2ml of a serum sample was added to 18ml of phosphate buffered saline (pbs) in tube 1. 10ml of tube 1 was added to 40ml of pbs in tube 2. what is the dilution of serum in tube 2? 3. four tubes were placed in a rack. to all four tubes, 9ml of pbs buffer was added. iml of a protein lysate was added to the first tube and mixed well. iml of the first tube is transferred to the second tube and again mixed. the transfer of 1ml from one tube to each subsequent tube continues until 1ml is discarded from the last tube. what is the dilution of protein lysate in the fourth tube?