Broad Spectrum Analysis of Resin Extracts - American Chemical Society

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Broad Spectrum Analysis of Resin Extracts: A Base Extraction Cleanup Procedure J. Gibs and I. H . Suffet 1

Environmental Studies Institute, Drexel University, Philadelphia, PA 19104 A base extraction procedure was developed to minimize the degradation of the performance of fused-silica capillary chromatographic columns used to analyze XAD resin extracts. The degradation of the capillary gas chromatographic column was apparently caused by acidic nonvolatiles called humic materials. The humic materials were absorbed on XAD resins and eluted by nonpolar solvents along with the nonpolar organic compounds of interest in the samples. The base extraction procedure removed approximately 84% of the humic materials present in the ether extract.

BROAD SPECTRUM GAS CHROMATOGRAPHC I CAPILLARY ANALYSIS of

trace organic chemicals i n water can be defined as a method that analyzes, at one time, the largest possible number of chemicals contained in a sample. Broad spectrum organic analyses are best applied to (1) samples needing m i n i m u m pretreatment and (2) samples containing a large molecular weight range of organic components. The usefulness of broad spectrum analysis is based upon being able to observe the changes in water quality data represented b y differences between chromatograms. Thus, the data analysis involves the interpretation of large quantities of chromatographic data at one time. Therefore, sample and instrument quality assurance becomes extremely critical for reliable comparison of interchromatographic data (J). Broad spectrum chromatographic analysis (2-8) has been used to determine order of magnitude changes between samples b y (1) evaluating peak-to-peak changes i n gas chromatographic (GC) detector Current address: Water Resources Division, U.S. Geological Survey, Trenton, NJ 08628

0065-2393/87/0214/0327$06.00/0 © 1987 American Chemical Society

Suffet and Malaiyandi; Organic Pollutants in Water Advances in Chemistry; American Chemical Society: Washington, DC, 1986.


328

ORGANIC POLLUTANTS IN WATER

response (2, 4, 5, 8), (2) noting visual impressions of the overall relative numbers of peaks and their areas (6), (3) noting changes in the response in parts of different chromatograms (2-4, 6,7), and (4) counting peaks that have different concentration levels (7). The change from packed to capillary G C columns made possible the separation of extremely complex mixtures of organic chemicals at nanograms-per-liter concentrations. Capillary G C has increased the number of peaks observed in a typical drinking water sample f r o m the Delaware River at Philadelphia, P A , f r o m 60 peaks with packed-column G C to more than 250 (8). This situation indicates the increased information n o w available with capillary G C analysis. T h e increased ability of capillary G C to resolve organic species caused an unanticipated drawback for broad spectrum analysis. The amount of stationary phase on a capillary G C column is much less than on a packed column. This condition increases the likelihood of observing a decrease in chromatographic performance caused b y the sample matrix. The altered stationary phase may cause reduced precision of retention times and peak areas. The changes in the chromatographic performance of the stationary phase are measured b y the G r o b general purpose test mix (9). Part of the strategy of broad spectrum analysis is to minimize sample pretreatment (and potential artifact production). This strategy was satisfactory for packed-column G C analysis (2). However, minimal sample pretreatment has pitfalls. The problems involved in the broad spectrum capillary G C analysis of ethyl ether elutions of X A D macroreticular resins used for environmental sampling were described b y Bean et al. (JO). The five major chromatographic problems found were (1) unresolved G C peaks on a 30-m O V - 1 0 1 capillary column with a flame-ionization detector ( F I D ) , (2) apparent decomposition of the stationary phase, (3) chromatographic column plugging at the injector end, (4) an injection port glass liner covered with black residue, and (5) an unfruitful G C - m a s s spectrometric ( G C - M S ) investigation because of a lack of clean spectra. Bean et al. (JO) assumed the cause to be "significant quantities of high molecular weight materials present in the sample". These workers used a two-step method to solve the problem: (1) gel permeation chromatography to remove materials having molecular weights ( M W ) greater than 800 (standardized versus polypropylene glycol, M W = 880) and (2) deactivated silica gel to fractionate compounds of M W 1 min. The smallest standard deviation occurred at C-14, which was in the middle of the chromatogram. This pattern was not the same for the retention-time standard deviations of the internal standards alone in ether (see Figure 3). For the internal standards alone, the standard devia tion of retention times was fairly uniform over the entire chromatogram. A significant bias was also seen in the mean retention times in the first 38% of the chromatograms (through the C-12 internal standard). Signifi cant interferences appeared to exist in the G C analysis of concentrates from the ethyl ether elution from the macroreticular resin isolation method. The interferences created other chromatographic problems besides poor retention-time precision. These problems included the presence of a large rising and falling base line (a hump between C-10 and C-17), an unexpected rise in the level of noise from the flame ionization detector, and contamination of the stationary phase of the chromato graphic column. These problems were especially severe for extracts of 150-L samples of chlorinated drinking water. The contamination of the


