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Anal. Chem. 1980, 52, 1256-1260
Application of a Modulated Magnetic Field to a Graphite Furnace in Zeeman Effect Atomic Absorption Spectrometry P. R. Liddell" and K. G. Brodie Varian Techtron Fty. Ltd., P.O. Box 222, Springvale 3 171, Victoria, Australia
required (13, 17, 18) although conventional hollow cathode lamps have been used (19). There is a more serious disadvantage of using a fixed field. This is evident with those transitions which exhibit the anomalous Zeeman effect. In these cases, the x component is split and this causes a loss of sensitivity (11,13) and linear concentration range (13). Attempts have been made to reduce this problem by selecting the magnitude of the magnetic field according to the element being determined (11, 13). Another potential problem with this approach is base-line instability. Potentially, the use of a fixed transverse field allows convenient compensation for drift or fluctuations in the light source by ratioing the u and x components. However, this ratio is affected by self-absorption in the lamp (16, 20, 21) and any fluctuation in the degree of self-absorption (due, for example, to fluctuation in the vapor pressure of the element) will cause the base line to fluctuate. A special lamp design has been proposed to overcome this problem ( 2 2 ) . Fixed Longitudinal Field on Furnace. A fixed longitudinal field is not generally applicable for reasons already discussed, and there have been no reports of this approach being used. Fixed Transverse Field on Furnace. This has been another common approach (7,23-26). The application of the field to the furnace rather than the lamp means that conventional lamps can be used but that special furnaces which fit between the poles of a magnet may need to be built (24-26). I t also means that background correction is carried out a t exactly the same wavelength as the atomic absorption, which is an advantage in correcting for narrow molecular absorption lines ( 2 , 3 ) . As discussed, the use of a fixed field means a loss of sensitivity for those transitions where the T component is split. Modulated Field on Lamp. When a modulated field is used (6, 7 ) , it is necessary to take the ratio of the lamp intensity with the field on, t o the intensity with the field off, rather than the ratio of the c and T components as is done with a fixed field. Any variations in this ratio will produce an unstable base line. Attempts to construct a lamp which gave a stable base line in the presence of a modulated magnetic field have been unsuccessful (27). In a commercial mercury detector (Mercury Analysis Spectrometer Model HGP-2, Scintrex Limited, Concord, Ontario, Canada), a second photodetector was used t o monitor the source lamp intensity. Modulated Longitudinal Field on Furnace, With this approach, (7),the T components are absent, there is no need for a polarizer and the full intensity of the lamp can be used for both analyte and background measurements. The absence of a polarizer is particularly beneficial a t low wavelengths where most polarizers have low transmission. A disadvantage of a longitudinal field is that there is a practical limit to the length of the furnace. It would be difficult to generate a usable field strength over the large pole gap required for a long furnace. Modulated Transverse Field on Furnace. If a fixed linear polarizer is used to eliminate the T components, this approach (7,28) is equivalent to the modulated longitudinal
An electromagnet is used to apply a modulated longitudinal magnetic field to a graphite furnace. Compared with some other configurations which have been used for Zeeman atomic absorption spectrometry, this approach offers more accurate background correction, a more stable base line and higher light throughput. The Sensitivity is better than approaches which use a fixed field but is poorer than conventional atomic absorption sensitivity. Curvature of the analytical curves is greater than that obtained with conventional instruments but the reflex curvature previously observed with Zeeman systems is eliminated by the use of the peak height mode. Compared with a conventional deuterium arc background correction system, the Zeeman approach gives significantly better background correction.
