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APDC-Mibk extraction system for the determination of copper and iron in 1 cm3 of sea water by flameless atomic-absorption spectrometry
Am!\vic’cr Chitnictr Acru. 70 ( 1974) 3.5-39 ((2 Elscvier Scientific Publishing Company,
Amsterdam
- Printed
in The
Ncthcrlands
35
APDC-MIBK EXTRACTION SYSTEM FOR THE DETERMINATION OF COPPER AND IRON IN 1 CM3 OF SEA WATER BY FLAMELESS ATOMIC-ABSORPTION SPECTROMETRY
K. KREMLING
and H. PETERSEN
It~srirur ,fiir Mceresktordc~. (Rcccived
1st October
Diister.rrhrookrr~
Wq
20. 23 K irl (Cierr~rcor~~)
1973)
In spite of the increasing attention that trace metals in sea water have received in recent years, our knowledge of their role in the biochemical and geochemical cycles of the oceans, and of their horizontal, vertical and seasonal variations is only fragmentary. Many of these efforts have been hindered or prevented by difficulties in analytical techniques. One of the modern instrumental methods promising more success in this field is flameless atomic-absorption spcctrometry. The development of atomizers such as the graphite tube furnaces of Massmann’ and L’vov’ has made it possible to determine very much smaller amounts than can be done by conventional atomic-absorption techniques. The smaller samples required need only small containers which are more convenient for cleaning, sampling and preventing contamination. This paper describes the development of a method for the determination of copper and iron in l-cm3 samples of oceanic waters based on the chelation of the metals with ammonium pyrrolidinedithiocarbamate (APDC) and extraction of the chelates into methyl isobutyl ketone (MIBK). The method was first applied to sea water by Brooks et ~1.~ and improved by Brewer ef al.‘; the two procedures required volumes of about 750 and 400 cm3 of sea water, respectively. This technique was selected for further study in view of the advantage of simultaneous extraction of several metals and the sensitivity of flameless atomic-absorption spectrometry, although the concentrations of trace metals reported by this method represent only that available to the chelation-solvent extraction process. With a phase ratio of nearly 1 (volume ratio of the aqueous to the organic phase), the extraction proceeds completely and a second extraction becomes unnecessary 3. The standard addition method4 is employed for evaluation of the measurements, thus eliminating interferences from different chemical compositions of samples. Attempts to analyze for trace metals by direct atomization of sea water were shown by Segar and Gonzales’ to be unsatisfactory, owing to scattering interferences by the major components. EXPERIMENTAL
Reagert ts Arntnortiutn p_v,.~olitiirtetlithioca~banrote
(A PDC).
Prepare
a 2% solution,(w/v)
36
K. KREMLING.
I-l. PETERSEN
by dissolving 1 g of APDC (Merck) in 50 cm3 of distilled water. add 10 cm” of MIBK and shake for 10 min in a funnel to purify the reagent. After separation run off the aqueous layer into a quartz vcsscl. The solution must bc prepared daily. Methyl isohlrt~*l kc~torw (MIBK). Redistil the commercial-grade reagent (Riedel-de-Haen AG) very slowly and carefully in a quartz still: no blank value for copper and iron should then be detected. Store the MIBK in a quartz vessel. Test the solvent before starting the extraction process. Hyctwclttor-ictrc-id. Redistil from purified hydrochloric acid. (“Suprapur”, Merck). standardize and dilute to a 0.01 N solution. Star~lar~l twtul sohrtior~. Prepare a standard stock solution (pH 2) of 1000 p.p.m. copper and 1000 p.p.m. iron (Titrisol. Merck). From thi3 stock solution. prep& daily a 0.1 p.p.m. working standard (pH 2) by dilution. Itlstrirtwetttntiort
A Perkin-Elmer Model 403 atomic absorption spectrophotometer equipped with a Rikadenki Kogyo Model Mark II recorder. a deuterium arc background corrector and a Perkin-Elmer HGA-72 heated graphite atomizer was employed. The instrumentation has been fully described by Welz”. For better precision and higher sensitivity. the special grooved graphite tube was used. A simple modification was made to the spectrophotometer for balancing the energies of the copper cathode lamp and the deuterium arc corrector. For this purpose. a shutter (diam. 7. mm) was installed behind the cathode lamp to lower its energy flow. Argon was used as the purging gas with ii flow rate of 1.7 dm” min-‘. All measurements were made in air-filtered rooms. Extractions were carried out in quartz tubes (length 7 cm, diam. 0.7 cm). which however remained untouched by The stoppers were .of polypropylene, solvent during the mixing and extraction processes. The tubes were cleaned with an APDC-MIBK mixture, rinsed with MIBK and dried with purified acetone. no blank value should be detected after If this cleaning process is well done, shaking with MIBK. All additions of reagents and sample injections were made with Eppendorf microlitre pipettes. Their plastic tips must be cleaned in the same manner as otherwise large contamination effects can be described for the quartz tubes. observed. The mixing and extraction process was done with an Eppendorf rotating mixer Model 3300 which allowed the simultaneous handling of 24 tubes.
