Chromatography Zone Broadening and Resolution
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Questions and Answers

What is the formula for the Separation Factor (α)?

α = k’2/k’1

The Selectivity factor α indicates how well compounds will __________.

separate

What is the purpose of chromatography?

to separate or resolve compounds

What is resolution in chromatography?

<p>the separation or distance between two peaks</p> Signup and view all the answers

What factors affect resolution in chromatography?

<p>All of the above</p> Signup and view all the answers

What is the formula for calculating resolution (Rs) in chromatography?

<p>Rs = ¼ (α-1/α) (k/k+1) N½</p> Signup and view all the answers

Increasing ______ in chromatography tends to increase resolution.

<p>selectivity</p> Signup and view all the answers

What did Mikhail Tswett coin in 1906?

<p>chromatography</p> Signup and view all the answers

What is a good method for determining the identity of an accelerant at a suspected arson scene?

<p>chromatography</p> Signup and view all the answers

What technique is used to purify and collect components of a sample?

<p>chromatography</p> Signup and view all the answers

Which of the following statements is true about chromatography?

<p>Chromatography involves interactions with both a mobile phase and a stationary phase.</p> Signup and view all the answers

Liquid chromatography uses organic solvent as the mobile phase.

<p>True</p> Signup and view all the answers

Thin-Layer Chromatography separates based on ________________.

<p>mode of sorption</p> Signup and view all the answers

What is the main reason different compounds can be separated in gas chromatography?

<p>Interaction of the compound with the stationary phase</p> Signup and view all the answers

Which gases are commonly used in gas chromatography?

<p>All of the above</p> Signup and view all the answers

In gas chromatography, compounds are separated based on their boiling points, affected by polarity and weight. Lower boiling compounds are lightest in weight and least polar, traveling the fastest, spending more time in the mobile phase, and less time 'stuck' to the __________ phase.

<p>stationary</p> Signup and view all the answers

Integration of peaks in chromatography can give peak height, peak area, and peak width.

<p>True</p> Signup and view all the answers

What is the retention time of compound A?

<p>Unknown</p> Signup and view all the answers

What is the retention time of compound B?

<p>Unknown</p> Signup and view all the answers

Which compound is present in a larger amount?

<p>Unknown</p> Signup and view all the answers

Which compound has the lower boiling point?

<p>Unknown</p> Signup and view all the answers

What would happen to the retention times of compounds A and B if the column temperature were raised?

<p>Unknown</p> Signup and view all the answers

You suspect that compound B is octane. What can you do to provide supporting evidence for this hypothesis?

<p>Unknown</p> Signup and view all the answers

If they are run on a DEGS GC column, which compound will have the lowest retention time?

<p>Unknown</p> Signup and view all the answers

If they are run on a DEGS GC column, which compound will have the highest retention time?

<p>Unknown</p> Signup and view all the answers

Why is the internal standard calibration more common when using manual injection than injection with an autosampler?

<p>Unknown</p> Signup and view all the answers

What is the primary factor that determines the value of N in the equation N = L/H?

<p>Plate height</p> Signup and view all the answers

What is the effect of increasing α on the resolution Rs?

<p>It increases Rs</p> Signup and view all the answers

What is the significance of a resolution value of 1.0?

<p>The peaks overlap by about 4%</p> Signup and view all the answers

What is the term used to describe the maximum number of theoretical plates in a column?

<p>Number of theoretical plates</p> Signup and view all the answers

What is the effect of increasing N on the resolution Rs?

<p>It increases Rs</p> Signup and view all the answers

What is the formula for calculating N from a chromatogram?

<p>N = 5.54 (tr/W1/2)^2</p> Signup and view all the answers

What is the effect of increasing k on the resolution Rs?

<p>It increases Rs</p> Signup and view all the answers

What is the physical property that affects the separation of compounds in chromatography?

<p>Polarity</p> Signup and view all the answers

What is the primary mechanism responsible for zone broadening in chromatography, according to Rate Theory?

<p>Mass Transfer processes</p> Signup and view all the answers

What is the main limitation of Plate Theory in chromatography?

