Wednesday, September 18, 2019

Resistive Heating Elements for Gas Chromatography

Gas Chromatography Heater
Custom gas chromatograph heater by BCE.

What is Gas Chromatography?

The most basic understanding of gas chromatography (GC) is that of a separation technique. It is used in analytical chemistry for the prompt separation, identification and quantification of compounds that can be vaporized without decomposition. Common uses of GC include testing the purity of a substance or separating and identifying the different components of a mixture.

The basic components of the GC system are:
  1. A long thin tube, referred to as a "column", and an oven or direct heating element that provides accurate temperature control and profiling. 
  2. The injector, which provides the means for the sample to enter the column.
  3. The detector, which provides the means for the sample to exit the column.
  4. The temperature control system.

Basic Understanding of Gas Chromatography

The sample (analyte) is transported through the column by the flow of an inert, gaseous mobile phase (typically nitrogen, helium, argon, or carbon dioxide). The GC column contains a liquid referred to as the "stationary phase" which, as it traverses the column, is repeatedly adsorbed onto the surface of an inert solid and desorbed back into the carrier gas stream. Based on an adsorbent's composition, it can have varying affinities to "hold". Because some constituent components are likely to spend more time in the stationary phase, while others are likely to spend less time, the separation process occurs.

Constant demand for optimized analytical throughput, GC portability, and less costly analysis propels the development of new gas chromatography designs and technologies. In recent decades, resistive heating technologies designed for the sample column heating have become commercially available and have grown in popularity. Resistive heaters have clear advantages over traditional air bath ovens, most notably direct, low-mass heating through conduction. The growth of portable GC systems has also benefitted from small, compact, low power resistive heaters.

The temperature control system is at the heart of gas chromatography accuracy, sensitivity and speed. This system is comprised of the electric heating elements, temperature controllers, and temperature sensors. All are critical in sample and column temperature regulation.

Gas Chromatography Heaters

This specialized category of electric heating element is designed to provide extremely uniform temperature profiles across the length of the GC column. This uniformity accommodates a stable temperature environment for the capture of repeatable and consistent data between runs. Gas chromatograph heaters allow for very precise temperature ramping and set point control to very tight tolerances. This is critical because the slightest fluctuation in column temperature during analysis will have significant effect on analysis outcomes.

Controlling the Temperature of the GC Column

Once the temperature profile of the column is established, a precision thermal control system is required to provide precision temperature control during the operation of the profile. The control system must be highly accurate and responsive, and must include advance control algorithms to handle a wide variety of column profiles.

Temperature Programmed Elution

In most cases, a ramping temperature profile is required when the analyte enters the column. The ramping temperature profile starts at a lower temperature and increases over time in a precise and linear fashion. At different points along the temperature profile, compound movement changes according to temperature exposure. This precise exposure to changing temperature enhances analyte separation and reduces cycle time.  Considering this, the heating element must be sized properly and have sufficient wattage to sustain the power requirements of the ramping temperature profile at all points along the ramp.

For more information about the application of resistive heaters for gas chromatography, contact a BCE applications engineer.