Filters typically are manufactured to meet specifications for removal of particles to result in defined solution purity characteristics. For example, a 0.1µm rated pore size filter is typically defined as capable of removing 99.98% of particles 0.1µm from a liquid. The pores present on the surface, however, may be much larger or smaller than 0.1µm. Maximum peak resolution in alpha spectroscopy requires as uniform a filter surface as possible to ensure a uniform deposition of the rare earth fluoride precipitate. Recently reported resolution quality concerns indicate that manufacturing specifications and quality control procedures of existing suppliers may not address the specific needs of the alpha spectroscopist. Eichrom’s line of Resolve™ Filters (0.1µm polypropylene, 25mm dia.) are manufactured to specifications appropriate for alpha source preparation.
Scanning Electron Microscopy (SEM)
Figure 1 is an SEM photograph of a typical lot of Resolve Filters at 5000x magnification. Figures 2 and 3 show photographs of another supplier’s product at the same magnification. One was reported to have poor alpha peak resolution performance (figure 2); the other reported to have excellent performance (figure 3). The poorly performing lot shows a number of very large pores (>1µm), while the well performing lot shows a much more uniform pore size distribution. Eichrom’s Resolve filters exhibit this more uniform pore size across all product lots.
Performance in Alpha Spectroscopy
Maintaining a more homogeneous pore size distribution in the membranes used in Resolve Filters, has allowed Eichrom to produce filters that give reproducibly good performance in alpha spectroscopy. We have evaluated the performance of multiple lots of filters in source preps of 232U and 241/243Am. Figures 4 and 5 show alpha spectra for a 232U and 241/243Am on a typical lot of Resolve™ Filters.
Incoming filter material is monitored and screened using scanning electron microscopy (SEM) to ensure necessary homogeneity of surface pore size distribution in our finished product. Additionally, a test measuring resolution of 241Am and 243Am is performed on each filter lot manufactured. Specifications on this test are set at FWHM 50 keV for each peak.
Some scientists consider that the two sides of filters may perform differently in the preparation of alpha sources. We have investigated whether different sides of Resolve Filters give different peak resolution. Two studies have been run, one internally at Eichrom and a second by Dr. Barry Stewart, at the Carlsbad Environmental Monitoring Research Center. In the study conducted in Carlsbad, various parameters were considered, including, “glossy” side vs. “non-glossy” side and the use or absence of substrate. Dr. Stewart made source preps of 241Am and 243Am on Resolve Filters using ASTM procedure C1163-98 (50 µg La carrier.) His study matrix evaluated the two sides of the filter and the effect of substrate use. His results are shown in the table below.
|Sample||Substrate?||Orientation||FWHM (keV)||243Am Recovery||241Am Recovery|
The Carlsbad results indicate that each side of the filter works equally well and that excellent peak resolution of the 243Am peak (< 30 keV) could be achieved with or without the use of substrate on the filter. A similar study conducted at Eichrom evaluated the peak resolution of the two Am isotopes on each side of the filter. In this case, the geometry of the filter was described by the orientation of that filter surface in the roll of material prior to cutting into filter discs, inside and outside. In this study source preps were made following Eichrom's procedure SPA01which uses 50 µg Ce. As shown in the following table, the FWHM of both the 241Am and 243Am did not vary as a function of the filter geometry.
|243Am Outside||243Am Inside||241Am Inside||241Am Inside|
|mean = 34||mean = 33||mean = 36||mean = 39|
|Std. Dev = 7||Std. Dev. = 8||Std. Dev. = 4||Std. Dev. = 8|
The pore size distribution of Resolve Filters and their performance in alpha spectroscopy are monitored in Eichrom’s QA/QC program. While the actual peak resolution that is observed is a function of many factor, Eichrom’s QC program ensures that the filter itself will not contribute to poor resolution in alpha spectrometry. Resolve Filters are available in packages of 100, part number RF-100-25PP01.