<p>It does not account for the mechanism causing peak broadening</p> Signup and view all the answers

What is the relationship between standard deviation in distance (σ) and standard deviation in time (τ) in chromatography?

<p>τ = L / tR</p> Signup and view all the answers

What is the equation used to determine the number of theoretical plates (N) in chromatography?

<p>N = 16 tR / W</p> Signup and view all the answers

Why does Gas Chromatography normally have high H but also high overall efficiency?

<p>Due to high flow rates</p> Signup and view all the answers

What is the main factor affecting column efficiency in chromatography?

<p>Flow rate of mobile phase</p> Signup and view all the answers

What is the relationship between plate height (H) and column length (L) in chromatography?

<p>H = L / 16</p> Signup and view all the answers

What is the effect of increasing the flow rate of the mobile phase on peak broadening in chromatography?

<p>It increases peak broadening</p> Signup and view all the answers

What is the primary assumption of the plate model in chromatography?

<p>Equilibrium is infinitely fast</p> Signup and view all the answers

What does the height equivalent of a theoretical plate (H) represent?

<p>The length of the column that corresponds to one theoretical plate</p> Signup and view all the answers

Which of the following is NOT a factor contributing to peak broadening according to the rate theory of chromatography?

<p>Sorption</p> Signup and view all the answers

What is the effect of increasing the number of theoretical plates (N) on a chromatographic column?

<p>Improved separation of solutes with small differences in retention times</p> Signup and view all the answers

How does the length of the column affect the number of theoretical plates (N)?

<p>N is directly proportional to the column length</p> Signup and view all the answers

What is the effect of increasing the height equivalent of a theoretical plate (H) on a chromatographic column?

<p>Increased peak broadening</p> Signup and view all the answers

Which of the following is a correct equation relating the number of theoretical plates (N) and the height equivalent of a theoretical plate (H)?

<p>H = L/N</p> Signup and view all the answers

What is the purpose of the plate model in chromatography?

<p>To compare the efficiencies of different chromatographic columns</p> Signup and view all the answers

What is the physical meaning of the plate height (H) in chromatography?

<p>The average distance a molecule travels in one diffusion step</p> Signup and view all the answers

What is the effect of increasing the column length (L) on the plate height (H) in chromatography?

<p>H decreases, resulting in better separation</p> Signup and view all the answers

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

<p>N is inversely proportional to H</p> Signup and view all the answers

What is the effect of mass transfer resistances on peak broadening in chromatography?

<p>Mass transfer resistances increase peak broadening</p> Signup and view all the answers

What is the relationship between the retention factor (k) and the distribution constant (K) in chromatography?

<p>k is directly proportional to K</p> Signup and view all the answers

What is the effect of increasing the flow rate (v) on the retention time (tR) in chromatography?

<p>tR decreases, resulting in better separation</p> Signup and view all the answers

What is the relationship between the column efficiency (N) and the resolution (Rs) in chromatography?

<p>N is directly proportional to Rs</p> Signup and view all the answers

What is the effect of peak broadening on the resolution (Rs) in chromatography?

<p>Peak broadening decreases Rs, resulting in poorer separation</p> Signup and view all the answers

What is the physical significance of k' in chromatography?

<p>It is a measure of the ratio of the amount of solute in the stationary and mobile phases at equilibrium.</p> Signup and view all the answers

Which of the following statements is true about peak broadening?

<p>It is a result of the rate of mass transfer between the stationary and mobile phases.</p> Signup and view all the answers

What is the relationship between column efficiency and peak width?

<p>A more efficient column produces narrower peaks.</p> Signup and view all the answers

What is the significance of the standard deviation (σ) in chromatography?

<p>It is a measure of the peak width.</p> Signup and view all the answers

What is the effect of increasing k' on the separation efficiency?

<p>It decreases the separation efficiency.</p> Signup and view all the answers

Which of the following is a measure of column efficiency?

<p>Number of theoretical plates (N).</p> Signup and view all the answers

What is the relationship between plate height (H) and column efficiency?

<p>A lower plate height indicates a more efficient column.</p> Signup and view all the answers

What is the effect of increasing the amount of time a solute spends in the column on the separation efficiency?

