I am working with snake venom, specifically targeting toxins around the 15 kDa range. I need to remove higher molecular weight toxins (above 20 kDa). Would it be advisable to use a 20 kDa MWCO membrane to separate proteins above 20 kDa and then use a 10 kDa MWCO filter to concentrate the lower molecular weight toxins around 15 kDa which is of my interest?