Non-atomic absorption has long been recognized as a problem in atomic absorption spectrometry using graphite furnace atomizers. It is usually corrected by the use of a continuum source such as a deuterium arc lamp. However, this technique has some limitations. The intensity of the continuum source is sometimes inadequate ( I ) , background caused by narrow line molecular absorption spectra cannot be accurately corrected (2, 3 ) and significant errors are sometimes observed in practical analyses ( 4 ) . An alternative approach, which has attracted considerable interest, is the use of the Zeeman effect, where a magnetic field is used to split the atomic spectral line. By this means, the background correction errors which occur with the continuum source technique can be minimized or eliminated. There are several possible configurations for Zeeman atomic absorption spectrometry (ZAAS) - the magnetic field may be applied to the light source or the furnace, the field may be fixed or modulated, and the direction of the field may be parallel to the light path (longitudinal field) or perpendicular t o t h e light path (transverse field). Fixed Longitudinal Field on Lamp. In a longitudinal field, the atomic spectral line is split into circularly polarized uf and c- components. This configuration is not generally applicable because there is no unshifted component with which to measure the atomic absorption. However, it has been used in conjunction iith the isotope shift for mercury to produce a spectrum in which the u- component coincided with the absorption peak of naturally occurring mercury (5). Modulation was accomplished by a fixed linear polarizer plus a rotating quarter-wave plate. Fixed Transverse Field on Lamp. In a transverse field, the atomic line is split into one or more R components, which are linearly polarized parallel to the field, and two or more u components which are linearly polarized perpendicular t o the field. The T components can be used to measure the atomic plus background absorption and the c components to measure the background only. T h e application of a fixed transverse field to the lamp has been a common approach to ZAAS (6-16). I t avoids the difficulties of modulated fields and allows any atomization system to be used. A disadvantage, however, is that specially designed light sources are usually 0003-2700/80/0352-1256$01 .OO/O
C
1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
1257
.
Figure 1. Schematic diagram showing the position of t h e magnet and furnace relative to t h e light path
field approach, except that the noise will be higher due to the light loss at the polarizer. Based on these considerations the configuration which we have chosen to investigate is the modulated longitudinal field applied to the furnace. Application of the field to the furnace means t h a t background correction is carried out a t exactly the same wavelength as the atomic absorption, special lamps are not required, and lamp intensity fluctuations can be compensated. Use of a modulated rather than a fixed field means that better sensitivity is obtained for transitions where t h e P component is split. Use of a longitudinal rather than transverse field means t h a t no polarizer is required and the full intensity of the lamp can be used, with resultant lower base-line noise. THEORY T h e theory of ZAAS has been discussed in detail (29) and will not be reproduced here. However a brief analysis is necessary in order to define the terms used in this paper. The measured light intensity, I , in the absence of a magnetic field is given by I = (1- ~ r ) I oexp(-k - kb) CUI,exp(-hb) (1) where Io = measured intensity of primary light source in the absence of atomic and background absorption, k = analyte absorption coefficient in the absence of a magnetic field, kb = background absorption coefficient, and cy = fraction of light from the lamp which is absorbed by the background but not t h e analyte. In t h e presence of a magnetic field, the measured light intensity, I H , is given by I H = (1 - a)rO exp(-hH - kb) d o exp(-kb) (2) where k H = analyte absorption coefficient in the presence of a magnetic field. A “Zeeman absorbance”, AZ, can be defined such that AZ= log I H / I (1 - o ( ) eXp(-kH) = log (3) (1 - a ) exp(-k ) a
+
+ +
