LITERATURE CITED (1) E. M. Schulman and C. Walling, Science, 178, 53 (1972). (2) E. M. Schulman and C. Wailing, J. Phys. Chem., 77, 902 (1973). (3) R. A. Paynters, S. L. Wellons, and J. D. Winefordner, Anal. Chem., 46(6), 736 (1974). (4) S. L. Wellons, R. A. Paynter. and J. D. Wlnefordner, Spectrochlm.Acta, Part A, 30, 2133 (1974).
(5) "The United States Pharrnacopela", 18th rev., Mack Publishing Co., Easton,
Pa., 1970, p 999.
(6)W. L. Paul, J. Pharm. Sci., 62(4), 660 (1973). (7) J. D. Wlnefordnerand M. Tin, Anal. Chlm. Acta, 31, 239 (1964). (8) G. G. Guilbauit, "Practlcal Fluorescence,Theory, Methods and Technique", Marcel Dekker, Inc., New York, N.Y., 1973, p 190. (9) J. J. Aaron and J. D. Winefordner, Talanfa, 19, 21 (1972).
for review October 249 1975. Resubmitted May 10, 1976. Accepted June 28,1976.
Determination of Tin by Gas Phase Atomization and Atomic Absorption Spectrometry Prem N. Vijan* and Chris Y. Chan Air Quality Laboratory, Ministry of the Environment (Ontario), Toronto, Ontario, Canada
A semiautomated method for the determination of tin In suspended air particulate matter is described. The dissolved tin is reacted with sodium borohydride solution and converted Into gaseous hydrides. The resultinggaseous mixture is combusted in a tube furnace and the atomic absorptlon of tin is measured at 286.3 nm. More than 20-fold concentrations of copper, nickel, antimony, and arsenic interfere. The interferences are eliminated by the addition of sodium oxalate to the sample solutions or by prior copreclpitatlon with hydrated manganese dioxide. Flfty Hi-Voi air filters can be analyzed per day. The relative standard deviation of the method is 6 % with a sensitivity of 0.45 pg/l. and a detection ilmit of 0.1 pg/l.
There is a need for a simple and sensitive method for measuring tin concentrations in environmental, food, and geological materials. The colorimetric methods for the determination of tin are tedious, nonspecific and involve lengthy distillation ( 1 ) . The sensitivity of the conventional flame atomic absorption methods for tin is approximately 0.5 Mg/ml which is insufficient for accurate analysis. Besides, they are prone to chemical, spectral, and molecular interferences (2-4). Tin belongs to a family of metals and metalloids that form gaseous hydrides with a fair degree of ease. The research papers based on the use of gaseous hydrides for atomic absorption measurements by flame and nonflame atomization devices have shown a considerable increase over the past few years. Improvements of sensitivity by several orders of magnitude have been reported compared with conventional atomic absorption methods. Fernandez ( 5 ) reported an absolute sensitivity of 7 ng/ml for tin in dilute hydrochloric acid solution using sodium borohydride pellets for hydride generation and a balloon reservoir for collection. He used an argonhydrogen-entrained air flame and measured the atomic absorption signal a t 224.6 nm. Heated quartz tubes are stable atomizers as compared with flames. They do not require the use of high pressure gases and are better suited for unattended operations (6). Schmidt et al. (7) have briefly listed the advantages of the automated approach. They used an argon-hydrogen-entrained air flame to establish the precision and the detection limits for arsenic, selenium, bismuth, antimony, and tin in aqueous solutions. Pierce et al. (8) applied the same idea to the determination of selenium and arsenic in surface waters using a tube furnace, as in Ref. 6, together with a stripping column used by Goulden e t al. (9). Smith (IO)carried out a systematic study of the interferences encountered in the determination of volatile hydride17aa
0
forming elements using the manual method and the argonhydrogen flame. Very little work, however, was done on tin, and large blanks were obtained. No attempt has been made to investigate the elimination of these interferences. Burke ( I 1 ) successfully used the scavenging properties of the oxide of Mn(1V) for quantitative coprecipitation of traces of antimony, bismuth, lead, and tin from solutions of nickel prior to their determination by atomic absorption spectrometry. The present paper describes a method for determination of tin in airborne dust collected on a glass fiber filter, and a few other matrices by a continuous hydride generation technique with special emphasis on the study and elimination of interferences.
