with the present system use of a 2 0 0 4 sample volume and a pulse time of 1.5 seconds provides optimum absorbance for a given solution concentration. However, absolute sensitivity for a given weight of a n element decreases with sample size. Manganese is cited as a typical example of this behavior. Sensitivity dropped from 0.025 ppm at 200 pl down to 0.06 ppm for a 50-p1 sample volume. Thus, sensitivity was decreased by a factor of two, while sample size was decreased by a factor of four. Reproducibility also suffered somewhat; however, the per cent deviation for any number of readings never exceeded 10% and was typically 6 to 8%. This represents a substantial improvement in the volume of sample normally required for flame spectrometric techniques. The loss in reproducibility at smaller sample volumes can be attributed primarily to the manipulation of the very small volumes. Second, the piezoelectric crystal can produce nodal points, depending upon the exact frequency applied. With very small samples, liquid falling on these portions of the crystal is not efficiently nebulized and erratic data can result. These points may be eliminated either by slightly changing the frequency of the radio frequency power generator or by simply not applying sample to effected areas. Either procedure eliminates the problem, yielding reproducible data. Cross-contamination from one sample to the next has not been observed; nor has solution been observed collecting on the inside of the burner. This is attributed to the geometry of the burner and to the reduced likelihood of very small droplets produced by ultrasonic nebulization at 3 MHz being impacted on a surface. These droplets possess so little inertia that they
are easily swept away from stationary boundary layers (existing next to the burner walls) by the flowing gas stream. CONCLUSIONS
Use of a high-power pulse ultrasonic nebulizer incorporated into the base of a low flow rate long-path multihole burner has proved to be a n efficient and reproducible system for converting small samples into aerosol and delivering virtually the entire sample into the flame. A 25- to 200-p1 sample injected upon the piezoelectric transducer is converted to fog in 1.5 seconds and swept by the support gases into the flame. The increase in sensitivity with this system is attributed to the production of a dense aerosol composed of uniform small droplets by the nebulizer. This results in an increase in the efficiency of production of atomic vapor. Knowledge gained from this study indicates that additional sensitivity should be possible by lengthening the burner path length or by a multipass optical system. ACKNOWLEDGMENT
The authors wish to thank Denco Research (Tucson, Ariz. 85715) for providing the radio frequency power unit and machine shop facilities and Dale Mack for assistance with electronic systems. RECEIVED for review August 21, 1972. Accepted October 20 1972. This work was supported in part through funds provided by the Arizona Mining Association to the University of Arizona Atmospheric Analysis Laboratory.
Rapid and Selective Pyrocatechol Violet Method for Tin Homer B. Corbin M & T Chemicals Inc., Subsidiary of American Can Co., Rahway Research Laboratory, P.O.Box 1104, Rahway, N.J. 07065 A spectrophotometric pyrocatechol violet method has been developed that is more rapid and selective than previous methods. Conventional methods are used to prepare a sulfuric acid solution. Either directly or after an iodide extraction separation, the absorbance of the tin(lV)-PCV complex is measured at the 660-nm maximum in a sulfuric and citric acid solution. The method has been applied to the direct measurement of tin in metals and after acid oxidation and separation to the determination of 0.01 to 1 part per million tin in biological samples. For different types of samples, the detection limit varied from 0.026 to 0.076 pg Sn. Relative standard deviations of 0.55% to 1.4% were obtained in the 1 to 10 pg Sn range on samples receiving various treatments. THISANALYTICAL LABORATORY has been involved in the determination of tin in a great variety of materials and over a range from decigram to microgram quantities. In recent years, the requirements have been increasingly in the direction of more analyses and lower concentrations. Major areas involved are the determination of residues of organotin compounds used as pesticides and the evaluation of organotin stabilizers for plastic food-wrap materials. The need for measurement at levels below 0.1 part per million in materials difficult to destroy, such as oils, composite food materials, and some cosmetic preparations containing highly reactive materials, prompted the evaluation of 534
ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
other more sensitive methods to serve as a complement to the dithiol methd which is used when the Sn easily available is about 5 pg or more. Recently a fluorometric method for Sn has been described ( 1 ) . This showed a detection limit of 0.007 microgram of Sn at a concentration of 0.3 part per billion in solution. This method, whether for pure tin or tin separated by extraction of the iodide, involves several steps including evaporations, fuming, and transfers. The reaction requires close control of temperature. The two most widely used of the more sensitive colorimetric reagents are phenyl fluorone (2-4) and pyrocatechol violet (5-7). Methods using these reagents have required separation of tin from a large number of interferences to obtain the required degree of selectivity. Because the reagent-tin complex is water soluble, the pyrocatechol violet-tin reaction was studied in an effort to develop a method reliably sensitive t o sub-microgram quantities of tin with less manipulation necessary than published methods. (1) T. D. Filer, ANAL.CHEM.,43, 1753 (1971). (2) C. L. Luke, ibid., 28, 1276 (1956). (3) R. L. Bennett and H. A. Smith, ibid., 31, 1441 (1959). (4) J. D. Smith, A m l y s t (Lotidoti),95, 347 (1970). (5) W. J. Ross and J. C. White, ANAL.CHEM., 33, 421 (1961). (6) E. J. Newman and P. D. Jones, Amlyst (Londo)?).91,406 (1966). (7) Analytical Methods Committee, ibid., 92, 320 (1967).