To each quartz tube, add 1.000 cm3 of sea water, 0.1 cm3 of 0.01 M hydrochloric acid, 0.050 cm3 of APDC solution: and 0. 0.030, 0.060 and 0.090 cm3 of the working standard solution. The chelation proceeds at pH 34. After mixing for 2 min. add 1.000 cm3 MIBK to the tube, and mix for 3 min. Allow the phases to separate (about 10 min). and start measurement of the metal content by the atomic absorption spectrophotometer with the temperature programme described in Table 1. For stability of complexes, iron should be whereas the copper measured within 3 11 after the extraction process,
COPPER TAI3LE
AND IRON
IN SEA WATER
37
I
PROGRAMME -._.-__.._-_..-
FOR THE CIGA-72 FURNACE ..__.-_.__ __-__.-._-. __-_- ..-. /-c (.?48.3 lrrrr) c!ficr. IO r,lirr - 3 /I ------_--_-.--_. ..___..._____ _._. -.._-_ -.._...._ step ( 2 )” (3)’ (1)” (4)” 2’00 Temp. ( ‘) 100 1400 GO IO 60 90 Times (s) ___._______. ._._.__ ______. ______ .____.-.. - .__-- - ..” Evapor:~tion or MIBK (3 x 0. IO cm.‘). h Heat combustion of’ organic malcrial. ” Atomization (with GAS STOP). 1’ Cooling pcrioil.
..-.. - .--- -----.-. ..---_.--_-_C-If (324.7 IIIII) cc/iw .?-2.? II ----.. .-.- -.-. -- ---. -.-------.---(Iy’ (2Y 900 IO0 00 140 ..-.---.-.---
..-_ -.----.---
(4)” (JY 1100 IO 60 -_.---.-- _..._ .____._
..-.. --
.--. -^--__
- --.
complex is stable for abdut 22 h without any loss of absorbance. This is in good agreement with the results of Brooks et ~1.~. Inject the MIBK solvent with a 0.100 cm3 pipette and evaporate by step (1). To obtain higher absorbance. the metal quantity in the graphite tube is enriched by repeating this step 3 times, so that the overall volume needed is 0.300 cm3. The temperature programmes for iron and copper differ in the heat combustion step (2). A lower temperature is necessary for copper because it starts to atomize above 950”; therefore more time is needed for complete removal of the organic material. The atomization step (3) is cclrried out with the GAS STOP programme. which provides an increased sensitivity of nearly lOOY<. Because of the higher impedance of the grooved furnace. its maximum temperature is limited to about 2200”. Step (4) is provided for cooling the tube with which about 100 measurements may be made. 0.175 o.lso
Iron
Copper
o.lzs 2 s 8 s 4
0.100 ‘0.075 o.oso 0.02s
JJ SAMPLE
Fig.
+3
+6
+9
--A? SAMPLE
+3
I. A set of atomization peaks (with dcuterium arc corrector) peak of the copper “Sample” probe indicates the f’urn;lco cm&ion. axis represent the mctnl added in ~(g dm-‘.
