Cannon Instruments wear metal standard solutions, and a third sample made from the standard metal caprates to approximate actual wear metal oil concentrations. Data were based on triplicate measurements and were collected on five separate days. The 2659-A line of Pt was found to be the best internal standard line and was used throughout. The relative standard deviations are as follows: Al, 21%; Cu, 9.4%; Fe, 4.2%; Cr, 11%; and Mg, 5.1%. Table IV compares the data from the Air Force jet engines, and Table V compares the data from the internal combustion engine oil from Trimble’s laboratory. There was sufficient jet engine oil for one triplicate analysis; therefore, these data should be considered as preliminary. The two samples from Trimble’s laboratory were run according to a standard procedure and can be considered to be a better indication of the method’s capabilities. These results are generally comparable within experimental error. Several points should be made with respect to Table IV. First, the data for the jet oil have been collected from 30 laboratories and indicate lab to lab variation rather than precision of a particular method. Second, agreement of results from lab to lab is not necessary for a meaningful wear metal analysis program. The important requirement is self-consistent results within a particular laboratory. Third, regardless of the “true” values for the elements in the jet engine samples, the data obtained by the plasma jet technique presented here give answers in the same ball park as those of the other labs. These data generally show acceptable results compared to other work (27). In the case of Al, the results are partly (27) Applications Report, “The Quantometric Analysis of Oil.” Applied Research Laboratories, Sunland, Calif. 1972.
explained by the rather flat analytical curve. The consistently high results for Fe, Cr, and, generally, Mg may in part be due to the metal particles which escape analysis in other wear metal techniques. The overall variations in the results may be the result of the lack of precision in the method rather than its inaccuracy. It must be realized that the specific procedures used in this study are subject to a rather large amount of inherent uncertainty. The more important sources of error were: the standard solution instability; the volumetric sampling procedure; the variation in syringe diameter (1.12 to 1.16 cm); and the variations involved with the photographic process. The variation in syringe diameter results in about 2% relative standard deviation from sample injection alone. Therefore, relative standard deviations of less than 10% can be considered quite good. It is felt that more sophisticated experimental design could lower this variation by a significant amount. Further research with a direct reading instrument using stable standard solutions, gravimetric sample preparation, and a more reproducible sample injection system should substantially improve the results of this technique.
ACKNOWLEDGMENT The authors would like to thank J. D. Winefordner and R. C. Trimble for samples used in this project. Thanks are also due to G. K. Wittenberg for helpful comments and suggestions with the manuscript. Received for review August 16, 1973. Accepted February 28, 1974. The authors would like to acknowledge partial support by an A.S.U. Faculty Grant-in-Aid as well as computer time provided by the University Computing Center.
Fluorescence Detection of Sulfur Dioxide in Air at the Parts per Billion Level F r e d e r i c k P. S c h w a r z ’ and H i d e o O k a b e Physicai Chemistry Division. Nationai Bureau of Standards. Washington. D . C. 20234
Julian K. W h i t t a k e r Nuclear Sciences Division, Nationai Bureau of Standards. Washington. D . C. 20234
A previously reported detector capable of rapid and continuous measurement of SO2 in air has been modified to extend the detection limit to the low ppb range. The principle of detection is based on photon counting of the SO2 fluorescence excited by the Zn 21 38-A line. Fluctuation of the lamp intensity was accounted for by measuring the ratio of the fluorescence photon counts to that of the excitation source. At 8.6 ppb, the standard deviation is 29% for a counting time of about 1 minute. The detector response is linear from at least 8.6 ppb to 1.8 ppm. The major source of measurement error at low ppb concen1024
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974
trations is the statistical fluctuation of the low scattered light and signal counts, whereas at high SO2 levels, it is due to fluctuations in the sample preparation. The cell design was modified to reduce the scattered light. The inside of the cell was coated with a non-water-absorbing black Teflon to reduce possible H2O-SO1-wall interactions. With the Zn lamp as an excitation source, the quenching effect of water vapor on the SO2 fluorescence signal previously observed with Cd 2288-A excitation was found negligible. The result can be reasonably explained by the shorter life time of the SO2 fluorescence.
5 0 ) INLlf
The measurement of atmospheric SO2 in the parts per billion (ppb by volume) range is an important and formidable problem. The two most commonly used techniques are the West-Gaeke (pararosaniline) ( 1 ) and flame photometric methods (2). The West-Gaeke method commonly requires a minimum sampling time of 30 minutes and can measure 21 ppb SO2 with a standard deviation of 50% ( 3 ) . The lower limit of detection is about 10 ppb ( 3 ) . The flame photometric detector responds from 5 ppb to 1 ppm (parts per million by volume) to all sulfur compounds with a quadratic signal dependence on the concentration
1
a
POWER SUPPLY
(4).
