developed for serum analysis ( 5 ) ,alternative flow systems require 60-1L samples at a rate of 125 per hour or 30-pL samples at 80 per hour. For phosphate in serum ( 5 ) ,the sample size is 200 pL and the rate is 130 per hour. An alternative method for inorganic phosphate consumes a 100 pL sample a t 50 per hour, and the results from that method are unsatisfactory. In the words of Hansen and Ruzicka (5,p 361), “However, even when the long dialysis unit and pumping speeds as low as 0.8-1 mL min-’ were used with a measuring cell with an optical path length of 20 mm, the highest absorbance signals recorded were only of the order of 0.1 absorbance unit.” Unsegmented flow systems have been known for many years, particularly as reaction systems following ion-exchange chromatography columns. The limitations of unsegmented flow have kept these systems from coming into general use. T h e renewed interest in unsegmented continuous flow, principally by the Copenhagen group, may eventually develop
facts to invalidate the conclusions I drew in my article. I leave it to the readers to decide if the information published to date has had this effect.
LITERATURE CITED
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J. Ruzicka. E. H. Hansen. H. Mosbaek. and F. J. Krua. Anal. Chem.. 40. preceding paper L R. Snyder, J. Levine, R Stoy, and A. Conetta, Anal. Chem.,48, 942A (19761 - -, M. Margoshes, Anal. Chem., 49, 17 (1977). J Isreeli and W. Smythe, in “Advances in Automated Analysis”, Mediad, Inc.. Tarrytown, N. Y.. 1973, Vol. 1, D 13; also 5 following in the - .DaDers . same volume. E. H. Hansen and J. Ruzicka, Anal. Chim. Acta, 87, 353 (1976). \
Marvin Margoshes Technicon Instruments Corporation, Tarrytown, New York 10591 RECEIVED for review April 12, 1977. Accepted July 18, 1977.
Stabilizing the Manganese Tetramethylenedithiocarbamate/Methyl Isobutyl Ketone Extract Sir: Tweeten and Knoeck ( I ) recently studied the chelation of six trace metals by sodium diethyldithiocarbamate (NaDEDTC) and their extraction into isoamyl alcohol for analysis by atomic absorption spectrophotometry. They obtained poor precision and extraction efficiency and concluded: “Based on these experiments, the method of solvent extraction was considered unacceptable for use for multiple metal analysis of a natural water system. Copper is the only trace metal which can be quantitatively determined by this method.” These poor results are rather surprising since many other investigators have used dithiocarbamate chelating agents quite successfully with a variety of extracting solvents. I have also experienced similar precision problems though when extracting Mn with ammonium tetramethylenedithiocarbamate (ATMDTC, commonly but incorrectly known as ammonium pyrrolidine dithiocarbamate) into methyl isobutyl ketone (MIBK). One advantage of ATMDTC over other dithiocarbamates is its much slower decomposition rate in acidic solution (2-10). There is also some evidence that its extracts may be more stable (10, 11). The reproducibility problem I’ve experienced has been traced to the instability of the extracts. Previous investigators have also reported that the Mn dithiocarbamate extracts exhibit a rapid decrease in absorbance (11, 14-19). Other metal dithiocarbamates are more stable. Brooks et al. (12) reported that iron and nickel complexes were stable for 3 h, lead and zinc for 5 h, and cobalt and copper for 24 h when extracted from seawater with ATMDTC into MIBK. Kremling and Peterson (13),also using the ATMDTC/MIBK system, obtained similar results for