+6
+9
for copper ;md iron. The second The numbers on the horizontal
K. KREMLING.
38
H. PETERSEN
A typical set of recorded curves is shown in Fig. 1. The recorder speed the atomization peak for copper was normally 2 cm min - ’ : but, to separate clearly from the emission signal of the heated graphite tube. a speed of 8 cm min- l probe. Optical modilications recently made to was used for the copper “Sample” Perkin-Elmer double-beam spectrophotometers seem to overcome the emission problem’. CALCULATIONS
The calculation of the metal concentrations is illustrated in Fig. 2. The absorbance values and .the quantity of standard in nanograms. measured The linear added to each l-cm3 aliquot of sea water. are plotted as coordinates. regression lines are fitted by the method of least squares. The intercept gives the in ccg amount of metals in 1 cm3 of seawater. which equals the concentration dme3. The precision of the method was tested by 10 replicate measurements of a filtered sample (0.4 Ctrn Nuclepore filter) with mean concentrations of 3.5 /cg Fe dms3 and 1.6 116 Cu dm -‘. The standard deviations of the whole procedure were found to be 17 and .14”/,. respectively. Some results for sea water are shown in Table II. The samples were obtained from the area of upwelling of the tropical north-east Atlantic Ocean during Cruise 26 of the R.V. “Meteor” in 1972. The copper and iron concentrations measured are in the same range as found by Riley and Taylor” in this area, despite the different filtration and pre-concentration steps used in the two investigations. Although the locations are nearly identical, substantial variations can but seems to be observed from station to station. This fact is, as yet, unexplained, be mainly caused by adsorption processes. 4200 1
E g
0.125-
8 2
O.lOO-
0.07 5 -
Metal
Fig. 2. Method
ADDITIONS
of calculation
In ng
of results.
COPPER TABLE
AND IRON
IN SEA WATER
39
II
ANALYSIS L0cc/tiorr
19” IO'N IG”54’W
I9^04’N I G”5O’W
19’OG’N 16”52’W
The gemeinschaft.
OE
SEA
YATER
DC~III (III)
0 7 15 30 200 300 400 0 15 30 75 200 300 400 0 11 23 75 200
authors
FROM
THE
TROPICAL
Coruwrtrutiot~ Fc
Cli
5.5 1.3 8.5 3.2 3.9 1.6 3.1 2.G 6.6 4.8 2.6 3.7 4. I 3.0 5. I 3.7 3.9 8.1 2.8
2.2 1.n 1.2 3.0 3.5 2.x 1.4 4.2 2.8 0.7 1.2 2.3 1.3 2.9 2.0 2.0 3.4 3.1
acknowledge
(pg
NORTH-EAST
ATLANTIC
OCEAN ___.
dm- “)
1.9
the
financial
support
of Deutsche
Forschungs-
SUMMARY
A micro APDC-MIBK extraction method is described for the simultaneous of sea water by flameless determination of copper and iron in l-cm3 samples atomic-absorption spectrometry with a HGA-72 graphite atomizer. Replicate analyses of Atlantic sea water with mean concentrations of 1.6 jcg Cu dmm3 and showed standard deviations of 14”/” and 17”/,, respectively. 3.5 llg Fe dm-”
REFERENCES 1 2 3 4 5 6 7 8
.H. Mzmnann. Specrrochitt~. Acrtr. 23B ( 1968) 2 15. B. V. L’vov. Spectrorhir~~. Acvu. 24B (19G9) 53. R. R. Brooks. B. J. Presley and I. R. Kaplan, Tcdatrru, 14 (1967) 809. P. G. Brcwcr. D. W. Spencer and C. L. Smith. Aromic Ahsorprim Specrrw.~-op): ASTM/STP 443, American Society for Testing and Mntcrials, 1969. p. 70. D. A. Scgar and J. G. Gonzales, .Ifrul. Chh. Acur, 58 (1972) 7. 8. Wclz, CZ-C/le,,li~-T~chrrik. I (1972) 455. J. D. Kcrber. A. J. Russo. G. E. Pctcrson and R. D. Edigcr. At. Ahsorprio~t Newslerr.. 12 ( 1973) 106. J. P. Riley and D. Taylor, Dwy+Seu Res.. 19 (1972) 307.