The measurement of the UV fluorescence intensity of SO2 has been shown to be a sensitive technique for the determination of SO2 concentrations in air ( 5 ) .The intensity of the SO2 fluorescence produced by absorption of the Cd 2288-A line is linearly proportional to the SO2 concentration from 0.1 to 1600 ppm. There is relatively little interference in the measurement from other pollutants in air. It has been shown, however, that the intensity of the fluorescence excited by the Cd lamp is decreased by the presence of H20. In this report, we present an improved version of this detector: The detection limit is extended to the ppb range by means of photon counting and an improved cell design; the interference from water vapor is minimized by using a Zn lamp as a light source and by coating the cell wall with non-water-absorbing low reflectivity black Teflon; and signal variations due to the excitation light fluctuation are reduced by measuring the ratio of the intensity of fluorescence to that of the excitation source.
EXPERIMENTAL Detector. The detector, shown schematically in Figure 1, consists of an excitation source, a fluorescence cell, and two photomultipliers. One photomultiplier is used to monitor the excitation intensity while the other photomultiplier measures, simultaneously. the SO2 fluorescence intensity. The excitation source consists of a commercial Zn glow discharge lamp with a quartz envelope. A P h i l i p Zn discharge lamp with the quartz envelope or an Osram zinc lamp without the outer Pyrex window were used. The interference filter was from Optical Coating Laboratory, Inc. This interference filter with 18% transmittance a t the Zn 2138-A line and a half band width of 225 A was placed between the cell window and the excitation lamp. The transmission of this filter above 3000 A is less than 0.001%. An EM1 62568 photomultiplier was used to measure the fluorescence. A Corning 9863 (CS7-54) filter was used to isolate the fluorescence band from 2400 to 4200 A ( 5 ) .The fluorescence spectrum without air extends from the exciting wavelength to about 4300 A with a broad maximum a t about 3200 A (5, 6). Although the spectral distribution may change with the addition of air and with the exciting wavelength, the fluorescence signal in the spectral region 2400 to 4200 A is found proportional to SO2 concentrations in air u p to 500 ppm with the 2138-A line and u p to 1600 ppm with the 2288-A line ( 5 ) . The distance between the excitation source and the quartz (Suprasil) lens is slightly greater than the focal length (3 inches). Also the center of the cell is a little more than 6 inches away from the lens. Considerable efforts have been made to improve the signal to background ratio by a factor of 10 over that of the previously used cell ( 5 ) .Major modifications
To whom reprint requests should be sent (1) P. W West and G . C. GaekecAnai Chem . 28, 1816 (1956). (2) R K Stevens. J D. M u l i k , A . E. O'Keeffe, and K . J. Krost, Anai. Chem 43,827-31 (1971) (3) t i C McKee. R . E. Childers, and 0. Saenz, J r . , "Collaborative Study
of Reference Method for Determination of Sulfur Dioxide in the At(4)
mosphere (Pararosaniline Method)," prep. for E.P.A. by the Southwest Research Institute. Houston, Texas, Sept. 1971 R . K . Stevens, A . E. O'Keeffe and G . C Ortman, Environ. Scr. Tech-
no/ . 3,652 ( 19 6 9 ) . (5) H . Okabe, P. L Splitstone, and J. J. Ball, J . Air Poiiut. Contr. 23, 514 (1973). (6) H 0kabe.J Amer Chem Soc.. 93,7095 (1971).
