11.26

SECTION 11

The Kovats retention

indices

(R.I.) indicate where compounds will appear on a chromatogram with

respect to unbranched alkanes injected with the sample. By definition, the R.I. for pentane is 500,

for hexane is 600, for heptane is 700, and so on, regardless of the column used or the operating

conditions, although the exact conditions and column must be specified, such as liquid loading,

particular support used, and any pretreatment. For example, suppose that on a 20% squalane column

at 100 C, the retention times for hexane, benzene, and octane are found to be 15, 16, and 25 min,

respectively. On a graph of

(naperianlntlogarithmR

of the adjusted

retention

time) of the

alkanes

versus their retention indices, a R.I. of 653 for benzene is read off the graph. The number 653 for

benzene (see last line of Table 11.13 in

the column headed “1” under “Reference compounds”)

means that it elutes halfway between hexane and heptane on a logarithmic time scale. If the exper-

iment is repeated with a dinonyl phthalate column, the R.I for benzene is found to be 736 (lying

between heptane and octane), which implies that dinonyl phthalate will retard benzene slightly more

than squalane will; that is, dinonyl phthalate is slightly more polar than squalane by

units

I 83

(the entry in Table 11.13 for dinonyl phthalate in the column headed “1” under “Reference com-

pounds”). The difference gives a measure

of solute-solvent interaction due to all intermolecular

forces other than London dispersion forces. The latter are the principal solute-solvent effects with

squalane.

TABLE 11.14

Characteristics of Selected Supercritical Fluids

Critical

Critical

Fluid

temperature, K (

C)

pressure, atm (psi)

Ammonia

406 (133)

111.3 (1636)

Argon

151 (

122)

48.1 (707)

Benzene

562 (289)

48.3 (710)

Butane

425 (125)

37.5 (551)

Carbon dioxide

304 (31)

72.8 (1070)

Carbon disulfide

552 (279)

78.0 (1147)

Chlorotrifluoromethane

379 (106)

40

(588)

2,2-Dimethylpropane

434 (161)

31.6 (464)

Ethane

305 (32)

48.2 (706)

Fluoromethane

318 (45)

58.0 (853)

Heptane

540 (267)

27.0 (397)

Hexane

507 (234)

29.3 (431)

Hydrogen sulfide

373 (100)

88.2 (1296)

Krypton

209 (

64)

54.3 (798)

Methane

191 (

82)

45.4 (667)

Methanol

513 (240)

79.9 (1175)

2-Methylpropane

408 (65)

36.0 (529)

Nitrogen

126 (

147)

33.5 (492)

Nitrogen(I) oxide

310 (37)

71.5 (1051)

Pentane

470 (197)

33.3 (490)

Propane

470 (197)

41.9 (616)

Sulfur dioxide

431 (158)

77.8 (1144)

Sulfur hexafluoride

319 (46)

37.1 (545)

Trichloromethane

536 (263)

54.9 (807)

Trifluoromethane

299 (26)

47.7 (701)

Water

647 (374)

217.6 (3199)

Xenon

290 (17)

57.6 (847)