New Ultraviolet Ratio Spectrophotometric System for the Determination of Trace Amounts of Phenolic Compounds J.
E. Fountaine, P.
6. Joshipura, a n d P. N. K e l i h e r l
Chemistry Department, Villanova Universify. Villanova. Pa. 79085
J. D. Johnson Spectrogram Corporation. 385 State Street. North Haven. Conn 06473
A novel type of instrumental system, which uses two conventional sealed hollow cathode lamps to monitor the ultraviolet bathochromic shift which occurs when phenolic compounds are made basic, is described. The ultraviolet ratio spectrophotometric system has been used for the determination of several phenols at the low ppb range including phenol, o-cresol, p-cresol, resorcinol, thymol, p-methoxy phenol, and tyrosine. Comparison is made with the standard ASTM Method D-1783-70 (colorimetric reaction with 4-aminoantipyrine), and results for both the ASTM method and the ultraviolet ratio spectrometric method are presented. In general, results with the ultraviolet ratio spectrometric method are higher, due to the presence of para-blocked phenols which do not react' with 4-aminoantipyrine.
Phenols are widely distributed by man and, a t trace levels, by nature. They are readily detected by taste and odor, particularly when the unsubstituted phenol is chlorinated in public water supplies. Phenols have been determined routinely for about thirty years by the condensation reaction of 4-aminoantipyrine (4-AAP). This reaction was first reported by Emerson (1-3) and is the basis for the method used at present by the American Society for Testing and Materials ( 4 ) and the American Public Health Association ( 5 ) . Other colorimetric reagents which have been used include quinonechloroimide (6, 7), diazotized sulfanilic acid (8),diazotized p-nitroaniline ( 9 ) , p dimethylaminobenzaldehyde ( I O ) , nitrous acid (11). and 3-methyl-2-benzothiazolinonehydrazone (12). At relatively high concentration levels, phenols may be determined by gas chromatography (13, 14). Gas chromatography is limited to the low ppm level, however, and is better used for separation rather than group determination. To whom
correspondence s h o u l d b e addressed.
E. Eisenstaedt (Emerson). J. Or9 Chem 3, 153 (1938). E. Emerson, J O r a Chem.. 8, 417 (1943). E. Emerson and K. Kelly, J. Org. Chem , 13, 532 (1948). American Society for Testing and Materials. Method D-1783-70, Philadelphia, Pa.. 1970 "Standard Methods for Examination of Water and Waste Water." 13th ed., American Public Health Association, New York, N.Y., 1971. H. D. Gibbs.J. Biol Chem . 71, 445 (1927). H. V . Burba. Anal Biochem , 24, 344 (1968) E. Sundt. J Chromafogr . 6, 475 (1961). E. Stahl, "Thin Layer Chromatography," Academic Press. New York. N.Y.. 1965. G. J. Kapadia. J. R . Mosby, G. G . Kapadia. and T. B. Zalucky. J. Pharm Sci 54, 41 (1965). L. Lykkon. R. Treseder, and V. Zahn. Ind. Enq. Chem Anal Ed 18, 103 (1946) H. 0. Friestad, D. E. Ott. and F. A. Gunther, Anai C h e m . 41, 1750 (1969). American Society for Testing and Materials, Method D-2580-68, Philadelphia. Pa., 1968. R. A. Baker and B. A. Malo, Environ Scr Techno/ 1, 997 (1967).
