behavior of an unknown when some easily obtainable information is provided. When individual peaks are obtained from a product of an addition reaction, it is much simpler to calculate their expected molecular weights than the effective carbon numbers or molar volumes. This information can then be used in combination with retention volumes determined experimentally, to identify the individual components based on the straight line relationships. A single column, 610 mm X 8 mm, was used for the present study and a number of compounds were examined to establish the elution volume and molecular weight relationships for small molecules. These data were then used to calculate the size factors for different types of molecular structures and functional groups.

EXPERIMENTAL Apparatus. Liquid Pump. An ISCO Model 314 (Instrument SpecialitiesCompany, Lincoln, Neb. 68505) pulseless flow syringe pump was used to provide a constant flow rate of 0.5 mL/min. Column. A stainless-steelcolumn 610 mm x 8 mm i.d. packed with TSK-Gel G2000H8 was provided by Toyo Soda Manufacturing Company, LM., Toso Building 1-7-7Akasaka, Minato-ku, Tokyo 107, Japan. The polystyrene-divinyl benzene gel has a nominal porosity of 100 A according t o the manufacturer’s literature. Differential Refractive Index Detector. The Fresnel type refractive index detector, Model No. 00-430003-00 was obtained from Varian Associates, Instrument Division, Palo Alto, Calif. 94303. The detector was thermostated at 32.9 “C by circulating water from a Temptrol Model 250 (GCA/Precision Scientific, Chicago, Ill. 60647) constant temperature circulating bath. Ultrauiolet Detector. A dual-beam UV detector Model UA-5 (Instrument Specialities Company, Lincoln, Neb. 68505) was operated at 254 nm with 10-mm path length high pressure microcells (Catalog No. 0090 U10, ISCO).

PRESSURE GAUGE

Table 11. Retention Volumes, Molar Volumes, and Size Factors for n-Alkanes

0 REFILL

Calcd molar Reten- vol, tion mL/mol Size vol, mL 20 “ C factor 18.95 49.2 1.00 18.95 65.4 1.00 18.20 81.6 1.00 17.70 97.8 1.00 17.33 114.0 1.00 16.90 130.2 1.00 16.48 146.5 1.00 16.18 162.8 1.00 15.80 179.0 1.00 15.20 211.5 1.00 14.95 227.8 1.00 14.30 276.7 1.00 13.85 325.7 1.00 13.10 423.8 1.00 12.03 621.0 1.00

Mol

Component Methane Ethane n-Propane n-Butane n-Pentane n-Hexane n-Heptane n-Octane n-Nonane n-Undecane n-Dodecane n-Pentadecane n-Octadecane n-Tetracosane n-Hexatriacontane

SOLVENT RESERVOIR

Figure 1.

Block diagram of the components

Table I. Specific Resolution Mol wt, (MI i- M , )/2 80 140 300 47 5 900 1500

Specific resolution 7.3 13.4 16.3 17.4 4.8 2.6

Recorder. The signals from the two detectors were recorded on a dual-channel Omniscribe recorder (Houston Instrument, Bellaire, Texas 77401). Injection Port. The injection port block (Catalog No. 820069) was obtained from DuPont Instruments, Wilmington, Del. 19898). Syringes. Samples were injected using 10 or 25-fiL syringes (Hamilton Company, Reno, Nev. 89502). Assembly of Apparatus. The block diagram (Figure 1)shows the assembly of apparatus using a single pump to provide both the reference and analytical flows at constant volume without stream splitting. The refractive index detector was operated with a static reference cell. Reagents. Chemicals used as standards were obtained from commercial laboratory supply houses. The condensation products and other oligomeric materials were provided by courtesy of the Organic Chemicals Department, Research Division, The Goodyear Tire & Rubber Company. Procedure. A sample of 300-350 mg was dissolved in 5 mL of tetrahydrofuran. This solution was injected wiith a 10-fiL syringe. A sample injection volume was selected between 1and 5 fiL to obtain suitable responses from the detectors. R E S U L T S AND DISCUSSION Column Characteristics. The exclusion and pore volumes were 8.76 mL and 18.95 mL, respectively as determined from the retention volumes for a polystyrene sample of 97200 molecular weight and methane. The capacity factor was calculated from these values to be 2.2. The efficiency of the column was examined throughout its operating range by injecting various known compounds. These values were converted to specific resolution, R,, as defined by Bly (21).