Amsterdam
- Printed
in The
Ncthcrlands
35
APDC-MIBK EXTRACTION SYSTEM FOR THE DETERMINATION OF COPPER AND IRON IN 1 CM3 OF SEA WATER BY FLAMELESS ATOMIC-ABSORPTION SPECTROMETRY
K. KREMLING
and H. PETERSEN
It~srirur ,fiir Mceresktordc~. (Rcccived
1st October
Diister.rrhrookrr~
Wq
20. 23 K irl (Cierr~rcor~~)
1973)
In spite of the increasing attention that trace metals in sea water have received in recent years, our knowledge of their role in the biochemical and geochemical cycles of the oceans, and of their horizontal, vertical and seasonal variations is only fragmentary. Many of these efforts have been hindered or prevented by difficulties in analytical techniques. One of the modern instrumental methods promising more success in this field is flameless atomic-absorption spcctrometry. The development of atomizers such as the graphite tube furnaces of Massmann’ and L’vov’ has made it possible to determine very much smaller amounts than can be done by conventional atomic-absorption techniques. The smaller samples required need only small containers which are more convenient for cleaning, sampling and preventing contamination. This paper describes the development of a method for the determination of copper and iron in l-cm3 samples of oceanic waters based on the chelation of the metals with ammonium pyrrolidinedithiocarbamate (APDC) and extraction of the chelates into methyl isobutyl ketone (MIBK). The method was first applied to sea water by Brooks et ~1.~ and improved by Brewer ef al.‘; the two procedures required volumes of about 750 and 400 cm3 of sea water, respectively. This technique was selected for further study in view of the advantage of simultaneous extraction of several metals and the sensitivity of flameless atomic-absorption spectrometry, although the concentrations of trace metals reported by this method represent only that available to the chelation-solvent extraction process. With a phase ratio of nearly 1 (volume ratio of the aqueous to the organic phase), the extraction proceeds completely and a second extraction becomes unnecessary 3. The standard addition method4 is employed for evaluation of the measurements, thus eliminating interferences from different chemical compositions of samples. Attempts to analyze for trace metals by direct atomization of sea water were shown by Segar and Gonzales’ to be unsatisfactory, owing to scattering interferences by the major components. EXPERIMENTAL
Reagert ts Arntnortiutn p_v,.~olitiirtetlithioca~banrote
(A PDC).
Prepare
a 2% solution,(w/v)
36
K. KREMLING.
I-l. PETERSEN
by dissolving 1 g of APDC (Merck) in 50 cm3 of distilled water. add 10 cm” of MIBK and shake for 10 min in a funnel to purify the reagent. After separation run off the aqueous layer into a quartz vcsscl. The solution must bc prepared daily. Methyl isohlrt~*l kc~torw (MIBK). Redistil the commercial-grade reagent (Riedel-de-Haen AG) very slowly and carefully in a quartz still: no blank value for copper and iron should then be detected. Store the MIBK in a quartz vessel. Test the solvent before starting the extraction process. Hyctwclttor-ictrc-id. Redistil from purified hydrochloric acid. (“Suprapur”, Merck). standardize and dilute to a 0.01 N solution. Star~lar~l twtul sohrtior~. Prepare a standard stock solution (pH 2) of 1000 p.p.m. copper and 1000 p.p.m. iron (Titrisol. Merck). From thi3 stock solution. prep& daily a 0.1 p.p.m. working standard (pH 2) by dilution. Itlstrirtwetttntiort
A Perkin-Elmer Model 403 atomic absorption spectrophotometer equipped with a Rikadenki Kogyo Model Mark II recorder. a deuterium arc background corrector and a Perkin-Elmer HGA-72 heated graphite atomizer was employed. The instrumentation has been fully described by Welz”. For better precision and higher sensitivity. the special grooved graphite tube was used. A simple modification was made to the spectrophotometer for balancing the energies of the copper cathode lamp and the deuterium arc corrector. For this purpose. a shutter (diam. 7. mm) was installed behind the cathode lamp to lower its energy flow. Argon was used as the purging gas with ii flow rate of 1.7 dm” min-‘. All measurements were made in air-filtered rooms. Extractions were carried out in quartz tubes (length 7 cm, diam. 0.7 cm). which however remained untouched by The stoppers were .of polypropylene, solvent during the mixing and extraction processes. The tubes were cleaned with an APDC-MIBK mixture, rinsed with MIBK and dried with purified acetone. no blank value should be detected after If this cleaning process is well done, shaking with MIBK. All additions of reagents and sample injections were made with Eppendorf microlitre pipettes. Their plastic tips must be cleaned in the same manner as otherwise large contamination effects can be described for the quartz tubes. observed. The mixing and extraction process was done with an Eppendorf rotating mixer Model 3300 which allowed the simultaneous handling of 24 tubes.
