Chromatography Techniques

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Questions and Answers

What does the number of theoretical plates (N) in a chromatography column indicate?

The length of the column

The flow rate of the mobile phase

The particle size of the stationary phase

The ability of the column to separate two compounds with small differences in retention (correct)

What is the relationship between the plate height (H) and the number of theoretical plates (N) in a chromatography column?

H is equal to N

H is inversely proportional to N (correct)

H is directly proportional to N

H is independent of N

According to the rate theory of chromatography, which of the following factors contributes to peak broadening?

Mobile phase mass transfer and particle size

Equilibrium between stationary and mobile phases

Length and diameter of the column

Eddy diffusion and stagnant mobile phase mass transfer (correct)

Which of the following is NOT a factor affecting the plate height (H) in a chromatography column?

Temperature of the column Signup and view all the answers

What is the assumption behind the plate model in chromatography?

Equilibrium is infinitely fast Signup and view all the answers

What is the relationship between the number of theoretical plates (N) and the separation of two compounds with small differences in retention?

N is directly proportional to the separation Signup and view all the answers

What is the unit of the plate height (H) in a chromatography column?

Length units (e.g. meters, centimeters) Signup and view all the answers

What is the difference between the plate model and the rate theory of chromatography?

The plate model takes into account the time taken for the solute to equilibrate between the stationary and mobile phase, while the rate theory does not Signup and view all the answers

What is the primary basis for dividing chromatographic techniques into different types?

Type of mobile phase used Signup and view all the answers

Which of the following is an example of a liquid chromatography method?

Liquid chromatography Signup and view all the answers

What is the purpose of analytical chromatography?

To determine the chemical composition of a sample Signup and view all the answers

What is a characteristic of packed columns?

They are packed with tiny particles Signup and view all the answers

Which of the following is an example of a gas chromatography method that uses a liquid-coated support?

Gas-liquid chromatography Signup and view all the answers

What is the purpose of preparative chromatography?

To purify and collect one or more components of a sample Signup and view all the answers

What is the characteristic of capillary columns?

They have a small diameter tube coated on the inside with stationary phase Signup and view all the answers

Which of the following is an example of a chromatography method that uses a supercritical fluid mobile phase?

Supercritical fluid chromatography Signup and view all the answers

What is the primary factor that determines the retention of solutes in chromatography?

The strength of the solute's interaction with the mobile and stationary phases Signup and view all the answers

What is the term for the volume of the mobile phase that it takes to elute a non-retained component?

Void volume Signup and view all the answers

What is the formula for calculating the capacity factor (k')?

k' = (tR - tM)/tM Signup and view all the answers

What is the purpose of the capacity factor (k') in chromatography?

To compare results obtained on different systems Signup and view all the answers

What is the fundamental definition of the capacity factor (k')?

k' = moles Astationary phase / moles Amobile phase Signup and view all the answers

What is the relationship between the capacity factor (k') and the strength of the interaction between a solute and the stationary and mobile phases?

k' is directly proportional to the strength of the interaction Signup and view all the answers

What is the benefit of using the retention volume instead of the retention time in chromatography?

It allows for easier comparison between different systems Signup and view all the answers

What is the purpose of having a sufficiently narrow width of the solute peaks in chromatography?

To improve the efficiency of the separation system Signup and view all the answers

What is the formula to calculate the retention factor (kA)?

kA = K AV S / V M Signup and view all the answers

What is the definition of the selectivity factor (α)?

α = kB / kA Signup and view all the answers

What is the formula to calculate the relative retention time (RRT)?

RRT = tR / tRs Signup and view all the answers

What is the formula to calculate the column efficiency (N)?

N = L / H Signup and view all the answers

What is the unit of the plate height (H)?

micrometers Signup and view all the answers

What is the relationship between the retention factor (kA) and the retention time (tR)?

tR = kA × tM Signup and view all the answers

What is the definition of the variance per unit length (σ²/L)?

