Anal. Chem. 1984, 56, 1689-1691
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Dithiocarbamate Extraction of Gallium from Natural Waters and from Biological Samples for Neutron Activation Analysis J. C. Yu and C. M. Wai* Department of Chemistry, University of Idaho, Moscow, Idaho 83843 Traces of galllum in aqueous solution can be extracted wlth a mixture of pyrrolldlnecarbodlthloate and dlethyldlthlocarbamate In the pH range 4-6 Into chloroform. Galllum In the organlc phase can be back-extracted Into the aqueous phase wlth a lead nitrate solution for neutron actlvatlon analysls (NAA). Thls two-step extraction process provides a large preconcentratlon factor for galllum and eliminates potential interferences from the matrix species and other metals present in natural water and blologlcal samples. Because lead ls Insensitive to NAA, lts presence In the flnal solution does not cause any Interferences for gallium determination. By use of this extractlon method and NAA, a detection limlt of lob3 pg/L gallium In water can be achleved.
Neutron activation analysis (NAA) is a sensitive method for gallium determination. However, in complex natural systems, spectral interferences caused by matrix species such as sodium and bromine severely limit the sensitivity of gallium detection by NAA. Wangen and Gladney employed epithermal neutron irradiation to enhance 72Gaproduction relative to 24Nain biological samples in order to improve the detection limit of gallium (I). Using instrumental epithermal neutron activation analysis, these authors were able to detect gallium a t the 0.1 pg level. Radiochemical separation after neutron irradiation can greatly improve the sensitivity of gallium determination. Extraction of 72Gain HC1 solution with isopropyl ether is a method commonly used for determining gallium in geological samples by neutron activation (2). The use of tri-n-octylphosphine in cyclohexane for the substoichiometric extraction of 72Gafrom HC1 for quantitation of gallium by NAA has also been reported ( 3 ) . With radiochemical separation, detection of gallium at the parts-perbillion level can be achieved. Preconcentration of metals by solvent extraction prior to neutron irradiation has been applied to several systems (4, 5). Extraction before neutron irradiation has the advantages of obtaining a large preconcentration factor, eliminating potential interfering reactions from other elements, and minimizing radiation exposure, as compared with postirradiation radiochemical separation. The preconcentration technique is especially useful for determining trace metals in natural systems with concentrations close to the detection limits of NAA. This paper describes a two-step dithiocarbamate extraction method applied prior to neutron irradiation for determining sub-part-per-billion levels of gallium in aqueous solution. The proposed method first extracts gallium dithiocarbamate complexes into an organic phase, followed by back-extraction of gallium with a lead nitrate solution, Since lead is insensitive to NAA, its presence in the final solution should not interfere with gallium determination. The method is sensitive and quite selective for gallium and is suitable for measuring low levels of gallium in natural waters and in biological samples. EXPERIMENTAL SECTION Reagents. All chemicals used were of reagent grade. Standard gallium solution (1000 Kg/mL) was prepared by dissolving 1 g of a pure gallium metal (99.9999%) in a minimum volume of 0003-2700/84/0356-1689$01.50/0
concentrated nitric acid and diluting to 1L with deionized water. A mixture of ammonium pyrrolidinecarbodithioate(APDC) and sodium diethyldithiocarbamate (NaDDC) solution (1%each) was prepared by dissolving 1g of each compound in 100 mL of water. This extraction solution was always prepared fresh prior to use. A pH 4.7 buffer was prepared by combining 35 g of ammonium acetate with 30 mL of glacial acetic acid and diluting to 500 mL. A lead nitrate solution (500 pg/mL) was prepared by dissolving 0.7995 g of lead nitrate in 1 L of a dilute nitric acid solution at pH 2. Surface seawatm was collected from Meadow Point in western Seattle. River water was collected from the Snake River in Lewiston, ID, above the confluence of the Clearwater River. The samples were filtered through a 0.45-~mfilter and acidified to pH 2 after collection. For gallium extraction, 400-mL aliquots of sample solution were placed into 500-mL ground-glassstoppered Erlenmeyer flasks. The samples were adjusted to pH 4 to 5 with ammonium hydroxide, followed by addition of 5 mL of the acetate buffer to each sample. After this, 20 mL of chloroform and 10 mL of the dithiocarbamate extraction solution were added. The mixture was shaken vigorously for 5 min on a wrist-action mechanical shaker (Burrell Model 7 5 ) and the phases were allowed to separate. The aqueous layer was then drained from the flask and the chloroform phase was washed with deionized water several times. After being washed, a 15-mL portion of the chloroform solution was pipetted into a 20-mL Beckman polyvial with a fast-turn cap. To back-extract gallium, 1.5 mL of the lead nitrate solution was added to the vial, and the mixture was shaken for 2 to 3 min. After phase separation, 1 mL of the aqueous phase was pipetted into a 2/b-drampolyethylene vial and heat-sealed for neutron irradiation. According to this procedure, a preconcentration factor of 200 for gallium could be obtained. Standards were made of 1 mL of the lead solution spiked with proper concentrations of gallium and sealed in 2/5-drampolyethylene vials. A procedure blank (400 mL of deionized