Critical Study of the APCD-MIBK Extraction System for Atomic Absorption S. R. Koirtyohann and John W . W e n ' Department of Agricultural Chemistry and fnvironmental Trace Substances Center, University of Missouri, Columbia, Mo. 65207
Atomic absorption response from metals extracted into methyl isobutyl ketone decreased as the pH of the aqueous phase prior to extraction was increased, in spite of the fact that the extraction efficiency was near 100% for all pH values used. The change in response was the same for copper, lead, and zinc. The difference was critically dependent on the nebulizer adjustment, and errors were avoided by addition of HC104 to all samples and standards prior to pH adjustment. The effect appears to be associated with nebulization and transport of the organic solvent to the flame, but no satisfactory explanation for the observations has been found.
Chemical enrichment of trace elements by extraction prior to their determination by atomic absorption is a very convenient way to improve sensitivity and remove interferences. The most useful extractions generally are those which reject the bulk matrix but which are rather nonselective for trace elements. Several determinations can then be made on a single extract, the atomic absorption process providing the necessary selectivity. Ammonium pyrrolidinecarbodithioate (APCD), which was first described by Malissa and Schoffmann ( I ) , meets this need quite well. It forms extractable chelates with many metals and is more stable in acid solution than other similar reagents. Extraction of APCD chelates into methyl isobutyl ketone (MIBK) prior to atomic absorption determination was described by Allan (2) and by Willis (3, 4 ) and has since become very popular ( 5 ) . Unfortunately, the name by which this reagent is most commonly known, ammonium pyrrolidinedithiocarbamate (APDC), violates accepted rules of chemical nomenclature. Two nitrogen atoms are implied from the name and only one is present in the molecule. The more correct name, ammonium pyrrolidinecarbodithioate will be used here. In our laboratory, results which were sometimes not consistent were obtained when the APCD-MIBK extraction was applied to complex samples such as plant or animal tissues. A detailed study was initiated to explain the inconsistencies.
EXPERIMENTAL Apparatus. Most atomic absorption measurements were made on a Perkin-Elmer Model 303 atomic absorption spectrophotometer equipped with a deuterium background corrector, Model 3030295. Emission measurements were made on an instrument which has been described previously (6) and which has the same premixed burner assembly as on the Model 303 (P.E. 303-0010). Present address, Arc0 Chemical Co., P.O. Box 328, Fort Madison, Iowa 52627. H . Malissa and E. Schoffmann, Mikrochim. Acta. 1, 187 (1955) J. E. Allan, Specfrochim. Acta, 17, 467 (1961). J. B. Willis, Nature, 191, 381 (1961). J. B. Willis, Anal. Chem.. 34, 614 (1962). W. Slavin, "Atomic Absorption Spectroscopy." Interscience. New Y o r k , N . Y . , 1968, p 75. (6) E. E. Pickett and S. R. Koirtyohann, Spectrochim. Acta. 238, 235
(1) (2) (3) (4) (5)
(1968).
1986
Nebulizers with stainless steel capillary (P.E. 303-0070) and platinum capillary (P.E.303-0073) were used. Radioactivity measurements were made with a thin-walled G-M tube (Nuclear Chicago D-52) mounted as a dip counter in a reservoir into which solutions were placed for counting. A Nuclear Chicago scaling unit, Model 161A was used to record the counts. Reagents. Copper-64 was obtained from the University of Missouri Research Reactor. Stable copper was added to give a solution containing 1.0 pg of copper and about 0.8 pCi of 64Cu per ml. Zinc-65 was obtained from Mallinckrodt Chemical Co. and was used along with stable zinc to prepare a solution which contained 1.0 pg of Zn and 1pCi of 65Zn per ml. APCD was prepared in the laboratory by the method given by Slavin ( 7 ) . The reagent was also obtained commercially from Eastman Kodak. No difference in performance was noted between the reagents. A 4% aqueous solution was prepared and extracted several times with 5-ml portions of MIBK to remove metal contaminants. Reagent grade MIBK was used throughout without further purification. Ammonium acetate buffer ( 5 N ) was prepared and metallic impurities were removed by adding a few milliliters of APCD solution and extracting with MIBK. Procedure. The aqueous phase containing sample or standard with or without radioisotope labeling was buffered with ammonium acetate and the pH adjusted to the desired value. The pH range used was 0-9 with most experiments done in the pH 2-6 region. Extractions were done in separatory funnels shaken for 1 minute by hand with 50 ml of aqueous volume, 1 ml of APCD solution, and 10 ml of MIBK. Atomic absorption and emission measurements were made on the separated organic phase. When the highest possible accuracy was required. the organic phase was placed in a 10-ml volumetric flask and diluted to volume with water-saturated MIBK. This eliminated the error caused by variations in the volume of organic phase lost because of solubility. For atomic absorption measurements, fuel and air flows were reduced so that a stable, nearly nonluminous flame was obtained on a 10-cm single slot burner when water-saturated MIBK was aspirated. Solvent was aspirated to establish the base line between sample readings. The three-slot Boling burner head is often recommended for work with organic solvents. With the extracts used here, however, the multiple-slot burner showed excessive memory (see later section) and its use was abandoned. The only other departure from the manufacturer's recommendations was in the use of the 2833-.&lead line rather than the more sensitive but noisier line a t 2170 A. It should be emphasized that all references to pH, foreign ion concentrations, etc. are for the aqueous phase prior to extraction and that all atomic absorption measurements were on the separated MIBK phase unless otherwise stated.
