Question 1

How does HPLC separate components in a liquid mixture?

Accepted Answer

HPLC separates components based on their different interactions with a stationary phase. A liquid mobile phase is pumped through a column packed with small beads of stationary phase. Components interact differently with these beads and travel through the column at different rates, causing separation. The time each component takes to exit the column, called retention time, identifies the component.

Question 2

What is the difference between analytical and preparative HPLC?

Accepted Answer

Analytical HPLC identifies components in a small sample volume, which is then discarded as waste. Preparative HPLC purifies a mixture and collects a desired amount of each component in fractions for later use. Both use the same separation principle but differ in their purpose and sample handling after analysis.

Question 3

Why is degassing the mobile phase important in HPLC?

Accepted Answer

Degassing removes dissolved gases from the mobile phase before it enters the column. Bubbles in the mobile phase can create voids in the stationary phase or travel to the detector cell, causing measurement instability. Filtering the mobile phase through a membrane filter under vacuum effectively removes gases and solid particulates that could clog the column.

Question 4

What does the chromatogram tell you about separated components?

Accepted Answer

A chromatogram plots retention time versus signal intensity. The retention time identifies each component, while the peak area quantifies the amount of compound in the original solution. By correlating peak areas to known concentrations using calibration curves principles and applications, you can determine unknown sample concentrations.

Question 5

What is reversed-phase chromatography and when is it used?

Accepted Answer

Reversed-phase chromatography uses a hydrophobic stationary phase, typically silica beads with C18 chains bonded to the surface, paired with a polar mobile phase of water and organic solvent like acetonitrile. Components elute in order of decreasing polarity. This is the most common HPLC mode and works well for water-soluble samples.

Question 6

How does gradient elution improve HPLC separation?

Accepted Answer

Gradient elution changes the mobile phase ratio linearly or stepwise during separation, increasing the organic solvent concentration over time. This prevents peak broadening of less polar components that occurs with constant isocratic conditions, thereby improving separation quality and shortening elution time for complex mixtures.

Question 7

Why are smaller stationary phase beads used in HPLC compared to column chromatography?

Accepted Answer

HPLC operates at higher flow rates and pressures than classical column chromatography, enabling the use of smaller stationary phase beads. These micro-scale beads have a greater surface area to volume ratio, which greatly increases interaction between the stationary phase and sample components, improving separation efficiency and resolution.

כרומטוגרפיה נוזלית בעלת ביצועים גבוהים (HPLC) - חידון הערכה