How to Maximize Column Oven Accuracy and Stability
Tips & Techniques for Long-term Instrument Performance
- Improve long-term data reproducibility through calibration.
- Reduce temperature variation by maintaining oven insulation.
Maintenance is a mantra in chromatography, and like almost every aspect of a gas chromatograph, the column oven requires routine preventative maintenance for optimal performance. This is important to keep in mind because column temperature has a profound effect on compound behavior inside the column. Many variables influence compound retention time, but column temperature is one of the few that can affect both absolute retention time and elution order (see “Retention Cross-over Phenomenon in Gas Chromatography Can the Mystery be Revealed?” for more about this effect). The simple tips and techniques presented here may improve the accuracy and precision of your data by keeping your oven at peak performance.
Oven Temperature Calibration
Ideally, your gas chromatograph arrived with a factory calibration where, for example, the actual oven temperature was within one percent of the oven set point. Most modern gas chromatographs are equipped with ovens that can control their temperatures at least to a tenth of a degree through a sophisticated sensor-heater feedback loop, so it is possible to achieve actual temperatures with less than a one percent deviation from the set point for a wide range of temperatures. Over time, however, the actual oven temperature can drift farther away from the set point. This can cause noticeable chromatographic effects, especially for laboratories running routine or quality-related analyses. The data shown in Figure 1 demonstrate the temperature variation in an uncalibrated column oven. To maintain a tighter temperature range around the set point, calibrate your oven using the following tips.
Use a temperature probe that is more sensitive than the oven temperature control system.
- Get a probe that measures to at least 0.01°C.
Be consistent with the temperature probe placement.
- Despite smaller oven volumes and forced air convection heating, temperature heterogeneity still exists inside the oven, so consistent probe placement will help when evaluating performance.
Place the temperature probe in a region occupied by the column.
- Because of the temperature gradient inside the oven, it is a good idea to measure temperature where the column will be. Note that it also is important to hang the column in the oven so that it isn’t touching the oven walls!
Figure 1 Inconsistent temperature inside an oven can reduce the accuracy and precision of your data.
In addition to reducing temperature variation from the set point, increasing temperature uniformity in the oven can improve data reproducibility. The observed temperature gradient shown in Figure 1 is created in part by the air flow pattern inside the oven. Introducing turbulence inside the oven can help homogenize the oven temperature, except at the point nearest the heating element (Figure 2). Keep in mind that some instruments only will allow the operator to enter temperature corrections for a single oven set-point temperature. As you’ll see in the next section, the ability of the oven to maintain an accurate temperature throughout a temperature programmed run will depend on more than just oven calibration.
Figure 2 Introduce turbulence to improve temperature uniformity in the oven.
Maintaining insulation is another important factor in achieving stable and accurate oven temperatures. If you perform maintenance on the sample injector, or on the detector, or insert temperature probes into the oven cavity, you can damage the insulation that surrounds the oven. This insulation is critical for keeping the oven isolated from the other heated zones of the chromatograph, as well as from the surrounding environment; use care when performing maintenance and inspect the oven insulation regularly. In addition to the insulation packed around the oven, gas chromatographs typically have a vent that opens and closes, allowing the hot air to be purged from the oven during a cool-down step. The vent (also known as a flapper or damper) has its own insulation which can, with time and usage, become damaged or fall off. Checking and replacing oven and vent insulation keeps your oven well-isolated and working at its peak across a range of typical working temperatures.
You can see from Figure 3 that oven calibration performed at only one temperature can’t always fix the more fundamental problem of maintaining known and accurate oven temperatures over a temperature range. Single-point calibration will merely fix a systematic error, not address the temperature drift observed in Figure 3. However, by keeping your oven insulation in good shape you can keep your oven’s performance consistent across the range of typically used temperatures. By proper oven maintenance and a better understanding of what is happening inside the oven cavity, you can make sure that your column is being heated evenly and consistently run after run.
Figure 3 Replacing oven and vent insulation dramatically improves consistency between the actual oven temperature and the set point over a wide operating range.