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ductants for this purpose. Besides, hydrochloric acid solutions are very suitable for hydride generation. Of other hydride-forming elements, tellurium will be reduced to the tetravalent state by hydrochloric acid, and the proposed method should therefore be applicable for this element too. This should also be the case for antimony and arsenic provided the hydrochloric acid also contains some iodide, as this is necessary to reduce these elements from pentavalent to the trivalent state. With minor modifications the method should also be useful for copper alloys as brass (Sn02 formed during dissolution must be filtered off; lead should end up on the anode and thus be removed). For copper-nickel alloys, however, nickel will remain in the solution during electrolysis and interfere seriously during the subsequent hydride formation step. However, a method for complexing a large amount of nickel in such analyses is to be published (14);but the method has not yet been tested on real samples.
Registry No. Se, 7782-49-2; Cu, 7440-50-8. LITERATURE CITED (1) Haynes, 8. H. A t . Absorpt. News/. 1979, 18, 46. (2) Ohta, K.; Suzuki, M. Anal. Chim. Acta 1975, 7 7 , 288. (3) Siu, K. W.; Berman, S. S. Anal. Chem. 1984, 5 6 , 1808. (4) BBdard, M.; Kerbyson, J. D. Can. J . Spectrosc. 1976, 21, 64. (5) Muir, M. K.; Anderson, T. N. A t . Spectrosc. 1982, 3 , 149. (6) Manning, D. C. At. Absorpt. News/. 1976, 1 7 , 107. (7) Fernandez, F. J.; Beaty, M. M. Spectrochim. Acta, Part 8 1984, 398, 519. (8) Bye, R.; Engvik, L.; Lund, W. Anal. Chem. 1983, 5 5 , 2457. (9) Vijan, P. N.: Leung, D. Anal. Chlm. Acta 1980, 120, 141. (10) Holak, W. J . Assoc. Off. Anal. Chem. 1976, 5 9 , 650. Adeloju, S. B.; Bond, A. M.; Hughes, H. C. Anal. Chim. Acta 1983. 148, 59. (12) Campell, A. D. Pure Appl. Chem. 1984, 5 6 , 645. (13) Bye, R. Talanta 1983, 30,993. (14) Bye, R. Analyst (London) 1985, 110, 85.
Received for review December 10,1984. Accepted February 11, 1985.
Semiautomated Method for Determination of Selenium in Geological Materials Using a Flow Injection Analysis Technique C h r i s C. Y. C h a n
Geoscience Laboratories, Ontario Geological Survey, Ministry of Natural Resources, 77 Grenville Street, Toronto, Ontario, Canada Following the development of an automated method (1)for the determination of Se in rocks using hydride generation and atomic absorption techniques, an effort has been made to improve sensitivity and reduce analysis time. The new approach is based on the adoption of a nonsegmented stream (2, 3), instead of an air-segmented stream, for performing chemical analysis in a continuous flow system in conjunction with the use of hydride generation and AAS techniques. A flow injection module was utilized for insertion of a sample segment into a continuous flowing stream. Since air bubbles are not required as an agent for mixing and segmenting the solutions, narrow tubing can be used throughout the entire flow system. This offers several advantages: (1) the flow volume being miniaturized allows the transportation of sample to be completed more rapidly; (2) the sample zone is well defined with little dispersion a t the boundary; (3) the sample segment is not being diluted by the carrier solution; (4) the volumes of sample segments are extremely reproducible; and ( 5 ) the sample and the reagent solutions can be well mixed within the narrow stream. Beacuse of these flow characteristics, the signal response is not only rapid and precise but the resolution and peak height are improved. This paper describes the new method. The method permits the accurate determination of Se in geological materials a t levels as low as 5 ppb with a rate of more than 50 digested samples per hour. Se values on 40 international geological reference samples are reported. EXPERIMENTAL S E C T I O N Instrumentation. A Varian Model AA6 atomic absorption spectrometer was equipped with a Model 9176 strip-chart recorder. A Technicon Sampler-I1 and a Proportioning Pump-I were used to sample and propel the solutions. A flow injection module (Model No. 1000-600, Lachat Chemical, Inc.) was utilized for sample injection. In performing flow injection analysis, samples are alternately pumped into and flushed out of the sample loop that is mounted on the valve of the flow injection module. The