To each quartz tube, add 1.000 cm3 of sea water, 0.1 cm3 of 0.01 M hydrochloric acid, 0.050 cm3 of APDC solution: and 0. 0.030, 0.060 and 0.090 cm3 of the working standard solution. The chelation proceeds at pH 34. After mixing for 2 min. add 1.000 cm3 MIBK to the tube, and mix for 3 min. Allow the phases to separate (about 10 min). and start measurement of the metal content by the atomic absorption spectrophotometer with the temperature programme described in Table 1. For stability of complexes, iron should be whereas the copper measured within 3 11 after the extraction process,
COPPER TAI3LE
AND IRON
IN SEA WATER
37
I
PROGRAMME -._.-__.._-_..-
FOR THE CIGA-72 FURNACE ..__.-_.__ __-__.-._-. __-_- ..-. /-c (.?48.3 lrrrr) c!ficr. IO r,lirr - 3 /I ------_--_-.--_. ..___..._____ _._. -.._-_ -.._...._ step ( 2 )” (3)’ (1)” (4)” 2’00 Temp. ( ‘) 100 1400 GO IO 60 90 Times (s) ___._______. ._._.__ ______. ______ .____.-.. - .__-- - ..” Evapor:~tion or MIBK (3 x 0. IO cm.‘). h Heat combustion of’ organic malcrial. ” Atomization (with GAS STOP). 1’ Cooling pcrioil.
..-.. - .--- -----.-. ..---_.--_-_C-If (324.7 IIIII) cc/iw .?-2.? II ----.. .-.- -.-. -- ---. -.-------.---(Iy’ (2Y 900 IO0 00 140 ..-.---.-.---
..-_ -.----.---
(4)” (JY 1100 IO 60 -_.---.-- _..._ .____._
..-.. --
.--. -^--__
- --.
complex is stable for abdut 22 h without any loss of absorbance. This is in good agreement with the results of Brooks et ~1.~. Inject the MIBK solvent with a 0.100 cm3 pipette and evaporate by step (1). To obtain higher absorbance. the metal quantity in the graphite tube is enriched by repeating this step 3 times, so that the overall volume needed is 0.300 cm3. The temperature programmes for iron and copper differ in the heat combustion step (2). A lower temperature is necessary for copper because it starts to atomize above 950”; therefore more time is needed for complete removal of the organic material. The atomization step (3) is cclrried out with the GAS STOP programme. which provides an increased sensitivity of nearly lOOY<. Because of the higher impedance of the grooved furnace. its maximum temperature is limited to about 2200”. Step (4) is provided for cooling the tube with which about 100 measurements may be made. 0.175 o.lso
Iron
Copper
o.lzs 2 s 8 s 4
0.100 ‘0.075 o.oso 0.02s
JJ SAMPLE
Fig.
+3
+6
+9
--A? SAMPLE
+3
I. A set of atomization peaks (with dcuterium arc corrector) peak of the copper “Sample” probe indicates the f’urn;lco cm&ion. axis represent the mctnl added in ~(g dm-‘.