Conventional ultraviolet absorption spectrophotometry has also been used (15-19) but is, generally, not as sensitive as the colorimetric methods. High sensitivity to approximately 10 ppb was achieved by Schmauch and Grubb (16) and further tested by Mochler and Jacob (17) using a dual beam ultraviolet instrument, but only at the expense of employing a tri-n-butyl phosphate concentrate extract from a sample of 1 liter or more. At least 1 hour was required for an analysis. A rapid and direct measurement on a minimal sample is desired. To date, conventional dual beam and split beam photometers have not performed direct analysis with adequate precision below the level of approximately 50 ppb. ASTM Method D-1783-70 ( 4 ) uses the 4-AAP reaction to analyze phenols a t the parts-per-billion and higher levels. There are two versions of this method. A direct colorimetric method (510 nm) is recommended for samples containing more than 0.1 mg/liter (100 ppb) of phenolic compounds and a chloroform extraction method is used for those samples which contain less than 100 ppb of phenolic compounds. Phenol itself is used as the standard. In practice, for convenience, many analytical laboratories use the chloroform extraction procedure, with appropriate dilutions, for all samples. In this procedure, the phenolic compounds are separated from nonvolatile interferences by a distillation at pH 4. Copper(I1) sulfate is usually added to the sample to prevent interference by sulfur-containing compounds and to inhibit biochemical oxidation. If the sample contains oil and/or tar. an alkaline (pH 12) extraction with carbon tetrachloride [in the absence of copper(I1) sulfate] must be performed before the distillation. The CC14 layer, containing the oil or tar, is then discarded and the aqueous layer, after acidification. is then distilled. A suitable aliquot of the distillate is then adjusted to a pH of 10 and reacted with 4-AAP in the presence of potassium ferricyanide. The color is allowed to develop for 3 minutes, after which the complex is extracted into chloroform (usually 25 ml) and the absorbance measured a t 460 nm on a suitable colorimeter. It has been shown, however, (4, 22, 20) that a "total phenol" determination is not possible with this technique, since the presumed condensation at the para position is blocked by many phenols having groups in the para position not replaced by the reagent at the pH values employed in this procedure. In fact, the ASTM Method states that "this method can be regarded only as an approximation of phenolic compounds W. Stendstrom and M . Reinhard, J Phys. Chem . 29, 1477 (1925). L. J. Schmauch and H.M . Grubb, Ana/. Chem.. 26, 309 (1954). E. F. Mochler. Jr.. and L. N. Jacob, A n a / Chem , 29, 1369 (1957). J. M. Martin. Jr., C. R. Orr, C. B. Kincannon, and J. L. Bishop, J. Wafer Poilut Confr Fed 39, 21 (1967) (19) A. S. Wexler, Anal Chem . 35, 1936 (1963). (20) S. D Faust and F C. Lorentz, "Factors Influencing the Condensation of 4-Aminoantipyrine with Phenols." Paper presented at the 11th Ontario Industrial Waste Conference, Bigwin Inn. Ontario. June 21. 1964. (15) (16) (17) (18)
in industrial waters and industrial waste waters. The concentration of phenolic compounds obtained represents the minimum amount of phenolic compounds present in the sample and, therefore, an expression of the accuracy of this method cannot be made" ( 4 ) . Although the most serious criticism of the standard 4-AAP method for phenols has been its discrimination against most para substituted phenols (4, 12, 20), two other deficiencies should be noted here. Gottlieb and March ( 2 1 ) have commented on the sensitivity of the reaction to pH variations and, more recently, Stroehl (22) has observed that losses of 10 to 207'0 phenolic compounds may occur when normal and steam distillation procedures are employed. Consideration of the above mentioned difficulties with the conventional 4-AAP procedure has led us to the development of a specially designed ultraviolet ratio spectrophotometer, which senses the change in intensity of a hollow cathode lamp due to the bathochromic shift which occurs when a phenolic compound is made basic. In this paper, we wish to describe this instrumentation, for which patent is pending, and to discuss the response of the instrumentation to phenolic compounds.
iI
l ri
Figure 1.
T I I 1
1
-
Schematic diagram of ultraviolet ratio spectrophotom-
eter