POWER IUPPLY
-R A T E MITIR
DISCRIMINATORS
AMPLIIIIRS
DI = '&in,, D2 = '/*-in. diameter, cell inside is coated with black Teflon: lens, 3-in. diameter fused silica, f / l . Honeycomb is a collimator. T h e incident light intensity is attenuated to an appropriate level by a filter. Figure 1. Schematic diagram of the detector
of the prototype cell are the addition of two light trapping horns, one opposite the fluorescence measuring photomultiplier window and the other opposite the excitation light entering window; the addition of diaphragm Dz (Yz-in. inner diameter); and an inside coating of black Teflon. A honeycomb (an assembly of tubings, YB-in. diameter, 1-in. length) is used to collimate the fluorescent light beam. The two electrostatically shielded photomultipliers are connected for photon counting. The UV sensitive photomultipliers were selected for a low dark count rate and both have quartz UV windows. As shown in Figure 1, each signal is amplified by a voltage gain of 1000. The amplifier chain consists of 3 cascaded, low noise x 10 wideband pulse amplifiers similar to those in reference (7). Amplitude discrimination of the amplified output signals are performed in a dc coupled 100-MHz discriminator which in turn is connected to a dual counter/timer to measure the count rates. For convenience, commercially available NIM modules were used. The modules consisted of an EG&G TR204A/N Updating Dual Discriminator, an Ortec 715 Dual Counter/Timer, and a Canberra Model 1480 Ratemeter. The amplifiers were of NBS manufacture. The excitation light signal after attenuation by a filter is fed into Counter A of the dual counter/timer. The fluorescence signal is fed into Counter B. Counter A which serves as the time base for Counter B is preset to terminate counting a t a given number of excitation photon pulses. In this way, variation of the excitation intensity is compensated for and the photon counts displayed on Counter B are only a function of the SO2 concentration. Another output of the fluorescence discriminator channel is fed to an analog ratemeter (Canberra Model 1480) and displayed on a strip chart recorder. Direct comparisons between the ratio counting method and the fluorescence signal are therefore possible. Calibration of the Detector. The detector was calibrated by flowing pure dry air around a calibrated 2-cm SO2 permeation tube and into the cell. The NBS-SRM No. 1627 permeation tube has a permeation rate of 0.536 pg per minute at 24 "C. Since the room temperature remained a t 24 "C throughout the experiment, there was no need to use a thermostat to regulate the air temperature. Concentrations from 1.8 ppm to 17 ppb were achieved by varying the flow rate of air around the tube from 116 ml to 11,800 ml per min. The 17-ppb sample was diluted to 8.6 ppb by mixing the sample with a nearly equal flow rate of air. In the same manner dry SOz-air samples were mixed with Hz0 saturated air in the S02-Hz0 experiments. A static system for the preparation of the S O Z - H ~ Oair mixtures consisted of attaching the cell t o a vacuum rack. The cell was pumped down to Torr. Various SOz/HzO/air mixtures were made in the vacuum system and then introduced into the cell. In the measurement of the SO2 concentrations, several time integrated background counts, Cg, were first taken by flowing pure dry air through the cell. Then the sample was introduced into the
Ass.. (7) W
Rev
R Dodge J A Coleman, S R Scr instrum 37, 1151 (1966)
Domen. and
J K
Whittaker.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974
1025
XI
20000
16000
e
3 12000
Figure 3. Recorder traces of the signal at various SO2 concentrations; X 10 signifies ten times attenuation of the sensitivity
we assume Poisson statistics, the statistical error is equal to the square root of the number of counts. The relative probable error, Q, is given by
4'/,
Q
= [(Cso,
+ CRPAW'W-~+ CB + (Cso, + CB>I"'[CSJ'
If indeed the experimental error is due to the above fluctuations, then Q should be very nearly equal to the experimental error. Figure 2. Signal counts as a function of SO2 Concentration. Cir-
cles are based on the ratio counting method. Triangles indicate recorder output using a ratemeter. The full line is drawn with a slope of 11 counts per ppb SO2 cell and several time integrated counts, (signal, and background, C g ) were recorded. Several background counts were again taken and were subtracted from the integrated counts. To eliminate the fluctuation of the light intensity during the measurement, Counter A, which monitors the light intensity, is preset as described. Counter B, which measures the fluorescence intensity, stops when Counter A reaches the preset number. The integration time was about 1 minute a t the lowest concentration of measurement (8.6 ppb). The integrated fluorescence count, Cs,, when absorption is small, is given by
