Gas Chromatography - Tandem Techniques - Modern GC/IR Systems > The Cryostatic Interface > Page 54



The device can collect very small quantities of material and still provide excellent spectra, and consequently, is extremely sensitive. The carrier gas can be helium mixed with a small amount of argon (ca 0.5 %), although the helium can be replaced by nitrogen if more convenient, The carrier gas, on leaving the column, is arranged to impinge onto a rotating gold plated drum, situated in a evacuated box maintained at about 12 űK by a liquid helium thermostat. The drum, while rotating, also moves very slowly in an axial direction and, thus, the samples are deposited as a thin helical trail on the outer walls of the drum. The sample on the drum surface is frozen with the argon and thus trapped in a cage of solid argon. The argon insulates the sample from the gold surface and so there can be no catalytic decomposition, or molecular rearrangement. The major component of the carrier gas (i.e., helium or nitrogen) is continuously removed by a low-pressure turbo-molecular pump. The pump contains no oil and, thus, the possibility of any long-term sample contamination is eliminated.