Practical Aspects

Plots such as in Figure 4 are quite difficult to obtain and calculations from these curves may be tedious. An easier way is to plot z2/t versus t, which yields a straight line as shown in Figure 5.

Curve 4 has a negative slope, which means that vaporization predominates. From the slope kvv/e (kv

Figure 4 Combination of adsorption and vaporization. Layer thickness, 0.01 cm; velocity constant kv = 0.0235 cm2 s~1; porosity of the dry layer, 0.75.

Curve 1 Adsorption speed v' = 2x 10~5cm s~1, vaporization speed v = 0. The particles are filled at A (st=0.40). The total porosity is 0.30 at C, 0.20 at D and 0.10 at E.

Curve 2 v' = 0, st = 0.4, v = 0, eqn [21] is applied.

Curve 4v' = 6 x 10~6 cm s~1, v = 0. The particles are filled at B (st= 0.40). The porosity is 0.30 at F.

Curve 6 v' = 0, v= 1 x 10~5cm s~1. (Reprinted with permission from Guiochon G and Siouffi A (1978) Journal of Chromatographic Science 16: 598-609.)

is known from the ordinate of the plot, e by plate fabrication) the determination of v is possible (see Table 2). With benzene and toluene (curves 1 and 2) a slight positive slope is observed, which means adsorption. The 'best' curve is curve 3, corresponding to carbon tetrachloride. It should be pointed out that measurements are very imprecise on short development (1 cm or less) as it is difficult to check accurately the solvent front. Table 2 displays the data gathered from Figure 5.

A developing solvent must be carefully chosen. Besides the retention requirements it is necessary to discard solvents that may give high adsorption or vaporization rates. Some properties of common TLC solvents compiled from Ripphahn and Halpaap and e

0.04

0.03

0.04

0.03

d

o

0

1

_

Ù

3

Û

1000

1000

Table 2 Deviations from the quadratic law (plates 0.2 mm thick of silica Si 60)

Solvent

Curve in Figure 5

v' -2v (m s~1)

Benzene

1

3 x 10~8

Toluene

2

3 x 10~8

Carbon tetrachloride

3

0.8 x 10~8

Dichloromethane

4

- 1.3x10~8

may be calculated from:

Figure 5 Plots of z2/t versus t with different solvents on plain silica, dp = 5 |im. Layer thickness 0.2 mm, porosity £ 0.80. Solvents: 1, benzene; 2, xylene; 3, carbon tetrachloride; 4, dich-loromethane. (Reprinted with permission from Guiochon G and Siouffi A (1978) Journal of Chromatographic Science 16: 598-609.)

our own data are reported in Table 3. 6 has been normalized to 5 cm development distance. It is worth checking the time necessary to fill the particles with solvent when adsorption predominates. This time s -0.4

and may be compared with the time needed for the solvent to move by 10 cm (t3).

With diethyl ether, for example, ts > t3 (see Table 3); with «-pentane, chloroform, dichloromethane and ethyl acetate t3 is between the time necessary to fill an internal porosity of 0.35 and one of 0.40. A correlation should be possible between v' and P0, but this relation is not linear. It is unfortunate that most TLC

Table 3 Velocity constants of various solvents

Solvent Viscosity t (cP) Surfacetension y/t/x10-5 10'xv' 102kv 102d (m s 1)

n-Pentane

0.24

16.0

n-Hexane

0.33

18.4

n-Heptane

0.41

20.4

Cyclohexane

0.97

25.5

Benzene

0.65

28.9

Toluene

0.59

28.5

Xylene (P)

0.65

28.4

Carbon disulfide

0.36

32.3

Carbon tetrachloride

0.97

27.0

Chloroform

0.58

27.1

Dichloromethane

0.43

26.5

Trichloroethylene

0.58

29.3

Diethyl ether

0.23

17.0

Diisopropyl ether

0.35

32.0

Dioxane

1.26

35.4

Tetrahydrofurane

0.47

26.4

Methanol

0.60

22.6

Ethanol

1.20

22.7

1-Propanol

2.26

23.8

2-Propanol

2.50

21.7

1-Butanol

2.95

24.6

i-Butanol

3.95

23.0

t-Butanol

2.82

20.7

Acetone

0.32

23.7

2-Butanone

0.43

24.6

Ethyl acetate

0.46

23.9

Butyl butyrate

1.03

24.0

Acetonitrile

0.36

29.3

Pyridine

0.97

38.0

6.7

10.9a

9.2

115

5.6

5.3

11.8

95

5.0

6.4

8.4

80

2.6

3.7

5.0

43

4.4

4.9

8.0

72

4.8

9.3b

7.1

77

4.2

4.3

6.1

70

8.7

14.9b

11.4

140

2.8

2.2

5.8

46

4.7

8.3a

7.9

75

6.2

10.4a

8.7

99

5.1

8.6a

6.9

82

7.1

17.0b

8.8

114

9.1

5.9

10.2

145

2.8

3.9

4.7

45

5.6

4.3

10.3

88

3.8

4.6

5.0

65

1.9

2.3

3.1

32

1.05

1.9

2.03

18

0.87

1.1

1.94

15

0.83

-

-

13

0.58

0.85

1.20

10

0.73

0.21

1.0

11.5

7.4

11.0

11.2

118

5.7

8.3

10.0

91

5.2

9.8a

8.0

85

2.3

-

-

37

7.5

8.1

11.4

125

3.9

5.3

5.6

64

aTime t3 (z = 10 cm) is larger than the time necessary to fill the inside of particles in a bed of inner porosity 0.35 (see text). bFor these solvents ts > t3.

data only quote RF and no information is given on the velocity constant as is usual in HPLC.

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