This term, AZ,is independent of background absorption and lamp intensity and illustrates how a single beam Zeeman instrument can perform the function of a conventional double-beam instrument with background correction. There is a potential problem at high analyte concentrations, where both eXp(-kH) and exp(-k) become small compared with a and, consequently, AZ approaches zero. A plot of AZ against analyte concentration passes through a maximum ( I I , 2 6 ) and measured values of AZ may, therefore, correspond to two different values of concentration. The normal (not Zeeman corrected) absorbance, AN,is given by AN= log V / I
= log
V (1 - 0)Io eXp(-h - kb)
+d
o eXp(-hb)
(4)
where V is a reference signal. It is usual to set V equal to Io when the instrument is zeroed but any drift in I, will show up as ai change in AN. In general, AZ is less than Ah. and a Zeeman sensitivity ratio, RZ,can be defined such that If kb = 0 and
cy
= 0, then
Usually, kH decreases as the magnetic field increases and the Zeeman splitting increases. At a sufficiently high field, k H = 0, RZ = 1 and t h e Zeeman calibration curve follows the normal calibration curve. Note that RZ,as defined here, is not the same as the “R value’‘ which has been associated with the fixed transverse field configuration (26). R was defined there t i s the ratio of absorbances of the u and a components. Complete separation of the u components would give RZ = 1 and I2 = 0. In the modulated field case, RZ = 1 implies AZ = AN. However, in the fixed field case, w e n with R = 0, the Zeeman absorbance could be significantly less than the normal absorbance, owing to splitting of the a component. EXPERIMENTAL Furnace. The graphite furnace was a Varian CRA-90 on which the water and gas inlets, the electrode clamping mechanism, and the electrical connections had been modified to enable the workhead to f i t between the poles of a magnet. A standard CRA-90 power supply was used. Samples were dispensed with an Oxford Ultramicro Sampler. Magnet. A specially constructed electromagnet was used. It comprised a C-shaped, laminated core on which were mounted two copper wire coils, each having 392 1,urns. The core was split to place the coils symmetrically about the pole pieces and finally clamped in position. The pole gap was 14 mm. Holes were drilled through the pole pieces and profiled to match the light beam. The hole diameter at the pole face was 5 mm. The magnet was mounted as shown in Figure 1with the field parallel to the light path. The original power supply provided a 50-Hz sine wave (field modulation at 100 Hz) and the peak field was 1.0 T. The power supply was later modified to pause at zero field for about 1.5 ms each half-cycle. The modulation frequency remained at 100 Hz and the peak field dropped to 0.73 T. Measurement of Magnetic Field Strength. A Hall-effect probe (F. W. Bell Inc., Columbus, Ohio) was used to measure the instantaneous strength of the modulated field. This probe was calibrated using a dc magnet and a Bell 610 Gaussmeter. Spectrometer. The monochromator, optical rail, lamp mounting, and lamp power supply were From a Varran AA-6. The electronic processing module was specially constructed. It triggered the lamp power supply to provide 1.5-ms pulses at a frequency of 200 Hz, synchronized to coincide with peak and zero field. Recorder outputs were provided for both the normal and Zeeman absorbance. A soft iron housing was constructed to shield the photomultiplier from magnetic interference. With this shield
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980 2 0,
Table I. Zeeman Sensitivity Ratios
element Cd
Cr cu Fe Pb Zn
wavelength, nm 228.8 357.9 324.8 248.3 283.3 213.9
lamp spectral current, bandwidth, mA nm 1
0.5
5 3 5 3
0.2 0.5 0.2
5
0.5
0.5
I
RZ,76 81 100
I
56 91 82 75
~
7
I
I i
0
Y
2 CONCENTRATION ( p g l m l )
1
3
Figure 3. Analytical curves for chromium (m) and lead ((.)A,,
J
(0)AN).
A , and A , are identical for chromium
CONCENTRATION
(,pg i m l )
Flgure 2. Analytical curves for cadmium ((.)A,, @)A,) and zinc ((.)A,,
