I 2dOo
C C
Figure 4. Spectrum of 0.9 ng of cellulose acetate with 10% triphenyl phosphate in a 50-pm aperture scanned 96000 times at 8 cm-'
resolution to 100 times the weight of the samples being considered. It was decided for these reasons that the samples would have to be examined neat. Sodium chloride was chosen as the most suitable support for the sample because of its infrared transmission and ease of cleavage. T h e use of microscopic techniques throughout the sample preparation has two advantages; the analyst is able to work with materials not visible to t h e naked eye, and microscopic physical and chemical separations enable relatively pure samples to be used, which greatly increases the probability of successful identification by infrared spectroscopy. Figure 1 is an infrared curve of 6 ng of triphenyl phosphate. This curve was used to identify an isolated 6-ng sample found in a manufacturing environment. Total analysis time was approximately 2 h of which 80% involved sample preparation. Figure 2 is the spectrum of 3.4 ng of cellulose acetate film base in a 100-pm aperture, and that in Figure 3 is of 0.9 ng
of the same sample in a 50-pm aperture. Both of these spectra are unsmoothed and were recorded after 2000 scans. The increased noise in Figure 3 shows a reduction in sample size and aperture. T h e spectrum in Figure 3 is identifiable as cellulose acetate even though less than 1 h of spectrometer time and less than 1 ng of sample were used. Figure 4 shows the improvement obtained by prolonged scanning (-40 h) of the sample described in Figure 3. The noise has been reduced to the extent that absorptions due to 90 pg of triphenyl phosphate, which is present a t approximately 10% in the film base, are recognizable. These absorptions have been marked with x's and may be compared with those in Figure 1. Microscopic sample manipulation combined with t h e sensitivity of FTIR spectroscopy allows us to identify previously unidentifiably small amounts of material. Work is under way to construct a stronger beam condenser to reduce data collection time and to improve sensitivity. T h e combination of this condenser with refinements in sample preparation should result in a t least a n additional 10-fold reduction in minimum identifiable sample size. The obtainable spectra are of sufficiently high quality that spectral subtractions will be feasible. Work is now under way to demonstrate the practicality and utility of this technique. Successful infrared spectroscopic analysis of materials at the nanogram to picogram level indicates that this technique, with all its advantages, can be considered as an ultra micro method.
ACKNOWLEDGMENT We thank Rex Wooton for his assistance in producing the spectra in Figures 1 through 4.
LITERATURE CITED (1) P. R. Griffiths and F. Block, Paper 329, 23rd Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, 1972. ( 2 ) S. S. T. King, J . Agric. Food Chern., 21, 526 (1973). (3) D. H. Anderson and T. E. Wilson, Ana,. Chem., 47, 2482 (1975).
RECEIVED for review July 18, 1977. Accepted September 26, 1977.
Spectrophotometric Determination of Acrylonitrile Maynard E. Hall" and John W. Stevens, Jr.' Univeristy of Arkansas, Graduate Institute of Technology, Post Office Box 30 17, Little Rock, Arkansas
A spectrophotometric method for the determination of acrylonitrile based on the absorbance of visible light of a pyridine-acrylonitrile complex at 41 1 nm has been developed. This complex Is formed in the presence of a basic hypochlorite solution at 60-65 OC. The molar absorptivity, based on acrylonitrile concentration, is 635.4. The precision of the method is f3.18% of the acrylonltrile present in the 5-30 ppm range. The complex color is stable for at least 20 mln after development.
T h e usefulness underlying t h e chemical analysis of acrylonitrile lies in the fact that it is widely used in the 'Present address, Dow Chemical Company, P.O. Box 520, Magnolia,
Ark. 7 2 7 5 3 .