<p>It increases the separation efficiency.</p> Signup and view all the answers

What is the primary mechanism responsible for zone broadening in chromatography, according to Rate Theory?

<p>Eddy diffusion</p> Signup and view all the answers

What is the effect of increasing the number of theoretical plates (N) on a chromatographic column?

<p>Resolution (Rs) increases</p> Signup and view all the answers

What is the relationship between plate height (H) and column length (L) in chromatography?

<p>H is directly proportional to L</p> Signup and view all the answers

What is the main factor affecting column efficiency in chromatography?

<p>Particle size of the stationary phase</p> Signup and view all the answers

What is the effect of increasing the flow rate of the mobile phase on peak broadening in chromatography?

<p>Peak broadening increases</p> Signup and view all the answers

What is the primary assumption of the plate model in chromatography?

<p>The column is homogeneous</p> Signup and view all the answers

What does the height equivalent of a theoretical plate (H) represent?

<p>The minimum distance required for a solute to separate</p> Signup and view all the answers

Which of the following is NOT a factor contributing to peak broadening according to the rate theory of chromatography?

<p>Sample injection volume</p> Signup and view all the answers

What is the primary mechanism responsible for zone broadening in chromatography, according to the Rate Theory?

<p>Eddy diffusion</p> Signup and view all the answers

Which of the following mass transfer processes is responsible for peak broadening due to differences in the rate of diffusion of solute molecules between the mobile phase outside and inside the pores of the support?

<p>Stagnant mobile phase mass transfer</p> Signup and view all the answers

What is the effect of increasing the column length on the number of theoretical plates (N) in chromatography?

<p>It increases N</p> Signup and view all the answers

What is the relationship between plate height (H) and column length (L) in chromatography?

<p>H is directly proportional to L</p> Signup and view all the answers

What is the effect of increasing the flow rate of the mobile phase on peak broadening in chromatography?

<p>It increases peak broadening</p> Signup and view all the answers

What is the primary assumption of the plate model in chromatography?

<p>The plate is a hypothetical zone</p> Signup and view all the answers

What does the height equivalent of a theoretical plate (H) represent?

<p>The average distance a solute molecule travels between two consecutive equilibria</p> Signup and view all the answers

Which of the following is NOT a factor contributing to peak broadening according to the Rate Theory of chromatography?

<p>Stationary phase mass transfer</p> Signup and view all the answers

What is the primary mechanism responsible for zone broadening in chromatography, according to Rate Theory?

<p>Stagnant mobile phase mass transfer</p> Signup and view all the answers

Which of the following factors affects the degree of band-broadening due to stagnant mobile phase mass transfer?

<p>All of the above</p> Signup and view all the answers

What is the term used to describe the maximum number of theoretical plates in a column?

<p>Column efficiency</p> Signup and view all the answers

What is the relationship between plate height (H) and column length (L) in chromatography?

<p>H is directly proportional to L</p> Signup and view all the answers

What is the effect of increasing the flow rate of the mobile phase on peak broadening in chromatography?

<p>It increases peak broadening</p> Signup and view all the answers

What is the effect of increasing the number of theoretical plates (N) on a chromatographic column?

<p>It increases the resolution</p> Signup and view all the answers

Which of the following is NOT a factor contributing to peak broadening according to the rate theory of chromatography?

<p>Adsorption energy</p> Signup and view all the answers

What is the primary assumption of the plate model in chromatography?

<p>The column is divided into an infinite number of theoretical plates</p> Signup and view all the answers

What is the primary limitation of Plate Theory in chromatography?

<p>It cannot account for the mechanism causing peak broadening.</p> Signup and view all the answers

Which of the following processes contributes to zone broadening in chromatography, according to Rate Theory?

<p>Mass Transfer</p> Signup and view all the answers

What is the relationship between plate height (H) and column length (L) in chromatography?

<p>H is directly proportional to L</p> Signup and view all the answers

What is the effect of increasing the flow rate of the mobile phase on peak broadening in chromatography?

