Atomic Absorption Spectrometric Determination of Sub-Part-per-Million Quantities of Tin in Extracts and Biological Materials with a Graphite Furnace Helen L. Trachman," Albert J. Tyberg, and Patrick D. Branigan M&T Chemicals Inc., Subsidiary of American Can Company, Research Laboratory, P.O. Box 1104, Rahway, New Jersey 07065
Sub-ppm levels of tln in solvent extracts from plastic barrier films stabilized with organotins and in blologlcai tissues after feeding studies can be determined by atomlc absorption measurement using the graphite furnace. The solvent extractions require little or no sample preparatlon except acidification with hydrochloric acid. The dlgestion of the biological tlssues utilizes a quaternary ammonium hydroxide soiubillzer In toluene. Introduction of thls type of digest dlrectly Into the graphite furnace represents a novel approach. Recoveries of tln added to various extraction medla average 99.8 YO. Determinations of samples in the range of 1 ppb to 20 ppm are possible. The precislon of the method Is 2.31 % relative standard devlatlon when measured at the 250-pg analytical level.
In recent years, the need for measurement of tin levels below 0.2 part per million in a variety of materials has prompted the search for alternative methods of analysis. Sensitive spectrophotometric methods are generally used for trace tin analysis utilizing pyrocatechol violet (1,2)or dithiol for color development (3-5). These analytical reagents are nonspecific and best suited to samples where tin values on materials are no lower than about 0.2 ppm. As demands are made for the determination of increasingly lower concentrations of tin, larger sample sizes must be used, adding to the analytical difficulties and increasing the significance of reagent blanks. Wet oxidation procedures for large size samples become troublesome and limit the utility of the method. Efforts have been made to analyze tin by flame atomic absorption methods. Schallis and Kahn (6,7)have determined tin in oils using a nitrous oxide-acetylene flame. A sensitivity of only 3 ppm for 1%absorption was obtained using the 286.3 nm resonance line. These workers (7), using a fuel-rich air-hydrogen flame, reported a sensitivity of 0.15 ppm at the 224.6 nm resonance line; however, this analytical wavelength is subject to interference. A sensitivity of 0.7 ppm using an argon-hydrogen flame was reported by Nakahara and coworkers (8) using the 286.3 nm line. Concentration and isolation methods for tin have been used by Mensik and Seideman (9) and Engberg (10) followed by atomization in air-hydrogen flames. Lack of sensitivity precludes the use of flame atomic absorption spectrophotometry for the analysis of tin a t the parts-per-billion level. The flameless atomic absorption analytical methods have resulted in numerous recent papers; however, little has appeared in the literature for tin analysis. Everett and West (11) investigated tin as a wear metal in oils using a carbon filament AAS at 5-20 ng tin levels. J. C. Meranger (12) has described a screening method to detect tin stabilizers in alcoholic beverages using a graphite furnace technique. The 0.04-ppm detection limit reported is adequate for monitoring the Canadian regulation which allows up to the l-ppm Sn level (13). A method with greater sensitivity was needed for monitoring levels required by the United States Food and Drug Regulations (14). 1090
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Approval to use the organotin compounds, di(n-octy1)tin S,S'-bis(isoocty1mercaptoacetate) and mixtures with di(nocty1)tin maleate as stabilizers for polyvinyl chloride resins used for packaging food items, has been granted by the Food and Drug Administration (14). In order to simplify the analyses of complex food items, the concept of food-simulating solvent extractions of the plastic packaging materials was initiated. In lieu of analyzing actual food samples, the finished plastics intended for contact with food are end-tested with an appropriate solvent under specified conditions of time and