plies suitable for this circuit. The lamp current supply consisted of two floating *200 V-40 MA (0.04zregulation) power supplies connected in series. The +15-V power supply should s~ipplyat least 60 mA current with 0.02 % regulation against line or load changes. Transistor T1 should be mounted on a heat sink insulated from chassis ground and supplied with a cooling air stream from a fan. Unless otherwise noted, R1-R6 are one-half watt metal film resistors; R8-R9 (20 watt) and R10-Rl3 (10 watt) are wirewound resistors. R14 is a ten-turn potentiometer equipped with an appropriate dial. It is necessary t o fit each lamp with a cable and connector wired t o the proper pins to place the appropriate load resistor, R8-Rl3, in series when the cable is plugged into the chassis lamp connector.
In this way inadvertant overloading and possible damage to the lamp is avoided. The approximate maximum possible current is 400 volts divided by the sum of the load resistor and the information resistor, R5. The actual current will be less than this due t o a potential drop across the lamp.
RECEIVED for review December 11, 1971. Accepted March 20, 1972. This work was performed under the auspices of the U. S. Atomic Energy Commission. Reference to a company or product name does not imply approval or recommendation of the product by the University of California or the U. S. Atomic Energy Commission to the exclusion of others that may be suitable.
Pyrolytic Graphite-Tube Micro-Furnace for Trace Analysis by Atomic Absorption Spectrometry K. I. Aspila,’ C. L. Chakrabarti,2 and M. P. Bratzel, Jr. Department of Chemistry, Carleton Uniuersity, Ottawa, Ontario KIS 5B6. Canada
CONSIDERABLE INTEREST has been shown recently in carbon rod atomizers (1-6) and graphite furnaces (7-11) for analysis of samples in various matrixes by atomic absorption and atomic fluorescence spectrometry, because of the increased sensitivity and lower detection limits obtained with such atomizing devices over conventional flames usually used for atomization and because only extremely small volume samples (Le., microliter) are required for analysis. Kirkbright (12) has recently reviewed the field. L‘vov (7, 8) has studied the use of a graphite furnace lined with pyrolytic graphite. He observed that pyrolytic graphite has several desirable advantages over standard graphite (7, 8): low gas permeability, which reduces the loss of atomic vapor by diffusion through the furnace walls ; high thermal conductivity, which permits uniform heating of the furnace and therefore a more uniform temperature of the atomic vapor; and high resistance to oxidation and a high sublimation point (3700 “C), which give the furnace a longer lifetime. Also, pyrolytic graphite is pure and has the advantage that the aqueous solution of the sample does not soak irreproducibly into the walls of the furnace as is 1 Present address, Food and Drug Directorate, Custom House, Room 725, Pesticide Lab., 1001 West Pender Street, Vancouver 1, B.C. 2 To whom all correspondence should be directed.
(1) T. S. West and X. K. Williams, Ami. Clzim. Acta, 4527 (1969). (2) M. D. Amos, P. A. Bennett, K. G. Brodie, P. W. Y . Lung, and J. P. Matousek, ANAL.CHEM., 43, 211 (1971). (3) K. G. Brodie and J. P. Matousek, ibid., p 1557. (4) M. P. Bratzel, Jr., C . L. Chakrabarti, R. E. Sturgeon, M. W.
McIntyre, and Haig Agemian, ANAL.CHEM., 44, 372 (1972). (5) M. P. Bratzel, Jr., and C. L. Chakrabarti, Anal. Chim. Acta, in press. (6) Gwen Hall, M. P. Bratzel, Jr., and C. L. Chakrabarti, unpub-
lished data. (7) B. V. L’vov, Spectrochim. Acta, 24B, 53 (1970). (8) B. V. L’vov, “Atomic Absorption Spectrochemical Analysis,” J. H. Dixon, translator, Adam Hilger, London, 1970. (9) H. M. Donega and T. E. Burgess, ANAL.CHEM.,42, 1521
(1970). (10) H. Massmann, Spectrochim. Acta, 23B, 215 (1968). (11) R. Woodriff and G. Ramelow, ibid., p 665. (12) G. F. Kirkbright, Aialyst (Lodon),96, 609 (1971). 1718
the case with standard graphite (3, 5-8). However, the L’vov furnace is complicated. A resistance-heated pyrolytic graphite tube furnace of much simpler design was evaluated for atom production in atomic spectrometry for trace analysis. An argon atmosphere was maintained inside the furnace. Parameters studied include the applied voltage for resistance heating, preheating for solvent removal prior to atomization, and the flow rate of argon through the cell. The system was evaluated using samples of gold as a chloro complex in both aqueous and organic solvents. Design changes and improvements are suggested to improve the sensitivity, detection limit, and the precision of the measurements. EXPERIMENTAL