Suffet and Malaiyandi; Organic Pollutants in Water Advances in Chemistry; American Chemical Society: Washington, DC, 1986. 1000

INDEX

1 1 00

Id KOVATS

ι . ilillll

IL i i .

i i ILiil.l.l.l,

Β . E X T

1 300

14/5/B3

1 4ΘΘ

Β . E X T

i l l

1 -4/S/B3

I MNIlllliiilllillllll

1 200

ILL

4/S

14/5

I

Figure 1A. Effect of base extraction on GC capillary chromatographic profiles.

930

111 ii 111,

iiihlii j !

800

il

-,i

i l

11

ιε-ai

LOG

1E-2L 1E2

LOG

1E-2JL 1 E2

LOG

1È-2L 1E2

LOG

!E2


GIBS A N D SUFFET

A Base Extraction Cleanup Procedure


15.


333

Suffet and Malaiyandi; Organic Pollutants in Water Advances in Chemistry; American Chemical Society: Washington, DC, 1986. 2000

1E-2L

LOG

1 E 2

ι E-S

LOG

1E2

ιε-a. à

LOG

1E2

tE-2L

LOG

1E2

ll II I I •

iL

2200

KOVATS

INDEX

2 3 0 0

iL

11

2 5 0 0

14/5/B3

26ΘΘ

Β.EXT

Β.EXT

t 4/5/B3

1 A/S

-L

Figure 1C. Effect of base extraction on GC capillary chromatographic profiles.

21 0 0

11

1 4 / 5


30

> H m

Ο 33 Ο > Ζ η s r r G H > Ζ H


15.

GIBS A N D SUFFET


335

-2 *-

6

8

18

12

14

16

18

28

22

24

26

28

INTERNAL STANDARD CARBON NO.

Figure 2. Changes in mean retention time due to sample matrix. column stationary phase was determined b y a Grob column-performance test mixture (9). The interferences appeared to be similar to those of Bean et al. (JO). This observation suggested that high molecular weight, nonchromatographable materials were present in the ether concentrate from the X A D resin accumulator. The dramatic effects of the interferences were not anticipated because previous X A D - 2 resin accumulator analyses b y packed-column G C reported b y Suffet et al. (2) and Yohe et al (5) showed acceptable results. Evaluation of G C Capillary C o l u m n . The G r o b test was used to evaluate G C capillary column performance (9). Degradation of the G C columns occurred after one or two capillary G C analyses of the ether extracts. The methyl esters, used to measure the Trenzahl (TZ) number and effective stationary-phase f i l m thickness, showed severe tailing and reduced areas. The tailing increased with increasing carbon number. This finding indicated that the stationary phase was being altered b y the X A D resin extract. The area of the acid peak (2-ethylhexanoic acid) in the G r o b test mix decreased markedly and almost disappeared. This


336


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»-

8Q0. This method of cleanup was rejected because size-exclusion chromatography might introduce new artifacts and cause additional delays in sample analysis time. Because the matrix problem appeared to have acidic components, it was decided that a base extrac tion procedure might remove these materials. Optimization of Base Extraction. The base extraction, sample cleanup procedure was optimized b y taking into account the following facts: 1. The material to be removed was probably humic substances, w h i c h may be considered diprotic acids having a pK\ = 4.8 and a pK = 10.5 + 0.3 (J9, 20). 2. The reaction to remove the interferences f r o m the ether sample was a simple acid-base reaction. 3. The amount of acidic materials in the sample varied con siderably from sample to sample. 4. The amount of acidic materials in any ethyl ether extract was unknown. 5. H u m i c materials absorbed in the UV-vis spectral region because of an aromatic structure containing phenolic and carboxylic functional groups. 2

The following decisions were made to achieve consistent removal of interferences: 1. The base should be a strong buffer. A comparison of the titration of 0.2 Ν K O H and 0.2 Ν K C 0 adjusted to p H 12 shows that it is easier to exceed the acid-neutralizing capacity of the K O H compared with the buffered solution of K C 0 (see Figure 4). 2

3

2


3

338



Q

9

1θ

II

12

13

pH TITRATION OF BASE EXTRACTANT

Figure 4. Buffer capacity of two bases.