West et al. (8) have investigated the use of various dispersing agents and found that cetyl trimethyl ammonium bromide (CTAB) sensitized the reaction, increasing the molar absorptivity from 65,000 t o 95,600 cm-1 mole-' liter and shifting the absorption maximum from 555 t o 662 nm. The use of this reagent has been incorporated in the present method.
Tin. pg
__ (8) R. M Dagnall, T. S. West, and P. Young, A m l y s t (Lo/zdoir),92,
27 (1967).
0.018 0.018 0.053 0.052 1 .o 0.122 0.123 2.0 0.291 0.290 4.0 0.622 0.624 6.0 0.959 0.955 8 .O 1.292 1.290 10.0 1.622 1.625 11 .o 1.788 1.784 Volume 50 ml, cell length 100 mm. Wavelength 660 nm. 0
0.5
EXPERIMENTAL
Apparatus. The photometric instrumentation used was a Beckman DU-2 Spectrophotometer provided with compartment and holder for 100-mm cells. By using 100-mm cells, 60% of the original sample may be made use of in absorbance measurements when samples are made up in standard 50-ml volumetric flasks. This is an important consideration when the tin available is less than 1 pg. Reagents. STAXDARD TIN SOLUTIONS.500 pglml. Dissolve 0.2500 gram of pure tin in 150 ml of concentrated hydrochloric a d d . Dilute to 500-ml volume in HzO. 10 wg/ml. Prepare from the 500 pglml solution by eliminating HCl and making up in 20% w/v H2S04and 10% w/v citric acid. 0.5 pg/ml. Prepare in 20% wjv HUS04and 10% wiv citric acid. 0.025 pg,'ml. Prepare fresh in 5 % w/v H 2 S 0 4and 2.5% w h citric acid. SULFURIC-CITRIC ACID SOLUTION.Five grams H?S04and 2.5 grams citric acid per 100 ml in H 2 0 . This mixed solution is used in preparing the calibration curve. CTAB SOLUTION.This was 5.5 mgiml in HzO. SENS~TIZED PYROCATECHOL VIOLET SOLUTION.For each 100-ml solution, dissolve 12 mg of pyrocatechol violet in water, add 2 ml CTAB solution, and dilute to volume. Prepare fresh daily as needed. Mix one volume of sulfuric ACID-IODIDE WASHSOLUTION. acid with two volumes of water and cool. Add 1 volume of potassium iodide, 20% wjv. Prepare fresh as needed. ASCORBICACID. Use 5 % w/v in HzO. Prepare fresh daily as needed. Preparation of Calibration Curve. Place a measured amount, 1 ml to 20 ml, of standard tin solution, 0.5 pglml, in a 50-ml volumetric flask. Dilute to 20 ml with a measured amount of sulfuric-citric acid, 5 and 2.5 % w!v. Add 2 m of ascorbic acid, 5 % w/v and dilute to 40 ml. Add 5 ml of sensitized pyrocatechol violet solution and dilute to 50-ml volume. After 30 minutes, measure the absorbance at 660 nm in a 100-mm cell. After each reading, rinse the cell twice with water, once with hydrochloric acid, 2 M , twice with water, and three times with the following solution. Plot absorbance readings against amount of tin. Preparation of Samples. General procedures are indicated, but the exact details will vary with the nature of the material. In general, in all cases where solutions are taken to fumes of H2S04,it is important that sufficient acid be present to cover the bottom of the flask or beaker at all times. If natural tin oxide or calcined synthetic tin oxide is present, the sample will require additional treatment with peroxide or bisulfate fusion. ACID-SOLUBLE INORGANIC MATERIALS.If no interferences are present, place a sample in a weighed 30-ml beaker. Acidify with a few milliliters of nitric acid and 0.5 ml of sulfuric acid. Evaporate to fumes of sulfuric acid. Fume until all water is removed. Cool and adjust to 1 & 0.05 gram of sulfuric acid. Dilute with water, add 5 ml of citric acid, 10% w/v, and mix. Transfer to a 50-ml volumetric flask. METALS. Treat a suitable sample with 5 ml of hydrochloric acid. Allow to react with the aid of slight warming. After the reaction slows or if there is no appreciable reaction, add 2 ml of nitric acid and warm. When the reaction is as
