lamp, thus, minimizes water vapor interference in the determination of SO2 concentrations in air. In the SOz/HZO-air measurements in the non-Teflon coated cells, the quenching of the SO2 fluorescence must be due partly to an uptake of the SO2 by the HzO on the surface. It has been observed (9) that the rate of SO2 depletion from laboratory air onto walls of the experimental (9) D. J. Spedding,Nature (London), 224, 1229 (1969)
container increases very markedly as the humidity of the air increases. More studies have to be done to delineate the interaction of SO2 with water layers on surfaces. Received for review September 7, 1973. Accepted February 21, 1974. This work was supported by the Measures for Air Quality Program a t the National Bureau of Standards. Standard Reference Material 1626 was supplied by the Office of Standard Reference Materials, National Bureau of Standards, Washington, D.C. 20234.
Temperature Controlled Heating of the Graphite Tube Atomizer in Flameless Atomic Absorption Spectrometry Gillis Lundgren, Lars Lundmark, and Gillis Johansson Department of Analytical Chemistry, University of Umea, 901 87 Umea, Sweden
A temperature controller for graphite rods or tubes in flameless atomic absorption is described. An infrared detector senses the radiation from the graphite and the power is regulated by a triac. The temperature of the graphite tube is raised rapidly and then kept constant within f10". The atomization procedure can be optimized which is important when interfering substances are present. It is thus shown that cadmium can be determined in sodium chloride at sea water concentrations with a detection limit of 0.03 gg Cd/l. at an atomization temperature of 820". The determination of lead was made both with the common constant voltage heating and with the described controller and the results were compared.
rate and the final temperature will change. The operating conditions can thus be selected until the best analytical result is obtained for an element. A rapid atomization cannot, however, be obtained together with a low final temperature. This paper will describe a device capable of giving a temperature-time relation as that shown in the right part of Figure 1. The temperature rises rapidly until a preselected value is obtained, then the power is controlled so that the temperature remains constant. The heating rate and the final temperature can be set independently. The instrument to be described utilizes an infrared sensor to measure the temperature of the graphite tube and a triac for power regulation.
EXPERIMENTAL In the flameless atomic absorption technique, a graphite rod or tube is heated electrically to a temperature where sample atomic vapor is formed in the light path. The atomization temperature depends on the element to be determined; for cadmium, the temperature must be W, and for chromium, it must be 1700". At these temperatures the atomization is fairly rapid but it starts much earlier, for example, cadmium begins to evaporate at about 500". It is important to reach a sufficiently high temperature before a sizeable fraction of the element disappears out of the light path by diffusion. As an example, at temperatures between 500 and 800", the signal for cadmium will be very dependent on the temperature and its time course. On the other hand, the temperature should not be increased more than necessary to completely vaporize the element, because other ions or sample components may interfere. In analyzing biological samples, a close control of the ashing temperature is important to prevent losses of easily vaporized elements during sample pretreatment. The instruments described in the literature are made so that a constant voltage is applied over the graphite rod or tube. The temperature will increase with time until the heat losses balance the supplied power. The left part of Figure 1 shows a typical temperature variation obtained with a constant voltage. By selecting various voltages, e.g., by tuning a variable transformer, both the heating 1028
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 8 , J U L Y 1974
Spectrometer. A Heath EU-700 E monochromator and a Hamamatsu R 456 photomultiplier were used together with a sample-and-hold amplifier. By chopping the light electrically, the background emission from the graphite tube can be subtracted from the signal. Two time-separated channels are used, one for the metal hollow-cathode lamp and the other for a hydrogen lamp. The principle of light measurement is similar to that described by Cordos and Malmstadt (I). The values of the light intensities before the run starts are stored in a sample-and-hold memory so that the quotient and absorbance values are available continuously for both channels. The nonselective absorption can be subtracted from the total absorption and the time integral can be taken. The time for the various steps in a cycle can be set on a control unit; drying, ashing, pressure regulation, atomization, and integration. The base-line stability of the spectrometer corresponds to about &0.003 absorbance unit for the time corresponding to an analytical cycle. Furnace. The furnace can be used both as a Massman and L'Vov type but in this paper only the Massman mode of operation will be treated. The furnace is described in more detail in another paper ( 2 ) . There are quartz windows in the light path and the furnace is gas-tight. It was filled with argon a t atmospheric pressure during the experiments described in this paper. The graphite tubes were machined from Ringsdorff RWO spectrographic graphite. The length of the graphite cuvettes was 10 cm and the i.d. 4 mm. The furnace was water-cooled. A photodiode, HP 4220, was mounted in a holder as shown in Figure 2 with a glass lens, f = 10 mm, to concentrate the radiation on the small sensitive area of the diode. This combination of (1) E. Cordos and H . V . Malmstadt, Anal. Chem., 44, 2277 (1972) (2) G. Lundgren and G. Johansson, Talanfa, in press.