iron and copper when extracted from seawater. Shendrikar et al. (14) found the ATMDTC extracts of zinc, cadmium, and lead to be stable for less than 1 day. It is apparent that the finite time stability of the dithiocarbamate extracts is a factor that must be taken into account when designing extraction procedures. For most dithiocarbamate extractable metals, it is inadvisable to use a lengthy extraction procedure (several hours or overnight) because of this instability problem. This may be the reason that Tweeten and Knoeck ( I ) experienced difficulties in their study. I t is unfortunate that the long-term storage of dithiocarbamate extracts is not possible. If they were more stable, 1862
A N A L Y T I C A L CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
then extractions of water samples could be performed at the sampling sites. This would considerably reduce the volume and weight of samples returned to the laboratory for analysis. There have been few attempts to improve the stability of the dithiocarbamate extracts. Yanagisawa et al. (11)found the Mn-ATMDTC/MIBK extract to be more stable than the Mn-NaDEDTC/MIBK extract. They also observed that the ATMDTC and NaDEDTC extracts of Mn were more stable (90 min) when extracted into alcohol and ester solvents. Tweeten and Knoeck ( I ) noted that metal-DEDTC standards in isoamyl alcohol could be used over a 2-day period with reproducible absorption values. Jenne and Ball (19) increased the stability of the Mn-ATMDTC/MIBK distilled water extract from 6 h to at least 3 days by the addition of 20% (v/v) acetone. Olsen and Sommerfeld (20) circumvented the problem by the lengthy process of evaporating the MIBK from the extract and redissolving the residue in 1:l acet0ne:O.l N HC1. Of the many metals extractable by dithiocarbamates, manganese seems to exhibit the worst stability characteristics. I have briefly investigated the Mn-ATMDTC/MIBK system with the objective of improving the stability of the extracts. Hopefully a method for stabilizing the Mn extracts will also be applicable to other dithiocarbamate extractable metals.
EXPERIMENTAL Apparatus. All analyses were performed with a Jarrell-Ash single beam atomic absorption spectrophotometer equipped with a Perkin-Elmer nebulizer and single-slot burner. Reagents. ATMDTC from Fisher Scientific Company was used throughout, and all other chemicals were reagent grade. All manganese solutions were prepared from a Fisher Scientific Company 1000 ppm manganese atomic absorption standard. Procedure. Sixty mL of distilled water, river water, or seawater were placed in a 100-mL volumetric flask and spiked with 10 pg of Mn. Two drops of bromophenyl blue indicator in 50% ethanol were added and the pH was adjusted to about 3.5 with 0.5 N HC1. Three mL of 5% ATMDTC were then added and the solution was allowed to sit for 2 to 3 min. After the addition of 10 mL of MIBK, the volumetric flasks were shaken vigorously on a horizontal shaker for 3 min. The phases were then allowed to separate and distilled water was added to bring the MIBK into the neck of the flask. The analysis was then done directly on the MIBK extract or on the treated solution as described in the
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Figure 1. (+),
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Influences of distilled water (O), seawater(.), and river water
on the stability of the Mn-ATMDTC/MIBK extract
Results and Discussion section. In all of the time stability studies, the "aged" samples were compared to newly extracted samples.