temperature. Depending upon the food product packaged, extractions are made in water, 3% acetic acid, heptane, 8% and/or 50% alcohol. The amended law (15) places a 0.5-ppm tolerance of the permitted octyltin stabilizers in food-simulating solvents, which equates to 0.08 ppm Sn. Many tin analyses in the ppb range are required to study formulated plastic systems. The analysis of animal tissues also represents an area where large numbers of tin analyses are required in order to follow the fate of the organotin compounds during feeding studies. Wet ashing procedures using various acids are time-consuming and require large quantities of samples. We have explored the use of a solvent-based quaternary ammonium hydroxide solubilizer (16) for sample digestion. Barlow and Khera ( I 7)have reported the use of this reagent as a solubilizer for liver and placenta in lead studies. EXPERIMENTAL Apparatus. A Perkin-Elmer Model 503 atomic absorption instrument equipped with a Perkin-Elmer Model HGA-2100 graphite furnace atomizer and its atomization control unit was used. A deuterium discharge lamp was required to correct for the nonatomic absorption signal. A tin electrodeless discharge lamp was used for improved stability and sensitivity at the low tin levels. To reduce noise, an electronic filter, Spectrum Model 1021, was used. This unit has an amplification feature which extends the working range when needed. A Sargent SR recorder which has a variable 1-10 mV input was used for peak height readout. Reagents. The chemicals used were concentrated hydrochloric acid, 97% glacial acetic acid, citric acid, toluene, heptane, 3A alcohol and tin metal, all certified ACS grade. Tin compounds used were di(n-octy1)tin maleate (Thermolite 813), 25.82% tin; di(n-octy1)tinS,S'-bis(isoocty1mercaptoacetate) (Thermolite 831), 16.10% tin; and dimethyltin dichloride, 54.03% tin. These organotin compounds were analyzed by standard wet chemical methods in the M&T Analytical Research Laboratories. Soluene-350 (purchased from the Packard Instrument Company) is a quaternary ammonium hydroxide tissue solubilizer in toluene. The standard stock solution of inorganic tin at 1000 ,ug/mL concentration was prepared in 10% v/v hydrochloric acid. Serial dilutions were made using 10% hydrochloric acid with 8% w/v citric acid as a stabilizer to obtain working standards of 0.10 to 0.01 T/mL tin concentration. 'I standard solutions for use with tissue digests were made in toluene from the specific organotin compound used for a feeding study. The working standards of 0.50 to 0.01 wg/mL were made in Soluene-350 which contains toluene. The standard stock solutions for di(n-octy1)tin maleate and di(n-octy1)tin S,S'-bis(isoocty1mercaptoacetate) were prepared at 1000 wg/mL Sn using 97% acetic acid as solvent. Serial diA-u
Table I. Instrumental Operating Conditions Atomic absorption spectrophotometer conditions Analytical wavelength: 286.3 nm (Perkin-Elmer EDL lamp, 7 mA) Background correction : Deuterium arc, to balance Slit width: 1.0 mm Band pass: 0.7 nm Mode: Absorbance Graphite furnace operation Atomization tube: Untreated graphite tube Inert gas: Nitrogen, about 300 cm'/min, interrupt position for atomization Cooling water: 4 L/min Sample size: 1 0 to 5 0 pL;100 pL Program: Temperature Time 100 "C 30 sa Dry 700 "C 30 s (extracts and aqueous solutions) Ash 60 s (tissue solutions) Atomize 2700 "C 8s Readout: Sargent SR Recorder Time constant 1.0 s Expansion 1 O X or 1 mV (1 pg Sn and below) Readout Peak height as an expression of absorbance a Increase to 60 s when greater than 50-pL sample is used. Table 11. Guide for Sample Injection of Extracts Estimated concn, Sn in soln, MglmL
Volume inject, lJ La
Total Sn to furnace, ng
0.001-0.01
100-25 20- 5 20-10 50-10 25- 5
0.01-0.1 0.1 -1.0
0.01 -0.10 0.10 - 0 . 5 0 0.50 -2.0 2.0 -20 a
Expansion Via Recorder or Via mV Instrument 1 1 10 5
1.0 -5 5.0 - 2 0 20-200
2
1ox 1ox
1x 5x
2x
Gas at atomization Interrupt Interrupt Interrupt Normal Normal
For tissue samples, the volume inject must be approximately doubled.
