surface waters should be particularly attractive, especially since sulfonated surfactants do not interfere. Determination of sulfate in biological fluids should also be quite practical. The low level Of sulfate this method permits One to reach should offer an opportunity to study the role of this common ion in many biological processes. While the method should be able to accept most such Samples without any treatment, it might be well to precipitate the protein material and run the analyses on the filtrate to avoid fouling the digestion vessel with organic material. The possible interferences due
to oxidizing agents and the ions listed in Table I should be kept in mind when choosing a protein precipitating agent.
for review May 19, 1971. Accepted October 29, 1971, Presented in part at the Combined Southeast-Southwest Regional Meeting, A.C.S., Memphis, Term,, December 1965. Taken in part from the dissertation,@ 1965, submitted by Joe B. Davis to the Graduate School of Ciemson University in partial fulfillment of the requirements for the degree of doctor of philosophy, July 1965. Work supported in part by National Defense Education Act Fellowship.
Spectrophotometric Analysis of Phenols and of Sulfonates by Formation of an Azo Dye L. Ronald Whitlock,I Sidney Siggia, and Janice E. Smo1a2 Department of Chemistry, University of Massachusetts, Amherst, Mass. 01002 Phenols and sulfonic acids are analyzed by a method based on the coupling reaction between a diazotized amine and the phenol. The concentration of the azo dye formed was measured by visible spectrophotometry. Sulfonic acids and their salts were converted to phenols via alkali fusion at 360 O C using potassium hydroxide, prior to coupling. Diazotized sulfanilic acid and p-phenylazoaniline, a previously unreported reagent, were particularly useful reagents for quantitative phenol analysis. With the latter, the optimum pH and reaction time for the phenols were remarkably similar and the molar absorptivities of the dyes formed were significantly higher. The coupling reaction conditions for a wide variety of phenols and the c and Xmsx values for the azo dyes are presented. The limit of detection was about 1 pmole of phenol per liter of solution and the minimum amount detectable was 0.3 pg. This method for sulfonic acid analysis will extend the useful range of application of the alkali fusion method to include polysulfonates and higher molecular weight sulfonates. Alkali fusion of halogenated sulfonates resulted in substitution of both the halogen and sulfonate groups. To help determine which sulfonates can be analyzed by the fusion procedure, the thermal stabilities of a large number of sulfonates and several phenolates are given.
SULFONIC ACIDS have represented a difficult analysis because of their limited reactivity and lack of volatility. A method for their analysis has been described by Siggia, Whitlock, and Tao (1) based on alkali fusion giving quantitative conversion to sulfite and the corresponding phenol. By utilizing direct gas chromatographic analysis for the phenols formed, many sulfonic acids and salts including their mixture could be analyzed with speed, and good precision and accuracy. However, with the alkali fusion method some sulfonates give phenols of low volatility that cannot be measured by gas chromatography. These include the polysulfonated compounds, those sulfonates containing other very polar functional groups, and certain higher molecular weight sulfonates. To extend the alkali fusion method to include these sulfonates, l Present address, Research Laboratories, Eastman Kodak Company, Rochester, N.Y. 14650 Present address, W. R. Grace & Co., Cambridge, Mass. 02140
(1) S. Siggia, L. R. Whitlock, and J. C. Tao, ANAL.CHEM.,41, 1387 (1969).
