1518
Anal. Chem. 1981, 53, 1516-1519
Table VII. Detection Limits (mg/L)
elem and line elem and line (nm) detect limit (nm) detect limit Cu 324.754 Mo 281.615 Mg 279,553 Fe 259.940
0.001
0.006 0.0002 0.004
Mn 257.610 A1 236.705 Cd 228.802 Zn 213.856
0.0008
0.083 0.003 0.009
for multielement analysis and simplifies instrument development. Although slewing time is increased relative to other systems of this type owing to the use of the BASIC programmable interface, overall analysis time of 7 min is satisfactory for the determination of approximately nine elements. The instrumentation and software developed have proven quite adequate for the trace analysis of drinking water. Apill be given plications to other sample types are complete and w in an upcoming report.
ACKNOWLEDGMENT We acknowledge EPA for providing water quality control references and drinking water samples analyzed. We express our appreciation to Kevin Mulligan for his help in the preparation of this manuscript. LITERATURE CITED (1) Dahlquist, R. L.; Knoll, J. W. Appl. Spectrosc. 1978, 32, 1-29. (2) Butler, C. C.; Kniseiey, R. N.; Fassei, V. A. Anal. Chem. 1975, 47, 825-829.
Ward, A. F.; Marciello, L. F. Anal. Chem. 1979, 51, 2264-2272. Woinik, K. A.; Kuennen, R. W.; Frlcke, F. L. I n "Developments In Atomic Plasma Spectrochemical Analysis"; Barnes, R., Ed.; Heyden & Son: London, in press. McQuaker, N. R.; Kluckner, P. D.; Chang, G. N. Anal. Chem. 1979, 51. 888-895. Barnes, R. M.; Genna, J. S. Anal. Chem. 1979, 51, 1085-1070. Duika, J. J.; Risby, T. H. Anal. Chem. 1976, 48, 640A-853A. Greenfield, S.; Jones, I. L.; Berry, C. T. Analyst (London) 1964, 80, 713-720. Wendt, R. H.; Fassei, V. A. Anal. Chem. 1965, 37, 920-922. Abercrombie, F. N.; Siivester, M. D.; Cruz, R. B. I n "Ultratrace Anaiysis In Blologlcal Sciences and Environment"; Risby, T. H., Ed. American Chemical Society: Washington, DC, 1979; Chapter 2. Floyd, M. A.; Fassel, V. A.; Wlnge, R. K.; Katzenberger, J. M.; D'Silva, A. P. Anal. Chem. 1980, 50, 431-438. Kawaguchi, H.; Masushi, 0.; Ito. T.; Mlzuike, A. 0. Anal. Chlm. Acta 1977.-95, 145-152. Johnson, D. J.; Piankey, F. W.; Winefordner, J. D. Anal. Chem. 1975, 47. 1739-1743. Spilman; R. WI; Maimstadt, H. V. Anal. Chem. 1976, 48, 303-311. Boumans, P. W. J. M.; Van 0001, 0. H.; Jansen, J. A. J. Analyst (London) 1976, 101, 585-587. Ediger, R. 0.; Wilson, D. L. At. Abs. News/. 1979, 18, 41-45. Savage, R. N.; HleftJe,G. M. Anal. Chem. 1979, 51, 408. Woodward, W. S.; Rellley, C. N. Pure Appl. Chem. 1976, 50, 785. Savltzky, A.; Goiay, M. J. E. Anal. Chem. 1964, 36, 1627-1639. Kawaguchi, H.; Ito, T.; Ito, A.; Mlzuike, A. Anal. Chim. Acta 1980, 122, 75-79.
RECEIVED for review January 7,1981. Accepted May 18,1981. We wish to acknowledge the National Institute of Occupational Safety and Health without whose support through Grant R01-OH-00739 this project would not have been possible.
Two-Phase Mixed Indicator Titration Method for the Determination of Anionic Surfactants Zhi-ping Li' and Miiton J. Rosen" Department of Chemistry, Brooklyn College, Cify University of New York, Brooklyn, New York 11210
The two-phase, mixed indicator titration method for anlonlc surfactants has been reexamlned. The method is not quantitative for surfactants wlth alkyl chains containing less than 12 carbon atoms but may be extended to them by using 2:3 (v/v) ch1oroform:l-nitropropane as the organic phase and a muitlple extraction-titration technlque. A screening test is proposed for determining the suitability of the standard method for determining an anionic surfactant of unspecified chain length. CI-at concentrations in excess of 5 X M in the aqueous phase interferes with both the standard and the extended methods. Na', Mg2+, and Sod2- at concentrations of up to 4 X lo-' M do not interfere with the determination of sodium dodecanesuifonate.
