I am measuring Calcium concentrations in Ca-EGTA solutions that are used in single fiber measurements. I am observing different outputs on different days of measuring. Is that due to the activity of Calcium going down due to cycles of warming up and cooling down or is it due to the washing of the probe with milliQ inbetween measurements that the concentration might be diluted?

I hope somebody can help me. I could not find any articles talking about the shelf life of Ca-EGTA buffer solutions. But among the people here in the group, it is known that buffers degrade in quality over time.

I also found out the pH differs after not using the buffers for years. They were stored in the -80 degrees celcius freezer the whole time.