Liquid Chromatography Questions and Answers

Answer: C

Combining the three terms:

\[ \begin{aligned} R_{s} &= \tfrac{\sqrt{10000}}{4} \cdot \tfrac{0.05}{1.05} \\ &\quad \cdot \tfrac{4}{5} \approx 0.95 \end{aligned} \]

giving about 0.95, short of the 1.5 wanted for baseline separation. The plate number is respectable and the capacity factor sits in its useful range, so the selectivity term of 0.05 over 1.05 is plainly what is doing the damage. Improving it by changing the organic modifier or the pH would cost nothing in time, whereas reaching the same resolution through efficiency alone would need four times the plate number and four times the run.

Answer: D

Only the selectivity term changes:

\[ \begin{aligned} R_{s} &= 25 \cdot \tfrac{0.10}{1.10} \cdot 0.8 \\ &\approx 1.8 \end{aligned} \]

giving about 1.8, comfortably resolved. A change from 1.05 to 1.10 has nearly doubled the resolution, where quadrupling the plate number would have been needed for the same gain - which is exactly why selectivity is the first lever reached for.

Answer: A

Selectivity depends on the chemistry - the organic modifier, the pH, the buffer and the bonded phase - and not on the column's dimensions. Length and particle size change efficiency, and flow rate changes efficiency and time. That the most powerful lever is a chemical one rather than a physical one is why method development begins with solvent screening rather than with hardware.

Answer: B

Band spreading is a random process, so a band's variance grows in proportion to the distance travelled and its width as the square root. Plate number rises with length rather than falling, the detector is not the limit, and selectivity is independent. A square-root return means doubling resolution costs four times the column, four times the time and four times the pressure - which is a poor bargain beside a change of solvent. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: C

Below about two the peaks crowd near the void volume where matrix components elute, and above about ten the analysis takes longer without improving resolution appreciably. The capacity term rises steeply at first and then flattens, which is exactly what makes the range useful rather than arbitrary. A lever with diminishing returns has a sensible operating window, and knowing where it lies saves a great deal of time. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: D

The term is k over one plus k, which approaches one asymptotically, so beyond about ten there is almost nothing left to gain while the analysis time grows without limit. Selectivity and plate number are not systematically affected, and detector response does not fall. Recognising that a term is saturating is what tells you to stop pushing it and move to another lever.

Answer: A

Isocratic means the mobile phase composition is held fixed from injection to the last peak. A changing composition is gradient elution, and constant temperature and pressure are separate conditions that usually apply either way. An isocratic method is simpler, re-equilibrates instantly and transfers between instruments more reliably, which is why it is preferred wherever the sample's range of retention allows. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: B

A sample spanning a wide range of polarity either crowds its early peaks or keeps its late ones for hours at any one composition, and ramping the organic content serves both. Pump, detector and column behaviour are separate concerns. This is the general elution problem, exactly as in gas chromatography where temperature programming answers it - the same difficulty solved by changing whichever variable that technique controls. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: C

After each run the column must be returned to the starting composition and given several column volumes to settle, which adds substantially to the cycle time. Gradient peaks are actually sharper than late isocratic ones, absorbance detectors work well provided the solvents are transparent, and dilution is unrelated. Re-equilibration is the hidden cost that makes a fast gradient less of a saving than the run time suggests. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: D

A compound that would sit near the head at the initial composition is pushed off as the eluent strengthens, so it travels most of the column as a compressed band rather than spreading over a long isocratic retention. Flow, detector response and temperature are held constant. The band compression that a gradient produces is a genuine benefit beyond solving the elution problem, and it is why gradients often give better peak shapes as well as shorter runs. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.