72203
synthetic rubber, fiber, and plastic industries. In the United States alone in 1976 over 1.5 billion pounds were produced ( I ) . Sensitive and reliable analytical methods for acrylonitrile are necessary for economic and toxicological purposes. Acrylonitrile is known to be toxic when ingested, inhaled, or applied to the skin, and the U.S. Public Health Service has set the maximum allowable limit for the monomer in the air at 20 ppm (2). Recent studies by Du P o n t have linked acrylonitrile to cancer in workers and steps are being taken by Du Pont to reduce exposure to workers to 2 ppm ( 3 ) . Analytical procedures which have been used most for acrylonitrile are titrimetric (4-6), gas chromatographic (7-9), and polarographic (10-13). T h e titrimetric procedures are good for relatively high concentrations while the others can measure as little as 5 ppm in aqueous solutions. The few gas chromatographic procedures reported are sensitive but have been limited to nonaqueous systems. Polarographic methods ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
2277
are subject to many interferences and lack the necessary sensitivity a t the ppm level. I t was found in this laboratory that acrylonitrile forms at elevated temperatures, a highly colored (yellow) complex in t h e presence of pyridine, hydroxyl ions, water, and sodium hypochlorite. This report describes a sensitive method developed for the quantitative determination of acrylonitrile in aqueous systems based on the yellow colored pyridine complex. EXPERIMENTAL Apparatus. The recorded spectra were obtained with a Bausch and Lomb SP 600 spectrophotometer in conjunction with a Sargent SRL recorder. Some single wavelength absorbances were made with the Bausch and Lomb ‘io Manual spectrophotometer. Absorption cells were mainly 2.3-cm path length and in some work 1-cm cells were used. Ten-mL volumetric flasks were used for all sample solutions on which absorbances were measured. Reagents. Pyridine, obtained from Fisher Scientific Company, was purified by distillation over potassium hydroxide. The first 1 2 % and last 20% were discarded. Lithium hydroxide, purified grade, was obtained from Fisher Scientific Company. Solutions were prepared in molar strengths as specified. Sodium hypochlorite, brand name Purex Bleach, was obtained from a grocery store as a 6% solution in water. Acrylonitrile,Chromatographic grade 99+ mol (7c , was obtained from Analabs Incorporated. Preparation of Stock Solution of Acrylonitrile. Stock solutions of acrylonitrile were made by pipetting approximately 0.5 mL into a tared 100-mL volumetric flask containing 25 to 50 mL of distilled water, and reweighing to obtain the weight of acrylonitrile. In order to ensure the accuracy of weighing and the purity of the acrylonitrile, the titration method reported by R. P. Taubinger ( 4 ) was used to analyze the stock solutions. Procedure. To a 10-mL volumetric flask, add 7.5 mL of pyridine, 1 mL of 0.05 N lithium hydroxide solution, and 0.1 mL of 0.6% solution of sodium hypochlorite. To this mixture add 1.0 mL of the unknown acrylonitrile solution and then dilute to volume with distilled water. Mix thoroughly, and place in a water bath at 65 “C for 10 min. After cooling, absorbance values were taken as the difference between absorbances at 411 and 500 nm. This technique of measuring absorbances was chosen so that it could he readily adapted t o nonrecording spectrophotometers. DISCUSSION AND RESULTS Pyridine is known to react with several substances in t h e presence of base and hypochlorite to form highly colored complexes. The Fujiwara (14) reaction as reported by Fugg (15) is probably the best known. The Fujiwara reaction employs sodium hydroxide and pyridine in the determination of chlorinated hydrocarbons. A complex between the pyridine and the chlorinated hydrocarbons is formed which absorbs at a wavelength of 535 nm. The complex formed is not very well understood but the method is quite sensitive for some chlorinated hydrocarbons. The complex formed by acrylonitrile and pyridine under similar conditions described for chlorinated hydrocarbons absorbs at a much lower wavelength, 411 nm, and is probably quite different. T h e color forming complex with acrylonitrile is believed to be more similar to cyanogen chloride complexes formed with pyridine (16). Attempts were made to isolate the complex by extraction and column chromatography to establish its composition, hut it became unstable when removed from the pyridine-water solution. The acrylonitrile complex absorbs a t only a slightly higher wavelength than the cyanogen chloride complex shown below.