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a method for phenol measurement based on the phenolic group reactivity has been developed. It is commonly known that substitution of a hydroxyl group on an aromatic ring, as occurs during alkali fusion of sulfonates, facilitates rapid electrophilic substitution. The position of attack is either para or ortho to the hydroxyl group. This effect has resulted in several methods for phenol measurement (2-6). The method described here is based on the coupling reaction between the phenol and a diazonium salt, producing a highly colored azo dye. The resulting dye concentration is measured quantitatively by visible spectrophotometry. This method of phenol analysis has been known for many years (7-11). The work concerning the diazo coupling reaction reported here was carried out, in part, so that a complete and workable procedure for use with the alkali fusion method for sulfonate analysis could be developed in detail. This includes a determination of the proper coupling reaction conditions for a wide variety of phenols and the E and Amax values for the azo dyes formed. In addition, a critical survey of several diazo compounds, including some not previously used, was made to determine which give the best results for phenol measurement. The problems commonly found in the analytical utility of the coupling reaction are nearly all derived from the coupling reaction conditions that must be controlled. A more extensive use of this method for phenol measurement has been hindered for two reasons. First, there is the general lack of information defining the proper reaction conditions needed for the many possible phenol-diazo combinations. Second, (2) B. Smith, Acta Chern. Scand., 10, 1589 (1956). (3) L. Tykken, R. S. Treseder, and V. Zahn, IND.ENG.CHEM., ANAL.ED., 18, 103 (1946). (4) H. H. Willard and A. L. Wooten, ANAL.CHEM., 22, 423, 670 (1950). (5) C. U. Houghton and R. G. Petty, Analyst, 62, 117 (1937). (6) C. J. B. Smit, M. A. Joslyn, and A. Lukton, ANAL.CHEM.,27, 1159 (1955). (7) R. H. DeMeio, Science, 108, 391 (1948). (8) G. A. Lugg, ANAL. CHEM., 35, 899 (1963). (9) W. Lee and J. H. Tumbull, Talarzfa,3, 318 (1960). (10) T. Takeuchi, M. Furusawa, and Y.Takayama, Jap. Anal., 4, 568 (1955). (11) G. B. Crump, ANAL. CHEM., 36,2447 (1964).
there is a need for diazo reagents that are more stable and whose coupling reactions are less sensitive to reaction conditions, such as p H and time. The method developed meets the need for a sensitive analysis for both sulfonic acids and phenols. The range of application of the alkali fusion reaction is increased to include the polysulfonates and higher molecular weight sulfonates. EXPERIMENTAL Reagents. Sulfonic acids and sulfonate salts were obtained from Eastman Organic Chemicals, J. T. Baker Chemical Company, and the K & K Laboratories in the purest grade available. The sulfonate salts were further purified by recrystallization from alcohol-water or water alone and dried in a vacuum over anhydrous calcium chloride before use. Each was checked for purity by elemental analysis for sulfur. All amines used for diazotization were obtained from Eastman Organic Chemicals. p-Phenylazoaniline was recrystallized from acetone-water. Sulfanilic acid was recrystallized from alcohol-water. The other amines were used as received. Phenol and the related aromatic hydroxy compounds were obtained from Eastman Organic Chemicals, Aldrich Chemical Company, and J. T. Baker Chemical Company. The phenols were purified either by recrystallization from alcohol-water or by low pressure distillation. Identity and purity were determined by infrared analysis, melting point measurements, and elemental analysis. Potassium hydroxide (Mallinckrodt Chemical Works) and sodium acetate (Fisher Scientific) were reagent grade and used as received for the fusion reaction. Diazotization of Sulfanilic Acid. A five-millimolar solution of diazosulfanilic acid was prepared by dissolving 0.1 16 gram of sulfanilic acid sodium salt and 0.035 gram of sodium nitrite in approximately 50 ml of water. The solution was cooled to 0 "C on ice. To this solution was added 2 ml of 2N HC1 with vigorous stirring. The reaction was complete after a few minutes. The final volume was adjusted to 100 ml with additional cooled ( 5 "C) distilled water. If excess nitrous acid was present, as evidenced by the starch-iodide