lutions to 10 pg/mL Sn were made in 97% acetic acid. Working standards at 0.10 t o 0.01 pg/mL were prepared using foodsimulating solvents, heptane and 3% acetic acid. The working standards for the food-simulating solvents, 8% and 50% alcohol, and water were made to also contain 10% v/v hydrochloric acid. Instrumental Operating Procedure. The instrumental operating conditions for the atomic absorption spectrophotometer are shown in Table I. Sample volumes were introduced into the HGA-2100furnace with an appropriately-sized Eppendorf syringe, fitted with a disposable plastic tip. Whenever possible, the inject volume was kept at 10-25 pL. It was possible t o employ larger sample volumes, up to 100 pL, although longer drying periods were required and, sometimes, fogging of the end windows due to condensation occurred. A guide to sample injection is shown in Table 11. Nitrogen was used as the sheath and purge gas. Since the internal gas interrupt mode was used for the lowest tin levels determined, the gas flow was not critical. Interruption of the gas flow occurs at the start of the atomization of the sample to maintain the atoms in the optical path of the instrument for a longer period of time. The spectrum filter for reducing noise was operated at a cutoff frequency of 0.1 Hz. After samples were introduced, the three-stage heating cycle was initiated. The operating parameters for the graphite furnace are shown in Table I. All absorption measurements were made in triplicate to confirm the reproducibility using peak height measurements. Procedure for Preparing Extract Samples. The detailed procedure for the food-simulating solvent extraction of flexible barrier materials intended for contact with food can be found in the literature (18). Our primary concern was with extracts of octyltin stabilizers used in the stabilization of poly(viny1chloride)
and vinyl chloride copolymers. When received, the 3% acetic acid and heptane extract samples required no pretreatment before analysis. Water and the aqueous alcohol extracts were acidified with hydrochloric acid to a 10% level before injection into the furnace. Procedure for Preparing Tissue Samples. Representative tissue samples, in glass vials, were digested readily with Soluene-350 at about 20% w/v concentration, by placing in a 65 "C water bath for 1'/*to 3 h. The clear digest was made to 10% concentration with toluene for injection into the graphite furnace.
RESULTS AND DISCUSSION Optimization of Instrumental Conditions. All peak measurements were obtained using the internal gas in the interrupt mode for the 1-100 ppb tin levels. The cessation of internal purge gas in the HGA-2100 prior to and during atomization cycle greatly increased the absorbance signal. About one third of full chart signal is obtained for the atomization of 0.5 ng of tin using expansion and the interrupt mode; whereas, with normal flow, no signal is observed. The atomization temperature and time were individually adjusted to give maximum values for the absorbance, observed as peak height. The effect of the atomization temperature on the peak height is presented in Table 111. The importance of this study has further significance beyond the obtaining of an optimum signal. This study implied there was no detectable loss of tin during ashing at temperatures up to 1200 " C . Confirmation was obtained by varying the ashing temperatures from 500 "C to 1200 "C and atomizing a t 2700 'C and obtaining a constant signal. The reduction of atomization temperature can be used as a simple way to ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
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Table 111. Effect of Atomization Temperature on Signal for 0 . 5 ng Tin Absorbance, Temperature peak height, in mm "C" ( l o x expansion) 875 None 1000 None 1200 None 1500 1.5 1700 3.5 2000 8 2200 16 2400 20 2600 32 2700 40
0'
TIME-
Figure 2. Signal obtained for 0.5 ng of background
a The temperature shown is in accordance with the meter scale of the atomization control unit.
I
I
0
!
30
!
50
1 60
!
70
100
INTERNAL GAS FLOW - CC/MIN
Figure 1. Response for 20 ng of Sn at various nitrogen internal gas flows
extend the working range of analysis to higher tin levels. At the higher atomization temperature used in these studies, no memory effects in the graphite furnace have been observed. Both nitrogen and argon were used as sheathing gas at about 300 cm3/min; comparable signals for tin were obtained. It was concluded tin does not form nitrides in the furnace and, therefore, nitrogen was used for subsequent studies. When analyzing for higher levels of tih where gas flow is used during the atomization cycle, a control for the gas flow is necessary as shown in Figure 1. The lifetime of the graphite furnace tube decreases when the interrupt is used, but replacement is usually not required until after approximately 200 firings. Lower sensitivity is obtained after prolonged use of a