EXPERIMENTAL Instrumentation. A block diagram of the apparatus used is shown in Figure 1. 'The instrumentation consists of three basic units: a dual hollow cathode lamp power supply (Spectrogram Model L P S - l D ) , a phase sensitive photometric amplifier with an integral high voltage power supply (Spectrogram Model LPA-1). and a n analyzer section consisting of two hollow cathode lamps. a small monochromator, t h e sample cell compartment. and a photomultiplier tube and housing. The photometric amplifier generates a 100-Hz square wave signal which triggers t h e dual hollow cathode l a m p power supply so that the two lamps are pulsed alternately 130' out of phase, with a square wave current pulse of 50% duty cycle. The lamps are never completely extinguished since a small d c signal is left continuously on each lamp. Hieftje and coworkers (-3.'?1 have recently shown this increases lamp s t a bility. Lamp A is chosen to be in the "pH sensitive" region for phenols and L a m p B is chosen to be in the " p H insensitive" region. Although different hollow cathode l a m p combinations were used in the beginning stages of this work. it was ultimately decided t h a t the most promising combination for phenol analysis was Lamp A-platinum and L a m p B-chromium. Conventional neon filled Westinghouse hollorv cathode lamps were used mounted inside specially designed housings to minimize drift, Three platinum lines a t 289.1. 239.8. and 29:3.0 n m are selected by the 0.2-meter monochromator which has a reciprocal dispersion of 1 2 angstroms per millimeter. These wavelengths are then beam combined with three chromium lines from Lamp B. 357.9. 359.3. and 360.2 nni. which are selected by a filter. The combined beams pass through a standard quartz windowed cylindrical a b sorption cell of I - to IO-cm path length. depending upon the concentration range being measured. The beams go through a protective filter, and finally strike the photosensitive area of the photomultiplier tube ( H a m a m a t s u T\'Co. Ltd.. Type K2112). If the cell contains a solution of a phenolic compound under normal ( i . e . . acidic) conditions, and the solution is subsequently made basic. light from the platinum hollow cathode l a m p will be absorbed. proportional to the amount of phenolic compound. while light from the chromium l a m p will not be absorbed. This is further described in the Procedure section. A "Set-Read" switch is incorporated into the analyzer section of the instrumentation. 'This consists of a small mechanical s h u t ter which can be used to block the beam from L a m p A and thus maintain the reference intensity a t full scale. 4 Model 202 U\'-\'isible Spectrophotometer (Perkin-Elmer Corporation. Norwalk. Conn.) was used to record phenolic spectra a t relatively high concentration levels. en. 20 p p m . A "Spectronic (21) S
Gottlieb and P B March ind Eng Chem
A n a / Ed
18, 16
(1 946)
Stroehl Mikiochim Acta 1969, 130 (23) G M Hieftje B E Holder A S Maddux Jr A n a / Chem 45, 238 (1 973) (22) G W
and Robert Llm
nm
-
Figure 2. Bathochromic shift for phenol: A . Acidic; B . Basic
20" colorimeter (Bausch and Lomb, Analytical Systems Division, Rochester, S.Y)was used in the comparison studies with 4-aminoantipyrine. Conventional distillation assemblies. as recommended in the ASTM procedure ( 4 )were used. Reagents. Reagent grade chemicals were used throughout. The phenolic compounds were obtained from a variety of sources and were purified before using. Distilled water was passed through a 4-foot activated charcoal column to remove trace amounts of phenolic materials and this distilled water was used to make u p all solutions including the 4M N a O H solution and for rinsing and cleaning the cells. A 10-ppm "synthetic phenol" solution was prepared from 60% phenol. 1 5 7 ~o-cresol. 15% rn-cresol, and 1070 p cresol. and all subsequent dilutions nere made from this stock solution. All stock phenolic solutions were kept in a refrigerator at below 10 "C a n d were also checked periodically for deterioration. Procedure. Figures 2 and 3 show hathochromic shifts which occur when phenolic compounds are made basic with NaOH. This shift is relatively independent of the amount of sodium hydroxide added. Although bathochromic shifts and relative molar absorptivities under acidic and basic conditions will vary with each phenolic compound. see Figure 3, t h e spectra for phenol itself are typical for phenolic compounds. Consideration of Figure 2 shows that the relative light intensity from the platinum hollow cathode lamp will be dependent upon t h e p H shift, while the chromium lamp will not be affected. In the operation of the instrumentation. the mechanical shutter is used to block the beam from Lamp A and. with a sample in the acid condition in the cell. current from L a m p B is set to some arbitrary value. usually 2 m h . High voltage to the photomultiplier is set at 500 V a n d amplifier variables are adjusted to yield a full scale deflection on the photometric amplifier meter. The shutter blocking t h e light from Lamp A is then opened ("Read" position). and the current to Lamp A increased or decreased so as to balance the intensities of
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 1 , J A N U A R Y 1974
63
A bs
1
305
- nm
I
IO0
300
CDnC
500
PPb.
Figure 4. Response curves for phenolic compounds chosen to represent a wide variety of phenolic materials
A bs
I
1 ) Phenol, 2) 0-cresol. 3 ) resorcinol, 4) p-cresol, 5) thymol, 6) m-methoxy phenol, 7 ) tyrosine, 8) p-methyl thiophenol
I
nm
-
Bathochromic shifts for p-methoxyphenol (top) and thymol (bottom):A Acidic; 6. Basic Figure 3.