2. T h e l i q u i d - l i q u i d extraction should have a constant volume ratio of base to ethyl ether. T h e volume ratio should be kept constant i n order to keep the fraction of the sample solute extracted into the base a constant (21). K C 0 was chosen as the buffer for the base because it is stable and has no interferences i n the UV-vis spectrum. The extraction procedure was optimized b y diluting the aqueous extract to twice its original volume and scanning it ùi a 5-mm cell from a wavelength of 190 n m to 501) n m with a P e r k i n - E l m e r 559 U V - v i s spectrophotometer with background correction. The U V spectrum showed that the basic solution maximum absorbance wavelength changed f r o m 225 n m before extraction to 240 n m after extraction. In addition, the shape of the spectrum also changed. This result indicated that a variety of organic compounds with differing absorbance maxima were being removed f r o m the ethyl ether sample. The total absorbance (A ) of the base extractant at any wavelength may 2

3

T


15.

GIBS A N D SUFFET


be represented as A = ΣΑ, where A is the absorbance of a constituent in the mixture. Thus, the area under the spectrum w o u l d be a better measure of the overall efficiency of the extraction procedure than the absorbance at a single wavelength. The area was integrated b y the trapezoidal area rule over the wavelength range 210-280 n m . This integration avoided the low transmittance at wavelengths below 200 n m . The parameters to be optimized for the extraction of the concen trated ethyl ether elution are (1) base-to-solvent volume ratio, (2) p H , and (3) number of extraction steps. Figure 5 shows a series of spectra for p H 11 base extracts of ethyl ether with different volume ratios. These curves are not corrected for dilution due to the change in volume of base. The increase in the ratio of the volume of base to the volume of ethyl ether decreases the spectral area; for example, increasing the volume ratio b y threefold decreases the spectral area b y 292. If no additional acidic materials were ex tracted, the expected decrease in spectral area w o u l d be 67%. A further increase in the volume ratio to fivefold decreases the spectral area b y 58% as opposed to the expected 80$ b y dilution. Thus, adding more volume of base removes more acidic materials f r o m the ethyl ether resin extract. The effects of the p H of the base (10.5, 11, 12) and two-step extrac tions were also investigated at a base-to-ether volume ratio of 9:1. As the number of milliequivalents of base increased, the absorbance also in creased. Sixteen percent of the total extractable material f r o m the twostep extraction (9:1) at p H 11 was left in the ether after the first extrac tion step. A comparison of packed-column 10% SE-30 gas chromatograms before and after base extraction is shown in Figure 6 for p H 12. T h e rise a n d fall of the base line hump in the later portion of the chromatogram was dramatically reduced. This result was verified b y observing the same phenomenon with chromatograms at p H 11 and 11.5. The cleanup procedure used was discussed in the section entitled Resin Elution Concentration. The extraction procedure was not the op timum procedure that would result f r o m the observations. T h e data indicated that a base extraction at p H >12 and a base-to-solvent ratio of 15:1 with multiple extraction steps were needed to maximize removal of the matrix interferences. However, the extraction efficiency must be balanced against minimal sample destruction and operational ease. Therefore, p H 12 was chosen to minimize base hydrolysis of organic compounds. T h e 10:1 volume ratio was used to assure suffi cient recovery of the ether phase from a larger volume ratio, and only one extraction step was used to minimize loss of volatiles during the cleanup method. T


339



340 ORGANIC POLLUTANTS IN WATER


GIBS A N D SUFFET



BEFORE EXTRACTION

C&LUM_N_ELLAiiK _&ASJ_L^"~~ Spectra-Phytic*

IT) CO

AFTER EXTRACTION /

COLUMN B L A Ν K_ B_A S_E_L l_N E. Figure 6. Effect of pH 11.5 extraction on chromatogram base line



