Normally glass vials are less prone to adsortion than plastic. In addition, you can use silanized vials to reduce the sorption.

Performing blanks and standard addition on blank vials may give you an idea.

i am not exactly, but some way i suggest use of kinetics and isotherm equations for linear plots may be helpful to requirements. before use these equations, should clearly understood that constant terms explains such as driving force, heat of adsorbtions, no of acivite sites per unit area. These constants comes arries everywhere in langmuir, Frendluich, Dubinin, Tempkin isothems etc. before we do such experiments of analyte adsorptions, we shold clear stability of sorbate and substrate in practicle purpose. thank you.

Oh, for the detection of terpene glucoside can be used a complexing agent by it titrations to quantifies amount of absorbents. UV-Vis might be helpful, but cost of complexing agents are high, or else HPLC is better choice to quantitative purpose shows good results. but the controlling of sorptions were tedious engineering works. need more time spend for proper understaing these, might not helpful for medical pepole.As i pricribed early just only theoritical explanations help to what actuvally happends for substrates sorptions, not use for controlling of analytes. sorry.

do you ask adsorption capacity or not ?

Could you please elaborate your question?

I want to compare adsorption capacity of two polymers at different concentrations. I use PP tubes and use blank and standard solutions but I have not found reproducible results yet.

The blank does not show any results mean to think, the surface of theta coverage may not be happend due to insufficient time to reach equillibirium or inert activity of the polymer. it also might be think an analyte concentration some times higher not obey for sorbate interaction rather than molecule molecule interaction (analyte). Most of the isotherms inversly proportional to the temperature, for this one more experiment conduct under room temp or at low pressure may expect sorprion capacity of your desired results. Thank you.

If you're having not reproducible results with PP tubes, I would suggest trying with glass vials.