Open-Tube-Column Gas Chromatography of Rosin Fluxes John C. Hasson and Murlidhar V. Kulkarni IBM Components Diuision, East Fishkill Facility, Hopewell Junction, N. Y. 12533 The major components in “water white” rosin flux have been separated with resolutions of two or better using open-tube capillary columns. This method was used to distinguish between acceptable and unacceptable fluxes for application in microelectronic processing. The optimum conditions for the analysis were investigated and a possible mechanism for the reaction between rosin flux and metal oxides was proposed.
RESINACIDS are the principal organic compounds found in rosin fluxes, besides the various inert solvents. These acids, the major ones being pimaric, palustric, isopimaric, dehydroabietic, abietic, and neoabietic acids, occur naturally in the sap or oleoresin of pine trees from which the rosin fluxes are prepared. They are three-ring, isomeric, monocarboxylic acids, differing mainly in the position of the double bonds and the attached alkyl groups (1). They have a hydrophenanthrene nucleus and they fall into two types: the abietic types, which have conjugated double bonds; and the pimaric types, in which the double bonds are not conjugated. Several papers (2-8) have appeared on the gas-liquid chromatography of these resin acids, starting with Hudy’s paper in 1959. Most of these workers used packed columns to study the resin acid composition of pine tree oleoresins, with varying degrees of success in resolving the resin acids. The best resolution was obtained by Brooks, Fisher, and Joye with a Versamid 900 liquid phase at 250 O C (4). Weismann et al. ( 5 , 6 ) experimented with capillary columns but did not get any better resolution than that available from packed columns. In the present work, we were able, using open-tube capillary columns, to separate with a resolution of at least two ( i e . ,no overlap of adjacent gaussian peaks) the major resin acids found in electronic grade rosin fluxes. Electronic grade fluxes are prepared with high-purity solvents and rosin that is “water white” (the ASTM designation for a high-purity rosin). Our objective was t o find a correlation between the chemical composition of a flux and its performance in processing microelectronics devices so that a “bad” flux could be isolated by chemical analysis before it was released for use on the manufacturing line. Because of the fragile nature of the metallization on these devices, even small amounts of corrosion or conductive residues resulting from “bad” fluxing could lead to reliability exposures and electrical failures of the microelectronic devices. Using gas-liquid chromatography, six fluxes which had given satisfactory soldering on the manufacturing line were (1) H. I. Enos, G. C. Harris, and G. W. Hedrick, “Rosin and Rosin Derivatives,” “Encyclopedia of Chemical Technology,” Vol. 17, Interscience Publishers, New York, N.Y., 1968, p 475. (2) J. A. Hudy, ANAL.CHEM., 31, 1754 (1959). (3) F. H. M. Nestler and D. F. Zinkel, ibid., 35, 1747 (1963). (4) T. W Brooks, G S. Fisher, and N. M. Joye, ibid., 37, 1063 (1965). Lack, 71, 713 (5) W. Sandermann and G. Weissmann, Farbe (1965). (6) G. Weissmann. H. H. Dietrichs, and W. Sootichanchai, Holz Roh- Werkst., 27, 189 (1969). (7) N. M. Joye and R. V. Lawerence, J . Chem. Eng. Data, 12, 279 (1967). (8) F. H. i. Nestler and D. F. Zinkel, ANAL.CHEM.,39, 1118 (1967).
