Thermal desorption technology increases the sensitivity of gas chromatography, but it also can concentrate contaminants from any gas stream that passes over a trap.
If contaminants interfere with the elution of the compound of interest, it is impossible to get a clean blank run (no sample applied yet there is still a peak) and the calibration curve will not pass through zero. This may be the result of contamination in either the gases used to blend the standards (trap tubes) or gases used internally by desorber (cold trap). However, when combined with an inability to get a clean zero, the evidence suggests that the problem is with gases internal to the instrument. The carrier gas, which passes through the cold trap at several stages of operation is the most likely source. We compared contamination from two He standards.
The total hydrocarbon contamination specification in helium cylinders is more important that using UHP Grade helium.
Romagnano, Joseph F. and Bugbee, Bruce, "Helium Quality Affects Thermal Desorber Calibration" (2008). Techniques and Instruments. Paper 6.