Ln Resin, like most of Eichrom’s products, grew out of the transformation of a selective organic extractant into an extraction chromatographic resin. The extractant used in Ln Resin is di(2-ethylhexyl) orthophosphoric acid (HDEHP). References to HDEHP in a chromatographic system date back more than thirty years. A number of these references are shown in the bibliography below.
Early work by Horwitz et al. (1975)1 , provided distribution coefficients (Kd) versus nitric acid for various metal ions using HDEHP on a hydrophobic support. This data is reproduced in figure 1 and has been used as the starting point for a number of radiochemical separations in use today. Note that for Eichrom’s Ln Resin, Kd can be converted into k’ (an approximation of free column volumes to peak maximum) by dividing by 4.33. Ln Resin has been applied to the analysis of radium, neodymium, promethium and as the data in figure 1 might indicate, many more analytical applications are possible and may already be operational.
Bill Burnett et al.(1995)2 provided an excellent reference for the determination of 228Ra using Ln Resin. Although this reference also demonstrates that Eichrom’s TRU Resin can be used to separate 228Ra from its direct beta particle-emitting daughter, 228Ac, Ln Resin may find favor with American regulators as it relies on the same HDEHP extractant used in EPA Method Ra-05-1 (EPA Procedures Manual 520/ 5-84-006) without the generation of mixed organic waste. An outline of Bill Burnett’s method is provided as figure 2. After a barium sulfate precipitation and conversion to barium carbonate, the sample is loaded in 10 mL 0.095M HNO3 onto a prepackaged Ln Resin column (0.7 g). The column is rinsed with 15 mL of the same nitric acid to remove alkali earths, lead and some other interference. (The load and rinse fractions can be collected and analyzed for Ra-226 by radon emanation.) Finally 10 mL of 0.35M HNO3 is used to quantitatively elute actinium. A source can then be prepared for gas flow proportional counting. For routine work a minimum detectable activity (MDA) of <1.0 pCi/L (0.037 Bq/L) may be obtained for a 2-L sample with 80% barium recovery with less than a 30 minute count. This method was used to analyze samples from a US-EPA inter-laboratory comparison for a number of Ra-228 samples. All results agreed with the expected values at the 95% (2 sigma) confidence interval. Additional development of this method at Eichrom has resulted in an improved procedure. The BaSO4 precipitation/carbonate metathesis steps have been replaced with a cation exchange concentration step. Alpha spectrometry is also indicated for Ra-226 as well has the shorter-lived Ra-224. This method has been published as RAW03, Radium in Water, and is available in the methods section of this website. Additional performance datawas presented at the Eichrom North American Users’ Meeting in May, 2002.
Ln Resin is also useful in the analysis of Pm-1473,4,5 . In analysis of environmental and waste samples from nuclear facilities it is necessary to separate promethium from other fission products and the actinides to obtain an accurate measurement. A method for water is outlined by Cable, et al. (1997.) A 0.5-2 L water sample can be concentrated by a CaHPO4 scavenge or by evaporation. The resulting sample is dissolved in 0.2M HNO3 with ascorbic acid. The ascorbic acid reduces any Fe(III) to Fe(II). A test with 55 mg Fe(II) in solution resulted in no breakthrough of Pm after 35 mL of 0.2M HNO3. A typical procedure would use a total of 18-20 mL to load and rinse a standard prepackaged Ln Resin column. Potentially interfering americium is rinsed through the column along with strontium. Promethium is retained along with bismuth, yttrium and the potential tracers samarium or gadolinium. The promethium along with any samarium or gadolinium tracer is eluted with 5 mL of 1 MHNO3. Corrected recoveries of Pm-147 were >88% using Gd-148 as a yield monitor in one reported test. Excellent decontamination were achieved from Co-60, Cs-134,137 and Sr-89,90. One other potential interference is Ce-144, a beta and gamma emitter. A check with gamma spectroscopy could be used to eliminate this as a potential false positive.
Christian Pin et al. (1996)6 reported a method for the sequential separation of Sm, Nd, Th and U in silicate rocks. The method uses Eichrom’s TRU Resin in series with Ln Resin. For samples high in iron, such as basaltic samples, a 50WX4 strong acid cation exchange column (available from Eichrom) is used up front. After dissolution and possible treatment with cation exchange resin, the sample is loaded onto a TRU Resin column in 1M HNO3. Unwanted cations are eluted with rinses of the same acid. The light rare earth elements (LREE) can then be stripped with 0.05M HNO3 . This fraction from the TRU Resin column can be loaded directly onto an Ln Resin column to sorb the LREE. The Pin work used a 0.3 gram Ln Resin column of our S-grade 50-100 micron bulk resin. Figure 3 shows the elution of La, Ce, Pr, Nd, Sm and Eu. A total of 5.5 mL of 0.25M HCl was used to strip the La, Ce, Pr and Nd with no detectable Sm (ID-TIMS). Then after passing 0.75 mL of 0.75M HCl, Sm appeared in the next 0.5 mL. Finally Eu was collected in the next 0.5 mL of 0.75M HCl. Comparison with 15 international standard reference materials of silicate rocks showed good agreement. Element concentrations were at the mg/g to µg/g level.
Ln Resin has a calculated maximum capacity for Nd of approximately 22 mg/mL of resin. In practice it is normally not recommended to exceed 10-20% of this amount in analytical procedures. Therefore a useful working capacity for Ln Resin would be 2-4 mg/mL or 4-8 mg/ 2mL pre-packaged column or cartridge.
Ln Resin is manufactured in three particle sizes (20-50µ, 50-100µ, and 100-150µ) and is sold in bottles or ready to use in prepackaged columns (for gravity flow) and cartridges ( for vacuum assisted flow.) Click here for part numbers and descriptions.