paper test, it was destroyed by the addition of a few drops of sulfamic acid solution. The reagentwas stable for three to five days when stored on ice (0 "C) and in darkness. Diazotization of p-Phenylazoaniline. A five-millimolar solution of diazo-p-phenylazoaniline was prepared by dissolving 0.099 gram of p-phenylazoaniline in 10 ml of acetone followed by the addition of 30 ml of water and 5 ml of 2N HC1 to the acetone solution. The temperature was then lowered to 15 "C. A solution of 20 ml of water and 0.035 gram of sodium nitrite was added with stirring over a twenty-minute period. A very slight excess of nitrous acid should be indicated on starchiodide paper. The final solution volume was adjusted to 100 ml with cooled distilled water. This reagent was stable for approximately one week when stored on ice (0 "C) and in darkness. Apparatus. All azo dye absorption measurements and spectra were recorded on a Perkin-Elmer 202 UltravioletVisible spectrophotometer. Matched NIR Silica cells (Beckman Instruments) with 1-cm path length were used to hold all solutions for measurement. Wavelength measurements were corrected to the 461 nm absorption band of holmium oxide glass. All LR spectra of the fusion reaction products were recorded on a Model 137 sodium chloride spectrophotometer (PerkinElmer). The apparatus and sample boats used for the fusion reaction were identical to those described previously (I), with the exception that the fusion unit was not connected to a gas chromatograph. Helium flowing at 40 ml/min. through the fusion oven assembly was used as a purge gas during the fusion reaction procedure. This was necessary since phenols
are known to oxidize by the action of oxygen, especially in alkaline solution (12). Procedure. PHENOLANALYSIS.Stock phenol solutions were prepared by dissolving several milligrams of phenol, accurately determined, in 100-ml volumetric flasks to give solutions containing approximately 0.5 millimolar phenol. Aliquots of these solutions or of further diluted stock solutions were taken for analysis. The coupling reaction was carried out in 50-ml volumetric flasks by first adding to the flask a 5-ml aliquot of the phenol stock solution to give a final concentration of 2 to 50 micromolar phenol. Next 20 ml of water was added along with a predetermined amount of 0.1 M sodium bicarbonate solution used to adjust the pH to the optimum value for coupling. When using diazotized p-phenylazoaniline, 25 ml of tetrahydrofuran was added to prevent precipitation of the azo dye formed and to accelerate the coupling reaction. Finally, a 1.0-ml aliquot of the diazonium stock solution was added with thorough mixing, and the solution volume was adjusted to the mark. For each phenol tested, the optimum reaction conditions were established by following a standard procedure. First, the effect of pH was determined by using p H values of 7.1, 7.4, 7.7, 8.1, and 8.5. Once the pH value at which maximum reaction occurred was found, additional coupling reactions were run to determine at which time interval after reaction the solution absorbance reached a stable maximum. Additionally, for some phenols, the amount of excess diazo was varied over wide limits to determine if this had an effect on the reaction. Usually none was found and a 3- to 5-fold molar excess of the diazo was normally maintained. SULFONICACID ANALYSIS.Samples were prepared for fusion by first weighing into a platinum sample boat between 3 and 10 mg of sulfonate. Approximately 30 mg of solid potassium hydroxide was added along with 5 to 8 mg of sodium acetate to act as a flux. After the sample boat was placed in the fusion oven, the system was flushed with helium gas for several minutes and the oven temperature was increased slowly to 360 "C for fusion. After the fusion reaction was complete, usually in 15 minutes, the sample boat was removed from the fusion apparatus and quickly placed into a 50-ml volumetric flask containing water and sufficient hydrochloric acid to neutralize the caustic. The final pH was adjusted to 7.0 with either dilute hydrochloric acid or sodium bicarbonate and the soluion volume was adjusted to the mark. Aliquots of this solution were then taken for spectrophotometric measurement following the same procedure used for the phenol standards. Quantitative results were obtained by comparing the absorbance measurements to a standard curve. RESULTS AND DISCUSSION