FAIL-SAFE CIRCUIT r
-
i
CURRENT TO VOLTAGE
-
DIFFERENTIAL
'
1
AMPLIFIER
CIRCUIT
Temp setting
Figure 1. Schematic picture of different ways of graphite tube heating. The left part shows heating with constant voltages and the right part, the principle of temperature controlled heating
P
a
/
ATOM I E
-15V
a erture
filter
- 15
R3
h p2Pl
photod1odt
R2
& -.
0
diode lens Figure 2. Temperature sensing unit lens and photodiode results in both high sensitivity and low dark current, both of which are necessary requirements for operation down to 550". A red filter, Kodak Wratten No. 92, cuts off wavelengths shorter than 620 nm. This filter is necessary in part of the temperature interval to avoid ambiguity which results from the combination of the sensitivity curve of the diode and the wavelength variation of the black body radiation. An aperture restricts the viewed area to the center of the graphite tube. The holder is mounted so that i t receives infrared radiation via a gas-tight window from the middle of the graphite rod. Temperature Control Unit. Figure 3 shows a block diagram of the control unit. The signal from the photodiode is converted to a voltage which is compared with a preset value. If the photodiode voltage is less than that prescribed, a gate current flows to a triac and full power is applied to the graphite tube. The power delivered when the triac conducts can be varied by changing the tapping of a step-down transformer. When the preset voltage is reached or exceeded, no gate current is passed to the triac and conduction stops. The circuit is thus a n on-off regulator where the shortest power unit is a full mains cycle. A fail-safe circuit stops conduction if the photodiode is improperly connected or seriously damaged. Circuit Diagrams. The circuit diagram is shown in Figure 4. The potentiometer PI balances the dark current of the photodiode, the amplifier A converts the net current to a voltage. This voltage is compared with the voltage set on P2 in the amplifier B. C is a comparator which changes the polarity. If the output from B is less than zero, a current passes the diode D2 into the trigger circuit, which then stops conduction of the triac. To adjust the fail-safe circuit, the photodiode is disconnected and the bias of amplifier A is adjusted to a slightly negative output which is a m plified further in D so that conduction of the triac is prevented. The photocurrent changes over several orders of magnitude when the graphite tube is operated from 550" to 2600". A coarse setting is made by changing R, with a switch and a fine control is made with Pp. The trigger circuit, shown in Figure 5, uses two photocouplers OCI. The first provides an electrical insulation between the mains and the control circuit. When it conducts, a trigger voltage makes the thyristor fire which shorts the alternating current through C2. R3, and Rq. If no gate current is passed through the thyristor, it
I
GRAPHITE TUBE
PHOTODIODE
time
time
TRIGGER
R10 Dl
Figure 4. Circuit diagram of temperature controlling unit Parts list: C, = 0.47 r F : D T - D ~= 10 D 1 ; A = Analog Devices 502 J; B = Analog Devices 741 K ; C. D = Analog Devices 741 C; P1 = trimpot. 10 kQ; P2 = trimpot. 1 k i l ; R1 = 10 MQ; R2 = 47 MQ; R, = 1 MR. 500, 200, 100, 50, 20, 10, 5, 1 kQ and 500 Q mounted on a 10-step switch; R3 = 14 kR; R1, R5 = 50 k Q ; R6, R g = 5 kQ; R7 = 500 kQ; Re = 1 MQ; Rlo. R1, = 1 kR; Photodiode = HP4220 will represent an open circuit and the positive half-period of the mains will start conduction through the light emitting diode in OCI2. The transistor TI will then conduct and deliver gate current to the power triac. When the triac fires, there will be no voltage difference across the triac and the current through the light emitting diode in OCI2 will cease. A current will then pass through De and Re to charge the capacitor Ca as there now will be a voltage drop over the main transformer when the triac conducts. On the negative half-period, the capacitor CJ will discharge through D5 and the light emitting diode causing the triac to fire again. Once fired, the circuit will conduct for the full power period, which is a necessary precaution in order to prevent dc magnetization of the transformer. The thyristor can stop conduction only when the mains voltage passes zero and, therefore, the triac can be fired only at the start of a period. An auxiliary transformer delivers energy to the gate circuit. The triac was an International Rectifiers, type 60 AC60, which can deliver 60 A continuously. The load requires normally about 55 A for the fastest heating rate. This triac requires a negative gate current, other types requiring a positive gate current may be fired after changing the direction, i.e., polarity of the gate current.