<p>Peak broadening increases</p> Signup and view all the answers

What is the relationship between standard deviation in distance (σ) and standard deviation in time (τ) in chromatography?

<p>σ is inversely proportional to τ</p> Signup and view all the answers

What is the primary assumption of the plate model in chromatography?

<p>The plate height is constant</p> Signup and view all the answers

What does the height equivalent of a theoretical plate (H) represent?

<p>The minimum distance required for separation</p> Signup and view all the answers

Why does Gas Chromatography normally have high H but also high overall efficiency?

<p>Because of the high flow rates used</p> Signup and view all the answers

What is the main reason for peak broadening in chromatography, according to the rate theory?

<p>Mass transfer effect</p> Signup and view all the answers

What is the relationship between the linear velocity of the mobile phase and the mass transfer effect?

<p>The faster the mobile phase moves, the more mass transfer effect</p> Signup and view all the answers

Which of the following is a consequence of the mass transfer effect on peak broadening?

<p>Some of the analytes are swept ahead of the main band</p> Signup and view all the answers

What is the purpose of the distribution constant (K) in chromatography?

<p>To describe the equilibrium between the mobile and stationary phases</p> Signup and view all the answers

What is the effect of increasing the column length (L) on the number of theoretical plates (N)?

<p>N increases</p> Signup and view all the answers

What is the effect of increasing the plate height (H) on a chromatographic column?

<p>The number of theoretical plates (N) decreases</p> Signup and view all the answers

Which of the following is a factor contributing to peak broadening, according to the plate theory?

<p>Longitudinal diffusion</p> Signup and view all the answers

What is the effect of increasing the flow rate of the mobile phase on peak broadening in chromatography?

<p>Peak broadening increases</p> Signup and view all the answers

Study Notes

History of Chromatography

  • Russian botanist Mikhail Tswett coined the term "chromatography" in 1906 to describe his experiments in separating different colored constituents of leaves.

Principles of Chromatography

  • Chromatography is a separation technique based on the different interactions of compounds with two phases: a mobile phase and a stationary phase.
  • Components of Chromatography: mobile phase, stationary phase, and supporting medium.
  • Chromatography separates components based on their interactions with the stationary and mobile phases.

Types of Chromatography

  • Gas Chromatography (GC): uses a gas as the mobile phase.
  • Liquid Chromatography (LC): uses a liquid as the mobile phase.
  • Supercritical Fluid Chromatography (SCFC): uses a supercritical fluid as the mobile phase.
  • Classification of Chromatography based on stationary phase:
    • Gas-Solid Chromatography: uses a solid stationary phase.
    • Gas-Liquid Chromatography: uses a liquid-coated stationary phase.
    • Bonded-Phase Gas Chromatography: uses a chemically-derivatized stationary phase.
  • Classification of Chromatography based on attractive forces:
    • Adsorption Chromatography: for polar non-ionic compounds.
    • Ion Exchange Chromatography: for ionic compounds.
    • Partition Chromatography: based on the relative solubility of analyte in mobile and stationary phases.
    • Size Exclusion Chromatography: based on the size of the analyte.

Column Chromatography

  • Packed Column: uses tiny particles as the stationary phase.
  • Capillary Column: uses a small diameter tube coated with the stationary phase on the inside.

Planar Chromatography

  • Thin-Layer Chromatography (TLC): uses a flat, uniform layer of stationary phase on a support.

Detectors

  • UV-Vis Detector: detects absorbance of light by the analyte.
  • Refractive Index (RI) Detector: detects changes in the refractive index of the mobile phase.
  • Mass Spectrometry (MS) Detector: detects the mass-to-charge ratio of ions.
  • Electrochemical (EC) Detector: detects the electrical signal generated by the analyte.
  • NMR Detector: detects the nuclear magnetic resonance signal of the analyte.