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compared to one which had allowed large amounts of corrosion to form during soldering, resulting in the electrical failure of many microelectronic devices. The use of open-tube capillary columns with their inherently high efficiency made it possible to carry out the analysis in a short time, at temperatures 50 to 75 “C lower than the temperature used with packed columns, and still get good resolution. The lower temperatures minimized the chances of thermal isomerization of the flux while it was passing through those columns. EXPERIMENTAL
An F and M Gas Chromatograph (Model 5750) was adapted for open-tube capillary columns by adding an inlet splitter with a nominal 100: 1 split ratio. Flame ionization detection was used with a carrier gas makeup sufficient to keep the sensitivity in the optimum range. Stainless steel open-tube capillary columns of standard size (150 feet x 0.01 inch) were purchased from the Perkin-Elmer Corporation coated with either Apiezon N or Versamid 930. The column and splitter were maintained at 200 “C while the injection port and detector were kept at 250 “C. Helium was used as the carrier gas with an inlet pressure of 100 psig. A 10-pl syringe was used to inject samples of 2-4 pl of methyl ethyl ketone containing about 100 pg per p l of the esterified rosin part of the flux. The attenuation setting was usually 10 X 4 or less. Since the resin acids were not volatile enough for gas chromatography, the methyl esters were prepared by reacting 1 gram of the rosin part of the flux and an excess of ethereal diazomethane at 65 “C (9). The solvents were pumped away from the fluxes in a vacuum oven at temperature less than 120 “C. Yields of the esters exceeding 95 were usual. The esters were then dissolved in the methyl ethyl ketone spiked with diethyl ether. Standards for the identification of the individual estersexcept for methyl palustrate and methyl sandaracopimaratewere prepared from water white rosin by the amine salt methods of Harris and Sanderson (10). D. F. Zinkel of the U.S. Forestry Services Forest Products Laboratory kindly provided a mixed standard of 89 % sandaracopimaric and 11 % isopimaric acids. A 14 sample of palustric acid was made by isomerizing pure abietic acid under vacuum at 200 “C for two hours to give a mixture of 14% palustric, 5 % neoabietic, and 81 % abietic acids (11). RESULTS AND DISCUSSION
Table I shows the relative retention times of the methyl esters of the resin acids for the 150-foot Apiezon N and Versamid 930 columns operated at 100 psig and 200 “C; the relative retention times were referred to methyl pimarate. In addition to the principal resins acids, we identified one of the three minor peaks as being due to sandaracopimaric acid, a minor ~
(9) Aldrich Co. Technical Information Bulletin on Diazald, based on T. J. deBoer and J. J. Backer, Reel. Trac. Chim., Pays-Bas, 73, 229, 582 (1954). (10) G. C. Harris and T. F. Sanderson, J. Amer. Chem. Soc., 70, 334, 2079 (1948). (11) H. Takeda, W. H. Schuller, and R. V. Lawrence, J . Org. Chem., 33, 1683 (1968).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9,A U G U S T 1972
Table I. Relative Retention Time Column APN V930
Sandaracopimarate 1.09 1.10
Pimarate 1.OO 1.00
Palustrate 1.36 1.33
Isopimarate 1.36 1.43
Dehydroabietate 1.58 1.80
Abietate 1.98 2.06
Neoabietate 2.30 2.39
Table 11. Weight Per Cent Composition of Fluxes Unidentified Sandara- Unidentified DehydroAbietate Neoabietate Fluxesa Pimarate impurity copirnarate impurity Palustrate Isopimarate abietate 15.6 24.9 1.8 0.9 17.2 20.0 5.8 3.7 Acceptable 1 9.9 15.0 7.5 3.3 2.1 1.2 18.0 18.5 24.4 2 10.4 13.1 32.0 3 6.7 3.7 2.0 0.9 17.0 18.0 6.6 12.8 30.2 4 5.8 3.0 2.2 1.0 18.2 18.9 7.9 12.0 30.9 5 5.3 2.7 1.0 0.0 17.7 18.8 11.6 11.o 25.0 6 5.5 3.5 0.0 18.8 20.6 1.7 13.8 0.5 4.5 6.7 31.4 1.7 1.3 51.0 1.3 1.5 Unacceptable 7 a Fluxes 1-6 gave acceptable fluxing while flux No. 7 gave unacceptable fluxing. Fluxes 1-3 were analyzed on an Apiezon N column while fluxes 4-7 were analyzed on a Versamid 930 column.
