MCAT Chemical and Physical Foundations of Biological Systems Flashcards: 5c Chromatography Techniques

What this deck covers

This deck focuses on 5c Chromatography Techniques, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Chemical and Physical Foundations of Biological Systems.

How to use these flashcards

Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

Flashcard 1: What does the dead time ($t_M$) represent in chromatography?

Answer: Time for an unretained species to pass through the column. It indicates the minimum time for any substance to traverse the column without retention, accounting for void volume.

Flashcard 2: What does the retention time ($t_R$) represent in gas or liquid chromatography?

Answer: Time from injection to the analyte peak maximum at the detector. It measures how long an analyte is retained in the column before reaching the detector, reflecting its interaction with the stationary phase.

Flashcard 3: Which component elutes first: one with higher affinity for mobile phase or stationary phase?

Answer: Higher affinity for the mobile phase elutes first. Greater interaction with the mobile phase allows the component to move faster through the column, resulting in earlier detection.

Flashcard 4: In normal-phase TLC on silica, which compound travels farther: less polar or more polar?

Answer: Less polar compound travels farther (higher $R_f$). In normal-phase setup, less polar solutes have weaker interactions with the polar stationary phase, allowing farther migration with the mobile phase.

Flashcard 5: What is the stationary phase in paper chromatography and TLC (thin-layer chromatography)?

Answer: A polar solid surface (cellulose paper or silica/alumina on a plate). These materials provide a hydrophilic surface that interacts with polar compounds via adsorption or partitioning.

Flashcard 6: What does baseline resolution between two chromatographic peaks indicate?

Answer: Two peaks are fully separated with minimal overlap at baseline. It signifies complete distinction between analytes, ensuring accurate quantification without interference.

Flashcard 7: State the formula for selectivity (separation factor) $\alpha$ between two analytes.

Answer: $\alpha = \frac{k'_2}{k'_1}$ with $k'_2 > k'_1$. It measures the relative retention of two analytes, where a value greater than 1 indicates potential for separation.

Flashcard 8: State the formula for the retention factor (capacity factor) $k'$ in chromatography.

Answer: $k' = \frac{t_R - t_M}{t_M}$. This ratio quantifies the time an analyte spends retained relative to the unretained time, indicating partition strength.

Flashcard 9: Identify which has larger $k'$: analyte with $t_R = 6\,\text{min}$ or $t_R = 3\,\text{min}$ if $t_M = 1\,\text{min}$.

Answer: Analyte with $t_R = 6\,\text{min}$ has larger $k'$. Longer retention time yields a higher capacity factor, as $k'$ increases with greater $(t_R - t_M)$.

Flashcard 10: What property most directly determines elution order in chromatography?

Answer: Relative affinity for stationary phase versus mobile phase. Components with greater affinity for the stationary phase are retained longer, while those preferring the mobile phase elute sooner.

Flashcard 11: Which condition commonly elutes bound proteins from an ion-exchange column: higher salt or lower salt?

Answer: Higher salt (increased ionic strength) elutes bound proteins. Elevated ionic strength competes with bound proteins for resin sites, disrupting electrostatic interactions to release them.

Flashcard 12: In cation-exchange chromatography, what type of analyte binds most strongly to the resin?

Answer: Positively charged analytes (cations) bind most strongly. Cation-exchange resins have negatively charged groups that attract and retain positively charged species more effectively.

Flashcard 13: What is the key separation principle of ion-exchange chromatography?

Answer: Separation by net charge via reversible binding to charged resin. Charged analytes bind electrostatically to oppositely charged sites on the resin, with elution controlled by ionic conditions.

Flashcard 14: In reversed-phase HPLC, which compound elutes first: more polar or more nonpolar?

Answer: More polar compound elutes first. Polar compounds interact more with the polar mobile phase, leading to weaker retention on the nonpolar stationary phase.

Flashcard 15: What is reversed-phase chromatography defined by regarding stationary and mobile phase polarity?

Answer: Nonpolar stationary phase with relatively polar mobile phase. This setup reverses the polarity gradient, enabling separation based on hydrophobicity rather than polarity.

Flashcard 16: Which change in normal-phase TLC generally increases $R_f$ values: more polar or less polar mobile phase?

Answer: More polar mobile phase increases $R_f$ values. Increased polarity enhances solute solubility in the mobile phase, reducing retention on the polar stationary phase.

Flashcard 17: Calculate $R_f$ if a TLC spot travels $3.0\,\text{cm}$ and the solvent front travels $6.0\,\text{cm}$.

Answer: $R_f = 0.50$. The value is the simple ratio of distances, independent of plate size, for consistent comparison across experiments.

Flashcard 18: State the formula for TLC retention factor $R_f$.

Answer: $R_f = \frac{\text{distance traveled by spot}}{\text{distance traveled by solvent front}}$. This ratio quantifies the relative migration of a compound compared to the solvent, reflecting its partitioning behavior.

Flashcard 19: Identify the detector commonly paired with GC for molecular mass information: FID or MS.

Answer: MS (mass spectrometry). MS provides fragmentation patterns and mass-to-charge ratios, enabling structural identification beyond FID's combustion-based detection.

Flashcard 20: Which sample property is most required for GC analysis: high volatility or high polarity?

Answer: High volatility and thermal stability are required for GC. Analytes must vaporize without decomposition at column temperatures for effective gas-phase separation.

Flashcard 21: What is the key separation principle of size-exclusion (gel filtration) chromatography?

Answer: Separation by size using porous beads that exclude larger molecules. Smaller molecules enter pores and take longer paths, while larger ones are excluded and elute faster.

Flashcard 22: In size-exclusion chromatography, which elutes first: larger or smaller molecules?

Answer: Larger molecules elute first. They are sterically excluded from pores, traveling a shorter path through the column compared to smaller molecules.

Flashcard 23: What is the key separation principle of affinity chromatography?

Answer: Specific ligand–target binding; elution by competitor or condition change. It exploits highly specific interactions for purification, with elution disrupting the complex without denaturing the target.

Flashcard 24: What distinguishes GC (gas chromatography) from LC (liquid chromatography) in the mobile phase?

Answer: GC uses an inert carrier gas; LC uses a liquid solvent. The gaseous mobile phase in GC suits volatile compounds, while liquid in LC allows broader analyte solubility.