2008

Improved Thermal Modulator for Gas Chromatography

Varying temperature can be controlled precisely at relatively low power demand.

An improved thermal modulator has been invented for use in a variant of gas chromatography (GC). The variant in question — denoted as two-dimensional gas chromatography (2DGC) or GC-GC — involves the use of three series-connected chromatographic columns, in the form of capillary tubes coated interiorly with suitable stationary phases (compounds for which different analytes exhibit different degrees of affinity). The two end columns are relatively long and are used as standard GC columns. The thermal modulator includes the middle column, which is relatively short and is not used as a standard GC column: instead, its temperature is modulated to affect timed adsorption and desorption of analyte gases between the two end columns in accordance with a 2DGC protocol.

In general, what is required of a thermal modulator is to vary the temperature of the middle capillary tube in the following cycle:

  1. Maintain the tube at a specified low temperature — typically between –10 and –40 °C for a specified time (typically between 1 and 10 seconds);
  2. Heat (within tens of milliseconds) the tube to a specified high temperature (typically between 180 and 350 °C) and maintain this temperature for a specified time (typically between 10 and 200 milliseconds); then
  3. Cool (preferably within 200 milliseconds) the tube back to the low temperature.

Image
In the Thermal Modulator, the capillary tube is heated ohmically and is in direct contact with a circulating coolant liquid. The electrical resistance of thetube is used as temperature feedback for controlling the applied voltage or current to obtain the desired temperature-versus-time profile.
The degree to which this heating-andcooling profile can be exactly controlled can have significant effects on performance, because of an exponential dependence of gas-elution speed on modulator temperature.

What distinguishes the present thermal modulator from prior thermal modulators is an improved design that enables the required rapid cyclic heating and cooling with greater precision of temperature control and less power demand. The capillary tube is made of metal coated on its inner surface with a protective layer and a suitable stationary phase. Along most of its length, the tube lies in a cylindrical cavity in a housing (see figure). Rapid cooling is achieved through contact between the tube and a coolant liquid that is continuously pumped through the cavity. Rapid heating is achieved by passing a controlled electric current along the tube.

Because of the large radial temperature gradient occasioned by the narrowness of the capillary tube (typically no more than 2 mm wide) and the presence of coolant liquid in contact with the tube, it is difficult or impossible to measure the temperature of the tube accurately by use of a thermocouple, thermistor, or other conventional temperature sensor. In the improved design, no attempt is made to use a conventional temperature sensor. Instead, for purposes of monitoring and feedback control of the temperature, the electrical resistance of the tube is measured as a function of time and the temperature is computed in real time by use of temperature-vs.- resistance data obtained in prior calibration measurements on the tube.

This work was done by Ernest Frederick Hasselbrink, Jr.; Patrick J. Hunt, and Richard D. Sacks of the University of Michigan for Goddard Space Flight Center.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
University of Michigan
2350 Hayward Street Room 2150
Ann Arbor, MI 48019
Refer to GSC-14855-1, volume and number of this NASA Tech Briefs issue, and the page number.