2(V2 - VI) + Wz)(log MI - log Mz) where V , W , and M represent the retention volumes, peak

R, =

(Wl

widths, and molecular weights of two components. The data are given in Table I. The peak widths for most of the compounds were about 0.5 mL. The theoretical plates for the column were calculated to be 16 520 from the acetone peak. The column was calibrated with n-alkanes and the data are given in Table 11. The molar volumes for the hydrocarbons

0

5

wt, log,,

1.205 1.478 1.644 1.764 1.858 1.935 2.001 2.058 2.108 2.194 2.231 2.327 2.406 2.529 2.705

I

I

I

10

15

20

RETENTION VOLUME ( M L ~

Gel permeation chromatogram of *alkanes. 610 mm X 8 mm TSK G2000 H 8 column. Tetrahydrofuran eluant at 0.5 mL/min. Differential refractive index detector. (1) I F C ~ ~ H(2)~ *CZ4H5,,. ~. (3) nC18H38. (4) nC12H26. (5) &&is. (6) (7) *HgHi+ (8)n C g H i 2 Figure 2.

shown in the table were calculated by the following equation (19)

Molar Volume ( m L / m o l at 20 “C) = 33.02 + 16.18 (CAU) + 16.58 (CAU)’ where CAU is size in carbon atom units. The values for the molar volumes for the various compounds were obtained from the calibration curve for n-alkanes prepared from these data. The precision for the retention volume values was determined to be 0.3% by replicate injections of the same sample. n-Alkanes. The high efficiency of the column was demonstrated by the separation of a mixture of n-alkanes from pentane to hexatriacontane (Figure 2). The relationship of retention volume and the logarithm of the molecular weights, as shown by plot I in Figure 3, is a straight line throughout this range. It suggests the absence of adsorption on the gel, and solvent-solute or solute-solute molecular associations. The n-alkanes were chosen as reference standards and arbitrarily assigned a “size factor” of 1.00 (Table 11). The size factor for other molecules which do not fall on the same line can then be easily obtained from the plot for the n-alkanes in Figure 3. The size factor simply represents a multiplier ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977

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3.5

1

10

Table IV. Retention Volumes, Molar Volumes, and Size Factors for Alcohols and Polyols Molar Reten- vol, Mol wt, tion mL/- Size Component log,, vol, mL mol factor Methanol 1.506 17.95 91 1.59 Ethanol 1.663 17.33 112 1.56 n-Butanol 1.940 16.43 147 1.18 Ethylene glycol 1.793 16.15 169 1.85 Diethylene glycol 2.026 15.68 185 1.26 Glycerol 1.964 15.50 195 1.73 n-Octanol 2.115 15.35 207 1.15 Triethylene glycol 2.177 15.17 218 1.05 n-Dodecanol 2.260 14.45 268 1.12 n-Tetradecanol 2.331 13.98 313 1.13 n-Hexadecanol 2.385 13.70 345 1.10

11

12

13

14

15

16

I

17

I

18

I

19

RETENTION VOLUME JML)

Figure 3. Molecular weight vs. retention volume relationships. (I) &Alkanes, C,HI2 to C38H74. (11) Toluene, p-xylene, diethyl phthalate. (111) n-Alcohols, C4H90H to C16H330H. (IV) 2,6-Di-te~-butyl-p-cresol, dibutyl, adlpate, ndidecyl phthalate, n-didodecyl phthalate. (V) Nonylphenol, formaldehyde adducts. (VI) TMDQ oligomers, dimer to hexamer

Table 111. Retention Volumes, Molar Volumes, and Size Factors for Aromatic and Cyclic Hydrocarbons Reten- Molar tion v01, Size Mol wt, vol, mL/- facmol mL tor Component log,, Benzene 1.893 18.80 68 0.44 78 0.41 To1u ene 1.965 18.50 89 0.47 2.026 18.00 p-Xylene 91 0.43 2.073 17.95 or-Methylstyrene 9 3 0.41 1,2,3,5-Tetramethyl- 2.128 17.85 benzene 2.121 17.82 94 0.41 Dicyclopentadiene 2.297 17.33 114 0.36 Cyclopentadiene trimer Hexylbenzene 2.200 15.78 180 0.76 or-Methylstyrene 2.347 15.33 207 0.68 dimer or-Methylstyrene 2.550 14.33 277 0.60 trimer

