Thermal management in supercritical fluid chromatography - the HP G1205A supercritical fluid chromatograph - Technical

Hewlett-Packard Journal,  August, 1994  by Connie Nathan,  Barbara A. Hackbarth

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Supercritical fluid chromatography (SFC) is a technique that has gained acceptance in the analytical chemistry marketplace as a complement to gas chromatography (GC) and liquid chromatography (LC). In the development of the HP G1205A supercritical fluid chromatograph (Fig. 1), leveraging of major components from HP GC and LC products was a primary goal. As a result of this goal, thermal management in the system was a challenge because the components were not intended to operate in the temperature range required for SFC. For example, the LC pumping module was designed to pump fluids at room temperature, while for SFC fluids optimal delivery is at 5[degrees]C or lower.

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Modifications were made to the components to integrate them into one product. Some components were optimized for SFC. This helped to improve the chromatographic technique by incorporating new ways to manage and control temperature. This paper examines the design modifications made to components to meet the thermal requirements for SFC while leveraging current HP analytical product components.

SFC System

An SFC unit consists of four major systems: fluid delivery (pumps), separation, detection, and data collection. A brief description of chromatography accompanies this article to provide the reader with an overview of the technology (see page 39).

What is SFC?

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).

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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.

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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.

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.

The HP G1205A SFC project goal of using, wherever possible, components from already proven HP GC and LC instrumentation resulted in reuse of the HP 5890 GC oven, the HP 1050 LC pumping module, a variety of both GC and LC detectors, and the HP ChemStation instrument control software. The major components of the HP G1205A SFC system will now be described in further detail.

Pumping Module. The HP G1205A SFC system is available as a single-pump system using [CO.sub.2] (or other fluids) for its supercritical mobile fluid or as a dual-pump system that allows modifiers to be added to the [CO.sub.2]. Both pumps consist of reciprocating dual pistons in series, allowing for continuous, reliable, and unattended pumping. This eliminates the inconvenience of refilling syringe pumps and allows for control of the flow and changing of the composition. The pumps have feedback control algorithms that dynamically compensate for optimum fluid compressibility and minimize the pressure ripple of the reciprocating pistons.