Apparatus. The atomic absorption instrumentation used was similar to that used previously (4-6) except that the usual atomizer burner was replaced with the graphite tube furnace. The signal was recorded on a strip chart recorder with a full scale pen response time of one second, which was adequate for accurately following the transient absorption signal. The furnace was evaluated for gold using the 242.80 nm resonance line. Description of Furnace. A cross-sectional schematic diagram of the graphite-tube micro-furnace is shown in Figure l. The furnace is held together with quartz tubing. The radiation from a hollow cathode discharge lamp enters the cell through the quartz window [3] and exits through the vapor exit port [12], and is focused at the pyrolytic graphite furnace tube [SI into which the sample is placed with a conventional microliter syringe. The furnace tube is about 36 mm long and has a n inner diameter of about 4 mm. Other furnace dimensions are not critical. The furnace is externally cooled with a continuous flow of compressed air aimed at the power supply leads where they connect with the graphite electrodes. Power was supplied to the furnace from a step-down transformer variac which could provide up to 400 amperes through the cell. The normal operating range was 20 to 150 amperes. An auxiliary variac of low power provided about 10 to 15 amperes for preheating the furnace prior to atomization of the sample to evaporate the solvent. Switching was accomplished with a manual double-pole, double-throw switch, and timing was done with a stopwatch.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
L
I
.6
3 12
w
0 2
U m K
.4
8 .3 m U
.2
Figure 1. Cross-sectional schematic diagram of graphite-tube furnace
.I
1. Argon inlet ports Ouartz - cell 3. Quartz end window 4. Pyrolytic graphite electrodes 5. Power supply leads 6. Graphite support blocks 7. Viewing port 8. Pyrolytic graphite furnace tube 9. Sample entry port 10. Sample entry port stopper 11. Graphite bames 12. Atomic vapor exit port
0 100 120 140 160 180 INPUT VOLTAGE ON POWER VARIAC
3
I.
Figure 2. Variation of absorbance of gold as a function of applied voltage through graphite cell Argon flow rate
=
1.12 l./min
c
.30
Reagents. All chemicals used were of analvtical reagent grade purity. The 1000 pg/ml gold stock solu6on contained a dilute aqua regia matrix. The acid concentration of all aqueous dilutions was maintained such that the final solutions (v/v) hydrochloric acid. Distilled contained about 20 water was used. Procedure. Aqueous solutions were analyzed either directly or the gold was extracted into freshly acid-equilibrated methyl isobutyl ketone (MIBK), which was then analyzed. The volume of sample which could be syringed into the furnace was 0.1 to 50 pl. The peak height of the transient signal was taken as the measure of absorption. The various experimental parameters were optimized as discussed below. The time required between sampling was 3 to 4 minutes because the cell temperature has to return to room temperature before the next sample is injected.
I
0
I
1.0
2.0
3.0
ARGON FLOW RATE IN V M I N
Figure 3. Variation of absorbance of gold as a function of argon flow rate Data obtained with preheating cycle (60 V for 20 sec) and atomization (160 V for about 5 sec)
RESULTS AND DISCUSSION
Optimization of Experimental Parameters. All studies to determine the optimum experimental parameters were made with a sample volume of 5 pl. The variation in the absorbance of gold ( 5 pg/ml solution) with the input voltage is shown in Figure 2. No pre-heating of the cell was done for this portion of the study. The location (Le., the time to reach the peak absorption after the power was on) and the shape of the peak were dependent on the input voltage, longer times being required for lower voltages. At higher voltages (Le., with current above about 140 amperes), the peak became quite sharp and occurred about one second after the power was on. There was no optimum voltage for atomization over the range studied. Higher voltages were not studied because switching was accomplished manually, and also because, upon switching, current surges of 200 amperes or greater tend to accelerate the deterioration of the graphite tube. The selection of 160 volts was considered a reasonable compromise as this voltage delivered 150 amperes in 5 to 6 seconds. Several factors control the atomization and the concentration of the atomic vapor in the furnace. At low voltages, there is insufficient heat to form atomic vapor, and also the gold chloro complex is not completely dissociated at low tem-
peratures. Since there is argon gas flowing through the cell, the atomic vapor is swept out of the cell and thus out of the optical path. Also, in the cooler regions of the system, condensation of gold vapor can occur, thereby creating potential memory effects. At higher voltages, the atomization is faster because of the supply of a greater quantity of heat per second; this results in a higher concentration of atomic vapor in the cell. For better atomization, temperature programming was used in the form of cell pre-heating at 15 i 5 amperes for twenty seconds prior to the atomization step. The advantage of preheating is that the solvent was evaporated prior to the atomization step, reducing sputtering of the sample and improving the precision and the sensitivity (about 10 to 2 0 2 of the absorbance). The effect of argon flow rate through the furnace on the sensitivity is shown in Figure 3. The absorbance decreases with increasing gas flow rate because the sample is diluted and swept out of the cell (and hence, out of the optical path) more quickly. The maximum absorbance should be observed in a
