Liquid Chromatography Questions and Answers

Answer: C

Every millilitre of volume between column and detector lets the separated bands spread again, undoing the resolution the column achieved. Pressure rating depends on wall thickness, pump capacity is a flow matter, and eluent consumption is set by the flow rate. That the plumbing can destroy a separation is why connections are made with the shortest possible lengths of the narrowest practical tubing. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: D

An absorbance detector is cheap, robust, sensitive and suits the great majority of pharmaceutical and biological analytes, which contain conjugated systems. Refractive index is universal but insensitive and incompatible with gradients, light scattering suits involatile analytes, and electrochemical detection is selective. A detector that suits most analytes most of the time, at low cost, is generally preferred to a universal one costing ten times as much. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: A

The signal is the difference in refractive index between sample and reference cells, and a changing eluent composition changes that difference enormously, so the baseline is unusable during a gradient. It is universal rather than selective, non-destructive, and needs no optical transparency. Being confined to isocratic work is a severe restriction, and it is why the detector survives mainly for sugars and polymers that nothing else sees. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: B

The eluent is nebulised and evaporated, leaving particles of anything less volatile, which scatter a light beam in proportion to their amount. Absorbance, fluorescence and charge are the bases of other detectors. Near-universality for involatile analytes, and compatibility with gradients, is what recommends it - at the cost of a response that is not linear and depends on particle size. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: C

Absorbance measures a small difference between two large intensities, while fluorescence measures emitted light against essentially nothing, so the noise behaves quite differently and detection limits fall by two or three orders of magnitude. Source, path length and range are not the mechanism, and its range is far narrower. Whether a measurement is a difference or an absolute reading decides how its noise scales, which is the whole advantage. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: D

Natural fluorescence is uncommon, so the technique either suits a fortunate analyte or requires a derivatisation step to attach a fluorophore. Gradients are fine, nothing is destroyed, and eluent transparency is easily arranged. Extraordinary sensitivity available only for a minority of compounds is the trade, and derivatisation extends the range at the cost of another chemical step. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: A

Phosphate and other involatile buffers deposit in the source as the solvent evaporates, suppressing ionisation and eventually blocking the interface. Electron absorption, viscosity and retention are not the problem. Ammonium formate and acetate are used instead, which constrains the pH range available and is one of the compromises that coupling to a spectrometer imposes on the chromatography. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: B

Something eluting with the analyte competes for charge in the electrospray droplet, so the analyte's signal falls without any change in the amount present. Mass filtering, flow limits and charge loss are separate matters. Because the suppression varies between samples of different matrix, an isotopically labelled internal standard is the only reliable correction - which is why such standards dominate quantitative work. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: C

A labelled analogue elutes at essentially the same time and experiences exactly the same suppression, so the ratio of the two signals is unaffected. Eluting elsewhere would defeat the purpose, higher efficiency is not wanted, and nothing is kept out of the source. Having a companion that shares the analyte's fate in every respect except mass is what makes the correction exact rather than approximate. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: D

With less distance to diffuse into and out of the particle, equilibrium is reached faster and the mass transfer term falls, so the plate height drops roughly in proportion to the particle diameter. Length is limited rather than extended by small particles, the flow is slower through a tighter bed, and film thickness is a surface property. Shortening the distance a molecule must diffuse is the most direct attack on the dominant band-broadening term. Framing the question as which quantity the change acts upon makes the answer follow without calculation.