Dr. M said:
Diablo -
None of your testosterone should bind to the filter, but you may lose a bit due to retention of volume (i.e. that last little bit you can't get out of the filter). If this is a worry, make your test solution to about 9mL instead of 10mL, then after filtration take about a mL of sterile oil and run it through the filter to 'clean' it out. Testosterone is extremely hydrophobic, and nylon or PVDF filter membranes tend to bind highly charged protein species... testosterone is a totally different beast.
Viruses are measured in the tens (very rare, if any) to hundreds of nanometers (10E-8 to 10E-7 m). Viruses which thrive in mammalian cells are usually around 400-1200nm (0.4 to 1.2um). If you look at the molecular size of cholesterol (the nucleus of which is the basis for testosterone and other steroids), it's 44.5 Å (Ångstroms, which are 10E-10 m) in its longest dimension, if I remember correctly from my lipid biophysics classes. So, testosterone and other steroids, including their esters, are on average let's say almost a thousand times smaller than some of the smallest viruses which infect mammalian cells.
They whizz right through a 0.22um PVDF filter, while most (if not all) viruses and bacteria are retained by the filter membrane.
Does this clear things up? Karma if you're diggin the info...
-M
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