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Anal. Chem. 1982, 5 4 , 1029-1032
Correction for Double Valued Calibration Curves in Zeeman Effect Atomic Absorption Spectrometry Hldeakl Kolzuml, * Hltoshl Sawakabu, and Masataka Koga Naka Works, Hita,chi, Ltd., Katsuta, Ibaraki, 312, Japan
The bending back of a callbratlon curve toward the concentration axls at a very hlgh concentration Is a general problem In Zeeman effect atomic absorptlon spectrometry (ZAAS), especially with a i flame atomlrer. We found that fast translent absorptlon peaks appear at the beginning and the end of sample aspiratloin Into a flame when the sample concentration exceeds the point of the roll over of a calibration curve. Detectlorn of these peaks alerts the analyst that sample dllutlon Is necessary to obtaln the correct concentration values. The operatlon procedures for callbratlon and the measurement are not dHferent from those of conventional flame AAS Interfaced with a mlcroprocessor.
Table I. Instrument Components and Experimental Parameters light source polarizer magnet field strength: material : gap width: gap cross section: burner slot length gas fuel
The Zeeman effect atomic absorption (AA) spectrometry technique is widely used because of its accurate and precise background correction capability. Recently, it was found that the Zeeman effect AA technique is highly effective not only for furnace AA but also for flame AA because it fulfills the optimal conditions for double beam optics; Le., the reference beam passes through the flame unlike the case with conventional double beam optics (1-3). However, all Zeeman effect AA techniques have one problem in common which has been reported by a number of researchers (2-12). At very high concentrations (around lo4times higher than the lower limit of detection) the calibration curve starts bending back toward the concentratioin axis resulting in a double valued curve. This behavior is observed with all Zeeman designs, regardless of how the magnet is positioned or operated. The roll over happens because the calibration curves are made by the differential absorption between the a and the u components in ZAA techniques. Saturation of the absorption of the a component starts occurring at lower concentrations than that for the u component, resulting in a decrease of differential absorption. As Liddle et al. have pointed out a double valued curve does not cause a serious problem in furnace ZAA ( 4 ) . However, in flame ZAA, if an unknown sample contained a very high concentration of the atoms of interest, the double valued curve might lead to erroneous results. Namely, the result might be interpreted to be as small as the other value on a double value curve, if the signal is integrated or recorded on a strip chart, recorder with an ordinary response time. An example of a double valued calibration curve and the related signals in illustrated in Figure 1. Point A and point B of different concentration values give the same absorbance. The absorption signal corresponding to point A shows no difference from the signal obtained by conventional flame AA. At point E%,however, transient absorption peaks should appear at the beginning ,and the end of sample aspiration. The height of the both peaks should be equal to the absorbance a t the point of roll over. These transient signals are too fast to be fully detected by a usual recording system. At the beginning of sample aspiration the concentration of atoms in the flame increases rapidly from zero to point B via the point of roll over. This produces the first peak. At the end of the sample aspiration the concentration of atoms in a flame decreases 0003-2700/82/0354-1029$01.25/0
auxiliary nebulizer photomultiplier signal processing modulation freq band-pass filter VIF converter
microprocessor ROM RAM read out recorder printer character display
hollow cathode lamp (Hitachi HLA-4) quartz Rochon prism (optical contact) permanent 9.5 kG HIMAG (Hitachi Metal Co.) 1 0 mm 100 mm X 1 0 mm premix type 100 mm (C,H,/air); 50 mm (C,H,/N,O) C,H,, 0-4.6 L/min for air; 0-8.9 L/min for N,O air, 0-10.6 L/min; N,O, 0-6.6 L/min concentric type with glass bead Hamamatsu T.V. R456A 1.5 kHz for light intensity,
55 Hz for polarization mechanical filter 1.5 i: 0.1 kHz SL-W22 (Daini Seikosha Co.) 5 MHz, M4707 (Teledyne Phylbrick) PACE 1 6 bit 32 kbyte 8 kbyte M056 (Hitachi) PUllOOE (Olivetti) K9C-1030G (Hitachi)
rapidly from B tto zero via the point of roll over. This produced the second peak. The peak height of the transient absorption corresponds to the maximum of the double valued calibration curve. The increasing and decreasing concentrations of sample atoms in the flame depend upon diffusion of the sample mist in a mixing chamber. We studied the detail of these transient absorption peaks by using fast electronics and a micropi~ocessor and found that the detection of these transient absorption peaks is useful to determine if the concentration of a sample exceeds the point of roll over. In this paper, we will report a new technique for roll over detection in Zeeman atomic absorption spectrophotometry. EXPERIMENTAL SECTION Table I lists the instrument components and experimental parameters. Figure 2 shows the block diagram of the data processing instrumentation. A polarized Zeeman effect AA spectrophotometer fabricated in our laboratory was used. With this instrument, a static magnetic field of 9.5 kG is applied to a flame and differential absorption is observed for radiation polarized perpendicular and parallel to the magnetic field, respectively. The signal from a photomultiplier was processed by a PACE microprocessor after A/D conversion by a V / F converter. Analytical results are shown on a character display and hard copied by a printer. The program was written in assembler language for real time signal processing. Two kbytes of programs were added to the basic software for correction of the double valued calibration 0 1982 American Chomical Society
1030
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982 DOUBLE VALUED
ABSORPTION SIGNAL
CALIBRATION CURVE
CONCENTRATION
-
TIME
Flgure 1. Relation between points on the double valued calibration
curve and the absorption signals.