RESULTS Preliminary results indicated significant differences in absorption response depending upon the pH prior to extraction. Some typical data are presented in Table I. Similar behavior of absorption response as a function of extraction p H has been reported by others ( 4 ) and the assumption made that the differences reflect variable extraction efficiency. The similarity of behavior for three elements and the absence of any obvious reason for extraction efficiency to be reduced at higher pH led to a more thorough investigation of the extraction and measurement. Tests for incomplete extraction of the metals were made by direct aspiration of the extracted aqueous phase to test for residual metals, by multiple extractions of a single (7) W. Slavin,A t . Absorption Newsiett, 3, 141 (1964)
ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973
I
1'
100
0
b U E 5 e
80
60
40
20
Figure 1. Extraction efficiency and absorption response for zinc extracted from plant ash as a function of p H A Absorption response of Zn extracted from plant ash solution. 0 65Znextracted from simple aqueous solution. 0 "Zn extracted from plant ash solution
Table I . Absorbance of Copper, Zinc, and Lead Extracted at Different pH Absorbance
PH
Cu (1 ppm)"
Zn (1 p p m ) a
Pb (1 ppm)"
1 2 3 4
0.153 0.153 0.150 0.146 0.144 0.134
0.1 7
0.026 0.026 0.025 0.024 0.023 0.021
5 6 a
0.50 0.50 0.48 0.48 0.46
C D
w/ml in the organic solvent.
Table 11. Extraction of 65Znas a Function of pH (1 .O pCi G5Zn, 1.0 pug total Zn) Net count, PH
aq
0.0
5.0
1786 414 2 24 10 4
6.0
0
1 .o 2.0 3.0 4.0
"Net standard count ~
~~
Net Count 0 rg
0 1418 1822 1872 1856 1852 1862
%aq
% org
YOtotal
98.0 22.7 0.1 1.3
0
98.0 100.3 99.8 103.6 100.6 100.5 100.4
0.5 0.2
0.0
77.6 99.7 102.4 100.1 100.3 100.4
= 1820.
~
aqueous phase with tests for metals in the second and third extract, by wet ashing of the organic phase followed by determination of the total metal in the resulting aqueous phase, and by the use of radioisotopically labeled copper and zinc added to samples and standards. The conclusion from all four of these tests was the same. The extraction of the three metals tested is complete or nearly so in a single extraction except a t low p H and it does not decrease as t h e p H is increased. Typical data from the radioisotope studies are presented in Table 11. Data from extraction and atomic absorption response for zinc extracted from plant ash solutions are given in Figure 1. The extraction approaches 100% throughout the range pH 2-6 but the atomic absorption response decreases quite significantly above p H 3. Similar changes in response as a function of extraction p H were observed in copper emission when the MIBK phase was aspirated into the nitrous oxide-acetylene
Zn
I ,ugh1
cu 2 wgiml
Pb 8 ygimi
Figure 2. Effect of p H and perchlorate on absorption response for zinc, copper, and lead A . pH 3, no C104-: 8. pH 6, no c104-: C. pH 3, 0.2M C104-; 0.2M c104-
D.pH
6,
flame. Changing the height of observation had no effect on the relative response from the various extracts in either flame. There were no consistent pH dependent changes in the volume of organic phase recovered and no change in behavior was noted when the MIBK phase was adjusted to a constant volume. Perchloric acid was added to the aqueous phase prior to pH adjustment to investigate possible effects of ionic strength. A slight enhancement of the absorption under the more acidic conditions resulted and the effect due to differences in extraction p H was removed. Typical recorder tracings are presented in Figure 2 . During the course of the investigation, the differences in atomic absorption response suddenly and mysteriously disappeared after the position of the capillary in the nebulizer had been readjusted for another purpose. Further investigation revealed that the observed variation in atomic absorption response was critically dependent on nebulizer adjustment and could be made more or less severe a t will. Figure 3 gives the adsorption response for copper as the
ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973
1987
Table IV. Effect on Copper Absorption of Perchlorate Ion Added to the Aqueous Phase Relative absorbance
c104 -,
w
0
20 100 500 5,000 10,000 100,000 1,000,000
PH 3
PH 6
0.80 0.90 0.94 0.95 0.96 1. o o a 1. O O U 1. O O U
0.60 0.69 0.78 0.85 0.97 1.ooa 1. o o a 1. o o a
Readings differed less than the noise level in each measurement.