Coupling Reaction. The coupling reaction was studied using a wide variety of phenols to determine the range of application of the method. In addition, several different diazotized amines were studied to evaluate their analytical utility for phenol measurement. The diazo coupling reaction is often dependent on reaction conditions. The proper set of conditions will vary depending on the phenol being determined and the choice of diazo used for coupling, Also, the molar absorptivity, e, and wavelength of maximum absorption, A,., of the azo dye formed will depend on the diazonium salt used for coupling. One amine chosen for study was sulfanilic acid. This is a commonly used reagent [13), It was studied here to determine its full range of utility as well as the optimum coupling conditions for quantiative phenol measurement. Sulfanilic acid gives a fairly active diazo (many times stronger than (12) J. Cason, "Organic Reactions," Vol. 14, John Wiley and Sons, New York, N.Y., 1948, Chapter 6. (13) J. J. Fox and J. H. Gauge, J. Chem. Ind., 39, 206 (1920). ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
533
Table I. Reaction Conditions, A,, and e Values for Phenols Coupled with Diazotized Sulfanilic Acid Reaction Molar Compound min pH absorptivity Amax, nm Phenol 2 8.5 21 ,000 450 o-Cresol 2 8.5 22,100 458 m-Cresol 2 8.5 18,200 434 2 7.6 43,500 440 Resorcinol Phloroglucinol 2 7.1 51,000 440 m-Hydroxybenzoic acid 15 8.1 14,100 414 p-Hydroxybenzoic acid 15 7.8 13,800 430 m-Aminophenol 5 7.5 30,000 450 m-Chlorophenol 5 8.5 22,500 432 2 7.5 25,200 448 o-Iodophenol o-Phenylphenol 2 7.5 19,300 460 7.1 25,000 520 a-Naphthol 15 0-Naphthol 15 7.1 21,800 492 2,7-Naphthalenediol 15 7.1 21,500 492 Table 11. Reaction Conditions, A,, and c Values for Phenols Coupled with Diazotized pPhenylazoanilinea,b Molar A,, nm Compound absorptivity Resorcinol 58,500 48 5 Phloroglucinol 85,000 495 m-Aminophenol 38,500 496 o-Cresol 27,500 535 m-Cresol 26,000 545 a-Naphthol 37,500 520 0-Naphthol 30,000 512 2,7-Naphthalenediol 30,000 510 1,5-Naphthalenediol 38,000 650 a pH for reaction of each phenol was 7.5. Time between reaction and spectrophotometric measurement was three minutes for each phenol.
aniline), possesses high water solubility, is easily diazotized, and the reagent solution remains stable for several days if stored at 0 "C. in darkness. The analytical data collected for each phenol is shown in Table I. The data include pH, time between reaction and spectrophotometric measurement, A,, and e. Also for each phenol listed, a linear plot consisting of four or five different phenol concentrations us. their absorbance was constructed which indicated the compound followed Beer's law. Many phenols, both those activated as well as deactivated by the presence of other functional groups, coupled satisfactorily. High molar absorptivities, up to 51,000, ensured that low concentrations of phenols could be determined with confidence. However, some phenols not included in Table I gave incomplete coupling and cannot be measured using diazotized sulfanilic acid. These include the nitrophenols, hydroquinone, pyrogallol, and some para substituted phenols. Diazotized p-nitroaniline is considerably more electrophilic than diazosulfanilic acid and is known to couple with some less active phenols (7, 9, 14). The results obtained, however, indicated that this reagent is not useful for quantitative analysis for those phenols not already easily determined using diazosulfanilic acid. Difficulties include multiple coupling, much lower molar absorptivities, and the Amax values shifted to considerably shorter wavelengths. Many of the dyes formed were water insoluble, requiring the use of organic solvents (14) J. A. Pearl and P. F. McCoy, ANAL.CHEM., 32, 1407 (1960). 534
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
Table 111. Spectrophotometric Determination of Various Phenols by Formation of an Azo Dye Moles ( X 106) Diazonium Compound Taken Found Phenol a 0.90 0.91 m-Cresol a 1.63 1.66 Resorcinol a 1.31 1.28 p-Hydroxybenzoic acid a 2.63 2.55 m-Hydroxybenzoic acid a 1.15 1.11 o-Iodophenol a 5.50 5.45 0-Naphthol a 3.08 3.10 m-Aminophenol U 4.21 4.26 o-Phenylphenol a 1.05 1.04 Resorcinol b 0.35 0.35 Phloroglucinol b 0.24 0.25 o-Cresol b 1.88 1.85 m- Aminophenol 2.00 2.04 b a-Naphthol b 1.65 1.66 Diazosulfanilic acid. b
Diazo-p-phenylazoaniline.