Table I. Precision Measurement at Different Concentration Levels Mean YO Phenol absorption in water, for 15 Std ppb measurements d ev 200 100 50
10.44 5.55 2.92
10
0.66
Re1 std dev, YO
0.21 6 0.192 0.190
2.0 3.4 6.5
0.13
19.7
the two wavelength regions. Since the lamps are 180” out of phase with respect to each other, the amplifier meter then reads zero. Upon making the sample basic with sodium hydroxide. to a p H of about 12. the beam from Lamp A is absorbed, while the beam from Lamp B is not. The meter then reads a per cent absorption (relative signal) proportional t o t h e concentration of the phenolic compound.
RESULTS AND DISCUSSION In the early stages of this work, it was found necessary to use the same (identical) cell for standards and unknowns; it was also found necessary to place the cell in the same position in the optical path each time. The extent.of bathochromic shift was, as expected, relatively independent of sodium hydroxide concentration, 2-3 drops of 4M NaOH being sufficient to bring the p H to about 12 when a 10-cm cell is used. When a 1-cm or 2-cm. cell is used (for higher concentration levels), one drop of 4M NaOH is sufficient to bring about the pH shift. Figure 4 shows response curves for eight compounds chosen to represent a wide variety of phenolic materials. Of these, only phenol, o-cresol, resorcinol, and p-cresol are apt to he pollutants in real samples. Since the instrumentation has been optimized for phenol, by the choice of the particular lines from the platinum hollow cathode lamp, 64
100
and since the phenols differ in their extent of bathochromic shift and relative molar absorptivities (at the platinum beam wavelengths), it was expected and found that slope differences would be observed in the response curves. It is possible to choose a different set of Lamp A lines (either platinum or some other hollow cathode lamp containing lines in the 270- to 320-qm range, e.g., bismuth) to maximize the slope for a different phenolic compound. For the practical analysis of phenolics, however, it seemed more useful to maximize for phenol since it is conventional in the analysis of phenolics to set up a calibration curve based on phenol itself as the standard ( 4 ) . In addition to the phenols shown in Figure 4, response curves were also obtained for m-chlorophenol, m-cresol, and p-phenyl phenol. Negative results were obtained for o-nitrophenol, due to the interference of the nitro chromophore in the chromium lamp region, and 1-naphthol, which shows a greatly elongated bathochromic shift. By adjustment of the beam from Lamp B to longer wavelengths, however, it should he possible to set up a system for o-nitrophenol and 1-naphthol. Precision data for phenol at different concentration levels are summarized in Table I. This shows the results of fifteen determinations at each of four different concentrations. Table I1 shows the results of a comparison study, for a “synthetic phenol” mixture, between the 4-AAP method, following conventional ASTM procedure with extraction into chloroform ( 4 ) , and the ultraviolet ratio spectrophotometric method. The “synthetic phenol” mixture consisted of 60% phenol, 15% o-cresol, 15R rn-cresol, and 10% pcresol, It is recognized that the choice of this particular phenolic mixture is somewhat arbitrary, but it is felt that this represents a “typical” phenolic mixture which might be found in some real samples. Ten distillations were performed a t each of the six concentration levels shown in Table 11. A 500-ml portion was taken in each distillation, 450 ml then distilled over, the solution allowed to cool, 50 ml of phenol free water added to the distillation flask. and the distillation continued until 500 ml was collected. A 250-ml aliquot of the distillate was then taken for the 4-AAP procedure, with the remainder available for the ultraviolet ratio spectrophotometric method. As only ca.
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 1, J A N U A R Y 1 9 7 4
Table II. Comparison of Standard 4-AAP Method and Ultraviolet Ratio Method for Phenols Ultraviolet ratio method
Standard 4-AAP method
Concn added to blank water, ppba 500 150
50 25
15 5
Mean of
tenC
Std dev
devh
Mean of tenC
25.4 6.01 5.12 1.25 0.84 1.69
5.63 5.03 12.7 6.30 7.40 25.2
491.7 141 7 48.2 24.6 13.1 5.1
Rei std
f
451.5 119.5 40.4 19.8 11.4 6.7
Synthetic mixture consisting of 60% phenol, 15% 0-cresol, 15% in-cresol, 10% p-cresol. centrations Drovided a common distillate for measurement with both techniques.
Rei std f
Std dev
devh
9.15 6.62 3.66 2.16 2.28 2.00
1.86 4.67 7.59 8.78 16.4 39.2
Per cent. Note: Each of the ten samples of the six con-
Table I l l . Analysis of Some Real Samples No.
