English to Spanish Science Chemistry; Chem Sci/Eng Laboratory equipment

This is a catalogue which has been translated into Spanish, but the translator forgot to translate the following phrases, and I don't know what a 'tube spacer' is. Apparently a 'rotor' is a part of a centrifuge.

For rotor CRC-4-8:
Tube spacer 2mL, 3mL, 4mL - 8 pcs. of each
Tube sleeve for 6 mL - 8 pcs
- - - - -
For rotor CRC-4-16:
Tube sleeve for 9mL, 10mL - 16 pcs. of each
Tube sleeve for 6mL - 16 pcs.
Tube spacer 2mL, 3mL, 4mL - 16 pcs. of each

Many thanks!
Carlos C.