Gas Chromatography Questions and Answers

Answer: C

Blood, food or a polymer would foul an injector and column within a few injections, and taking only the vapour leaves all of it in the vial. Dilution, filtration and particulate retention are incidental. Choosing a sampling method that excludes the troublesome part of a matrix, rather than removing it afterwards, is the cheaper approach whenever the analytes happen to be volatile.

Answer: D

An analyte must partition appreciably into the vapour to be sampled at all, so involatile compounds are invisible however much is present. Solids work perfectly well, the vial is heated well below boiling, and many analytes are measured together. That the technique's chief advantage and its chief limitation are the same property is worth recognising before it is chosen for an unfamiliar analyte. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: A

A short length of coated fused silica is exposed to the sample or its headspace, adsorbs the analytes, and is then desorbed thermally in the injector. Cartridges are conventional solid phase extraction, and membranes and resins describe other approaches. Combining extraction, concentration and injection into one solventless operation is what made the technique popular for environmental work.

Answer: B

Nothing is eluted with solvent, so there is no solvent peak, no disposal problem and no dilution of the analyte. Extraction is deliberately incomplete and equilibrium-based rather than quantitative, it works with semi-volatiles too, and calibration is still needed. That an equilibrium extraction can be quantitative provided the conditions are reproduced exactly is the point most often misunderstood. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: C

Only a fraction of the analyte transfers to the fibre, and that fraction depends on time, temperature, agitation and salt content, so every sample and standard must experience the same conditions. Coating degradation is gradual, decomposition is uncommon, and the injector desorbs whatever arrives. A method whose result depends on conditions rather than on completeness must control those conditions rigidly. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: D

Dissolved salt reduces the solubility of organic compounds in water, pushing more of them into the headspace or onto the fibre and improving the recovery. Freezing, reaction and pH are other matters. The salting-out effect is a simple and cheap way to shift a partition equilibrium in the wanted direction, and it costs nothing but the need to add the same amount every time.

Answer: A

Converting a carboxylic acid to its methyl ester, or an alcohol to a silyl ether, replaces hydrogen-bonding groups and lowers the boiling point enough for the compound to pass through a column. Concentration, clean-up and identification are separate steps. Chemically modifying an analyte so that it suits the technique, rather than choosing a different technique, is worthwhile when the rest of the method is already established. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: B

Any incomplete or variable reaction introduces an error that the chromatography cannot detect, so the derivatisation must be reproducible to the same standard as everything else. The analyte is modified rather than destroyed, mass spectrometry works well on derivatives, and the phase is chosen freely. Adding a chemical step to a physical method imports the chemistry's variability, which is why derivatisation is avoided where an alternative exists.

Answer: C

Replacing an active hydrogen with a bulky silyl group removes the hydrogen bonding that causes both low volatility and tailing on active sites. Molecular weight rises, which is incidental and slightly unhelpful, boiling point falls rather than rises, and electron capture response would need a halogen. Treating the cause of two symptoms at once, volatility and peak shape, is why silylation is the commonest derivatisation of all. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: D

The carrier must move the sample without taking any part in the partition, so helium, nitrogen and hydrogen are used and oxygen is rigorously excluded. Solubility in the phase would destroy the separation, reactivity would change the analytes, and density is irrelevant. That the mobile phase contributes nothing chemically is the defining difference from liquid chromatography, where it contributes a great deal. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.