New SPE column packing material retention assessment method and its application for the radionuclide chromatographic separation

DOI: 10.1007/s10967-007-7131-1 Journal of Radioanalytical and Nuclear Chemistry, Vol 277, No.3 (2008) 651–661 New SPE column packing material: Retention assessment method and its application for the radionuclide chromatographic separation Le Van So*, N Morcos Radiopharmaceutical Research Institute, Australian Nuclear Science and Technology Organisation, New Illawarra Road, Lucas Heights, P.M.B 1, Menai, NSW 2234, Australia (Received October 29, 2007) The preparation of the OASIS®-HLB sorbent based solid phase extraction (SPE) resins and their application for the 177Lu radioisotope separation were investigated Di-(2-ethylhexyl) orthophosphoric acid (HDEHP) impregnated OASIS-HLB sorbent based SPE resins (OASIS-HDEHP) were successfully developed from this investigation The wettable porosity structure of the moderately extractant impregnated OASIS-HDEHP resins is favorable for the effective diffusion of polar and ionic solutes giving good separation performance Its good wetting ability offers ease of packing into conventional chromatographic columns Their off-gassing-free operation makes OASIS-HDEHP columns good for long term use with highly consistent elution dynamics (several dozens of separations perfectly achievable on the same column) The simple method for the capacity factor (k’) evaluation was developed to facilitate the characterization of the SPE chromatographic resin column A competent procedure using OASISE30RS resin (one member of the OASIS-HDEHP resin group) for the separation of no-carrier added (n.c.a) 177Lu from the bulk amount of Yb target was developed This separation procedure has showed very good performance with several prominent advantages such as the much shorter separation time (5–6 hours) and high reproducibility Its high adsorption capacity for Yb and Lu makes it ideal for the separation of the bulky sample (50 mg Yb target for the 20 g weight resin column) for the production of the several Ci of 177Lu radioactivity Introduction Among the column packing materials used for solid phase extraction (SPE) chromatography the extractant (liquid organic extractant and/or a liquid ion-exchanger of high molecular weight) coated inert sorbent based SPE resins are widely used for the inorganic solute separation These resins have been recently used for radiochemical separations in radioisotope production processes Normally, these SPE resins are useful in reverse phase chromatography which requires a polar liquid mobile phase for the elution A hydrophobic inert substrate is currently used to form the SPE resin with a relevant less hydrophilic extractant This has caused non-wetting and off-gassing problems during the column packing and running, respectively The nonwetting property of this SPE resin type is unfavorable for the diffusion of polar and ionic solutes to be separated, so the column separation performance will be seriously affected Even if the hydrophilic inert substrate is alternatively used, the non-wetting problem will also increase if an improper process of the extractant impregnation is applied If the strongly hydrophilic inert sorbent is to be applied for SPE resin preparation, then longer elution times will be experienced due to the unwanted side effect of strong polar-polar interaction between the sorbent and polar (or ionic) solutes in reverse phase chromatography For example the US patent based separation process using the strongly polar Amberchrom CG-71 sorbent based LN resin (Eichrom Technologies) and HCl acid eluent for the separation of n.c.a 177Lu radioisotope from mg Yb target took around 12 hours.8,9 Logically, an inert substrate rendering a hydrophilichydrophobic balanced property will be preferable to overcome the strong interaction and long retention time problems mentioned above Among such expected inert substrates, the OASIS®-HLB sorbent (a hydrophiliclipophilic balanced co-polymer) was the first choice for preparation of the SPE resin The OASIS ®HLB sorbent is a macroporous copolymer made from a balanced ratio of two monomers, the lipophilic divinylbenzene and hydrophilic N-vinylpyrrolidone It provides reversephase capability with a special “polar-hook” for enhanced capture of polar solutes and excellent wetting ability.15 This sorbent has the unique capabilities of remaining wetted with water and retaining a wide spectrum of non-polar organic extractant and several liquid ion-exchangers which are used as stationary phase for SPE chromatography Wetting with water into porosity structure of OASIS-HLB sorbent particle is an advantageous feature to facilitate the diffusion process of polar and ionic solutes during elution with aqueous mobile phase In this paper the preparation of the OASIS®-HLB sorbent based SPE resins and their application for the 177Lu radioisotope separation are reported The HDEHP impregnated OASIS-HLB sorbent based SPE resins (OASIS-HDEHP) were prepared and their adsorption properties and chromatographic performance are investigated * E-mail: slv@ansto.gov.au 0236–5731/USD 20.00 © 2008 Akadémiai Kiadó, Budapest Akadémiai Kiadó, Budapest Springer, Dordrecht LE VAN SO, N MORCOS: NEW SPE COLUMN PACKING MATERIAL The basic retention calculation method for SPE chromatography is also refined to formulate the generalized equations matching the specific requirements of the SPE resin development and application for chromatographic separation To demonstrate the prominent performance regarding the predictable chromatographic properties of OASIS-HDEHP resins, the column separation of n.c.a 177Lu radioisotope from the bulk ytterbium target was performed The separation of trace level radioactive impurities from the 176Lu (n,γ)177Lu reaction produced 177Lu solution is well within the capabilities of the OASIS-HDEHP resin columns.14 The separation of the bulk Yb target can also be carried out with Eichchrom’s LN resin (the Amberchrom CG-71 resin substrate impregnated with HDEHP), however, the performance of LN resin columns may be affected by the strong polarity of the acrylic ester based inert resin substrate leading to long elution times due to the stronger adsorption of the Yb–Lu pair.8–11 The hydrophilic Oasis-HLB sorbent based OASIS-HDEHP resin reported here has surpassed the shortcomings of the LN resin mentioned above Experimental Reagent and material The commercially available OASIS®-HLB resin (WATERS’s sample extraction products) with average particle size of 54.4 µm, pore diameter of 86.0 Å, specific surface area of 813.0 m2/g15 and di-(2ethylhexyl)orthophosphoric acid (97%, BDH) were used for our di-(2-ethylhexyl)orthophosphoric acid (HDEHP) impregnated OASIS-HLB sorbent based SPE resins’ (OASIS-HDEHP) preparation The analytical grade hydrochloric acid, HPLC grade methyl alcohol and Milli-Q purified water were used for the whole experimental process The isotopically enriched 176Y O and 176Lu O targets for neutron activation 3 were purchased from Trace-Sciences International Inc., USA.13 The 176Y2O3 target isotopic compositions were 176Yb (97.6%), 174Yb (1.93%), 173Yb (0.18%), 172Yb (0.22%), 171Yb (0.07%), 170Yb (