'

for the molecular weight, required to bring other molecules into conformity with the straight line for n-alkanes. It can be defined as F = AIM; where F is the size factor, M is the molecular weight of the compound, and A is the molecular weight of a real or hypothetical n-alkane that would elute at the same retention volume as the compound. Numerically it is the antilogarithm of the vertical distance of the experimentally obtained point from the reference line for the n-alkanes. The lack of availability of pure n-alkanes of higher molecular weight necessitated the extrapolation of the curve as shown by the dotted line in plot I in Figure 3. The molar volume data also are given in Table I1 for correlation with data from other sources if needed. Aromatic a n d Cyclic Hydrocarbons. Changes in the size of the molecule may be expected when aromatic and cyclic molecules are compared to n-alkanes of similar molecular weight. All of the aromatic and cyclic compounds shown in Table I11 exhibited a decrease in their effective size and were 900

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Table V. Retention Volumes, Molar Volumes, and Size Factors for Esters Reten- Molar vol, tion Mol wt, vol, mL/- Size mol factor mL Component log,, Diethyl phthalate Dibutyl maleate Dibutyl phthalate Dibutyl adipate Methyl stearate Di-(2-ethylhexy1)phthalate Didecyl phthalate Didodecyl phthalate Tristearin

2.347 2.359 2.445 2.412 2.475 2.582

15.83 15.23 14.80 14.13 13.45 13.25

177 214 236 298 373 402

0.57 0.68 0.65 0.89 0.98 0.83

2.650 2.701 2.950

12.65 12.30 11.00

500 568 1127

0.89 0.91 1.02

retained longer on the column than n-alkanes (Plot I1 in Figure 3). The effect of the aromatic ring was decreased with larger side chains as is evident in the case of hexylbenzene. Alcohols a n d Polyols. n-Alcohols from butanol to hexadecanol (Table IV) all exhibited a larger size (1.12) as compared to n-alkanes and thus formed a straight line parallel to those for the alkanes (Plot I11 in Figure 3). The effect of the association of the hydroxyl groups with the solvent was quite pronounced in ethylene glycol but it became relatively less pronounced as the chain length was increased in di- and triethylene glycols. Esters. Significant variations in the retention behavior of phthalate esters were observed (Table V) from diethyl to didodecyl. The increasing chain length of the ester increased the size factor and the larger esters eluted closer to the nalkanes (Plot IV in Figure 3). A similar effect was evident when comparing dibutyl esters of maleic and adipic acids. The long chain in methyl stearate made it behave identically to an n-alkane of similar molecular weight. Phenols a n d Amines. The retention volumes obtained for a few phenols and amines are given in Table VI. Marked differences were observed when comparing quinone to hydroquinone, octyl quinone to octyl hydroquinone, and dioctyl quinone to dioctyl hydroquinone. In each case the hydroquinone eluted before the corresponding quinone, probably because of association of the hydroxyl groups with tetrahydrofuran. Although the separation of octyl quinone from hydroquinone and dioctyl quinone from octyl hydroquinone was not adequate, it was possible to distinguish the quinones from the hydroquinones because of their enhanced UV response at 254 nm. As shown in Table VII, the hydroquinones had a relatively low UV response while the quinones had much higher UV responses. Comparison of UV and RI responses for an unknown peak, say at 14.15 mL can thus help to establish its

Table VI. Retention Volumes, Molar Volumes, and Size Factors for Phenols and Amines Component

Mol wt, log,, 2.034 2.168 1.969 1.974 2.338 2.341 2.042

Quinone Dicyclopentadienol Aniline Phenol Octyl Quinone N-Phenyl-2-naphthylamine Hydroquinone 4-tert-Butyl-l,2-pyrocatechol N,N'-Diphenyl-p-phenylenediamine Dioctyl quinone Octyl hydroquinone Dinonyl phenol 2,2'-Methylene-bis(6-tert-butyl-4-methyl phenol) Dioctyl hydroquinone

Table VII. Comparison of UV and RI Responses for Quinones and Hydroquinones Elution UV/RI Component v01, mL response Quinone 17.38 3.75 Octyl quinone 15.38 3.20 Hydroquinone 15.23 0.05 Dioctyl quinone 14.18 2.62 Octyl hydroquinone 14.10 0.05 Dioctvl hvdroauinone 13.15 0.05 Table VIII. Retention Volumes, Molar Volumes, and Size Factors for Nonylphenol-Formaldehyde Condensation Products Reten- Molar Mol tion vol, wt, vol, mL/Component log,, mL mol Size factor p-Nonylphenol 2:l Nonylphenol:CH, 3:2 Nonylphenol:CH, 4:3 Nonylphenol:CH, 5:4 Nonylphenol:CH, 6:5 Nonylphenol:CH, 7:6 Nonylphenol:CH,