0 N+
-cf 2278
C ‘N
ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
Table I. Effect of Concentration of Water on Absorbance %
%
Sample
H,O
Pyridine
Absorbance
1 2
20 25 30
80 75
0.32
70
40
60
0.31 0.19
50 60 70
50 40
3 4 5 6
7
30
0.35 0.09 0.04 0.03
Table 11. Effect of Base Strength on Absorbance Sample 1 2 3
4 5
Final LiOH concn, N 0.0025 0.0050 0.0100 0.0200 0.0250
Absorbance 0.517 0.580 0.527 0.410
0.370
Effect of W a t e r Concentration. The solvent system for the acrylonitrile determination is a mixture of pyridine and water with pyridine being present in the greatest amount. The ratio of pyridine to water affects the amount of color forming species produced and this, of course, affects the sensitivity of the method. In order to achieve maximum sensitivity, the amount of water in the analysis mixture must be controlled. Seven solutions were prepared where the pyridine-water ratio was varied, but the amounts of lithium hydroxide and sodium hypochlorite were held constant. T h e absorbance values were measured for these seven solutions and are listed in Table I. The data show that approximately 25% water in the reaction mixture gives maximum color formation. Effect of L i t h i u m Hydroxide Concentration. In order for the yellow color reaction to develop for acrylonitrile in the presence of pyridine, water, and sodium hypochlorite, the solution must be basic. Lithium hydroxide was chosen as the base because of its greater solubility in organic solvents than sodium hydroxide. T h e pyridine-water ratio was held constant a t a 75-25 vol % , the hypochlorite was kept constant a t 0 . 2 % , and the amount of acrylonitrile was kept constant a t 45 pg. One mL of base was added to each solution but the amount of base was varied by using LiOH solutions of different concentrations. At lithium hydroxide concentrations of 0.25 N and higher, the LiOH precipitated when added to the 75-25 pyridine-water solvent. All solutions were made up to a 10-mL volume just as described in the study of water effects. The results of the base concentration studies are given in Table 11. These data indicate that a concentration of 0.005 N lithium hydroxide gives the maximum absorbance. E f f e c t of S o d i u m Hypochlorite Concentration. The formation of the yellow pyridine-acrylonitrile complex is more sensitive to the hypochlorite concentration than to t h e concentration of the other variables. In order to achieve maximum sensitivity, linearity, and reproducibility, the required amount of hypochlorite is very small and must be carefully controlled. T h e hypochlorite studies were made on several solutions of different acrylonitrile concentrations since the amount of hypochlorite is so critical. The pyridine-water ratio and the amount of base were held constant. T h e acrylonitrile concentrations ranged from 48 down to 10 ppm. One-tenth mL of sodium hypochlorite solution was added to each test solution and the amount of hypochlorite was varied by using hypochlorite solutions of different strengths. T h e results of these tests are given in Table 111. Effect of Time a n d T e m p e r a t u r e of Heating. Elevated temperatures were necessary to produce a colored complex,
Table 111. Effect of Sodium Hypochlorite Concentration on Absorbance Values Final concn
of NaOCl x 100, % 1.00 0.85 0.75 0.66 0.60
48 P
P ~
Absorbance A , , , - A 500 1.36 1.35 1.41 1.28 1.22 25 PPm
2.00 1.20 1.00 0.85 0.75 0.66 0.54
0.44 0.56 0.57 0.64 0.68 0.69 0.70 20 PPm
2.00 1.20 1.00 0.85 0.75 0.66 0.54
0.36 0.47 0.52 0.54 0.60 0.59
0.61 10 PPm
2.00 1.20
1.00
-
0.85 0.75 0.66 0.54
0.16 0.21 0.25 0.26 0.27 0.29 0.29
but if the temperature became too high, side reactions apparently developed. At temperatures of 75 to 80 "C, the products of the acrylonitrile pyridine complex broke down and the resulting red solution showed no linearity with concentration. In order to study the effects of temperature, stock solutions were prepared as described earlier, then the samples were treated with the remaining reagents and heated in a water bath. The time of heating and temperature were carefully monitored and controlled. I t was found that heating a t 60-65 "C developed the color quite satisfactorily and heating time from 5-10 min made very little difference in the absorbance value of 30 ppm acrylonitrile sample. A more detailed study a t 65 'C showed that over a concentration range of 0 to 25 ppm a heating time of 10 min gave the best reproducibility. T i m e S t a b i l i t y of t h e Complex. Immediately after development of the complex as described, absorbances were monitored for a period of 20 min for samples containing 5 to 15.5 ppm acrylonitrile. Absorbance values remained constant over this time period. Interferences. The following interferences were studied because these compounds, or similar types in small concentrations, are known to accompany pyridine and acrylonitrile in their manufacture. Cyanide. The cyanide standard solution was prepared by dissolving 3.77 g of potassium cyanide into 500 mL of water to which 1mL 1.0 N sodium hydroxide had been added. This solution was then diluted down to a concentration of 3 ppm cyanide ion. T h e absorbance for a 3 ppm cyanide solution was 0.66 a t 411 nm. The absorbance for a 3 ppm acrylonitrile solution
