Method for processing a fluid and fluid processing device

A fluid processing device for use in a centrifuge having a rotor, said fluid processing device comprising:

• a) a first holder for holding a first tube having a first cross section;
• said first holder having a first stopper and said first tube having a first cap;
• and
• b) at least one first cap fixture means for holding said first cap in a defined position with respect to said first holder during centrifugation in said centrifuge, and
• c) a second holder for holding a second tube, said second holder having a second stopper,
• wherein the first and second holders are cylindrically shaped,
• wherein the first stopper and second stopper extend above and below a surface connecting said first holder and said second holder;
• wherein the at least one first cap fixture means is arranged such that the first cap is held at a position that leaves the first tube's inlet opening open;
• and
• wherein said at least one first cap fixture means is respectively integral with the first holder.