The two-phase mixed indicator titration method for the analysis of anionic surfactants, developed by Reid and coworkers ( I , 2) for the Commission Internationale d'Analyses of the Comit6 International des DBrivBs Tensioactifs (70 Champs Elysees, Paris, France) is an excellent and well-established method. However, it has been apparent for some Visiting Scholar from Xinjiang Chemical Institute, Scientific Academy of the People's Republic of China. 0003-2700/81/0353-1518$01.25/0
time now that the method is not quantitative for all anionic surfactants, particularly when the hydrophobic group is short. In an attempt to overcome this deficiency, the method was reexamined and its limitations explored. A number of different types of surfactants, of approximately 99% purity, were analyzed by the method, under controlled conditions. In addition, the effect of the partitioning of the surfactant-indicator complex between aqueous and organic phases, the presence of various electrolytes, and the nature of the organic phase were explored. Finally, various modifications were made in the procedure to extend the range of application of the method.
EXPERIMENTAL SECTION Materials. Anionic Surfactants. Sodium decanesulfonate (Anal. Found C, 49.40; H, 8.71; S, 13.28. Calcd: C, 49.16; H, 8.66; S, 13.12),sodium dodecanesulfonate(Anal. Found C, 52.99; H,9.29; S, 11.74. Found C, 52.92; H, 9.25; S, 11.77),and sodium tetradecanesulfonate (Anal. Found: C, 56.0; H,9.76; S, 10.48. Calcd: C, 56.0; H, 9.67; S, 10.66) were purchased from Research was synthesized Plus, Bayonne, NJ. P-(C~H,~OC~H~~)C,H,SO,N~ in this laboratory (3) (Anal. Found C, 51.14; H, 6.45; S, 10.19. Calcd.: C, 51.84; H,6.53; S, 9.88). Aerosol AY (sodium diamyl sulfosuccinate), Aerosol MA (sodium dihexyl sulfonsuccinate), and Aerosol OT [sodium di(2-ethylhexyl) sulfosuccinate] were obtained from American Cyanamide Co. These materials were 0 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981
Table I. Assaya and Solubility of Surfactants with Different Alkyl Groups; % of surfactant in assay, CHC1, surfactant structure % phase 89.2, 5.62 C10H21S03Na 89.8, 6.18 C,H,,OC~H,OC,H,SO,Na C,H,, OOCCH,CH(SO,Na)COOC,H,, 95.9, 16.43 C,H,,OOCCH,CH(SO,Na)COOC,H,, 96.8, 17.61 c C,H,,00CCH2CH(S~3,Na)COOC,H,,98.2, 99.1, 19.2 C12H25S03Na 99.0, c 1' 4H29 so3 Na a Procedure A; CHCl, organic phase; surfactant concenAfter the addition of 10 mL of tration = 1 X l o w 3M. Not determinable; acid mixed indicator solution. stable emulsion formed.