Sample location
Stream-lthan Rd. & Bridge Booth School 2. Stream-Bryn Mawr 3. Schuylkill River, Conshohocken ( r u n n i n g water) 4. Schuylkill River, Conshohocken ( r u n n i n g water) 5. Schuylkill River, Conshohocken (still water) 6. Philadelphia Public Water supply, Overbrook Area (Tap water sample)) 7. Villanova University (Tap water sample) 8. Schuylkill River, Conshohocken ( r u n n i n g water) 9. Stream-Evens Road & Gypsy Hill Road. Gwynedd Valley, Pa. 10. West Virginia Industrial Plant Influent 11. West Virginia Industrial Plant Effluent (Influent after treatment) 1.
000
C0“l
VVL
Figure 5. Relative standard deviations vs
concentration for 4-AAP and U V ratio methods and log-log plots for phenol at high (1-cm cell) and low (10-cm cell) concentration ranges
40 ml of sample is required (for a 10-cm cell) for the ultraviolet ratio spectrophotometer, it was possible to run several analyses on the second portion of the distillate. In order to make a fair comparison with the ASTM procedure, however, where 250 ml is required for one analysis, only results for the first aliquot used for the ultraviolet ratio spectrophotometer are reported in Table 11. As expected, results with the ASTM method are generally lower than with the ultraviolet ratio spectrophotometric method, this is because there is no reaction between 4-AAP and p-cresol. At the 500- through 15-ppb levels, the standard 4-AAP method averages approximately 15% lower than the ultraviolet ratio method. This is due not only to the 10% p-cresol in the mixture but also to the differing molar absorptivities between these particular compounds. At the 5-ppb level. a 34% enhancement is observed with the 4-AAP method; this is probably due to the deficiencies of the colorimeter used a t this very low concentration. Again, there is a better comparison to the amount added with the ultraviolet ratio spectrophotometer. The relative standard deviations with the ultraviolet ratio spectrophotometer compare favorably with the 4-AAP method at the higher concentrations and are only slightly worse a t the lower concentrations. This is shown in Table I1 and in Figure 5 which also shows log-log plots of phenol at low and high concentrations. In the ultraviolet ratio spectrophotometric system, the “weak link” will be the stability of the lamps; however, with the specially designed housings, the drift of the lamps can be kept to a minimum. The platinum hollow cathode lamp which was used for the studies in Table I1 and Figure 5 was about two years old and had previously been used in conventional atomic absorption equipment. It has recently been replaced by a new platinum hollow cathode lamp and a cor-
Date collected
uv
ratioa
ASTMa
1 - 17 - 7 3
11.0
11.0
1-17-73 2-6-73
8.0 16.0
7.0 20.0
2- 1 4 - 7 3
20.0
27.0
2- 1 4 - 7 3
22.0
32.0
4-6-73
14.0
22.0
4-6-73
18.0
28.0
4-6-73
16.0
24.0
4-6-73
10.0
15.0
4-1 0 - 7 3
11800.0
15000.0
4-1 0-73
5500.0
8000.0
a Expressed a s parts per billion.
-
responding increase in stability has been noted. It is expected that the operating lifetimes of the hollow cathode lamps in this apparatus will be comparable with the lifetimes of these sources in conventional atomic absorption apparatus where the lamps are normally used. Table I11 shows some analysis results for samples taken from various locations in the Philadelphia area and also shows some results for an industrial “influent” and “effluent” from West Virginia. .411 of the samples were analyzed on the day of collection except the industrial samples which were analyzed approximately 36 hours after collection. Analysis results with the ultraviolet ratio spectrophotometer are generally higher than when the ASTM procedure is employed, indicating that many of these samples contain high percentages of para-blocked phenols. The Schuylkill River water samples were collected just below a large industrial plant in Conshohocken, Pa. Samples :3. 4. and 8 were collected from the same location, at a spot where the water is running fairly rapidly, while Sample 5 was located at a spot about 50 feet away where the water
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1 , J A N U A R Y 1 9 7 4
65
is essentially stationary. Sample 3 was collected just after a rainstorm while the other Schuylkill River samples were collected on relatively warm sunny days. The two industrial samples, No. 10 and 11, contain relatively high amounts of phenolic compounds as would be expected. The “influent” is after an in-plant process and the “effluent” has received a preliminary in-plant cleaning treatment. This cleaning treatment is primarily designed to remove metallic contamination (chromium, iron, zinc, etc.) and atomic absorption studies on the samples show it is quite effective in removing metals. As can be seen in Table 111, it is only partially successful in removing phenolics. It should be noted. however, that both analytical methods show an approximately 30% decrease in phenolic content after the cleaning treatment indicating that “total phenols” are removed. There are several significant advantages to be gained by the use of the ultraviolet ratio spectrophotometric system for the analysis of trace amounts of phenolic compounds. 1) The system will respond to para-substituted phenols and therefore more accurate analyses are obtained when samples contain para-blocked phenols. 2) The amount of sample required for analysis is only ea 50 ml cs the 500 ml required when the ASTM procedure is employed and,
therefore, the time required for distillation is considerably reduced. 3) Only one simple reagent is required. sodium hydroxide, and the amount of this reagent added is not critical. 4) The system is very simple to use. Point 4 deserves some further explanation. Several undergraduate students a t Villanova University have been trained to operate the ultraviolet ratio spectrophotometer in about 15 minutes. The difficulties with the ASTM procedure have been well documented ( 4 , 12, 212).