was 0.087, thus the cyanide was 7.6 times as sensitive as acrylonitrile to this reaction. This means, of course, that cyanide ion is a serious interference and a separation procedure has to be found to eliminate cyanide interference if the latter is present. Fortunately the interference of cyanide in solution is easily detected because of the characteristic green color which is formed on the addition of the sodium hypochlorite a t room temperatures. Acrylonitrile does not react in these solutions a t room temperature. This warning enables one to detect the presence of cyanide before any analysis is made for acrylonitrile. In the analytical procedure reported by Kroeller (In, he describes a method for the separation of cyanide from acrylonitrile samples using cotton wool and copper sulfate. This method may be used to remove cyanide from acrylonitrile samples. Acetone. The acetone standard solutions were prepared in the same manner as the acrylonitrile standards. One-half mL of acetone was placed in approximately 50 mL of distilled water contained in a 100-mL volumetric flask which had been weighed. The resulting solution was then reweighed to ascertain the amount of acetone. The stock solution was diluted 500-fold to yield a standard solution of 10 ppm. The test was then carried out as in the acrylonitrile determination. Acetone gave no indication of interference either a t the 10 or 100 ppm level. Ammonia. The ammonia standard solutions were prepared by dissolving 0.3141 g of ammonium chloride into 50 mL of water. This solution was then diluted to one hundred mL to yield a stock solution to 100 ppm in ammonia. This initial stock solution was then diluted ten fold to yield a 10 ppm stock solution. After preparation of the stock solution the ammonia sample was tested in the same manner as acrylonitrile samples. The tests gave no indication of interference from ammonia either a t the 10 or 100 ppm level. Reproducibility a n d Statistics. Statistical calculations were made in accordance with the method of Smith and Mathews ( I @ , assuming a constant percentage error in the data. According to this method, the percentage error instead of absolute error is assumed to be constant. In chemical analyses, the experimental conditions are generally controlled so as to make the percentage error a constant. In this case the sum of the weighted squares of the deviations from the least squares line is minimized. The standard deviation was found to be *0.255, and is the fractional error along the calibration line. In terms of percent, the standard deviation is f3.18. The lowest concentration of acrylonitrile measured in this work was 5 ppm using 2.5-cm cells. Obviously, by using 10-cm path length cells, a concentration approaching 1ppm could be measured. Molar Absorptivity. The molar absorptivity of the acrylonitrile-pyridine complex, based on the acrylonitrile concentration, was calculated to be 635.4. Applications. Analysis of Acetonitrile. The method developed in this report was applied to the analysis of acetonitrile (Fisher Scientific Company) for its acrylonitrile contlsnt. A content of 250 ppm acrylonitrile was found. This compares favorably with 230 ppm found on the same sample by gas chromatographic techniques developed in this laboratory. ACKNOWLEDGMENT We appreciate the assistance of E. D. Smith in making the gas chromatographic analyses. LITERATURE CITED (1) Chem. Eng. News. 55 (le),37 (1977). (2) H. Brieger, W. A. Hodes, and F. Reiders, Id.t-&g. Occup. Med., 6 128-140
(1952).
ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
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(3) Chem. Eng. News, 55 (21), 6 (1977). (4) R. P. Taubinger, Analyst (London), 94, 628-633 (1969). (5) D. W. Beesing, W. P. Tyler, D. M. Kurtz, and S. A. Harrison, Anal. Chem.,
21, 1073 (1949). (6) M. Wronski. and E. Bald, Chem. Anal. (Warsaw),15, 357-359 (1970). (7) A. J. Gorerea, Gas Chromatogr., 3, 138 (1965). (8) (9) (10) (11) (12)
C. E. R. Jones, Proc. Gas Chromatcgr. Symp., 3rd, Edinburgh,501 (1960). H. Nestler and W. Berger, Chem. Tech. (Berlin), 17, 169 (1965). W . L. Bird, and C. H. Hale. Anal. Chem., 24, 586-587 (1952). S. I . Meklev. Aferbneft Kohz. 6, 33-34 (1968). I. G. Sevast and A . P. Tomilov, Zavod. Lab., 32 1210 (1966).