connections of these components and the analytical system are shown in Figure 1. A gas-liquid separator was installed to separate the hydride from the waste solution. An impinger partly filled with concentrated sulfuric acid served to remove moisture and to homogenize the hydride-argon mixture. The quartz tube, 16 cm long and 10 mm i.d., with a 10 cm long inlet tube (3 mm i.d.) fused into it at the center, was wound with a 22-gauge chrome1 A heating wire and insulated with a layer of wrapped asbestos string. It was mounted on the burner of the spectrometer. The temperature of the quartz tube atomizer was controlled at 850 *20 "C by a variable transformer. Reagents. All chemicals used were reagent grade, and water was distilled from glass. Mineral acids were hydrofluoric acid (48%),perchloric acid (60%),nitric acid (70%), and hydrochloric acid (38%). Digestion Mixture. In a polyethylene bottle mix hydrofluoric acid, perchloric acid, nitric acid, and water in the ratio of 4:4:1:1, respectively. Reducing Solution. Dissolve 5 g of sodium borohydride and five pellets of sodium hydroxide in 500 mL of water. Store in a refrigerator when not in use. The solution is stable for at least a week at 4 "C. Masking Reagent. Dissolve 1g of 1,lO-phenanthroline in 100 mL of 0.1 M HC1. Stock Se Standard Solution, 1000 wg/mL. Dissolve 0.100 g of powdered selenium in 100 mL of 10% nitric acid. Working Se Standard Solutions. Prepare 0.25,0.50, 1.0, 2.0, and 4.0 ng/mL solutions by serial dilution of the stock standard solution with 3.6 N HC1. Decomposition of Samples. Digest 0.200 g of rock sample with 5 mL of digestion mixture in a 30-mL Teflon beaker on a hot plate for about an hour until white fumes of perchloric acid appear and the volume of the contents reduces to ca. 1 mL. Cool and add ca. 2 mL of water and 4.5 mL of concentrated HC1. Heat the contents t o just under boiling for several minutes to reduce the Se that is in the + 6 oxidation state to the + 4 state. Cool and transfer the contents to a test tube calibrated at 15 mL. Make up to volume with water. Seal the test tube with a piece of Parafilm, and mix the solution thoroughly. The concentration of HC1 in the sample solution is now 3.6 N (30% v/v). Prepare
0 1985 American Chemical Society 0003-2700/85/0357-1482$01.50/0
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985 Heated
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Chart I Atomic Absorption Spectrometer wavelength, nm 196.0 lamp current, mA 8 300 slit width, pm C (maximum) damping 6 expansion Flow Injection Module sampling time, s 30 washing time, s 40
n Sampler
Variable Transformer 50 dial (set to produce 850 2 20 "C in the atomizer)
POSITION 1 (Sampling)
Recorder span,mV chart speed, cm/min
5% HCI
10 1
Argon flow rate, mL/min
300
1% Phenanthroline 1% NaBH4
U Proportioning Pump
Sampler POSITION 2 (Injection)
5%HCI
out the tailings of the sample solution, in preparation for a new cycle of sampling. When the analysis is complete, calculate the Se concentration from a calibration plot of peak height vs. concentration, which is linear up to 4 ng of Se/mL. Multiply the net value (nanograms/milliliter) by 75 to obtain the Se content (nanograms/gram) in rock. The practical detection limit, expressed as 3 times the overall blank value, is 0.2 ng/mL in solution, equivalent to 10 ng/g in rock. If the sample matrix is not unusual, the detection limit can be lowered to 5 ng/g by doubling the sample weight to 0.4 g.
