Several of the cuvette holder products of Quantum Northwest may be modified for extended temperature operation. Suitable products are the Versa 20, Versa 20×2, Luma 40, CD 250, qpod and qpod 2e. Modification requires changes in design, materials and firmware. Proper use of these products requires knowledge of how to effectively operate them under extreme conditions. The finished product must be able to withstand many cycles of heating and cooling over a huge temperature range.
We begin with high quality components for all our cuvette holders. We use only Peltier units that are precisely flat and capable of use up to typically 200 °C. Metal cuvette towers are machined with precision, and contacting surfaces are flattened to high precision for good thermal contact. We use proprietary methods for ensuring long-term functionality. Peltier failures are very rare in our products, even after years of heavy use. The insulating covers on all our cuvette holders are composed of thermally annealed urethane or glass-filled nylon that distort little at the temperatures to which they will be subjected.
The temperature sensor used for PID temperature control is typically located in the metal cuvette tower at a height near one of the optical windows. Usually this sensor has a range of -40 °C to +125 °C. For the /E option, we replace the sensor with one that has a range of -55 °C to +150 °C. We also program the firmware of the corresponding temperature controller so that temperatures cannot be set to exceed the a maximum and a minimum value. Achievable temperatures will depend on the specific cuvette holder design and the conditions under which it is used. Often, special accessory products are required to achieve certain temperature ranges, and usually it is not possible to achieve the entire factory set range. Please see product specifications for each individual product for relevant temperatures in °C for “factory set range”, “range easily achievable” and “range achievable using extended temperature techniques”.
Other changes to the cuvette holder include, as needed, the use of silicone rather than vinyl tubing, physical changes to move the stirrer motor away from the Peltier heat exchanger and the use of wire with insulation rated at high temperature.
Use at very high temperatures
The Peltier unit is sandwiched between a brass heat exchanger and a flat surface under or beside the metal tower that holds the cuvette. For most use, water is flowed through the heat exchanger. Liquid flow is generally required to transfer the relatively large amount of heat required for rapid temperature changes over wide temperature ranges. When the temperature is raised, heat is transferred from the water to the tower. When working above 80 °C, or so, we recommend that the water flow be shut off. Otherwise the temperature difference between the flow and the cell holder becomes great enough to impede raising the temperature further. With the flow curtailed, it is no longer possible to drive the cell holder temperature down, since the heat quickly deposited in the empty heat exchanger will quickly shut down temperature control to prevent damage to the unit. However, temperatures may be raised all the way to the limit of the device. When you want the temperature to go down, you simply need to restart circulating flow. Once flow is restarted, then the temperature may be driven to the new desired value.
Please be careful. The extended temperature range is typically up to 150 °C! That is hot enough to cause a serious burn! When you are working at these temperatures, do not touch the cuvette or any hot surfaces!
Just remember that without flow you may drive to higher temperatures, but you may not drive it lower without restarting flow. If you are working in an extended temperature range, we recommend that you let the unit cool down by itself to below 100 °C, before restarting flow, to avoid unnecessarily shocking the system.
Use at very low temperatures
To work at low temperatures using the /E option, you must overcome four issues: (1) Cold objects become covered with condensed water or ice, when operated in ambient air. (2) Convection of air against the cuvette surface will transfer heat to your sample. (3) Insulation must be adequate to minimize the heat that must be transferred to achieve the desired temperature. (4) For the Peltier unit to be efficient, you must minimize the temperature difference between cuvette tower and the heat exchanger on the opposite side of the Peltier.
(1) A dry gas purge is critical for reducing condensation on the cuvette surfaces. Even a trace of condensation on a cuvette surface can alter your measurement. The user must provide a flow of dry gas (typically nitrogen) at a rate on the order of 50 cc/minute. In Quantum Northwest cuvette holders, dry gas flows up through holes the temperature-controlled base under the cuvette. Thus, the base acts as a heat exchanger for the gas. The cooled gas flows up through holes placed at the bottom of each optical port next to a cuvette. For some cuvette holders, gas flow serves a second function of reducing internal condensation around the Peltier and magnetic stirring motor. Without dry gas flow, please do not set temperatures below the dew point temperature or internal damage may occur.
Some cuvette holders are operated on an open workbench, whereas some are operated within and enclosed spectrometer sample compartment. Operated in the open, the cuvette holders are much more vulnerable to condensation. Operation in a sample compartment will often benefit from complete purging of the entire compartment.
(2) Windowed jackets are available for the Versa 20, Versa 20×2, Luma 40 and Flash 300 products. Windows will trap purge gas, eliminating condensation and reducing transfer of heat to the sample by convection. Even when using a windowed jacket on a bench top application, some condensation may form on the outside of the jacket’s windows. An additional small stream of dry gas may be required to eliminate this condensation.
(3) The windowed jackets, when available, are an easy way to add insulation to a cuvette holder. The more insulation that is added, the better the performance achieved when working at low temperatures. Other insulation, such as foam materials should be added around the cuvette holder to aid in reaching very low temperatures. Be sure not to block the light beams.
(4) Circulating ice water through the Peltier heat exchanger will usually permit routine use of a Quantum Northwest cuvette holder down to at least -25 °C or even -35 °C. For work at colder temperatures, you will need a refrigerated circulator. For fluid, we recommend 30% methanol/water, due its high heat capacity and low viscosity, although ethylene glycol mixtures will also work. Be sure to circulate fluid at a temperature within 25 °C or so of the temperature you wish to achieve.
The lowest temperature that we have reached was -52 °C using a Flash 300/E, circulating 30% methanol water at -15 °C. These cuvette holders are not cryogenic devices, but with proper care, you should be able to achieve the low temperatures that you require.