all dried to constant weight in vacuo before use. The Aerosols are stated to have a purity of 99% or better. Cationic Surfactant. Hyamine 1622 (Rohm and Haas Co.). The purity of this compound was first detemined by titration against sodium dodecyl sulfate (BDH Chemicals, Ltd.) whose assay had been determined by acid hydrolysis (4). On the basis of this, the molar absorptivity of the sample of Hyamine 1622 used throughout this investigation was determined to be 1266 at 269.5 nm and the concentrations of all subsequent Hyamine solutions were checked by ultraviolet absorbance. Acid Mixed Indicator Solution. This was prepared from Dimidium bromide, Erioglaucine, and sulfuric acid in the usual fashion (1, 4). Analytical Method. )Procedure A: To 10 mL of surfactant M concentration in a 250-mL, glasssolution of (1-4) X stoppered, Erlenmeyer fla3k were added 10 mL of the acid mixed indicator solution and 16 mL of chloroform (or ch1oroform:lnitropropane mixture). The mixture was titrated with a solution of Hyamine 1622 of concentration approximately that of the surfactant solution with vigorous shaking after each addition,until the pink color was just discharged from the chloroform layer. Procedure B The procedure was identical with that described in A, above, except that the containing vessel was a 150-mL separatory funnel. (Emulsions, which sometimes form, are readily broken by warming the mixture.) When the titration end point had been reached, the lower (chloroform or ch1oroform:l-nitropropane) layer was drained out through the stopcock at the bottom of the funnel and 15 mL of fresh chloroform was added. The mixture was vigorously shaken and then titrated with the Hyamine solution to a new end poinit. The lower layer was again removed. Titration and removal of the lower layer were repeated until no significant further addition of Hyamine solution was needed to reach the end point. The final assay of the surfactant was determined from the total volume of Hyamine added. Partitioning of Surfactant between Aqueous and Organic Phases in the Presence of Acid Mixed Indicator. For determination of the percentage of surfactant that is soluble in the organic (chloroform or ch1oroform:l-nitropropane)phase, 10 mL M concenof the surfactant solution, of approximately 1 X tration, 10 mL of the acid mixed indicator solution, and 15 mL of the organic phase were shaken together vigorously in a 150-mL separatory funnel for 10 min. The mixture was allowed to stand in a constant temperature bath at 25 OC until the two separated phases were clear (2-3 h). The lower organic phase was drained into a 250-mL Erlenmeyer flask, 10 mL of acid mixed indicator solution was added to it, arid the anionic surfactant content was determined by titration with approximately 1 X Hyamine 1622 in the usual fashion. RESULTS AND DISCUSSION Results obtained by using the standard two-phase mixed indicator titration method (procedure A, with chloroform as the organic phase) are shovvn in Table I. The assay percentage is based upon an assumed 100% purity for the vacuum-dried surfactant. It is apparent that the standard procedure is not
1517
Table 11. Effect of CHC1,:l-Nitropropane Ratio and Surfactant Concentration on Sodium Decanesulfonate Assay (Procedure A ) % of surfactant
CHCl,/NP ratio (v/v)
surfactant concn, M
100% CHCI, 100% CHCl, 100%CHCl, 4:l 3:2
1.017 X lo-, 2.035 X lo-' 4.109 X lo-, 1.013 X 1.013 X lo-, 1.013 X 1.017 X lo-' 2.035 X lo-, 4.109 X lo-, 1.013 X lo-'
1:l
2:3 2:3 2:3 1:4
in organic phasea 5.62
15.45
assay, %
89.2, 92.9, 93.7, 90.4, 90.9, 92.6, 93.9, 95.2, 95.4, 93.7,
After addition of 10 mL of acid mixed indicator solution. 20 mL of surfactant solution used. in aqueous phose
R-
R-. D++ D-
+D-
fD+
($;icay/
(indicator (anionic surfactant1 1 1
/ /
in organic phase
+ R,N+
R-.D+ (pink1
Z=
R,N+.
R-
(Hyaminel (colorless1
ot end m i n t
R 4 N + + D-
R, N'.
D'
( b l u e , organic phose I
Figure 1. Equilibria involved in the two-phase, mixed indicator titration method for anionic surfactants.
adequate for surfactants with alkyl chains containing less that 12 carbon atoms. The last column shows the percentage of the anionic surfactant present in the chloroform phase after the addition of the acid mixed indicator solution. It is noteworthy that, in the absence of acid indicator solution, almost all of the surfactants in Table I show no significant solubility in the CHC1, layer. There is an almost linear increase in the assay percentage with increase in the percentage of anionic surfactant soluble in the chloroform layer in the presence of acid mixed indicator solution. For the titration to be quantitative, it appears that about 19% of the surfactant must be in the chloroform phase prior to the addition of the Hyamine solution. The procedure for determining the percentage of surfactant soluble in the chloroform layer in the presence of acid mixed indicator can, therefore, be used as a screening test for determining the suitability of the two-phase mixed indicator titration for analyzing an anionic surfactant of unspecified chain length. In view of these results, it was felt that the substitution of some of the chloroform by water-immiscible organic solvent of greater polarity might permit the extension of the method to surfactants of shorter chain length. Several solvents were tried and 1-nitropropane (NP) was chosen for further investigation. Table I1 shows some results obtained with different chloroform:nitropropane ratios, using sodium decanesulfonate. Since very low ch1oroform:nitropropane ratios tended to produce emulsions when shaken with the surfactant solution, the 2:3 chloroform:nitropropane ratio was used in subsequent work. Table I1 also shows the effect of change in the concentration of the surfactant solution. Similar experiments on sodium dodecane and tetradecanesulfonate showed no significant change in the assay with change in the concentration of the surfactant solution in the (1-4) X lo5 M range when either CHC13or 2:3 CHC13:NPwas used as the organic phase. The reactions which form the basis for this analytical method can be formulated as shown in Figure 1. It is clear, from the results obtained, that the reason for the low assays in the case of the surfactants with short alkyl
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ANALYTICAL CHEMISTRY, VOL. 53, NO.
