1032
Anal. Chem. 7982, 5 4 , 1032-1037
Works for their helpful discussions.
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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 P b in streamwater containing substantial amounts of organic acids, Briese and Giesy (11) eliminated matrix effects due to NaOH on P b response by using equal volumes of 1%HN03 and 50% NH4N03. Frech and Cedergren (12) also found that for P b 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 P b with Mo and pyrolytically coated graphite tubes. Other modifiers, notably H3P04 and ascorbic acid, reduce signal interference in P b 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 P b and Cd signals
This article not subject to U.S. Copyright. Published 1982 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
'1033
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Table I. HGA.500 Graphite Furnace Program for Pb Determination time, s
-step
base line,
temp, "C
ramp
hold
tread
BOC
(1)dry ( 2 ) clhar
110 900b
10 12
30
36
(3) atomize (4)burnoutf ( 5 ) cool down
2300' 2700 20
30 25 7e 4
Id
1 1
0
Rec
int flow,n mL/min
-6
300 300 20
15
a Prepurified argon (Linde) purge gas. b , c Actual. optimization temperature used depended upon sample matrix and atom0 s if MAX POWER. e 9 s if platform used. f Used only with MAX POWER and platform. ization conditions.
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in samples of water, urine, and blood. hscrobic acid has been used to seduce suppression of the P b signal to less than 5% during analysis of natural water samples and synthetic water matrices (16, 1 7 ) . Another moans of reducing matrix interference is by physically altering the graphite tube atomizer to achieve more isothermal conditions during analyte atomization. Two approaches most often used are introduction of the sample into a specially constructed constant temperature furnace and atomization of the sample from a platform placed within the graphite tube of a commercially available atomizer. The first technique (18) is not convenient to use and is difficult to automate for improved sample throughput. The platform method uses a thin sheet of graphite placed inside the graphite tube. By atomizing from a platform, L'vov (19) increased Pb pulse amplitude and eliminated NaCl interference effects and Slavin and Manning (20) reduced interferences on P b from NaCl, Na2S04,and NaH2P04. Recently, the combination of chemical modifiers and the platform has largely eliminated interferences on the P b signal in various matrices. Kaiser et al. (21) used NH4H2P04and a modified L'vov platform to greatly reduce P b signal suppression during analysis of natural water. The average ratio of the additions plot slope to the standard curve slope (hereafter referred to as "slope ratio") was 0.96 with a platform + modifier combination, as compared with 0.49 for atomization from the tube wall. Using a Ta-treated pyrolytic tube and platform + NH4H2P04modifier, Hinderberger et al. (22, 23) achieved average slope ratios of 0.97 and 0.94 for P b in digests of urine and blood. To date, most work with chemical modifiers, L'vov platforms, and platform + modifier combinations has involved either synthetic matrices or natural water samples. We work routinely with whole ground fish tissues and wished to determine if a platform f modifier combination would eliminate matrix interferences on P b signals in these and other tissue matrices. Using the slope ratio approach, we investigated various tube, modifier, and platform combinations to determine the contribution of each component and the most effective combination for reducing matrix interferences during the determination of P b in fish tissue samples.
ElXPERIMENTAL SECTION Sample Selection. We obtained three types of fish samples--blood, liver, and whole fish-from archived collections to provide different levels of sample matrix. Selected samples of blood were collected from adult longear sunfish from the Big River, which drains Missouri's Old Lead Belt. A fish liver composite was prepared from adult common carp from each of five sites on the Upper Mississippi River. Five samples of whole freshwater fish, each a different species composite of 2-5 fish, were collected from the Upper Ohio River. Species selected were channel catfish, freshwater drum, common carp, spotted sucker, and largemouth bass. Sample Protection. Heparinized blood samples required no preparation before acid digestion. Carp livers were placed with deionized water (15-18 M a cm specific resistivity) into a 200-mL
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vertical glass flute flask and blended at 10000 rpm with a Tekmar SD-45 homogenizer equipped with a G-450 generator and Teflon bearing shaft (SD-45N). Fish from frozen composites were reduced to ice-cube-sized blocks with a meat cleaver, and then passed twice through a Hobart meat grinder. A 60-g aliquot of the ground product was then further reduced and blended with the Tekmar SD-45 homogenizer. Whole fish and fish liver homogenates were frozen at -30 "C in an FTS Systems freeze dryer. Lyophilization was initiated with a condenser coil temperature at -65 "C and vacuum at