Theory of Chromatography

  • Chromatogram: a graphical representation of the detector signal vs. retention time or volume.
  • Retention Time (tR): the time taken for an analyte to elute from the column.
  • Void Time (tM): the time taken for a non-retained component to elute from the column.
  • Capacity Factor (k'): a measure of the retention of an analyte, calculated as k' = (tR - tM)/tM.
  • Efficiency (N): the number of theoretical plates in a column, calculated as N = (tR/σ)².
  • Plate Height (H): the length of column required for one theoretical plate, calculated as H = L/N.
  • Van Deemter Equation: a mathematical equation that relates the plate height to the linear velocity of the mobile phase.

Physicochemical Principles of Chromatographic Separation

  • Adsorption Chromatography: separates based on Van der Waals forces, electrostatic forces, hydrogen bonds, and hydrophobic interactions.
  • Partition Chromatography: separates based on differences in solubility in mobile and stationary phases.
  • Ion-Exchange Chromatography: separates based on differences in affinity of ions for the exchangers.
  • Size Exclusion Chromatography: separates based on the size of the analyte.
  • Affinity Chromatography: separates based on bioselective interactions between the analyte and the ligand.### Chromatography Theory
  • Partition Coefficient (K): ratio of the concentration of a solute in the organic phase to its concentration in the aqueous phase, represented by the equation K = Co/Cw, where Co is the concentration in the organic phase and Cw is the concentration in the aqueous phase.

Column Chromatography

  • Dilution and Peak Broadening: occur when the mobile phase moves too quickly, causing the analyte molecules to spread out and overlap, resulting in poor resolution.

Distribution Constant (K) and Retention Times

  • K: a measure of how much a solute is retained in the stationary phase, affecting its retention time.
  • Retention Time (tR): the time it takes for an analyte to pass through the column, influenced by the distribution constant (K) and the mobile phase velocity.

Velocities and Retention Times

  • Linear Rate of Solute Migration: the velocity of a solute is affected by the mobile phase velocity and the fraction of time spent in the mobile phase.
  • Velocity of Solute: influenced by the distribution constant (K) and the mobile phase velocity, represented by the equation v = μ × (1 + KVS/VM).

Selectivity Factor (α)

  • α: a measure of how well two analytes are separated, influenced by their respective distribution constants (K) and retention times, represented by the equation α = kB/kA.

Column Efficiency - Theoretical Plates

  • Theoretical Plates (N): a measure of column efficiency, influenced by the plate height (H) and column length (L), represented by the equation N = L/H.
  • Plate Height (H): a measure of the zone broadening, influenced by the mobile phase velocity (μ) and the plate height, represented by the van Deemter equation.

Zone Broadening

  • Zone Broadening: the spreading of the analyte zone as it moves through the column, influenced by the mobile phase velocity (μ) and kinetic processes.
  • Kinetic Processes: include multiple pathways, eddy diffusion, and longitudinal diffusion, which affect zone broadening.

Resolution

  • Resolution (Rs): a measure of the separation between two peaks, influenced by the retention times, peak widths, and column efficiency, represented by the equation Rs = 2(tR,B - tR,A)/(wB + wA).

Gas Chromatography

  • Injector: where the sample is introduced into the column.
  • Detector: where the separated compounds are detected.
  • Column Oven: where the column is heated to control the separation process.
  • Carrier Gas: the mobile phase in gas chromatography, which must be inert to prevent interference with the separation process.

Advantages and Disadvantages of Gas Chromatography

  • Advantages: only requires a small sample size, can analyze any compound that can be vaporized, is a fast and inexpensive method.

  • Disadvantage: is a destructive technique, meaning the sample is lost after analysis.### Gas Chromatography

  • Too hot temperatures can cause all compounds to vaporize and be carried rapidly through the column, resulting in no partitioning effect and thin, narrow peaks.

  • Too cool temperatures can cause compounds to condense more than they should, resulting in broad and overlapped peaks.

Types of Analysis

  • Qualitative Analysis: Identification of compounds using retention time (similar to Rf values for TLC).
  • Retention time: The amount of time it takes for a compound to travel through the column, from the point of vaporization to the detector.
  • Example: Compound X has a retention time of 3.1 minutes, Compound Y has a retention time of 4.8 minutes, and Compound Z has a retention time of 5.3 minutes.

