Anal. Chem. 1982, 54, 1211-1214
1211
Determination of Arsenic, Bismuth, Cadmium, Selenium, and Thallium by Atomic Absorption Spectrometry with a Volatilization Technique Hartmut Heinrlohs" and Helner Keltsch Geochemisches Instituf, Goldschmidtstrasse 1, 0-3400 Gottingen, West Germany
For our studies on the concentration of arsenic, bismuth, cadmium, selenium, and thallium in ores, metals, minerals, soils, rocks, coals, ashes, cement, urban particulate matters, plants, etc., a method was needed that allows the analysis of these elements djown to the lower part-per-billion level. In recent studies (1--5),several authors have adequately documented that volatilization techniques offer several advantages for the analysis of volatile elements in sample types mentioned above. This principle, originally introduced by Bunsen, attracted an awakened interest in the 1950s under the name "volatilization analysis". Nanogram to microgram amounts of volatile elements or compounds can be separated in a stream of carrier gas fro'm relatively nonvolatile matrices at temperatures up to a practical limit of 1400 "C and condensed in a capillary or on a cold finger (6). These new techniques are particularly attractive because of the very low associated danger of contamination from reagents. Taking advantage of the ease with which arsenic, bismuth, cadmium, selenium, and thallium can be separated from powdered samples by volatilization, the authors have modified a technique described by Geilmann (1) and by Geilmann and Neeb (2). The elements are ultimately determined by flameless atomic absorption spectrornetry. EXPERIMENTAL SECTION Apparatus and Equipment. (a) A Perkin-Elmer graphite tube HGA 70 mounted on a Perkin-Elmer 400 atomic absorption spectrometer equipped with a deuterium arc background corrector was used. The instrumental parameters are listed in Table I. (b) The easily constructed apparatus for the separation of the elements is shown in Figure 1. The volatilization is carried out in a quartz tube, ending in an orifice at one end, which is attached to a water-cooled condenser. The surface of the condenser is concave. TQavoid strong cooling, the tip of the central jet goes into the opening of the condenser only for 1mm. The distance between jet and the condenser tip has to be kept constant to obtain reproducible yields. All parts of the apparatus consist of quartz, including condenser. One end of the 65-cm quartz tube has a removable plug with the gas inlet tube. A small outlet tube at the opposite end allows the carrier gas to escape. The quartz tube is heated in a resistance type furnace at 1000-1300 "C. Once used, a standby temperature of 700-900 "C extends the working life of the quartz tube. Alkalies attack the quartz tube; thus it has to be replaced periiodically. The temperature is measured with a thermocouple on1 the quartz tube above the jet. A mixture of 20% hydrogen and 80% nitrogen or oxygen is used as carrier gas, The flow rates are 2-10 L/h. They can be regulated by a needle valve and controlled by a suitable flow meter. The sample is weighed into a quartz boat. Table I1 shows the operating conditions on volatilization. Standards and Reagents. Commercial stock solutions (100 ppm) of arsenic, bismuth, cadmium, selenium, and thallium are diluted to get workimg standard solutions in the range of 1ng/mL, to 500 ng/mL. Then 2% (v/v) concentrated nitric acid and 2% (v/v) hydrogen peroxide (30%) are added to each flask of calibration standards. A nickel solution of 2.7% (w/v) in 2% (v/v) concentrated nitric acid is used as a matrix modification reagent to prevent preatomization volatilization of arsenic and selenium. All reagents (analytical grade) are used without additional purification. Deionized water is needed for all the procedures. Procedure. Sample boats are heated up to 1300 "C for 10 min prior to their first usage. The quartz tube is preflushed by the carrier gas. The water-cooled condenser is inserted. A 0.1-1.0 g portion of the powdered sample is weighed into the quartz boat and transferred with a quartz rod to the middle part of the quartz 0003-2700/82/0354-1211$01.25/0
Table I. Instrumental Parameters on Flameless AAS EDL 197 ( A s ) , 223 (Bi), 229 (Cd), 196 (Se), 277 (Tl) 0.7 (Bi, Cd, Tl), 2.0 (As, Se)
lamp type wavelength, nm spectral bandwidth, rim drying temp, "C drying time, s charring temp, "C
100 45 330 (Bi, Cd, Tl), 490 ( A s ) , 750 (Se) 2000 (Bi, Cd, Tl), 2200 (Se), 2400 (As) 8 20
atomization temp, "C atomization time, s vol of sample solution,
d-. gas
20% H,/80% N,
gas flow rate, L/mina background correction
1.7
(As, Bi, Cd, Tl), N, (Se)
Yes For atomic absorption measurement the gas flow is reduced, a
Table 11. Operating Conditions on Volatilization for As, Cd, Se, TI amt of sample, rng time of volatilization, min temp,a "C carrier gas gas flow, L/h flow of cooling water, L/h a
200-1000
for Bi, Cd, TI
50-70
100-200 30-40
1000-1300 02
N,/H, (80%/20%)
5-10 30-40
1000-1200 2-4 30-40
The temperature i s raised by steps of 100 "C.
