182
Anal. Chem. 1983, 5 5 , 182-186
LITERATURE CITED (1) Benninghoven, A.; Sichtermann, W. Anal. Chem. 1978, 5 0 , 1180. (2) Eicke, A.; Sichtermann, W.; Benninghoven, A. Org. Mass Spectrom. 1960, 15, 289. (3) Sichtermann, W.; Junack, M.; Eicke, A.; Benninghoven, A. Fresenlus' 2.Anal. Chem. W 8 0 , 301, 115. (4) Benninghoven, A.; Sichtermann, W. Int. J . Mass Spectrom. Ion Phys. W 8 1 , 38, 351. (5) Ens, W.; Standing, K. G.; Chait, B. T.; Field, F. H. Anal. Chem. 1981, 53. 1241. (6) Hunt, D . F.; Bone, W. M.; Shabanowitz, J.; Rhodes, J.; Baiiard, J. M. Anal. Chem. 1981. 53. 1704. (7) Kambara, H.; Hlshida, S. Org. Mass Spectrom. lg81, 16, 167. (8) Liu, L. K.; Busch, K. L.; Cooks, R. G. Anal. Chem. 1981, 53, 109. (9) Kambara, H.; Hishida, S . ; Naganawa, H. Org. Mass. Spectrom. 1082, 17, 67. (10) Sichtermann, W.; Eicke, A.; Junack, M.; Benninghoven, A. Fresenius' 2.Anal. Chem. 1882. 371, 410.
(11) Barber, M.; Bordoll, R. S.; Eliott, G. J.; Sedgwick, R. D.; Typer, A. N. Anal. Chem. 1982, 5 4 , 845A. (12) Jaenicke, L.; Kutzbach, C. fortschr. Chem. Org. Naturst. 1963, 27, 183. (13) Smith, R. G.; Pegues, J. C.; Farquhar, D.; Loo, T. L.; Wang, Y.-M. Biomed. Mass Spectrom. 1981, 8 , 144. (14) Junack, M.; Eicke, A.; Sichtermann, W.; Benninghoven, A,, unpub-
lished work. (15) Lange, W.; Jirikowsky, M.; Benninghoven, A,, submitted for publication in Surf. Sci. (16) "The Merck Index" 9th ed.; Merck & Co., Inc.: Rahway, NJ, 1976; p
5852.
RECEIVED for review August 12, 1982. Accepted October 19, 1982.
Direct Determination of Cadmium in Coastal Seawater by Atomic Absorption Spectrometry with the Stabilized Temperature Platform Furnace and Zeeman Background Correction Ewa Pruszkowska, Glen R. Carnrick, and Walter Slavln" Perkin-Elmer Corporation, Main A venue, Norwalk, Connecticut 06856
A simple and dlrect method is described for the determination of Cd in coastal seawater. A platform graphite furnace and Zeeman background correction were used. We obtained a detection llmlt of 0.013 pg/L in 12-pL samples, or about 0.16 pg of Cd in the coastal seawater sample. The characteristlc integrated amount was 0.35 pg of Cd per 0.0044 A s . A matrix modifier containlng (NH,),HPO, and HNO, was used. Concentrations of Cd in coastal seawater were calculated directly from a calibration curve. Standards contalned NaCl and the same matrix modifier as the samples. No interference from the matrix was observed.
