Question 1

Analytical chemistry — gas chromatography In gas chromatography of volatile mixtures, components are separated primarily due to differences in which property?

Accepted Answer

Introduction / Context: Gas chromatography (GC) separates volatile analytes as they partition between a mobile gas phase (carrier gas) and a stationary phase (often a liquid film on capillary walls). Understanding the controlling physicochemical parameter explains retention and resolution. Given Data / Assumptions: Analytes are volatile or made volatile. Columns contain a stationary phase with which analytes interact. Carrier gas flow transports analytes through the column. Concept / Approach: The central parameter in GC is the distribution (partition) coefficient, K, which reflects the tendency of a compound to spend time in the stationary phase versus the mobile phase. Higher affinity for the stationary phase increases retention time; lower affinity elutes earlier. Step-by-Step Solution: Model elution as repeated partitioning events between phases.Relate retention time to partition coefficient: greater interaction → longer retention.Choose the option explicitly invoking partition coefficients. Verification / Alternative check: Temperature programming alters partition coefficients by changing analyte vapor pressures and interaction strengths, confirming that partitioning governs separation. Why Other Options Are Wrong: Conductivity (option b) is not the GC separation principle.Molecular weight (option c) can correlate with retention but is not the fundamental basis.Molarity of injection (option d) influences peak shape/overload, not intrinsic separation. Common Pitfalls: Assuming GC is size-exclusion or purely mass-based; neglecting temperature effects on partitioning. Final Answer: Partition coefficients between the mobile gas and stationary liquid/solid phase.

Question 2

Principle of ion-exchange chromatography Ion-exchange separations rely primarily on which interaction between solutes and the stationary phase?

Accepted Answer

Introduction / Context: Ion-exchange chromatography (IEX) is a workhorse for protein, peptide, and small ion separations. Understanding its governing interaction guides buffer choice, pH, and salt gradients. Given Data / Assumptions: Stationary phase bears fixed charges (e.g., sulfonate for cation exchange, quaternary ammonium for anion exchange). Analytes carry pH-dependent net charges. Mobile phase composition (salt, pH) tunes retention. Concept / Approach: The primary mechanism is Coulombic attraction: oppositely charged solutes bind to the stationary phase. Elution occurs by screening with salt (increasing ionic strength) or by changing pH to alter analyte charge. Step-by-Step Solution: Identify charge on stationary phase (cation or anion exchanger).Relate analyte net charge (function of pH) to binding strength.Select the option describing electrostatic attraction. Verification / Alternative check: Conductivity gradients cause predictable elution windows; pH titrations shift retention consistent with electrostatic binding theory. Why Other Options Are Wrong: Electrical mobility (option b) describes electrophoresis, not chromatography.Neutral adsorption (option c) fits normal-phase/adsorption chromatography, not IEX.Partitioning without charge (option d) fits partition chromatography, not IEX. Common Pitfalls: Confusing ion-exchange with capillary electrophoresis; ignoring the pH dependence of analyte charge. Final Answer: Electrostatic attraction between charged analytes and oppositely charged functional groups.

Question 3

Reversed-phase liquid chromatography (RPLC) In reversed-phase chromatography, how is the stationary phase typically characterized?

Accepted Answer

Introduction / Context: Reversed-phase liquid chromatography (RPLC) is the most widely used HPLC mode. Knowing that the stationary phase is hydrophobic clarifies why more hydrophobic analytes are retained longer and why mobile phases are water–organic mixtures. Given Data / Assumptions: Stationary phase is typically alkyl-bonded silica (C18, C8, C4). Mobile phase is relatively polar (water with methanol or acetonitrile). Elution strength increases with higher organic content. Concept / Approach: In RPLC, “reversed” refers to the polarity reversal relative to normal-phase chromatography: the stationary phase is non-polar and the mobile phase is polar. Hydrophobic interactions dominate retention. Step-by-Step Solution: Identify hydrophobic bonded phases (e.g., C18).Relate increased organic solvent to decreased retention of hydrophobic solutes.Select the option that explicitly states a non-polar stationary phase. Verification / Alternative check: Gradient runs from high water to high organic shorten retention of hydrophobes, consistent with a hydrophobic stationary phase. Why Other Options Are Wrong: Polar stationary phase (option b) describes normal-phase/HILIC, not reversed phase.Option c is imprecise; the stationary phase chemistry does not change between runs.Option d contradicts established RPLC principles. Common Pitfalls: Confusing RPLC with HILIC; assuming polarity depends only on solvent, not bonded phase. Final Answer: Non-polar (hydrophobic) surface such as C18-bonded silica.