tube, which is preheated to 1000 "C. The temperature is raised by steps of 100 "C. The heating period is 30-70 min (Table 11). The separated elements are transported through the jet by means of the carrier gas and deposited on the water-cooled condenser. Upon completion of the flushing, the condenser is pulled out of the quartz tube. The precipitate on the end of the condenser is dissolved with a hot aqueous solution of 2% (v/v) concentrated nitric acid. The solution is transferred to a 1-mL flask with a micropipet. A 20-pL portion of hydrogen peroxide is added. The total volume including washings is 1 mL. This method is applicable for the separation of two element groups (Table 11). Organic materials are ashed in the volatilization tube at 400 "C in oxygen. The materials are covered with quartz powder to reduce the oxidation rate. The separation of the elements is accomplished in oxygen or in a mixture of nitrogen and hydrogen at 1000-1300 "C. The quartz powder is preignited at 1300 "C to avoid contamination.
DISCUSSION Theoretical volatilization rates can of course only be realized under vacuum, and then only with pure substances in bulk. The composition of the vapor phase of binary solid systems is, however, normally not the same as that calculated from the ratio of the volatilization rates of the pure substances, mainly because the volatilization rate of the faster volatilizing component decreases as the surface becomes depleted. Separation will therefore be the more efficient the larger the surface area. Yields differ for different sample sizes, if the dimension of the sample boat is held constant. Moreover, fiial 0 1982 American Chemical Society
1212
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982 D
\
I J
A
Flgure 1. Volatilization tube with jet and water-cooled condenser (mm): electric furnace (A), carrier-gas inlet (B), Teflon plug (C), quartz boat (D), thermocouple (E), centering rlngs (F), carrier-gas outlet (G),water outlet (H), water inlet (I), packing rlng (J), rubber tube (K).
I
100 *I,
90 9.
90 %
80 9.
a 0 -1,
Yield
51,Cd,TI
10
20
30
40
50
60 m i n
+ Flgure 2. Volatilization rates of bismuth, cadmium, and thallium dependent on time: sample, synthetic sample with the elements as oxides; carrier gas, N,/H, (80%/20%); temperature, 1100 "C. TlrnO
T 800'
900'
1000*
1100"
1200'
1300'
C
Figure 4. Volatilization rates of cadmium from sllicate rocks In oxygen (solid line) and in a mixture of nitrogen and hydrogen (dashed he); volatilization time, 60 min.
10
20
30
40
50 Tima
60 m i n
Flgure 3. Volatilization rates of arsenic, cadmium, selenium, and thallium dependent on time: sample, synthetic sample with the elements as oxides; carrier gas, 02: temperature, 1300 "C.