EXPERIMENTAL Apparatus. A Techtron Model AA-5 atomic absorption spectrophotometer was used to obtain all the data. The analytical wavelength used was 286.3 nm and the light source was a tin hollow cathode tube (Westinghouse) operated at recommended current. Argon gas flow rate was regulated at 240 f:30 ml/min by means of a calibrated flow meter. An electricallyheated open ended quartz tube, 10 cm long and 10-mm i.d. with a 4-mm diameter inlet tube was used for atomizing tin in the gaseous stream. The temperature of the tube furnace was controlled by a Variac transformer. A Technicon sampler, proportioning pump, and manifold were used in conjunction with a 10-mV variable range recorder for achieving an automatic operation as previously reported ( 6 ) .The system diagram is shown in Figure 1. The optimum instrumental parameters are summarized in Table I. Reagents. Tin standard solutions were prepared by diluting 1000 ppm stock solution (BDH)and 1%HCl. (All other acids and salts used were reagent grade.) A 1%solution of sodium borohydride (98%Fisher Scientific) was used for hydride generation. Demineralized distilled water was used for preparing all the reagent and sample solutions. Procedure. Sample Decomposition. Hi-Vol Filters. Circular filter disks measuring 5 cm2 were cut from 33 exposed Hi-Vol glass fiber filters by means of a stainless steel punch. They were placed in 18 X 150-mm test tubes held in a 40-hole aluminum heating block. Seven blank disks were placed in the remaining test tubes and 0,0.05,0.10, 0.20, 0.30, 0.50, and 1.00 ml of 15 rg/ml tin solution were added by means of a microburet. Two-ml portions of concentrated hydrochloric acid were added to each test tube. The loaded aluminum block was transferred to a hot plate and the contents of the test tubes were maintained at near boiling temperature for 2 h. The digestates were diluted to 15 ml with demineralized distilled water and mixed thoroughly. The test tubes were centrifuged and 1:lO dilutions of the clear solutions were transferred to the sampling cups for automated tin analysis. The final solutions contained approximately 1%hydrochloric acid. If lower reportable limits are desired, the dilution step is eliminated and instead the sample digestates are neutralized with sodium hydroxide, reacidified with 1.5 ml of 10% hydrochloric acid and made up to 15 ml with distilled water using one drop of phenolphthalein as indicator.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
ARGON 270 mlimln
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NaBH4 2.60 mlimin 0 . I N HCl 1.20 mlimin
IMPINGER
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Sample 3.90 mlimln
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Alr 3.90 mllmln
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Air 3.80 ml/mln Water 3.90ml/mln
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Figure 1. #AutoAnalyzer-AAS system for tin
Minerals. The samples were fused with sodium peroxide in vitreous carbon crucibles. The fusions were dissolved in 1:l hydrochloric acid and diluted to known volumes. The prepared solutions were further diluted with 1%hydrochloric acid to bring their concentrations within the optimum range of working standards. Rocks. About 100 mg of accurately weighed sample was heated with 2 ml of 5:2:10 mixture of H&Od:HN03:HF in a 25-ml Teflon beaker. The contents were taken to fuming. The residue was dissolved in distilled water and neutralized with 5% sodium hydroxide until a precipitate appeared. The solution was acidified with 1ml of 1:lhydrochloric acid and the volume was made up to 50 ml with distilled water. Sludges. About 100 mg of accurately weighed sample was digested with 2 ml of 1:4 perchloric-nitric acid mixture in a covered 25-ml beaker. The contents were heated until white fumes appeared. The digestate was made up to 100 ml with 1%hydrochloric acid. The prepared solution was further diluted with 1%hydrochloric acid as required. Liquid Food and Water Samples. No sample preparation was required. Samples were diluted with 1%hydrochloric acid and analyzed. For solid foods, decomposition procedures recommended by Sandell ( I ) may be used. Coprecipitation with Hydrous Manganese Dioxide. A suitable aliquot of the prepared sample solution containing 0.3 to 1.0 pg of tin was transferred to an 18- X 150-mm test tube and diluted with 10 to 12 ml of distilled water. One-ml volume of 1%manganese sulfate solution was added and the contents were heated to boiling. A half-ml portion of 0.25%potassium permanganate solution was added dropwise to the contents of the test tube and heated to boiling again. The test tube was set aside for 30 min at room temperature. The precipitated manganese dioxide was filtered, under gentle suction, through a disk of glass fiber or Whatman 40 filter fitted into a Gooch crucible. The test tube was rinsed 3 to 4 times with small volumes of distilled water and the rinsings were passed through the same Gooch crucible. The precipitate was wetted with 0.3 ml of 6 M hydrochloric acid followed by 2 ml of 10%hydrogen peroxide to dissolve it completely. The solution was received in the same test tube in which the precipitation was performed. The filter and Gooch crucible were washed several times with small quantities of water so as not to exceed the volume of 15 ml. The final volume was made to the 15-ml mark on the test tube.
