Gases for Chromatography
The function of a gas chromatography depends on the quality of the auxiliary gases used. Impurities can result in contamination of column packing and or damage to detectors.
Contamination of the carrier gas can damage the packing materials and lead to instability and drift of the base line, column bleeding, the creation of tailing and poor separation. Heavy hydrocarbons can create ghost peaks, while oxygen and moisture can oxide or hydrolyse the stationary phase.
Impurities in the carrier gas or the gases used in the detector may produce noise, drift and reduced sensitivity. Impurities can result in contamination or, in the worst case, irreparable damage to the detector. The table below illustrates which types of contamination are most critical for the various detectors.
|Detector||Critical contamination Product number|
|FID||-||-||X||(X)||-||Hydrocarbons either in the carrier gas or fuel gas can produce increased noise and thus reduced sensitivity.|
|TCD||X||-||-||-||-||Oxygen and moisture can oxidize the filaments and produce reduced sensitivity and may damage the detector.|
|ECD||X||X||(X)||X||-||Halogen compounds can produce noise and negative peaks. Oxygen and moisture may affect the detector response and increase noise. Hydrocarbons may produce quenching of the signal and ghost peaks.|
|FPD||-||-||X||(X)||X||Sulphur compounds, carbon dioxide and hydrocarbons can produce increased background noise and quenching of the signal.|
|PID||X||X||X||X||-||Hydrocarbons can contaminate the optical window. Electronegative compounds reduce the signal.|
|HID/DID||X||X||-||-||X||Traces of impurities will produce base line noise, reduced sensitivity contamination can produce negative tops.|
|MS||X||X||X||X||X||Any impurity that have the same mass number as the analyzed component will give inaccurate analysis.|
**HHC – Halogenated Hydrocarbons
In order to achieve optional gas chromatography, it is essential to choose the correct gas quality with regards critical impurities.
Furthermore, the equipment leading from the gas cylinder to the chromatography must be of high quality to prevent impurities. Thus applies to all equipment and tubings/pipes.
The following table provide guidelines for the choice of gas for the different types of detectors.
|Detector||Carrier gases and detector gases|
|FID||He, N2, Ar, Air, H2|
|TCD||He, H2, N2, Ar|
|FPD||He, N2, Ar, Air, H2|
|PID||He, N2, Ar, Air, H2|
|MS||He, H2, N2|