Resin Acids Gas Chromatography of Their Methyl Esters J. A. HUDY Research Center, Hercules Powder Co., Wilmingfon, Del. ,The high temperature gas-liquid partition chromatography of resin acid methyl esters has been studied on packed columns containing various polyesters and Apiezon grease as stationary phases. Relative retention data are presented for unsaturated and saturated abietic- and pimarictype acids. In general, separations based on the expected selectivity for unsaturation are shown, although steric differences and relative volatility of the resin acids preclude any clear correlations between structure and retention volume. Although partial isomerization of methyl levopimarate and methyl palustrate occurs, quantitative analysis of the thermally stable acids may be obtained.
A
LTHOUGH gas chromatography has
been applied extensively to the analj.sis of fatty acids by various workers (a,8,9 ) , nothing has apl)carrd in the 1iter:iture concerning its application to the resin acids. Gengc ( 4 ) describes the anal>sis of resin acids by mass spectroni-
etry of their methyl esters. Supplementing this work, gas chromatography has been applied to these same methyl esters with successful results. Resin acids are monocarboxylic acids of alkylated hydrophenanthrene structure occurring in nature as the major components in rosin. They are generally classified into two types, abietic and piniaric. The structures of the various acids have been illustrated and discussed by Harris and Sanderson (6, 7 ) and other workers (3, 5, 10, 11). Accurate analysis for the individual species in mixtures has been hindered largely by the similarity of the various isomers, and the lack of pure standards as well as the ease of isomerization and oxidation of many of the conjugated diene acids. I n addition, the piniarictype acids and the saturated acids have no absorption in the ultraviolet region. I n conjunction with the techniques of heated inlet mass spectronietrj., gas chromatography appeared to be n promising new approach to the scp:iration and mialysis of these mixtures.
APPARATUS A N D PROCEDURE
The apparatus used a conventional hot-wire thermal conductivity cell, consisting of a Gow-Mac TR-2-B stainless steel cell separately thermostated a t 260" C. Current to the filament x a s supplied by a 12-volt storage battery. The heating bath for the column consisted of a Glass-Col 31 640 mantle. The temperature was controlled to within 1" C. by a variable transformer and input yoltage was stabiliwd by a constant voltage transformer. The heating bath n-as enclosrd by a 10-inch Transitr disk drillcd to accept the tubing leads from the thcrmal conductivity cell. The sample vaporizcr and exit tubing were hrated to 300" and 260" C., resprctively, b!. 75-watt hrating cartridgcs and an aluminum 111ock. Column packing was prcparctl in the conventional niannrr using untreated 60 100-mesh Celitc (.Johns-\Ianvillr Chroniosorh C48560). Thc stationary phase \\'as applied a t a conccntration of 28.4% hy w i g h t . The columns w r e 3.7-mctcr spirals of coppor tulling 4 mm. in intrrnal diamctw. Tht, packing tl(msit!. was 5.45 grams pcr nictcr of Apiczon N packing and 6.2 to 6.9 grams pvr mctcr of poljc&)r packing. In the caw of the polyester columns. I
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Figure 2. Gas chromotograms of resin acid methyl esters on Resoflex 446 column
Figure 1 . Gas chromatograms of resin acid methyl esters on Apiezon N column
* Derived 1754
from isomerization or disproportionation
ANALYTICAL CHEMISTRY
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conditioning of the column priur to use at 225' C. was required. Samples were delivered t o the heated inlet system by a Hamilton 701 N syringe through a silicone rubber seal. The sample volume was adjusted so that 100 to 200 Y of the resin acid ester was delivered. The following stationary phases were studied : Apiezon N Grease, Metropolitan-Vickers Electrical Co., Ltd. Reoplex 400, poly(oxyalky1ene adipate), Geigy Chemical Co. Resoflex 446, poly(diethy1ene glycolYntaerythritol adipate), Cambridge ndustries. Craig Polyester, poly( 1,4-butylene succinate), prepared by Wilkens Instrument and Research, Inc., Walnut Creek, Calif. The following are the preferred operating parameters for the analysis. Column Type ____ Apiezon N Polyester Cell teFperature, C. 260 260 Cell filament current, ma. 150-200 150-200 Carrier gas Helium Helium Inlet pressure, p.s.1.g. 20 24 Exit pressure Atmospheric Atmospheric Flow rate at ex.it, ml./ 100 100 min . Column length, meters 3.66 3.66 Column temperature, c. 270 225 Retention volume of methyl palmitate internal standard, ml . 653 6450 Approximate, varied depending on type and condition of column. O
Standard Sample Preparation. The methods of preparation and the compounds used as standards were identical t o those described by Genge (4). In general, the primary resin acids were separated from various rosin sources as the amine salts and regenerated according to the procedures of Harris and Sanderson ( 7 ) . Palustric acid and A8a,8-isopimaric acid were obtained from R. V. Lawrence, United States Department of Agriculture, Olustee, F!a. The A4b,8a-isopimaric acid was obtaiLiedfrom 0. E. Edwards, National Research Council, Canada. Hydrogenation of enriched resin acid fractions using platinum and palladium catalysts was used to obtain the partially saturated and saturated acids, respectively. The acids were esterified with diazomethane according to the procedure described by deBoer and Backer ( 1 ) . Approximately 25% of methyl palmitate was added to each of the samples as an internal standard. A few drops of acetone were also added as a solvent and to reduce the viscosity of the sample.