σ²/L = H / L Signup and view all the answers

What is the formula to calculate the adjusted retention time (tR - tM)?

tR - tM = kA × tM Signup and view all the answers

What is the primary advantage of using peak area over peak height in quantitative chromatography?

Peak area is independent of retention time and peak width Signup and view all the answers

What is the primary purpose of using an internal standard in chromatography?

To correct for instrumental drift in detector response Signup and view all the answers

What is the major limitation of using peak height for quantitation in chromatography?

It is dependent on retention time and peak width Signup and view all the answers

What is the primary advantage of using external standard calibration in chromatography?

It provides the best results when the unknown concentration is within the calibration range Signup and view all the answers

What is the purpose of calibrating a chromatography instrument using an external standard?

To prepare a calibration curve for quantitation Signup and view all the answers

What is the equation that relates the peak area ratio of the analyte to the internal standard to the concentration ratio?

Ax / As = F(Cx / Cs) Signup and view all the answers

What is the primary disadvantage of using manual integration in chromatography?

It is more time-consuming than auto integration Signup and view all the answers

What is the purpose of manually integrating peak areas in chromatography?

To determine the start and end points of peaks Signup and view all the answers

Study Notes

Chromatography Basics

Primary division of chromatographic techniques based on the type of mobile phase used:
- Gas chromatography (GC): gas mobile phase
- Liquid chromatography (LC): liquid mobile phase
- Supercritical fluid chromatography (SCFC): supercritical fluid mobile phase
Further divisions based on the type of stationary phase used:
- Gas-solid chromatography: solid, underivatized support
- Gas-liquid chromatography: liquid-coated support
- Bonded-phase gas chromatography: chemically-derivatized support

Purpose of Chromatography

Analytical: determine chemical composition of a sample
Preparative: purify and collect one or more components of a sample

Chromatography Columns

Packed Column: typical HPLC columns, packed with tiny particles
Capillary Column: typical GC column, small diameter tube coated with stationary phase

Factors Affecting Chromatography

Separation of solutes depends on:
- Difference in retention of solutes (time or volume of elution)
- Sufficiently narrow width of solute peaks (efficiency of separation system)
Retention time or volume directly related to strength of solute interaction with mobile and stationary phases

Theory of Chromatography

Solute Retention

Capacity factor (k'): measures solute retention, independent of column length and flow rate
k' = (tR - tM) / tM or k' = (VR - VM) / VM
k' is directly related to strength of interaction between solute and stationary and mobile phases

Efficiency

Theoretical plates (N): compare efficiencies of systems for solutes with different retention times
Plate height or height equivalent of a theoretical plate (H or HETP): relates column length to efficiency
H = L / N, where L = column length and N = number of theoretical plates
H is dependent on column length and kinetic processes that give rise to peak broadening

Plate and Rate Theories

Plate model: assumes infinitely fast equilibrium
Rate theory: takes into account time taken for solute to equilibrate between stationary and mobile phases
Factors affecting band spreading:
- Eddy diffusion
- Mobile phase mass transfer
- Stagnant mobile phase mass transfer

Selectivity Factor

Measures ability to separate two solutes, α > 1 if B is retained more than A
α = k_B / k_A, where k_B and k_A are retention factors for B and A, respectively

Column Efficiency

Theoretical plates (N): relates to column length and efficiency
Plate height or height equivalent of a theoretical plate (H or HETP): relates column length to efficiency

Quantitation in Chromatography

Integration

Auto integration or manual integration methods
Issues in integration:
- Noise peaks
- Start and ends of peaks
- Overlapping peaks

Peak Height vs. Peak Area

Peak area is independent of retention time and peak width, while peak height decreases with increased retention time and column overload
Peak height may be used when samples are close to detection limits or with manual injection

Calibration Methods

External standard: most common method, standards run separately and calibration curve prepared
Internal standard: useful with GC and manual injection, standard added to sample and calibration based on peak area ratio

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Description

This quiz covers the primary division of chromatographic techniques based on the type of mobile phase used and further divisions. It includes gas chromatography, liquid chromatography, and supercritical fluid chromatography.