Page 169 - Multidimensional Chromatography

Unified Chromatography 161
Very volatile fluids (such as CO 2 and others mentioned earlier) are often gases at
ambient conditions. However, to be useful as mobile phases in unified chromatogra-
phy they have to be pumped as liquids and later heated, if necessary. This requires
delivering the fluids to the pump inlet under elevated pressure. The vapor pressure of
the fluid is usually sufficient if the pump head is cooled 10–20°C below the temper-
ature of the fluid reservoir. It is also believed that lowering the pump-head tempera-
ture reduces the possibility of cavitation occurring in the pump, but there is some
disagreement on the overall role of pump temperature (25). Regardless, pumping is
accomplished in an HPLC-like fashion very successfully with these fluids by cooling
the pump head and compensating for the compressibility of the fluid.
Many HPLC instruments are already furnished with temperature controls for the
column. Unified chromatography requires a much wider temperature range than is
currently practiced in HPLC. Until better defined by experience, a temperature range
from about 60 to about 350°C seems reasonable as a specification. Since this is
well in the range of a GC oven with subambient temperature capability, no new tech-
nology is required.
Pressure is controlled by the use of a back-pressure regulator or programmable
valve used in conjunction with a pressure transducer and placed under control of a
pressure-regulating electronic circuit or a computer. Positioning the pressure-control
point downstream (following the column outlet or the detector) allows the mobile-
phase fluid to be independently flow-controlled at the column inlet. This provides the
ability to carry out volumetric mixing of two fluid components and to program
mobile-phase-composition gradients in a fashion exactly like that already done in
HPLC (26).
When a flow-through detector that can be operated at outlet pressure is used, such
as a UV or fluorescence detector, it is most effective to control pressure at the detec-
tor outlet. In these cases, the pressure drop through the detector is negligible, so the
column outlet pressure is still controlled. Since this arrangement does not require
lowering the pressure before detection, the possibility of solute precipitation or an
unwanted phase change occurring during transport to the detector is minimized. In
addition, transport through a pressure regulator or valve always introduces some
degree of mixing which, when significant, contributes to peak broadening and loss of
resolution. This worry is eliminated when pressure control follows detection.
When low-pressure detectors such as an evaporative-light-scattering detector or a
mass spectrometer are necessary, other means are preferred for controlling pressure
and delivering solutes to the detector to eliminate the possibility for solute precipita-
tion or peak broadening. One of these means involves replacing the pressure regula-
tor with a tee-joint in which the less-volatile mobile-phase component is mixed
under pressure control from a third pump (27). A flow restrictor must be added at the
inlet of the low-pressure detector to prevent the pressure from dropping until the last
instant and to limit the amount of make-up fluid needed to maintain the pressure at
the set point. Care must be taken with the order of mixing and temperature change to
make sure the mobile phase never enters the two-phase region anywhere on the path
between the column outlet and the detector inlet. Other arrangements also work well