Cadmium is one of the most sensitive furnace AA determinations. For conventional furnace work, a characteristic amount of 1 pg/0.0044 A and a detection limit of 0.3 pg for Cd in water was reported ( I ) which, in a 20-pL seawater sample would be 0.015 pg/L. It is therefore not surprising that the determination of Cd in seawater most frequently utilizes the graphite furnace. However, most workers separated the Cd from the salt matrix prior to analysis. Some have used chelation and extraction (2-9), ion exchange (3,5,6, IO), or electrodeposition (11, 12). The various approaches to determination of trace metals in seawater were recently reviewed (13). The direct determination of Cd in seawater is particularly difficult because the alkali and alkaline earth salts cannot be fully charred away at temperatures that will not also volatilize Cd. Most workers in the past (6,14-17) who have attempted a direct method have volatilized the Cd at temperatures which would leave sea salts in the furnace. This required careful setting of temperatures and was disturbed by situations which caused temperature settings to change with the life of the furnace tubes. Lundgren et al. (14) showed that the Cd signal could be separated from a 2% NaCl signal by atomizing a t 820 "C, below the temperature where the NaCl was vaporized. This
technique has been called selective volatilization. They detected 0.03 pg/L Cd in the 2% NaCl solution. They used an infrared optical temperature monitor to set the atomization temperature accurately. Campbell and Ottaway (16) also used selective volatilization of the Cd analyte to determine Cd in seawater. They could detect 0.04 pg/L Cd (2 pg in 50 pL) in seawater. They dried at 100 "C and atomized at 1500 "C with no char step. Cd was lost above 350 "C. They could not use NH4N03because the char temperature required to remove the NaN03 volatilized Cd also. Na and Mg chloride salts provided reduced signals for Cd at much lower concentrations than their concentration in seawater if the atomization temperature was in excess of 1800 "C. The determination required lower atomization temperatures to avoid atomizing the salts. Even this left the Mg interference which required the method of additions. Guevremont et al. (17) used a direct, selective volatilization determination of Cd in seawater. They used 20-pL seawater samples, 1g/L of EDTA, an atomization ramp from 250 "C to 2500 "C in 5 s, and the method of additions. Their detection limit was 0.01 yg/L (0.2 pg in 20 pL), the characteristic amount was 0.7 pg/0.0044 A . The EDTA promoted the early atomization of Cd, below 600 "C. Their test seawater sample (0.053 pg/L Cd) was confirmed by other methods. These authors were unable to separate reliably the Cd and background signals by using the method of Campbell and Ottaway; the EDTA made this possible. In similar work, Sturgeon et al. (6) compared direct furnace methods with extraction methods for Cd on two coastal seawater samples. They found 0.2 and 0.05 pg/L Cd and could have measured Cd down to 0.01 pg/L. They used 10 pg/L ascorbic acid as a matrix modifier. Various organic matrix modifiers were studied by Guevremont (15)for this analysis. He found citric acid to be somewhat preferable to EDTA, aspartic acid, lactic acid, and histidine. The method of standard additions was required. The standard deviation was better than 0.01 pg/L in a seawater sample containing 0.07 pg/L Cd. Generally, he charred at 300 "C and atomized at
0003-2700/83/0355-0182$01.50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 2, FEBRUARY 1983
Table I. Zeemiin Graphite Furnace Conditions clean dry char atomize out temp, ’C ramp, s hold, s int gas flow mL/min recorder, s
160
550
1
1 15
60 300
300
cool
1600 0
2600 1
20 1
5
6 300
20 300
0
-5
1500 OC. The method required compromise between char and atomization temperatures, sensitivity, heating rates, etc., but the analytical r?sults seemed precise ar.d accurate. Nitrate added as NaN03 delayed the Cd peak and suppressed the Cd signal. Sperling has :reported extensively on the determination of Cd in seawater as well as in other biological samples and materials. HE (18) added (NH4)&08 which permitted charring seawater a t 430 OC without lcss of Cd. For work below 2 pg/L Cd in seawater, he (7, 19) recommended extraction of the Cd t o separate it from the matrix. He found no change in the measured levels over many months when the seawater was stored in dense polyethy1e:ne or polypropylene. In this paper we extended our stabilized temperature platform furnace work from our earlier Mn in seawater (20) to Cd in seawater. Zeeman background correction has made this determination more convenient. It may be possible t o d o this determination with conventional background correction but we have not tested this possibility. T h e method reported here i:; simple and direct. Seawater samples are quantitated against standards, and the :method of additions is not used. Careful temperature setting is required. This paper is directed specifically to Cd in coastal seawater and we are publishing separately (21) more general experience involving the determination of Cd in a Zeeman furnace, as well as Cd in urine (22). EXPERIIVLENTAL SECTION Equipment. All test; were performed on a Perkin-Elmer Zeeman/5000 equipped with an AS-40 autcsampler and a Model 56 strip-chart recorder. .A Cd EDL was used and was run at 5 W. All experiments were performed at 228.8 nm, with 0.7 nm spectral slit width. Peak iibsorbance and integrated absorbances were calculated by a Da.ta System 10 and a Hewlett-Packard 7225A graphics plotter. Char and atomization temperature reported here were taken from an optical pyrometer. Pyrolytically coated tubes, Perkin-Elnier part no. B0109-322, with solid pyrolytic graphite platforms, P-E part no. 0290-2311, were used. The furnace conditions are summarized in Table I. The integration