(OM,) in place, there was no shift in the base line or change in the noise level when the magnet was switched on. RESULTS AND DISCUSSION Zeeman Sensitivity Ratio. The initial experiments were performed using a sinusoidal field. The problem with this approach was that a zero field was not maintained during the measurement of I. Consequently, the normal absorbance with the magnet on was lower than that with the magnet off. With Cr, for example, the value of A N measured with the magnet on was about half that with the magnet off. To overcome this problem, the modified power supply, which maintained zero field during the measurement of I , was used. With this supply, AN was the same with the magnet on or off. As well as varying with magnetic field, RZ also varies with lamp current, spectral bandwidth and concentration. Table I shows the instrumental conditions used and the Zeeman sensitivity ratio for six elements measured a t a Zeeman absorbance of about 0.1. A n a l y t i c a l Curves. Plots of both AZ and Ah. against concentration are shown in Figures 2 to 4. Measurements were also made a t higher concentrations but these are not shown in the figures. The sample volume was 2 pL in each case and the instrumental conditions were the same as in Table I. The absorbance values plotted were peak heights measured from a recorder chart. Equation 3 predicts that the AZ curves should reflex a t high concentrations if RZ is less than 100% and a is greater than zero. In the case of a furnace atomization peak, the density of atoms in the furnace builds up to a maximum and then decays to zero. Therefore, at high analyte concentration, AZ should pass through a peak, drop to a lower value corresponding to the peak atom density, and then pass through a second peak as the atom density decays. T h a t this does occur can be seen by the recorder traces for Fe reproduced in Figure 5 . As the concentration increases, the Zeeman absorbance corresponding to the peak atom density decreases (almost to zero at 30 hg/mL). However, the
CONCENTRATION (,ug i m l )
Figure 4. Analytical curves for copper ((.)A,,
(O)AN)and iron ((.)A,,
(0)AN)
A
B
C
0
H
5s
Figure 5. Recorder trace showing A , for iron. (A) 5 pg/mL, (6)10 p g / m L , (C) 30 p g / m L
peak height remains unchanged. I t follows that analytical curves plotted using peak heights should asymptote to a fixed absorbance. Figures 2 and 4 show that this does occur for Cd and Fe. The same behavior should be observed for Cu and P b a t higher concentrations. The analytical curve for Zn, shown in Figure 2, shows a dip a t high concentration. This is because the Zn atomization peak is very fast and the chart recorder did not respond quickly enough to detect the true height of the double peak. S t a t i c B a c k g r o u n d Correction. The main impetus behind the study of ZAAS has been the promise of improved background correction. The ability of a system to correct for a static background can be tested using gauzes or optical glass
ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
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Table 11. Static Background Correction background,
% error,
AN
( A z I A N )X 100
0.5 1.0 2.0
0.02 0.06
A
0.01
h A
B
w V
"I
T
m
Q 0 N
A,+
Jc
h-
H
30s
(.
.zos' Figure 6, Recorder traces showing A , and A , for lead in blood. (A)
and (E)Duplicate traces with lead lamp;
(C)
hydrogen lamp
filters. When this test was applied to the Zeeman system, no error in background correction could be detected above the base-line noise under normal operating conditions. However, when the maximum lamp current (30 mA) and spectral bandwidth (1 nm) were used in order to minimize the base-line noise, a very small shift in the base line could be detected. Table I1 shows the background correction error (A, expressed as a percentage of AN), measured when various gauzes were inserted in the light path. Dynamic Background Correction. Although the Zeeman system has been shown to give excellent correction for a static background signal, there is a possibility of error with a background signal which changes with time. The time between the measurements of I and IH in this system was 5 ms. The Zeeman absorbance was determined by taking the ratio of each I measurement t o the average of the preceding and following IH measurements. In this way, a background signal which varied linearly with time could be corrected. However, there is still a possibility of error with furnace atomization as the background signal passes through a peak. Figure 6 shows recorder traces for both AN and AZ for the analysis of P b in blood. The blood was diluted 1:l with 5% Triton X-100, thoroughly agitated in a vortex mixer and lightly centrifuged (30). A 2-KL volume of the supernatant liquid was sampled. In order to estimate the accuracy of background correction, the P b lamp was replaced by a hydrogen hollow cathode lamp. The traces obtained with this lamp are also shown in Figure 6. With the hydrogen lamp, only the background absorbance is detected and the trace for AZ shows the excellent correction obtained. During the ash peak, there is a small residual signal on the Zeeman channel of about 0.002 absorbance. The ash peak is about 1.0 absorbance, so the error is about 0.2%. For the case of P b in blood, there is little background during the atomization stage and the ash peak is fairly slow. To assess the accuracy of correction for fast background peaks during