9, AUGUST 1981
Table 111. Results Obtained by Procedure B (Surfactant Concentration surfactant structure
-
organic phase
CIOH,, SO3Na
1 X lo-, M)
assay, % 1st end point 2nd end point 3rd end point
CHC1, CHC1, CHC1, 2 :3 CHC1, :NPC 2:3 CHCl,:NPC 2:3 CHCl,:NPC 2:3 CHC1,:NPC 2:3 CHC1,:NPC 2:3 CHC13:NPC 2:3 CHC1,:NPC 2:3 CHCl,:NPC 2:3 CHCI,:NPC
C10H21S03Na
C,,H,,SO,Nab C10H21S03Na
C,,H,,SO,Naa C,,H,,SO,Nab C6H,,0C,H,0C,H,S0,Na C,H,, OOCCH,CH( SO,Na)COOC,H,, C,H,,OOCCH,CH( S03Na)COOC,H,, C,H1,OOCCH,CH( SO,Na)COOC,H,, C12H2$03Na C14H2’3S03Na
a Concentration of surfactant solution is approximately 2 x mately 4 X lo-, M. 1-Nitropropane.
lo-, M.
89.2, 92.9, 93.7, 93.9, 95.2, 95.4, 94.8, 97.8, 98.2, 99.5, 99.3, 99.2,
93.8, 96.1, 96.4, 98.5, 99.4, 99.4, 99.1, 99.2, 99.2, 99.5, 99.3, 99.2,
91.5, 99.4, 99.4, 99.4, 99.1, 99.2, 99.2,
Concentration of surfactant solution is approxi-
Table IV. Effect of Electrolyte on Assay of Sodium Dodecanesulfonatea % at concn of electrolyte in aqueous phase (molar)
electrolyte
a
0
1 x 10-3
5 x 10-3 2 x
10-2
5 x 10-2
NaCl 99.1, 99.1, 99.1, 99.3, Na,SO, 99.1, 99.1, 99.1, 99.1, NaH,PO, 99.1, 99.1, 99.2, 99.1, CaC1, 99.1, 99.1, 99.1, 99.3, 99.1, 99.1, 99.1, 99.3, MgCl, 99.1, 99.1, 99.1, 99.1, MgSO 4 Procedure A; CHC1, organic phase; surfactant concentration
1x
10-1
99.4, 99.6, 99.1, 99.1, 99.1, 98.9, 99.6, 100.6, 99.6, 100.4, 99.1, 99.1, 1 X lo-, M.
-
2 x 10-1
3 x 10-1
100.4, 99.1, 98.6, 102.2, 101.4, 99.1,
101.1, 99.1, 98.3, 103.1, 102.4, 99.1,
4x
10-1
101.9, 99.1,
99.1,
Table V. Effect of Electrolyte on Assay of Sodium Dodecanesulfonatea % at concn of electrolyte in aqueous phase (molar)
electrolyte
0
1 x 10-3
5 x 10-3
2x
10-2
5x
10-2
1x
2x
10-2
io-,
NaCl 99.3, 99.3, 99.3, 99.6, 99.8, 102.0, 105.2, 99.0, 99.3, 99.3, 99.3, 99.3, 99.3, 99.3, Na,SO, 98.8, 98.5, 99.3, 99.3, 99.4, 99.2, 99.2, NaH,PO, 110.6, 100.9, 102.6, 105.3, CaC1, 99.3, 99.5, 100.0, 111.0, 102.4, 105.3, 99.5, 100.6, 99.3, 99.3, MgC1, 99.1, 99.1, 99.1, 99.1, 99.1, 99.1, MBSO, 99.3, a Procedure A; 2:3 CHC1,:l-nitropropane organic phase; surfactant concentration 1 X M.