1% Phenanthroline 1 % NaEH4 U
Flgure 1. Analytical manifold for the determination of flow injection and hydride AA techniques.
Se by automated
a reagent blank simultaneously. Decant a portion of the solution to a sample cup for subsequent AAS determination as described below. Determination of Se. Select the parameters listed in Chart I. Set up the hydride generation equipment and connect the tubing that leads to and from the injection valve of the flow injection module according to the layout shown in Figure 1. Mount the quartz tube on the burner with ita side arm connected to the hydride generator with Tygon tubing. Align the quartz tube with the light beam to allow maximum radiation to reach the detector. Switch on the preset variable transformer to provide the desired temperature in the atomizer. Turn on the proportioning pump with all the reagent tubes dipped in water. Introduce the argon immediately with its flow rate regulated at 300 mL/min. Turn on the flow injection module which is interfaced with the sampler. The motions of the injection valve and the sample probe are sychronized. Set the sampling time at 30 s and the washing time at 40 s on the flow injection module. Insert the reagent tubes into the corresponding solutions (see arrangement in Figure 1). As soon as the system has stabilized and the base line is established, the standard and sample solutions which have been loaded in the sampler can then be run sequentially. In position 1,Figure 1,sample solution is picked up from the sample cup and is drawn through the sample tubes (lines 4a and 4 in Figure 1)by the action of a peristaltic pump. It enters port A of the injection valve, filling up the sample loop AB, and the excess exits through port B. The roter of the injection valve then rotates a quarter turn so that ports A and B interchange places with ports C and D (position 2, Figure l),permitting the sample solution in the loop AB to be propelled by the 5% HC1 carrier into the reacting stream. A t the same time, the sample probe swings into the sampler reservoir in a synchronized motion with the roter. The solution in the sample line 4a, which is now 5% HCl, enters port C and exits through port D. This flow will flush
RESULTS AND DISCUSSION Interferences and Limitations. Tests for interferences were performed as they were during the development of the analytical method for Se (method 1) which was reported in ref 1. Once again freedom from chemical interferences due to major constituents, anions, and most of the trace elements in rocks is found. An improvement has been noticed in controlling the effects of Cu and Ni, the two interferents most likely to be encountered. The tolerance limit (tl)of Cu has been increased twofold from 0.15% to 0.30% in rock and that of Ni from 2.25% to 4%. Such high concentrations are normally not to be expected in rock samples. Imai et al. ( 4 ) reported the use of Fe3+ (1000 hg/mL) for controlling interferents, but they did not elaborate on the extent to which it works. To evaluate its effectiveness, Fe3+ was added to Se standard solutions containing different levels of interferents, and the test solutions were run in the absence of 1 , l O phenanthroline. The t l s thus established were compared with those of the present method. The results indicate that both complexing agents exhibit equal ability in controlling interferences. However, one advantage of using phenanthroline over Fe3+remains. During the reduction process that occurs in the reacting stream, starting at the junction of the NaBH, and the sample solutions (point F in Figure l),the cations (Cu, Ni, Fe, etc.) are being partially reduced to metals. If phenanthroline is not being introduced, these metals will deposit on the inside wall of the T joint (point F). As the analyses progress, particularly on samples with high concentrations of metals, the black deposits accumulated may block the passage and reduce the signal-response sensitivity. By using phenanthroline, the cations are chelated and are protected from being reduced to metallic form, thus keeping the flow system clean and the sensitivity consistent. Although the tl of As is 40 ppm, based on the study of the effects between the pure chemicals, in practice rock samples containing much higher levels of As do not render interference. The most obvious reason is that As, particularly As3+,is lost in the acid digestion step, cutting the total As content to a
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Table I. Selenium Concentration (ppb) in Geological Reference Materials this study ref sample
literature values (ref)
mean
n
rsd 70
112 1160 6O