RESULTS AND DISCUSSION Effects of Experimental Parameters. Manifold Design. The tube sizes required t o build the manifold shown in Figure 1,were determined by t h e trial and error method t o provide optimum flow rates of various reagents. Use of a two-lobe sampler cam allowed 1 min for sampling and 2 min for wash cycles. These intervals were best suited for obtaining well formed signal peaks returning smoothly t o a reasonably stable
Table I. Optimum Instrumental Parameters Damping Wavelength Slit width Sample time Wash time Recorder span Recorder speed Atomizer temp.
Maximum (D) 286.3nm 100 pm 1 min 2 min 5-mV Full scale 0.5 cm/min 850 "C
baseline. A draught- and smoke-free environment around the ends of t h e quartz tube, and a minimum opening of t h e exhaust pipe damper were necessary to reduce the baseline drift t o half a chart paper division (0.001 absorbance). A sudden drop in sensitivity was always attributed t o either t h e condition of the quartz cell or the manifold tubes. After a prolonged use, a greyish powdery deposit built u p on the inner walls of the cell, possibly due to a reaction between quartz and hydrogen at high temperature. This affected the performance of t h e system. An occasional brushing with a mild detergent and heating t h e quartz tube t o a high temperature restored its performance. T h e manifold tubes required occasional replacement or cleaning with hydrochloric acid. Introduction of air into the system was necessary. Complete blockage of air tubes produced signal peaks which were irregular in shape and took a longer time t o return t o the baseline. Use of two manifold tubes for air supply gave slightly better performance than one tube. T h e system was found t o be quite rugged and trouble free over an operating period of 3 t o 4 months. T h e presence of sulfuric acid dryer was not essential but was desirable because it prevented the condensation of water vapor in the gas delivery tube. Carrier Gases. T h e optimum argon flow rate was between 220 to 280 ml/min depending on the size of t h e quartz cell. A continuous decrease of argon flow rate from 600 ml/min to 100 ml/min increased t h e signal progressively by approximately 60%. However, lower flow rates produced distorted peaks with unstable baseline and higher flow rates decreased sensitivity partly due to dilution and partly due t o t h e shortened residence time in the heated atomizer. T h e use of nitrogen as sweep gas at the same flow rate as argon gave approximately t h e same response (12). Atomizer Temperature. The temperature of t h e quartz tube was 850 f 10 " C for maximum sensitivity and precision.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
1789
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Flgure 2. Effect of acid concentration on the absorbance signal of tin; (-) sample acid concentration vs. absorbance: (- - - -) background acid concentration vs. absorbance
Table 11. Effect of Foreign Elements on Recovery of Tina % Recovery
Interfering element Ni
Amount added, pg ml-1
With Direct copptnb
100 100 100 100 100 100 cu 100 100 100 100 75 100 85 50.0 50 100 65 As 0.3 100 100 100 0.5 82 100 82 5.0 50 98 50 a All solutions contained 0.03 pg ml-l tin in 1%HC1.10 000 pg ml-l Na, K; 1000 gg ml-I P043-, S042-, NO3-, C1-; 40 pg ml-I Pb, Ca, Mg, Zn, Fe, Al; 5 yg ml-l Se, Cr, Mo do not interfere. 1000 pg ml-I of Cu, Ni, Cr, Zn, Fe, A1 gave 100%recoveries. Sb behaves similarly. 0.5 2.0 5.0 10.0 0.5 2.0 5.0
100 90 60 30 100 85
With Na&04, 600 pg ml-1
100 100