RESULTS AND DISCUSSION Regulator Performance. Figure 6, lower trace, shows an oscilloscope record of the temperature rise as a function of time. The oscilloscope was connected to the output of amplifier A, Figure 4, and the temperature scale is not linear as shown to the right. The upper trace shows the voltage over the load, and it is seen that when the desired temperature is reached, only about every eighth period is passed on through the load. There is no temperature overshoot at this or a t other temperatures. A N A L Y T I C A L CHEMISTRY, VOL. 46, N O . 8, J U L Y 1974
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Figure 5. Circuit diagram of trigger circuit for triac PartslistC, = 1oogF/35V;C2=0.1pF/400V;C1 = 0 . 3 5 ~ F / 4 0 0 V ; C ~ = 5 0 0 , t F / 3 5 V : C r = 1 0 0 , t F / 3 5 V ; D ~ - D s = 1 0 D 6 ; D 7 - O l o = 1 0 D 1 ; O C l r , 0 C 1 2 = Optically coupled isolator (Faimhild FPLA 820): R, = 31 k n . 2 W: R2 = 56 kn. W R3 = 1 kR, 'A W; R4 = 10 k R , 9 W R6 = 910% 1 W; Rs = 13 kfl.'~W:Ri=llO~;R~=22~;ThvrIsto1 r= 0 6 0 1 1IRI:T. = C 4 2 6 : T r i a c = 6
-
~
T e
1555 1445 1585 1825
2000 and the voltage across the furnace (upper trace) The smallest heat unit that can he passed through the graphite tube is one power cycle. At 22 V output from the transformer, the heating rate was ahout 900"/sec which corresponds to a swing of 18" in temperature of the tube for each power cycle. These variations can he seen on the oscilloscope traces in Figure 6. It can also he seen that the cooling is about five times slower than the heating. The transformer is also tapped a t 11 V and if this output is used, a lower heating rate 300"/sec, will he obtained as well as a much smaller temperature swing. Normally an output voltage of 22 V was used from the transformer. The control unit was calibrated with thermoelements a t lower temperatures and with an optical pyrometer a t higher, using a value of the emissivity of carhon of 0.95. Table I shows the accuracy of the regulation, including the swing, a t various operating temperatures. The results show that the graphite cuvette can he rapidly heated to a preset temperature between 150-2000" which is then kept constant within &loD. Analytical Applications. Determination of Lead. Figure I shows a comparison between a furnace using an almost constant voltage on the right, and a temperaturecontrolled furnace as described above, The conventional heating was made using a Perkin-Elmer HGA 72 together with a Perkin-Elmer 300 atomic absorption spectrometer. In order to increase the heating rate, the HGA 72 delivers more power at the start than a purely constant voltage circuit. Figure I shows that in the temperature-controlled furnace, there is a rapid increase in absorption starting a t 950". If the temperature is increased further, a constant 1030
ANALYTiCAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974
4
IO
5
15 12 15 12
signal is obtained above 1060". At 1015", the vapor press u e of Ph is 1 Torr (3) and, according to calculations by L'Vov ( 4 ) , a vapor pressure of a few Torr will he sufficient to obtain a vaporization rate which is larger than diffusion out of the light path. In the Perkin-Elmer HGA 72 graphite furnace, appreciable losses of lead occur during the heating so that a constant absorption signal is not ohtained until above a setting of 2000". The settings represent the final temperatures and the peak will appear before this is reached. The settings alone cannot he used for measuring the atomization temperature, and the evaluation is further complicated by a simultaneous change of heating rate and final temperature. Determination of Cadmium. Figure 8 shows the ahsorption for Cd a t 228.8 nm as a function of.the temperature. The samples were 10 pl of 3 p g / l . Cd in 2% NaCl solution. It is seen that a sufficiently rapid vaporization is obtained a t 820". The amount of nonselective absorption is .also shown in the Figure, and it increases with temperature to a value of 0.6 absorbance unit a t 900". This absorbance value is close to the maximum which can he handled with sufficient reliability by the background corrector of our instrument. Cadmium can thus be determined a t 820" in 2% NaCl using the temperature controlled heating. Figure 9 shows the absorption as a function of the amount of NaCl in the sample, and it is seen that an almost constant signal is obtained. The results have been corrected for the cadmium in the sodium chloride which (3) See e.g. "Handbook of Chemistry and Physics." The Chemical Rubber Co., Cleveland. Ohio, 1966, p D l l l . (4) 0. v. LIVov. "Atomic Absorption Spectrochemical Analysis," Adam Hilger, London, 1970.