tube. Light scattering by organic combustion products present in the extracts required the use of a deuterium discharge lamp as a corrector for the nonatomic absorption signal. In the case of solubilized tissues, the molecular absorption of volatilized inorganic salts required compensation, although we were not operating at maximum sensitivity. Figure 2 shows the signal and background for 0.5 ng Sn using the combined apparatus described. Linearity f o r Tin. The analytical calibration curve was examined using inorganic tin solutions as the chloride. Ten (10) 1 L injections of the standard tin solutions resulted in a linear response for 0.1 ng to 0.75 ng of tin. By doubling or halving the volume, one can analyze extracts covering the specification limits for simulated-solvent extractions. The reproducibility of the atomic absorption method, obtained by statistically evaluating the peak height in millimeters on five consecutive injections for each level of tin, showed a standard deviation of 0.560 (average). The standard deviation is independent of the total tin being evaluated for the analytical 1092
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Sn showing reproducibilityand
Table IV. Recovery of Organic Tin in Extraction Solvents 0.50 ng Sn added [di(n-octy1)tin maleate] Sn obtained Recovery, Media ng % 3% Acetic acid 0.496 99.2 Heptane 0.487 97.4 Water (10% HCl) 0.500 100.0 8% Alcohol (10% HCl) 0.499 99.8 50% Alcohol (10% HCl) 0.500 100.0 0.50 ng Sn added [di(n-octy1)tin S,S'-bis(isooctylmercaptoacetate ] 3% Acetic acid 0.503 100.6 Heptane 0.475 95.0 Water (10% HC1) 0.503 100.6 8% Alcohol (10% HCl) 0.503 100.6 50% Alcohol (10% HCl) 0.502 100.4 range of 0.10 to 0.75 ng. These results are well within the reproducibility limits necessary for measuring the tin concentration of extracts from plastic surfaces. Organic T i n Compounds. It was considered desirable to determine whether instrument response to both organic tin and inorganic tin was close enough to permit the use of inorganic tin standards for the simulated-solvent extraction samples. Standards containing 0.05 Fg/mL of tin for each of the organotins, di(n-octy1)tin maleate and di(n-octy1)tin S,S'bis(isooctylmercaptoacetate),were prepared in 3 70acetic acid, heptane, 8% and 50% alcohol, and water as described in the reagent section. These solutions were analyzed according to the procedure. The recovery of tin is shown in Table IV. It was concluded that the inorganic tin standard stabilized with citric acid can be used for the analysis of the extracts. The slightly poorer recovery from heptane is attributed to the incomplete transfer of solvent from the Eppendorf pipet, as there is a tendency for the solvent to adhere to the tip. Other tin compounds which we have studied that give equivalent response in the graphite furnace to inorganic tin chloride standards are triphenyltin fluoride and tributyltin fluoride. Application t o P l a s t i c Extracts. Numerous solvent extract samples from plastic films have been analyzed. The accuracy of the proposed method was checked by determination of the tin content by the proposed instrumental method and by the chemical dithiol colorimetric method. The true tin content was unknown. Analyses were obtained on eight water extractions, tin 0.008 to 0.160 ppm; thirteen 3% acetic acid extractions, tin 0.006 to 0.071 ppm; seven heptane extractions, tin 0.018 to 0.430 ppm; five 8% alcohol extractions, tin 0.005 to 0.060 ppm; and eight 50% alcohol extractions, tin 0.036 to 0.057 ppm. The value of the correlation coefficient was 0.9936.
Table V. Total Tin (pg/g) in Beef Tissue after Feeding with Me,SnCl,
Organ Liver Liver Fat Kidney Kidney Brain
AAS graphite furnace (Av. of 3 injects) 0,156 0.056
Chemical
0.016
0.016 0.236
0,099
0.106 0.027
0.016 0.222 0.094
AAS standard addition 0.157
0.024
anal.
pyrocatechol violet 0.156 0.066
0.025
Application to Tissue Digests. The standards for analysis of tissue digests in Soluene-350 are prepared from the particular organic compound used in the feeding study, as described in the reagent section. Table V shows typical data obtained from organ digests of beef cattle fed dimethyltin dichloride. As interference effects of the inter-element type have been reported in flameless atomic absorption studies, some verification using the standard method of additions has been made, as well as comparisons with standard wet chemical colorimetric procedures. There is insufficient data for statistical evaluation at this time, but work looks very promising. Further studies for specific interferences in the graphite furnace analysis of tin are planned as we expand the method to other systems.
ACKNOWLEDGMENT The authors thank Kenneth Buxton, Bionomics, for his valuable discussions and J. Pekola and H. Corbin of the M&T Central Analytical Department for providing the colorimetric tin determinations.
LITERATURE CITED H. B. Corbin, Anal. Chem., 45, 534-537 (1973).