cso, = % f v l [SO2IC* where a is a proportionality constant, & is the fluorescence yield, the absorption coefficient, I the path length, [SO21 the SO2 concentration, and CA the integrated photon count of the light source, Since Ci\ is made constant, Cso2 is proportional to [SO21 and is independent of the source fluctuation. Measurement of the SO2 concentration by monitoring just the fluorescence photon count with a ratemeter was done in the same manner. In this case. c
RRI,
=
df~lrS021~e
where Rsoz is the ratemeter output and 1, the intensity of the light source. Rso? is proportional to both the concentration and the excitation light intensity. Error Analysis. The probable error is due to fluctuations, Aw, in the flow rate, w , of the air around the permeation tube, the statistical fluctuation of the background count, A CB, and the statistical fluctuation of the fluorescence count, ACso2 + CR).If
Table I. SOnin Air Fluorescence Measurements
as a
RESULTS Calibration of the Detector. In Table I and Figure 2, we present measurements of SO2 in air concentrations ranging from 8.6 ppb to 1.8 ppm. The measurements presented in Table I and the solid circles in Figure 2 were taken by the ratio counting method. Each CsoZ signal represents the average value of 9-10 C,,, determinations taken in a random fashion over a period of three days. During this period, the excitation lamp intensity fluctuated by factors of 1 to 2. The corresponding calculated probable error, Q, of each concentration measurement is also presented in Table I. The linearity of the detector response is shown in the last column of Table I and by the solid line of Figure 2, which is drawn with a slope of 11 counts per 1 ppb SO2. The maximum integration time of these measurements was two minutes. Longer integration times improve the counting statistics. For example, for an integration time of 5 minutes, a 8.6-ppb concentration measurement has a 12% standard deviation whereas for a 1-minute integration time, the standard deviation is up to 29%. In Figure 3 we present some recorder traces of the output of the ratemeter which monitors the fluorescence measuring photomultiplier output. The recorder response for various SO2 concentrations is shown in Figure 2 by triangles. The broken line in Figure 2 demonstrates the detector linearity. Effect of H2O on the Calibration. Initial determinations of the effect of water vapor on the SO2 fluorescence measurements were made in the static system and with the earlier uncoated Monel sampling cell (5). The results
F u n c t i o n of Concentration
+
Concentration S o d a i r , ppb
8.6 17 27 43 60 ppmb 0.15 0.21 0.43 0.90
1.8
'' ';7c 1026
Signal background counts, Cso, CR
-
Average background counts, Cn
Signal, Cso?
366 441 605 786 921
259 229 274 270 248
107 212 331 516 673
1,808 2,500 4,070 11,700 19,024
260 265 240 260 252
1,549 2,235 3,833 11,484 18,772
Probable Error = 100 X [Cso,]
-1
[CB
Error of, signal, %
f 31 f 39 f 31 f 54 f 75
29 18 9.4 11
f 56 f 193 f 544 i 1370 f 1825
3.6 8.6 14 12 9.7
11
Max. flow fluctuation, %, Aw/w
Signa1,'concn countsippb SO),
Cso?/~so!l
2 .o 2 .o 5.7 6.4 5.9
25 12 13 12 9.6
12 f 4 12 i 2 12 1 12 f 2
3.5 5.9 12 4.4 8.7
5.1 6.9 13 4.7 8.6
11 f 1 11 f 1 9 * 1 13 f 2 11 f 1
+ (Cso, + CBj + (Aw,/wj?(Cso, + CB~?]'/:.' ppm is parts per million.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, J U L Y 1974
Probable Error c/c, Q"
11
*1
varied from 8 to 11%quenching of the 2138-A excited SO2 fluorescence for 2% H2O (71% relative humidity a t 24 "C) air-SO2 mixtures and were somewhat dependent on the mixing time of the SOz/air and the H20 vapor. The SO2 concentrations of the mixtures were changed by diluting a known SOa/air mixture with known volumes of H2O saturated air. It is uncertain how much of the observed signal decrease can be attributed to the loss of the SO2 by the water vapor or to the walls. In the flow system and with the uncoated Monel cell, several measurements showed a quenching variation from 4% to 10% a t 50% relative humidity with the same excitation source (Zn). It took approximately 10 minutes from the start of the humid air flow into the cell to register the same humidity at the exit opening of the cell, indicating that some of the water vapor was lost onto the walls of the cell. In order to avoid the loss of H20 and SO2 to the walls, the cell wall was coated with black Teflon. In this case, it took several minutes for the humid air flow to register the same humidity a t the exit port, indicating that water vapor was less adsorbed onto the walls of the cell. The signal ratios with and without water vapor a t 10 ppm SO2 were measured as a function of relative humidity a t the 2138-A and 2288-A excitation wavelengths and the results are given in Figure 4. In the range 50 to 75% relative humidity, 0 to 5% quenching by H2O is found with the 2138-A line. On the other hand 20 to 44% quenching is observed when the relative humidity increases from 50 to 75% with the Cd excitation source. Average quenching of the fluorescence