Liquid Chromatography Questions and Answers

Answer: C

These are core-shell particles, sometimes called superficially porous. A monolith is a single continuous rod rather than a packed bed, fully porous particles are the conventional kind, and polymeric packings are made from a different material altogether. Four packing constructions with four different pressure and efficiency characteristics, and the choice between them is usually decided by what pressure the instrument can deliver. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: D

A monolith is one continuous piece of porous silica or polymer with large flow-through channels and fine pores for retention, so the eluent passes readily while the surface area stays high. It contains no particles, no second phase and is not defined by its casing. Separating the flow path from the retentive surface is what lets a monolith run at high flow with modest pressure.

Answer: A

The large through-pores let a monolith be run at several millilitres a minute where a packed column would exceed the pump's limit, so a separation can be made much faster. Efficiency is comparable rather than superior, silanols remain, and capacity is lower. Buying speed through a structural change rather than through pressure is the alternative route to the same end, and it suits an instrument that cannot reach high pressure.

Answer: B

A five-minute gradient followed by five minutes of re-equilibration is a ten-minute cycle, so the saving is half what the run time suggests. Detector settling and pump response are quick, and no cooling is required. Counting the cycle rather than the run is what makes a throughput comparison honest, and it is why a shallow gradient on a fast column sometimes beats a steep one on a slow column.

Answer: C

Roughly ten column volumes restores the initial composition throughout the packing, though a strongly retained modifier can need more. One volume merely displaces the liquid without re-establishing the equilibrium on the surface, and a hundred would waste a great deal of time and solvent. Knowing the figure as a number of volumes rather than as a time lets it be scaled correctly when the column or the flow changes.

Answer: D

An additive such as an ion-pairing reagent coats the surface and comes off only slowly, so the column's selectivity drifts for many volumes after the gradient returns. Viscosity, reaction and void volume are not the mechanism. That an additive improving one separation can make a column difficult to use for anything else afterwards is why such reagents are often confined to a dedicated column.

Answer: A

A long-chain ion of opposite charge pairs with the ionised analyte and lends it enough hydrophobic character to be retained on a C18 column. Buffering, chelation and absorbance suppression are other additives' functions. Making an analyte behave as though it had a property it lacks, rather than changing the column to suit it, is the technique - and the slow desorption is the price paid.

Answer: B

A polar stationary phase with a mostly organic eluent retains polar and ionic compounds without any additive, so the column remains usable for other work and the eluent suits mass spectrometry. Normal phase, size exclusion and hot reversed phase do not address the problem. Replacing an additive with a different mode avoids the contamination the additive causes, which matters most when a mass spectrometer is downstream.

Answer: C

A non-volatile ion-pairing reagent competes for charge in the electrospray droplet and contaminates the source, so the analyte's signal falls sharply. Absorbance is an issue for an optical detector rather than a spectrometer, viscosity is unremarkable, and no reaction occurs. That an additive solving a chromatographic problem creates a detection one is exactly the coupling between the two halves of a hyphenated method that has to be thought through in advance.

Answer: D

The first two are correct and the third is the reverse: pressure varies inversely with the square of the particle diameter, so finer particles demand far more of it. That is precisely why monoliths and core-shell particles exist - both offer a route to speed that does not require a thousand-bar instrument. Getting the direction of the pressure relationship wrong would invert the whole logic of column selection.