yields depend on the time, on the melting point of the sample matrix, on the temperatures, and on the carrier gas or, rather, on the reaction gas. To get optimum volatilization in practice, it is necessary to raise the temperature by steps of about 100 "C. Otherwise bismuth and thallium can be trapped in the melt. Dependence on Time and Different Carrier Gases. Figures 2 and 3 show the volatilization rates of arsenic, bismuth, cadmium, selenium, and thallium from a synthetic sample. The quartz powder contained the elements as oxides. Portions of this synthetic standard were treated in the volatilization tube at 1100 OC or 1300 "C in a mixture of nitrogen and hydrogen or oxygen. In a mixture of nitrogen and hydrogen Bi, Cd, and T1 were recovered with yields higher than 90% within 30 min. Under the same operating conditions the yields of As and Se were far below 60%. In oxygen at a temperature of 1300 "C, however, the yields of As, Cd, Se, and T1 were higher than 90% within 40 min. Fifty percent of the
800'
900m
1000'
1100'
1200'
1300'
C
Flgure 5. Volatilization rates of thallium from silicate rocks in oxygen (solid line) and in a mixture of nltrogen and hydrogen (dashed line): volatilization time, 60 min.
bismuth was trapped. Subsequent tests (Figures 4-8) show that the yields of As, Bi, Cd, Se, and T1 from rocks depend on temperature and different carrier gases. Standard reference samples were chosen for these experiments. The recoveries of the elements were calculated on the basis of recent literature data. A t temperatures of 1100-1200 "C, cadmium and thallium volatilized completely in both oxygen and a mixture
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
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Table 111. Arsenic, Bismuth, Cadmium, Selenium, and Thallium in Standard Reference Samples orchard leaves N.B.S. S.R.M. 1571
Bowen's Kale
Diabase U. S.G. S.
Basalt U.S.G.S. BCR-1
1.9 2.1
0.77 0.72
0.15 0.12
11.5
(9-13)
(8, 11, 14-16)
0.044
(18) 0.064
0.050
0.046 0.046
(1 7 ) 0.072
( 31
(3
(3)
Cd (PPm) lit. av selected ref.
0.061 0.060
0.17 0.158
0.13
0.72
0.134
( 31
(3
(3)
0.85 (1 7 )
Se (PPm) lit. av selected ref
0.074
0.070
0.13 0.13
Granodiorite U. S.G.S. GSP-1 0.55
As (PPm) lit. av selected ref
0.66 ( 81 0.045 0.037
Bi (PPm) lit. av selected ref
T1(P P ~
lit. av selected ref
w-1
As
(18)
0.111
(4,5,19-21)
(9, 1 9 , 2 1 , 2 4 , 2 5 )
(4,19-23)
(17)
1.28 1.29
0.11 0.114
0.28
0.53
0.294
( 31
(3
(3)
0.15 (17)
100%
50'1'1 i 1 900.
0.11
0.069
40 7.
800'
__
0.082 0.099
f
1000'
1100'
40%
1200'
of nitrogen and h,ydrogen (Figures 4 and 5). The complete yields of arsenic and selenium depend mainly on the reaction with oxygen at temperature of 1200-1300 "C (Figures 6 and 7). Bismuth, however, was completely recovered only in a mixture of nitrogen and hydrogen a t temperatures of 1100-1200 OC (Figure 8). From the different volatilization and condensation behaviors, arsenic and selenium can completely be trapped only as their oxides, cadmium and thallium as their oxides and as the elements, and bismuth only as the element. Under the operating conditions described here, the yields of arsenic and selenium in both nitrogen and a mixture of nitrogen and hydrogen are below 60% although arsenic and selenium are distinctly volatile below 700 "C. Bismuth, however, cannot completely be trapped in oxygen although Bi203is distinctly volatile above 950 OC. These discrepancies reflect the incomplete condensation of those metals or their compounds on the water-cooled condenser at temperatures of 100-200 "C. Precision, Acciuracy, and Detection Limit. The results for the determination of As, Bi, Cd, $e, and T1 in some standard reference samples are summarized in Table 111. The rates of recovery are entirely acceptable. Analytical precision varied with the elements and their abundance. Standard deviations have been computed for different ranges of concentration on the basis of duplicate values according to the procedure proposed by Kaiser and Specker (7): 4-20% As; 5-30% Bi, Cd; 7-20% $e; 2-25% T1. With electrodeless
0.13 0.122
(18 1 0.068 0.080 (18) 0.074
VdO4 2
A Basalt 1 0 Granile
,/
1
1300' C
Figure 6. Volatlllzatlon rates of arsenic from silicate rocks in oxygen (solld line) and In a imixture of nltrogen and hydrogen (dashed h e ) ; volatilization time, 60 mln.
