decreasing 230/260 Ratio

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Questioner25.08
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decreasing 230/260 Ratio

I´ve isolated DNA from FFPE Tissue using the QIAamp DNA FFPE Tissue Kit, using water instead of ATE Buffer. In first measurement after DNA-isolation with ND1000 the results were good. I`ve stored the DNA at 4°C. In a remeasurement some days later the 230/260 ratio was not as good as in the first measurement and decreased further in the following weeks. Examples 230/260: 2,03 to 1,58; 2,17 to 1,63 in 20 days.
Anyone who has an idea? Somebody suggested the water may have evaporated leaving relatively more salt in the tubes, but samples isolated with same Kit, stored in same tubes did not show this problem.
Thanks