Table I. Calibration Data' Absorbance
'1
complete as possible, add 5 ml of sulfuric acid. Heat t o fumes of sulfuric acid and fume lightly only. Cool and adjust to 10 grams of sulfuric acid. Transfer to a 100-ml volumetric flask and dilute t o volume. Allow any insoluble material to settle or, if the sample shows considerable carbon or does not settle to a clear solution, it may be passed through a filter. Place a suitable aliquot containing less than 12 pg of tin in a 50-ml volumetric flask. Add sufficient sulfuric acid to bring the total present t o 1 gram. Add 5 ml of citric acid, 10% wiv. If the total interferences are too high (e.g., considerable molybdenum) or if it is required to measure trace quantities, it is more convenient t o separate by the iodide extraction procedure, described below. BIOLOGICAL MATERIALS.For this class of materials, the interest extends down to the parts-per-billion level. In general, the iodide extraction separation is used to remove the larger amount of sulfuric acid and the insoluble material which is often present following conventional wet-digestion with nitric and sulfuric acids. The sensitivity of the determination is limited by the control of the blank as well as by the sensitivity of the color measurement. The final stage, after destruction is complete, is the concentrated sulfuric acid solution of the residue. Iodide Extraction of Tin. The general procedure has been described (9, IO), but modification is necessary to allow for the conditions used in this colorimetric measurement. Add to the known volume of sulfuric acid resulting from the sample preparation, two volumes of water and cool. Add one volume of potassium iodide, 20% w/v, and transfer to a suitable separatory funnel. Extract for one minute with 2-4 volumes of n-hexane and allow to settle for 5 to 10 minutes. Transfer the hexane to a second 125-ml separatory funnel by pouring from the neck. If there is considerable insoluble material in the sample, a second extraction is made and combined with the first. Add to the hexane solution exactly 20 ml of standard tin solution, 0.025 pglml. Extract 30-60 seconds and allow to settle well. The solutions should be a t or slightly above room temperature rather than below. Draw the acid solution into a 50-ml volumetric flask. Wash the hexane twice using 10-ml portions of water and add to the volumetric flask. Allow each washing to settle for 5 minutes. Allow the solution to stand in the flask for 2-3 hours or overnight before developing and reading the color. Color Development and Measurement. Add to the sample i n the volumetric flask, 2 ml of ascorbid acid, 5 w/v, freshly prepared. Add 5 ml of sensitized pyrocatechol violet solution to the solution. dilute to 50 ml, and mix. After 30 minutes, read absorbance a t 660 nm in a 100-mm cell. Deduct absorbance of blank(s) carried through the entire procedure (9) H. B. Corbin, J . Ass. Ofic.Arral. Clzem., 53, 140 (1970). (10) M E. Getzendaner and H. B. Corbin, J . Agr. Food C/zem., 20, 881 (1972) ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
535
Table 11. Evaluation of Factor Variations
Table VI. Recovery of Tin from Biological Samples 9
1
Errora Time
+0.012 +o ,022
Citric acid +O ,008 +O ,054 Pyrocatechol violet CTAB +0.012 pg tin per 10% variation in factor.
-0.003 +0.041 $0.024 -0.034 -0.028
Table IV. Tolerance to Interfering Anions Acid Tolerance, mgR F(U 0.2 1. BFa(1) PnOdIV) 1. POa(II1) 20. Cl(1) 50. CIO,(I) 1. Error
