Gas Chromatography Questions and Answers

Answer: A

Each compound spends part of its time dissolved in or adsorbed on the stationary phase and part moving with the carrier, and the proportion decides how long it takes to emerge. Molecular weight and boiling point influence that partition without being the mechanism, and the carrier gas is chosen precisely because nothing dissolves in it. That separation depends on an equilibrium rather than on any single molecular property is what allows compounds of similar boiling point to be resolved.

Answer: B

Dead time is how long the carrier gas itself takes to traverse the column, measured with a compound that does not interact with the stationary phase. Injection intervals, detector response and oven equilibration are other quantities. Because every retention time contains this transit as a floor, subtracting it is what converts a raw time into a measure of how strongly a compound is actually retained. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: C

The capacity factor is the adjusted retention time divided by the dead time, so it counts how many times longer a compound stays than the carrier does. Sample capacity, plate number and the ratio between two compounds are separate quantities, the last being selectivity. Expressing retention relative to the dead time makes it a property of the chemistry rather than of the flow rate, which is why it transfers between instruments. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: D

Subtracting the dead time before dividing:

\[ k = \frac{300-60}{60} = 4 \]

giving 4, meaning the compound spends four times as long in the stationary phase as in the mobile. Five would follow from forgetting to subtract, and 240 is the adjusted time in seconds. A value between two and ten is usually the target, since below that resolution suffers and above it the analysis simply takes longer.

Answer: A

Selectivity is the ratio of the two capacity factors, so a value of one means the two compounds are indistinguishable however good the column is. Peak count, detector response and peak sharpness are separate ideas. Because no amount of efficiency can separate compounds with a selectivity of one, changing the stationary phase is the only remedy in that case - which makes selectivity the first thing to check when a separation fails. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: B

Plate number is computed from a peak's retention time and its width, and a large value means narrow peaks and a sharp separation. It is a figure of merit rather than a physical count, borrowed from distillation theory where the plates were real. Peak capacity and coating layers are different things. That an imaginary quantity from another technique became the standard measure of column quality is a historical accident worth knowing. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: C

Applying the standard relation for a base-width measurement:

\[ N = 16\left(\frac{t_{R}}{w}\right)^{2} = 16(30)^{2} = 14400 \]

giving 14400. The factor of sixteen belongs to the base width; a half-height measurement uses 5.54 instead. Quoting which width was used is essential, since the two constants differ by nearly threefold and a plate number without its method is meaningless.

Answer: D

Plate height is the length of column equivalent to one theoretical plate, so a small value means an efficient column. Inverting the ratio gives plates per metre, which is a related but distinct figure. Peak height and film thickness are quite different quantities. Expressing efficiency as a length lets columns of different lengths be compared directly, which a bare plate number cannot do. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: A

Dividing length by plate number:

\[ H = \frac{30 \ \mathrm{m}}{90000} = 0.33 \ \mathrm{mm} \]

giving 0.33 millimetres, typical of a good capillary column. A packed column might give ten times that. Converting to a length per plate is what makes a thirty-metre capillary and a two-metre packed column directly comparable.

Answer: B

The equation gives plate height as the sum of three terms, one falling with velocity and two rising, so a minimum exists at an optimum flow. Temperature, detector response and length dependence are described elsewhere. That an optimum velocity exists at all, rather than faster always being worse or better, is the equation's central message and the reason carrier flow is a tuned parameter rather than a convenience. The shared properties explain why these belong to one family, and the differences explain why more than one exists.