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Figure 1. Typical chromatogram for rosin flux using an Apiezon N liquid phase
resin acid. The adjusted retention times were measured from the diethyl ether peak, which was found to have the same retention time as methane. With the aid of a thermal conductivity detector, we found that both methane and ether had the same retention times as air (1.2 minutes). Besides completely resolving the methyl palustrate and methyl isopimarate peaks, which were superimposed when an Apiezon N liquid phase was used, the Versamid 930 liquid phase allowed the analysis to be completed in three-fourths of the time needed with Apiezon N. The composition-by-weight-per cent of six satisfactory electronic grade fluxes obtained on both Apiezon N and Versamid 930 columns is compared with that of the unsatisfactory flux in Table 11. Except for the unfortunate superimposition of the palustrate and isopimarate peak in the Apiezon N column, the results are about the same. The values are based directly on the normalized areas of the peaks, since the c-factors are the same for all of the isomers. Peaks which contributed less
than about 1 to the total area were not considered. In the case of the Apiezon N column, the individual weight percentages of the palustrate and isopimarate peaks were calculated by obtaining their ratio from a Versamid 930 column. Table I11 gives the operating characteristics of Versamid 930 columns of various lengths and inlet pressures. The resolution was calculated for the separation of the methyl palustrate and isopimarate peaks, since they were the most difficult to separate. Table 111 shows that the least efficient column (Le.,largest HEETP) was the 150-foot column operated at 100 psig, although it gave good resolution. The gas velocity in this column was 76 cmisec, which was well above the usual optimum practical gas velocity, indicating why ,the efficiency was poor. The best resolution was obtained with a 100-foot column and an inlet pressure of 33 psig but the analysis time was excessively long. The best combination of resolution and analysis time was given by a 50-foot column operated at 16 psig. However, the 50-foot column operated at 33 psig should
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
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zM)~-2ooo-2w~c 1w' x ,010" 100 p i g
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Figure 2. Typical chromatogram for rosin flux using a Versamid 930 liquid phase
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Figure 3. Rosin flux chromatogram obtained using Versamid 930 liquid phase under the conditions that gave the best resolution be perfectly satisfactory for routine analyses since its resolution was 1.2 ( i t . , 0 . 8 2 x area contribution from an adjacent equal peak) and the analysis time was only 12 minutes. Figure 1 shows a typical chromatogram obtained on a 150foot column with an Apiezon N liquid phase at 200 "C and a pressure of 100 psgi, while Figure 2 shows the chromatogram obtained with a Versamid 930 liquid phase under the same conditions. Figure 3 shows a chromatogram obtained under the conditions which gave the best resolution (ix.,100-foot column, 200 "C, 33 psig) between the palustrate and the isopimarate peaks. Not only are the palustrate and isopimarate peaks completely resolved, but the sandaracopimaric peak is also completely resolved from the first impurity peak. The usual methods of characterizing rosins (Le., color, acid number, saponification number, ultraviolet and infrared spectra) had failed to show any difference between the bad flux and acceptable fluxes except that the bad flux had a larger absorption peak at 2765 A. This larger absorption, however, 1588
agrees with the gas chromatographic work, siace dehydroabietic acid has its maximum absorption at 2765 A. The acid and saponification number of 166 and 171, respectively, were about the same as for the acceptable fluxes and are what one could expect for rosin ( I ) . Such values indicate that the poor fluxing was not due to the presence of an unusual amount of neutral constituents or polymerized rosin acids, since both of these lead to low acid and saponification numbers. The infrared spectrum and color were also the same as for the acceptable fluxes. Thus the only significant difference between the acceptable and unacceptable fluxes was the high percentage of dehydroabietic acid in the bad flux, It is known ( I ) that dehydroabietic acid is the chief product of heat-degraded rosins above 270 OC. Since heat-degraded rosin fluxes, almost by definition, have a poor fluxing action, it might be expected that a flux which contained a high percentage of dehydroabietic acid would not be a good flux. If we assume that the attack of
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
Table 111. Calculated Resolution and Efficiency Dataa Analysis HEETP, P, time, L , feet P,,psig R mm cm/sec min 150 100 2.0 3.4 76 44 100 66 1.5 2.7 59 28 100 33 3.3 1.5 28 124 50 33 1.2 1.9 51 12 50 16 2.0 1 .o 24 24 50 8 0.9 22 50 2.2 a Resolution based on separation of the methyl palustrate and methyl isopimarate peaks. HEETP calculated for the isopimarate peak. the acid groups of the resin acids on metal oxides is the mechanism of fluxing, it is not clear why dehydroabietic acid should be any less effective than the other resin acids. However, it is known that the air oxidation of the resin acids takes place readily at the double bonds to give hydroxy and peroxy intermediates, which react further to give a complex mixture of
oxidation products (1). If fluxing action were also to involve the attack of the oxygen in metal oxides upon double bonds, then the great stability of the double bonds in the aromatic ring of dehydroabietic acid would cause it to be a poorer fluxing agent than the other resin acids. Although Apiezon N columns have a long life, their inability to separate the methyl palustrate and isopimarate peaks limits their usefulness for flux analysis. The Versamid 930 columns, on the other hand, are excellent for the analysis of rosin fluxes, but are very sensitive to oxidation. If they are not flushed thoroughly with the carrier gas before heating and then slowly brought to the operating temperature to allow dissolved oxygen to escape without reacting, they will lose their resolving power within a week.