time for the analytical work was limited to 2 s. The signals frcm the Data System 10 reported here are called ZAA signals for the analytical result and SB signals (single beam) for the backgrounds. The SB signals are expressed in absorbance units ( A ) and the ZAA signals are usually in absorbance units. seconds (Ass). The SB signal is signal plus background, but for the small analyte signals of this study, the SB signal is effectively background. The actual integrated absorbance signals that were used were calculated by software on the Data Station 10 from signals transmitted by the Zeeman/5000. The plots shown in later figures show typical signals but were not used for quantitative evaluation. Materials. TEe seawater samples, which were analyzed in our previous paper cn Mn (20) were received from the National Research Council, Canada. They were named Sandy Cove N2, 8, Sandy Cove No. 9, Bermuda, and NRC. The first two samples were filtered and they and the NRC sample were acidified to approximately 1‘3 HNO, and have been stored for 2 years. Generally, the NliC sample was used for studies. Our labeling of the samples may not correspond to the S R C labeling. The sample marked Bermuda is the NRC Reference Material NASS-1. The sample marked Sandy Cove No. 8 in this study is the sample marked Seawater sample B from their paper ( 6 ) . The samples
‘183
marked Sandy Cove No. 8 and No. 9 were used in our previous paper (20). The Cd standards were prepared from a 1000 mg/L standard from Alfa Products, Ventron, by dilution with 1%HNO, (Ultrex). Both (NH,),HPO,, and NH4H,P04 have been recommended as matrix modifiers for Cd (21). We chose to use (NH4),HP04, from Mallinckrodt Chemical Co., exclusively in this procedure because it had much smaller blanks in comparison to the particular NH4HzP04material!; which were available to us. Water from a Continental Water System was purified in a Millipore deionizer consisting of Milli-R04and Milli-Q systems. Temperature Settings. The conditions are similar to those reported for the stabilized temperature platform furnace (23) and are summarized in Table I. The temperatures have been set with an Ircon optical pyrometer. For char temperatures below 900 “C, the HGA-500 temperature setting system was calibrated at 900 “C and any discrepancy found between setting and measurement was used to correct the set temperature below 900 OC. Typically we found that the HGA-500 setting was 150 OC higher a t 900 “C than was measured with the pyrometer. At the 1600 OC atomization temperature, the discrepancy between the set value and the pyrometer value was less than 50 “C and can be ignored. The difference between set and measured temperature gradually increased with each firing during the life of the tube. If a pyrometer is not used, this may cause a gradual increase in background which is not a problem with Zeeman background correction. The photodiode which controls the discontinuation of “max power heating” must be set with the Zeeman magnet “on”. This is accomplished by turning the “Accessory” switch on and depressing the “Rec Man” button before entering the desired atomization temperature and holding down the “Man Temp” button. We used a new tube each day and completed 80 to 100 firings during the day. A dry cotton swab was used to remove salt from the windows and contact rings prior to starting each day. If a pyrometer is used, it in preferable to focus it upon a surface within the furnace tube through the fill hole and allow 30 s for the signal to reach equilibrium. Focusing upon the outside surface of the graphite tube may lead to an error since the emissivity of the graphite is not known. Procedure. To avoid a contamination problem, we cleaned the plasticware as previously described (20). The polypropylene cups, bottles, and tips were soaked for 24 h in 20% HNO,, rinsed in deionized water, oven dried at 50 “C, and covered with Parafilm for storage. The pipet tips were also rinsed several times with the solution to be dispensed, prior to taking the analytical aliquot. Yellow disposable pipet tips are known to be contaminated with Cd, so clear tips should be used. The determination of Cd in seawater was calibrated with a working curve. Standards for the working curve were prepared in 2% NaCl, 240 pg of NaCl on the platform. It proved convenient to use 12 WLof undiluted seawater on the platform. Smaller samples needlessly sacrificed sensitivity and larger samples provided increasingly troublesome background signals. The matrix modifier, dispensed in a 5-pL aliquot, contained 200 pg of (NH&HPO4 and about 28% HNO,. This aliquot was deposited on the platform by tlhe autosampler after the sample was dispensed. This yielded about 8% HNO, on the platform. Integrated absorbance values were used exclusively for construction of the working curve and for calculation of the concentration of Cd in seawater samples. RESULTS Matrix Modifiers a n d Char Studies. Matrix modifiers have been used as additives in furnace analyses either to reduce the effect of the particular matrix or t o increase the stability of the analyk. In the material below, we discuss both situations: “,NO, acting on the seawater matrix and (NH4),HP04 to stabilize the Cd to higher char temperatures. We are generally more interested in the stabilization of the analyte, although this work is not treated in depth in this paper (21). Seawater samples usually contain a total of 2 t o 3% of several alkali and alkaline earth salts with NaCl as a main constituent. A 2-pL sample of seawater, charred at 700 O C ,
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 2, FEBRUARY 1983
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Flgure 1. Background (SB) profiles for 2 pL of seawater alone, with 200 p g of (NH,),HPO,
and with 500 p g of ",NO,. The char temperature was 700 "C and the atomization temperature was 1700 "C.
