Chromatography and Detectors

Flashcards generated from lecture notes on Chromatography

Why is band broadening important to consider in chromatography?

Band broadening is an unwanted effect in chromatography.

What is the formula for theoretical plate height (H)?

H = L/N, where H is theoretical plate height, L is column length, and N is the number of theoretical plates.

What is the formula for linear velocity (u)?

u = L/Tm, where u is linear velocity, L is column length, and Tm is dead time.

What is the formula for reduced plate height (h)?

h = H/dp, where h is reduced plate height and dp is particle diameter.

What is the formula for reduced linear velocity (v)?

v = udp/Dm, where v is reduced linear velocity, u is linear velocity, dp is particle diameter, and Dm is the diffusion coefficient.

What is the equation for Fick's Law?

dM = −D grad c dF dt, where dM is mass of substance, D is diffusion coefficient, grad c is concentration gradient, dF is surface area, and dt is time.

What factors affect the Diffusion Coefficient?

Solvent viscosity, temperature, and solvent density.

What is the goal of plotting plate height versus linear velocity (van Deemter plot)?

To determine the optimum experimental conditions for the most efficient separation by achieving the lowest plate height (H) and optimum linear velocity (u).

What do A, B, and C represent in the van Deemter equation?

A = Eddy diffusion, B = Longitudinal diffusion, C = Mass transfer kinetics.

What is Eddy Diffusion?

It is the contribution to plate height due to molecules traveling along different paths through the column, also known as multipath diffusion.

What factors influence eddy diffusion (A)?

Particle size (size distribution) and geometry of the packing, and the packing procedure quality.

What primarily causes high eddy diffusion?

Inhomogeneities or small variations in the packing material.

How can eddy diffusion be decreased?

Selecting well-packed columns, using smaller stationary phase particles, and using particles with a narrow size distribution.

What is Longitudinal Diffusion (B)?

It describes the contribution to plate height due to the diffusion of molecules traveling along the column.

On what does longitudinal diffusion depend?

Type of stationary phase, type of mobile phase, type of analyte, temperature, and pressure.

How can longitudinal diffusion be minimized?

Using higher mobile phase flow rates; keeping system tubing short and as narrow as possible; using correct nuts, ferrules, and fittings.

What governs mass transfer (C)?

Absorption kinetics and mass transfer (mainly due to diffusion) inside the particles.

What is a Detector?

Sensor that responds to components in the effluent.

What is a Universal Detector?

Responds to every component in the effluent except the mobile phase.

What is a Selective Detector?

Detector which responds to a related group of components in the effluent.

What is a Specific Detector?

Responds to a single sample component or to a limited number having similar chemical characteristics.

What does FID stand for and what type of detector is it?

Flame Ionization Detector - Specific

What is the disadvantage of using an FID?

Destroys sample components.

What is the principle of operation for an FID?

Compounds are burned and the flame change is registered.

On what does the response of an FID depend?

Number of C atoms.

What type of sensitivity does an FID have?

Rate sensitive (mass flow dependent).

What does TCD stand for and what type of detector is it?

Thermal Conductivity Detector - Universal

How does a TCD work?

Two cell detector, one for the sample, the other for a reference.

What type of sensitivity does a TCD have?

Concentration sensitive.

What analytes are difficult to measure using TCD?

Analytes with thermal conductivity close to the carrier gas.

What does MSD Stand for and what type of detector is it?

Mass Selective Detector - Selective

What does ECD Stand for?

Electron Capture Detector

How does an ECD work?

It uses a radioactive nickel foil that constantly emits fast β- particles (i. e. fast electrons) and a collector electrode at the opposite side.

What is added to the carrier gas in ECD to slow down electrons?

Argon gas or other decelerant gas.

What is needed for interactions in ECD?

Components need affinity to e-, defined as the amount of energy released if a single atom captures an electron in the gas phase at 0 K.

What does FPD stand for?

Flame Photometric Detector

What is chemiluminescence in FPD?

Burning N-, S-, P- produces chemiluminescent species.

What does either NPD, PND, TID, or TSD stand for?

Thermionic Detector

What does a Thermionic Detector look like?

Looks like a FID but has the addition of alkali metal bead.

What is a concentration sensitive detector?

The signal or response depends on the concentration of a solute in the detector cell.

What is a mass or rate sensitive detector?

The signal depends on the number of ionizable molecules in the detector (the mass flow).