StationaryPhases
39
nitropropane andpyridine. Eachof these compounds canbe considered standard for certain
classes of substances,which are similar in terms of chromatographic behaviour, as follows:
-
benzene
for unsaturatedhydrocarbons and aromatichydrocarbons;
-
1-butanol
for alcohols, phenols, carboxylic acids;
-
2-pentanona
for aldehydes, ketones, ethers, esters;
-
nitropropane
for nitroderivates andnitriles;
-
pyridine
for aromatic bases andheterocycleswithnitrogen.
The steps to determinate the constantsMcReynolds for a particular stationary phase A, for
packed columns, are as follows:
-
each reference compound is analyzed on a column with 20% squalane as stationary
phase, isothermal at 100
˚
C and the corresponding retention indices aredeterminate;
-
reference compounds are analyzed on a column containing 20% stationary phase A,
under the same conditions and in such casesKovats retention indices are established;
-
McReynolds constants of the stationaryphaseA are calculated as follows:
for benzene (denoted by x '): the difference between the retention index of benzene on
stationaryphaseA andon squalane:
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=
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(1)
for a butanol (denoted by y '): the difference between the retention index on phase
butanol on stationaryphaseA andon squalane;
The same applies for 2-pentanona (z'), nitropropane (u') andpyridine (s').
These McReynolds constants values were determined for a great number of common
stationaryphases and they are tabulated. Somevalues arealsogiven inTable 5.
Nameof the stationaryphase
X’
Y’
Z’
U’
S’
SqualaneC
30
H
62
0
0
0
0
0
MethylsiliconeOV-1
16
55
44
64
42
MethylsiliconeSE-30
15
53
44
64
41
Methyl-phenyl-silicone (20%phenyl)OV-7
69
113
111
171
128
Methyl-phenyl-silicone (50%phenyl)OV-17
119
158
162
243
202
Cyanopropyl-methyl-phenyl-siliconeOV-225
228
369
338
492
386
Carbowax 20M (Polyethylenglycol)
322
536
368
572
510
Nitroterephtalic ester of PEG (FFAP)
340
580
397
602
627
Diethyleneglycol succinate (DEGS)
496
746
590
837
835
Table 5.McReynolds constants values for someusual stationaryphases
Currently there are hundreds of stationary phases used in gas chromatography and
McReynolds constants allow selecting the one that promises the best separation of the
components analyzed. Because obviously there are a number of stationary phases that have
similar McReynolds constant values, they can be substituted in between without affecting
separation. Thus, if there is a stationaryphase recommended in the literature for separation,
but isunavailable ina laboratory, itwill be replacedwith an equivalent.