9.6
800'
900'
1000'
1100'
1200'
1300'
C
Figure 7. Volatilizatlon rates of selenlum from silicate rocks In oxygen (solid line) and In a mixture of nltrogen and hydrogen (dashed line); volatlllzatlon time, 60 min.
60%
Bi 50%
1
1
,/
:,
/P
L ; 800'
900'
1000'
1100'
O2
0 '
1200.
1300. C
Figure 8. Volatilization rates of blsmuth from silicate rocks In oxygen (solld line) and in a mixture of nitrogen and hydrogen (dashed line); volatlllzation time, 60 min.
discharge lamps, the limits of detections are 20 ppb As, 1 ppb Bi, 0.1 ppb Cd, 5 ppb Se, and 1 ppb T1. The detection limits can be improved by increasing the sample size. Here, however, a prolongation in heating period may become necessary, since the yields also depend on the sample size.
1214
Anal. Chem. 1982, 5 4 , 1214-1215
ACKNOWLEDGMENT The authors gratefully acknowledge financial support by the German Research Foundation (Grant 145/31). LITERATURE CITED Gellmann, W. 2.Anal. Chem. 1956, 760. 410-426. Gellmann, W.; Neeb, K. H. Z . Anal. Chem. 1959, 765, 251-268. Heinrichs, H. Z . Anal. Chem. 1979, 294, 345-351. Erzlnger, J.; Puchelt, H. Geostandards News/. 1960, 4 , 13-16. Meyer, A.; Hofer, Ch.; Tolg. G. Z . Anal. Chem. 1978, 290, 292-298. Tolg, G. Talanta 1974, 27, 327-345. Kaiser, H.; Specker, H. Z . Anal. Chem. 1956, 149, 46-66. Feldmann, C. Anal. Chem. 1977, 49, 825-828. Subramanian, K. S. Z . Anal. Chem. 1981, 305, 382-386. Smith, R. G.; van Loon, J. C.; Knechtel, J. R.; Fraser, J. L.; Pltts, A. E.; Hodges, A. E. Anal. Chlm. Acta 1977, 9 3 , 61-87. (11) Aslin, G. E. M. J . Geochem. Explor. 1976, 6 , 321-330. 112) Abbev. S.: Glllieson. A. H.: Perrault. G. Can. Met. Mineral Enerov Techno/., MRP-MSL 1975, ' 7 , 75-132. (13) Simon, F. 0.; Brown, F. W.; Greenland, L. P. J . Res. U S . Geol. SUW. 1975, 3, 187-190.
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
. .
(14) Terashlma, S.Anal. Chim. Acta 1976, 86, 43-51. (15) Wangen, L. E.; Gladney, E. S.Anal. Chlm. Acta 1978, 9 6 , 271-277. (16) Morrison, G. H.; Gerard, J. T.; Travesl, A.; Currie, R. L.; Peterson, S. F.; Potter, N. M. Anal. Chem. 1969, 42, 1633-1637. (17) Bowen, H. J. M. "Environmental Chemistry of the Elements"; Academic Press: New York, 1979. (18) Gladney, E. S. Anal. Chim. Acta 1980, 178, 385-396. (19) Brunfelt, A. 0.; Steinnes, E. Geochim. Cosmochlm. Acta 1967, 3 1 , 283-285. (20) Schnepfe, M. M. J . Res. U . S . Geol. Surv. 1974, 2 , 631-636. (21) Golembeskl, T. Talanta 1975, 2 2 , 547-549. (22) Keays, R. R.; Ganapathy, R.; Iaul, J. C.; Krahenbuhl, U.; Morgan, J. W. Anal. Chlm. Acta 1974, 72, 1-29. (23) Laul, J. C.; Ganapathy, R.; Anders, E.; Morgan, J. W. Geochim. Cosmochlm. Acta 1973, 3 7 , 329-357. (24) Nadkarni, R. A.; Haldar, B. C. Radlochem. Radioanal. Lett. 1971. 7 , 305-31 1. 1976, 840, (25) Gregory, J. E.; Lavrakas, V. Geol. Surv. Prof. Pap. (US.) 163.