2.343 2.656 2.836 2.973 3.060 3.141 3.208

14.56 258 13.18 409 12.35 560 11.80 711 11.40 887 11.10 1057 10.90 1180

0.90 0.72 0.66 0.60 0.61 0.61 0.61

Table IX. Retention Volumes, Molar Volumes, and Size Factors for 2,2,4-Trimethyl-1,2-dihydroquinoline (TMDQ) Oligomers Reten- Molar tion vol, Molecular vol, mL/Size Component wt log,, mL mol factor TMDQ 2.239 15.63 190 0.79 TMDQ dimer 2.540 14.23 288 0.64 TMDQ trimer 2.716 13.35 387 0.58 TMDQ tetramer 2.840 12.80 518 0.54 TMDQ pentamer 2.938 12.25 578 0.54 TMDQ hexamer 3.016 11.90 673 0.52

2.211

2.416 2.523 2.347 2.540 2.532 2.526

Retention vol, mL 17.38 16.55 16.46 16.33 15.38 15.30 15.23 14.50 14.20 14.18 14.10 13.59 13.50 13.15

Molar vol, mL/mol 110

115 147 153 203 208 214 263 290 291 301 357 370 41 5

Size factor 0.64 0.66 1.09 1.14

0.68 0.69 1.41 1.23 0.86 0.68 1.04 0.81 0.85 0.97

identity-dioctyl quinone or octyl hydroquinone or a mixture of the two. The use of dual detectors-UV and differential refractive index-makes this possible.

Nonylphenol-Formaldehyde Condensation Products. Condensation products derived from a phenol and formaldehyde provide a series of compounds with increasing molecular weights. The major components result from successive additions of phenols with methylene bridges. The products derived from nonylphenol and formaldehyde were separated on the gel column and the retention volumes for the individual peaks obtained were plotted vs. the logarithm of the expected molecular weights (Plot V in Figure 3). The separation factors (Table VIII) and the position of the plot relative to n-alkanes are in conformity with the general behavior of aromatic compounds and indirectly substantiate the molecular weight assignments made for the various components.

2,2,4-Trimethyl-l,2-dihydroquinoline(TMDQ) Oligomers. TMDQ is derived from the condensation of aniline and acetone and then polymerized to produce species of increasing complexity. Even the dimer is not volatile enough to be gas chromatographed conveniently (22). Examination of different samples of polymerized TMDQ by gel permeation chromatography indicated the presence of a number of peaks. The major components were assigned the expected molecular weights for the polymerization reaction (Table IX). The plot of the retention volumes for the individual peaks vs. the logarithm of the molecular weights resulted in a line similar in slope t o that for n-alkanes (Plot VI in Figure 3). Miscellaneous Compounds. The retention volumes and the size factors for a number of accelerators and other materials are given in Table X. Ureas and maleic anhydride exhibit an enhanced size effect. The gel permeation technique was also applied to analysis of various plasticizers. Although gas chromatography is directly applicable to lower molecular weight materials, the higher molecular weight compounds require hydrolysis and esterification for analysis (23). A mixture of phthalate esters along with dibutyl adipate and epoxidized soybean oil was separated by GPC (Figure 4). The use of dual detectors readily detected the presence of UV-

Table X. Retention Volumes, Molar Volumes, and Size Factors for Miscellaneous Compounds Molecular Retention Molar vol, Component wt, log,, vol, mL mL/mol N-bis(Dioxyethy1ene)thiobenzothiazole sulfenamide 2.454 16.68 138 Urea 1.779 16.55 142 Maleic anhydride 1.992 16.29 155 2-Mercaptobenzothiazole 2.224 16.03 166 tert-Butyl urea 2.065 15.33 207 Di-tert-butyl urea 2.236 14.58 257 Tris-(Nonylpheny1)phosphite 2.838 11.84 705

Size factor 0.33 1.63 1.11

0.72 1.30 1.13 0.81

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elution of all components is fixed; thus there is no loss of time waiting for elution of unknown components unless adsorption becomes a problem for some highly polar compounds. Limitations. Solubility of the sample in the carrier solvent is essential for analysis by this technique. Size differences among isomers may not be sufficient to provide adequate separations. Any reactive compounds which may chemically attack the column packing should not be injected onto the column. The efficiency of the column may be impaired if the solvent is changed. This difficulty can be avoided by obtaining an appropriate column packed with the desired solvent.

