AdvancedGasChromatography–Progress inAgricultural, Biomedical and Industrial Applications
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Therewas an impulseof consolidating thenumber of stationaryphasesused in themid-1970s.
Leary et al. (1973) reported the application of the statistical technique
the nearest neighbour
for
the 226 of the stationary phases of McReynolds study and suggested that a total of only 12
phases could replace the 226. Later it has been found that four phases,OV-101,OV-17,OV-225
and Carbowax 20M could provide GC analysis, satisfying 80% of a wide variety of organic
compounds, and a list of six favourite stationary phases on which almost all gas-liquid
chromatographic analysis could be performed: (1) dimethylpolysiloxane (OV-101, SE-30, SP-
210); (2) 50% phenyl-polysiloxane (OV-17, SP-2250); (3) poly-ethylene-glycol of molecular
weight> 4000 (Carbowax); (4) diethyleneglycol succinate (DEGS); (5) 3- ciano-propyl-
polysiloxane (Silar-10C, SP-2340) and (6) tri-fluor-propyl-polysiloxane (OV-210, SP-2401).
Another quality of McReynolds constants is guiding the selection of columns to separate
compounds with different functional groups, such as ketones by alcohols, ethers by olefins
and esters by nitriles. If the analyzer wants a column to elute an ester after an alcohol, the
stationary phase should have a value Z' greater than the value Y'. Also, a stationary phase
shouldhave avalueof Y’ greater thanZ' inorder tomake ether elutebefore the alcohol.
6. Column type
Different chromatography types of column arepresentedby their diameters. Thus, there are
threemain types of column: packed columns, capillary columns andmicro-packed columns.
Packed columns
The most used column materials are stainless steel, glass, quartz, nickel and
polytetrafluorethylene (PTFE). Because of their good thermal conductivity and easy
manipulation, stainless steel columns are widely applied. Glass columns are more inert.
PTFE columns are used to separate mixtures of halogens and derivates. Classical packed
columns’ inner diameters range from
1.5
mm to
6
mm. Very important for packed columns
is the load; the load is directly proportional to the column cross-section and hence to the
squareof the inner diameter (Leibnitz&Struppe, 1984).
Capillary columns (open tubular columns)
The terminology used for capillary column includes names such asWCOT (well coated open
tubular)which are capillary columnswhere the stationaryphase isdepositeddirectlyonto the
inner surface of the wall not including substances that could be considered support. SCOT
(support coated open tubular) includes those capillaries that have deposited on the inner
surface a finely divided support (NaCl, BaCO3, SiO2, etc.) in which stationary phase is
submitted. PLOT (porous layer open tubular) columns contain capillaries with stationary
phase deposited on a porous support consisting of a finely divided support for packed gas
chromatography columns (Celite, Chromosorb, etc.). Porous material is deposited in the
capillary tubes from thevery firstmoment of column conditioning. Capillary columnsBP,DP,
SPB (bond phase) include capillary tubes that have stationary phase chemically immobilized
on the surface of the capillary innerwall.When choosing a certain type of capillary column it
is necessary to take into account the nature of the sample being analyzed, the equipment
available and the effective separation. Capillary columns WCOT have better efficiency in
separation than SCOT because the inner column diameter is smaller. SCOT columns can be
considered as subgroups of PLOT columns, since all SCOT are PLOT columns, but not vice
versa (Ciucanu, 1990). For capillary columns the innerdiameter ranges from0.10 to0.20mm.