+6
+9
for copper ;md iron. The second The numbers on the horizontal
K. KREMLING.
38
H. PETERSEN
A typical set of recorded curves is shown in Fig. 1. The recorder speed the atomization peak for copper was normally 2 cm min - ’ : but, to separate clearly from the emission signal of the heated graphite tube. a speed of 8 cm min- l probe. Optical modilications recently made to was used for the copper “Sample” Perkin-Elmer double-beam spectrophotometers seem to overcome the emission problem’. CALCULATIONS
The calculation of the metal concentrations is illustrated in Fig. 2. The absorbance values and .the quantity of standard in nanograms. measured The linear added to each l-cm3 aliquot of sea water. are plotted as coordinates. regression lines are fitted by the method of least squares. The intercept gives the in ccg amount of metals in 1 cm3 of seawater. which equals the concentration dme3. The precision of the method was tested by 10 replicate measurements of a filtered sample (0.4 Ctrn Nuclepore filter) with mean concentrations of 3.5 /cg Fe dms3 and 1.6 116 Cu dm -‘. The standard deviations of the whole procedure were found to be 17 and .14”/,. respectively. Some results for sea water are shown in Table II. The samples were obtained from the area of upwelling of the tropical north-east Atlantic Ocean during Cruise 26 of the R.V. “Meteor” in 1972. The copper and iron concentrations measured are in the same range as found by Riley and Taylor” in this area, despite the different filtration and pre-concentration steps used in the two investigations. Although the locations are nearly identical, substantial variations can but seems to be observed from station to station. This fact is, as yet, unexplained, be mainly caused by adsorption processes. 4200 1
E g
0.125-
8 2
O.lOO-
0.07 5 -
Metal
Fig. 2. Method
ADDITIONS
of calculation
In ng
of results.
COPPER TABLE
AND IRON
IN SEA WATER
39
II
ANALYSIS L0cc/tiorr
19” IO'N IG”54’W
I9^04’N I G”5O’W
19’OG’N 16”52’W
The gemeinschaft.
OE
SEA
YATER
DC~III (III)
0 7 15 30 200 300 400 0 15 30 75 200 300 400 0 11 23 75 200
authors
FROM
THE
TROPICAL
Coruwrtrutiot~ Fc
Cli
5.5 1.3 8.5 3.2 3.9 1.6 3.1 2.G 6.6 4.8 2.6 3.7 4. I 3.0 5. I 3.7 3.9 8.1 2.8
2.2 1.n 1.2 3.0 3.5 2.x 1.4 4.2 2.8 0.7 1.2 2.3 1.3 2.9 2.0 2.0 3.4 3.1
acknowledge
(pg
NORTH-EAST
ATLANTIC
OCEAN ___.
dm- “)
1.9
the
financial
support
of Deutsche
Forschungs-
SUMMARY
A micro APDC-MIBK extraction method is described for the simultaneous of sea water by flameless determination of copper and iron in l-cm3 samples atomic-absorption spectrometry with a HGA-72 graphite atomizer. Replicate analyses of Atlantic sea water with mean concentrations of 1.6 jcg Cu dmm3 and showed standard deviations of 14”/” and 17”/,, respectively. 3.5 llg Fe dm-”
REFERENCES 1 2 3 4 5 6 7 8
.H. Mzmnann. Specrrochitt~. Acrtr. 23B ( 1968) 2 15. B. V. L’vov. Spectrorhir~~. Acvu. 24B (19G9) 53. R. R. Brooks. B. J. Presley and I. R. Kaplan, Tcdatrru, 14 (1967) 809. P. G. Brcwcr. D. W. Spencer and C. L. Smith. Aromic Ahsorprim Specrrw.~-op): ASTM/STP 443, American Society for Testing and Mntcrials, 1969. p. 70. D. A. Scgar and J. G. Gonzales, .Ifrul. Chh. Acur, 58 (1972) 7. 8. Wclz, CZ-C/le,,li~-T~chrrik. I (1972) 455. J. D. Kcrber. A. J. Russo. G. E. Pctcrson and R. D. Edigcr. At. Ahsorprio~t Newslerr.. 12 ( 1973) 106. J. P. Riley and D. Taylor, Dwy+Seu Res.. 19 (1972) 307.