Recorder traces showing A, and A, for cadmium in seawater and fresh milk (B)
Figure 7. (A)
atomization, measurements were made with seawater and fresh milk using a Cd lamp. Figure 7 shows recorder traces obtained with 2 p L of seawater and 1 pL of fresh milk. I t was anticipated that the level of Cd in these samples would be below the detection limit and that the traces would show only background absorbance. This was verified using a hydrogen lamp in place of the Cd lamp. With both samples, there was a small shift in the base line of the Zeeman channel corresponding to the atomize signal. With seawater, the AZsignal was -0.004, corresponding to a correction error of 0.4%. With milk, the AZ signal was -0.0015, giving a correction error of 0.2%. For comparison, the same samples were analyzed on a Varian AA-375 equipped with a conventional deuterium arc background corrector. The measured errors were 3.3% with the seawater sample and 1.7% with the milk sample. It is clear that, even though the accuracy of correction may deteriorate as the speed of the background peak increases, the Zeeman system gives significantly better correction than the conventional system. These studies are the first which have been reported on the configuration of a modulated magnetic field applied longitudinally to a furnace. The features of ZAAS are the improved accuracy of correction for non-atomic absorption, and the achievement of a double-beam effect with single beam optics. The disadvantages are poorer sensitivity and linearity and the physical limitation of a furnace which is partially enclosed by a magnet. ACKNOWLEDGMENT We are greatly indebted to the late A. S. Pearl for introducing us to this subject and for his encouragement and advice. We also thank J. A. Chidzey for designing the processing electronics and the magnet. LITERATURE CITED ( I ) Segar. D. A,; Gonzalez, J. G . Anal. Chim. Acta 1972, 58, 7-14. (2) Massmann, H: El Gohaw. 2.: Gucer. S. Smctrochh. Acta, Part 6 1976, 31, 399-409. (3) HendrikxJongerius, C.:d e Galan, L. Anal. Chim. Acta 1976, 8 7 , 259-71. (4) Guthrie, B. E.; Wolf, W. R.; Veillon, C. Anal. Chem. 1978, 5 0 , 1900-1902.
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Anal. Chem. 1980, 52, 1260-1264
(5) Hadeishi, T. Appl. Phys. Lett. 1972, 21, 438-40. (6) Prugger, H.; Torge, R. United States Patent 3676004, filed Dec. 17, 1970; filed in Germany Dec. 23, 1969. (7) Parker, C.; Pearl, A. Australian Patent 474204, filed Jan. 5, 1971. (8) Hadeishi, T.; McLaughlin, R. D. Science 1971, 174, 404-7. (9) Held, A. M. Ph.D. Thesis, Montana State University, 1972. (10) Hadeishi, T.: Church, D. A.; McLauahlin, R. D.; Zak, B. D.; Nakamura, M.; Chang, R. Science 1975, 187,-348-9. (11) Stephens, R.; Ryan, D. E . Taianta 1975,22, 659-62. (12) Koizumi, H.; Yasuda, K. Anal. Chem. 1975, 47, 1679-82. (13) Koizumi. H.: Yasuda. K. Soectrochim. Acta. Part81978.3 1 . 237-55. i14j Koizumi; H:; Yasuda, K. Anal. Chem. 1976, 48, 1178-82. (15) Uchida, Y.; Hattori, S. Bunko Kenkyu 1977,26, 266-71; Chem. Abstr. 1978,88, 163211a. (16) Stephens, R. Talanta 1978, 25,435-40. (17) Stephens, R.; Ryan, D. E. Talanta 1975, 22, 655-8. (18) Stephens, R. Talanta 1977,24. 233-9. (19) Murphy, G. F.; Stephens, R. Talanta 1978,25, 223-5. (20) Koizumi, H.; Katayama, M. Phys. Lett. 1977, 63A, 233-4.
(21) Koizumi, H.; Katayama, M . Phys. Lett. 1977, 64A. 285-6. (22) Koizumi. H. Japanese Patent 145 181-1975, filed May 10, 1974. (23) Dawson, J. 8.;Grassam, E.; Ellis, D. J.; Keir, M. J. Ana/yst(London) 1978, 101, 315-16. (24) Koizumi, H.; Yasuda, K. Spectrochim. Acta, Part 8 1978,31, 523-35. (25) Koizumi, H.; Yasuda, K., Katayama, M. Anal. Chem. 1977, 49, 1106-12. (26) Grassam, E.; Dawson, J. B.; Ellis, D. J. Ana/yst(London) 1977, 102, 804-18. (27) Brodie, K. G. Unpublished work, Varian Techtron, 1971. (28) Otruba, V.; Jambor, J.: Komarek, J.; Horak, J.; Sommer, L. Anal. Chim. Acta 1978, 101, 367-74. (29) de Loos-Vollebregt, M. T. C.; de Galan, L. Spectrochim. Acta, Part 8 1978,33. 495-512. (30) Kubasik, N. P.; Volosin, M. T. Clin. Chem. 1974,20, 300-301.
