Liquid Chromatography Questions and Answers

Answer: A

Varying the pH, the organic content, the flow and the temperature slightly shows whether the method survives the ordinary variation of routine use. Range, intermediate precision and stability are separate parameters. A method that only works when every condition is exact will fail somewhere else, so deliberately disturbing it before release is what predicts whether it will transfer.

Answer: B

Silanol interaction is the usual cause, and protonating the silanols at low pH or using a column with fewer of them addresses it directly. Flow, injection volume and detector settings change other things. Because the mechanism is chemical rather than physical, the remedy must be chemical too, which is why altering the eluent or the column is tried before anything else.

Answer: C

Particulates from samples and from pump seals accumulate on the frit at the column head, restricting the flow while the separation itself remains acceptable at first. Worn seals reduce pressure by leaking, phase loss reduces retention, and a dirty cell affects the baseline. That a gradual pressure rise points at a filter rather than at the column proper is worth knowing before an expensive column is discarded. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: D

Hydrolysis at low pH or dissolution at high strips bonded chains from the silica, so there is less stationary phase and everything elutes sooner. A leak would lengthen retention by reducing flow, detector drift affects response, and concentration does not shift retention. Because the change is irreversible, recognising it early is what prompts a review of the eluent's pH before the next column goes the same way. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: A

The first two are correct and the third overstates: the capacity term approaches one asymptotically, so beyond about ten there is almost nothing left to gain while the run grows longer. Knowing that one lever saturates, another returns only a square root, and the third can transform a separation is what makes method development a matter of judgement rather than of trying everything in turn. Checking the units of each quantity catches a wrong formula before the arithmetic begins. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: B

The first three are correct and the fourth is not, so three match. Affinity chromatography exploits a specific biological interaction with an immobilised ligand, and volatility plays no part in any liquid chromatographic mode - it is the basis of gas chromatography instead. Each mode is named after the property it separates by, which makes the family easy to hold once that is noticed. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: C

This is the dwell volume, and it delays the gradient's arrival at the column. Void and dead volume both describe the liquid held within the column itself, and extra-column volume is everything outside it that broadens peaks. Three volumes with three distinct effects, and confusing the dwell volume with the others is the commonest reason a gradient method fails to transfer between instruments. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: D

A larger injection overloads the column and broadens or distorts the peaks, reducing resolution rather than improving it. The other three act on efficiency or selectivity. Because the resolution equation contains no term for sample amount, any change to it can only hurt - which is why injection volume is minimised subject to detecting the analyte.

Answer: A

Anything larger than the packing's interstices lodges on the frit and raises the back pressure, eventually making the column unusable. Detection, concentration and degassing are unaffected by filtration. A two-minute step that protects a component costing several hundred pounds is among the best returns available in the laboratory, and skipping it is the commonest cause of a prematurely blocked column. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: B

Passing the sample through a small bed retains the analyte while interferences wash through, and eluting into a small volume concentrates it at the same time. Isomer separation is the column's job, derivatisation is a chemical step, and degassing is unrelated. Achieving two objectives in one operation is why the technique so often precedes a difficult determination.