temperature controlled heating (0.5 ng Pb)
o*'t,:/ 1000
max temp for HGA (2650')
OC
'
temperature 1500 setting in atomization ' ' ' 2000 step I - . - '
4
Figure 7. Comparison between temperature-controlled heating and conventional heating in analyzing lead at 283.3n m
t
Apeak
P
I
f d
*peak
nonspecif I C \absorp!ion
I
0 I
I
"C
Figure 8. 0 ) Cd signal at 228.8 nm (45 picogram C d ) in 10 pI 2% NaCI; 0 ) nonspecific absorption from 1 0 pl 2% NaCl as a function of final temperature in atomization step
was 0.035 pg/gram. The results plotted represent mean values, and the dotted lines give the limits for *la around a mean value of all the samples, n. = 20. The relative standard deviation was found to be 4.770, which is of the same size as for this technique in general. There was, therefore, no significant dependence of cadmium absorption on the amount of NaCl present. The detection limit in this experiment was 0.03 pg Cd/l., calculated from sensitivity data as the amount of Cd giving twice the signal when running a blank. These results should be compared to those of Segar and Gonzalez ( 5 ) who used a conventional heating with a Perkin-Elmer HGA 70. They tried to determine Cd, Ag, Pb, and Sn in sea water in the range 0.03-0.17 pg/l. for Cd. They found that it was impossible because of interference from the salts. Standard additions could not be recovered at all. Segar and Gonzalez (5) also studied the copper absorption as a function of the salinity and found a decrease by an order of magnitude from zero salinity to 350/&,. This obviously depends on differences in heat transport through the sample. The conventional heating of the graphite tube may be slow in comparison with the losses of the element
n
I
I
1
2
3
, OIO NaCl
4
Figure 9. Determination of Cd ( 4 5 picogram C d ) in various sodium chloride concentrations. Sample volume was 10 pl. The dotted lines indicate the deviation 1 u (0.023) from the mean value ( 0 . 4 7 4 ) of the whole material (n = 2 0 ) . Final temperature of atomization was 820"
through diffusion. If then the element is vaporized with a slower heating rate as caused by salt matrix, the peak absorbance value is decreased. The salt in the sample causes a negative interference. If the heating rate is higher than the rate of loss, this type of interference should be absent. Figure 9 shows that, in the determination of cadmium, the temperature controller effectively removes the interference from salt in the sample. The calibration is independent of the salt concentration within experimental errors.
CONCLUSION The temperature controller described can easily be added to commercial graphite furnaces. It will increase the performance of the system and it will facilitate several types of analysis which are impossible using a constant voltage heating. This has been demonstrated for the analysis of cadmium in sodium chloride. As there is a possibility of obtaining defined heating rate and a defined constant operating temperature, the temperature controller is essential in evaluating reactions and physical effects in a graphite tube. Received for review October 15, 1973. Accepted January
(5)
D.A
Segar and J G Gonzalez. Anal. Chim. Acfa, 58, 7 (1972)
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