RESULTS A N D DISCUSSION T h e stability of the Mn-ATMDTC/MIBK extract was found to be significantly affected by the solution matrix of natural waters (Figure 1). The extraction of Mn from seawater and a river water sample resulted in more than a 50% decrease in detectable Mn in 3 h. Mn extracted from distilled water showed greater stability. T h e addition of 20% acetone to the seawater extract improved its stability as reported earlier (19),but there was still a 15% decrease in absorbance in only 3 h (Figure 2 ) . Since only the complexed ATMDTC is extracted from the aqueous phase into MIBK, little uncomplexed ATMDTC is present in the MIBK extract. I t was thought that the MnTMDTC chelate might be made more stable in the MIBK if, after the extraction, uncomplexed ATMDTC could be added to the MIBK. This might maintain the Mn in a chelated state, thereby preventing its oxidation to MnOe (22) which may not be completely atomized in the air-acetylene flame. T h e addition of ATMDTC was accomplished by first dissolving the ATMDTC in a polar organic solvent and then adding this t o the MIBK extract which had been separated from the aqueous phase. One volume of saturated ATMDTC (about 2.570) in 95% ethanol added to four volumes of the MIBK extract from seawater resulted in good stabilization of the manganese complex in MIBK (Figure 2 ) . This ATMDTC-EtOH-MIBK mixture was found to be capable of maintaining 10 pg of Mn in solution for up to 1week with no measurable decrease in absorbance. The addition of 95% ethanol without ATMDTC was not sufficient to completely maintain the Mn complex in MIBK (Figure 2). I t was also determined that a minimum of 1.5% ATMDTC in the 95% ethanol additive is required to stabilize 10 Fg of Mn extracted into MIBK from seawater for 24 h. CONCLUSION Although the Mn-ATMDTC/MIBK extract can be significantly stabilized by the addition of excess ATMDTC, the
Figure 2. Effects of additives on seawater Mn-ATMDTC/MIBK extract stability: ( 0 )seawater, no additives, (A)acetone, (0)ATMDTC + 9 5 % ethanol, (V) 9 5 % ethanol
method has not yet been extended to the extraction of trace levels of Mn or any other metals from large volumes of natural waters. The method described here has the disadvantages that two new steps are added to the usual procedure and the extract is slightly diluted by the ATMDTC-EtOH additive. The method does suggest though that the use of a different dithiocarbamate chelating agent or combination of chelating agents with the appropriate solubility characteristics could provide stable extracts without added procedural steps.
LITERATURE CITED (1) (2) (3) (4)
(5) (6) (7) (8) (9) (10) (1 1) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21)
T. N. Tweeten and J. W. Knoeck. Anal. Chem.. 48, 64 (1976). A. E. Martin, Anal. Chem., 25, 1260 (1953). A. Hulanicki, Talanfa, 14, 1371 (1967). S. J. Joris, K. I. Aspila, and C. L. Chakrabarti, Anal. Chem., 41, 1441 (1969). K. I. Aspila, V. S. Sastri, and C. L. Chakrabarti. Talanta. 16. 1099 (1969). S. J. Joris, K. I. Aspila, and C. L. Chakrabarti, J . Phys. Chem., 74, 860 (1970). K. I . Aspila, S. J. Joris, and C. L. Chakrabarti, J. Phys. Chem., 74, 3625 (1970). K. I . Aspila, S. J Joris, and C. L. Chakrabarti, Anal Chem.. 43, 1529 (1971). K. I . Aspiia, C. L. Chakrabarti. and V. S. Sastri, Anal. Chem., 45, 363 (1973). R. J. Everson and H. E. Parker, Anal. Chem., 46, 1966 (1974). M. Yanagisawa. M. Suzuki, and T. Takeuchi, Anal. Chim Acta, 43, 500 (1968). R. R. Brooks, E. J. Presley, and I. R. Kaplan, Talanta, 14, 809 (1967). K. Kremiing and H. Peterson, Anal. Chlm. Acta, 70, 35 (1974). A. D. Shendrlkar, V. Dharmarajan, H. Walker-Merrick, and P. W. West, Anal. Chlm. Acta, 84, 409 (1976). J. Nix and T. Goodwin, At. Absorp. Newsl., 9, 119 (1970). R. E. Mansell, At. Absorp. Newsl., 4, 276 (1965). R . E. Mansell and H. W. Ernmel, A t . Absorp. Newsl., 4, 365 (1965). J. D. Kinrade and J. C . Ban Loon, Anal. Chem.. 46, 1894 (1974). E. A. Jenne and J. W. Ball, A t . Absorp. News/.. 11. 90 (1972). R. D. Olsen and M. R. Sommerfeld. At. Absorp. News/.. 12. 165 (1973). J. Stary. "Extraction of Metal Chelates", Macmiilan Co., New York, N.Y.. 1964, p 160.
Raymon F. Roberts Department of Chemistry University of South Florida Tampa, Florida 33620
RECEIVED for review January 14, 1977. Accepted July 1,1977. ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1 9 7 7
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