The pressure cell is connected to a source of compressed gas (such as Argon, Helium or Nitrogen).  A capillary is placed through a ferrule in the cap so that one end is in contact with the liquid in the tube or vial.  The distal end of the capillary is fritted to retain the particles while packing.  Pressure from the compressed gas can be regulated to adjust the flow rate of the sample into the capillary.