Question 4

Gel filtration / size-exclusion chromatography relation For a solute in size-exclusion chromatography, which general relationship correctly gives the elution volume V in terms of void volume V0, distribution constant kD, and internal water volume Vi?

Accepted Answer

Introduction / Context: In size-exclusion (gel filtration) chromatography, molecules separate by their ability to access pore volume. The classic linear relationship between elution volume and distribution behavior is fundamental for estimating molecular size and calibrating columns. Given Data / Assumptions: V0 is the void volume (outside the beads). Vi is the internal solvent volume within the beads accessible to solute. kD is the distribution constant ranging from 0 (totally excluded) to 1 (fully included). Concept / Approach: The total volume sampled by a solute equals the volume outside beads plus a fraction of the internal volume it can enter. Hence, elution volume is V = V0 + kD * Vi. This relationship allows plotting V versus log(MW) using standards. Step-by-Step Solution: Recognize limiting cases: a very large solute has kD = 0 → V ≈ V0.For a very small solute, kD = 1 → V ≈ V0 + Vi (total accessible volume).Interpolate for intermediate sizes: V = V0 + kDVi.Select the expression consistent with these boundary conditions. Verification / Alternative check: Empirical calibration curves using proteins of known molecular mass produce linear plots when V (or Kav) is related to log(MW), supporting the stated formula. Why Other Options Are Wrong: V = V0 / Vi (option b) has incorrect units and ignores kD.V = V0 - kDVi (option c) would predict decreasing volume with greater inclusion, which is opposite of reality.V / V0 = kDVi (option d) mixes dimensions and is not the standard relationship. Common Pitfalls: Confusing V0 (outside beads) with Vi (inside beads); forgetting that increased pore access increases elution volume (later elution), not decreases it. Final Answer: V = V0 + kDVi.

Question 5

Planar chromatography principle Thin-layer chromatography (TLC) primarily operates via which chromatographic mechanism?

Accepted Answer

Introduction / Context: TLC is a rapid, inexpensive method for separating small molecules. Recognizing its underlying mechanism informs solvent selection and interpretation of Rf values. Given Data / Assumptions: The plate is coated with a polar adsorbent (silica gel or alumina). Mobile phase is a solvent or solvent mixture that ascends by capillarity. Analytes distribute between being adsorbed on the surface and moving with the solvent front. Concept / Approach: TLC relies on adsorption to a solid stationary phase, with competition by the mobile solvent for binding sites. More strongly adsorbed compounds move more slowly, yielding lower Rf values. Step-by-Step Solution: Identify stationary phase: polar solid (silica/alumina) → adsorption.Consider solvent strength: stronger eluent disrupts adsorption and increases Rf.Select the option that names adsorption chromatography. Verification / Alternative check: Switching from hexane to ethyl acetate (more polar) typically increases Rf for polar analytes, consistent with adsorption competition. Why Other Options Are Wrong: Partition chromatography (option a) better fits paper chromatography or certain bonded-phase systems.Electrical mobility (option b) describes electrophoresis, not TLC.None of the above (option d) is incorrect as adsorption is correct. Common Pitfalls: Assuming TLC is identical to paper chromatography; ignoring how solvent polarity affects adsorption strength. Final Answer: Adsorption chromatography on a polar solid such as silica or alumina.

Question 6

Hybrid separation technique A combination of paper chromatography with electrophoresis separates analytes based on which principles?

Accepted Answer

Introduction / Context: Some analytical methods intentionally blend mechanisms to increase resolution. Coupling paper chromatography (partition-based) with electrophoresis (field-driven migration) is a classic example used for amino acids, peptides, and small metabolites. Given Data / Assumptions: Paper acts as a hydrophilic support containing immobilized water. A solvent front can induce partitioning; an electric field can simultaneously or sequentially drive ionic movement. Analytes may differ in both polarity and charge. Concept / Approach: Paper chromatography relies on partitioning between water held in cellulose fibers (stationary) and a moving organic/aqueous solvent (mobile). Electrophoresis adds a second dimension: separation by electrophoretic mobility, which depends on charge and size in the buffer. Step-by-Step Solution: Identify partition component: distribution between immobilized water and mobile phase.Identify electrophoresis component: migration under electric field based on net charge.Select the option acknowledging both mechanisms. Verification / Alternative check: Two-dimensional separations (first electrophoresis, then chromatography, or vice versa) produce orthogonal patterns consistent with the two distinct principles. Why Other Options Are Wrong: Option a ignores the electrophoretic contribution.Option b ignores the chromatographic partitioning.Option d is wrong because a combined mechanism is indeed correct. Common Pitfalls: Assuming a single mechanism explains complex separations; overlooking the impact of pH on electrophoretic mobility. Final Answer: Both partition chromatography and electrical mobility of ionic species.

Chromatography Questions