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[MZZ42-P04] Distribution coefficients of 29 elements on anion-exchange resin in HCl–2-propanol mixtures
Keywords:Distribution coefficient, Ion exchange chromatography, HCl–2-propanol mixture, iron meteorite, potassium
Cosmogenic nuclides are produced by the interactions of galactic and solar cosmic-rays with planetary materials. Potassium (K) is often abundant in planetary materials. However, native K is so rare in iron meteorites that the cosmogenic fractions of K isotopes can be determined (Voshage 1978). Since the radionuclide 40K has a long half-life of 1.3 Gyr, the cosmogenic K isotope ratios of iron meteorites have potential to study the long-term variation of galactic cosmic-ray flux on a timescale of 108 to 109 years (Wieler 2013).
Ion exchange chromatography is a powerful technique for separating an element of interest from other matrix elements, which is required for accurate determination of isotope ratios by mass spectrometry. However, it is difficult to separate K and Ni, one of the major elements of iron meteorites, by ion exchange chromatography because of similar distribution coefficient (Kd) values for K and Ni under various acid types and acid concentrations (Saito 1984). In a previous study, K–Ni separation was performed by anion exchange using a 3:1 mixture of 2-propanol and conc. HCl (Nitoh et al. 1980). In this study, we determined the Kd values of 29 elements, including K and Ni, for anion exchange resins in various mixing ratios of 2-propanol, HCl, and water to investigate the best condition for K–Ni separation by ion chromatography.
Pre-washed Eichrom AG1x8 resin (100–200 mesh) in the chloride form was dried at 50 ℃. A multi-element standard solution XSTC-13 (10 mg L−1, SPEX CertiPrep Inc.) was accurately weighed into Teflon vessels and dried at 90 ℃. The resin (100 mg) and reagents (3 mL) containing various concentrations of 2-propanol (0–90%) and HCl (0.5–8 M) were added to the vessels and weighed. The lids of vessels were tightly closed and left at room temperature for several days. The vessels were thoroughly shaken every day. Then 600 μL of the supernatants were collected, weighed into the other Teflon vessels, dried at 100 ℃, and then diluted with 3 mL of 0.5 M HNO3. 50 μL of 100 ppb Rh standard solution (0.5 M HNO3) was added as an internal standard for the subsequent inductively coupled plasma mass spectrometry (ICP-MS). The concentrations of 29 elements in the solutions were measured by ICP-MS (iCAP TQ; Thermo Fisher Scientific) at Science Tokyo. The Kd values of each element were determined from the concentrations in the solutions.
Here we divide elements into four groups based on the Kd values: (1) no or weak adsorption (Kd < 20), (2) medium adsorption (20 < Kd < 200), (3) strong adsorption (200 < Kd < 2000), (4) very strong adsorption (2000 < Kd). The results for 1 M HCl, 8 M HCl, and 2-propanol (90%)–HCl (1 M) mixture are as follows:
1 M HCl: (1) Li, Be, Na, Mg, Al, K, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, As, Se, Rb, Sr, In, Ba, Th, U; (2) Pb; (3) Zn, Ag; (4) Cd, Tl, Bi.
8 M HCl: (1) Li, Be, Na, Mg, Al, K, Ca, V, Cr, Mn, Ni, As, Rb, Sr, Ag, In, Ba, Pb, Th; (2) Co, Cu, Zn, Cd, Bi; (3) Fe, Se; (4) Ga, Tl, U.
2-propanol (90%)–HCl (1 M): (1) Li, Be, Na, K, As, Rb; (2) Mg, Al, Ca, V, Cr, Ni, Se, Sr, Ba; (3) Fe, Ga, Ag, Tl, Th; (4) Mn, Co, Cu, Zn, Cd, In, Pb Bi, U.
The anion exchange behavior for 29 elements in 1 M HCl and 8 M HCl is qualitatively in agreement with literature (Kraus and Nelson, 1956). Significant increases in Kd values of many elements were observed with increasing 2-propanol concentration. Ni and K showed almost no adsorption on resin in HCl at any concentration. However, significant adsorption of Ni (Kd ~70) was observed in the mixtures of 2-propanol (90%) and HCl (0.5 M, 1 M), while K showed weak adsorption (Kd ~10). The Kd value of Ni in 0.5M HCl is slightly higher than that in 1 M HCl. In addition, the other major elements of iron meteorites (Fe, Co, Cu) are strongly adsorbed on resin in this condition. Therefore, anion exchange chromatography using a mixture of 2-propanol (90%) and 0.5 M HCl as an eluent may be useful for the separation of K from iron meteorites.