RECEIVED for review December 13,1971. Accepted March 20, 1972. Presented at the 160th National Meeting of the American Chemical Society, Chicago, Ill., September 18,1970.
Raman Spectra-Structure Correlations for Pyrazines New Method for Obtaining Spectra of Trapped Nanoliter Gas Chromatograph Fractions Richard P. Oertel and D. V. Myhre The Procter & Gamble Company, Miami Valley Laboratories, P.O. Box 39175, Cincinnati, Ohio 45239 Raman spectra-structure correlations have been formulated (based on data for 32 pyrazines variously substituted with alkyl, alkoxy, and vinyl groups) which allow convenient, unequivocal determination of the ring-substitution pattern of pure pyrazines and, in many cases, components of a pyrazine mixture. A method is described for trapping GC fractions in glass capillary tubes for use in the coaxial ( 1 8 0 O ) mode of a Cary 81 laser-Raman spectrometer. Useful structural data are thereby obtainable from samples of less than one nanoliter, and the microsamples are preserved for further spectrometric analysis.
SINCEPYRAZINES (lP-diazines) are among the flavor components of many natural and heat-processed foods, detection of their presence and elucidation of their structure are often important. Mass spectrometry bas been useful in identifying pyrazines recovered from gas chromatographic (GC) effluents ( 1 , 2 ) ,although results from our laboratories indicate that this method frequently does not give unambiguous proof of the ring-substitution pattern. Nuclear magnetic resonance could be of help in this regard if it displayed the sensitivity required for the routine study of nanoliter amounts of pyrazines (often the maximum sample size available). Vibrational spectroscopy should be applicable in locating ring substituents, as certain pyrazine vibrations are expected to be sensitive to (1) H. A. Bondarovich, P. Friedel, V. Krampl, J. A. Renner, F. W. Shephard, and M. A. Gianturco, J. Agr. Food Cliern., 15, 1093
(1967). (2) P. Friedel, V. Krampl, T. Radford, J. A. Renner, F. W. Shephard, and M.A. Gianturco, ibid., 19, 530 (1971).
ring substitution (1-3). There are limitations, however, to the use of a conventional dispersion infrared spectrometer to investigate volatile pyrazines at the nanoliter level, owing to problems in sample handling and instrument sensitivity. The alternative of interferometrically recording infrared spectra of “on-the-fly” G C effluents only recently has become commercially available. We have found that examination of trapped G C effluents by Raman spectrometry offers a convenient and sensitive means to obtain the desired structural information, while preserving the microsample for further spectrometric analysis. Presented here are Raman spectra-structure correlations which allow easy and unequivocal determination of the ringsubstitution pattern of pure pyrazines and, in many cases, components of a pyrazine mixture. Severdl of these Raman correlations recently were used in our laboratories to help identify the three methoxymethylpyrazine isomers (4). In the course of this work, we devised a simple method for trapping nanoliter G C fractions in glass capillary tubes for use in the coaxial (180”) mode of a Cary 81 laser-Raman spectrometer. EXPERIMENTAL
Synthesis of Pyrazines. The pyrazines in Table I were obtained commercially (Aldrich Chemical Company) or were synthesized according to accepted literature methods (3) G. Varsanyi, “Vibrational Spectra of Benzene Derivatives,” Academic Press, New York, N.Y., 1969. (4) G . M. Nakel and L. V. Haynes, J . Agr. Food Cltem., 20, 682 (1972).
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