Gas Chromatography Questions and Answers

Answer: C

Mass transfer in the stationary phase happens inside the column and is one of the van Deemter terms, not an extra-column effect. The other three broaden a band after or before the separation and are not improved by a better column. Distinguishing what the column does from what the plumbing does is what tells you whether a new column would actually help.

Answer: D

A larger internal diameter carries a thicker and longer band of stationary phase, so more analyte can dissolve in it before the partition isotherm curves and the peak fronts. Velocity, plate height and temperature are separate or work the other way - a wide column is less efficient. Trading efficiency for capacity is the bore decision, and a dirty or concentrated sample usually settles it in favour of the wider column. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: A

With little analyte present the priority is resolution rather than capacity, so a narrow column with a thin film gives the sharpest peaks and the lowest detection limit. Wide bores and thick films buy capacity that a dilute sample does not need, and a packed column is far less efficient. Matching the column's capacity to the sample's concentration, rather than always choosing the most robust, is what gets the last factor of two in sensitivity.

Answer: B

Retention depends on how much stationary phase there is to partition into, so halving the film roughly halves the capacity factor. Carrier velocity is set by the pressure, temperature is a separate control, and diffusion out of a thin film is quicker rather than slower. That film thickness moves retention and capacity together, while affecting efficiency in the opposite direction, makes it one of the more consequential choices in specifying a column.

Answer: C

A peak must exceed about three times the noise before it can be said to be present, and ten times before it is quantified, so noise sets the detection limit directly. Retention, capacity and resolution depend on other things. Expressing the smallest measurable amount as a multiple of the noise rather than an absolute figure is what makes detection limits comparable between instruments and days. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: D

Impurities in the hydrogen or air, or deposits on the jet, produce a fluctuating current that appears as noise and raises the detection limit. Column length, ramp rate and sample concentration affect the chromatography rather than the detector's stability. That a detection limit can be spoiled by the purity of a gas rather than by anything in the instrument's design is a reminder that consumables are part of the method. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: A

Stationary phase degradation accelerates with temperature, so the bleed and hence the baseline climb steadily as the programme proceeds. Detector temperature is controlled separately, flow is regulated, and injector decomposition would give peaks rather than a slope. Because the rise is reproducible, it can be subtracted by running a blank programme - which is exactly what a mass spectrometer's background subtraction does. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: B

Fractions eluting from a first column are transferred in turn onto a second of quite different selectivity, so compounds unresolved on one are separated on the other. Two detectors, two samples and two programmes describe other arrangements. Multiplying the peak capacity by separating on two independent properties is what makes very complex mixtures such as petroleum tractable.

Answer: C

If both phases retained compounds in the same order, the second column would simply re-perform the first separation and add nothing. Length, carrier gas and oven programming are practical details. The gain comes entirely from the two dimensions being independent, which is why a non-polar column is typically paired with a polar one rather than with another of the same kind.

Answer: D

A modulator collects each narrow slice of the first column's effluent, focuses it and injects it rapidly onto the second, many times a minute. Detector switching, gas splitting and cooling are not its function. Because the second separation must finish before the next slice arrives, that column is very short and very fast, which is the engineering constraint the whole technique turns on.