CSEC>
Figure 3. Background (SB) profiles for 12 pL of seawater, with 200 h g of (NH,),HPO, and varied amounts of "0,. The char temperature was 550 "C and the atomization temperature was 1600 "C.
S B 2T
1.25 MG N H 4 N 0 3 2.5 MG\ 5. 0 MG, ' 7.5 MG.
0
: I Q
0 4
0
T I M E
4
0
TIME
CSEC)
Figure 2. Background (SB) profiles for 10 p L of seawater, with 200 p g of (NH,),HPO, and varied amounts of ",NO,. The char temperature was 600 "C and the atomization temperature was 1600 "C.
Flgure 4. Background (SB) profiles for 10 hL of seawater, with 8 % HNO, and varied amounts of (NH,),HPO,. The char temperature was 550 "C, and the atomization temperature was 1600 "C.
has a SB background signal so high, over 2 A , that even the Zeeman correction system cannot handle it (Figure 1). The large amounts of NaCl present in seawater are reportedly volatilized below 950 "C (24), but even with ammonium phosphate, the matrix modifier recommended for Cd, it is not possible to char at so high a temperature. We found in Figure 1that 200 pg of (NH4)2HP04reduced the SB signal of 2 p L of seawater to 0.5 A , but 500 pg of ",NO3 reduced the background more effectively to 0.16 A. We found no reduction of the Cd signal in the presence of NH4N03if the char temperature was below 600 "C and phosphate was used as a matrix was used without phosphate, the Cd modifier. If ",NO3 was lost at temperatures below 500 "C. The addition of the phosphate stabilized the Cd while the NH4N03promoted the release or conversion of the bulk of the background producing material. The addition of the phosphate produced a background signal that appeared much later than the Cd peak. Guevremont (15) reported that NaN0, suppressed the Cd signal. Our experience indicated that his observation was more likely explained as a reduction in the char temperature that produced a loss of Cd. In order to estimate how much NH4N03was needed, we ran 10 p L of seawater with different amounts of NH4N03, Figure 2. We found that 1.25 mg of NH4N03is enough to keep the SB signal below 1.5 A and there are no large differences in SB absorbances for amounts from 1.25 to 7.5 mg of NH4NO3. Since NH4N03reduced the effect of the background, HNO, should work as well (21). We analyzed 12 p L of seawater with increasing concentrations of HNO,, also in the SB mode,
Figure 3. The figures show percent acid in the total sample plus modifier deposited on a platform. Two percent HNO, reduced the background to a level which can be handled by the Zeeman correction system. From 4 to 8% "OB, the changes in SB signal shapes were not very large. In the first 2 s the SB signal is not higher than 0.5 A. In the next experiments we used 8% "OB, which is very conservative, and smaller amounts may be adequate. We also ran, Figure 4,10 pL of seawater with 8% HNO, but with different amounts of (NH4)2HP04,from 80 to 240 pg. Higher concentrations of (NH4)2HP0,caused the SB peak to appear earlier and also caused a considerable increase in the SB signal. With a smaller amount of phosphate, a very fast, small peak appeared in the first second, which affected the Zeeman signal. Considering these facts, we decided to use 200 pg of (NH4)2HP02for this method. It is very important to determine a char temperature that will not lose the analyte element. While from previous experience (21) we knew that small amounts (micrograms) of the salts of several alkali and alkaline earth metals in the presence of phosphate increased the permissible charring temperature for Cd, large amounts decreased the maximum permissible charring temperature. For example, Cd was lost a t lower char temperatures in the presence of increasing concentration of Mg. In Figure 5 we compare the char curves for 20 pg of Cd both in water and in seawater. We added 5 pL of the matrix modifier consisting of 200 pg of (NH4),HP04 and enough H N 0 3 to provide 8% on the platform. We used 1600 "C for atomization. Curve 1 is the 20-pg standard plus the modifier.
