Page 35 - Tandem Techniques

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better solution for some samples might be, where possible, to employ a packed column.

Other types of columns have been developed, such as the SCOT (Solid Coated Open Tubes) columns.
These are open tubular columns that have a coating of celite deposited on the internal surface on which
the stationary phase has been coated. This type of column attempts to retain the low flow impedance of
the open tube, while increasing the stationary phase load to recover some of the advantages of the high
stationary phase loading of the packed column. The basic difference between the capillary and the
packed column is their flow impedance. Due to the continual change in direction of the flow of gas in a
packed column, as it winds its way between the particles of packing, considerable energy is utilized and
this is obtained at the expense of a greater pressure drop across the packed column. Thus, the flow
impedance of a packed column is much greater than that of the open tube. This is reflected in the
relative magnitudes of the constants in the d'Arcy equation, for the flow of fluids through packed beds,
compared with that of the Poiseuille equation, for fluid flow through open tubes. It follows that for a
given inlet gas pressure, a capillary column can be much longer than a packed column, perhaps by as
much as one to two orders of magnitude, and thus the capillary column has a far greater potential for
high column efficiencies. There are, however, in addition to injection difficulties, certain disadvantages
to capillary columns so that they are not always optimum for tandem systems; these will be discussed a
little later.

The characteristics of the GC system, that are important for tandem operation, are the solute
concentration leaving the column and the volume flow rate. The concentration of solute in the eluting
gas from a GC column can range from 1 x 10-6 g/ml to 5 x 10-12 g/ml. In any particular GC system,
the actual concentration will depend both on the mass of sample injected and the dimensions of the
column. The flow rate, depending on the column diameter and the carrier gas employed, will range
from about 0.5 ml/min. for a capillary column 0.25 mm I.D. to about 25 ml/min. for a standard packed
column 4.6 mm I.D.. It is clear that the interface will need to be carefully designed, on the one hand, to