A direct resistively heated gas chromatography column with heating and sensing on the same nickel element

Nickel clad or nickel wired fused silica column bundles were constructed and evaluated. The nickel sheathing or wire functions not only as the heating element for direct resistive heat, but also as the temperature sensor, since nickel has a large resistive temperature coefficient. With this method the temperature controller is able to apply power and measure the temperature simultaneously on the same nickel element, which can effectively avoid the temperature overshoot caused by any delayed response of the sensor to the heating element. This approach also eliminates the cool spot where a separate sensor touches the column. There are some other advantages to the column bundle structure: (1) the column can be heated quickly because of the direct heating and the column's low mass, shortening analysis time. We demonstrate a maximum heating rate of 13 °C/s (800 °C/min). (2) Cooling time is also short, increasing sample throughput. The column drops from 360 °C to 40 °C is less than 1 min. (3) Power consumption is very low – 1.7 W/m (8.5 W total) for a 5 m column and 0.69 W/m (10.4 W total) for a 15 m column when they are kept at 200 °C isothermally. With temperature programming, the power consumption for a 5 m column is less then 70 W for an 800 °C/min ramp to 350 °C. (4) The column bundle is small, with a diameter of only about 2.25 in. All these advantages make the column bundle ideal for fast GC analysis or portable instruments. Column efficiencies and retention time repeatability have been evaluated and compared with the conventional oven heating method in this study. For isothermal conditions, the column efficiencies are measured by effective theoretical plate number. It was found that the plate number with resistive heat is always less than with oven heat, due to uneven heat in the column bundle. However, the loss is not significant – an average of about 1.5% for the nickel clad column and 4.5% for the nickel wired column. Separation numbers are used for the comparison with temperature programming, with results similar to those observed for isothermal conditions. Retention time repeatability for direct heat were 0.010% RSD for isotheral and 0.037% RSD for temperature programming, which is similar to those obtained by oven heat. Applications have been demonstrated, including diesel and PAH analysis.