Ion exchange chromatography is a powerful technique for separating an element of interest from other matrix elements, which is required for accurate determination of isotope ratios by mass spectrometry. However, it is difficult to separate K and Ni, one of the major elements of iron meteorites, by ion exchange chromatography because of similar distribution coefficient (Kd) values for K and Ni under various acid types and acid concentrations (Saito 1984). In a previous study, K–Ni separation was performed by anion exchange using a 3:1 mixture of 2-propanol and conc. HCl (Nitoh et al. 1980). In this study, we determined the Kd values of 29 elements, including K and Ni, for anion exchange resins in various mixing ratios of 2-propanol, HCl, and water to investigate the best condition for K–Ni separation by ion chromatography.
Pre-washed Eichrom AG1x8 resin (100–200 mesh) in the chloride form was dried at 50 ℃. A multi-element standard solution XSTC-13 (10 mg L−1, SPEX CertiPrep Inc.) was accurately weighed into Teflon vessels and dried at 90 ℃. The resin (100 mg) and reagents (3 mL) containing various concentrations of 2-propanol (0–90%) and HCl (0.5–8 M) were added to the vessels and weighed. The lids of vessels were tightly closed and left at room temperature for several days. The vessels were thoroughly shaken every day. Then 600 μL of the supernatants were collected, weighed into the other Teflon vessels, dried at 100 ℃, and then diluted with 3 mL of 0.5 M HNO3. 50 μL of 100 ppb Rh standard solution (0.5 M HNO3) was added as an internal standard for the subsequent inductively coupled plasma mass spectrometry (ICP-MS). The concentrations of 29 elements in the solutions were measured by ICP-MS (iCAP TQ; Thermo Fisher Scientific) at Science Tokyo. The Kd values of each element were determined from the concentrations in the solutions.
Here we divide elements into four groups based on the Kd values: (1) no or weak adsorption (Kd < 20), (2) medium adsorption (20 < Kd < 200), (3) strong adsorption (200 < Kd < 2000), (4) very strong adsorption (2000 < Kd). The results for 1 M HCl, 8 M HCl, and 2-propanol (90%)–HCl (1 M) mixture are as follows:
1 M HCl: (1) Li, Be, Na, Mg, Al, K, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, As, Se, Rb, Sr, In, Ba, Th, U; (2) Pb; (3) Zn, Ag; (4) Cd, Tl, Bi.
8 M HCl: (1) Li, Be, Na, Mg, Al, K, Ca, V, Cr, Mn, Ni, As, Rb, Sr, Ag, In, Ba, Pb, Th; (2) Co, Cu, Zn, Cd, Bi; (3) Fe, Se; (4) Ga, Tl, U.
2-propanol (90%)–HCl (1 M): (1) Li, Be, Na, K, As, Rb; (2) Mg, Al, Ca, V, Cr, Ni, Se, Sr, Ba; (3) Fe, Ga, Ag, Tl, Th; (4) Mn, Co, Cu, Zn, Cd, In, Pb Bi, U.
The anion exchange behavior for 29 elements in 1 M HCl and 8 M HCl is qualitatively in agreement with literature (Kraus and Nelson, 1956). Significant increases in Kd values of many elements were observed with increasing 2-propanol concentration. Ni and K showed almost no adsorption on resin in HCl at any concentration. However, significant adsorption of Ni (Kd ~70) was observed in the mixtures of 2-propanol (90%) and HCl (0.5 M, 1 M), while K showed weak adsorption (Kd ~10). The Kd value of Ni in 0.5M HCl is slightly higher than that in 1 M HCl. In addition, the other major elements of iron meteorites (Fe, Co, Cu) are strongly adsorbed on resin in this condition. Therefore, anion exchange chromatography using a mixture of 2-propanol (90%) and 0.5 M HCl as an eluent may be useful for the separation of K from iron meteorites.