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Flashcards in Chromatography Deck (58)
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1

Who initially designed chromatography, and for what purpose?

M. Tswett (botanist); separate pigments in plants using column made of calcium carbonate

2

origin of the term 'chromatography'

greek;

chroma = color
graphein = to write

3

the 2 phases in chromatography:

mobile phase

stationary phase

4

What is the mobile phase? What does it do?

fluid that passes through/over the stationary phase
(liquid, gas, or supercritical fluid)

elutes out the various components in the sample (also called the 'eluent')

5

What is the stationary phase? What does it do?

column or surface - gel, solid, (or liquid distributed, bound, or immobilized on solid)

immobile (fixed) phase that interacts with compounds passing through (pushed by mobile phase)

6

What is the definition of chromatography?

separation based on difference in rate (speed) that components migrate through stationary phase (pushed by mobiled phase)

7

What is 'elution?'

process of separating components of mixture using appropriate solvents

8

When a sample is passed through a chromatography column, the ____ chemicals will pass through fastest, while ____ chemicals move slowly

unretained

retained (greater affinity for column)

9

The results (output) diagram of chromatography is called a _____. Different compounds are shown as ______

chromatogram
different peaks

10

What is tR?

retention time (time between injection and max peak of analyte)

11

The larger the tR, the greater the ______

affinity for the column (retention)

12

What is u?

linear velocity of mobile phase (or non retained analyte)

13

The linear velocities of retained analytes 'B' and 'C' would be expressed as:

vB
vC

14

What does 'L' represent?

length of column

15

What is tM?

dead/void time
time needed for mobile phase to pass through

16

L divided by tM =

u (linear velocity of mobile phase)

17

What is k'B?

retention factor
time the sample component stays in stationary phase vs time it resides in mobile phase

(indicates how much longer it is held back by stationary phase vs travelling straight through with mobile phase)

18

How to calculate k'B: (2)

ratio of amounts in 2 phases (stationary/mobile)

(tR(B) - tM)/tM

19

different types of interaction between the stationary phase and compounds (4)

adsorption
partitioning (affinity based)
ion exchange (charge based)
sieving (size based)

20

For ____ and ____ interactions, the stationary phase is immobilized on an inert base

ion exchange
partitioning

21

sieving interactions will have greater retention for (larger/smaller) particles

smaller (caught up inside sorbent )

22

types of liquid chromatography: (5)

liquid solid (adsorption)
liquid liquid (partitioning)
ion exchange
size exclusion (sieving)
affinity (partitioning)

23

types of gas chromatography techniques: (2)

gas liquid (partitioning)
gas solid (adsorption)

24

types of supercritical fluid chromatography:

SF liquid (partitioning)
SF solid (adsorption)

25

What theory did Martin and Synge come up with?

Plate theory:
consider chromatography as a series of 2 phase extractions (without reaching actual equlibrium due to continued movement through)

26

What is the K distribution constant?

K = [A] in stationary phase / [A] in mobile phase

27

A high K distribution constant means: _____

greater distribution into the stationary phase (more affinity); longer retention

28

the greater the number of theoretical 'plates' in a column, the _____ the extraction

more efficient (better yield)

29

What is 'alpha?'

selectivity factor

30

What is the selectivity factor?

measure of the separation of 2 components
ratio of the K distribution constants (or retention factors) for each

31

T/F: alpha (selectivity factor) depends on column efficiency

False; independent of column efficiency
(only consider the chemistry of compounds, eluent type, adsorbent chemistry)

32

What factors affect 'alpha' (selectivity factor)? (3)

chemistry of compounds
eluent type
adsorbent chemistry

33

'alpha' (selectivity factor) must be _______, other wise ______

>1

if alpha = 1, no chance to improve column efficiency to better separate

34

The chromatogram peak should be a ____ shape

gaussian

35

The number of _______ is used to assess column performance

theoretical plates (N)

36

How is 'N' calculated?

N = 16 x (tR/W)^2
tR (retention time)
W (peak width at base)

37

The height of each theoretical plate is given as:

H = L/N

38

T/F: the theoretical plate approach does not explain peak broadening

true

39

What is the equation used to describe chromatographic effiency in 'rate theory?'
Who was behind it?


H = A + B/u + C*u
u (linear v of mobile phase)
A: multipath coefficient
B: longitudinal diffusion coefficient
C: mass transfer coefficient

van Deemter

40

what does 'A' (multipath coefficient' in rate theory describe?

describes variation in analyte flow path (molecules flow down slightly different pathways, not travelling exactly same distance)

41

what does 'B' (longitudinal coefficient' in rate theory describe?

describes tendency of molecules to diffuse out in random directions (slightly drift in other directions rather than straight through column)

42

what does 'C' (mass transfer coefficient' in rate theory describe?

resistance to mass transfer; analyte is trying to equilibrate between stationary and mobile phase, but not instantaneous, and mobile phase keeps moving
=> molecules leaving stationary into mobile phase will lag behind slightly

43

identical molecules will not elute out at the same time. This is known as ___ _____, and can be explained by ______

peak broadening

van deemter equation (A, B, C terms)

44

As flow velocity increases (high flow), what happens to 'A' factor in the van deemter equation?

remains constant

45

As flow velocity DECREASES, what happens to 'C' factor in the van deemter equation?

decreases

46

The C term in van deemter equation can be further divided into (2)

Cs (stationary phase)
Cm (mobile phase)

47

What happens to B factor in van deemter equation if flow is increased?

exponential decrease

48

T/F: at a high flow rate, resistance to mass transfer will have a larger impact on band broadening than the longitudinal diffusion coefficient

True

49

The van deemter equation for high flow?

The van deemter equation for low flow?

high flow:
H = A + B/u + C*u

low flow:
H = B/u + (Cs + Cm)*u

50

Describe the van deemter graph:

plot u (mobile phase velocity) vs theoretical plate height (H)

J shaped curve (optimal efficiency is at lowest point)

51

What is 'resolution?'

denoted by 'R'

characterize separation of 2 adjacent peaks

52

How is R calculated?

(distance from mid-peaks)/(sum of peak widths divided by 2)

or

2 x (difference in retention time)/(sum of peak widths)

53

A (higher/lower) resolution means better separated peaks

higher

54

A greater number of theoretical plates is considered to be (more/less) efficient

more

55

How is the plate number affected by column length?

increases with length

56

How does the size of the column (stationary phase and internal diameter) affect efficiency?

smaller diameter -> more efficient (more plates)

57

examples of non-ideal peaks (2)

tailing (stretched out slope after peak)
fronting (stretched out slope before peak)

58

What are some practical problems that lead to non-ideal peaks?

overload
flow rate too high
slow injection...