Temperature is a key piece of the equation in any microwave acid digestion. Not only is accurate temperature measurement essential for a successful digestion, but it can also help to ensure reproducibility and safety. In addition to the traditional methods that have been developed to measure temperature in microwave digestion, several innovative solutions have recently made their way to market.
Tried and tested
Contactless infrared (IR) sensors are the most widely used technique to measure the temperature of a reaction vessel. By measuring surface temperature, these sensors can be useful for determining temperature differences between reaction vessels that have been loaded in a single batch. However, being an outer surface measurement method, contactless IR sensors are unable to directly monitor the internal temperature of the solution inside the vessel.
Additionally, manufacturers will often provide the option of an immersing temperature probe, which is immersed in the reaction vessel. “This is technically the most accurate way of measuring temperature and thereby controlling the microwave reaction,” says Reynhardt Klopper, an analytical and synthetic chemistry product specialist at Anton Paar (Ashland, VA). Still, this method has its limitations. Klopper points out the high cost of probes, along with the fact that a probe can only be applied to a single reaction vessel in a batch.
Bob Lockerman, a global product manager at CEM Corporation (Matthews, NC), notes that probes are also commonly used in combination with IR. When a probe is used to calibrate IR, the IR becomes more reliable and accurate, he explains.
Beyond the traditional sensors and probes
According to Art Ross, a product specialist in automated digestion systems at SCP Science (Baie-D’Urfé, Quebec, Canada), being able to measure temperature with IR in real time for 12 samples simultaneously is one of the more recent upgrades to microwave digestion. With new technology from SCP Science, researchers can “run multiple methods on the same rack at the same time in the digestion tunnel,” says Ross. For example, one could run a regulated method like a USEPA 3051 at 175 degrees Celsius and concurrently digest an oil sample at 200 degrees Celsius on the same rack.
Just last year, CEM launched a new product that is similar to IR in that it involves a single sensor, but Lockerman notes that unlike IR, this technology can see though the vessel and directly measure the temperature of the solution. “Instead of being a surface measurement like IR, iWave measures the actual solution temperature, and by doing that, can provide the functionality of a fiber optic probe but with the use of a sensor,” he says. Lockerman adds that the new technology provides accurate temperature information on each vessel, so if any one were to overheat, the power of the microwave would immediately adjust in response.
In a similar vein, Anton Paar has developed a technology in its microwave systems that can determine the temperature of the solution inside of the vessel with a noninvasive IR sensor. As Klopper explains, “A special algorithm, which is programmed into the microwave system software, will actually calculate the internal temperature of the solution.” The technology converts IR temperature values to corresponding sample temperatures based on several models, providing real internal temperature information without the need for a temperature probe.
Whereas temperature probes normally feature fiberoptic cables, which often get tangled and break when the user attempts to connect them to the inside surface of the microwave system, Anton Paar offers sensors that transmit temperature data wirelessly to the microwave software. And while CEM’s new product does require a wire to connect to the processor, it is completely wireless within the microwave cavity.
“Microwaves have been around for about 30 years, and temperature control has been around for about 25 years,” says Lockerman. Nonetheless, temperature measurement solutions for microwave digestion appear to be continually advancing.
For additional resources on microwave digesters, including useful articles and a list of manufacturers, visit www.labmanager.com/microwave