Liquid Chromatography Questions and Answers

Answer: A

Pressure drop varies as the inverse square of the particle diameter, so moving from five to three micrometres nearly triples it and to sub-two micrometres demands a wholly different instrument. Selectivity is a chemical property, the detector is unaffected, and bonding is the same. That an efficiency gain is paid for in pressure is what defined the boundary between conventional and ultra-high-performance instruments. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: B

Pressure varies inversely with the square of the particle diameter:

\[ \left(\frac{5}{3}\right)^{2} \approx 2.8 \]

giving about 2.8 times the original pressure at the same flow and length. A column already near the pump's limit therefore cannot simply be repacked with finer particles, which is why shorter columns accompany smaller particles in practice.

Answer: C

Since plate height falls with particle size, a shorter column gives the same plate number, and shortening it brings the pressure back within reach. Eluent consumption falls as a welcome side effect, selectivity is unchanged, and capacity falls rather than rising. Trading length against particle size to hold efficiency constant while managing pressure is the design logic behind every modern fast method. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: D

A non-porous core carrying a thin porous shell shortens the diffusion path without making the particle itself smaller, so efficiency approaches that of a much finer packing at a fraction of the pressure. The other descriptions do not correspond to a real packing. Getting most of a benefit while avoiding most of its cost is what made these particles so widely adopted so quickly.

Answer: A

Efficiency depends on how far a molecule must diffuse, which the thin shell makes small, while pressure depends on the overall particle diameter, which stays comparatively large. Surface area is actually lower, and neither bonding density nor pore width is the mechanism. Separating the two properties that a solid particle couples together is the whole idea, and it is a good example of a structural solution to a physical trade. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: B

Sub-two-micrometre particles demand a thousand bar or more, and every seal, fitting and pump head must be built for it. Wavelength range, temperature and injection volume are unchanged or reduced. That an incremental improvement in a column forced a complete redesign of the instrument around it is why the technique arrived as a new platform rather than as a new column.

Answer: C

A very efficient column produces peaks only a few microlitres wide, and a connection of comparable volume can undo much of the separation. Pump filling, detector saturation and equilibration are secondary. The better the column, the more the plumbing matters, which is why these instruments use finer tubing and smaller cells throughout rather than merely a stronger pump.

Answer: D

A resin carrying fixed charges retains counter-ions of the opposite sign, and they are displaced by raising the salt concentration or changing the pH. Size, polarity and volatility are the bases of the other modes. Because charge depends on pH, both the retention and the elution can be controlled through it, which makes the mode particularly suited to proteins and inorganic ions.

Answer: A

Competing ions displace the analyte from the fixed charges, so a salt gradient elutes in order of binding strength, and a pH change achieves the same by altering the analyte's own charge. Organic content is the reversed phase lever, temperature has a modest effect, and reversing the flow is not a technique. Eluting by competition rather than by solubility is what distinguishes the mode's method development entirely. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: B

Large molecules cannot enter the pores and pass through quickly, while small ones penetrate and take longer, so elution order is from large to small. Charge, polarity and affinity define the other modes. Because nothing is retained in the usual sense, the whole separation occurs within one column volume, which limits the peak capacity severely but makes the run time entirely predictable. Framing the question as which quantity the change acts upon makes the answer follow without calculation.