CONCLUSIONS 1

The high efficiency of the GPC column affords a separation of components as distinct separate peaks, in a short time, and provides a very useful technique for extending the molecular weight range beyond that covered by gas chromatography. The analysis time remains constant for all analyses and usually all components are eluted from the column.

ACKNOWLEDGMENT The authors offer their appreciation to Toyo Soda Manufacturing Company for providing the GPC column. The permission of The Goodyear Tire & Rubber Company to publish is also gratefully acknowledged. RETENTiON VOLUME JMLI

Flgure 4. Gel permeation chromatogram of plasticizers. 610 mm X 8 mm TSK G2000 H8 column. Tetrahydrofuran eluant at 0.5 mL/min. Differential refractive index and ultraviolet (254 nm) detectors. (1) Epoxidized soya bean oil. (2) Ddodecyl phthalate. (3) Didecyl phthalate. (4) Di-(2-ethylhexyl)phthalate. (5) Dibutyl adipate

absorbing phthalate esters. Identification of the polymeric ester type of plasticizers was greatly facilitated by the simultaneous estimation of their approximate molecular weights by GPC. Column Life. The gel permeation column used in this investigation has been in operation for 20 months and has not shown any sign of deterioration or loss in efficiency. Advantages of GPC. The range of molecular weights covered by the GPC column overlaps that of normal gas chromatography in the lower range. This provides a convenient means of detecting the presence of any heavier components which do not elute from the column in gas chromatography. In addition to providing the means for separating individual components, GPC furnishes information regarding the approximate molecular weight of all the components. The simultaneous use of both a differential refractive index and an ultraviolet detector, affords additional information about the chemical type. The total time for

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LITERATURE CITED J. C. Moore, J . Polym. Sci., Part A , 2, 835 (1964). C. W. Wadelin and G. S. Trick, Anal. Chem., 43, 334R (1971). C. W. Wadelin and M. C. Morrls, Anal. Chem., 45, 333R (1973). C. W. Wadeiin and M. C . Morris, Anal. Chem., 47, 327R (1975). K. J. Bombaugh, W. A. Datk, and R. F. Levangie, Sep. Sci., 3,375 (1968). J. L. Muider and F. A. Buytenhuys, J . Chromatogr., 51, 459 (1970). T. L. Chang, Anal. Chem., 40, 969 (1968). (8)T. L. Chang, Anal. Chlm. Acta, 42, 51 (1968). (9) W. Heitz, 8.Bomer, and H. Ullner, Mekromol. Chem., 121, 102, (1969). (10) J. C. Hendrickson, Anal. Chem., 40, 49 (1968). (11) F. Spagnolo and W. M. Maione, J . Chromatogr. Sci., 14, 52 (1976). (12) N. D. Kornbau and D. C. Ziegier, Anal. Chem., 42, 1290 (1970). (13) V. F. Gaylor, H. L. James, and H. H. Weetal, AM/. Chem., 46,44R, (1976). (14) R . L. Bartusiewicz, J. Paint Techno/., 39, 507 (1967). (15) . . Y. Kato. S. Kid0 and T. Hashimoto. J . Polvm. Sci.. folvm. Phvs. Ed.. 11, 2329 (1973). (16) Y. Kato, S. Kldo, M.Yamamoto, and T. Hashimoto, J . Poiym., Scl., foiym. Phys. Ed., 12, 1339 (1974). (17) H. Hatano, Res.lDev., 51, 28 (1973). (18) J. G. Hendrickson and J. C. Moore, J . Polym. Sci., Part A- 1 , 6, 167 (1966). (19) A. Lambert, J . Appl. Chem., 20, 305 (1970). (20) A. Lamben, Anal. Chim. Acta, 53, 83 (1971). (21) D. D. Bly, J . Polym. Sci., Part C ,21, 13 (1966). (22) J. P. Brown and B. K. Tidd, J . Chem. SOC. C , 1075 (1968). (23) A. Krishen, Anal. Chem., 43, 1130 (1971). (1) (2) (3) (4) (5) (6) (7)

RECEIVED for review July 28,1976. Accepted March 14,1977. Presented, in part, at the 172nd National Meeting, American Chemical Society, San Francisco, Calif., August 31, 1976.