in the procedure. Also, the diazonium salt has very limited stability. This reagent may be useful for qualitative detection since partial coupling was observed with a larger number of phenols. Diazotized 4-amino-1-naphthalenesulfonic acid was studied primarily to increase the molar absorptivity of the resulting azo dyes. This would increase the sensitivity of the method. The results obtained, however, indicated that this reagent too, is not useful for quantitative phenol measurement. Several distinct advantages were realized when diazotized p-phenylazoaniline was used for phenol analysis. This amine apparently has not been used before as a reagent for phenol analysis. It possesses a considerable amount of extended conjugation; once coupled to phenols, azo dyes with very high absorptivities are formed. Increases of 10,000 to 30,000 absorptivity units over dyes formed from diazo sulfanilic acid were common. The Amax and e values for each phenol studied using this reagent are shown in Table 11. Besides the higher e values obtained with this reagent, the pH dependence of the coupling reaction was eliminated. For all phenols studied, the optimum pH of coupling was 7.5. Similarly, the optimum time between reaction and measurement was the same for each phenol. An additional advantage is that the diazonium salt is very stable. It was commonly used for a week or longer when stored on ice (0 "C) and in darkness with no noticeable change in its ultraviolet or visible spectrum. The reagent solution has a light yellow color above pH 7.0. An absorbance value between 0.05 and 0.10 was normally observed in the wavelength region used for the phenol measurement. This blank value' was measured and subtracted from the dye absorbance value for each phenol determination. In order for the coupling reaction to proceed smoothly to completion, and also to prevent possible precipitation of the azo dye, tetrahydrofuran was added to the phenol sample before the diazo reagent was introduced. The final solution volume contained a 50: 50 mixture of tetrahydrofuran and water. Phenol Analysis, Some analyses of phenol samples in water solution are given in Table 111. Data using both diazotized sulfanilic acid and p-phenylazoaniline are included. The results were obtained by reacting known concentrations of the phenols and comparing their absorbance values to a calibration curve prepared for each of the phenols. An advantage of this method of phenol measurement is that very small amounts can be measured. For example,
Table V. Sulfonate Analysis by Alkali Fusion and Measurement of the Phenol by Formation of an Azo Dyea
1
I
I
4 00
I
I
I
I
6r30 WAVELENGTH (nm)
I
i
700
500
Figure 1. Visible absorption spectrum of a mixture of 1,5- and ZY7-naphthalenediolcoupled with diazo-p-phenylazoaniline Spectrum A. Solvent blank Spectrum B. Reagent blank (diazo-p-phenylazoaniline at pH 7.5) Spectrum C. Mixture Solvent: THF/H20, 50 :50 Table IV. Spectrophotometric Analysis of Naphthalenediol Isomers by the Method of Simultaneous Equations 1,5-Naphthalenediol, 2,7-Naphthalenediol, moles ( X 105) moles ( X 105) Mixture Taken Found Taken Found A 0.75 0.72 1.15 1.20 1.73 1.78 B 1.50 1.44 2.88 2.89 C 2.25 2.16
assuming an e of 5 x 104 and that 0.05 absorbance unit can be accurately measured, the limit of detection is 1.0 micromole of phenol per liter of solution. For a cell volume of 2 ml, the minimum amount detectable is 0.3 pg. The total phenolic content of a sample containing mixtures of phenols can be measured from a single coupling reaction. This is done by comparing the absorbance value of the unknown mixture to a calibration curve prepared using any chosen standard, such as phenol. The resulting concentration is then expressed as per cent of the sample as phenol. For example, a solution containing 1.6 X 10.-5Mphenol, 1.1 X 10-5M o-cresol, and 2.5 X lO-6M m-cresol was determined to contain 4.9 X lO-6Mas phenol. The error is about 5 %. The use of simultaneous equations permits the analysis for individual components present in a mixture. A unique application of this technique is shown for the analysis of the 13- and 2,7-naphthalenediol isomers using diazo-p-phenylazoaniline for coupling. The ,A, values for the 1,5- and 2,7- isomers are 650 nm and 510 nm, respectively, an unusually large separation. The simultaneous equations constructed based on their azo dye absorption spectra with the appropriate substitution of molar absorptivities at each wavelength are as follows: A610
=
8,850 c i , s f 30,000
A650 =
+0
38,000c1,5
c2,7
c2,7
-
- B6io B650
where B H Oand Beso are the reagent blank absorbance values for the diazonium solution alone, and ClVsand C?,, are the molar concentrations of each isomer. The results showing three different mixtures of these isomers are given in Table IV. A typical absorption spectrum for the mixture is shown in Figure 1. Sulfonic Acid Analysis. Once a reliable procedure was developed for phenol measurement, the method was combined with the alkali fusion reaction for sulfonate analysis, To
Analysis, Compound mole %* 2-Naphthalenesulfonic acid sodium salt 99.2 (4) Benzenesulfonic acid sodium salt 98.7(5) rn-Benzenedisulfonicacid disodium salt 92.5 (6) rn-Benzenedisulfonicacid disodium salt (purified) loo.2 (5) 1,3,5-Benzenetrisulfonicacid trisodium salt 97.3 ( 5 ) 2,7-Naphthalenedisulfonicacid disodium salt 98.4(6) Diazotized sulfanilic acid was used for coupling. * Figure in parentheses represents number of trials.