ILL/
ANALYTICAL CHEMISTRY, VOL. 55, NO.
0.
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/
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2
185
2, FEBRUARY 1983
U Z
A -
'500
400
600
700
800
900
:
:
:
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TEMPERATURE ("C)
Figure 5. Char temperature study for 20 pg of Cd in H,O (curve l), 20 pg of Cd plus 240 pg of NaCl (curve 2), and 20 pg of Cd in 12 pL of seawater (curve 3). The atomization temperature was 1600 OC. The modifier was 200 p g of (NH,),HP04 and 8% "0,.
:
:
1500 /0.19/
:
.
4 0
0
1000
n
1400 '0'21/
4
4
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Figure 7. The atomization temperature study for 20 pg of Cd in 18 pL of seawater. The modifier was 200 pg of (NH,),HPO4 and 8% "0,. ~~
~
~
Table 11. Quantitative Results and Recoveries amt found, pg/L this work Sandy Cove No. 8 0.040 Sandy Cove No. 9 0.058 0.029 Bermuda NRC 0.049 a
NRC
i
0.006 0.05 t O . O l a
t
0.004 0.029 t 0.004
t
0.005
* 0.007
%
recoverY 103 95
103 94
Sample B in the paper by Sturgeon et al. (6).
-~
k t
5
0.02
t
I ' 80 160 240 I
I
I
320
400
pg NoCl ON THE PLATFORM
Figure 6. Losses of Cd as a function of salinity In the 5-pg standard solution at three char temperatures: 500, 600, 700 OC. The modlfier was 200 p g of (NH,),HPO, and 8% "0,.
Curve 2 is the 20-pg standard with 240 pg of NaCl plus modifier. To provide 240 pg of NaCl on the platform required 2% NaCl in the standard solutions when a 12-pL aliquot was dispensed onto the platform. Curve 3 represents 1 2 p L of seawater plus 20 pg of Cd with the same modifier as in the other curves. Without the NaC1 (curve 1) there was a small loss of Cd at 600 "C and the data were imprecise and variable. The addition of 240 pg of NaCl to the standards kept the Cd on the platform to 600 OC and improved the precision of the measurement. The shape and the maximum usable char temperature for the standards and samples, curves 2 and 3, were very similar. Therefore we have added NaCl to the standards in preparing the calibration curves. We varied the amount of NaCl added to standards containing 5 pg of Ccl and charred at three different temperatures. The results are plotted in Figure 6. We did not observe significant interference to 300 pg of NaCl on the platform at 500 or 600 OC, but at 700 OC interference was observed when the amount of hlaCl was higher than 80 pg. In Figure 7 are plotted absorbance profiles of the seawater sample (18 pL) at several atomization temperatures. Twenty picograms of Cd and the matrix modifier containing 200 kg of (NH4)*HP04and 8% H N 0 3 were added to the platform. The char temperature was 500 "C. The integrated absorbance signal decreased slowly with increasing atomization temperature. At 1400 O C the curve did not reach the base line after 4 s, i.e., at the end of the atomization step. We decided to
use 1600 OC as the atomization temperature in order to use shorter integration times. With the conditions chosen, we found complete recovery of added Cd for 12 pL of seawater samples and for smaller sample amounts. The Cd peaks were easily integrated for. 2 s and the signals were precise and reproducible. Larger quantities of sample gave generally good results but we decided to be conservative and limit the sample to 12 pL for this method. It is quite likely that the detection limit can 1Se improved somewhat if necessity demands it. Determination of Cd in Seawater. Using the optimized conditions, i.e., char temperature 550 "C, atomization ternperature 1600 "C, and the matrix modifier that delivered 200 pg of (NH&HP04 and 8% HN03 to the platform, we prepared working curves. The standards with 1, 2,3, and 4 pg of Cd and 240 pg of NaCl were prepared. The blank was less than 0.013 pg/L and was independent of the concentration of NaC1. The slope of the curve provided a sensitivity, expressed as the characteristic amount, of 0.35 pg of Cd per 0.0044 A-s, using integrated absorbance measurements. The results of the determination of Cd in four seawater samples are shown in Table 11. The determination was done directly from the calibration curve and 12 pL of seawater