RECEIVED for review December 21, 1979. Accepted March 7 , 1980.
Determination of the Absolute Quantum Efficiency of Luminescence of Solid Materials Employing Photoacoustic Spectroscopy M. J. Adams,' J. G.
Highfield,* and G. F. Kirkbright"
Chemistry Department, Imperial College of Science and Technology, London S W7, U.K.
Photoacoustic spectroscopy is employed as a calorimetric method for the determlnation of the luminescent quantum efficiency of several solid materials: tetraphenylbutadiene (TPB), yellow liumogen, and sodium salicylate. The values obtained for the quantum efficiency of TPB are shown to be dependent upon the physical nature of the sample. The technique employed utilizes the phenomenon of photoacoustic saturation, in which the amplitude of the photoacoustic signal is independent of the sample absorption coefflcient. With the aid of a graphical extrapolation procedure, the proposed method is rapid, accurate, and precise. The method may be employed for the examination of solid samples and materials in solutlon.
The problems associated with the study and determination of photoluminescence quantum efficiency values of solid materials have always been more difficult to overcome than for materials in solution. Tregellas-Williams (1)has reviewed the methods employed for the determination of luminescence efficiencies for inorganic phosphors and, more recently, Lipsett has provided an extensive summary of the subject ( 2 ) . The majority of techniques employed today use photometric methods, Le., the detection and measurement of a certain fraction of the emitted luminescence following excitation of the sample and an examination of the literature suggests that the most serious errors in these methods occur in applying corrections necessary to account for the experimental geometry employed. Wrighton e t al. ( 3 ) have utilized a conventional scanning emission spectrophotometer to determine absolute fluorescence efficiences of powdered samples. By measuring Present address, Macaulay I n s t i t u t e for Soil Research, Craigiebuckler, Aberdeen AB9 2QJ, U.K. Present address, Chemistry Department, U n i v e r s i t y of Bath, B a t h . U.K. 0003-2700/80/0352-1260$01 .OO/O
the diffuse reflectance of the sample relative to that of a nonabsorbing standard material a t the excitation wavelength and by recording the emission of the sample under identical conditions, the luminescence efficiency was determined as the ratio of the intensity of emitted radiation to the difference in intensity of the diffuse radiation from the sample and the nonabsorbing standard. Following calibration of the detector sensitivity as a function of wavelength, however, and following introductions of corrections for the nonideality of the absolute reflectance standards, the error in the method was reported to be k25%. Considering the extensive application of photometric methods, and their inherent disadvantages, there have been few developments in alternative techniques for determining luminescence quantum efficiencies of solids. Most calorimetric methods are modified versions of Bodo's ( 4 ) and employ thermocouples or thermistors to monitor the heating effect within the sample following absorption of electromagnetic radiation. A piezoelectric calorimeter has been described by Callis ( 5 ) and used to determine the triplet yield for anthracene dissolved in a rigid matrix of polymethylmethacrylate. In this paper we wish to report the use of photoacoustic spectroscopy, PAS, for the determination of absolute quantum efficiencies of solid materials. PAS employs the optoacoustic effect in which the absorption of modulated electromagnetic radiation produces a periodic temperature wave within the sample. The magnitude of this temperature fluctuation is dependent upon the optical absorption characteristics of the material under study and upon the efficiency of radiationless conversion following excitation. In PAS the temperature of the sample, contained within a sealed cell, is monitored with the aid of a microphone transducer via the periodic pressure wave produced in the gaseous atmosphere surrounding the sample (6, 7). Adams et al. have employed the PAS technique for the determination of the absolute fluorescence quantum efficiency of aqueous solutions of quinine bisulfate (8), and Malkin and Cahen have used PAS to study radiant energy 8 1980 American Chemical Society