-. for review December 7, 1981* Accepted March 2, 1982.
Varlable Temperature Controller Scott L. Buell and J. N. Demas" Department of Chemistry, University of Virginia, Charlottesville, Virginia 22903
Spectroscopy often requires a stable temperature in the cell compartment to ensure reproducibility of data from experiment to experiment. For example, our work involves luminescence and kinetic studies on luminescent molecules interacting with micelles. These systems are notoriously sensitive to temperature effects and reasonably reproducible temperature control is essential for reliable results. We describe a variable temperature controller which provides relatively stable temperature control (A-0.5 "C), is inexpensive and simple to construct, and is easily adaptable to a variety of spectrophotometer and fluorimeter configurations. Further, the system is designed to survive a harsh EM1 environment. This latter feature was dictated by the spectrofluorimeter's 40-kV rf xenon arc lamp igniter. The ignition of the lamp regularly destroyed all inadequately protected electronics in its vicinity.
EXPERIMENTAL SECTION The schematic is shown in Figure 1. The system is a simple on-off controller with a DC Wheatstone bridge and a thermistor sensor. Both the thermistor and the flexible Thermofoil heater are attached to the sample cell holder. The trimpot resistor R2 provides for variable temperature control by changing the balance point of the bridge. Bridge imbalance is detected by an operational amplifier. The 8-pin DIP 741 was chosen because it is readily available and operates over a wide voltage range. The variable power supply voltage allows for optimization of the system. When an imbalance in the bridge occurs, the op amp switches on the power transistor which applies power to the heater. The current remains on until the bridge is balanced again. The LED shows when the heater is on. For off-on controllers optimum temperature regulation occurs when the duty cycle is 50%. The duty cycle can be varied by adjusting the power supply which controls the power dissipated in the heater. In our applications, however, we have not found the performance to vary appreciably for duty cycles of -10-90%. The flexible Thermofoil heaters are adaptable to a wide variety of existing sample cell holders and compartments. Figure 2 shows two custom designs used in our work. Figure 2a shows a holder for 2.0 cm diameter cells used in our lifetime apparatus. The design of Figure 2b is for 1.0-cm square cells used in our spec0003-2700/82/0354-12 14$01.2510
trofluorimeter. The Thermofoil heaters are folded to conform to the surface and cemented in place with GE Silastic RTV silicone rubber. The thermistor is either inserted into a hole in the base of the aluminum block or cemented to the outside of the holder with Silastic. The controller circuit is carefully designed to suppress EM1 transients. During the development of this controller, we were plagued by frequent, but random, destruction of the 741 op amp by the rf noise generated by the ignition of our xenon arc lamp. The following features were added to eliminate the failures. D4 and D5 protect the amplifier's inputs. R6, R7, C3, C4, C5, and C6 provide suppression of power supply transients. R8, D6, and D7 protect the output from Q1 switching transients and from EM1 pick up in the heater connections. These features improved the amplifier's life span but did not completely eliminate the problem. The GE MOV transient suppressor was then added t o the ac power line to prevent transients from entering the controller through the ac lines. No failures have occurred since this addition. The layout for the controller printed circuit board is available on request and the printed circuit board can be purchased. For details contact the authors.
RESULTS AND DISCUSSION The system has proved to be robust and flexible. We have three units with accumulated running times of -3 years and have experienced no failures. Figure 3a shows the temperature vs. time profile under actual experimental conditions; a 10-mL sample was used in the holder of Figure 2a. Only minimal insulation was used and precise control of the room temperature was not possible. The exterior of the aluminum holder was covered with a in. sheet of styrofoam and wrapped with plastic tape. The solution temperature was monitored directly with a thermistor. The thermistor's resistance was followed with a Keithley 177 DVM interfaced to an HP-85 microcomputer. During a 12-h period, the temperature varied over a 0.54 "C range with a mean temperature of 25.43 "C. The control can be increased by removing effects of the room air conditioning system. Figure 3b shows the time vs. temperature profile of the controller when the heater and sample cell are well insulated from ambient temperature 0 1982 American Chemical Society