water put through the same procedure as the water samples) was run for each set of the experiments. Gallium was not detectable in any of the blanks. Biological samples were digested with a mixture of nitric acid and sulfuric acid in a reflux apparatus similar to that described by Bethge (6). The effectivenessof this wet-ashing method for digesting trace metals in organic materials has been discussed in a recent paper by Bajo et al. (7). After digestion, samples were neutralized with sodium hydroxide, followed by extraction with the dithiocarbamate solution according to the procedure described above for natural waters. Because of the relatively high concentrations of gallium in biological systems, normally 1g of dry sample is sufficient for each determination. All samples and standards were irradiated for 3 h in a 1-MW TRIGA reactor at a steady flux of 6 X 10l2n cm-2 s-l. To avoid interferences of 24Naand other radioactivities produced in the plastic material of the irradiated vials, samples and standards were transferred into new 2/S-dramvials, using 5-mL disposable syringes. Each sample was counted for 2 x lo3s in a large-volume coaxial ORTEC Ge(Li) detector with a resolution of about 2.3 keV at the 1332-keV y from 6oCo. The 834.0-keV y from 72Ga was used to determine the gallium concentrations in various samples. Data analysis was carried out by use of the SPANprogram in an IBM 370 computer. Experiments were also carried out to determine the extracted gallium by graphite furnace atomic absorption spectrometry (GFAAS). The extraction procedure was the same as that described above for NAA except the back-extraction agent used in this case was a 1000 Fg/mL Hg2+solution. The advantages of using Hg2+as a back-extraction agent for GFAAS have been described in a previous paper (8). Because of its high extraction 0 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST
1984
x L
P,
>
0 100-
u
(b)
0,
L1!
60-
60
-
40
-
PH Figure 1. pH dependence of the extraction of gallium by (a) NaDDC (0)and APDC (m) separately; (b) 1:l mixture of NaDDC and APDC (0).
constant, Hg2+is very effective for back-extracting many trace metals complexed with dithiocarbamate. The extraction constant of gallium dithiocarbamate is not known. Our experiments indicate that Hg2+can quantitatively replace gallium in the organic phase. A Perkin-Elmer Model 603 AA spectrometer with deuterium arc background correction and a HGA 2100 graphite furnace were used for the gallium determination. Samples were placed in the furnace with a 25-pL Eppendorf micropipet. The operation conditions followed those reported by Pelosi and Attoline (9).
RESULTS AND DISCUSSION The complexation of gallium with NaDDC was known more than 30 years ago (10). However, very little work has been done to combine this extraction method with modern analytical techniques such as NAA. Preconcentration with dithiocarbamate extraction followed by neutron activation provides a very sensitive way of determining some ultratrace metals in natural waters. Interfering matrix species such as the alkali metals, alkaline-earth metals, and the halogens can be effectively removed, and the metals of interest can be concentrated by 2 to 3 orders of magnitude. The extraction generally depends on the pH of the solution and the form of dithiocarbamate used. The effects of pH on the extraction of gallium with NaDDC and with APDC are shown in Figure la. The experiments were carried out with 1 pg/L gallium in a synthetic seawater sample according to the extraction procedure described in the Experimental Section. The composition of the synthetic seawater is described elsewhere (11). Quantitative extraction of gallium was observed in a narrow pH range around 4 to 5 with NaDDC and around 5 to 6 with APDC. However, when a mixture of NaDDC and APDC was
used, the pH range for quantitative extraction of gallium was found to be much broader. According to Figure Ib, total recovery of gallium can be achieved in the pH range 4 to 6 using a 1:l mixture of NaDDC and APDC. Based on these results, a pH of 4.7 was chosen as the standard condition of the extraction of gallium with the dithiocarbamate mixture. Because the extraction is quite pH dependent, it is necessary to use a buffer to control the pH of the solution during extraction. The acetate buffer was chosen because of its high buffer value in this pH range. Our experiments also indicate that there is no observable interference caused by the acetate buffer for the extraction of gallium using this method. The speed of extracting gallium under the specified pH condition is fast. Generally, a few minutes of shaking is sufficient to complete the extraction. The extraction constants of metal-DDC complexes are known to decrease in the order Au3+,Pd2+,Hg2+,Ag+, Po4+, Bi3+,Cu2+,Ni2+,Pb2+,Cd2+,As3+,Zn2+,Fez+,Co2+,Tl+, and Mn2+(12-16). The extraction constant of Ga3+is probably between that of Zn2+ and TP. This estimate was derived based on our experiments that Ga3+ could replace T1+ in TlDDC but was not able to replace Zn2+ in Zn(DDC)z. We chose to use Pb2+ for back-extraction of gallium from the organic phase because P b can replace Ga efficiently and is insensitive to NAA. In the back-extraction process, metals with extraction constants higher than that of Pb2+ would remain in the organic phase and therefore are separated from Ga. Furthermore, our experiments also showed that the rates of back-extracting Fe, Co, and Mn by Pb2+are much slower compared with that of Ga although the extraction constants of these metals are lower than that of Ga3+. With several minutes of shaking, only negligible amounts (