jrL_i
H I s
Figure 3. Observed shapes of flame ZAA signals at high concentrations (measured with a fast detection technique that stores a signal in RAM every 18 rns): (A) Cd 5 ppm, the concentration lower than that
at the point of roll over: (E) Cd 50 ppm, the concentration higher than that at the point of roll over.
(PACE)
C U , 3 0 ppn
INTERFACE
Cu, 1 0 0 pprr
Cu, 1 5 0 PPr
KEY BOARD
c u , 1 0 ppm
7
-VIDEO INTERFACE
CRT DISPLAY
Figure 2. Block diagram of the data processing instrumentation.
curves. For observation of the detail of the rapidly changing signal, Zeeman absorption signals were stored every 18 ms in RAM and the stored signals were slowly read out later. With this technique details of the rapid change in absorption signal can be observed on an ordinary strip chart recorder. The Cd absorption line at 228.8 nm and the Cu absorption line at 324.8 nm were used in this experiment. Cadmium is one of the most sensitive elements and the emission line of Cd at 228.8 nm is easily broadened by self-absorption in the light source. The Cu line at 324.8 nm shows anomalous Zeeman effect with the a component as well as the u splitting into two lines. Both types of the line broadenings (i.e., self-absorption in the emission line and anomalous Zeeman effect in the absorption line) cause bending and roll over of a calibration curve. The spray chamber used was longer than that for standard AA to be combined with the permanent magnet (I). Therefore, the decay and rise times of droplets through the spray chamber were longer than usual.
RESULTS AND DISCUSSIONS Figure 3 shows the observed shapes of absorption signals in flame ZAA. These absorption profiles were measured from a fast detection technique that stored signals in RAM every 18 ms. The samples contained Cd at 5 or 50 ppm. As shown in Figure 3A, the shape of absorption signal was not different from that of conventional AA when the Cd concentration was lower than that at the point of roll over. As shown in Figure 3B, however, transient absorption peaks appeared at the beginning and end of sample aspiration into the flame when the Cd concentration was higher than that at the point of roll over. The concentration of Cd at the point of roll over was around 10 ppm under these experimental conditions. In Figure 3, the widths of the transient absorption peaks at the beginning and the end of sample aspiration are about 110 and
w I s
Flgure 4. Observed translent absorption slgnais appearing at the end of sample aspiration of Cu samples.
250 ms, respectively. The widths of these transient absorption peaks depend upon the decay and rise times of droplets through the spray chamber. The peak at the end of sample aspiration is broader than that a t the beginning. We confirmed that this was also true for other elements. Therefore, the peak at the end of sample aspiration should be used to determine if the concentration of the element in the unknown sample exceeds the point of roll over. In the following we shall discuss the second absorption peak which appears at the end of sample aspiration. Figure 4 shows the observed transient absorption peaks for Cu samples. The concentration a t the point of roll over for Cu was 40 ppm under these experimental conditions. Ten parts per million of Cu does not cause any transient peak since this concentration is smaller than that at the point of roll over. However, 50,100, and 150 ppm samples cause transient absorption peaks since these concentrations are larger than the concentration at the point of roll over. As we expected, these transient absorption peaks have a height corresponding to the absorbance at the point of roll over. The most common way to measure absorbance in conventional flame AA is the time-weighed integration of the absorption signal for an interval during sample aspiration. If the measurement is carried out in this manner with a double valued calibration curve, the sample of a concentration corresponding to B in Figure 1 may result a wrong answer; i.e., it may be mistaken for the concentration at A in Figure 1. However, this mistake can be avoided by utilizing the transient absorption phenomenon described above. We developed computer software for judging whether or not the concentration of the sample exceeds the point of roll over without changing the ordinary operating procedures.
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982 F I R S T PEAK
11031
Cd CONCENTRATION
SECOND PEAK
IPPrn )
1.2 10
w
5 2
l5 2 0
25
1.0 5 1
v1
I 0.8
0.6
0.4
T START ITI 1
B
0.2
n I
0
0 -
L
I
1
I
2
3
TIVE (Pin)
Flgure 6. Absorption signals of high concentrations recorded with conventional detecting system.
