I am trying to refine a density separation method for quantifying the mass of microplastics in a water sample. I am encountering a persistent problem of recovering excess weight in all my spike samples, including in my method blanks. I know that ZnCl2 can precipitate zinc oxychlorides at higher pHs, so I have used acidic DI water (pH = 2) to rinse ZnCl2 through the filter during vacuum filtration. This has reduced the excess mass recovery, but I am still recovering 2-10 mg of excess mass, even in method blanks. This persistent bias is a problem because I expect the mass of the samples I hope to analyze is in mg. Does anyone have recommendations for improving the precision of density separation methods with ZnCl2?
Hi Chelsea. We had issues with high amounts of precipitates on our filters after high density salt separations. Could you you rinse the filters with DI water to help remove some of the salts from the particles? We follow our filtrations with DI water wash steps to help remove excess salt.
Michaela A. Cashman thank you for your response! I have been rinsing the filter 3X with DI water during the vacuum filtration after filtering out the ZnCl2. I still get excess weight from filter blanks where ZnCl2 has been used. How many DI rinses do you use?
Usually 3 rinses, but we use high volumes of DI (~100-300mL) per rinse to help clean particles. I'm not sure if the salt matters, but I found ZnCl2 more difficult to work with and onerous to properly dispose of. We use NaBr for our separations. I've seen other labs also successfully use CaCl. Maybe a less dense salt would help.
Thanks Michaela. Maybe higher volume rinses would help. I am using ZnCl2 because I need to achieve a density between 1.6-1.8 g/mL. Thanks For your help!
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