Table 1.
Retention Volumes of the Resin Acid Methyl Esters Relative to Methyl Palmitate on Apiezon N and Polyester Columns
Stationary Phase Poly( 1 , 4
Reoplex 400 (225' C.) 1 .OO
Resoflex 446 (225" C.) 1 .oo 4.50 4.95
Methyl Ester Palmitate A4b,8a-Isopimarate ... Dihydropimara te 4.78 Tetrahydropimarate ... ... Tetrahydroabietateb 4.92 5.11 Pimarate 5.00 5.20 Tetrahydroisopimarate 5.00 ... Dihydroabietate 5.85 6.10 Tetrahydroabietateb 5.85 6 14 Dihydropalustrate 5.96 6.20 Dihydroisopimarate 6.36 6.60 Palustrakc 6.36 6.63 A8a,8-Isopimarate 6.86 7.20 Dehydroabietate 9.70 10.1 Abietate 9.70 10.1 Levopimaratec 9.70 ... Neoabietate 10.6 ... a 2.5-meter column on Chromosorb C44860. Previously unseparated isomers present in standard. Main peak indicated, sample partially isomerized.
DISCUSSION
The nature of the resin acid molecule suggested that stationary phases based on selectivity for the differences in relative volatility and unsaturation might offer means of separation. Therefore studies were made using both nonpolar (Apiezon N grease) and polar (polyester) columns. Shown in Table I are the retention volumes relative to methyl palmitate for the 16 resin acid methyl esters studied. Figures 1 and 2 illustrate chromatograms of the various resin acid esters on Apiezon I\' and Resoflex 446 columns, respectively. Pimaric-Type Acids. The unsaturated pimaric acid methyl esters are completely separated on both the polar and nonpolar columns. T h e polyester columns show remarkable selectivity for A8a,8-isopimarateJ indicating t h a t the double bonds of this isomer are not severely hindered sterically. The internal double bond in A4b,8a-isopimarate, on the other hand, must be quite hindered, as evidenced by its much lower retention volume which is even less than t h a t of methyl pimarate. The partially hydrogenated pimaric acid esters in comparison with the unsaturated pimaric acid esters are more retarded on the Apiezon N column and less retarded on the polyester columns, as expected. The shift in retention volume is that expected for the loss of one double bond. However, the completely saturated tetrahydroisopimarate shows unexpected behavior, in that its retention is much lower than that expected for the loss of the final double bond. It is possible that the compound studied is the more volatile
butylene succinate) (225' C.). 1.00 4.64 4.88 4.95 5.16 5.28 5.30 5.91 6.06 5.80 6.65 6.60 7.16 9.80 9.80 9.80 11.1
Apiezon N (270" C.) 1.oo
3.10 3.95 3.99 4.18 3.53 4.32 4.48 4.83 4.37 4.58 4.00 4.30 4.40 5.17 4.37 6.00
configuration of two isomers as in the case of methyl tetrahydroabietate. Abietic-Type Acids. Mrthyl abiet a t e and methyl dehydroabietate are not separated on the polyester columns in spite of their structural differences, abietate being a diene and dehydroabietate a triene (aromatic) acid. This separation is easily accomplished on the Apiezon S column, however. Apparently the additional double bond in methyl dehydroabietate has sufficient effect in the case of thc polyester column to offset exactly the difference in volatility. Examination of the eluent by ultraviolet spectroscopy indicated no detectable isomerization of these resin acid esters. Methyl palustrate shows a much lower retention volume than methyl abietate, presumably due to the severely hindered interim1 double bond. Partial isomerization to methyl abietate occurs. Methyl levopimarate is also convcrted to methyl abietate and methyl clehydroabietate on the columns. Methyl neoabietate shows the longest retention volume of the resin acids studied on both the polar and nonpolar columns. The partially hydrogenated abietic acid esters in comparison with unsaturated abietic acid esters show a shorter retention time on the polyester columns, as expected. However, the retention time for methyl dihydroabietate is also shorter on the Apiezon h' column and methyl dehydroabietate is not resolved from the dihydro derivative. The structurally similar methyl dihydroabietate and methyl dihydropalustrate are not separated on either type of column. Tn-o peaks are exhibited by various VOL. 31, NO. 1 1 , NOVEMBER 1959
1755
inethyl tttrahydroabietate standards. 'l'hrse are believed to be geometric isomers brought about by the difference in the position of the hydrogen atom iluring Irydrogrnation of the last double boiid. The ratio of these isomers :Lppears to be related to the melting point of the sample, the more retarded isoiner prcdoniinating in samples of high ilielting point. .4side from the general relationship between degree of unsaturation and retention time, no clear correlation between structure and retention characteristics is shown for the resin acid methyl esters on a given type of column, probably because of the somrtime opposing rffects of unszturatinn and volatility on crrtain species. However, the rffects of solute and solvent polarity arc clearly illustrated in Figure 3. A logarithmic plot of the retention volumes of the wsiii acid methyl esters on polyester us. .lpicbzon N stationary phases shows the functionality differrnces for the satui'atcd and unsaturated voinpounds. N o significant differencr iii selectivity 01' rcstcntion time for the resin acid invthyl esters was shown for the various polgrst,er columns studied. Columns containing Reoplex 400, Resoflex 446, xnd poly(l,4-butylene succinatr) appo:iIed to he equnlly satisfactory after coiiditioning for 225' C. operation. 