PEAK D3TECTION
PRINT OUT and x2 I T 5 )
x1
Flgure 5. (A) Timing and (e) flow charts for determination if concentration exceeds tlhe point of roll over.
The chart in Figure 5B shows the timing in measurements and the flow of the software. The sample starta to be aspirated into the flame a t T1. A button for integration start is pressed by an operator (T2) a few seconds after the beginning of sample aspiratiion. The signal is integrated to measure X1 from T2 to T3 ([seeFigure 5A) which is a desired integration time selected beforehand. The operater is informed of the end of the integration (T3) by a small lamp and a buzzer. Then the sample aspiration is stopped (T4). The second transient absorption peak appears right after stopping the sample aspiration, and then the signal goes down to the zero level (T5). The peak height X2 is measured between T4 and T5. If a peak is detected, both the values of X1 and X2 are printed out. If no peak is detected, only the value of X1 is printed out. Supplementary programs were prepared for ensuring selection of the correct value from a double valued calibration curve. Several kinds of threshold values were considered to avoid a possible rnisjudgement. For example, in the case where the noise on the signal is considerably large, the supplementary program must distinguish the peak of the transient absorption signal from the noise. Shot noise is dominant in polarized Zeeman effect AA. The shot noise limitation does not necessarily mean thLe light intensity is very weak, but other kinds of noise are as small as 10-L10-6 aborbance units because the flicker noise from the lamp and the noise due to the fluctuations of the flame can be corrected with the ZAA technique. The noise due to signal processing and the electronics is around lo4. Therefore, the overall noise mainly consists of the shot noise. Shot noise from a photomultiplier is white noise. On the ohher hand, the width of the peak of transient
absorption is around 250 ms. Therefore, high frequency constituents of the shot noise can be eliminated by filtering the high-frequency constituents with little sacrifice to the peak height of the transient absorption. By averaging the signal every 100 ms, we decreased the magnitude of the noise to 310% while keeping 95% of the peak height of the transient absorption. With this software if the peak height X2 exceeds 105% of the integrated absorbance value X1, X2 was regarded as a transient absorption peak. This tolerance of 5% corresponds to absorbance of more than 0.0025 since the roll over happens at higher than 0.5 A. Considering both positive and negative values, the noise equivalent to this absorbancle is 0.005. Under the usual operating conditions, the noise love1 never exceeded this value of 0.005 with the response time of 100 ms. By use of this threshold value, peaks less than 105% of the integrated absorption value are neglected. However, in practice this does not pose a problem. In atomic absorption spectrophotometry, a calibration curve is made at the beginning of measurement using standard samples whose concentrations are known. If a double valued calibration curve is obtained in the standardization, the standards must be changed or diluted to get a calibration curve having a reasonably small bending. This process of standardization is the same as that with the conventional AA. The sample concentration should be smaller than the standards of the highest concentration. When the measured concentrtion of a sample exceeds the concentration of the highest standard, the sample must be diluted. So long as a calibration curve is not extremely bent, the concentration at the point of roll over is much higher than that of the highest standard. Therefore, there is no possibility of misjudging the roll over in an actual analytical procedure even if 5% tolerance is provided for peak detection. Table I1 shows the results that were obtained from Cd samples of 0,5,10, 15,20, and 25 ppm by the present technique, while Figure 6 shows the absorption signals recorded by an ordinary recorder. Because the ordinary recorder can not follow the fast transient signal, no transient peak may be observed in the signal from 15 ppm. In the signal from 20 or 25 ppm, a part of the peak was observed only at the end of sample aspiration. As shown in Table 11, however, X2 values were printed out for samples 15,20, and 25 ppm Cd which were higher than the concentration at the roll over. Therefore, we can clearly distinguish samples which must be diluted to obtain the correct concentration values.
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Anal. Chem. 7982, 5 4 , 1032-1037
Works for their helpful discussions.
