Gas Chromatography Questions and Answers

Answer: A

Organochlorine compounds capture electrons avidly, and each additional chlorine increases the response, so picogram quantities are detectable. Concentration, combustibility and molecular weight are not the mechanism. That a detector's selectivity happens to align with a class of compounds of great regulatory interest is why the technique dominated residue analysis for decades. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: B

Recording a spectrum of each peak gives structural information no other detector provides, turning a retention time into an identification. Electron capture is more sensitive for its own class, a vacuum is essential, and response varies widely between compounds. That a detector can answer what as well as how much is what made the hyphenated technique the reference method in so many fields. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: C

Thermal conductivity responds to water as readily as to anything else, while a flame ionisation detector is blind to it, electron capture nearly so, and a nitrogen-phosphorus detector is selective for other elements. Choosing a detector begins with asking whether it can see the analyte at all, and for water that question eliminates three of the four common ones immediately. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: D

Exposed silanol groups or a dirty liner adsorb a fraction of the analyte and release it slowly, drawing out the peak's trailing edge. Overloading produces a fronting or a flat-topped peak instead, excess flow broadens symmetrically, and a slow detector broadens every peak equally. Reading a peak's asymmetry as evidence about its cause is what makes a chromatogram diagnostic rather than merely a record. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: A

Beyond the column's capacity the partition isotherm curves, the excess sample travels faster than the rest, and the peak leans forward. Active sites produce tailing, the opposite asymmetry, and injector temperature and detector saturation give other symptoms. That overload and adsorption distort a peak in opposite directions is convenient, since the direction alone distinguishes them. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: B

A mismatch between solvent and stationary phase polarity can leave the sample band in two portions at the column head, which then elute as a doubled peak. True coelution gives one peak or two resolved ones, detectors do not double-count, and a leak reduces signal. That an artefact of injection can mimic a chemical observation is why the shape of the earliest peaks is worth inspecting before any conclusion is drawn from them. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: C

A flow far from the optimum raises the plate height for every compound, broadening all peaks together. Gain affects height rather than width, concentration affects shape at the extremes, and split ratio affects the amount reaching the column. That a fault affecting every peak equally points at something common to them all - flow, temperature or column condition - is the first division to make in any diagnosis. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: D

A slow leak reduces the flow through the column, so everything takes longer while peak shapes stay much the same. Detector sensitivity affects height, an over-fast programme would shorten times, and concentration does not shift retention. Retention time is a sensitive reporter of flow, which makes a drifting sequence one of the earliest signs of a leak developing somewhere in the system. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: A

The first two are correct and the third is backwards: the mass transfer term rises with velocity, because equilibrium has less time to establish. That one term falls and another rises is exactly what produces a minimum, and getting the direction wrong would predict that faster is always better. The existence of an optimum is the equation's whole practical content.

Answer: B

The first two are correct and the third reverses on-column injection's purpose: placing the liquid directly in the column removes the vaporisation step where discrimination arises, so it reduces the problem to nearly nothing. Eliminating the step responsible for an error, rather than optimising it, is the more reliable remedy, and this technique exists precisely to do that.