Table V. Copper and Zinc Determination in Laboratory, Reference Alfalfa Sample, (pg/gram dry weight) 1
2
3
APPROXIMATE ROTATION (TURNS)
Figure 3. Absorption response as a function of nebulizer capillary position, platinum capillary Table Ill. Absorbance from 2 W / m l Cu as a Function of Nebulizer Adjustment, Pt Capillary Absorbance at nebulizer positionn PH
3.0 6.0 3.0b 6.0b
a
b
C
d
e
f
0.229 0.164 0.237 0.237
0.268 0.204 0.280 0.280
0.284 0.284 0.284 0.284
0.211 0.211 0.211 0.211
0.201 0.201 0.201 0.201
0.187 0.187 0.187 0.187
Letters indicate position of nebulizer adjustment from Figure 3. 0.2M Clod- in the aoueous DhaSe.
capillary was turned clockwise from a position where blow-back occured through several maxima. This pattern varies somewhat for individual nebulizers but most are generally similar. The manufacturer recommends operating the unit near point e on the figure but adjustment to the exact peak cannot always be assured. The nebulizer was adjusted as carefully as possible to the positions corresponding to the letters a-f in Figure 3 and the data in Table I11 were taken. pH dependent variation in adsorption response was observed for adjustment a t points a and b but not for points e-f. In every case where pH dependence was observed, it was removed by addition of perchloric acid to the aqueous phase prior to p H adjustment and extraction. Similar experiments were performed using a different nebulizer with a stainless steel capillary. In this case, the effect of p H was observed with adjustment on and slightly beyond the first maximum but disappeared with adjustment on the second maximum. The effect on copper absorption of varying the amount of perchlorate ion added is given in Table IV. The data were taken with the platinum capillary adjusted near point a of Figure 3 in order to enhance the effect. Constant response was reached if the aqueous phase contained more than 10 mg of c l o d - . Other ions tested to see if they had an effect similar to perchlorate included sulfate, phosphate, citrate, nitrate. acetate, chloride, periodate, permanganate, and persulfate. Only periodate behaved like perchlorate, and extracts from aqueous solutions containing periodate ion tended to decompose within 15-20 minutes after extrac1988
Element
pH
Run l a
Run2'
Cu
3 6 3 6
14.3 13.2 38.5 32.8
16.2 16.2 39.0 39.0
Zn
Previously accepted R ~ n 3 ~ R ~ n 4 ~value
10.3 9.4 35.0 30.0
11.7 11.7 35.4 35.4
11.7e 35.5j
aRun 1, sample without HC104; standards without HC104. b R u n 2, sample with HCI04; standards without HCIO4. Run 3, sample without HCIO4; standards with HC104. Run 4, sample with HCI04; standards with HCI04. e Determined colorimetrically with diethyldithiocarbamate (9) and by atomic absorption. f Determined on a wet ash solution by atomic absorDtion. Averaae of several indeDendent runs.