342


Results After Base Extraction. Base extraction was found to eliminate the observed chromatographic hump (as shown in Figure 6) and improve peak resolution and detection. O n the negative side, the procedure could remove compounds of interest or add contaminants to sample. T o study the effects in more detail, a sample was chromatographed on a capillary column before and after base extraction (Figure 1). C o m puter reconstructed plots of the sample capillary chromatograms before and after base extraction were used to observe significant changes in the number and response of peaks. The detector response in Figure 1 was plotted logarithmically, and the m i n i m u m response was 0.01 (0.6 ppt). The comparison showed fluctuations in the number and response of peaks after base extraction primarily for the small peaks below 0.6 ppt. The more concentrated G C peaks were not greatly affected b y base extraction. The decreasing response of peaks does not appear to be due to the physical manipulation of the sample during the procedure. C h e m ical explanations are more likely, possibly including the conversion to new compounds or the loss due to solubility changes, that is, the hump materials dissolving into the aqueous base solution. The increase or appearance of peaks has several possible explanations. Addition of contaminants by the base during extraction is minimal, as proven b y G C analysis of blank ether extractions of the buffer used in the procedure. The base could convert compounds and thus account for some of the new peaks seen. However, the hump obscures the normal column base line, and the removal of the hump b y base extraction may cause previously undetected or unintegrated compounds to be measured. The hump-associated retention times have more detector noise than the other retention times in the chromatograms. The integrator usually defines the detector noise as a peak unless the sensitivity of the integrator is decreased (22). Although qualitative improvement in the packed-column G C was observed (Figure 6), the improvement in identifying the capillary column G C peaks b y their Kovat's indexes was determined b y using the Fisher F test. Table I shows the standard deviations of the retention times of the normal alkane internal standards f r o m packed-column G C analysis before and after a base extraction. The Fisher F test (P = 0.05) showed that only the standard deviation of the retention time of the C - 1 4 hydrocarbon of the base-extracted sample was not significantly smaller. The better precision of internal standard retention time i m proved the ability to compare G C peaks b y their retention times and, in turn, the Kovat's indexes throughout the whole chromatogram. Such comparisons are essential to the interpretation of broad spectrum analysis. The base extraction cleanup procedure changed the initial G C profile data contained in the original ethyl ether extract. Specifically,


15.

GIBS A N D SUFFET

343


Table I. Retention Time Standard Deviations of Internal Alkane Standards of Composite XAD Resin Water Samples With and Without Cleanup


Standard Deviation (min) Compound

With Cleanup (n = 7)

Without Cleanup (n=4)

F°

C-8 C-9 C-10 C-ll C-12 C-13 C-14 C-15 C-16 C-17 C-18 C-19 C-20 C-21 C-22 C-23 C-25 C-26

0.143 (n = 6) 0.114 0.120 0.131 0.139 0.136 0.140 0.135 0.134 0.138 0.136 0.123 0.126 0.129 0.133 0.119 0.136 0.132

0.69 1.30 1.89 2.20 1.57 0.56 0.09 0.44 0.95 0.65 0.77 0.87 0.85 1.13 1.27 1.40 3.05 2.01

23.28 130.04 248.06 282.03 127.58 16.96 0.413 10.62 50.26 22.19 32.06 50.03 45.51 76.73 91.18 138.41 502.95 231.87

N O T E : All compounds, except C-8 and C-14, exceeded the critical value of F 0.05 (df = 3, 6) = 4.76. C - 8 exceeded the critical value of F 0.05 (df = 3, 5) = 5.41, and C-14 did not exceed the critical value. F = (standard deviation without cleanup) /(standard deviation with cleanup) .

a

acidic sample constituents were removed i n the cleanup process. A measure of the total amount of acidic components in a sample can be estimated b y the amount of base neutralized during the extraction procedure. The following assumptions are necessary to make this estimate: 1. The compounds are soluble i n water at a p H between 2.9 and 3.5, which was the p H of the water sample. 2. The compounds can be sorbed b y the X A D - 2 and X A D - 8 resin at a p H between 2.9 and 3.5. 3. The compounds are soluble i n ethyl ether, which was used to elute the resin column. The amount of base neutralized was computed from the measurement of the p H of the buffer after the cleanup step. This p H value was entered on the acid titration curve of the buffer (see Figure 4), and the


