Chromatography- Paul Flashcards

Question

Peak symmetry:

Answer 1

normal distribution, molecules remain in a narrow band that passes through the column, Gaussian in nature

Answer 2

peaks that are clearly separated - molecules stay within a narrow band as it passes through the column, no diffusion of molecules form this band giving rise to a wider peak at detection

Answer 3

directional aspect of asymmetry or broadening of the peaks – indication that equilibrium cycles are not optimised

Answer 4

all about number of theoretical plates (N) – a theoretical plate represents the distance that a molecule travels within the column to complete an equilibrium cycle, the smaller the distance the molecule travels for an equilibrium cycle the smaller the ‘height equivalent of the theoretical plate’ (HETP)

Answer 5

the more efficient the separation is and the better optimised are all the above parameters

Answer 6

happens when one of the separation mechanisms is overloaded wrong solvents used wrong column used other issues

Answer 7

Adjusted Retention Time = tr -tm the retention time of an unretained compound, the retention time of the mobile phase or the hold up time

Answer 8

The characteristic time for a particular analyte to pass through the system (from the column inlet to the detector) under set conditions The time between sample injection and an analyte peak reaching a detector at the end of the column is termed the retention time (tr ) Time taken for mobile phase to pass through the column is called tm t’r is calculated as tr - tm

Answer 9

Vr, is the volume of mobile phase required to elute a particular solute from the column:

Answer 10

Often used to describe the migration rate of an analyte on a column, also called the capacity factor The retention factor for an analyte is defined as: k' = (tr - tm)/ tm tr and tm easily obtained from chromatogram When an analytes’ retention factor is less than one, elution is so fast that accurate determination of the retention time is very difficult

Answer 11

Described as the separating power – measure of selectivity a.k.a relative retention

Answer 12

To manipulate k in LC – mobile phase composition can be altered – k based on retention To manipulate k in GC – change the temperature of the mobile phase Other factors also include – chemical nature and quantity of the stationary phase Also the ratio of the amount of MP to SP – called the Phase Ratio For GC – optimum resolution occurs with k values between 1 and 10 For LC - optimum resolution occurs with k values between 1 and 5

Answer 13

Efficiency is a measure of the dispersion of the analyte band as it travels through the column and system Due to dispersion peaks take on the normal shape – Gaussian distribution Band Broadening occurs the longer the band remains on the column – it results from a loss of efficiency

Answer 14

Ideally we need a high value of N – the number of theoretical plates and a small ‘height equivalence’ of each plate The smaller the plate, and the more of them then the more efficient the separation

Answer 15

Plate theory refers to the equilibration of component molecules in the sample between the MP and the SP As the component molecules move down the column there is a transfer of the component molecules from the MP to the SP and back to the MP and then back to the SP on a continuous basis down the length of the column One cycle of equilibration is the transfer of the molecules completely from the MP to the SP and back to the MP again. The term plate refers to the distance over which the component molecules achieve one cycle of equilibration Separation of different components occurs because each component has a different efficiency of separation These plates are theoretical in nature

Answer 16

Height Equivalent to a Theoretical Plate = the distance it takes for once cycle of equilibration to occur HETP is also referred to simply as H in some equations Can be calculated as follows H = L / N L = length of the column N = Number of Theoretical plates The smaller the value of H the narrower the peaks

Answer 17

the van Deemter Equation and Plot

Answer 18

How the molecules move through the SP

Answer 19

how the band of molecules stay together and do not move apart, uniform motion as a narrow band

Answer 20

relates to the equilibrium cycle / height equivalent of theoretical plate: (i) molecule moves from MP to SP (ii) molecule interacts with SP (iii) molecule moves from SP to MP (iv) molecule moves with MP (v) back to (i) and repeat for the next plate

Answer 21

Relates to analyte flow path in the column: analyte molecules can take one of many paths due to small variations in particle size of SP, non homogenity of packing of particles etc

Answer 22

MP moves slower along walls of column than through the middle – cause of Laminar Flow Laminar Flow – particle size< 40mm, MP flow rate < 4ml/min Non Laminar (Turbulent) Flow – particle sizes bigger and flow rate >5ml/min

Answer 23

Relates again to MP flow rate. Analyte molecules at the edge of the band diffuse in every direction due to concentration gradients. Will happen in all tubing and gets worse when column is Too long Too wide Not connected properly

Answer 24

The rate at which analyte molecules move of diffuse in/out/within SP If SP is porous the MP in the pores is stagnant - analyte molecules in this stagnant MP get held up

Answer 25

The rate at which the analyte molecules move or diffuse within/in/out of the the SP - Also analyte molecules reside on surface of or within the SP for variable lengths of time – this also causes band broadening

Answer 26

Resistance to mass transfer is dependent on the speed with which the partition equilibrium between mobile and stationary phase is obtained. Since the resistance to mass transfer in the mobile phase is not the same for all molecules of one type of analyte (it depends on the location in the column at a particular time and the distance they have to travel to partition between phases) this will also result in peak broadening of that analyte in the column.

Answer 27

Known as the packing parameter Due to different pathways due to diffusion of MP gas Quality of packing in column

Answer 28

Again analyte molecules disperse at edges of the band in all directions

Answer 29

* Using shorter (smaller i.d.) columns. * Using higher mobile phase flow rates. * Ensure the inlet line and temperature are appropriate and the column is properly installed into the injector and detector (see Section 2.5). * Use carrier gas with a low diffusion coefficient (nitrogen). Note nitrogen is not the optimum carrier to use with capillary gas chromatography

Answer 30

Selecting well (tightly) packed columns Using smaller stationary phase particles (note that the smallest practical particle size is limited by the back pressure -pressure drop across the column. Smaller particles create higher column pressure) Using particles with a narrow size distribution

Answer 31

* Using smaller (diameter) stationary phase particles * Using lower mobile phase flow rates * Heating the column (at higher temperatures the diffusion processes are speeded up and the differences in elution time from the particle pore are reduced)

Chromatography- Paul Flashcards

(55 cards)