was used for each run. Every seawater sample was run 10 times. The standard deviation, (r, for each sample was calculateld. The recoveries from each sample were measured by adding 2 pg of Cd to each. Each recovery sample was also measured 10 times. The recoveries are in the range 94 to 103%. The detection limit for Cd in seawater, calculated as 2a for low concentrations, was 0.013 pg/L. Since we used 12 pL of seawater, this corresponded to 0.16 pg of Cd in the seawater. This was about half of what was previously reported as the detection limit for Cd in pure water solutions ( I ) , using the instrumentation witlh conventional background correctioin. Typical ZAA signals and SB signals for one seawater sample are shown in Figure 8. An integration time of 2 s was used in order to avoid the high background appearing in the third second. The high backgrounds introduced no analytical error
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 2, FEBRUARY 1983 S B
2-
w
' '''1 0
i
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1.12 U L SEAWATER
UL
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3512 Ut
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Figure 8. Zeeman profiles of a seawater sample (Sandy Cove No. 9) and SB profiles. The first palr of profiles represent a single 12-pL aliquot, the second pair, two aliquots, and the third pair, three aliquots. The modifier was 200 p g of (NH4),HP04, 8 % "OB, and 5 p g of Mg(NO,),. The char temperature was 550 "C, and the atomization temperature was 1600 "C.
but did reduce the precision by increasing the noise. Since the background is not limiting the determination using Zeeman background correction, we expected multiple aliquots of seawater to provide improved detection limits. We dried and charred a 12-pL sample of seawater with 5 KL of the modifier. Before the atomization step, we put another sample (12 pL) with the modifier (5 KL)on the platform. We repeated the drying and charring steps and then atomized the material from the platform. We obtained a ZAA signal larger by a factor of 2 and the SB signal was about 1.6 A . When we tried to put three 12-pL samples on the platform, we found that the SB signal was higher than 2 A , and that, therefore, the signal was probably too noisy to be used.
"C char temperature, increasing amounts of phosphate to 900 pg still produced char losses over 50%. In this paper we have not addressed the question of "normal" Cd levels in unpolluted seawater. Three of our samples were coastal waters, at levels of 0.04-0.06 pg/L of Cd. There is not yet general agreement on levels in unpolluted seawater but Rasmussen ( 5 ) ,using Chelex-100 and solvent extraction found a level of 0.02 pg/L in carefully collected samples. Determination of Cd in seawater by a direct method a t so low a level will require that the detection limits of our method be improved. We have used essentially the same method for the determination of Cd in natural stream waters with detection limits down to less than 0.02 pg/L. The method does not appear to be affected by variations in the hardness of the water nor by the various materials present. Typically such samples can be charred at higher temperatures than we used in this paper. It is probable that the same method used here would be applicable for Zn and P b in seawater to a detection limit lower than 0.02 pg/L for Zn or 0.5 pg/L for Pb. We have not yet tested this possibility. It appeared to us that the major advantage of the Zeeman background correction system for the determination of Cd in seawater was the insensitivity of the method to the choice of conditions. The fact that the backgrounds were large could not be ignored but the large backgrounds were more easily handled with the Zeeman correction system. The stabilized temperature platform furnace was at least equally important in providing stable and convenient analytical conditions.
ACKNOWLEDGMENT We thank Ralph Sturgeon of National Research Council, Canada, for providing the samples and some of the comparative analyses. Registry No. Cd, 7440-43-9;HzO, 7732-18-5;(NH.JZHPO4, 7783-28-0;"03, 7697-37-2.
