I need to measure the zeta-potential and protein mobility of lactate dehydrogenase, using Malvern ZSP, but the buffer it's in is actually cell culture medium with the following components:

0.002M HEPES

50ug/ml gentamicin

15% horse serum

0.4% fungizone

My main concern is the horse serum and whether the machine will measure zeta-potential of the culture medium instead of my enzyme. Is there a way to define buffer/culture media components which are not in the list of the Malvern software? How should I avoid background measurement of the medium, or if not possible, does it make sense to measure the zeta potential of only the cell culture medium first, and then substract that value from the zeta-potential of medium+enzyme? Doesn't seem like the like the right approach to me but wanted to check anyway.

Thanks!