Liquid Chromatography Questions and Answers

Answer: A

In reversed phase the packing is non-polar and the eluent is polar, so polar compounds elute first and non-polar ones are retained - the opposite of the original normal phase arrangement on bare silica. The mobile phase is liquid in both, and nothing flows backwards. The name records only that the polarity relationship was inverted from what came first, which is worth knowing because the word suggests something far stranger than it is. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: B

Pharmaceuticals, peptides and environmental analytes are mostly polar and water-soluble, and water-organic eluents dissolve them, run reproducibly and suit mass spectrometric detection. Column cost, efficiency and pressure are comparable across modes. That a mode won because its solvents happened to suit the samples people actually had, rather than for any theoretical superiority, is the honest explanation. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: C

Octadecylsilane, universally written as C18, is bonded to silica particles and provides the non-polar surface. Bare silica is the normal phase packing, alumina is little used now, and polymer beads serve ion exchange and size exclusion. That one packing chemistry accounts for the large majority of all separations makes it the sensible starting point for any method development.

Answer: D

Silica resists the hundreds of bars a packed bed of small particles demands, where a soft polymer would compress. It is emphatically not inert at extremes of pH, dissolving above about eight and hydrolysing below two, and neither transparency nor swelling is relevant. That the support's mechanical strength is what recommends it, while its chemistry is the source of most of the technique's difficulties, is a trade worth stating. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: A

Not every silanol can be bonded, and those remaining are weakly acidic and interact strongly with amines, drawing out the trailing edge of their peaks. Pressure follows from particle size, non-polar retention comes from the bonded chains, and eluent absorbance is a solvent property. End-capping with a small silane and working at low pH both address it, which indicates how persistent a nuisance it has been. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: B

A short silane such as trimethylsilyl reaches silanols that the bulky octadecyl chain could not, reducing the tailing of basic compounds substantially. It has nothing to do with the column's physical ends, adds no second phase, and does not alter the pores. Treating the residue of an incomplete reaction with a smaller reagent is a neat chemical solution to a chromatographic problem. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: C

Incorporating organic groups into the particle extends the usable pH from roughly two to eight up to one to twelve, which allows basic compounds to be run in their neutral form. Such particles cost more, give comparable pressure, and still carry a bonded phase. Extending the pH range matters because ionisation state controls retention so strongly, and being confined to a narrow window forces compromises elsewhere. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: D

A basic compound protonated at low pH becomes charged, far more soluble in the aqueous eluent, and elutes much earlier than its neutral form. The phase dissolves only outside its rated range, detectors are largely indifferent, and pumps handle acids routinely. Because retention can change several-fold across one pH unit near a compound's pKa, buffering the eluent is essential rather than optional. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: A

Resolution is the product of a term in the square root of the plate number, a term in the selectivity, and a term in the capacity factor. Flow, temperature and dimensions act through those three rather than appearing separately, and the detector plays no part. Separating the three levers is what makes method development systematic, because each responds to a different change and each gives a very different return. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: B

Because resolution depends only on the square root of plate number, quadrupling the efficiency merely doubles resolution, while a small change in selectivity can transform a separation. The capacity factor helps only while it is small. Detector sensitivity affects detection rather than separation. Knowing that one lever returns far more than the others is what stops a method developer buying a longer column when a different solvent would have sufficed. Most errors here come from carrying a result across from a neighbouring case where it does not hold.