GC Rising Baseline

The answer to the following question was provided by LCGC’s “GC Connections” columnist John Hinshaw.

Q: In gas chromatography, why might a rising baseline be observed when using temperature gradients?

John Hinshaw: Baseline rise during a temperature program can be attributed to three principal effects. First, at higher temperatures the stationary phase can vaporize slightly or may decompose under the influence of temperature, the presence of oxygen traces in the carrier gas, or due to catalytic decomposition from sample residues. This will cause the detector to respond. Stationary phase bleed may be identified with a GC–MS system by the presence of SiO- fragments in the mass spectrum.

A second cause of stationary phase bleed is the decomposition of sample residues themselves to produce a gradually increasing baseline during a temperature program. In both of these cases the bleed will tend to decrease once an isothermal final temperature is reached due to depletion of the source of the bleed.

The third cause is a change in the mass flow of the carrier gas from not using a constant flow mode. As temperatures increase the carrier gas viscosity increases as well, causing the flow to drop off if the pressure drop is held constant. The constant flow mode increases the pressure drop to compensate.

Many detectors are sensitive to the carrier flow; in a mass spectrometer the detector sensitivity may also be affected as the source pressure drops slightly during a temperature program. If hydrogen carrier is used then the total hydrogen flow through the detector will change without constant flow, potentially causing a shift in response factors with hydrogen-consuming detectors such as the FID.

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