DISCUSSION The determination of cadmium in seawater above 0.013 pg/L is simple and reliable. The technique which is described does not require the method of additions, a simple calibration curve is used. It is not sensitive to variations in salinity when the char temperature is not higher than 600 "C. When the proposed method is compared with direct methods reported by others, only the selective volatilization method of Guevremont et al. (15,17) reached a detection limit as low as 0.01 pg/L Cd in seawater. While we report here only a single set of experimental conditions, the samples were analyzed on many occasions with conditions that were somewhat different in the amount of the matrix modifier, the amount of seawater, and the char and atomization temperatures. In these situations the analytical result on the several seawater samples were always within 0.02 pg/L of the values in Table 11, usually much closer, and the recoveries were always between 90 and 110%. Although our determinations and experiments were performed on the Zeeman/5000, this method might be repeated on a standard Perkin-Elmer Model 5000 with deuterium background correction as long as the atomization time would not be longer than 2-2.5 s. At the end of this period the background starts to rise to a value which could not be handled by the deuterium correction system. The detection limits will probably be poorer than those reported here. We specifically noticed that lower atomization temperatures (1400 "C) appeared to delay the large background sign& until later than the third second, well after the Cd signal had passed. This condition might be preferable for instruments utilizing platform technology with deuterium-lamp background correction. At the 1400 "C atomization temperature and the 800
LITERATURE CITED Slavin, W.; Manning, D. C. Prog. Anal. At. Spectrosc. 1982, 5 , 243-340.
Sturgeon, R. E.; Berman, S. S.;Desaulniers, J. A. H.; Russell, D. S. Talanfa 1980, 27,85-94.
Bruland, K. W.; Franks, R, P.; Knauer, G. A,; Martin, J. H. Anal. Chim. Acta 1979, 705,233-245. Smith, R. G., Jr.; Wlndom, H. L. Anal. Chim. Acta 1980, 713, 39-46. Rasmussen, L. Anal. Chlm. Acta 1981, 125, 117-130. Sturgeon, R. E.; Berman, S.S.;Desaulniers, J. A. H.: Mykytiuk, A. P.; McLaren, J. W.; Russell, D. S. Anal. Chem. 1980, 52, 1585-1588. Sperllng, K.-R. Z . Anal. Chem. 1978, 292, 113-119. Bengtsson, M.; Danlelsson, L.-G.; Magnusson, B. Anal. Lett. 1979, 12, 1367-1384. Sperling, K.-R. 2 . Anal. Chem. 1980, 301, 294-299. Kingston. H. M.; Barnes, 1. L.; Brady. T. J.; Rains, T. C.; Champ, M. A. Anal. Chem. 1978, 50, 2064--2070. Lund, W.; Larsen, B. V. Anal. Chim. Acta 1974, 72, 57-62. Bately, G. E. Anal. Chim. Acta 1981, 724, 121-129. Slavln, W. A t . Spectrosc. 1980, 1 , 66-71. Lundgren, G.; Lundmark, L.: Johansson, G. Anal. Chem. 1974, 4 6 , 1026-1031. Guevremont. R. Anal. Chem. 1980, 52, 1574-1578. Campbell, W. C.; Ottaway, J. M. Analyst (London) 1977, 702, 495-502. Guevremont, R.; Sturgeon, R.E.; Berman, S.S.Anal. Chim. Acta 1980, 175, 163-170. Sperllng, K.4. 2 . Anal. Chem. 1977, 287, 23-27. Sperling, K.-R. 2 . Anal. Chem. 1980, 301, 294-299. Carnrlck, G. R.; Slavin. W.; Manning, D. C. Anal. Chem. 1981, 5 3 , 1866-1 872. Slavin, W.; Manning. D. C.; Carnrick G. R.; Pruszkowska, E.. unpub-
lished work.
Pruszkowska, E.; Carnrick, G. R.; Slavln, W. Clln . Chem, (WinsfonSalem, N . C . ) , in press. Slavln, W.; Mannlng, D. C.; Carnrick, G. R. At. Spectrosc. 1981, 2 , 137-145. Nakahara, T.; Chakrabarti, C. L. Anal. Cb/m. Acta 1979, 104, 99-111.
RECEIVED for review July 6, 1982. Accepted November 12, 1982.