-
chains is the low solubility of their comlex with indicator I in the organic phase. The anionic-indicator I complex remaining in the aqueous phase remains untitrated by the Hyamine 1622. (Measurements of the partition of Hyamine 1622 between the aqueous and organic phases used in these experiments in the presence of the acid mixed indicator in the absence of anionic surfactant showed that 85-95% of the Hyamine 1622 is in the organic phase.) In order to titrate this remaining anionic surfactant, the initial titration with Hyamine was conducted in a separatory funnel (procedure B, Experimental Section), rather than in an Erlenmeyer flask and, when the end point of the titration had been reached, the organic (lower) phase was withdrawn and replaced by fresh organic phase. After being shaken vigorously, the mixture showed a pink color in the organic phase indicating that additional anionic surfactant-indicator I complex had been extracted from the aqueous phase into the organic phase. The titration with Hyamine 1622 was continued to a second end point. This extraction-titration procedure wm repeated until no significant additional quantity of Hyamine was required to reach an end point. Table I11 shows some results obtained by use of this procedure. From the data, it appears that procedure B with a 2 3 (v/v) mixture of chloroform:l-nitropropaneas the organic phase is suitable for analyzing all of the surfactants tested. Only two titrations
3x
io-,
4x
io-,
109.1, 99.3, 98.2,
111.4, 99.3,
99.1,
99.1,
are required to analyze most of the surfactants at a concentration of 1 X low3M, in the case of sodium decanesulfonate, two titrations are required for concentrations of 2 X M M. or higher, three for concentrations of 1 X The last factor investigated was the effect of electrolyte on the end point of the titration, both with chloroform as the organic phase and with 2 3 chloroform:l-nitropropanemixture. For these experiments, sodium dodecanesulfonate,which gives satisfactory and comparable results with both these organic phases, was used. Results are given in Tables IV and V. The data indicate that C1- is the only ion tested that has an effect on the titration result, both in CHC& and in CHC13-NP. (The effect of HzPO; at high concentrations is most probably due to its effect on the pH of the aqueous phase.) C1- at concentrations in excess of 5 X low3M in the aqueous phase causes overtitration when either chloroform or 2:3 ch1oroform:l-nitropropaneis used as the organic phase. Na+, Mg2+,and SO:- at concentrations of up to 4 X 10-1M in the aqueous phase do not interfere with the analysis of sodium dodecanesulfonate. The C1- effect is probably due to its depression of the ionization of Hyamine 1622 which is a quaternary aminium chloride, thereby making the quaternary aminium ion less available for reaction with the anionic surfactant-indicator I complex. This common-ion effect of C1- has been noted previously by Longman (5) when methy-
Anal. Chem. 1981, 53, 1519-1522
1519
(3) Chiang, Y. C. Master’s Thesis, Brooklyn College, Clty University of New York, Brooklyn, NY, July, 1978. (4) Rosen, M.; Goldsmith, H. A. “Systematic Analysis of Surface-Actlve Agents”, 2nd ed.; Wiley-Interscience: New York, 1972; pp 427-428. (5) Longman, G. F. “The Analysis of Detergents and Detergent Products”;
lene blue is used as the indicator in the two-Dhase titration of anionic with quaternary aminium chloride but is in contradiction to his statement that this effect of C1- is not observed with other indicators.
Wilev-Intersclence: New York.
LITERATURE CITED Alston, T.; Heinerth, E. Tenslde 1967, 4 , 292-304.
(1) Reid, V. W.; (2) Reid, V. W.; Alston, T.; Heinerth, E. Tenslde 1968, 5, 90-96.
1975: D 227.
RECEIVED for review March 9,1981. Accepted May 18,1981.
Atmospheric: Pressure Active Nitrogen Afterglow as a Detector for Gas Chromatography G. W.