signal was 20 f 10% a t 50% relative humidity and 44 f 7% a t 75% relative humidity with the 2288-A line. DISCUSSION Calibration of the Detector. The SO2 fluorescence signal to concentration ratio in Table I is a measure of the detector linearity and the precision of the calibration. This ratio varies from 9.0 f 1.3 to 13 f 2 counts per 1 ppb with an average value of 11 f 1 (standard deviation) under the present experimental conditions. This standard deviation is within the error range 1 to 4. The variation of this ratio does not exhibit any trend, such as becoming larger as the concentration decreases. We may conclude that the ratio variation is due to the imprecision of the calibration technique and not due to nonlinearity of the detector response. Let us now examine the main causes of error in the calibration. Table I shows that the experimental error of each measurement correlates well with the calculated Q. Since there is a wide disparity between the flow rate error and Q in the 8.6- to 27-ppb range, we can attribute most of the experimental error to the statistical fluctuation of C, and Cso, C B ) . This statistical fluctuation is proportional ~ . can reduce the fluctuto CB1I2 and C,O, + C B ) ~ ' We ation by reduction of the background count, CB. In this sampling cell, the background count is just twice the dark count of the photomultiplier. We can reduce the dark count by cooling the photomultiplier and thus reduce CB. This would reduce the error in the background count by a factor of about 1.4. On the other hand, the major error contribution in the 0.15- to 1.8-ppm range is the fluctuation of the flow rate as observed by comparison of the flow fluctuation with the signal error in Table I. Better flow regulation can be achieved by the incorporation of flow controllers in the SO2 sampling system. We believe that with the presently available excitation sources and with limitation of the integration time to 1 minute, the limit of the detector sensitivity is 2 ppb (signal to noise ratio of 1). Reduction of the background count
+
Zn LAMP
1.0
-
0
50
100
RELATIVE HUMIDITY (70) AT P4'C
Figure 4. The fluorescence quenching ratios vs. relative humidity at 24 "C, [SO,] = 10 pprn
by cooling the photomultiplier could reduce the error of the 8.6-ppb measurement from Q = 25% to ([130 + 237 + 22]1/2/107) x 100 = 18.4%. The signal to noise ratio increases with the square root of the lamp intensity and integration time. The detector sensitivity can thus be increased.by a more intense lamp and/or longer integration times. The ratemeter output us. concentration shown in Figures 2 and 3 also illustrates the detector linearity over the short time when these measurements were taken (1 day). If lamp stability could be ensured for longer periods of time, this method of calibration could be used. Otherwise, i t is necessary to monitor the lamp intensity simultaneously. Effect of H2O on the Calibration. The fluorescence yield and the lifetime of SO2 decrease abruptly below the excitation wavelength 2200 A (6, 8). This is attributed to the predissociation of the excited SO2 below this wavelength. It is expected therefore that the effect of H2O on the fluorescence quenching would be less at the 2138-A line than that at the 2288-A line because the number of collisons is less when the lifetime is shorter. Our results show that 0 to 5% quenching is observed with the 2138-A excitation while 20% f 10% quenching is found at 50% relative humidity (11 Torr of H2O) with the 2288-A line, thus supporting our proposition. The approximate quenching constant for water may be obtained from the equation
+ hso,[S021 + h J a i r 1 )
If/Zf(HD) = 1 + k~,o[H@l/(kf
where Zf is the fluorescence intensity in dry air, If(HzO), that in humid air, kf, the fluorescence decay rate, and k~ (M = H20, S02, air), the quenching rate constant for HzO, SOz, and air. At 10 ppm SO,, k i o , [SO,] is equal to 2.7 x lo5 sec-l (8). The value k,,,[air] = 2.0 X lo8 sec- is obtained from
+
Zj/If,, = h,/(hf k,,Lairl) at 2288 A, where Zf/Zfo = 0.13 (7) and kf = 3 x 107 s e r 8 (8) (If and Ifoare the fluorescence intensities with and without atmospheric air). The observed ratio Zf/If(H,O) = 1.25 f 0.14 a t 11 Torr H2O and a t the 2288-A line gives k H n O = 5.2 f 2.6 x lo6 sec-' Torr-I which is close to unit quenching efficieky. On the other hand kf 5 x 108 sec-l at 2138 8, (extrapolated value from Ref. 8 ) gives Zf/Zf(HzO) = 1.08 f 0.05 indicating that the fluorescence intensity with H2O is 93% f 5% of that without H2O. Experimentally we found 0 to 5% quenching a t 50% relative humidity. Excitation of the SO2 fluorescence by the Zn
-
(8) Man-Him Hui and S. A . Rice, Chem. Phys. Lett.. 17, 474 (1972)
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974
1027
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
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8 , JULY 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.