Raising the temperature to 950 "C caused progressive reduction in response by 25% and lowering the temperature to 700 O C increased the sensitivity slightly but caused peak distortion and baseline instability. A 10- X 100-mm quartz tube was found to be twice as sensitive as an 18- X 140-mm tube. Sodium Borohydride Solution Concentration. The increase of sodium borohydride solution concentration from 0.6% to 5%, w/v, did not change the sensitivity significantly. A 1% concentration chosen for this work allowed for some spontaneous decomposition of the reagent solution without affecting the calibration. A fresh solution was prepared before use. The solution of sodium borohydride stored overnight a t room 1790
IO0
0
10
20
30
40 TIME (days)
50
60
70
80
Flgure 3. Effect of acid concentration on the stability of a standard solution of tin (0.1 pg per ml)
temperature produced a lower response. Storage in a refrigerator maintained the strength of this solution. Acid Concentration. The sensitivity of the present method was found t o be strongly dependent on the concentration of hydrochloric acid present in the sample and of the acid pumped through line 2 of the manifold diagram. This relationship is illustrated in Figure 2. T h e small plateau of the curve allows some variation in the acidity of the sample solution without adverse effect on the quality of results. Presence of some acid is essential for initiating the reaction. The background acid (line 2 of manifold) was necessary to obtain good signal peaks with stable baseline and to continuously subtract the reagent blank by setting the recorder pen to zero. On substituting distilled water for 0.5N hydrochloric acid in line 2, a reagent blank signal peak corresponding to 1.2 ng per ml tin was obtained. This peak was confirmed to be due to tin by the use of a neighboring 284.0-nm non-absorbing line (Figure 2). Acids such as nitric, hydrochloric, sulfuric, perchloric and their mixtures of equivalent concentration gave equal absorbance signals for 0.1 Kg per ml tin. Matrix Effects. The effect of several matrix elements likely to occur in Hi-Vol filters and other samples was investigated. Of the common elements studied only nicke?, copper, antimony and arsenic interfered. Up to 20-fold concentrations of these four elements did not suppress the signal. The amounts of nickel, copper, antimony and arsenic present in air, in excess of 0.5, 2.5, 1.0 and 2.0 pglrn3 respectively, would interfere. These concentrations are uncommon in typical air samples. Interference due to higher amounts of nickel and copper was effectively removed by the use of coprecipitation procedure and to a lesser degree by providing 600 pg per ml of sodium oxalate in the final sample solutions. The results are summarized in Table 11. Tin impurity was not detected in 600 pg per ml solution of sodium oxalate. All results reported in this paper were obtained without the use of coprecipitation or addition of sodium oxalate. T h e inherent sensitivity of the method allows very high dilutions of the sample solutions thereby reducing the concentration of interferents to a point
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
-
045
Table IV. Determination of Tin in Hi-Vol Filter Samples by Two Methods
0.35 -
040
030
1 f
Mean of Duplicates
-
OZ5:
020
-
0.15
-
010
-
a
~
-
Present method (pg/m3)
Flame AASa (pg/m3)
206.1 f 0 20.6 f 0 183.1 f 7.0 260.7 f 0 104.3 f 4.2 151.6 f 0 104.9 f 1.2 Nitrous oxide-acetylene.
215.2 f 21.2 20.0 f 0 184.9 f 9.1 266.8 f 0 97.0 f 0 145.5 f 0 106.1 f 3.0
~~~~
Table V. Determination of Tin in Different Sample Matrices Manufacturer I, Manufacturer 11, p g rnl-la wg ml-l
Foods Figure 4. Typical recorder tracings of tin; (A) triplicate peaks for sample No. 1; (6) triplicate peaks for sample No. 2
Table 111. Precision Data for Determination of Tin in Three Hi-Vol Filters Sample No.
I
I1
No. of detns Mean (ng ml-1) Mean (pg/m3) Std dev Std error Re1 std clev
12 21.3 0.126 0.86 0.25 4.06
12 48.2
0.285 2.86 0.83 5.94
111 12
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