344


Table II. Acid Neutralization of Base Extract Sampling Period


0

Sample Location

8/1/80

9/9/80

9/30/80

Chlorinated rapid sand filter effluent Chlorinated ozonator effluent Chlorinated GAC effluent Chlorinated ozonated GAC effluent Nonchlorinated rapid sand filter effluent Nonchlorinated ozonator effluent Nonchlorinated ozonated GAC effluent Nonchlorinated GAC effluent Lab still blank Transportation blank

1.0 0.84 0.16 0.20 0.44 0.34 0.21 0.21 0.18 0.22

0.45 0.65 0.13 0.295

0.70 0.31 0.65 0.04 0.11 0.0 0.0 0.0

0.20 0.285 0.26 0.0 0.0

Note: G A C denotes granular activated carbon. Values are the milbequivalents of base neutralized during the sampling period ending on the given date.

0

corresponding number of milliequivalents of acid was determined. An example of the results of this procedure is shown in Table II. Loss of Compounds D u r i n g Base Extraction. There are no perfect cleanup methods, and certain constituents in the sample extracts may react or be lost during the base extraction step. Gas chromatographic-mass spectrometric (GC-MS) analysis of base extracts of resin eluates revealed that base hydrolysis was occurring for at least one class of compounds, namely, phthalates (23). The two isopropylidene sugars undergoing losses were identified by G C - M S as the diacetone sugars of L-sorbose and D-xylose. The phthalate derivatives undergoing base hydrolysis were dimethyl, diethyl, and dibutyl phthalates. Rhoades et al. (24) showed that dimethyl phthalate and benzyl phthalate undergo significant storage losses at p H 10. The isopropylidene sugars are moderately polar compounds that probably were back extracted into the base. The back extraction occurred because isopropylidene sugars are moderately soluble in water, that is, 14 g of 1,2,3,5-di-O-isopropylidene-D-glucofuranose dissolves in 100 g of boiling water (25). The loss of compounds in any method limits the usefulness of the approach for broad spectrum screening. The loss of compounds during base extraction occurred either by dissolution back into the water or by reaction with base. These losses are deemed to be an acceptable trade-off for the following reasons: 1. Improved accuracy and reproducibility for the nonpolar synthetic organic chemicals that are being studied. 2. More productive use of analytical equipment because the capillary column is not degraded rapidly.


15.

GIBS A N D SUFFET


345

3. One source of organics in drinking water is studied. 4. Detection of compounds having a l o w concentration in a sample is enhanced b y a significantly lower base line noise level.


Conclusions The removal of chromatographic analysis interference increases the reliability of the broad spectrum approach to organic analysis. The improved retention-time precision and lower background noise level make it possible to use statistical significance-level testing of the broad spectrum data (26). The sample cleanup procedure is efficient and easy to perform. T h e cleanup procedure removes approximately 84% of the nonchromatographable materials. The losses f r o m the sample affect only a small number of the synthetic organic chemicals of interest in the field of drinking water treatment. The compounds undergoing the largest losses were phthalates and diacetone sugars. The use of UV-absorbance measurements to optimize the base extraction was demonstrated. U V measurements over a w i d e wavelength range were demonstrated to be necessary because many humic materials b e i n g r e m o v e d m a y not have the same m a x i m u m absorbance wavelength.

Acknowledgments Research support was provided b y the U.S. Environmental Protection Agency under Contract N o . 806256-02, J . Keith Carswell, Project Of ficer. W e wish to thank M . E . Post for her advice and help in developing the laboratory procedures and B. Najar for significantly contributing to the laboratory analyses.

Literature Cited 1. Gibs, J. Ph.D. Thesis, Drexel University, 1983. 2. Suffet, I. H.; Brenner, L.; Coyle, J. T.; Cairo, P. R. Environ. Sci. Technol. 1978, 12, 1315. 3. Suffet, I. H. In Activated Carbon Adsorption of Organics from the Aqueous Phase, Vol. 2; McGuire, M. J.; Suffet, I. H., Eds.; Ann Arbor Science: Ann Arbor, MI, 1980; Chapter 24, p 539. 4. Yohe, T. L.; Suffet, I. H.; Coyle, J. T. In Activated Carbon Adsorption of Organics from the Aqueous Phase, Vol. 2; McGuire, M. J.; Suffet, I. H., Eds; Ann Arbor Science: Ann Arbor, MI, 1980; Chapter 2, p 37. 5. Yohe, T. L.; Suffet, I. H.; Cairo, P. R. J. Am. Water Works Assoc. 1981, 73, 402. 6. Stevens, Α. Α.; Seeger, D. R.; Slocum, C. J.; Domino, M. M. J. Am. Water Works Assoc. 1981, 73, 548.