E. J. Newman and P. D. Jones, Analyst (London), 91, 406 (1966). H. B. Corbin, J. Assoc. Off. Anal. Chem., 53, 140-146 (1970). M. Farnsworth and J. Pekola, Anal. Chem., 26, 735-737 (1954). I.M. Kolthoff and P. J. Elving, “Treatise on Analytical Chem.”. Part 11, Vol. 3, Interscience, New York, 1961, p 363. J. E. Schallis and H. L. Kahn, At. Absorpt. Newsl., 7, 84 (1968). H. L. Kahn and J. E. Schallis, At. Absorpt. Newsl., 7, 5 (1968). T. Nakahara, M. Munemori, and S. Musla, Anal. Chim. Acta, 62, 267 (1972). J. D. Mensik and H. J. Seideman, Jr., At. Absorpt. Newsl.. 13, 8 (1974). A. Engberg, Analyst (London), 98, 137 (1973). G. Everett and T. West, Anal. Chim. Acta, 70, 296 (1974). J. C. Meranger, J . Assoc. Off. Anal. Chem., 58, 1143 (1975). Food and Drug Act and Regulations (1973), Information Canada, Ottawa, Ontario, Sec. B 23.003. Food, Drug and Cosmetic Law Reports, Food Additive Regulations, 21 CFR, Section 121,2602, paragraph 56,952. Food,Drug and Cosmetic Law Reports, Food Additive Regulations, Section 121.2602, Fed. Regist., 40, No. 11, January 16, 1975. Technical Bulletin, Packard Instrument Co., Ino., 2200 Warrenville Rd., Downers Grove, Ill., 60515. P. J. Barlow and A. K. Khera, At. Absorpt. Newsl., 14 (6) 149 (1975). Association of Official Analytical Chemists, “Official Methods”, 12th ed., pp 386-390.
RECEIVED for review November 29,1976. Accepted April 27, 1977.
Determination of Sub-microgram per Liter Quantities of Arsenic in Water by Arsine Generation Followed by Graphite Furnace Atomic Absorption Spectrometry AH U. Shaikh and Dennis E. Tallman” Department of Chemistry, North Dakota State University, Fargo, North Dakota 58 102
The arsenic in a large volume (50 mL) of water sample is subjected to NaBH4 reduction and the liberated arslne is trapped in a small volume of a chloroform/ephedrine solution of silver diethyldithiocarbamate(SDDC). The arsenlc in the SDDC solutlon Is then determined by graphlte furnace atomic absorption spectrometry. The relative standard deviation for ten replicate determlnatlons4s less than 3 % at the 5 ppb level. A detection limit of 10 ng is obtained, corresponding to 0.2 ppb for a 50-mL water sample. The detection llmit can be extended to below 0.05 ppb by employing a larger volume of water sample and by evaporation of solvent from the SDDC solution as a means of further preconcentratlon. The relatlve accuracy of the method at the 5 ppb level is better than 5 % as determined from the analysls of EPA reference samples.
The determination of arsenic in trace amount in the environment continues to be of considerable interest, due in large part to the known toxicity of this substance. A variety of methods have been developed for the trace determination of arsenic (I-3), often involving generation of arsine (AsH3)from As(ITI)/As(V) followed by atomic absorption spectrometry (AAS) in which the arsine is atomized by introduction directly into either a flame (2) or an electrical furnace ( 3 ) . Recent
reports ( 4 , 5 )describe an automated technique in which arsine is generated on a hot reaction bed a t high pressure followed by AAS detection. Using A1 and HC1 for reduction ( 4 ) , 40 specimens per hour can be analyzed with a detection limit of 0.1 ppb, whereas NaBH, and HC1 reduction ( 5 ) permits the analysis of 70 specimens per hour with a detection limit of 0.011 ppb. The automated approach, however, requires a somewhat elaborate experimental setup and thus may not be worth pursuing unless many arsenic samples are being determined routinely. It appears that a relatively simple and rapid method for arsenic determination with high sensitivity and precision employing readily available instrumentation is still in need. Such a method is described in this paper. It has been reported (6) that AsH3 forms a stable complex with silver diethyldithiocarbamate (SDDC). A standard method for the determination of As involves the dissolution of generated AsH3 in a pyridine solution of SDDC followed by colorimetric determination ( I ) . More recently, Kopp (7) has shown that 1-ephedrine in chloroform can be used as a solvent for SDDC in the colorimetric method (thus avoiding the disagreeable odor of pyridine) without loss of sensitivity, accuracy, or precision. We have found in our laboratory that AsH3 is very conveniently trapped in a chloroform/ephedrine solution of SDDC which can then be analyzed for As by Graphite-Furnace ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
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