I have a series of fluid samples that I have collected weekly for 290+ days. The samples were frozen after collection. Due to the experimental treatment I am expecting to observe changes in Fe2+ and Fe3+ concentrations however, I am not detecting the presence of either form of iron. I'm wondering if freezing the samples may cause iron to precipitate lowering the detection capability of ferrozine assay. Is anyone familiar with preserving fluid samples prior to ferrozine analysis?

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