Standard deviation 1.9 2.0 1.8 1.7 3.6 2.0
Table VI. Decomposition Temperature of Some Sulfonic Acids, Alkali Metal Sulfonates, and Alkali Metal Phenolates in Air and Helium Atmospheres4 Decomposition temperature, "C Compound Air Helium Potassium phenolate 215 420 Potassium 2-naphtholate 250 480 Sodium p-nitrophenolate ... 350** Benzenesulfonic acid sodium salt 520 520 m-Benzenedisulfonicacid disodium salt ... 570 1,3,5-€%enzenetrisulfonic acid trisodium salt ... 560 p-Sulfobenzoic acid monopotassium salt 390 405** m-Sulfobenzoic acid monosodium salt 430** ... p-Chlorobenzenesulfonic acid sodium salt 450 4 50 p-Aminobenzenesulfonicacid sodium salt 255 ... rn-Nitrobenzenesulfonicacid sodium salt 4oo** ... p-Acetylbenzenesulfonic acid sodium salt ... 350 Dodecylbenzenesulfonic acid sodium salt 400 400 p-Diphenylaminesulfonic acid sodium salt 420 420 1-Naphthalenesulfonicacid sodium salt ... 480 2-Naphthalenesulfonic acid sodium salt ... 510 2,7-Naphthalenedisulfonicacid disodium salt ... 520 4-Amino-1-naphthalenesulfonic acid sodium salt 285 ... 1-Anthraquinonesulfonic acid sodium ... 450** salt 2-Anthraquinonesulfonic acid sodium ... 430** salt ... 365* m-Aminobenzenesulfonic acid p-Aminobenzenesulfonicacid 260* ... p-Toluenesulfonic acid 270* No asterisk, determined by TGA; one asterisk, determined by DSC; two asterisks, determined by TGA, confirmed by DSC. I . .
properly test this combined procedure, two sulfonates which had been analyzed by the earlier, gas chromatographic procedure ( I ) were taken as test samples. Sulfanilic acid was used for the coupling reaction. The data from the analysis of benzenesulfonate and 2-naphthalenesulfonate are given in Table V. The results demonstrate that this new procedure works well for these sulfonates. An important group of sulfonates whose analysis was not previously attempted because of the low volatility of the phenols produced are the polysulfonates. The results of the analyses of this group of sulfonates are also given in Table V. In order to achieve complete conversion during the fusion of m-benzenedisulfonate and 1,3,5-benzenetrisulfonate, one ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
535
third longer fusion times were used (30 to 35 minutes at 380 “C). The fusion reaction products of each compound were clear white. An infrared analysis of each fusion product showed only resorcinol and phloroglucinol, respectively. The first samples of m-benzenedisulfonate were taken as received from the supplier. The average of six determinations was 92.5% as the disulfonate. However, when further purified samples were taken (elemental analysis showed 99.4% of the theoretical amount of sulfur) the average of five determinations was 100.4%. The fusion of 2,7naphthalenedisulfonate was found to go to completion without difficulty at the normal fusion conditions. The average of the standard deviations for all the compounds given in Table V is about 2 . 0 x for sample sizes between 3 and 10 mg. The alkali fusion of p-chlorobenzenesulfonategave 50 % conversion to resorcinol rather than the predicted product, p-chlorophenol. An NMR spectrum of the starting sulfonate showed that it was indeed, para substituted. However, an infrared identification of the fusion products showed only the presence of resorcinol. A probable explanation of this result is that both the halogen and sulfonate groups of halogenated sulfonates undergo alkaline hydrolysis. An eliminationaddition mechanism with formation of a benzyne intermediate which leads to the resorcinol formation (15) has been suggested. When utilizing a high temperature reaction such as alkali fusion, the possibility of thermal degradation of the starting
sample or of the reaction product is always present. To help more easily establish first, which sulfonates can be analyzed by fusion and second, the upper limit for their fusion temperature that can be safely used, an examination of their thermal stabilities is suggested. The data obtained for a large number of sulfonates and several phenolates are presented in Table VI. This aspect of the fusion reaction was studied using thermogravimetry in an atmosphere of both air and helium. Most sulfonate salts examined here were stable to temperatures well above 400 “C and some are stable above 500 “C. Sulfonic acids, however, start to decompose at much lower temperatures than their corresponding salts. The acids can be analyzed without difficulty, however, because they are quickly neutralized by the alkali, and fusion of the potassium sulfonate salt proceeds normally. The phenolates are much more stable in a helium atmosphere than in air. For this reason, the fusion reaction oven must be continuously purged with a flow of helium.
