Gas Chromatography Questions and Answers

Answer: A

Traces of oxygen oxidise the stationary phase whenever the oven is hot, raising the bleed and eventually destroying the column. Filament attack is a real hazard in a thermal conductivity detector but secondary, and neither absorption nor pressure is affected. An oxygen trap in the carrier line is standard, and a column that has lost its performance suddenly is often a leak that admitted air while the oven was hot. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: B

Water hydrolyses the siloxane backbone of a bonded phase at working temperatures, shortening the column's life and raising the bleed. Detector condensation, injector corrosion and sample dilution are not the concern. Traps for oxygen, water and hydrocarbons in series are standard on a gas line, each protecting against a different slow degradation that would otherwise be blamed on the column itself. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: C

Heating under flow removes solvent and low molecular weight fragments left from the coating process, which would otherwise appear as a rising baseline for the first several runs. The coating is already applied, and flow and retention are set separately. A preparation step that trades an hour once against a spoiled baseline on every early run is worth the wait, and skipping it is a common cause of an apparently faulty new column. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: D

Exceeding the rated limit decomposes the stationary phase irreversibly, and the column never recovers its efficiency or its low bleed. It is not a matter of speed, and neither efficiency nor bleed improves. A rating that describes where a component is destroyed rather than where it performs less well is one to treat as absolute, and the few degrees of margin are deliberately conservative.

Answer: A

A sample that vaporises slowly or incompletely enters the column over a long period and produces broad or distorted peaks. Sterilisation is irrelevant, the septum seals mechanically, and the carrier needs no preheating. The injector temperature is set high enough for the least volatile analyte and low enough not to decompose the most fragile, which is another window bounded from both sides.

Answer: B

The rubber septum releases volatile components when hot, and they enter the carrier stream and appear as ghost peaks or a raised baseline. Gas escaping is a leak, sample return is a different fault, and phase degradation is column bleed. Low-bleed septa and a purge flow that sweeps the septum region both exist to address it, which indicates how troublesome it once was.

Answer: C

A small continuous flow passes across the underside of the septum and out to vent, carrying its degradation products away before they can reach the column. Cooling, needle clearing and pressure control are other functions. Diverting a contaminant at its source, rather than trying to separate it later, is the cheaper approach, and it costs only a few millilitres a minute of carrier.

Answer: D

The first two are correct and the third is false: the phase decomposes irreversibly above its limit, and even a brief excursion costs efficiency and raises bleed permanently. A maximum temperature is a destruction threshold rather than a performance guideline, which is why the oven's programme is checked before an unfamiliar column is installed.

Answer: A

The first three are correct and the fourth is not, so three match. The septum purge protects the column from septum bleed rather than the detector, which sees whatever the column delivers. Each protective measure guards a specific component against a specific threat, and knowing which is which explains why none of them can be omitted as redundant.

Answer: B

This is a guard column, sacrificed to catch involatile residues that would foul the analytical column. Capillary and packed describe the analytical column's construction, and a reference column belongs to a dual-channel thermal conductivity detector. Naming a component after the job it does rather than what it is made of makes a schematic self-explanatory.