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
1719
0.3
w
CONCLUSIONS
-
0.2
-
0.1
-
0
z
m a K
zm U
0
5 CONC.
10
I5
20
25
OF GOLD IN MIBK. GxlO‘O
Figure 4. Calibration curve for gold as chloro complex in MIBK closed cell (furnace) in which higher pressure (than the atmospheric pressure) can be maintained but this cannot be done with the present design in which the exit port is open to the atmosphere. By flushing out air from inside the cell after sampling, the argon not only provides an inert atmosphere which prevents oxidation of the graphite and formation of compounds between gold and the gas phase species present in the cell, but also flushes the system clean of all atomic vapor after each analysis. The system is therefore self-purging. A typical calibration curve is shown in Figure 4, obtained by using a MIBK extract of gold chloro complex. The sensitivity for gold in the MIBK extract or in the aqueous solution (directly) is essentially the same, 2.9 X IO-l* gram (absolute). In view of the primitive heating circuitry used, this value compares favorably with the value of 1 x lo-” gram (absolute) obtained by Bratzel et al. (4) with a carbon rod atomization device, and of 1.0 X lo-’* gram (absolute) obtained by L’vov (7) with a graphite crucible. The coefficient of variation is about 10% for measurements at the 5.0 x gram (absolute) level, compared with 5-8 obtained by others using either a graphite furnace (7) or a carbon rod atomizer (4). The advantage of the solvent extraction step is that the gold is selectively removed from the matrix and potentially interfering species. A tracing of the signal obtained with gold in the aqueous phase without the pre-heating step shows a number of irreproducible scatter and molecular absorption bands which tend to obscure the gold absorption signal. Pre-heating removes the solvent and reduces the magnitude of these extraneous signals. A tracing of the signal obtained with the MIBK extract shows that the scatter and molecular absorption bands have been greatly reduced. Also, solvent extraction effects a very desirable and often needed pre-concentration of gold in the sample to be analyzed.
1720
ANALYTICAL CHEMISTRY, VOL. 44,
This preliminary study has demonstrated the potentialities of a simple pyrolytic graphite tube furnace as a nonflame atomization device for use in trace analysis by atomic absorption spectrometry. Aqueous samples can be analyzed satisfactorily and reproducible absorption signals obtained. Gold in either MIBK or dilute acid yields the same sensitivity. Studies with a “mini-Massmann” carbon rod atomizer have shown that standard graphite results in diminished and irreproducible signals (3, 5 ) with aqueous samples unless the rod is pretreated with an organic solvent to inhibit penetration into the rod. This is not necessary for a pyrolytic graphite furnace. This means that aqueous samples can be analyzed directly without pretreatment of the rod and that the sensitivity should be independent of the solvent employed. The higher cost of pyrolytic graphite would be offset by the increased lifetime of the furnace. As a result of this study, several modifications of this prototype furnace are suggested to improve the sensitivity, precision, and ease of use of the furnace. The use of more sophisticated electronic circuitry for temperature-programmed preheating and atomization is suggested so as to increase the rate of rise in temperature, to maintain the constancy of this rate, and to increase the maximum temperature attainable. The division of the preheating step into two distinct steps would permit selective drying to remove the solvent, followed by ashing of the sample to remove any matrix that is volatile or rendered volatile by combustion, such as organic materials. Thus, most of the extraneous signals observed above should be eliminated. Other suggested modifications are the use of water rather than air for cooling so as to effect a faster return to room temperature of the furnace and hence decrease the time between measurements. A longer portion of the tube could be heated so that condensation of the atomic vapor in the cooler regions of the furnace still within the optical path is eliminated. The addition of a window (either a movable quartz plate or a transverse inert gas stream) at the exit port would permit production of much higher concentration of atomic vapor, followed by its rapid removal. Such a modified nonflame atomization device will have obvious potentialities for quantitative determination of traces of other elements as well. ACKNOWLEDGMENT
The authors are grateful to J. P. Mislan, General Chemistry Branch, Chalk River Nuclear Laboratories, Atomic Energy Commission Limited, Canada, for the loan of the prototype graphite-tube micro-furnace. RECEIVED for review December 9, 1971. Accepted March 15, 1972. This work was done under a contract agreement with Cominco Limited, to whom the authors are grateful for financial support.
NO. 9, AUGUST 1972