D. Buckley and T. R . Crompton, Analyst(London), 99, 76 (1968). L. Fujiwara, Sitzungsber. Abh. Naturforsch. Ges. Rostock, 6, 33 (1914). G. A. Fugg, Anal. Chem., 36, 1532 (1966). F. Feigl, "Spot Tests in Organic Analysis". Van Nostrand Company, Inc., Princeton, N.J. 1956, p 280. (17) E. Kroeller, Dtsche Lebensm-Rundsch., 66, 11 (1970). (18) E. D. Smith, and D. M. Mathews, J . Chem. Educ., 44, 757 (1967).
RECEIVED for review June 28,1977. Accepted October 3,1977.
Determination of Optimum Compromise Flame Conditions in Simultaneous Multielement Flame Spectrometry D. F. Brost, 6. Malloy, and K. W. Busch" Department of Chemistry, Baylor University, Waco, Texas 76703
A composite response parameter, PJ,is developed for optimization of compromise excitation conditions in simultaneous multielement flame spectrometry. PJ is a direct measure of overall instrumental response for a particular multielement sample and is therefore sensitive to the detection powers and concentrations of each analyte species. Optimum compromise conditions are selected by simple maximization of the P J response surface. The behavior of P , is illustrated with experimental measurements made with a vidicon flame spectrometer on an ideal sample containing the spectrochemically divergent elements manganese and molybdenum. Practical application to several types of multielement samples is discussed on the basis of these measurements.
Simultaneous multielement analytical methods have played a n important role in survey analyses where information on a large number of elements is desired. Traditionally, this role has been filled by such techniques as spark-source mass spectrometry, neutron activation analysis, wavelength dispersive x-ray fluorescence, and emission spectrometry. Recently, there has been an interest in extending the capability of traditionally single element methods, such as flame techniques, to include simultaneous multielement determinations ( 1 , 2). One of t h e problems inherent in multielement flame spectrometry is the selection of the optimum compromise excitation conditions for the elements under consideration. Although several statistical methods could be applied to this problem, the simplest technique would involve some form of response surface analysis utilizing a single response parameter which directly measures the overall effectiveness of the instrument for the simultaneous multielement determination of all analytes in the sample. If such a composite parameter were available, simple maximization of the response surface would lead directly to the optimum compromise conditions without going through complex mathematical analysis of individual linear regression equations. This paper describes a n effective joint response parameter, developed from probability considerations, as well as a simple method for its use in the optimization of excitation conditions for simultaneous multielement determinations by flame emission.
THEORY The need for the determination of compromise excitation conditions in simultaneous multielement flame emission 2280
ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1 9 7 7
spectrometry arises from differences in the chemical and physical behavior of elements in the flame. Because of these differences, optimum detection power for some elements may be obtained by observing a volume element close to the burner top in a fuel-lean flame, while others may require observation higher in the same flame, or under more fuel-rich conditions. Since the simultaneous determination of all elements in a given sample is presently restricted to one set of conditions, a compromise must be found which provides adequate detection power for all analyte species. The severity of the necessary compromise depends upon the spectrochemical behavior of the individual elements as well as their respective concentrations in the sample. If all the elements are present in concentrations far in excess of their poorest detection limits over the useful range of experimental conditions, the selection of a particular set of conditions is not critical for their determination. On the other hand, if two or more spectrochemically divergent elements are present in a sample in low concentrations, the selection of an appropriate compromise becomes very important. The optimum excitation conditions for any element in flame emission spectrometry are those which yield the minimum possible detection limit. Under these conditions, the signal-to-noise ratio (S/N) is a t a maximum for a given concentration, and the emitted intensity a t the wavelength of interest is measured with the greatest possible precision. All that is needed, therefore, for optimization in single element determinations is the location of the minimum experimentally obtainable detection limit. For the case of multielement optimization, the problem becomes more complex. In order for a response parameter t o be useful for optimization of multielement systems, it must satisfy two criteria. First, the parameter must be a function of the detection power of the instrument for each element just as in the single element case. Second, it must be sensitive t o the prevailing concentrations of the analyte species. The necessity of concentration dependence is a direct consequence of the basic need to compromise. Consider a model sample containing two elements whose hypothetically divergent spectrochemical behaviors are shown in Figure 1 for various oxidant-to-fuel ratios (Ox/Fuel) at a constant observation height. The individual optimum for element X occurs a t low values (fuel-rich), while that for element Y occurs a t high values (fuel-lean). If element X is present in very high concentration in the sample (e.g., 100 ppm), its presence will easily be detected a t any Ox/Fuel setting. If element Y is present at very low concentration (e.g.,
