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Figure 2 shows the system response while cycling between two set point temperatures of 16°C and 40°C. In addition to system response testing, the assembled system was monitored using a thermal imaging camera to detect any temperature variation between each of the heating zones and the attached capillary. Any variation would be an indication that the transfer of heat between the modules and capillary, or vice versa, was inefficient and that the capillary was at a different temperature than the module.
Figure 2. System response during cycling between 16 and 40°C Figure 3. Thermal image of assembled array and capillary showing heating mode gradient from 50°C to 30°C
Figure 3 shows a thermal image of a capillary column attached to the device during a separation. The modules are set up to provide a temperature gradient of 20°C, starting from 50°C at the head of the column, down to 30°C at the end of the column. It can be seen from the image that the capillary column is not visible, confirming that there is an efficient thermal transfer between the column and the Peltier modules. By using the thermal imaging camera the temperature gradient along the column is highly visible and can be monitored in real-time. Although using thermal imaging to visualise thermal profiles and heat transfer was necessary during the construction and testing of the device, it is not used to monitor and control zone temperatures during normal operation.
Testing & Results In order to test the effectiveness of the heating array, a mixture of alkylbenzenes (ethylbenzene, butylbenzene, and pentylbenzene) was separated on a lauryl methacrylate based monolithic capillary column. During initial trials, several separations were run at various isothermal set points and the retention factor (k) and number of theoretical plates were calculated for each temperature. Figure 4 shows the retention times of the different alkylbenzenes at increasing temperatures.
Figure 4. Retention times of alkylbenzenes at increasing temperatures
mounted on the array. Although the thermoelectric modules are capable of operating up to 230°C, the temperatures limits are set by the driver circuits for each of the modules and also by the efficiency of the heat exchanger. In order to protect the circuitry from overheating, the maximum and minimum operating temperatures were limited to 65°C and 15°C respectively. Working at these levels with the current heat exchanger the maximum current load for each channel is approximately 500 mA.
System Performance The performance of the system was tested by measuring the individual response of each of the modules against time, and by also monitoring the thermal stability over time. System response during heating and cooling was found to be an impressive 6°Cs-1 during heating-to-cooling, or vice versa, and approximately 3°Cs-1
during power off to
power on cycles. During stability testing it was found that deviation from steady state temperatures was in the region of ±0.2°C.
The effectiveness of the device as a simple column heater is demonstrated by the expected retention data shown in Figure 4, with column equilibration time between injections being a matter of just seconds, in both increased and decreased directions.
Current work is underway to further extend the temperature range of the prototype device and explore a range of novel applications of the new platform. However, this low cost and versatile capillary column heating/cooling platform certainly sets high performance standards for current commercially available column heaters to match.
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