ACKNOWLEDGMENT We wish to thank Michael Cichetti. Jody Cummings, and Angie Pizzi. undergraduate students at Villanova University, for their experimental assistance. We also thank Herbert R. Gram, Spectrogram Corporation, for his advice and encouragement. Received for review May 11, 1973. Accepted July 20, 1973. Parts of this paper were presented a t the 19th Spectroscopy Symposium of Canada, Montreal, October. 1972. and a t the 24th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, March 1973. Paper S o . 110.
Liquid-Layer-on-Solid-Sample Spark Technique for Emission Spectrochemical Analysis R. M. Barnes1 and H. V. Malmstadt Department of Chemistry, University of Illinois, Urbana. //I. 6180 1
The application of a liquid layer on a solid sample electrode surface during sparking produces improved analytical results when compared to data obtained under identical spark conditions without the liquid layer. Examples are presented to show improved analytical sensitivity and precision, increased sample removal and depth of spark penetration, and decreased interferences from counterelectrode and atmosphere-line emission when the technique is used with conventional and custom-built spark sources. Additionally, a solute in the solution layer was used as an internal reference instead of the conventional matrix element in the solid sample, as illustrated with aluminum, iron, and nickel alloys. The technique is a development based upon submicrosecond time-resolved studies of the basic processes in spark discharges.
Submicrosecond time-resolved emission studies of solid and solution sampling methods ( I ) have demonstrated that when a solute was introduced into a n analytical spark by any one of the commonly-used solution techniques, the solute underwent processes quite similar to those previously established for solid materials (2). In adPresent address, Department of Chemistry, University of Massachusetts, Amherst, Mass. 01002. Author to whom correspondence should be addressed. (1) R. M . Barnes, Diss. Abstr. Int. B, 27,3806 (1967). (2) J. P. Walters and H . V. Malmstadt, Ana/. Chem., 37, 1784 (1965).
66
dition, the solvent decomposed and produced atomic hydrogen a t the surface of the electrode a t which the solution was introduced. The hydrogen interacted with the atmospheric nitrogen and oxygen ions to reduce their emission and contribution to background. In extending these basic findings to spectrochemical analysis, a single analytical curve was obtained for nickel in stainless steel and a nickel-base alloy when samples were determined either as solutions or solids ( 1 ) . Based upon these results, further work was undertaken to develop new methods of combined solution and solid sampling. A few Russian workers have reported experiments in which both solutions and solids were combined in a spark method. Petukh ( 3 ) , in a study of third-element effects, proposed that for the analysis of copper alloys, a n internal standard element could be introduced into the analytical gap as a spray which uniformly wetted the electrode surfaces. Cobalt and nickel solutions were sprayed through lower hollow counter electrodes in a pin-to-pin configuration. Podmoshenskii and Shelemina ( 4 ) applied a thin coating of water or salt solution from a moving gauze pad on the sample electrode surface in a high voltage spark. Some effects of general composition, structure, and heating were apparently eliminated for analysis of zinc in copper alloys. Also, Petukh (3) and Podmoshenskii and Shelemina ( 4 ) observed a n increase of electrode erosion in the presence (3) M . L. Petukh, Indust. Lab.. 30, 1813 (1964). ( 4 ) I . V . Podrnoshenskii and V M. Shelemina. indust. L a b , 29, 589 (1963).
A N A L Y T I C A L C H E M I S T R Y , V O L . 46, NO. 1, J A N U A R Y 1974
