Gas Chromatography Questions and Answers

Answer: C

In a packed column molecules take paths of different lengths between the particles, so they arrive spread out. Axial diffusion and mass transfer are the other two terms, and gas compressibility is a separate correction. This term vanishes in an open capillary, which has only one path, and that absence is a large part of why capillary columns are so much more efficient.

Answer: D

Molecules diffuse along the column at a rate set by the gas, so the faster the band moves, the less time it spends spreading and the smaller the contribution. Diffusion coefficients depend on temperature and gas identity rather than velocity, and neither compression nor absorption is involved. A term inversely proportional to velocity is what makes the plate height fall at low flow, and it is why a column left standing under flow loses resolution.

Answer: A

Partition takes time, and at high velocity a molecule is swept along before it has settled into equilibrium, so some molecules run ahead and others lag. Phase stripping, turbulence and pressure drop are not the mechanism. A term proportional to velocity, opposing one inversely proportional to it, is exactly what produces a minimum, and the balance point is the optimum flow. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: B

Hydrogen has the highest diffusivity, which both lowers the mass transfer term and flattens the curve, so a column can be run well above its optimum with little loss of efficiency and a much shorter analysis. Nitrogen gives the lowest minimum but a sharp curve, and helium sits between. Flatness matters more than depth when speed is wanted, which is why hydrogen has displaced helium wherever its flammability can be managed. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: C

Run exactly at its optimum, nitrogen delivers the highest efficiency of the three, which suits a difficult separation where time is not pressing. Its low diffusivity is precisely why the optimum is sharp and the analysis slow, and the detector imposes no such requirement. Choosing between a deep narrow minimum and a shallow broad one is a choice between efficiency and speed, and the separation's difficulty decides it. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: D

Resolution is twice the difference in retention times divided by the sum of the two base widths, so both how far apart the peaks are and how broad they are matter equally. Height and analysis time do not enter. Two peaks well separated but very broad are no better resolved than two close sharp ones, which is why efficiency and selectivity must both be considered when a separation is being improved. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: A

Applying the standard expression:

\[ R_{s} = \frac{2(320-300)}{8+9} \approx 2.4 \]

giving about 2.4, comfortably above the 1.5 usually taken as baseline separation. Omitting the factor of two gives 1.2, which would wrongly suggest a marginal separation. A figure above 1.5 means the peaks may be integrated independently, which is the practical reason the threshold matters.

Answer: B

At 1.5 two equal peaks are separated essentially to the baseline and may be integrated independently, which is what quantitation requires. A resolution of one leaves a noticeable overlap, and three is comfortable but usually costs unnecessary analysis time. A threshold chosen because it is where a practical consequence changes, rather than for mathematical neatness, is worth knowing as a number. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: C

A capillary is an open tube whose inner wall carries a thin film, leaving the centre clear, while a packed column is filled with coated particles. The absence of a packed bed removes the eddy diffusion term entirely and permits far greater lengths at modest pressure. That one structural change removes a whole term from the van Deemter equation is why capillary columns displaced packed ones for nearly all work. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: D

An open tube has a far lower pressure drop per metre than a packed bed, so thirty or sixty metres is practical where a packed column is limited to two or three. Film thickness, temperature and sample capacity are consequences or separate matters. Length multiplies plate number directly, so removing the obstacle to length is worth as much as improving the plate height itself. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.