a sample tube outlet end 9 extends through the passage 6 the sample tube 7 further comprises a locking portion 11 in the form of a collar about the cylindrical tube body 10 the tube interlocking portion 11 is adapted to interlock with a complementary interlocking portion 12 for the sample tube 7 on the base 2 the sample tube interlocking portion 11 is provided with a peripheral thread 13 which is received in a complementary threaded portion 14 of the base 2 the collar 11 provides a male threaded fitting 13 which is receivable within complementary female thread fitting 14 of the base 2 the sample tube interlocking portion 11 further comprises a sample tube abutment surface 15 which is narrowingly tapered toward the free end 9 of the tube body 10 a complementary abutment surface 16 on the base 2 provides a stop for the interlocking portion 11 of the sample tube 7 when the interlocking portion 11 of the sample tube 7 is locked by complementary threaded portions on the sample tube interlocking portion 11 and base 2 interlocking portion 12 interaction of the abutment surfaces 15 16 aid in ensuring alignment of the sample tube 7 in the concentric tube formation 2 a when locked in place the sample tube interlocking portion 11 may form an approximately co planar surface with the bottom of the base 2 the concentric tubes include a plasma tube 17 comprising a flared portion 18 a plasma tube inlet end 19 and a plasma tube outlet end 19 a plasma tube inlet end is located in a socket 21 within the base 2.