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7. Van Rensberg, J. F. J.; Hassett, Α.; Theron, S.; Wiecher, S. G. Prog. Water Technol. 1980, 12, 537. 8. Coyle, G. T.; Maloney, S. W.; Gibs, J.; Suffet, I. H. In Water Chlorination: Environmental Impact and Health Effects; Jolley, R. et al., Eds.; Ann Arbor Science: Ann Arbor, MI, 1983; Vol. 4, Chapter 30, p 42. 9. Grob, K., Jr.; Grob, G.; Grob, K. J. Chromatogr. 1978, 156, 1. 10. Bean, R. M.; Ryan, P. W.; Riley, R. G. Proc. Symp. High Resolut. Gas Chromatogr. Am. Chem. Soc. 1977, Aug. 28-Sept. 2, Chicago, IL; Academic: New York. 11. Schomberg, G. Chromatogr. Electrophor. 1980, 10, 327. 12. Aiken, G. R.; Thurman, Ε. M.; Malcolm, R. L.; Walton, H. F. Anal. Chem. 1979, 51, 1803. 13. Junk, G. Α.; Richard, R. J.; Grieser, M. D.; Witiak, D.; Witiak, J. L.; Arguello, M. D.; Vick, R.; Sveck, H. J.; Fritz, J. S.; Calder, G. V. J. Chromatogr. 1974, 99, 745. 14. Grob, K. J. High Resolut. Chromatogr. Chromatogr. Commun. 1978, 1, 263. 15. Rijks, J. A. Ph.D. Thesis, Technical University of Enidhoven, The Nether lands, 1973. 16. High Resolution Chromatography Products; J & W Scientific: Rancho Cordova, CA, 1981. 17. Van Den Dool, H.; Krantz, P. D. J. Chromatogr. 1963, 11, 463. 18. Glaser, E. R.; Silver, B. L.; Suffet, I. H. J. Chromatogr. Sci. 1977, 15, 22. 19. MacCarthy, P.; Peterson, M. J.; Malcolm, R. L.; Thurman, E. A. Anal. Chem. 1979, 51, 2041. 20. Perdue, Ε. M. Geochim. Cosmochim. Acta 1978, 42, 1351. 21. Suffet, I. H. J. Agric. Food Chem. 1973, 21, 288. 22. Environmental Protection Agency. Problems with Chromatographic In tegrators: Process Measurements Review; Industrial Environmental Research Laboratory: Research Triangle Park, NC, 1979; Vol. 2, No. 1, p 45. 23. Neukrug, Η. M.; Smith, M. G.; Coyle, J. T.; Santo, J. P.; McElhaney, J.; Suffet, I. H.; Maloney, S. W.; Chostowski, P. C.; Pipes, W. O.; Gibs, J.; Bancroft, K. Removing Organics from Drinking Water by Combined Ozonization and Adsorption; U.S. Environmental Protection Agency. U.S. Government Printing Office: Washington, DC, 1981; EPA-600152-83-048. 24. Rhoades, J. W.; Thomas, R. E.; Johnson, D. E.; Tillery, J. B. Determination of Phthalates in Industrial and Municipal Waste Waters; U.S. Environmental Protection Agency. U.S. Government Printing Office: Washington, DC, 1981; EPA-600/S4:81-063. 25. Dictionary of Organic Compounds; Buckingham, J., Ed.; Chapman-Hall: New York; 1982. 26. Gibs, J.; Suffet, I. H.; Najar, B. In Water Chlorination: Environmental Impact and Health Effects, Vol. 5, Jolly, R.; Bull, R. J.; Davis, W. P.; Katz, S.; Roberts, M. H. Jr.; Jacobs, V. Α., Eds.; Lewis: Ann Arbor, MI, 1985; Chapter 88, p 1099. RECEIVED

for review October 17, 1985.

ACCEPTED

May 14, 1986.


Broad Spectrum Analysis of Resin Extracts - American Chemical Society

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