tion. All other ions tested had no effect on the absorption response or on the differences due to pH of the aqueous phase. The effect of perchlorate ion was the same if it was added in more concentrated form to the organic phase after extraction. The desired amounts of C104- in 50 pl of aqueous volume were added to 10 ml of MIBK solution after extraction. The solution was shaken and aspirated. Except for the reduction in the amount of perchlorate required, the results are similar to those obtained by addition to the aqueous phase. The absorbance was unaffected by extraction pH or perchlorate concentration in the range 60-5000 pg of c104- added. The amount of perchlorate ion extracted with the organic phase was determined by the ferroin method (8). Significant amounts were found (up to about 2 mg) but it is not known if it was simply entrained with small droplets of the aqueous phase or if it was extracted as part of the chelate. It was always possible to get consistent results which did not depend on extraction pH, regardless of the nebulizer adjustment, if the aqueous phase contained perchlorate ion. In the analysis of practical samples, it was necessary to add perchlorate to both samples and standards. For example, portions of an alfalfa sample which had been carefully analyzed in our laboratory by several independent methods were dry ashed and extracted with and without addition of c104-. The results are given in Table V. It is readily apparent that the most reliable results were obtained when HC104 was added to both samples and standards prior to pH adjustment. The problem of excessive memory when using the three slot burner head was mentioned previously. Figure 4 shows recorder tracings from a single series of extracts (8) J. S. Fritz, J. E. Abbink, and P. A. Campbell. Ana/. Chem.. 36, 2123
(1964). (9) E. B. Sandell, "Colorimetric Determination of Traces of Metals," Interscience, New York. N.Y., 1944,p 469.
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 12, OCTOBER 1973
using the three-slot burner head, a single-slot burner head, and the same single-slot head after it was removed and thoroughly scrubbed. Water-saturated MIBK was aspirated to establish the base line in each case. The significant blank signal from the three-slot head is reduced by changing to the single slot and lowered even more by cleaning the single-slot head. Also, the time required for the reading to stabilize is considerably longer with the three-slot head even though the instrument time constant was unchanged. The blank signals in Figure 4 are caused by the presence of copper atoms in the flame because using the background compensation system had no effect on the signal. The copper was not present in the aspirated solution, however, because only a very small blank signal was observed with the single-slot burner. The problem could be reduced temporarily by scrubbing the three-slot burner but no lasting solution has been found. The blank signal for the three-slot burner increased as the amount of APCD in the extraction solution was increased but the same solutions gave consistently low readings on the single slot burner. Similar behavior has been observed for lead using a single slot burner on different instrument (P.E. Model 403 with 040-0146 burner and 303-0418 single-slot burner head) which had been used for large numbers of relatively high lead samples. The extraction procedure showed a blank equivalent to about 1 pg of P b in a 5-ml extract which persisted when the background compensation system was used and which remained rather constant in spite of considerable effort to clean all reagents. The blank signal was reduced to nondetectable levels after the burner head was scrubbed thoroughly and rinsed with dilute acid. DISCUSSION No satisfactory explanation for the observed changes in response due to extraction p H or for the effect of perchlorate ion has been found in spite of the fact that the behavior is quite reproducible and is consistent with results reported by others ( 4 ) . All evidence indicates an effect associated with nebulization and/or physical transport of spray to the flame. However, no measurable change in surface tension, viscosity, or solution uptake rate due to extraction pH or perchlorate was found. No serious attempt was made to measure differences in the amount of solvent transported to the flame because the errors expected in such measurements are nearly as large as the differences sought. Several different nebulizer-burners were used but all were of one basic design from a single manufacturer. Other equipment may or may not show similar behavior. The memory effects and blank signals due to the burner head appear to be caused by revolatilization of metals which collected in or on the head from previous samples.
a
a
Three Slot Burner Head
Figure 4.
Single Slot Burner Head
Single Slot Burner Head ( cleaned)
Burner head memory when organic extracts are aspi-
rated a. 2.5 pg Gu std; b. 0.5 p g Cu std; c. 0.0 gg Cu std. MIBK volume = 10
ml
The more obvious explanations such as background adsorption and reagent contamination are ruled out by the facts that the signal persists in spite of background compensation and disappears or is drastically reduced when the same solutions are aspirated with a cleaned single-slot burner head. The mechanism by which the deposited metal is volatilized remains a t least somewhat obscure. The signal appears only when solvent which has been used for an extraction is aspirated. Several metals, including copper, have been shown to form volatile compounds with APCD (10). It therefore seems reasonable to postulate that excess chelating agent is extracted and reacts a t least to some extent with metal deposited on the burner head, thereby causing it to become volatile. Recognition and elimination of the problem due to burner head memory with the APCD-MIBK extraction system should improve results. ,
ACKNOWLEDGMENT The authors wish to thank E. E. Pickett for his interest in this work and for several helpful discussions in which he took part. Received for review December 6, 1972. Accepted May 17, 1973. (10) D. C . Hilderbrand. Ph.D. Thesis, Universityof Missouri, 1971
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