Rice, J. J. Richard, A. P. D’Silva, and V. A. Fassel”
Ames Laboratory‘ and De)uartmentof Chemlstry, Iowa State University, Ames, Iowa 5001 1
An atmospheric pressuire active nitrogen (APAN) afterglow has been evaluated as a selective detector for gas chromatography (GC). Selective detection for organo Hg, Pb, and Sn species at trace levels has been achleved. The GC-APAN system has also been observed to be a nonselective detector for organic molecules at nanogram levels through the detection of CN (B2E+)emls!slon at 388.3 nm. The merits of the APAN detector In comparison to other emlsslon-selectlve detectors is discussed.
Gas chromatography coupled with element-selective detectors is widely utilized iin speciation studies of environmental pollutants, a subject of substantial current interest. In such studies an extended list of detection systems have been investigated to enhance the limits of detection. A class of such detectors is the microwave-induced plasma (MIP) operated a t low or atmospheric pressure with He or Ar as the plasma gas (1-4). Of the MIP detectors, the atmospheric pressure MIP in He, operated in a Beenakker type TMolo cylindrical resonant cavity is potentially the most versatile element-selective detector (5). Unfortunately, the MIP detectors have several operational limitations. The injection of relatively small amounts (microliters) of species into the plasma gas, such as a solvent plug from a GC or excessive hydrogen from dissociated hydrocarbons, can decouple the resonant cavity, resulting in total quenching of the discharge. Such a limitation has been circumvented by adopting one or more experimental manipulations. They are: (a) initiation of the discharge after the solvent peak elutes out of the resonant cavity, (b) selection of a solvent that elutes at the tail of the sample elution sequence, (c) utilization of elaborate gas switching instrumentation and routines to control the flow of GC effluents into the MIP, or (d) the use of fused silica capillary columns for reduced sample volumes. A limitation of MIP sources operated a t reduced pressure is the buildup of carbonaceous deposita\ on the walls of the discharge with effluents containing carbon. Such deposits have to be periodically burnt off in situ, by operating the discharge in a mixture of He with 0 2 or Nz (3). To minimize the above ‘Operated for the U.S. Department of Ener y by Iowa State University under Contract No. W-7405-En 82. khis research was su ported by the Division of Chemical biiences, Budget Code K8-03-02-03, Office of Energy Research. 0003-2700/81/0353-1519$01.25/0
limit-+ions, alternate element-specific detectors such as the direct current atmospheric pressure argon plasma (6, 7) and inductively coupled plasma (8,9) have also been evaluated. A chemiluminescence detector, based on the reaction of organic compounds with active nitrogen (AN) sustained a t low pressure (20-30 torr) to form excited CN (B22+)molecules, has been recently reported (10, 11). The low-pressure AN source, however, is susceptible to a build up of polymeric deposits containing CN and thus requires periodic cleaning similar in nature to the MIP. The addition of HC1 gas is also required to enhance the sensitivity for simple saturated hydrocarbon compounds. Finally, the fact that the “chemiluminescent” detector operates at low pressure whereas the GC operates at atmospheric pressure may also introduce some inconvenience in sample introduction. The reactions of a variety of organic and organometallic compounds with metastable atoms and molecules of nitrogen present in low-pressure active nitrogen sources have been extensively investivated over several decades (12). In such reactions, the fragmentation and excitation of species, which lead to characteristic atomic and/or molecular emission, has been predicted to be the result of energy transfer from metastable atomic and molecular nitrogen present in the discharge source. In a recent communication we described the operation and performance of an atmospheric pressure active nitrogen (APAN) afterglow as an efficient source for the excitation of atomic emission (13). In the present paper we report on the performance of the APAN afterglow as an element-selective GC detector.
EXPERIMENTAL PROCEDURES Apparatus. The electrodeless ozonizer type discharge tube utilized in these investigations was slightly modified from the design previously described (13). The outer tube diameter was reduced from 28 mm to 18 mm and cooling of the inner tube eliminated. A 2 mm i.d. sample inlet was used for connection to the GC interface. A conventional GC (Hewlett-Packard, Model 5700A) was modified by cutting a 3 cm diameter hole through the side of the oven wall to accommodate the transfer line from the GC column to the APAN discharge tube. The interface constructed for the transfer line is shown in Figure 1. The glass capillary transfer line minimized peak broadening from dead volume between the GC column and the discharge tube. Copper tubing wrapped with nichrome wire was used to produce uniform heating around the transfer line. A variable temperature control (Omega Engineering, 0 1981 American Chemical Society
