Liquid Chromatography Questions and Answers

Answer: C

The plate number has fallen below its limit, which points at a degraded or partly blocked column, and the batch must not proceed until it is resolved. Passing two criteria out of three is not passing, wavelength does not affect efficiency, and a larger injection would reduce the plate count further. A suitability test that is overridden when it fails serves no purpose at all, which is the point of setting the limits in advance. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: D

This is a system suitability test. Validation establishes a method's performance once, qualification demonstrates that an instrument is installed and operating correctly, and calibration relates a response to a concentration. Four activities on four different timescales, and using the right term matters because a regulator will ask which of them was performed.

Answer: A

Extraction does nothing for the column's efficiency, which is a property of the packing and the flow. The other three are its genuine purposes, and solvent exchange in particular matters when a sample in a strong solvent would otherwise distort the early peaks. Listing what a preparation step does and does not achieve is what prevents it being relied on for something it never addressed.

Answer: B

A few degrees shift retention times measurably and change the eluent's viscosity and hence the back pressure, so an uncontrolled column drifts with the laboratory. Detector response, pump capability and phase stability are far less sensitive. Controlling a variable that affects both the separation and the pressure is worth an oven, and it is why an unheated column is now unusual in routine work.

Answer: C

Viscosity falls markedly with temperature, so the same flow meets less resistance and the pressure drops - often by a third over thirty degrees. Expansion of eluent and packing is negligible, and the pressure certainly does change. Using temperature to buy pressure headroom is one way to run finer particles on an ordinary instrument, though the retention shift must then be accommodated in the method. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: D

Retention is an equilibrium, and raising the temperature generally shifts it towards the mobile phase, so compounds elute sooner. The pump maintains the set flow whatever the viscosity, plate number usually improves rather than falling, and detector timing is unaffected. That temperature is a selectivity lever as well as a retention one, since different compounds respond differently, makes it worth screening alongside pH and organic modifier. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: A

Hydrolysis of the siloxane bonds accelerates with temperature, so a column run hot at an unfavourable pH may last weeks rather than months. The eluent stays liquid under pressure, settling is prevented by the packing procedure, and the cell is thermostatted separately. Every gain from temperature is paid for in column life, and the exchange rate depends steeply on the pH chosen.

Answer: B

A collapsed or eroded bed lets the sample spread before the separation begins, so peaks emerge doubled or grossly misshapen while retention stays roughly right. Rising pressure indicates a blocked frit, falling retention indicates phase loss, and noise is a detector matter. Because the symptom is a peak shape rather than a number, it is recognised at a glance once it has been seen once.

Answer: C

The packed bed is held together partly by the liquid within it, and drying can shrink and fracture it irreversibly. Oxidation is slow, bacterial growth is a reason to avoid pure water rather than to store wet, and settling is a consequence of drying rather than a separate cause. Storing in the recommended solvent, usually acetonitrile and water without buffer, is a small discipline that determines how long a column lasts. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: D

As solvent evaporates from the ends the salt crystallises, blocking frits and scoring pump seals when the column is next used. Bacterial growth is a real but slower problem, dissolution needs extreme pH, and selectivity does not change on standing. Flushing with buffer-free eluent before storage is a two-minute step that prevents a whole class of expensive faults.