Faster, Cleaner, Safer: Why more labs are turning to microwave furnaces

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Conventional ovens and muffle furnaces are used daily in thousands of laboratories for many different applications from drying to ashing and are designed to provide continuous heating often for long periods. Microwave furnace systems can be used for exactly the same applications, albeit with significant advantages over conventional systems. Simon Osborne, from Analytix, examines the use of microwave furnace systems, the advantages they can offer over traditional systems, and why laboratories should consider these as an alternative when replacing their current technology.

To emphasise the benefits of microwave technology, we will examine one application that employs high temperatures, the determination of ash content. Whether for process control or as a preparation method for other analytical techniques, this is a relatively simple process. Many users however often comment on certain negative aspects of using conventional ovens or muffle furnace systems for this application, these being:

 • Long ashing times creating a potential bottleneck in sample preparation
 • Poor programmable temperature control and occasional incomplete ashing
 • Safety problems with fumes from samples escaping into laboratory
 • Heat from system affecting laboratory working conditions
 • High energy consumption with systems left on continuously

With increasing constraints - time and financial – on laboratories and testing facilities, these perceived drawbacks that hamper operating time, cost, and working conditions, can become seriously detrimental to overall performance and turnaround rate of samples and test subjects.

Microwave-enhanced sample preparation fundamentals

A special microwave-transparent ceramic muffle sits inside the microwave cavity that allows microwave energy to pass through so the sample can be heated directly. In the top or side sections of the muffle sits a silicon carbide plate that efficiently absorbs microwaves and rapidly heats up. Porous ceramic honeycomb frits built into the side walls of the muffle furnace allow a constant stream of air to pass over the sample crucibles. The combination of direct microwave heating, the radiated heat from the silicon carbide plates, and the “superheating” from the oxygen in the air flow through the frits, results in a dramatic reduction of ashing times.
A high-precision thermocouple located in close proximity to the bottom of the crucibles (and unaffected by the airflow) monitors the temperature inside the furnace. The thermocouple is used as a feedback control mechanism to regulate microwave power output and maintain the user selected ramping/ashing temperature. The high-efficiency microwave energy coupling characteristics of the silicon carbide plate allows the use of any type of crucible (metal, porcelain, disposable quartz fibre) within the microwave furnace.
Ashing times
Conventional ovens and muffle furnaces used in a huge number of laboratories for ashing, are inefficient in a number of respects that often leads to the process becoming tedious and time consuming. Users are often unaware how long the ashing process takes as samples are frequently left ashing overnight as the process cannot be completed during the working day. 
With microwave ashing instruments, ashing can be completed many times faster than traditional techniques have previously allowed and in some cases the process can achieve a 97% reduction in ashing times that in turn will dramatically improve upon existing workflows. By reducing ashing times by such a margin, laboratories can dramatically reduce turnaround time on incoming samples, for example, giving much faster feedback to production processes.
Temperatures of 800°C can be reached within 30 minutes, dramatically reducing ashing times, often from hours to minutes, and offers the laboratory the most versatile way to ash samples.