Gas Chromatography Questions and Answers

Answer: A

More stationary phase holds more sample before overloading and retains each compound longer, which suits very volatile analytes. Efficiency falls rather than rises, because the mass transfer term grows with film thickness, and analyses take longer. Trading efficiency for capacity and retention is the film thickness decision, and the right value depends on whether the analytes are too volatile or the sample too concentrated. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: B

At high temperature the stationary phase slowly degrades and its fragments reach the detector, raising the baseline as the oven is programmed upward. Leaks, carry-over and back-diffusion are separate faults. Because bleed rises steeply near a phase's maximum temperature, the rated limit is a real constraint rather than a cautious suggestion, and exceeding it shortens the column's life as well as spoiling the baseline. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: C

Siloxane fragments from a bleeding column appear as persistent background ions and gradually coat the source, reducing sensitivity. There is no flame, electrons are not absorbed appreciably, and a capillary's flow is well within pumping capacity. Low-bleed columns exist specifically for this pairing, which is a good example of one component being redesigned because of what it does to another. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: D

Polydimethylsiloxane is essentially non-polar and separates non-polar compounds largely in order of boiling point, which is why it is the default first choice. Polyethylene glycol and cyanopropyl phases are polar and retain polar compounds selectively. Matching the phase's polarity to the analytes' is the principle, and like dissolves like remains the most useful guide in choosing one. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: A

A polar phase retains hydrogen-bonding and dipolar compounds far more than their boiling points alone would suggest, so elution order departs from volatility. Molecular weight, carbon count and diffusivity are not the mechanism. That changing the phase changes the elution order, while changing the temperature or the length does not, is why selectivity is the lever reached for when two peaks refuse to separate. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: B

Enantiomers are identical in every ordinary physical property, so only a stationary phase that is itself chiral interacts with them differently and separates them. Polarity, volatility and gas separations are handled by ordinary phases. That a separation impossible by any achiral means becomes routine with the right phase is a striking illustration of selectivity doing what efficiency cannot. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: C

A short length of uncoated tubing catches non-volatile residues that would otherwise accumulate at the head of the expensive analytical column, and is cut back or replaced as it fouls. It adds negligible efficiency, does no cooling, and splitting happens in the injector. Sacrificing a cheap component to protect an expensive one is a standard economy, and it is why dirty samples are tolerable at all. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: D

A split injector vaporises the sample and passes only a small fraction to the column, discarding the rest, which prevents a capillary from being overloaded by a concentrated sample. It uses one column, one injection and produces one peak per compound. Deliberately throwing away most of a sample seems wasteful until the alternative - a badly overloaded and distorted peak - is considered. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: A

Sending the whole sample onto the column suits trace work, where discarding ninety-nine per cent of it would put the analytes below detection. A concentrated sample would overload the column, packed columns take direct injection, and thermal lability calls for on-column injection. Which injection mode to use is decided by concentration first and by thermal stability second, and the two questions are quite separate. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: B

If the needle and liner do not vaporise every component equally, the less volatile ones are under-represented on the column and the composition reported is wrong. Detector response, irreversible retention and split variability are separate problems. Because discrimination distorts a quantitative result without distorting the chromatogram's appearance, it is detected only by comparing against a standard of known composition. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.