-
Table 11. Results of Analvses of High Concentration Samples by the Present Technique sample Cd concn, no. PPm 1 2 3 4 5 6
0 5 10 15 20 25
LITERATURE CITED
absorbance
x, 0.000 0.923 1.055 1.013 0.946 0.875
x2
1.062 1.074 1.057
This technique can be generally used for other analyses that produce a double valued calibration curve, for instance, coherent forward scattering (CFS) and atomic fluorescence spectroscopy (AFS). ACKNOWLEDGMENT The authors are grateful to K. Oishi and K. Uchino of Naka
Koizumi, H.; Yamada, H.; Yasuda, K.; Uchino, K.; Oishi, K. Spectrochim. Acta, Part8 1981, 368, 603-614. Fernandez, F. J.; Myers, S. A.; Slavin, W. Anal. Chem. 1980, 52, 741-746. Brodie, K. G.; Liddell, P. R. Anal. Chem. 1980, 52, 1059-1064. Liddell, P. R.; Brodie, K. G. Anal. Chem. 1980, 52, 1256-1260. de Loos-Vollebregt, M. T. C.; de Galan, L. Spectrochim. Acta, Part 8 1978, 338,495-506. de Loos-Vollebregt, M. T. C.; de Galan, L. Appi. Spectrosc. 1979, 33, 616-626. de Loos-Vollebregt, M. T. C.; de Galan, L. Appl. Spectrosc. 1980, 34, 464-472. Uchida, Y.; Hattori, S. Oyo Butsuri 1975, 44,852-857. Uchida, Y.; Hattori, S. Bunko Kenyu 1977, 26, 266-271. Grassam, E.; Dawson, J. 8.; Ellis, D. J. Analyst (London) 1977, 702, 604-616. Grassam, E.: Dawson, J. B. Eur. Spectrosc. News 1978, 27, 27-30. Magyer, B.; Vonmont, H. Spectrochim. Acta, Part 8 1980, 358, 177-192.
RECEIVED for review January 4,1982. Accepted March 9,1982.
Matrix Modifier and L'vov Platform for Elimination of Matrix Interferences in the Analysis of Fish Tissues for Lead by Graphite Furnace Atomic Absorption Spectrometry Thomas W. May" and William G. Brumbaugh Columbla Natlonal Fisheries Research Laboratory, U.S. Fish and Wildlife Service, Route 1, Columbia, Missouri 6520 1
A chemlcai modifier (NH,H,PO,) and a modifled L'vov platform are investigated for possible aiievlatlon of matrlx interferences, whlch are normally severe during graphite furnace analysis of fish tlssues for lead. Slope ratlos of addltlons plot to standard curve were calculated under four atomizatlon condltlons with regular graphite tubes: tube wall, tube modlfler, platform, and platform modifier. The effects on slope ratlos of slow (1 s RAMP) vs. fast (MAX POWER) heating rates during atomlzatlon are also Investigated. Near unlty (1.00) slope ratios were produced with the MAX POWER platform modifier comblnatlon for whole ground fish, fish liver, and fish blood uslng carefully optimlzed charring and atomization temperatures based on sample matrix and atomlzation conditions. Most slope ratio Improvement was due to the addition of NH4H,P0,. Precision with the MAX POWER platform 4- modlfler combination was 0.5 % relative standard devlatlon.
+
+
+
Graphite furnace AAS is often subject to matrix interferences that can cause severe suppression of the analyte signal. When determining P b in whole freshwater fish, we commonly experience signal suppression ranging from 30 to 50% as compared with response in dilute acid. Because of the ease of application, the method of standard additions is frequently used to correct for matrix suppression in samples of unknown composition. However, the use of standard additions to correct for matrix interferences increases both cost and time of sample analysis. We therefore investigated other ways to remedy matrix interferences that occur when fish tissues are analyzed
for P b with a commercially available graphite furnace. Considerable research has been directed toward characterizing and reducing matrix interference in P b systems. Although different graphite tube coatings or liners (1-6) and variations in the type of furnace purge gas (7,8) have emerged as methods to reduce matrix interferences, chemical modifiers have received greater application with more successful results. Ediger (9, 10) first reported the use of ammonium salts to reduce NaCl interference effects during graphite furnace analysis of Cd and Cu. Since then, other workers also have successfully employed ammonium compounds to reduce matrix interferences in P b systems. While measuring Pb in streamwater containing substantial amounts of organic acids, Briese and Giesy (11) eliminated matrix effects due to NaOH on Pb response by using equal volumes of 1%HN03 and 50% NH4N03. Frech and Cedergren (12) also found that for Pb determinations in NaCl solutions, nonatomic absorption significantly decreased after ",NO3 and HNOBtreatment. Manning and Slavin (I) concluded that NH4N03was superior to the dicarboxylic acids for reducing interferences when measuring Pb with Mo and pyrolytically coated graphite tubes. Other modifiers, notably H3P04 and ascorbic acid, reduce signal interference in Pb systems. Hodges (13)used graphite tubes treated with H3P04 and HZ4Mo7NSOz4 to reduce matrix suppression effects during analyses of urine for Pb. Czobik and Matousek (14) proposed the conversion of interfering chlorides to pyrophosphates for reduced suppression on the P b signal during H3P04 treatment in a Pb-CuC1, system. Koirtyohann et al. (15) combined the suppression reducing properties of NH4N03and &Po4 by studying the effects of 1%NH4H,P04 in reducing suppression of Pb and Cd signals
This article not subject to U.S. Copyright. Published 1982 by the American Chemical Society