Quantitative Analysis

  • Determination of relative amounts of each compound (ratio of compounds).
  • Step 1: Calculate the area under the peaks using the equation: Area = (height of peak) x (width at half the height).
  • Step 2: Calculate the Percent Composition (Relative Amounts) by dividing each individual area by the sum total of all areas x 100.

Internal Standard

  • A compound added to the sample in a known and constant concentration.
  • Ratio of its retention time to the retention times of the target compounds and ratio of its peak area to the peak areas of target compounds are determined for various concentrations of the target compounds.
  • Preferred technique because it corrects for errors in sample preparation and variations in the amount of sample injected.

External Standard

  • Known amounts of analytes are run in separate analyses, and the resulting peak areas are used to obtain calibrated response factors.
  • Concentrations of the analytes are calculated by applying the response factors obtained from the standard runs.

Quantitation in Chromatography

  • Detector Response: Depends on the concentration of the analyte or mass of the analyte reaching the detector.
  • Detector Noise: Present in all detectors, can be high or low frequency.
  • Ability to detect small quantities depends on the signal (peak height) to noise ratio.

Levels of Detection and Quantitation

  • Noise can have high and low frequency parts.
  • Ways of defining noise: peak to peak (roughly 5σ) or standard deviation (more accurate way).

Data Smoothing

  • Data should be digitized with a frequency ~20/peak width.
  • High frequency noise can be removed by filtering, but over-filtering can result in reduction of signal and loss of resolution.

Integration

  • Integration of peak should give peak height, peak area, and peak width.
  • Difficulty comes from determining if a peak is a peak (or just noise) and when to start and end the peak.
  • Can use auto integration or manual integration.

Integration Issues

  • Other issues in integration: start and ends to peaks, how to split overlapping peaks.

Peak Height vs. Peak Area

  • Reasons for using peak area: peak area is independent of retention time, peak area is independent of peak width.
  • Reasons for using peak height: integration errors tend to be smaller if samples are close to the detection limits.

Calibration Methods

  • External Standard: Most common method, standards run separately and calibration curve prepared.
  • Internal Standard: Useful for GC with manual injection, standard added to sample, and calibration and sample determination based on peak area ratio.

Recovery Standards

  • Principle of use is similar to standard addition, useful for determining losses during extractions, derivatization, and with matrix effects.

Additional Questions/Problems

  • Questions and problems related to retention times, GC and TLC, and chromatograph analysis.

Chromatography Efficiency

  • Higher α (selectivity) means larger differences in retention time and more separation between peaks.
  • Efficiency (N) is a factor used to describe peak width, with high efficiency corresponding to narrow peaks.

Number of Theoretical Plates (N)

  • N is calculated using the formula: N = L/H, where L is the column length and H is the plate height (both in the same units).
  • N can be measured from peaks on a chromatogram using the formula: N = 5.54 (tr/W1/2)^2, where tr is the retention time and W1/2 is the peak width at half height.
  • N is independent of solute retention and dependent on the length of the column.

Resolution (Rs)

  • Resolution is the distance between two peaks and is a function of three factors: retention (k), selectivity (α), and efficiency (N).
  • Rs can be calculated using the formula: Rs = ¼ (α-1/α) (k/k+1) N½.
  • Increasing α, k, or N increases Rs.

Column Efficiency (N) and Plate Height (H)

  • Plate height (H) relates to the length of the column that corresponds to one theoretical plate.
  • H is dependent on kinetic variables such as flow rate and particle size.
  • Rate theory of chromatography takes into account the time taken for solute equilibration between the stationary and mobile phase.

Selectivity Factor (α)

  • α is a measure of how well two peaks can be separated.
  • B retained more than A → α > 1.
  • Selectivity factor can be calculated using the retention factors kA and kB.

Retention Time and Column Length

  • Retention time (tR) is related to the column length (L) and standard deviation (σ): σ = L/tR.
  • Column length is related to the standard deviation (σ) and plate height (H): H = L/N.

Theoretical Plates (Plate Model) and Rate Theory

  • Plate model assumes infinitely fast equilibrium between mobile and stationary phases.
  • Rate theory takes into account the time taken for solute equilibration between phases.
  • Factors affecting peak broadening include eddy diffusion, mobile phase mass transfer, and stagnant mobile phase mass transfer.

