GC Rising Baseline - - Chromatography Online
GC Rising Baseline

E-Separation Solutions

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.

If you have a question you'd like answered, please submit them at Ask LCGC. We look forward to hearing from you!

ADVERTISEMENT

blog comments powered by Disqus
LCGC E-mail Newsletters
Global E-newsletters subscribe here:




 

LCGC COLUMNISTS 2014

Column Watch: Ron Majors, established authority on new column technologies, keeps readers up-to-date with new sample preparation trends in all branches of chromatography and reviews developments. LATEST: Avoiding Reversed-Phase Chromatography Problems Through Informed Method Development Practices: Choosing the Stationary-Phase Chemistry


Perspectives in Modern HPLC: Michael W. Dong is a senior scientist in Small Molecule Drug Discovery at Genentech in South San Francisco, California. He is responsible for new technologies, automation, and supporting late-stage research projects in small molecule analytical chemistry and QC of small molecule pharmaceutical sciences. LATEST: Seven Common Faux Pas in Modern HPLC


MS — The Practical Art: Kate Yu brings her expertise in the field of mass spectrometry and hyphenated techniques to the pages of LCGC. In this column she examines the mass spectrometric side of coupled liquid and gas-phase systems. Troubleshooting-style articles provide readers with invaluable advice for getting the most from their mass spectrometers. LATEST: Radical Mass Spectrometry as a New Frontier for Bioanalysis


LC Troubleshooting: LC Troubleshooting sets about making HPLC methods easier to master. By covering the basics of liquid chromatography separations and instrumentation, John Dolan is able to highlight common problems and provide remedies for them. LATEST: Estimating Resolution for Marginally Separated Peaks


More LCGC Columnists>>

LCGC North America Editorial Advisory Board>>

LCGC Europe Editorial Advisory Board>>

LCGC Editorial Team Contacts>>


Source: E-Separation Solutions,
Click here