RECEIVED for review August 9, 1971. Accepted October 14, 1971. This work was supported by the National Science Foundation, Grant GP 12171.
(15) S. Oea, N. Furukawa, and T. Asari, BUN. Chew. SOC.Jap.,
42, 177 (1969).
A Compound Classifier Based on Computer Analysis of Low Resolution Mass Spectral Data Geochemical and Environmental Applications Dennis H. Smith’ Organic Geochemistry Unit, School of Chemistry, Bristol University, Bristol BSB ITS, England
A computer-based method fok determination of chemical compound class based on low resolution mass spectral data has been developed. The method relies on computer analysis of sets of standard spectra, reducing these large data sets to a much smaller “correlation set.” The correlation set, consisting of “ion series spectra” of each class, is used in subsequent automatic computer classification of mass spectra. This approach is particularly important in analysis of data from coupled gas chromatograph/ mass spectrometer systems where large numbers of spectra of separated components of complex mixtures can be classified rapidly and further structural information elicited based on this classification. Although initially programmed for compound classes relevant to geochemical and environmental studies, the correlation set and structural information programs can easily be expanded to include classes important in other areas of research. The simplicity of the method lends itself readily to small computer or semiautomatic methods of data reduction and analysis. COMPLEX MIXTURES of organic compounds are encountered in many chemical investigations. The analysis of these mixtures presents formidable problems of separation, isolation, and identification of the many individual components present. In investigations in organic geochemistry and environmental 1 Present address, Department of Chemistry, Stanford University, Stanford, Calif. 94305.
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
chemistry, the situation is frequently further complicated by the availability of only small amounts of material. In these cases, every chemical separation or isolation procedure carries with it great risks of contamination and sample loss. These analytical problems are being countered to some extent by development of sophisticated instrumentation capable of dealing with small amounts of material, the use of computers in data handling and analysis, and new chemical procedures for derivatization and volatilization of organic material. The combined gas chromatograph/mass spectrometer (GC/ MS) has made the most significant impact in the area of instrumentation. When such an instrument is coupled to a digital computer for data acquisition and reduction, large amounts of data on the separated components of mixtures can be acquired rapidly and accurately, thereby exploiting the full capabilities of GC/MS in terms of sample throughput and providing data in a format suitable for further analysis (1-5). (1) R. A. Hites and K. Biemann, ANAL.CHEM., 40,1217 (1968). (2) C. C. Sweeley, B. D. Ray, W. I. Wood, J. F. Holland, and M. I. Krichevsky, ibid., 42,1505 (1970). (3) D. H. Smith, R . W. Olsen, F. C. Walls, and A. L. Burlingame, ibid., 43, 1796 (1971). (4) D. Henneberg and G. Schomberg, “Advances in Mass Spectrometry, Vol. 5,” A. Quayle, Ed,, The Institute of Petroleum, London, in press. (5) W. E. Reynolds, V. A. Bacon, J. C. Bridges, T. C. Coburn, B. Habern. J. Lederberg, E. C. Levinthal, E. Steed, and R. B. Tucker, ANAL.CHEM., 42,-1122 (1970).
