In supercritical fluid chromatography, very high degrees of accuracy are
required for temperature control. On the fluid supply end of the system,
cooling is critical. On the separation end, heating is important. This paper
discusses temperature control in the HP G1205A supercritical fluid
chromatograph.
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The choice of which chromatographic method to use depends on the compounds
being analyzed. In gas chromatography (GC), the mobile phase that carries the
sample injected into the system is a gas. GC is generally a method for volatile and
low molecular weight compounds. High-performance liquid chromatography
(HPLC) is primarily used for analysis of nonvolatile and higher molecular weight
compounds. A combination of desirable characteristics from both of these methods
can be obtained by using a supercritical fluid as the mobile phase. A supercritical
fluid is a substance above its critical point on the temperature/pressure phase
diagram (see Fig. 2). Above the critical point, the fluid is neither a gas nor a liquid,
but possesses properties of both.
The advantage of SFC is that the high density of a supercritical fluid gives it the
solvent properties of a liquid, while it still exhibits the faster physical flow properties of a gas. In chromatographic terms, supercritical fluids allow the high efficiency and detection options associated with gas chromatography to be combined
with the high selectivity and the wider sample polarity range of high-performance
liquid chromatography. Applications that are unique to SFC include analysis of
compounds that are either too polar, too high in molecular weight, or too thermally
labile for GC methods and are undetectable with HPLC detectors. Another benefit
of SFC over LC is the reduction of toxic solvent use and the expense associated
with solvent disposal. This aspect has become increasingly important as environmental awareness becomes a larger issue.
Liquid
Pressure
Chromatography is a process in which a chemical mixture, carried by a mobile
phase, is separated into components as a result of differential distribution of the
solutes as they flow over a stationary phase. The distribution is the result of differing physical and/or chemical interactions of the components with the stationary
phase. On a very basic level, chromatography instrumentation consists of (1) a
delivery system to transport the sample within a mobile phase, (2) a stationary
phase (the column) where the separation process occurs, (3) a detection system
that identifies or distinguishes between the eluted compounds, and (4) a data
collection device to record the results (see Fig. 1).
Pc
Supercritical
Fluid
Region
Solid
Critical
Point
Gas
Temperature
Tc
Fig. 2. Phase diagram with critical point and supercritical region.
Hewlett-Packard developed and manufactured its first SFC instrument in 1982. For
the past decade, SFC has primarily been used in R&D laboratories. The market
has now expanded to include routine analysis for process and quality control as
SFC is continuing to gain acceptance as a complementary technique to GC and
LC.
Connie Nathan
Barbara Hackbarth
Development Engineers
Analytical Products Group
Sample
Injection
Pump
Detector
Column
Data Handling
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Table I
Refrigerant Cooling Comparison
Thermoelectric
Cryogenic Fluids
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Fan
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Peltier Cooler
Copper Cold
Plate
CO2
Tank
Nickel Heat
Exchanger
Pump Drive Assembly
Pump Head
Tank Valve
Fig. 3. )#2!& -$ 2&# .3+.
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Collector
Heated
Zone
Collector
Air In
Jet
Air In
Jet
Detector Block
Heated Zone
Heater
Heater
Hydrogen In
Hydrogen In
Detector Block
Heated Zone
(a)
(b)
Condensation forms from the combustion of hydrogen and
air when the detector block temperature is below 100°C.
Table II is a comparison of the GC and SFC detectors.
Table II
Comparison of SFC and GC Flame Ionization Detectors
SFC Detector
GC Detector
Physical Size
6 mm thick, compact
with jet permanently
attached
25 mm thick with jet
a separate part
Temperature
Gradient
1°C from sensor to
jet tip
17°C from sensor to
jet tip
Problem
Area
No spiking or
flameout
Spiking and flameĆ
out when used as an
SFC detector
Collector
Heated collector
zone
Collector not
actively heated
The SFC flame ionization detector is insulated from the oven
so that the heat from the oven has minimal effect on it. The
SFC detector's minimum operating temperature is the oven
temperature plus 10°C because of convection and conduction
of heat from the oven.
To achieve temperature stability, the SFC flame ionization
detector heated zone required changes in the temperature
control parameters. The controller uses the ZieglerĆNichols
algorithm1 to obtain the proportional (P), integral (I), and
derivative (D) terms in the control algorithm. This algorithm
defines the PID constants such that the object reaches its
desired steadyĆstate temperature without large offsets or
deviations. The PID controller is designed for larger masses
such as the GC heated detector zones. Although it takes
longer for larger masses to heat up, the oscillations are miniĆ
mal and therefore it is easier to dampen the response and
bring temperature under control. The SFC flame ionization
detector is 1/4 the size of the GC design, so it heats up very
quickly. Using the GC PID constants, large oscillations were
seen because of poor temperature control. New initial values
for the PID parameters for the SFC detector were derived
using the ZieglerĆNichols tuning algorithm. A trial and error
method was then applied to fineĆtune the PID parameters to
eliminate oscillations.
Fig. 4. Sketches of the SFC (a)
and GC (b) flame ionization deĆ
tectors. Note the difference in the
heated block size and the locaĆ
tion of the jet.
August 1994 HewlettĆPackard Journal
The major result was the introduction of an HP G1205A SFC
system in May 1992 that meets its performance goals. It was
a challenge to reuse existing components that were not deĆ
signed with SFC in mind. In SFC, control of temperature in
different regions of the system is critical to the success of the
chemical analysis and the performance of the instrument. To
make components perform under conditions for which they
were not originally designed was a major success for the
team. Another tangible result was the reduction of product
cost by consolidation and reuse of components. The product
is a compact modular design and maintains the integrity of
the existing designs it leverages. The introduction of a therĆ
moelectric PeltierĆcooled pump head provides an advantage
over designs that use a second source of CO2 to cool the
pump head. Other HP analytical products have incorporated
the Peltier device. The SFC flame ionization detector, with its
shortened zone, has improved the quality of the analytical
results.
The work described in this paper is the result of several
years of development efforts by the R&D project team. SpeĆ
cifically, Hans Georg Haertl is responsible for successfully
incorporating the Peltier device into LC pump design. Hans
Georg was an engineer on loan to the site from the HP
Waldbronn Division in Germany. The other members of the
project team included Chris Toney (project manager), Elmer
Axelson (software), Paul Dryden (firmware), Bill Wilson
(chemist), Terry Berger (chemist), Mahmoud AbdelĆRahman
(firmware and hardware), and Joe Wyan (hardware). Our
success must also be shared by several other functional
groups (manufacturing, marketing, purchasing, and adminisĆ
trative support) which were instrumental in the release of the
product. Without the combined efforts of these individuals,
the SFC would not have met its targeted release date.
1. G.K. McMillan, Instrument
Society of America, 1990, Chapter 1.
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