'I'he ,4piezon Tu' column required no conditioning for 270" C. operation. ITnder the conditions of analysis, column c8iciencies of 3400 theoretical plates were obtained a t methyl pimarate on Apiezon N columns, while efficiencies of 1600 theoretical plat,es wrrr obtained on polyester columns. Methyl levopimarate and inethyl palustrate are partially converted to methyl abietate and methyl dehydroabietate, the extent'of conversion appearing to be a function of the acidity of the column. Treatment of the Celite substrate with methanolic potassium hydroxide has in some cases appreciably reduced the interaction on the column. The inlet system is necessarily operated a t high temperatures to achieve column efficiency and this may play a large part in the isomerization or disproportionation of the thermally unstablc rcsin acid csters. I n all other cases, however, cdibration and quantitative analysis are possible, As a result of concurrent work
1756
ANALMlCAL CHEMISTRY
IO
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relative volatility is far too limited for such complcx mixtures. The two types of stationary phases complenient each other very w l l , however. Of the resin acid mrthyl esters studied, only two pr,irs--dihydropalustrate-dih?droabietnte and tlihydropimarate - tetrahytlrnpimarate-cannnt be st least, partially separated by either type of cnlumn. The probleni of the thermal or acid isomerization of some of the resin acids deserves continuing 2,tt.ention. This may never be solved unless adequate low temperature tcehniques are developed. However, there are many cases where thermal isonierisntion is no problem in the analysis ani1 g ~ chromatography s may be applied effectively.
R E T E N T I O N VOLUME, LITERS APIEZON N
Figure 3. Log retention volume in poly( 1,4-butylene succinate) vs. log retention volume in Apiezon N A. 8. C.
D.
Trienoic (aromatic) resin acid methyl ester Dienoic resin acid methyl esters Monoenoic resin acid methyl esters Saturated resin acid methyl esters
on fntty acids, methyl palmitate was used as an internal standard. In contrast to fatty acids, it was found that considerable correction was necessap in most' cases to convert the integrated pe:hk areas to w i g h t per cent. Where:Ls correction factors of 1.0 to 1.3 were obtained for the C16 to (% fatty acids, corrrctioii 1':tctors for the resin acid methyl esters generally ranged from I .3 to 1.6. It was a t first thought that ester interchange may have been taking phce on the polycstcr columns, but comparison with calibrations on Apiezon N columns showed sinilar results. That the correction factors are a function of the particular column is indicated by the fact that they appear to be the same for all the stable esters on a given column, although the!. may vary from column to colunin. I t is evident from these studies that the polyester columns offer the hest means of separation and characterization of the resin acids. Certain separations are obtained equally well or better on the nonpolar column, but in general the separating p o w r based primarily on
LITERATURE CITED
(1) deBoer, T. J., Backer, H. J., R e c . trai.
chim. 73,220,582(1954). ( 2 ) Cropper, F. R., Heywood, A., S a t i i r c 172,1101 (1953): 174,1063 (1954). (3) Edwards, 0. E.,Howe, R., Can. J Chem. 37, 760 (lO5w. ( 4 ) Genge, C. A , , A N A L .CHEsr. 31, 1750 (\ 1- -9.59) _ - ,
(5) Green, B.,Harris, A , , Whalley, W.E., J . Chem. Soc. 1958,4715. ( 6 ) Harris, G . C., "Rosin and Rosin Derivatives,' ' Encyclopedia of Chemical Technology, Vol. 11, p. 779, Interscience, Xew York, 1?51. 1 i i Harris, G. C., Sanderson, T. F., J . .Am Chern. SOC.20, 334, 339, 2079, 2081 f 1948). ( S i James, A . T >fartin, .4. J . l'.. Bzochem. J . 5 0 , 6 i 9 (1952). (9) Orr, C. H., Callen, J. E., J . .1m. Chem. SOC.80,249 (1058;. (10) Thoi, L. V., Lklepine, S I . , Compt. rend. 247, 1343 (1955). (11) Wenkert, E., Chamberlin, J. W., J . A m . Chem.Soc.81,688(1959).
RECEIVEDfor review July 8, 1959. Accepted September 3, 1959.
Rapid Determination of Organically Bound FluorineCorrection I n the article on "Rapid Determination of Organically Bound Fluorine" [B. Z. Senkowski, E. G. Wollish, E. G. E. Shafer, ANAL. CHEM.31, 1574 (1959)] on page 1575, wcond column, third paragraph under Reagents, the third sentence should read: Dissolvr 221.0 mg., etc.