Peak Width and Efficiency

  • Peak width (Wb) is related to efficiency, with narrower peaks indicating higher efficiency.
  • Peak width can be used to estimate standard deviation (σ) assuming a Gaussian-shaped peak.

Zone Broadening and Kinetic Processes

  • Zone broadening is related to kinetic processes, including eddy diffusion, mobile phase mass transfer, stagnant mobile phase mass transfer, stationary phase mass transfer, and longitudinal diffusion.
  • The Van Deemter Equation is H = A + B/μ + (CS + CM)μ, where H is the plate height, A is the multipath term, B is the coefficient of longitudinal diffusion, CS and CM are coefficients of mass transfer in the stationary and mobile phases, respectively, and μ is the linear velocity.

Eddy Diffusion

  • Eddy diffusion is a band broadening process that results from different path lengths passed by solutes through a packed column.
  • It is directly proportional to the diameter of the packing material.
  • Eddy diffusion is offset by ordinary diffusion.
  • Lower mobile-phase velocity leads to smaller eddy diffusion.

Mobile Phase Mass Transfer

  • Mobile phase mass transfer is a process of peak broadening caused by the presence of different flow profiles within channels or between particles of the support in the column.
  • It leads to band broadening due to differences in the rate of solute molecules moving through the column.

Stagnant Mobile Phase Mass Transfer

  • Stagnant mobile phase mass transfer is a band broadening process due to differences in the rate of diffusion of solute molecules between the mobile phase outside the pores of the support and the mobile phase within the pores.
  • The degree of band-broadening depends on the size, shape, and pore structure of the packing material, the diffusion and retention of the solute, and the flow rate of the solute through the column.

Stationary Phase Mass Transfer

  • Stationary phase mass transfer is a band broadening process due to the movement of solute between the stagnant phase and the stationary phase.
  • The degree of band-broadening depends on the retention and diffusion of the solute, the flow rate of the solute through the column, and the kinetics of interaction between the solute and the stationary phase.

Longitudinal Diffusion

  • Longitudinal diffusion is a band broadening process due to the diffusion of the solute along the length of the column in the flowing mobile phase.
  • The degree of band-broadening depends on the diffusion of the solute and the flow rate of the solute through the column.

Theory of Chromatography

  • The rate theory of chromatography explains the kinetics of the chromatographic process.
  • The theory of chromatography includes the Van Deemter Equation, eddy diffusion, mobile phase mass transfer, stagnant mobile phase mass transfer, stationary phase mass transfer, and longitudinal diffusion.

Column Efficiency

  • Column efficiency is a measure of the number of theoretical plates in a column.
  • The number of theoretical plates (N) is calculated using the equation N = 16(tR / W)², where tR is the retention time and W is the peak width.

Plate Theory

  • Plate theory successfully accounts for peak shapes and rates of movement.
  • However, it does not account for the mechanisms causing peak broadening, the effects of other parameters, or how to adjust experimental parameters.

Rate Theory

  • Rate theory relates zone broadening to mass transfer processes.
  • It explains the kinetics of the chromatographic process.

Column Efficiency and Flow Rate

  • The flow rate of the mobile phase affects column efficiency.
  • Liquid chromatography and gas chromatography have different flow rates and plate heights.

Mass Transfer Terms Cs and Cm

  • Cs and Cm are coefficients of mass transfer in the stationary and mobile phases, respectively.
  • Equilibrium between the mobile and stationary phases is never realized, leading to mass transfer effects on peak broadening.

Distribution Constant (K)

  • The distribution constant (K) is the ratio of the concentration of the solute in the stationary phase to the concentration in the mobile phase.
  • K is a constant in linear chromatography and affects retention times.
  • Manipulating K can be done by adjusting the stationary phase, mobile phase, or temperature.

Retention Times

  • Retention times are affected by the distribution constant (K).
  • The retention time of the mobile phase (tM) is the dead time, while the retention time of the analyte (tR) is the time spent in the stationary phase.

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